Dissertationen: „Physics, Low Temperature|Physics, Condensed Matter“

1

Lupien, Christian. „Piezoresistive torque magnetometry at low temperature“. Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1997. http://www.collectionscanada.ca/obj/s4/f2/dsk2/ftp03/MQ37143.pdf.

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2

Christian, Aaron Brandon. „Magnetic and Thermal Properties of Low-Dimensional Single-Crystalline Transition-Metal Antimonates and Tantalates“. Thesis, Montana State University, 2017. http://pqdtopen.proquest.com/#viewpdf?dispub=10268687.

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This work contributes to the study of magnetic interactions in the low-dimensional antiferromagnets M(Sb,Ta)₂O₆, where M is a transition metal. By virtue of the trirutile structure, M-O-O-M chains propagate along [110] at z = 0 and [110] at z = 1/2 of the unit cell. These chains are separated along [001] by sheets of weakly-interacting diamagnetic ions. The spin-exchange coupling perpendicular to the chains is weak, permitting the low-dimensional classification. Single crystals have been grown using chemical vapor deposition and the floating zone method. Magnetization, in-field heat capacity, and high-resolution thermal expansion measurements have been performed along various axes, revealing significant anisotropy due to the peculiar magnetic structures and low dimensionality.

The Neel temperature, T_N, at which long-range order occurs is found to be unstable against the application of magnetic field above 2 T. Large fields tend to lower T_N of the set of moments with projections along the applied field. Moments which are aligned perpendicular to the field are significantly less affected. This can lead to the formation of a secondary peak in heat capacity when magnetic field is along either [110] or [110]. The change in heat capacity at the location of the newly formed peak means there is a change in entropy, which depends upon the direction of applied field with respect to the magnetic moments. Consequently, an anisotropic magnetocaloric effect arises due to the unique magnetic structure. The anisotropic nature of this effect has potential applications in magnetic refrigeration.

3

Safranski, Christopher. „Transport measurements and fabrication of superconductor-exchange spring magnet-superconductor systems“. California State University, Long Beach, 2013.

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4

Carter, Faustin Wirkus. „A transition-edge-sensor-based instrument for the measurement of individual He2* excimers in a superfluid 4He bath at 100 mK“. Thesis, Yale University, 2016. http://pqdtopen.proquest.com/#viewpdf?dispub=10012481.

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This dissertation is an account of the first calorimetric detection of individual He*₂ excimers within a bath of superfluid ⁴He. When superfluid helium is subject to ionizing radiation, diatomic He molecules are created in both the singlet and triplet states. The singlet He molecules decay within nanoseconds, but due to a forbidden spin-flip the triplet molecules have a relatively long lifetime of 13 seconds in superfluid He. When He*₂ molecules decay, they emit a ~15 eV photon. Nearly all matter is opaque to these vacuum-UV photons, although they do propagate through liquid helium. The triplet state excimers propagate ballistically through the superfluid until they quench upon a surface; this process deposits a large amount of energy into the surface. The prospect of detecting both excimer states is the motivation for building a detector immersed directly in the superfluid bath.

The detector used in this work is a single superconducting titanium transition edge sensor (TES). The TES is mounted inside a hermetically sealed chamber at the baseplate of a dilution refrigerator. The chamber contains superfluid helium at 100 mK. Excimers are created during the relaxation of high-energy electrons, which are introduced into the superfluid bath either in situ via a sharp tungsten tip held above the field-emission voltage, or by using an external gamma-ray source to ionize He atoms. These excimers either propagate through the LHe bath and quench on a surface, or decay and emit vacuum-ultraviolet photons that can be collected by the detector.

This dissertation discusses the design, construction, and calibration of the TES-based excimer detecting instrument. It also presents the first spectra resulting from the direct detection of individual singlet and triplet helium excimers.

5

Alonzo-Proulx, Olivier. „Low-temperature thermal conductivity of the amorphous superconductor FexNi₁-xZr₂“. Thesis, McGill University, 2005. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=97890.

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Thermal conductivity is a powerful tool to probe the phonon and electron exitations in a solid, especially in superconductors were one can basically tune the respective electronic and phononic contributions by applying a magnetic field below Tc.
After a short review on the concepts of superconductivity, thermal conductivity and amorphous matter, we present a study of the thermal conductivity of an exotic material, the amorphous metallic superconductor Fe0.5Ni 0.5Zr2. The results indicate an unexpected dominant electonic contribution to the thermal conductivity across the superconducting transition, in accordance with an inhomogeneous sample composed of a bulk normal phase with inhomogeneous superconducting phases.

6

Hetel, Iulian Nicolae. „Quantum Critical Behavior In The Superfluid Density Of High-Temperature Superconducting Thin Films“. The Ohio State University, 2008. http://rave.ohiolink.edu/etdc/view?acc_num=osu1204918571.

