Tesis: "Superconductivity, Graphite"

1

Ballestar, Ana. "Superconductivity at Graphite Interfaces". Doctoral thesis, Universitätsbibliothek Leipzig, 2014. http://nbn-resolving.de/urn:nbn:de:bsz:15-qucosa-141196.

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The existence of superconductivity in graphite has been under discussion since the 1960s when it was found in intercalated graphitic compounds, such as C8K, C8Rb and C8Cs. However, it was only about 40 years ago when the existence of superconductivity in pure graphite came up. In this work we directly investigate the interfaces highly oriented pyrolytic graphite (HOPG) has in its inner structure, since they play a major role in the electronic properties. The results obtained after studying the electrical transport provide clear evidence on granular superconductivity localized at the interfaces of graphite samples. Zero resistance states, strong current dependence and magnetic field effect on the superconducting phase support this statement. Additionally, an abrupt reduction in the measured voltage at temperatures from 3 to 175 K has been observed. However, the upper value of this transition temperature seems to not have been reached yet. A possible method to enhance it is to increase the carrier density of graphite samples. In order to preserve to quasi-two-dimensional structure of highly oriented pyrolytic graphite, chemical doping has been dismissed in the frame of this work. We used an external electric field to move the Fermi level and, hence, try to trigger superconductivity in multi layer graphene samples. A drop on the resistance at around 17 K has been measured for a large enough electric field applied perpendicular to the graphene planes. This transition is strongly affected by magnetic field and only appeared at low temperatures. As a result of the studies included in this work, it appears clear that graphite has a superconducting phase located at certain interfaces with a very high transition temperature.

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2

Dean, M. P. M. "Superconductivity and electron-phonon interactions in graphite intercalation compounds". Thesis, University of Cambridge, 2010. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.598476.

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Systematic Raman scattering experiments were performed to compare the phonon properties in series of GICs BaC₆, SrC₆, YbC₆ and CaC₆. The key difference induced by changing the ions in this order is a reduction in the separation of the graphene layers, which is concurrent with an increase in the superconducting transition temperature T_c from < 80 mK to 11.5 K. It was possible to correlate the increase in T_cwith a softening of an out-of-plane carbon related phonon, which was explained in terms of increased charge in the carbon-related electronic band. This provides evidence that the carbon-related phonons and electronic-bands are crucial to the superconductivity in these compounds. An in-plane carbon phonon was also measured, which was shown not to follow the Born-Oppenheimer approximation. Recent theoretical attempts to explain these effects cannot fully account for the observed electron-phonon scattering rate. Neutron scattering was also used to measure the high energy carbon-related phonons in CaC₆. Due to the highly textured nature of the samples, special analytical techniques were developed to allow for the comparison between experiment and density functional theory (DFT). Overall, a good level of agreement between experiment and theory is found, which is significant in light of several other measurements of phonon related properties of CaC₆, which disagree with the theoretical predictions. YbC₆ was studied as a function of pressure to investigate the changes induced by reducing the layer separation. T_c initially increases consistent with the idea that moving the graphene layers closer increases T_c, however, at higher pressures T_c decreases disappearing at 7 GPa. These effects are discussed in light of a possible valence transition in YbC₆.

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3

Weller, T. E. "Superconductivity in the intercalated graphite compounds C6Yb and C6Ca". Thesis, University College London (University of London), 2007. http://discovery.ucl.ac.uk/1446134/.

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This thesis concerns the discovery of superconductivity in the intercalated graphite compounds C6Yb and C6Ca. A novel technique for synthesis of these intercalates has been developed, and is presented in detail. These two materials are shown to superconduct at 6.5K and 11.5K respectively. The superconductivity is demonstrated by measurements of the magnetisation and resistivity. Initial measurements of the superconducting transition of these materials as a function of pressure shows an increase in the transition with increasing pressure.

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4

Precker, Christian E., Pablo D. Esquinazi, Ana Champi, José Barzola-Quiquia, Mahsa Zoraghi, Santiago Muinos-Landin, Annette Setzer et al. "Identification of a possible superconducting transition above room temperature in natural graphite crystals". Universitätsbibliothek Leipzig, 2016. http://nbn-resolving.de/urn:nbn:de:bsz:15-qucosa-216014.

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Measuring with high precision the electrical resistance of highly ordered natural graphite samples from a Brazil mine, we have identified a transition at ∼350 K with ∼40 K transition width. The steplike change in temperature of the resistance, its magnetic irreversibility and time dependence after a field change, consistent with trapped flux and flux creep, and the partial magnetic flux expulsion obtained by magnetization measurements, suggest the existence of granular superconductivity below 350 K. The zero-field virgin state can only be reached again after zero field cooling the sample from above the transition. Paradoxically, the extraordinarily high transition temperature we found for this and several other graphite samples is the reason why this transition remained undetected so far. The existence of well ordered rhombohedral graphite phase in all measured samples has been proved by x-rays diffraction measurements, suggesting its interfaces with the Bernal phase as a possible origin for the high-temperature superconductivity, as theoretical studies predicted. The localization of the granular superconductivity at these two dimensional interfaces prevents the observation of a zero resistance state or of a full Meissner state.

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5

Ballestar, Ana [Verfasser], Pablo [Akademischer Betreuer] Esquinazi y Pablo [Gutachter] Esquinazi. "Superconductivity at Graphite Interfaces / Ana Ballestar ; Gutachter: Pablo Esquinazi ; Betreuer: Pablo Esquinazi". Leipzig : Universitätsbibliothek Leipzig, 2014. http://d-nb.info/1238601561/34.

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6

Camargo, Bruno Cury 1988. "Efeitos quânticos em semimetais de Dirac e heteroestruturas relacionadas". [s.n.], 2014. http://repositorio.unicamp.br/jspui/handle/REPOSIP/276954.

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Orientador: Iakov Veniaminovitch Kopelevitch
Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin
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Resumo: Neste trabalho serão apresentados os principais resultados obtidos peloautor no decorrer de seu doutorado. Os sistemas estudados eram compostos por grafite, grafeno, antimônio e interfaces de grafite/silício. Uma das partes do trabalho consistiu no estudo de efeitos de desordem estrutural sobre oscilações quânticas em grafite. O estudo revelou que a mosaicidade da grafite estudada, largamente utilizada para se determinar a qualidade de amostras de grafite pirolítico altamente orientado (HOPG), não apresenta correlação com a amplitude das oscilações quânticas no material. Ao invés disso, os experimentos mostraram uma clara correlação entre a rugosidade superficial, a mobilidade eletrônica média e a amplitude do efeito de Haas van Alphen no material. Os resultados indicam que deformações da superfície da grafite afetam fortemente a mobilidade eletrônica do material (reduzindo a amplitude de oscilações quânticas) sem reduzir sua anisotropia. No trabalho, também é discutida a possibilidade de que as oscilações quânticas em grafite estejam relacionadas com a existência de interfaces bem definidas na estrutura interna do material. Também foram estudadas propriedades de transporte elétrico interplanar em grafite no limite ultraquântico. Medidas de magnetorresistência interplanar para campos magnéticos de até 60 T acusaram a ocorrência de uma região de magnetorresistência positiva seguida de magnetorresistência negativa (MRN) para campos magnéticos suficientemente altos. O efeito persistia até temperatura ambiente. Ele é explicado considerando-se o tunelamento de férmions de Dirac entre níveis fundamentais de Landau de planos de grafeno adjacentes dentro da grafite. A região de MRN é mais pronunciada em grafites com menor mosaicidade, o que sugere que o alargamento de níveis de Landau seja responsável pela magnetorresistência positiva observada nas medidas ao longo do eixo c da grafite. Além disso, experimentos de magnetorresistência interplanar com campos magnéticos orientados paralelamente à direção dos planos da grafite apresentaram indícios de que o material se torna mais tridimensional com a redução da temperatura. Os resultados sugerem que a integral de overlap interplanar em grafite possui valor ?1 < 7 meV. Esse valor é muito inferior àqueles reportados na literatura considerando-se o modelo mais bem aceito para grafite, segundo o qual ?1 ? 380 meV. Nesta tese também são apresentados resultados inéditos obtidos pelo autor relacionados a efeito Hall quântico em grafeno crescido epitaxial mente sobre substratos de carbeto de silício, efeitos de desordem estrutural sobre as propriedades de transporte elétrico basal da grafite, supercondutividade em heteroestruturas de grafite e silício e supercondutividade em compósitos de antimônio-ouro
Abstract: In this thesis, experimental results obtained by the author during his PhD will be presented. The work consisted on the study of electrical and magnetic properties of Dirac semimetals and related heterostructures. Namely: graphite, graphene, graphite/silicon interfaces and antimony. Part of the work about graphite consisted on the study of the effects of structural disorder on the quantum oscillations in the material. Experimental results in the literature widely regard the mosaic spread in graphite as a good disorder parameter. However, in the present work, we report that the mosaicity of graphite samples does not correlate with their quantum oscillations¿ amplitude. Experiments have revealed a clear relation of surface roughness to the electronic mobility and the amplitude of the deHaas van Alphen effect in the material. The possibility that quantum oscillations in graphite are affected by the presence of sharp interfaces within its stacking structure is also discussed. We have also studied out-of-plane magnetoresistance properties in ultraquantum graphite. Experiments performed at magnetic fields B//c up to 60 T have shown the occurrence of positive c-axis magnetoresistance followed by a region of negative magnetoresistance (NMR). The NMR persists up to room temperature and has been explained in terms of the tunneling of electrons between zero-energy Landau levels of adjacent graphitic layers. The NMR is more evident in samples with low mosaicity, suggesting the positive c-axis magnetoresistance is induced by means of broadening of LL¿s by disorder. In addition, c-axis magnetoresistance measurements with magnetic fields perpendicular to c-axis (B?c) suggest that our samples undergo a 2D to 3D transition with the reduction of temperature. Based on our results, we estimate a value for the interplane hopping energy parameter ?1 < 7 meV. This value is at odds with the most accepted model for graphite, for which ?1 ? 380 meV. In this thesis, we also present unpublished results on the occurrence of quantum Hall effect in graphene grown epitaxially in silicon carbide substrates, on the effects of structural disorder in the basal electric properties of graphite
Doutorado
Física
Doutor em Ciências

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7

Merlo, Rafael Borges 1983. "Supercondutividade em materiais à base de carbono". [s.n.], 2011. http://repositorio.unicamp.br/jspui/handle/REPOSIP/277964.