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7

Brecht, Teresa Lynn. „Micromachined quantum circuits“. Thesis, Yale University, 2018. http://pqdtopen.proquest.com/#viewpdf?dispub=10783438.

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Quantum computers will potentially outperform classical computers for certain applications by employing quantum states to store and process information. However, algorithms using quantum states are prone to errors through continuous decay, posing unique challenges to engineering a quantum system with enough quantum bits and sufficient controls to solve interesting problems. A promising platform for implementing quantum computers is that of circuit quantum electrodynamics (cQED) using superconducting qubits. Here, two energy levels of a resonant circuit endowed with a Josephson junction serve as the qubit, which is coupled to a microwave-frequency electromagnetic resonator. Modern quantum circuits are reaching size and complexity that puts extreme demands on input/output connections as well as selective isolation among internal elements. Continued progress will require adapting sophisticated 3D integration and RF packaging techniques found in today's high-density classical devices to the cQED platform. This novel technology will take the form of multilayer microwave integrated quantum circuits (MMIQCs), combining the superb coherence of three-dimensional structures with the advantages of lithographic integrated circuit fabrication. Several design and fabrication techniques are essential to this new physical architecture, notably micromachining, superconducting wafer bonding, and out-of-plane qubit coupling. This thesis explores these techniques and culminates in the design, fabrication, and measurement of a two-cavity/one-qubit MMIQC featuring qubit coupling to a superconducting micromachined cavity resonator in silicon wafers. Current prototypes are extensible to larger scale MMIQCs for scalable quantum information processing.

8

Rao, K. Umar. „Positron interactions at low-dimensional condensed surfaces“. Thesis, Royal Holloway, University of London, 1988. http://repository.royalholloway.ac.uk/items/9042f7f2-0cb6-4d26-90ff-957fe870187a/1/.

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A microprocessor control system for automatic data acquisition in a photon spectroscopy system was developed. The control process performed Doppler broadening measurements over the temperature range 4.2-600K with simultaneous recording when necessary of pressure. After work on the chalcogenide semiconductor (Pb,Ge)Te, Doppler broadening studies concentrated on the phenomenon of gas condensation on cooled graphite substrates. This class of adsorbent, of proven homogeneity was found ideal to explore the interaction of positrons at surfaces. By measurement of positronium, adlayer formation of methane, krypton, argon, nitrogen and oxygen was followed. All gases produced peaks in positronium emission with an investigation of the substrate further revealing an underlying thermal variation. Sound evidence in the form of accurate simultaneous pressure measurements and isotherms was found for a coverage of one half at the maximum in positronium. These studies of gas physisorption were conducted under a variety of temperatures and with pressures embracing atmospheric to under one Torr. Although results for the gases were of the same form each produced different maximum levels of positronium which appeared to relate to substrate-adsorbate binding energy. Maximum emission in the case of argon showed a temperature dependence however this requires verification. Oxygen was a special case as it yielded 12[percent] para-positronium at half-coverage. The coupling of this to a low 1.5[percent] ortho-positronium emission was attributed to chemical quenching. A convolution procedure based on the superposition of an additional para-positronium derived narrow gaussian to the usual gaussian and inverted parabola components was employed for an estimate of the positronium energy. This was used to attack the question of whether positronium is bound at the surface or is created flying.

9

Greer, Allan J. Jr. „Low internal magnetic fields in anisotropic superconductors“. W&M ScholarWorks, 1994. https://scholarworks.wm.edu/etd/1539623852.

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This thesis is a theoretical, numerical study of the magnetic fields which exist in the anisotropic, high temperature superconductors like $YBa\sb2Cu\sb3O\sb{7-\delta}$, or YBCO for short, using both the anisotropic London theory and simulations based on existing muon spin rotation techniques. The thesis first describes the muon spin rotation ($\mu$SR) techniques, and then gives a brief discussion of superconductivity with regard to the London theory of anisotropic, type II superconductors. Next, numerical results of the application of this theory to YBCO are presented. Three dimensional surface plots of the magnetic field components within the flux line lattice (FLL) are shown, as well as the corresponding contour plots of the fields. Field distributions are calculated from these surfaces, and the graphs are presented. These distributions correspond to the real part of the Fourier transform of the muon histogram, and a comparison between data taken on a polycrystalline sample and the theoretical prediction is made. In addition, variation of the field distributions with parameters such as penetration depth, angle of the average field, and the magnitude of the average field is discussed. The last part of the thesis is a theoretical study of the behavior of muons which have stopped within a superconductor. The muons are assumed to stop uniformly throughout the FLL area, and the precession of each about its local field is recorded as the projection of its polarization along each of three mutually perpendicular "detectors." The depolarization of these signals as a function of time is an indication of the existence of transverse field components which exist within the FLL due solely to the anisotropy of the material. In order to further investigate these off axis fields, we have developed an extension of the usual $\mu$SR techniques, coupled with Fourier analysis, which yields new information. For example, with the proper analysis procedure, one may determine to good precision the direction of the average internal field B with respect to the applied field H$\sb{a}$. Other quantities, which we call moments of the field distribution, may also be determined.