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Orientador: Iakov Veniaminovitch Kopelevitch
Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin
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Resumo: Evidências experimentais e teóricas recentes de que a supercondutividade nas várias formas alotrópicas do carbono pode ocorrer em temperaturas próximas ou até acima da temperatura ambiente, desencadearam um grande interesse científico. Resultados do presente trabalho demonstraram a ocorrência de supercondutividade em compósitos de carbono vítreo-enxofre (CV-S) a T = 3 K. Nossas medidas revelaram que a supercondutividade ocorre em uma pequena fração da amostra, e que a grafitização do carbono amorfo parece ser uma condição necessária para disparar a supercondutividade. Apresentamos também evidências de supercondutividade à temperatura ambiente em sanduíches de grafite/Si, e demonstramos que a supercondutividade está associada à interface grafite/silício. O comportamento encontrado é semelhante ao conhecido para estruturas supercondutoras de baixadimensionalidade. Em particular, observamos oscilações do tipo Josephson em curvas características de corrente-tensão (I-V), bem como sua supressão pela aplicação de campo magnético. Além disso, o campo magnético perpendicular transforma as curvas características I-V do tipo supercondutor para tipo isolante, assemelhando-se à transição supercondutor-isolante induzida por campo magnético em redes de junções Josephson. Todos estes resultados indicam que a interface grafite/silício pode ser um material promissor para o desenvolvimento de dispositivos microeletrônicos sem dissipação à temperatura ambiente
Abstract: Recent both experimental and theoretical evidence that superconductivity in various allotropic forms of carbon can occur at temperatures near or even above room temperature, triggered a broad scientific interest. Results of the present work demonstrated the occurrence of superconductivity in carbon glassy-sulfur composites (CV-S) at T = 3 K. Our measurements revealed that the superconductivity occurs in a small fraction of the sample, and that the graphitization of the amorphous carbon seems to be a necessary condition to trigger the superconductivity. We also present evidence for the room temperature superconductivity in graphite/Si sandwiches and demonstrate that the superconductivity is associated with the graphite/silicon interface. The found behavior is similar to that known for low-dimensional superconducting structures. In particular, we have observed Josephson-type oscillations in current-voltage (I-V) characteristics as well as their suppression by applied magnetic field. Moreover, the perpendicular magnetic field transforms the superconducting-like to insulating-like I-V characteristics resembling the magnetic-field-driven superconductor-insulator transition in Josephsonjunction-arrays. All these results indicate that graphite-silicon interface can be a promising material for the development of microelectronic devices without dissipation at room temperature
Mestrado
Supercondutividade
Mestre em Física

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8

Gutierrez, Yatacue Diego Fernando. "Efeito de proximidade gigante entre supercondutor e grafite". [s.n.], 2009. http://repositorio.unicamp.br/jspui/handle/REPOSIP/277897.

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Orientador: Iakov Veniaminovitch Kopelevitch
Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin
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Resumo: No intuito de verificar a existência de correlações supercondutoras em grafite, estudamos a possível existência do fenômeno conhecido como efeito de proximidade gigante em amostras de grafite pirolítica altamente orientada (HOPG). Medidas de magneto-transporte realizadas em amostras de HOPG com eletrodos supercondutores de In ou In-Pb revelaram a ocorrência de efeito de proximidade em uma escala muito maior que o comprimento de coerência dos eletrodos supercondutores, o que indica que a grafite pode ser considerada um supercondutor com flutuações de fase. Além disso, nossos estudos revelaram uma supressão do efeito de proximidade para campos magnéticos da ordem de 1 kOe aplicado perpendicularmente aos planos de grafite. Adicionalmente, realizamos estudos comparativos do efeito de proximidade em bismuto metálico. Discutimos os resultados obtidos em termos de modelos teóricos propostos para este assunto.
Abstract: In order to verify the existence of superconducting correlations in graphite, in this work we studied the possibility of the so-called giant proximity effect in highly oriented pyrolytic graphite (HOPG) samples. Magnetoresistance measurements performed on various thoroughly characterized HOPG samples with attached superconducting In or Pb-In electrodes revealed the occurrence of proximity effect on a scale much bigger than a coherence length of superconducting electrodes, indicating that graphite can be considered as a phase-fluctuating superconductor, indeed. Besides, our studies revealed a suppression of the proximity effect in magnetic field H ~ 1 kOe applied perpendicularly to graphene planes. Additionally, we performed comparative studies of the proximity effect in semimetallic bismuth. We discuss the obtained results in terms of available theoretical models.
Mestrado
Física da Matéria Condensada
Mestre em Física

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9

El, Bana Mohammed Sobhy El Sayed. "Superconductivity in two-dimensional crystals". Thesis, University of Bath, 2013. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.589655.

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Since the first isolation of graphene in 2004 interest in superconductivity and the superconducting proximity effect in monolayer or few-layer crystals has grown rapidly. This thesis describes studies of both the proximity effect in single and fewlayer graphene flakes, as well as the superconducting transition in few unit cell chalcogenide flakes. Optical and atomic force microscopy and Raman spectroscopy have been used to characterise the quality and number of molecular layers present in these flakes. Graphene structures with superconducting Al electrodes have been realised by micromechanical cleavage techniques on Si/SiO2 substrates. Devices show good normal state transport characteristics, efficient back-gating of the longitudinal resistivity, and low contact resistances. Several trials have been made to investigate proximity-induced critical currents in devices with junction lengths in the range 250-750 nm. Unfortunately, no sign of proximity supercurrents was observed in any of these devices. Nevertheless the same devices have been used to carefully characterise proximity doping, (due to the deposited electrode), and weak localisation/anti-localisation contributions to the conductivity in them. In addition this work has been extended to investigations of the superconducting transition in few unit-cell dichalcogenide flakes. Four-terminal devices have been realised by micromechanical cleavage from a 2H-NbSe2 single crystal onto Si/SiO2 substrates followed by the deposition of Cr/Au contacts. While very thin NbSe2 flakes do not appear to conduct, slightly thicker flakes are superconducting with an onset ܶ௖ that is only slightly depressed from the bulk value (7.2K). The resistance typically shows a small, sharp, high temperature transition followed by one or more broader transitions, which end in a wide tail to zero resistance at low temperatures. These multiple transitions appear to be related to disorder in the layer stacking rather than lateral inhomogeneity. The behaviour of several flakes has been characterised as a function of temperature, applied field and back-gate voltage. The resistance and transition temperatures are found to depend weakly on the gate voltage. Results have been analysed in terms of available theories for these phenomena.

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10

Shajari, Hasti. "Gate-tunable superconductivity in thin films and layered crystals". Thesis, University of Bath, 2018. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.760970.

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Theoretical and experimental work on superconductivity has won a number of Nobel prizes in Physics, beginning with the prize to Kamerlingh Onnes in 1913 that included the initial discovery of superconductivity in mercury. Superconductivity has since been at the forefront of research in condensed matter physics. Furthermore, since the first isolation of graphene by Geim and Novoselov in 2004, there has been growing interest in other monolayer and few-layer crystals. Like graphene, other materials can be exfoliated due to the weak van der Waals interactions between layers, primarily the transition metal dichalcogenides (TMDs). Atomically flat and chemically stable thin two dimensional (2D) layers of TMDs have opened up new opportunities for discovering exciting new physics and ultimately developing thin flexible devices. Defect-free exfoliated TMDs are regarded to be ideal materials for use as channels for field effect transistors (FET), which have been shown to possess remarkable electronic properties. Recent advances in field effect-based TMD devices have been achieved using ionic liquid gating and the formation of electrical double layers. Using the techniques previously developed for isolating graphene, few-layer crystals of 1T- and 2H-TaS2 have been obtained in this project to be used as channel materials for FET and ionic field effect transistor (iFET) devices that incorporate DEME-TFSI ionic liquids as a top gate to control the carrier density. In the first experimental chapter (chapter 5) iFETs using a 1 μm thin film of a highly boron-doped diamond (BDD) as the channel material are introduced and the influence of top gating on the transition temperature using a DEME-TFSI ionic liquid is studied. An enhancement in the Tc of the BDD sample under positive top gate potentials is shown as a result of electron doping at the grain boundaries leading to stronger coupling between the grains. The following chapter (Chapter 6) describes low temperature measurements of graphene FET (GFET) devices. These devices were fabricated to enable a reliable and effective calibration for the DEME-TFSI top gate specific capacitance against the known back gate capacitance. This represents a valuable reference for ionic liquid gating studies of TMD materials. The last experimental chapter describes the electrical properties of few-layer 1T-TaS2 (initial section) and 2H-TaS2 (final section) samples used as channels in FET devices. Charge density wave (CDW) transitions in 1T- and 2H-TaS2 are investigated and gating measurements using ionic liquids on these samples are described and summarised. Although no gate influence was seen on the CDW in 2H-TaS2 , a suppression of the CDW transition in cooling cycles of a 1T-TaS2-based FET sample was observed. This suppression demonstrates that accumulation of additional charge carriers in the sample drives it into a metallic state. In a ∼15 nm 2H-TaS2 FET device, strong enhancement of the superconducting critical temperature from 0.8 to 4.7 K is observed with DEME-TFSI top gating. The influence of an additional back gate potential on the device enhances the transition temperature still further up to 5 K. This indicates a co-operative effect between the top and back gates of the sample. It was also demonstrated that 2H-TaS2 crystals are susceptible to intercalation by DEME+ cations in the ionic liquid; a clear enhancement of Tc was observed after simply placing a drop of ionic liquid on a 2H-TaS2 flake without application of a top gate bias. This research project has studied superconductivity in 2D materials and illustrates the capability of ionic liquid gating as a versatile tool to modify the carrier concentration and enhance the critical temperature of a wide range of different materials.