10

Buckingham, David Tracy Willis. „High-Resolution Thermal Expansion and Dielectric Relaxation Measurements on H2O and D2O Ice Ih“. Thesis, Montana State University, 2017. http://pqdtopen.proquest.com/#viewpdf?dispub=10607201.

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Ice Ih, formed by freezing liquid water below 273∼K at atmospheric pressure, is well-known and highly-studied, but some of its fundamental physical properties have mystified scientists since the early twentieth century. The thermal expansion is one of those properties; the low relative-resolution of past measurements has left questions regarding the structural isotropy and negative thermal expansion (NTE). Furthermore, the existence of relaxation phenomena near 100∼K, related to the residual entropy at 0∼K, may reveal itself through subtle features in the thermal expansion and, thus, warrants further investigation. Here we measure the thermal expansion of ultra-pure single crystal ice from 5–265∼K with 10⁶ times higher relative resolution than has previously been made. The data reveal a distinct crossover to NTE below 62∼K, and a third-order transition along the crystallographic $c$-axis near 100∼K, as evident by an unambiguous relaxational decrease in the thermal expansion coefficient on cooling. To further understand the nature of the transition, isotopic substitution and dielectric measurements were performed.

Three properties of the dielectric relaxation in ice were probed at temperatures between 80--250∼K; the thermally stimulated depolarization (TSD) current, static electrical conductivity, and dielectric relaxation time. The dielectric data agree with relaxation-based models and provide for the determination of activation energies which identify the dielectric relaxation in ice as being dominated by Bjerrum defects below 140∼K. An anisotropy was also found in the data which revealed that molecular reorientations, in the form of propagating Bjerrum point defects, are energetically favored along the $c$-axis between 80--140∼K. Furthermore, a similar relaxational effect to that observed in the thermal expansion was observed in the TSD along $c$, providing a strong correlation between dielectric relaxation and inherent thermodynamic relaxation in ice. Finally, isotopic substitution in both measurement sets indicates the transition is related the movements of hydrogen nuclei, not those of the whole molecule, and provides details about the low-temperature phonon modes. These findings paint a picture of ice as a proton-disordered crystal which undergoes a partial ordering on cooling near 100∼K but, before an ordered equilibrium state is realized, the exponentially increasing relaxation time rapidly slows the ordering and ultimately freezes-in the residual entropy, causing a continuous decrease in the thermal expansion coefficient.

11

Lee, Junhyun. „Novel Quantum Phase Transitions in Low-Dimensional Systems“. Thesis, Harvard University, 2016. http://nrs.harvard.edu/urn-3:HUL.InstRepos:33493318.

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We study a number of quantum phase transitions, which are exotic in their nature and separates non-trivial phases of matter. Since quantum fluctuations, which drive these phase transitions, are stronger in low-dimensions, we concentrate on low-dimensional systems. We consider two different two-dimensional systems in this thesis and study their phase transition. First, we investigate a phase transition in graphene, one of the most famous two-dimensional systems in condensed matter. For a suspended bilayer graphene in ν = 0 quantum Hall regime, the conductivity data and mean-field analysis suggests a phase transition from an antiferromagnetic (AF) state to a valence bond solid (VBS) state, when perpendicular electric field is increased. This AF to VBS phase transition is reminiscent of deconfined criticality, which is a novel phase transition that cannot be explained by Landau’s theory of symmetry breaking. We show that in the strong coupling regime of bilayer graphene, the AF state is destabilized by the transverse electric field, likely resulting in a VBS state. We also consider monolayer and bilayer graphene in the large cyclotron gap limit and show that the effective action for the AF and VBS order parameters have a topological Wess-Zumino-Witten term, supporting that the phase transition observed in experiments is in the deconfined criticality class. Second, we study the model systems of cuprate superconductor, which is effectively a two-dimensionalal system in the CuO_2 plane. The proposal that the pseudogap metal is a fractionalized Fermi liquid described by a quantum dimer model is extended using the density matrix renormalization group. Measuring the Friedel oscillations in the open boundaries reveals that the fermionic dimers have dispersion minima near (π/2,π/2), which is compatible with the Fermi arcs in photoemission. Moreover, investigating the entanglement entropy suggests that the dimer model with low fermion density is similar to the free fermion system above the Lifshitz transition. We also study the phase transition from a metal with SU(2) spin symmetry to an AF metal. By applying the functional renormalization group to the two-band spin-fermion model, we establish the existence of a strongly coupled fixed point and calculate critical exponents of the fixed point.
Physics

12

Roseman, Mark A. „Low temperature magnetic force microscopy studies of superconducting niobium films“. Thesis, McGill University, 2001. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=38266.