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11

Chapman, James Oliver. "Alkali metal doped graphene : superconductivity, structural, magnetic and optical properties". Thesis, University of Manchester, 2015. https://www.research.manchester.ac.uk/portal/en/theses/alkali-metal-doped-graphene-superconductivity-structural-magnetic-and-optical-properties(0ae09e60-4104-465a-a5ad-9cd2d6816661).html.

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Intercalation of graphite with alkali metals has previously been shown to, in some cases, produce superconducting compounds from the two non-superconducting components. The use of graphene as a basis to continue this research offers new possibilities as confinement of intercalant species is reduced from bulk graphite. Papers comprised of exfoliated graphene flakes were doped with Li, Cs, K and Ca atoms via vapour transport methods in order to investigate superconducting properties. While Li, Cs and K-doping showed no signs of a superconducting transition as low as 1.8 K, observed through magnetic measurements, Ca-doped graphene became superconducting below 6.4 K – a lower transition temperature than Ca-doped graphite, TC = 11.5 K. The carrier concentration could also be changed using composite papers made from graphene and various proportions of insulating boron nitride flakes, allowing TC to be varied. Optical reflectivity spectra were used to determine the level of doping present in each compound, directly calculated from their estimated plasmon energy. Ca-doped graphene paper exhibited a 20% lower carrier concentration than Ca-intercalated graphite, offering an explanation for the lower value of TC. To allow insight into the partial doping of graphene papers, samples were exposed to air and monitored via dynamic x-ray diffraction techniques and optical analysis during degradation. With prolonged reaction in air, the carrier concentration was found to drop monotonically, while the interlayer separations contracted as intercalant species vacated the structure, leaving an arrangement of flakes similar to that of the initial, un-doped, graphene paper. The range of carrier concentrations observed suggests that doping of graphene flakes is non-discrete, thus implying tunable TC.

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12

Holland, Kiar. "Doping as a Possible Means to create Superconductivity in Graphene". FIU Digital Commons, 2016. http://digitalcommons.fiu.edu/etd/2550.

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The possibility of creating superconductivity in Highly Oriented Pyrolytic Graphite (HOPG) by means of doping was investigated. Bulk HOPG samples were doped with phosphorous using either ion-implantation or by Chemical Vapor Deposition growth with phosphine in the gas mixture. The methods for testing the graphene samples, once doped, were done by performing R vs. T measurements, and determining via observation suppressed superconductive characteristics signaling the presence of the Meissner Effect in a strong applied magnetic field. Before doping, the resistance vs. temperature (R vs. T) characteristic of the HOPG was measured. The R vs. T characteristic was again measured after doping, and for surface multilayers of graphene exfoliated from the post doped bulk sample. A 100 to 350 mT magnetic field was supplied, and the R vs. T characteristic was re-measured on a number of samples. Phosphorous-implanted HOPG samples exhibit deviations from the expected rise in resistance as the temperature is reduced to some point above 100 K. The application of a modest magnetic field reverses this trend. A step in resistance at a temperature of approximately 50-60 K in all of the samples is clearly observed, as well as a second step at 100-120 K, a third at a temperature range of 150-180 K and a fourth from about 200-240 K. A response consistent with the presence of magnetic field flux pancake vortices in phosphorous implanted HOPG and in phosphorous-doped exfoliated multilayer graphene has been observed. The lack of zero resistance at low temperatures is also consistent with pancake vortex behaviour in the flux-flow regime. The presence of magnetic vortices requires, and is direct evidence of superconductivity.

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13

Ludbrook, Bartholomew Mears. "Electron-phonon mediated superconductivity probed by ARPES : from MgB2 to lithium-decorated graphene". Thesis, University of British Columbia, 2014. http://hdl.handle.net/2429/51260.

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This thesis traces a path from conventional superconductivity in a bulk material to the introduction of superconductivity and other novel phenomena in graphene. Magnesium diboride is a conventional superconductor, where the pairing is mediated by the electron-phonon coupling. ARPES (angle resolved photoemission spectroscopy) is shown to be an excellent probe to quantitatively study the momentum dependence of the electron-phonon coupling, demonstrating the origin of the distribution of superconducting gap sizes previously observed with other experimental techniques. Next, we exploit our understanding of the electron-phonon coupling to study how it can be tuned in a low dimensional system. It is shown that the electron-phonon coupling in graphene can be strongly enhanced by the deposition of alkali adatoms. High resolution, low temperature ARPES measurements provide the first experimental evidence of superconductivity in this two-dimensional system, showing a temperature dependent pairing gap, and an estimated Tc of ~ 6K. Finally, we present a study of another graphene-adatom system expected to show novel physics. Thallium on graphene has been predicted to enhance the spin-orbit coupling, leading to a robust topological insulator state. From ARPES measurements characterizing this system, we disentangle the long-range and short-range scattering contributions and show that thallium atoms act as surprisingly strong short-range scatterers. Our results are consistent with theoretical predictions for this system, indicating it is a good place to search for a two-dimensional topological insulator.
Science, Faculty of
Physics and Astronomy, Department of
Graduate

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14

Huder, Loïc. "Lien entre structure et propriétés électroniques des moirés de graphène étudié par microscopie à effet tunnel". Thesis, Université Grenoble Alpes (ComUE), 2017. http://www.theses.fr/2017GREAY083/document.

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Les dernières années ont vu l'avènement des couches cristallines bidimensionnelles, appelées matériaux 2D. L'exemple le plus connu est le graphène, d'autres étant le nitrure de bore hexagonal isolant et le diséléniure de niobium supraconducteur. Ces matériaux 2D peuvent être empilés de manière contrôlée sous la forme d'hétérostructures de van der Waals pour obtenir les propriétés électroniques désirées. L’une des plus simples hétérostructures de van der Waals est l'empilement de deux couches de graphène tournées. Cet empilement donne naissance à un moiré qui peut être vu comme un potentiel superpériodique dépendant de l'angle entre les deux couches. Les propriétés électroniques des couches tournées de graphène sont intimement liées à ce moiré.Le sujet de cette thèse est l'étude expérimentale du lien entre la structure et les propriétés électroniques des couches tournées de graphène par Microscopie et Spectroscopie à effet tunnel à basse température.Alors que l'effet de l'angle entre les couches sur les propriétés électroniques a déjà été étudié en détail, la modification de celles-ci par une déformation des couches n'a été envisagée que récemment. La première partie de ce travail expérimental étudie la modification par la déformation des propriétés électroniques de couches de graphène tournées d'un angle de 1.26° crûes sur carbure de silicium. La déformation en question est différente dans les deux couches et son effet apparait clairement dans la densité locale d'états électroniques du moiré. Contrairement à une déformation appliquée identiquement aux deux couches, une différence de déformations entre les couches (déformation relative) modifie fortement la structure de bandes même à faibles valeurs de déformations. Alors que la déformation relative était spontanément présente, la deuxième partie de cette thèse s'intéresse à l'effet d'une déformation appliquée directement aux couches de graphène. Cette déformation vient d'une interaction induite par l'approche de la pointe STM vers la surface de graphène. La modification active de la densité d'états qui en résulte dépend de la position de la pointe dans le moiré avec l'apparition d'instabilités périodiques lorsque la distance entre la pointe et l'échantillon est très faible.La troisième partie de cette thèse concerne l'étude d'un autre type de modification des propriétés électroniques consistant en l'induction de supraconductivité dans les couches de graphène. Cette modification est effectuée par une croissance du graphène en une seule étape sur du carbure de tantale supraconducteur. Les résultats montrent la formation d'une couche de carbure de tantale de grande qualité sur laquelle les couches de graphène forment des moirés. La mesure à basse température de la densité d'états de ces moirés montre la présence d'un effet de proximité supraconducteur induit par le carbure de tantale
Recent years have seen the emergence of two-dimensional crystalline layers, called 2D materials. Examples include the well-known graphene, insulating hexagonal boron nitride and superconducting niobium diselenide. The stacking of these 2D materials can be controlled to achieve desirable electronic properties under the form of van der Waals heterostructures. One of the simplest van der Waals heterostructures is the misaligned stacking of two graphene layers. Twisted graphene layers show a moiré pattern which can be viewed as a superperiodic potential that depends on the twist angle. The electronic properties of the twisted graphene layers are strongly linked to this moiré pattern.The subject of the present thesis is the experimental study of the link between the structural and the electronic properties of twisted graphene layers by means of low-temperature Scanning Tunneling Microscopy and Spectroscopy (STM/STS).While the effect of the twist angle has already been studied in great details, the modulation of the electronic properties by the deformation of the layers has been explored only recently. In the first part of this experimental work, a strain-driven modification of the electronic properties is probed in graphene layers with a twist angle of 1.26° grown on silicon carbide. The determined strain is found to be different in the two layers leading to a clear signature in the local electronic density of states of the moiré even at low strain magnitudes. Contrary to a strain applied in the two layers, this difference of strain between the layers (relative strain) modifies strongly the electronic band structure even at low strain magnitudes. While this relative strain is natively present, the second part of the work explores the effect of an applied strain in the layers. This is realized by approaching the STM tip to the graphene surface to trigger an interaction between the two. The resulting active modification of the density of states is shown to depend on the position on the moiré, leading to periodic instabilities at very low tip-sample distances.In the third part of the work, another type of modification of the electronic properties is studied when superconductivity was induced in the graphene layers. This is done by growing graphene on superconducting tantalum carbide in a single-step annealing. The results show the formation of a high-quality tantalum carbide layer on which graphene layers form moiré patterns. The low-temperature density of states of these moirés show evidence of a superconducting proximity effect induced by the tantalum carbide

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15

Löthman, Tomas. "Point defects in the (d+id)-wave superconducting state of heavily doped graphene". Thesis, Uppsala universitet, Institutionen för fysik och astronomi, 2013. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-213212.

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Previous studies have suggested that the material graphene might transition into an electron-electron interaction driven, unconventional, time-reversal-symmetry-breaking, (d+id)wave superconducting state upon either significant electron or hole doping, and, in particular, upon doping to the Van Hove singularity. As defects are likely to be introduced in the doping process, we are, in this text, concerned with the effects of defects on this superconducting state near the Van Hove singularity doping. To investigate the effects we use a mean-field treatment of a phenomenological resonant-valence-bond model. We find that the resonant-valence-bond amplitudes, which in the defect free graphene sheet are proportional to the superconducting pairing-potential, are suppressed near the defects, and that the recovery is well described by an exponential, yet anisotropic, recovery. In general, we find that the (d+id)-wave, superconducting state is quite resilient, and that even for strong defects, such as a vacancy, the recovery length is of the order of one lattice constant when extrapolated to weak pairing-potentials; this is compared to a conventional superconducting state of an attractive Hubbard model for which the same decay length is found to be of the order of a half lattice constant. For the defect free graphene sheet the (d+id)-wave state is a completely gapped state. The introduction of vacancies is, however, found to be accompanied by the appearance of midgap states. These states are shown to be localized around the vacancies. In accordance with the nature of this text, we will, for the benefit of students and non-experts, include an introductory section on the fundamental methods and concepts used. It gives a short and hopefully pedagogical introduction to the rudimentary concepts of solid state theory and the microscopic BCS theory of superconductivity.

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16

Bordaz, Julien [Verfasser] y H. von [Akademischer Betreuer] Löhneysen. "Proximity-induced superconductivity in single-layer and bilayer graphene / Julien Bordaz. Betreuer: H. von Löhneysen". Karlsruhe : KIT-Bibliothek, 2014. http://d-nb.info/1050767535/34.

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17

Hümpfner, Tobias [Verfasser], Torsten [Gutachter] Fritz, Jörg [Gutachter] Fink y Reinhold [Gutachter] Kleiner. "Superconductivity of potassium-intercalated epitaxial graphene / Tobias Hümpfner ; Gutachter: Torsten Fritz, Jörg Fink, Reinhold Kleiner". Jena : Friedrich-Schiller-Universität Jena, 2021. http://d-nb.info/1230468633/34.

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18

Hudson, David Christopher. "Two dimensional atomically thin materials and hybrid superconducting devices". Thesis, University of Exeter, 2014. http://hdl.handle.net/10871/16034.

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In this thesis a variety of topics concerning 2D materials that have been separated from bulk layered crystals are discussed. Throughout the thesis, single and few layers of graphene, fluorinated graphene, MoS2 and WS2 are used. Two new methods of freely suspending 2D materials are presented as well as a method of removing the background from optical images. This aids contrast measurements for the determination of the number of layers. Fluorinated graphene is found to be sensitive to beta radiation; the resistance of fluorinated graphene transistors is shown to decrease upon exposure to the radiation. This happens due to the carbon-fluorine bond breaking. The sp3 hybridised structure of the fluorinated graphene is reduced back into the sp2 hybridised structure of pristine graphene. The superconducting properties of molybdenum-rhenium are characterised. It is shown to have a transition temperature of 7.5 K. It is also discovered that the material has a resistance to hydrofluoric acid; the acid etches nearly all other superconducting materials. This makes MoRe a possible candidate to explore superconductivity in conjunction with high mobility suspended graphene. To see if the material is compatible with graphene, a supported Josephson junction is fabricated. A proximity induced super current is sustained through the junction up to biases of ∼ 200 nA. The temperature dependence of the conductivity is measured for both suspended MoS2 and WS2 on a hexagonal boron nitride substrate. The dominant hopping mechanism that contributes to the conductivity at low temperatures is found to be Mott variable range hopping, with the characteristic T−1/3 dependence. The hopping transport is due to impurities that are intrinsic to the crystals, this is confirmed by comparing the results with those of supported devices on SiO2.

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19

Cohnitz, Laura Ann [Verfasser], Reinhold [Gutachter] Egger y Carsten [Gutachter] Müller. "Flat bands, snake states and superconductivity in graphene monolayers / Laura Ann Cohnitz ; Gutachter: Reinhold Egger, Carsten Müller". Düsseldorf : Universitäts- und Landesbibliothek der Heinrich-Heine-Universität Düsseldorf, 2017. http://d-nb.info/114933035X/34.

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20

Du, Renjun [Verfasser] y H. von [Akademischer Betreuer] Löhneysen. "Quantum transport in bilayer graphene: Fabry-Pérot interferences and proximity-induced superconductivity / Renjun Du. Betreuer: H. von Löhneysen". Karlsruhe : KIT-Bibliothek, 2015. http://d-nb.info/1080246320/34.

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21

De, Cecco Alessandro. "Electronique quantique dans les nano-structures explorées par microscopie à sonde locale". Thesis, Université Grenoble Alpes (ComUE), 2018. http://www.theses.fr/2018GREAY035/document.

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Les nano-structures sont des systèmes physiques de premier intérêt pour les études de base et pour les applications, car elles montrent des effets quantiques comme le confinement, la discrétisation énergétique, la cohérence... Le comportement quantique des nano-dispositifs peut être cependant fortement influencé par le désordre, les effets thermiques et hors-équilibre. Dans cette Thèse, nous montrons, par exemple, comment la dissipation affecte le transport électronique dans les dispositifs supraconducteurs soumis aux fréquences micro-ondes.En utilisant un setup cryogénique AFM/STM fait maison, on peut étudier différents types de nano-structures. En premier, nous nous occupons de la réalisation d'un transistor à électron unique avec une sonde locale. Les nano-particules métalliques sont bien connues pour leur comportement comme boîtes quantiques zéro-dimensionnelles (QD), elles montrent du confinement quantique et des effets de charge, que l’on retrouve aussi dans nos mesures de microscopie à sonde locale à basse température. Nous démontrons comment un nouveau procédé de nano-fabrication peut être mis en œuvre avec l'introduction d' une électrode de grille suffisamment mince et sans-fuite, ce qui pourra fournir un réglage de précision des propriétés de la boîte quantique et permettre l'exploration résolue spatialement des phénomènes quantiques dans différents régimes de couplage. En deuxième, nous étudions le graphène épitaxial sur SiC comme un matériau 2D très prometteur pour l'électronique. En particulier, les nano-rubans de graphène obtenus par croissance épitaxiale sur des parois inclinées (GNRs) sont des nano-structures d'intérêt fondamental qui peuvent fournir un accès direct et contrôlable au graphène neutre. À cause du confinement quantique, ces systèmes peuvent montrer du transport balistique exceptionnel à température ambiante. Nous réalisons une technique novatrice de potentiométrie à sonde locale qui nous permet une résolution spatiale à l'échelle du nm et une résolution en tension à l'échelle du µV. Le potentiel locale et la résistance locale mesurés sur un dispositif unique basé sur des nano-rubans de graphène nous donnent des indications claires de transport non-diffusif.La physique explorée, les méthodes ainsi que les technique développées dans cette Thèse peuvent donc fournir des nouvelles visions aux nombreux (et assez divers) sujets en vogue
Nanostructures are physical systems of paramount interest for both fundamental studies and applications, since they display quantum effects such as confinement, energy discretization, coherence…The quantum behavior of nano-devices can however be strongly influenced by disorder, thermal and non-equilibrium effects. In this Thesis, we show, for instance, how dissipation deeply affects the electron transport in superconducting nano-devices at microwave frequencies.By using a home-made cryogenic AFM/STM setup, we are able to investigate different kinds of nanostructures. First, we address the realization of a Single Electron Transistor with a Scanning Probe. Metallic nanoparticles are well known for their behavior as 0D-Quantum Dots (QD), and they display quantum confinement and charging effects, which are evidenced in our low-temperature SPM measurements as well. We demonstrate how a novel nanofabrication process can be implemented with the addition of gate electrodes sufficiently thin and leakage-proof, which in the future can provide a fine-tuning of the QD's properties and allow spatially-resolved exploration of quantum phenomena in a variety of different coupling regimes. Second, we study epitaxial graphene on SiC as a very promising 2D material for electronics. In particular, epitaxial sidewalls graphene nanoribbons (GNRs) are nanostructures of fundamental interest which can provide direct and controllable access to charge neutral graphene. Due to quantum confinement, these systems can display exceptional ballistic transport at room temperature. We implemented an innovative Scanning Tunneling Potentiometry technique allowing for nm-scale spatial resolution and μ V-scale voltage resolution. Measured local potential and resistance of single GNRs devices provide clear indication of non-diffusive transport.The physics investigated and the methods and the techniques developed in this Thesis can thus provide a new insight on several (and quite diverse) on-trend topics