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Low temperature magnetic force microscopy studies of superconducting niobium films have been undertaken with the goal of studying the interplay between artificial pinning centers and magnetic vortices.
Measurements were performed using a custom built low temperature magnetic force microscope, capable of operation at temperatures ranging from 4.2 K to room temperature. Special attention has been paid to optimizing the instrumentation through a detailed study of the noise characteristics, with particular emphasis placed on achieving a large signal-to-noise ratio and corresponding high force gradient sensitivity.
Magnetic force spectroscopy data has been used to deduce the critical temperature of the superconducting samples, based upon the repulsive Meissner interaction between the magnetic tip and the sample. Images of vortices as a function of applied magnetic field demonstrate the expected linear relation between vortex density and field strength, and confirms that only single vortices, each carrying one flux quantum, are observed. Two different methods are put forward to determine the magnetic penetration depth; one using magnetic force spectroscopy, the other using constant height imaging of vortices. Images of vortices as a function of temperature demonstrate that as temperatures rise, vortices become more easily depinned during the scanning process through interactions with the magnetic field of the tip. Dissipation images of vortices suggest eddy current damping as well as vortex motion within potential wells as major sources of energy loss. Studies on a patterned niobium film show that only interstitial vortices are easily detectable by MFM, but that a strong tip influence results in significant tip induced motion of these vortices around the antidots.

13

Cai, Aiguo. „Study of surface flattening kinetics by low energy electron diffraction on rutile(110)“. Thesis, National Library of Canada = Bibliothèque nationale du Canada, 2000. http://www.collectionscanada.ca/obj/s4/f2/dsk2/ftp03/MQ57094.pdf.

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14

Roumiantsev, Ilia. „Many-body effects in low-order optical nonlinearities of semiconductor quantum wells“. Diss., The University of Arizona, 2003. http://hdl.handle.net/10150/280383.

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This dissertation addresses both fundamental aspects of Coulomb correlations in semiconductor quantum wells and more practical aspects of theoretical analysis of semiconductor optoelectronic devices. After introducing the subject, we present and evaluate a state-of-the-art theory of the third order coherent optical response of a semiconductor quantum well based on the Dynamics Controlled Truncation (DCT) scheme. Already in the third order (the so-called chi (3)) regime, semiconductors exhibit a number of many-body Coulomb correlation effects. Their manifestation in various multi-pulse experimental configurations, customarily used in ultrafast semiconductor spectroscopy, has been an important component of this thesis. Coherent optical effects in a semiconductor 3-band system based on the heavy-hole, light-hole and conduction bands were investigated. The quantum beats in the time-integrated differential transmission signal were analyzed and compared with experimental data obtained at the University of Iowa. Fundamental differences from corresponding quantum beats in atomic 3-level systems were found. Also, the analysis of experimental data (obtained at the University of Arizona) of the coupled heavy-hole-light-hole optical Stark shift revealed evidence of intervalence band coherences, an analog of Raman coherences in atomic 3-levels systems. A scheme for realization of electromagnetically-induced transparency (EIT) based on the interference of excitonic and biexcitonic coherences was proposed. Corresponding experiments performed at the University of Oregon showed indeed a considerable coherent reduction of excitonic absorption. Furthermore, an extension of the chi(3) analysis revealed an energy renormalization of the biexciton, in good agreement with the corresponding experiment. A microscopic analysis of polarization dynamics in time-resolved four-wave mixing signals was performed, revealing interesting implications for the biexciton dephasing in addition to the significance of many-body correlations. In the case of four-wave mixing in semiconductor microcavities, our theoretical analysis in conjunction with experimental data obtained at the University of Tokyo gave us indications for a significant shortcoming of the second Born approximation (2nd BA) applied to two-exciton Coulomb correlations in a thin semiconductor quantum well, in agreement with the general knowledge of the qualitative failure of the 2nd BA in systems with short-range interaction in two dimensions. We also analyzed a novel all-optical switching technique based on the nonlinear polarization rotation. Apart from identifying the many-particle processes relevant for the switch operation in the chi(3) regime, we proposed ways to further optimize the switch.

15

Rezeq, Mohammed (Moh'd). „Investigation of magnetothermal and critical current hysteresis in polycrystals of low and high T(c) type II superconductors“. Thesis, University of Ottawa (Canada), 2002. http://hdl.handle.net/10393/6371.