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22

Allain, Adrien. "Supraconductivité induite dans le graphène dopé par des nanoparticules métalliques". Phd thesis, Université de Grenoble, 2012. http://tel.archives-ouvertes.fr/tel-00845885.

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Cette thèse présente une étude des propriétés de transport à basses températures de matériaux hybrides composés de nano-clusters de métaux supraconducteurs (Sn et Pb) auto-assemblés à la surface d'une feuille de graphène. L'auto-assemblage du métal réalise un réseau bi-dimensionnel désordonné de jonctions Josephson. La caractérisation des propriétés supraconductrices révèle une transition de type 'BKT' avec une température de transition dépendant de la morphologie de la surface. Les propriétés supraconductrices de ce système sont fortement influencées par la grille arrière, qui contrôle la résistance dans l'état normal du graphène. Le résultat le plus marquant de cette thèse a été obtenu en utilisant du graphène désordonné. La présence de défauts structuraux dans la maille de graphène induit un régime de localisation forte à basses températures. En faisant varier le voltage de grille, la résistance de tels échantillons peut varier de 3 ordres de grandeurs. Cette grande dynamique a été mise à contribution pour la réalisation d'une transition de phase supraconducteur-isolant dans des échantillons décorés à l'étain. L'étude de cette transition de phase quantique révèle un comportement de type percolatif et une résistivité universelle prédite par la théorie à la transition. Enfin, un travail préliminaire visant à réaliser des résonateurs mécaniques supraconducteurs à l'aide des ces matériaux hybrides est également présenté.

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23

Kiesel, Maximilian Ludwig [Verfasser] y Werner [Akademischer Betreuer] Hanke. "Unconventional Superconductivity in Cuprates, Cobaltates and Graphene: What is Universal and what is Material-Dependent in strongly versus weakly Correlated Materials? / Maximilian Ludwig Kiesel. Betreuer: Werner Hanke". Würzburg : Universitätsbibliothek der Universität Würzburg, 2013. http://d-nb.info/1031630856/34.

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24

Fedorov, Alexander. "Electronic structure of doped 2D materials". Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2016. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-203500.

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Electronic systems are an indivisible part of modern life. Every day, new materials, devices, passive components, antennas for wireless communication are needed to be designed and developed. In particular, flexible and biocompatible wearable devices are urgent required for medical and industrial applications. The great hope lies in the materials with high crystalline quality and flexibility such as graphene and other 2D semiconductors and insulators. Doping is a conventional tool for tailoring of the electronic properties of the functional materials. Here we examine application of the widely used the electron donor species to the graphene and hexagonal boron nitride monolayer (h-BN). For each we determine surface-interface properties and the full electronic band structure using the combination of the surface science methods such as angle-integrated and angle resolved photoemission (XPS, ARPES), electron diffraction (LEED) and photo absorption (XAS). As the result we provided insight into mechanisms underlying the doping gating of the graphene h-BN monolayer by the alkali metals. We fully characterized their surface and interface structure. Finally we studied the interplay between electrons and phonons in the doped graphene and we demonstrated that Ca-doped graphene is the promising candidate for realizing superconductivity in graphene.

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25

Berggren, Peter. "Elastic and inelastic scattering effects in conductance measurements at the nanoscale : A theoretical treatise". Doctoral thesis, Uppsala universitet, Institutionen för fysik och astronomi, 2015. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-261609.

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Elastic and inelastic interactions are studied in tunnel junctions of a superconducting nanoelectromechanical setup and in response to resent experimental superconducting scanning tunneling microscope findings on a paramagnetic molecule. In addition, the electron density of molecular graphene is modeled by a scattering theory approach in very good agreement with experiment. All studies where conducted through the use of model Hamiltonians and a Green function formalism. The nanoelectromechanical system comprise two fixed superconducting leads in-between which a cantilever suspended superconducting island oscillates in an asymmetric fashion with respect to both fixed leads. The Josephson current is found to modulate the island motion which in turn affects the current, such that parameter regions of periodic, quasi periodic and chaotic behavior arise. Our modeled STM setup reproduces the experimentally obtained spin excitations of the paramagnetic molecule and we show a probable cause for the increased uniaxial anisotropy observed when closing the gap distance of tip and substrate. A wider parameter space is also investigated including effects of external magnetic fields, temperature and transverse anisotropy. Molecular graphene turns out to be well described by our adopted scattering theory, producing results that are in good agreement with experiment. Several point like scattering centers are therefore well suited to describe a continuously decaying potential and effects of impurities are easily calculated.

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26

Santos, Filipe Daniel Rodrigues. "Critical magnetic elds in superconducting systems with semi-metallic bands". Master's thesis, Universidade de Aveiro, 2015. http://hdl.handle.net/10773/16074.

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Mestrado em Física
In this work, we study the Zeeman splitting eﬀects in the parallel magnetic ﬁeld versus temperature phase diagram of two-dimensional superconductors with one graphene-like band and the orbital eﬀects of perpendicular magnetic ﬁelds in isotropic two-dimensional semi-metallic superconductors. We show that when parallel magnetic ﬁelds are applied to graphene and as the intraband interaction decreases to a critical value, the width of the metastability region present in the phase diagram decreases, vanishing completely at that critical value. In the case of two-band superconductors with one graphene-like band, a new critical interaction, associated primarily with the graphene-like band, is required in order for a second metastability region to be present in the phase diagram. For intermediate values of this interaction, a low-temperature ﬁrst-order transition line bifurcates at an intermediate temperature into a ﬁrst-order transition between superconducting phases and a second-order transition line between the normal and the superconducting states. In our study on the upper critical ﬁelds in generic semi-metallic superconductors, we ﬁnd that the pair propagator decays faster than that of a superconductor with a metallic band. As result, the zero ﬁeld band gap equation does not have solution for weak intraband interactions, meaning that there is a critical intraband interaction value in order for a superconducting phase to be present in semi-metallic superconductors. Finally, we show that the out-of-plane critical magnetic ﬁeld versus temperature phase diagram displays a positive curvature, contrasting with the parabolic-like behaviour typical of metallic superconductors.
Neste trabalho, estudamos o efeito de Zeeman nos diagramas de fases do campo magnético versus temperatura de supercondutores bidimensionais com uma banda de grafeno na sua composição, sobre a ação de campos magnéticos paralelos e os efeitos orbitais em supercondutores semimetálicos bidimensionais e isotrópicos sobre a ação de campos magnéticos perpendiculares. Mostramos que quando se aplica campos magnéticos paralelamente a uma camada de grafeno e à medida que a interação da banda diminui, a largura da zona de metastabilidade presente no diagrama de fases diminui, desaparecendo por completo quando a interação toma um valor crítico. No caso de supercondutors de duas bandas com uma banda de grafeno, existe uma nova interação crítica, associada principalmente à banda de grafeno, necessária ao surgimento de uma segunda zona de metastabilidade no diagrama de fases. Para valores intermédios dessa interação, a transição de primeira ordem que surge, no diagrama de fases, a baixas temperaturas, ramiﬁca-se, a temperaturas intermedias, numa transição de primeira ordem entre fases supercondutoras e numa transição de segunda ordem entre os estados não supercondutor e supercondutor. Em relação ao nosso estudo, sobre os efeitos orbitais em supercondutores semimetálicos genéricos, descobrimos que o propagador de pares decai, com a distância, mais depressa do que no caso de um supercondutor metálico. Como consequência, a equação de gap a campo nulo não tem solução para fracas interações da banda semimetálica indicando a existência de um valor crítico para a interação da banda necessária à existência de uma fase supercondutora em supercondutores semimetálicos. Mostramos ﬁnalmente que o diagrama de fases do campo crítico magnético versus temperatura exibe uma curvatura positiva em contraste com a curva parabólica do diagrama de fases típico de supercondutores metálicos.

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27

Ballestar, Ana. "Superconductivity at Graphite Interfaces". Doctoral thesis, 2013. https://ul.qucosa.de/id/qucosa%3A12424.