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The model of Clem and Hao and others is extended to account for the enhancement of the Meissner effect observed in single crystals of hysteretic type II superconductors upon thermal cycling below Tc in static applied magnetic fields. Predictions are made about the features of the final closed thermal hysteretis loop achievable by extensive cycling and their dependence on the temperature limits T1 and T2 < Tc chosen for the cycles. A large variety of observations, by several workers, of a narrow peak of enhanced Meissner effect near Tc in polycrystalline type II superconductors upon slow warming in static applied fields after fast field cooling, are qualitatively and quantitatively accounted for by a model where we introduced the scheme developed above for single crystals into a weak-linked intergranular network (matrix). This "two tier" framework is then extended to describe the enhancement of the Meissner effect observed by Hyun by thermal cycling of weak-linked Nb3Sn below Tc in a static field. A simple framework is presented which quantitatively develops the proposal of Evetts and Glowacki that the superposition of the applied field H a and the return field, Hr of the magnetized grains, is the cause of the hysteretic behaviour of Ic in weak-linked high T c superconductors and the occurrence of a peak in Ic versus Ha descending and reascending, after an excursion to various values, denoted Hcycle, or after field cooling in different H cool. Observations by several workers on the dependence, of the position of four categories of peaks of Ic, on Hcycle and Hcool are reproduced by this model and yield estimates of the "compression" factor C in the linear dipole approximation, H r = C Mg. We also show that, ratios of the measured plateau values for the position of these peaks, lead to an estimate for C which is independent of H·g, the penetration field into the grains, and of the model chosen to calculate the dependence of the magnetization of the grains, Mg, on Ha. Instead of the artificial pseudo-Josephson - junction expression generally used by other workers in the analysis of Ic hysteresis phenomena, we develop a family of formulae based on the critical state concept applied to idealized planar geometry. Exploiting an especially simple case from this family of formulae we reproduce a panoply of experimental curves of Ic versus Ha displayed in the literature and exhibiting a variety, of features. Analysis of the extensive data of List et al reveals that our approach leads to results in accord with observations whereas the Josephson junction format does not. (Abstract shortened by UMI.)

16

Lavelle, Christopher M. „The neutronic design and performance of the Indiana University Cyclotron Facility (IUCF) Low Energy Neutron Source (LENS)“. [Bloomington, Ind.] : Indiana University, 2007. http://gateway.proquest.com/openurl?url_ver=Z39.88-2004&rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&res_dat=xri:pqdiss&rft_dat=xri:pqdiss:3255512.

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Thesis (Ph.D.)--Indiana University, Dept. of Physics, 2007.
Title from PDF t.p. (viewed Nov. 20, 2008). Source: Dissertation Abstracts International, Volume: 68-03, Section: B, page: 1688. Adviser: David V. Baxter.

17

Roseman, Mark A. „Design and characteristics of a low-temperature atomic force microscope“. Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1997. http://www.collectionscanada.ca/obj/s4/f2/dsk2/ftp01/MQ44260.pdf.

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18

Benzaquen, Roberto. „Photoluminescence of heavily zinc-doped gallium arsenide and gallium indium arsenide grown by low-pressure metal organic vapour phase epitaxy“. Thesis, University of Ottawa (Canada), 1991. http://hdl.handle.net/10393/7891.

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Zn-doped, p-type, GaAs and $\rm Ga\sb{0.85}In\sb{0.15}As$ samples grown by low-pressure metal organic vapour phase epitaxy with free carrier concentrations in the range of n = 4.3 $\times$ 10$\sp {\rm cm}\sp{-3}$ (nominally undoped)--p = 1.95 x 10$\sp{20} {\rm cm}\sp{-3}$ at room temperature have been studied by temperature-dependent photoluminescence. At low doping levels, recombinations involving discrete impurity states and free excitons provided measurement of both the 5 K band gap ($E\sb{g}(5)=(1.296\pm0.003)$ eV) and the zinc acceptor binding energy $(E(Zn\sp0)=(0.025\pm0.003)$ eV) in the $\rm Ga\sb{0.85}In\sb{0.15}As$ alloy. At high concentrations, the discrete acceptor levels are replace by an impurity band which merges with the valence band above the Mott$\sp{\lbrack 1\rbrack}$ transition. This gives rise to a density of states band tail extending into the gap and containing both extended and localised states. In the presence of such a high density of impurities, potential fluctuations and interparticle interactions result in a band gap shrinkage $\vert \Delta E\sb{g}\vert$ which has been observed with photoluminescence experiments. A model based on the presence of Kane$\sp{\lbrack 2\rbrack}$ band tails and on the assumption of a constant matrix element for the relevant optical transitions has been fitted to the photoluminescence spectra of heavily doped layers of GaAs and $\rm Ga\sb{0.85}In\sb{0.15}As$ in the range of $p=1.6\times 10\sp $ cm$\sp{-3}-p=1.95\times 10\sp{20}$ cm$\sp{-3}.$ This model provided a good description of the experimental results. The 5 K band gap shrinkage has been found to be $\vert\Delta E\sb{g}\vert=2.7\times 10\sp{-8}p\sp{1/3}$ for GaAs and $\vert\Delta E\sb{g}\vert=1.4\times 10\sp{-8}p\sp{1/3}$ for $\rm Ga\sb{0.85}In\sb{0.15}As$ with $\vert\Delta E\sb{g}\vert$ in Ev and p in cm$\sp{-3}.$

19

Remmes, Nicholas B. „Design of the Small Angle Neutron Scattering instrument at the Indiana University Low Energy Neutron Source : applications to the study of nanostructured materials /“. [Bloomington, Ind.] : Indiana University, 2007. http://gateway.proquest.com/openurl?url_ver=Z39.88-2004&rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&res_dat=xri:pqdiss&rft_dat=xri:pqdiss:3297113.

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Thesis (Ph.D.)--Indiana University, Dept. of Physics, 2007.
Title from dissertation home page (viewed Sept. 30, 2008). Source: Dissertation Abstracts International, Volume: 69-02, Section: B, page: 1064. Adviser: David V. Baxter.

20

Sucharitakul, Sukrit. „2D ELECTRONIC SYSTEMS IN LAYERED SEMICONDUCTING MATERIALS“. Case Western Reserve University School of Graduate Studies / OhioLINK, 2017. http://rave.ohiolink.edu/etdc/view?acc_num=case1491497351482802.

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21

Atluri, Vasudeva Prasad 1959. „Hydrogen passivation of silicon(100) used as templates for low-temperature epitaxy and oxidation“. Diss., The University of Arizona, 1998. http://hdl.handle.net/10150/282650.

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Epitaxial growth, oxidation and ohmic contacts require surfaces as free as possible of physical defects and chemical contaminants, especially, oxygen and hydrocarbons. Wet chemical cleaning typically involves a RCA clean to remove contaminants by stripping the native oxide and regrowing a chemical oxide with only trace levels of carbon and metallic impurities. Low temperature epitaxy, T limits the thermal budget for the desorption of impurities and surface oxides, and can be performed on processed structures. But, silicon dioxide cannot be desorbed at temperatures lower than 800°C. Recently, hydrogen passivation of Si(111) has been reported to produce stable and ordered surfaces at low temperatures. Hydrogen can then be desorbed between 200°C and 600°C prior to deposition. In this work, Si(100) is passivated via a solution of hydrofluoric acid in alcohol (methanol, ethanol, or isopropyl alcohol) with HF concentrations between 0.5 to 10%. A rinse in water or alcohol is performed after etching to remove excess fluorine. This work investigates wet chemical cleaning of Si(100) to produce ordered, hydrogen-terminated, oxygen- and carbon-free surfaces to be used as templates for low temperature epitaxial growth and rapid thermal oxidation. Ion beam analysis, Tapping mode atomic force microscopy, Fourier transform infrared spectroscopy, Secondary ion mass spectroscopy, Chemical etching, Capacitance-voltage measurements and Ellipsometry are used to measure, at the surface and interface, impurities concentration, residual disorder, crystalline order, surface topography, roughness, chemical composition, defects density, electrical characteristics, thickness, and refractive index as a function of cleaning conditions for homoepitaxial silicon growth and oxidation. The wetting characteristics of the Si(100) surfaces are measured with a tilting plate technique. Different materials are analyzed by ion beam analysis for use as hydrogen standards in elastic recoil detection of hydrogen on sample surfaces. The results obtained in this study provide a quantitative optimization of passivation of Si(100) surfaces and their use as templates for low temperature epitaxy and rapid thermal oxidation. Ion beam analysis shows that the total coverage of H increases during passivation of Si(100) via HF in alcohol, while Fourier transform infrared spectroscopy indicates that more complex termination than the formation of simple silicon hydrides occurs.

22

Christiansen, David A. „Investigation of magnetic proximity effect in ferromagnet/superconductor thin films by low temperature Magneto Optical Kerr Effect measurement“. California State University, Long Beach, 2013.

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23

Herring, Patrick Kenichi. „Low Dimensional Carbon Electronics“. Thesis, Harvard University, 2014. http://dissertations.umi.com/gsas.harvard:11475.

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This thesis covers several different experiments that comprised my graduate career. The main focus of these experiments was the use of carbon as an electronic material and a steady evolution of fabrication recipes that allowed us to perform reliable and consistent measurements. The second chapter describes experiments with carbon nanotubes, where our goal was to produce devices capable of manipulating electronic spin states in order create quantum bits or "qubits." The third chapter covers the development of fabrication recipes with the goal of creating qubits within Si-Ge nanowire, and the bottom-gating approach that was developed. The fourth chapter begins graphene related research, describing one of the simplest uses of graphene as a simple transparent electrode on a SiN micromembrane. The remainder of the thesis describes experiments that develop graphene based optical and infrared detectors, study their characteristics and determine the physics that underlies their detection mechanism. Key in these experiments were the fabrication recipes that had been developed to create carbon nanotube and Si-Ge nanowire devices. Finally, we demonstrate how engineering of the device's thermal characteristics can lead to improved sensitivity and how graphene can be used in novel applications where conventional materials are not suitable.
Physics

24

Alexander-Webber, Jack A. „High magnetic field effects in low-dimensional carbon nanostructures“. Thesis, University of Oxford, 2013. http://ora.ox.ac.uk/objects/uuid:1f81947b-16d7-4ab4-ace3-6e8b192429c8.