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The existence of superconductivity in graphite has been under discussion since the 1960s when it was found in intercalated graphitic compounds, such as C8K, C8Rb and C8Cs. However, it was only about 40 years ago when the existence of superconductivity in pure graphite came up. In this work we directly investigate the interfaces highly oriented pyrolytic graphite (HOPG) has in its inner structure, since they play a major role in the electronic properties. The results obtained after studying the electrical transport provide clear evidence on granular superconductivity localized at the interfaces of graphite samples. Zero resistance states, strong current dependence and magnetic field effect on the superconducting phase support this statement. Additionally, an abrupt reduction in the measured voltage at temperatures from 3 to 175 K has been observed. However, the upper value of this transition temperature seems to not have been reached yet. A possible method to enhance it is to increase the carrier density of graphite samples. In order to preserve to quasi-two-dimensional structure of highly oriented pyrolytic graphite, chemical doping has been dismissed in the frame of this work. We used an external electric field to move the Fermi level and, hence, try to trigger superconductivity in multi layer graphene samples. A drop on the resistance at around 17 K has been measured for a large enough electric field applied perpendicular to the graphene planes. This transition is strongly affected by magnetic field and only appeared at low temperatures. As a result of the studies included in this work, it appears clear that graphite has a superconducting phase located at certain interfaces with a very high transition temperature.

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28

Precker, Christian Eike. "Superconducting Effects in the Electrical Transport Properties of Graphite". 2021. https://ul.qucosa.de/id/qucosa%3A75673.

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Supraleitung in Graphit ist kein neues Thema. Dieser Effekt wurde bereits in den 1960er Jahren in Interkalationsverbindungen von Graphit gefunden. Die Supraleitung in reinem Graphit wurde bereits vor etwa 50 Jahren beschrieben. Kürzlich wurden in zweischichtigem Graphen, in dem die Graphenschichten um einen 'magischen' Winkeln um die c Achse verdreht wurden, flache Bänder in der elektronischen Bandstruktur nachgewiesen, welche mit der Entstehung von Supraleitung zusammenhängen. Wir haben die elektrischen Transporteigenschaften in Graphitproben mit unterschiedlichen Elektrodenkonfigurationen untersucht. Wir haben den elektrischen Widerstand von hochgeordnetem natürlichem und synthetischem Graphit mit Elektroden auf der Oberseite der ab Basalebene und auch parallel zur c Achse mit hoher Präzision gemessen und den Einfluss der hochleitenden Stapelfehler untersucht, an denen, eingebettet zwischen den kristallinen Graphitschichten, 2D-Grenzflächen entstehen, die ebenfalls flache Bänder aufweisen. Die Existenz einer gut geordneten rhomboedrischen Graphitphase in allen gemessenen Proben wurde durch Röntgenbeugungsmessungen nachgewiesen. Die Grenzflächen mit der hexagonalen Phase stellen laut theoretischer Vorhersagen einen möglichen Ursprung für die Hochtemperatursupraleitung dar. Die experimentellen Ergebnisse liefern eindeutige Beweise für körnige Supraleitung in diesen Materialien, z. B. einen schrittweisen Temperaturübergang bei ~ 350 K, magnetische Irreversibilität, Zeitabhängigkeit nach einer Feldänderung, die mit dem eingeschlossenen Fluss und Flusskriechen übereinstimmt, und den teilweise abgestoßenen magnetischen Fluss, welcher in Magnetisierungsmessungen beobchtet werden kann. Die Lokalisierung der körnigen Supraleitung an diesen 2D-Grenzflächen verhindert die Beobachtung widerstandsfreier elektrischer Ströme oder eines vollständigen Meißner-Zustands. Der Grund ist, dass die körnige Supraleitung in abgegrenzten Regionen an den Grenzflächen entsteht, welche in eine Multigraphen-Halbleitermatrix eingebettet sind. In dieser Arbeit wird eine detaillierte Untersuchung des Magnetowiderstands in verschiedenen Arten von Graphitproben bei niedrigen und hoch gepulsten Magnetfeldern vorgestellt.
Superconductivity in graphite is not a new topic. Its existence goes back to the 1960s when this effect was found in intercalation compounds of graphite. Superconductivity in pure graphite was reported already around 50 years ago and recently proved in bi-layer graphene, related to 'magic' angles between the graphene layers, twisted around the c axis, with the electronic band structure exhibiting flat bands. We have studied electrical transport properties in graphite samples with different electrode configurations. Measuring with high precision, the electrical resistance of highly ordered natural and synthetic graphite, with electrodes placed on the top of the ab basal plane, and also parallel to the c axis, we investigated the influence of the highly conducting stacking faults, referred as 2D interfaces, embedded between the crystalline regions of graphite, which also exhibit flat bands. The existence of well ordered rhombohedral graphite phase in all measured samples has been proved by x-ray diffraction measurements, suggesting its interfaces with the hexagonal phase as a possible origin of high-temperature superconductivity, predicted by theoretical studies. The results provide clear evidence of granular superconductivity, e.g., a step-like transition in temperature at ~ 350 K, magnetic irreversibility, time dependence after a field change, consistent with trapped flux and flux creep, and the partial magnetic flux expulsion from magnetization measurements. The localization of the granular superconductivity at these 2D interfaces prevents the observation of a zero resistance state or a full Meissner state. The reason is that the superconducting distribution is a mixture of superconducting patches at the interfaces, and they are embedded in a multigraphene semiconducting matrix. A detailed study of the magnetoresistance in different kinds of graphite samples at low and high-pulsed magnetic fields is presented in this work.

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29

Chen, Chi. "Design and Construction of a Low Temperature Scanning Tunneling Microscope". 2010. http://hdl.handle.net/1969.1/ETD-TAMU-2010-08-8498.

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A low temperature scanning tunneling microscope (LTSTM) was built that we could use in an ultra high vacuum (UHV) system. The scanning tunneling microscope (STM) was tested on an existing 3He cryostat and calibrated at room, liquid nitrogen and helium temperatures. We analyzed the operational electronic and vibration noises and made some effective improvements. To demonstrate the capabilities of the STM, we obtained atomically resolved images of the Au (111) and graphite surfaces. In addition, we showed that the stable tunneling junctions can be formed between the Pt/Ir tip and a superconducting thin film PbBi. We observed the atomic corrugation on Au (111) and measured the height of the atomic steps to be approximately2.53Å, which agrees with published values. In our images of the graphite surface, we found both the β atoms triangular structure, as well as the complete α-β hexagonal unit cell, using the same tip and the same bias voltage of 0.2V. The successful observation of the hidden α atoms of graphite is encouraging in regards to the possibility of imaging other materials with atomic resolution using our STM. We also demonstrated that stable tunneling junctions can be formed at various temperatures. To demonstrate this, the superconducting current-voltage and differential conductance-voltage characteristics of a PbBi film were measured from 1.1K to 9K From this data, the temperature dependent energy gap of the superconductor was shown to be consistent with the predictions of the Bardeen, Cooper, and Schrieffer (BCS) theory.

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30

Efetov, Dmitri K. "Towards inducing superconductivity into graphene". Thesis, 2014. https://doi.org/10.7916/D8VX0F3T.

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Graphenes transport properties have been extensively studied in the 10 years since its discovery in 2004, with ground-breaking experimental observations such as Klein tunneling, fractional quantum Hall effect and Hofstadters butterfly. Though, so far, it turned out to be rather poor on complex correlated electronic ground states and phase transitions, despite various theoretical predictions. The purpose of this thesis is to help understanding the underlying theoretical and experimental reasons for the lack of strong electronic interactions in graphene, and, employing graphenes high tunability and versatility, to identify and alter experimental parameters that could help to induce stronger correlations. In particular graphene holds one last, not yet experimentally discovered prediction, namely exhibiting intrinsic superconductivity. With its vanishingly small Fermi surface at the Dirac point, graphene is a semi-metal with very weak electronic interactions. Though, if it is doped into the metallic regime, where the size of the Fermi surface becomes comparable to the size of the Brillouin zone, the density of states becomes sizeable and electronic interactions are predicted to be dramatically enhanced, resulting in competing correlated ground states such as superconductivity, magnetism and charge density wave formation. Following these predictions, this thesis first describes the creation of metallic graphene at high carrier doping via electrostatic doping techniques based on electrolytic gates. Due to graphenes surface only properties, we are able to induce carrier densities above n>10¹⁴cm⁻²(εF>1eV) into the chemically inert graphene. While at these record high carrier densities we yet do not observe superconductivity, we do observe fundamentally altered transport properties as compared to semi-metallic graphene. Here, detailed measurements of the low temperature resistivity reveal that the electron-phonon interactions are governed by a reduced, density dependent effective Debey temperature - the so-called Bloch-Grüneisen temperature ΘBG. We also probe the transport properties of the high energy sub-bands in bilayer graphene by electrolyte gating. Furthermore we demonstrate that electrolyte gates can be used to drive intercalation reactions in graphite and present an all optical study of the reaction kinetics during the creation of the graphene derived graphite intercalation compound LiC₆, and show the general applicability of the electrolyte gates to other 2-dimensional materials such as thin films of complex oxides, where we demonstrate gating dependent conductance changes in the spin-orbit Mott insulator Sr₂IrO₄. Another, entirely different approach to induce superconducting correlations into graphene is by bringing it into proximity to a superconductor. Although not intrinsic to graphene, Cooper pairs can leak in from the superconductor and exist in graphene in the form of phase-coherent electron-hole states, the so-called Andreev states. Here we demonstrate a new way of fabricating highly transparent graphene/superconductor junctions by vertical stacking of graphene and the type-II van der Waals superconductor NbSe₂. Due to NbSe₂'s high upper critical field of Hc₂= 4 T we are able to test a long proposed and yet not well understood regime, where proximity effect and quantum Hall effect coexist.

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31

Wang, Da. "Inducing Superconductivity in Two-dimensional Materials". Thesis, 2020. https://doi.org/10.7916/d8-d8qt-1v86.

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In this thesis, I firstly report high field measurements of graphene/NbN junctions, in which NbN makes edge contact to graphene. Transport measurements at zero field demonstrate clear features associated with both retro and specular Andreev reflection. By applying perpendicular magnetic field, field dependence of junction transparency at Quantum Hall (QH) / superconductor (SC) interface is calculated and explained by a picture of superposition of electron and hole edge excitation. Zeeman splitting is induced in graphene by applying in plane magnetic field. We observe changes in the Andreev reflection spectrum that are consisting with spin splitting of the graphene band structure. This edge contact technique provides the opportunity to create hybrid SC/graphene or SC/QH system to illustrate new physics such as non-Abelian zero modes of Majorana physics. Secondly, other potential material candidates for SC/graphene junctions are discussed, high field transport measurement of FeSeTe/graphene junction is discussed, Superconductor/quantum spin Hall (QSH) interface and superconductor-graphene-superconductor weak link are also discussed, respectively. At last, via contact, a new contact method for two-dimensional materials, especially air-sensitive materials is discussed, the via contact method provides a new and reliable fabrication technique for two dimensional materials.

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32

Kochat, Vidya. "Impact of Disorder, Magnetism and Proximity-Induced Superconductivity on Conductance Fluctuations in Graphene". Thesis, 2014. http://hdl.handle.net/2005/3048.

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The experimental discovery of graphene in 2004 has opened up a new research field in the direction of atomically thin two-dimensional layered materials for exploration of many fundamental research problems and technological applications. The charge carriers in graphene are massless Dirac fermions due to which they exhibit absence of localization, thereby giving rise to huge intrinsic mobilities and ballistic transport even at room temperatures. But it was observed that the extrinsic disorder and intrinsic structural disorder can significantly influence the transport in graphene films. This thesis focuses on three different aspects of graphene -disorder, magnetism and proximity-induced superconductivity. We have reported conductance fluctuations-based transport studies to investigate these aspects as they provide more detailed information than what can be obtained from the standard transport measurements. Even though these conductivity fluctuations pose a serious bottleneck for various applications, they can also yield useful insights into the various scattering mechanisms and the symmetry properties of graphene. In the first half of the thesis, we describe the measurement of low frequency 1/f noise in large area polycrystalline graphene films to understand the role of grain boundaries in charge carrier transmission in graphene. TEM studies on the low and high angled GBs formed in these graphene samples showed that they form distinct disordered regions of varying widths depending on the tilt angle of the GBs. At low temperatures, the 1/f noise measurements indicated spontaneous breaking of time reversal symmetry across graphene grain boundaries which suggests the magnetic nature of these grain boundaries. In the second half of the thesis, we will concentrate on the universal conductance fluctuations (UCF) in graphene which is the manifestation of quantum interference phenomena at low temperatures. We find that the absolute magnitude of the UCF is directly related to various symmetry-breaking disorder present in graphene. We also discuss how the UCF can be used to study the nature of proximity-induced superconducting correlations in graphene. In the end, we have proposed new device schemes for the integration of ferromagnetic and superconducting materials with graphene.

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33

Arora, Harpreet Singh. "Superconductivity in Graphene Hetero-Structures: From Fundamental Physics to Functional Devices". Thesis, 2020. https://thesis.library.caltech.edu/13781/14/PhD_Thesis_Harpreet_Arora_Final.pdf.

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While graphene has been dubbed as a "wonder material" because of its amazing characteristics, such as the ability to conduct electricity better than copper and being two hundred times stronger than steel, until recently, the key quantum phenomenon of superconductivity was missing from the list of properties exhibited by graphene. In 2018, an astonishing discovery showed that by placing two sheets of graphene on top of each other in a structure known as Twisted Bilayer Graphene, it is possible to realize superconductivity when the rotation angle between the sheets is close to the "Magic Angle" value of 1.1°. More surprisingly, superconductivity in the initial reports was observed in close proximity to insulating states - resembling the phase diagram of High T_c superconductors. This sparked a fierce debate about its origin and its possible relation to High T_c superconductors. In this thesis, we show that by carefully engineering the dielectric environment of TBG, it is possible to stabilize superconductivity in non-magic angle TBG devices without the presence of any insulating states. This discovery imposes severe constraints on the origin of superconductivity in TBG. We also report, for the first time, the successful induction of spin-orbit coupling in TBG and discuss its implications.

Superconductivity can also be induced into graphene via coupling to conventional superconductors, and the strength of the induced supercurrent depends strongly on temperature. We employ this thermal dependence by integrating graphene into superconducting circuits that serves two purposes a) to investigate graphene's thermal behavior at milliKelvin temperatures and b) to utilize its extremely low heat capacity in making functional devices that have the potential to achieve ultra-high thermal sensitivity.

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34

Borzenets, Ivan Valerievich. "Graphene-based Josephson junctions: phase diffusion, effects of magnetic field, and mesoscopic properties". Diss., 2012. http://hdl.handle.net/10161/5576.

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We report on graphene-based Superconductor-Normal metal-Superconductor Joseph- son junctions with contacts made from lead. The high transition temperature of this superconductor allows us to observe the supercurrent branch at temperatures up to 􏰆 2 K. We are able to detect a small, but non-zero, resistance despite the Josephson junctions being in the superconducting state. We attribute this resistance to the phase diffusion regime, which has not been yet identified in graphene. By measuring the resistance as a function of temperature and gate voltage, we can further charac- terize the nature of electromagnetic environment and dissipation in our samples. In addition we modulate the critical current through grapehene by an external magnetic field; the resulting Fraunhofer interference pattern shows several periods of oscilla- tions. However, deviations from the perfect Fraunhofer pattern are observed, and their cause is explained by a simulation that takes into account the sample design.

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35

Kiesel, Maximilian Ludwig. "Unconventional Superconductivity in Cuprates, Cobaltates and Graphene: What is Universal and what is Material-Dependent in strongly versus weakly Correlated Materials?" Doctoral thesis, 2012. https://nbn-resolving.org/urn:nbn:de:bvb:20-opus-76421.

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Eine allgemeingültige Theorie für alle unterschiedlichen Arten von unkonventionellen Supraleitern ist immer noch eine der ungelösten Kernfragen der Festkörperphysik. Momentan ist es nicht einmal bewiesen, dass es überhaupt einen gemeinsamen grundlegenden Mechanismus gibt, sondern es müssen vielleicht mehrere verschiedene Ursachen für unkonventionelle Supraleitung berücksichtigt werden. Der Einfluss der Elektron-Phonon-Wechselwirkung ist dabei noch nicht abschließend geklärt. In dieser Dissertation wird ein rein elektronischer Paarungsmechanismus untersucht, in welchem die Paarung durch Spin-Fluktuationen vermittelt wird, was nach dem aktuellen Stand der Forschung auf dem Gebiet der unkonventionellen Supraleiter am wahrscheinlichsten ist. Der Schwerpunkt liegt dabei auf der Bestimmung von Material-unabhängigen Eigenschaften der supraleitenden Phase. Diese können durch eine Auswahl sehr unterschiedlicher Systeme herausgearbeitet werden. Eine Untersuchung der Phasendiagramme gibt außerdem Auskunft darüber, welche konkurrierenden Quantenfluktuationen den supraleitenden Zustand abschwächen oder verstärken. Für diese Analyse von sehr unterschiedlichen supraleitenden Materialien ist der Einsatz einer einzelnen numerischen Lösungsmethode unzureichend. Für diese Dissertation ist dies aber kein Nachteil, sondern vielmehr ein großer Vorteil, da der Einsatz verschiedener Techniken die Abhängigkeit der Ergebnisse von der verwendeten Numerik reduziert und dadurch der grundlegende Mechanismus besser untersucht werden kann. Im speziellen werden in dieser Dissertation die Kuprate mit der Variationellen Clusternäherung ausgewertet, weil die Elektronen hier eine starke Wechselwirkung untereinander besitzen. Besonders die Frage eines möglichen Klebstoffs für die Cooper-Paare wird ausführlich diskutiert, auch mit einer Unterscheidung in retardierte und nicht-retardierte Beträge. Den Kupraten werden das Kobaltat NaCoO sowie Graphen gegenübergestellt. Diese Materialien sind jedoch schwach korrelierte Systeme, so dass hier die Funkionelle Renormierungsgruppe als numerisches Grundgerüst dient. Die Ergebnisse sind reichhaltige Phasendiagramme mit vielen verschiedenen langreichweitigen Ordnungen, wie zum Beispiel d+id-wellenartige Supraleitung. Diese bricht die Zeitumkehr-Symmetrie und besitzt eine vollständige Bandlücke, welche im Falle von NaCoO jedoch eine stark Dotierungs-abhängige Anisotropie aufweist. Als letztes wird das Kagome-Gitter allgemein diskutiert, ohne ein konkretes Material zu beschreiben. Hier hat eine destruktive Interferenz zwischen den Elektronen auf verschiedenen Untergittern drastische Auswirkungen auf die Instabilitäten der Fermi-Fläche, so dass die übliche Spin-Dichte-Welle und die damit verbundene d+id-wellenartige Supraleitung unterdrückt werden. Dadurch treten ungewöhnliche Spin- und Ladungsdichte-Ordnungen sowie eine nematische Pomeranchuck Instabilität hervor. Zusammengefasst bietet diese Dissertation einen Einblick in unterschiedliche Materialklassen von unkonventionellen Supraleitern. Dadurch wird es möglich, die Material-spezifischen Eigenschaften von den universellen zu trennen
A general theory for all classes of unconventional superconductors is still one of the unsolved key issues in condensed-matter physics. Actually, it is not yet fully settled if there is a common underlying pairing mechanism. Instead, it might be possible that several distinct sources for unconventional (not phonon-mediated) superconductivity have to be considered, or an electron-phonon interaction is not negligible. The focus of this thesis is on the most probable mechanism for the formation of Cooper pairs in unconventional superconductors, namely a strictly electronic one where spin fluctuations are the mediators. Studying different superconductors in this thesis, the emphasis is put on material-independent features of the pairing mechanism. In addition, the investigation of the phase diagrams enables a view on the vicinity of superconductivity. Thus, it is possible to clarify which competing quantum fluctuations enhance or weaken the propensity for a superconducting state. The broad range of superconducting materials requires the use of more than one numerical technique to study an appropriate microscopic description. This is not a problem but a big advantage because this facilitates the approach-independent description of common underlying physics. For this evaluation, the strongly correlated cuprates are simulated with the variational cluster approach. Especially the question of a pairing glue is taken into consideration. Furthermore, it is possible to distinguish between retarded and non-retarded contributions to the gap function. The cuprates are confronted with the cobaltate NaCoO and graphene. These weakly correlated materials are investigated with the functional renormalization group (fRG) and reveal a comprehensive phase diagram, including a d+id-wave superconductivity, which breaks time-reversal symmetry. The corresponding gap function is nodeless, but for NaCoO, it features a doping-dependent anisotropy. In addition, some general considerations on the kagome lattice are completing the discussion, where a sublattice interference dramatically affects the Fermi-surface instabilities, suppressing the usual spin-density wave and d+id-wave superconductivity. Thereby, some different fascinating charge and bond orders as well as a nematic are observable. In short, this thesis provides an insight to distinct classes of unconventional superconductors with appropriate simulation techniques. This facilitates to separate the material specific properties from the universal ones