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This thesis describes studies of graphene, single walled carbon nanotubes (SWNTs) and InSb. Optical and electronic measurements probe the effects of high magnetic fields on these low-dimensional systems. Chapter 1 introduces a theoretical description and background behind the materials and physical phenomena studied in this work. The structure and unique properties of carbon nano-materials are described. The experimental methods used in this thesis are described in Chapter 2. Chapter 3 describes magnetotransport measurements on InSb/AlInSb heterostructures revealing that the large energy gaps, and extremely high mobility, associated with this system leads to exceptionally well defined quantum Hall plateaux for both even (Landau level) and odd (spin-split) filling factors. Even higher cyclotron energy gaps are expected in graphene. Chapter 4 reveals that due to a combination of large cyclotron energy gaps and fast electron-phonon energy loss rates, the quantum Hall effect (QHE) in graphene can be observed to unprecedented current densities (43 A/m) and temperatures (> 45 K). The behaviour of epitaxial graphene grown on silicon carbide in the quantum Hall regime is shown to be characterised by a strongly magnetic field dependent carrier density due to charge transfer from surface donor states in the substrate. Chapter 5 shows that polymer wrapping of SWNTs can achieve high quality purified samples. Individual SWNTs were probed using micro-photoluminescence measurements in magnetic fields up to 30 T. The combination of high magnetic fields and high spectral and spatial resolution allowed a detailed study of exciton fine structure. High intensity laser irradiation is shown to induce bound excitons in pristine tubes. The optical properties of a number of tubes are dominated by defect sites which may be imaged along the tube using the magnetic brightening of dark excitons associated with such defects.

25

Downs, Christopher Stephen Charles. „A route to strain-engineering electron transport in graphene“. Thesis, University of Exeter, 2015. http://hdl.handle.net/10871/18897.

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Graphene, a single atomic layer of graphite, has many exciting electronic and mechanical properties. On a fundamental level, the quasi-relativistic behaviour of the charge carriers in graphene arises from the honeycomb-like atomic structure. Deforming the lattice changes the lengths of the carbon-carbon bonds, breaking the hopping symmetry between carbon sites. Mathematically, elastic strain in a graphene membrane can be described by additional terms in the low-energy effective Hamiltonian, analogous to the vector potential of an external magnetic field. Hence, certain non-uniform strain geometries produce so-called `pseudo-magnetic fields', leading to a predicted zero-field quantum Hall effect. These fictitious magnetic fields are distinct from an external magnetic field in that they are only observed by charge carriers within the membrane, and have opposing polarity for electrons in the K and K' valleys, preserving time-reversal symmetry of the lattice as a whole. Deforming graphene in the non-uniform manner required to produce a homogeneous pseudo-magnetic field has proven to be a huge technological challenge, however, restricting experimental evidence to scanning tunnelling spectroscopy measurements on, for example, highly deformed nanobubbles formed by the thermal expansion of an epitaxially grown sheet on a platinum substrate. These results stimulated a large amount of interest in strain-engineering electron transport in graphene, partly due to the extreme magnitude of the observed pseudo-magnetic field, a direct consequence of the strain components strongly varying over the space of a few nanometres, but the formation of nanobubbles is a highly stochastic process which cannot be reliably reproduced. Subsequent research found a way to fabricate nanobubbles with a high degree of consistency, but the measurements were still limited to local-probe techniques due to the nanoscale size of the devices. As such, a method to reliably induce a homogeneous pseudo-magnetic field within a micron-sized membrane would be an attractive proposition, and is the basis for the work presented within this thesis. The non-uniform strain required precludes a simple bending or elongation of the substrate, hence a more local method is required. A novel nanostructure consisting of suspended gold beams surrounding a graphene membrane will deform upon cooling to cryogenic temperatures, and crucially, the actuation mechanism can be designed to produce any configuration of strain, including uniaxial strain, triaxial strain and a fan-shaped deformation, the latter two of which are predicted to create homogeneous pseudo-magnetic fields within a membrane. Strain patterns which are predicted to produce experimentally significant pseudo-magnetic fields (~1 T) may be generated with complex actuation beams that are physically achievable. Furthermore, the actuation mechanisms may be utilised as electrical contacts to the membrane, allowing its conductivity to be measured in the context of a two- or multi-terminal measurement, in conjunction with an external magnetic field. The design of the devices was developed using finite-element analysis, and the behaviour verified by low-temperature imaging of prototypes. While, after careful annealing, some conventional two-terminal suspended devices exhibited quantum Hall features at very low fields, the fabricated strain-inducing devices did not display pseudo-Landau quantisation, nor Landau quantisation, due to the difficulties of using current annealing to clean devices post-fabrication. The presented work, however, could pave the way towards observing signatures of pseudo-magnetic fields in a range of experimental measurements, as well as creating alternative strain geometries.

26

Taychatanapat, Thiti. „From Hopping to Ballistic Transport in Graphene-Based Electronic Devices“. Thesis, Harvard University, 2013. http://dissertations.umi.com/gsas.harvard:10815.