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36

Pathak, Sandeep. "Ground State Studies Of Strongly Correlated 2D Systems". Thesis, 2010. http://hdl.handle.net/2005/1924.

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The quest for obtaining higher Tc superconductivity led to the discovery of cuprates about 20 years ago. Since then, they continue to puzzle the scientific community with their bizarre properties like non-BCS superconductivity, pseudo gap, Fermi arcs, linear T resistivity etc. Since these materials show unusually high Tc, a novel mechanism is at play and strong correlations are believed to play an important role. The theme of this thesis work is to study physics of such strongly correlated systems in two dimensions at T = 0 along with development of new theoretical tools necessary for the study. The focus of the thesis is on the ground state studies of strongly correlated models like t-J and Hubbard models using variational Monte Carlo (VMC) and renormalized mean field theory (RMFT). The general method is to propose a variational wave function, motivated by the physics ideas, to be a candidate ground state of the system. Methods to efficiently evaluate the ground state energy and minimizing it with respect to the variational parameters are developed in this work. Antiferromagnetism-superconductivity competition and electron-hole asymmetry in the extended t-J model is investigated. The main result of this work is that increasing the magnitude of the next neighbor hopping (t') on hole doped side strengthen superconductivity while it stabilizes antiferromagnetism on the electron doped side. It is also shown that it is possible to characterize the T = 0 phase diagram with just one parameter called as Fermi Surface Convexity Parameter (FSCP). Next, the possibility of phase separation in the t-J model on a square lattice is investigated using local RMFT technique. It is found that for certain doping, the system phase separates into regions with antiferromagnetic and superconducting orders. Next, the role played by crystalline anisotropy of orthorhombic YBCO cuprates on their properties is examined using anisotropic tx-ty-J model and this ground state study suggests that the anisotropies seen in their properties are plausible solely due to the crystalline anisotropy. A new general method to study strongly correlated systems with singlet ground states is developed and tested in this thesis work. The last part of the thesis explores the possibility of high Tc superconductivity in graphene which is a intermediate coupling resonating valence bond (RVB) system. It is found that undoped graphene is not a superconductor, consistent with the experiments. On doping, the ground state of graphene is found to be a superconductor with “d+id” symmetry whose strength shows a dome as a function of doping which is reminiscent of RVB physics.

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37

"Relativistic Matter Under Extreme Conditions". Doctoral diss., 2013. http://hdl.handle.net/2286/R.I.18686.

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abstract: In this thesis, we present the study of several physical properties of relativistic mat- ters under extreme conditions. We start by deriving the rate of the nonleptonic weak processes and the bulk viscosity in several spin-one color superconducting phases of quark matter. We also calculate the bulk viscosity in the nonlinear and anharmonic regime in the normal phase of strange quark matter. We point out several qualitative effects due to the anharmonicity, although quantitatively they appear to be relatively small. In the corresponding study, we take into account the interplay between the non- leptonic and semileptonic weak processes. The results can be important in order to relate accessible observables of compact stars to their internal composition. We also use quantum field theoretical methods to study the transport properties in monolayer graphene in a strong magnetic field. The corresponding quasi-relativistic system re- veals an anomalous quantum Hall effect, whose features are directly connected with the spontaneous flavor symmetry breaking. We study the microscopic origin of Fara- day rotation and magneto-optical transmission in graphene and show that their main features are in agreement with the experimental data.
Dissertation/Thesis
Ph.D. Physics 2013

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38

Sahoo, Anindita. "Electrical Transport in the Hybrid Structures of 2D Van Der Waals Materials and Perovskite Oxide". Thesis, 2016. http://etd.iisc.ernet.in/handle/2005/2948.

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Perovskite oxides have provided a wide variety of exotic functionalities based on their unique physical and chemical properties. By combining different perovskite oxides, interesting physical phenomena have been observed at the interfaces of perovskite heterostructures. The most interesting among these phenomena is the formation of two dimensional electron gas at the interface of two perovskite materials SrTiO3 and LaAlO3 which led to a number of fascinating physical properties such as metal-insulator transition, super-conductivity, large negative magnetoresistance and so on. This has raised the interest in exploiting the interface of various hybrids structures built on the perovskite oxide backbone. On the other hand, the two dimensional (2D) van der Waals materials such as graphene, MoS2, boron nitride etc. represent a new paradigm in the 2D electron-ics. The functionalities of these individual materials have been combined to obtain new enriched functionalities by stacking different materials together forming van der Waals heterostructures. In this work, we present a detailed study of the interface in hybrid structures made of vander Waals materials (graphene and MoS2) and their hybrids with a perovskite material namely, SrTiO3 which is known as the building block of complex oxide heterostructures. In graphene-MoS2 vertical heterostructure, we have carried out a detailed set of investigations on the modulation of the Schottky barrier at the graphene-MoS2 interface with varying external electric field. By using different stacking sequences and device structures, we obtained high mobility at large current on-off ratio at room temperature along with a tunable Schottky barrier which can be varied as high as ∼ 0.4 eV by applying electric field. We also explored the interface of graphene and SrTiO3 as well as MoS2 and SrTiO3 by electrical transport and low frequency 1/f noise measurements. We observed a hysteretic feature in the transfer characteristics of dual gated graphene and MoS2 field effect transistors on SrTiO3. The dual gated geometry enabled us to measure the effective capacitance of SrTiO3 interface which showed an enhancement indicating the possible existence of negative capacitance developed by the surface dipoles at the interface of SrTiO3 and the graphene or MoS2 channel. Our 1/f noise study and the analysis of higher order statistics of noise also support the possibility of electric field-driven reorient able surface dipoles at the interface.

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39

Fedorov, Alexander. "Electronic structure of doped 2D materials". Doctoral thesis, 2015. https://tud.qucosa.de/id/qucosa%3A29534.

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Electronic systems are an indivisible part of modern life. Every day, new materials, devices, passive components, antennas for wireless communication are needed to be designed and developed. In particular, flexible and biocompatible wearable devices are urgent required for medical and industrial applications. The great hope lies in the materials with high crystalline quality and flexibility such as graphene and other 2D semiconductors and insulators. Doping is a conventional tool for tailoring of the electronic properties of the functional materials. Here we examine application of the widely used the electron donor species to the graphene and hexagonal boron nitride monolayer (h-BN). For each we determine surface-interface properties and the full electronic band structure using the combination of the surface science methods such as angle-integrated and angle resolved photoemission (XPS, ARPES), electron diffraction (LEED) and photo absorption (XAS). As the result we provided insight into mechanisms underlying the doping gating of the graphene h-BN monolayer by the alkali metals. We fully characterized their surface and interface structure. Finally we studied the interplay between electrons and phonons in the doped graphene and we demonstrated that Ca-doped graphene is the promising candidate for realizing superconductivity in graphene.

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Tesis sobre el tema "Superconductivity, Graphite"

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