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This thesis describes electronic transport experiments in graphene from the hopping to the ballistic regime. The first experiment studies dual-gated bilayer graphene devices. By applying an electric field with these dual gates, we can open a band gap in bilayer graphene and observe an increase in resistance of over six orders of magnitude as well as a strongly non-linear behavior in the transport characteristics. A temperature-dependence study of resistance at large electric field at the charge neutrality point shows the change in the transport mechanism from a hopping dominated regime at low temperature to a diffusive regime at high temperature.
Physics

27

Quilliam, Jeffrey. „Specific Heat of the Dilute, Dipolar-Coupled, Ising Magnet LiHo_xY_1-xF₄“. Thesis, University of Waterloo, 2006. http://hdl.handle.net/10012/2964.

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The system LiHo_xY_1-xF₄ is a nearly perfect example of a dilute, dipolar-coupled Ising magnet and, as such, it is an ideal testing ground for many theories in statistical mechanics. At low holmium concentration (x = 0. 045) an unusual spin liquid or "anti-glass" state was discovered in previous work [1]. This state does not exhibit a spin glass freezing transition as is expected for a long-range interaction. Instead, it shows dynamics which are consistent with a collection of low-frequency oscillators [2]. It was also seen to have sharp features in its specific heat [3].

We present heat capacity measurements on three samples at and around the concentration of the spin liquid state in zero magnetic field and in a temperature range from around 50 mK to 1 K. In contrast to previous measurements, we find no sharp features in the specific heat. The specific heat is a broad feature which is qualitatively consistent with that of a spin glass. The residual entropy as a function of x, obtained through a numerical integral of the data, however, is consistent with numerical simulations which predict a disappearance of spin glass ordering below a critical concentration of dipoles [4].

Also presented here, is ac susceptibility data on an x = 0. 45 sample which exhibits a paramagnetic to ferromagnetic transition and is found to be consistent with previous work.

28

McClure, Douglas. „Interferometer-Based Studies of Quantum Hall Phenomena“. Thesis, Harvard University, 2012. http://dissertations.umi.com/gsas.harvard:10300.

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The fractional quantum Hall (FQH) effect harbors a wealth of unique phenomena, many of which remain mysterious. Of particular interest is the predicted existence of quasi-particles with unusual topological properties, especially in light of recent proposals to observe these properties using electronic interferometers. An introduction to quantum Hall physics and electronic interferometry is given in Chapter 1 of this thesis. The remaining chapters, summarized below, describe a set of experiments in which FQH systems are studied using electronic Fabry-Perot interferometry and related techniques. Since prior studies of electronic Fabry-Perot interferometers revealed unexpected behavior even in the integer quantum Hall (IQH) regime, we began our measurements there. Our initial experiment, presented in Chapter 2, disentangles signatures of Coulomb interaction effects from those of Aharonov-Bohm (AB) interference and provides the first measurement of pure AB interference in these devices. In our next experiment, presented in Chapter 3, we measure AB interference oscillations as a function of an applied dc bias, use their period to study the velocity of the interfering electrons, and study how the oscillations decay as a function of bias and magnetic field. Moving to the FQH regime, applying a similar-sized bias to a quantum point contact leads to long-lasting changes in the strengths and positions of FQH plateaus. The involvement of lattice nuclear spins in this effect, suggested by the long persistence times, is confirmed using NMR-type measurements. Although the exact physical process responsible for the effect remains unclear, its filling-factor dependence provides a striking illustration of composite fermion physics. These measurements are described in Chapter 4. In certain devices, interference oscillations associated with several FQH states are observed. Interpretation of their magnetic-field and gate-voltage periods provides a measurement of quasi-particle charge, and temperature dependence measurements suggest differences between the edge structure of IQH and FQH states. These measurements are described in Chapter 5. Finally, Chapter 6 presents some recent, not-yet-published observations that may shed light on ways to improve the visibility of existing oscillations and potentially observe interference at additional FQH states. This chapter concludes with a discussion of possible next steps toward achieving these goals.
Physics

29

Alnoor, Hatim. „Toward the Optimization of Low-temperature Solution-based Synthesis of ZnO Nanostructures for Device Applications“. Doctoral thesis, Linköpings universitet, Fysik och elektroteknik, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-141753.

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One-dimensional (1D) nanostructures (NSs) of Zinc Oxide (ZnO) such as nanorods (NRs) have recently attracted considerable research attention due to their potential for the development of optoelectronic devices such as ultraviolet (UV) photodetectors and light-emitting diodes (LEDs). The potential of ZnO NRs in all these applications, however, would require synthesis of high crystal quality ZnO NRs with precise control over the optical and electronic properties. It is known that the optical and electronic properties of ZnO NRs are mostly influenced by the presence of native (intrinsic) and impurities (extrinsic) defects. Therefore, understanding the nature of these intrinsic and extrinsic defects and their spatial distribution is critical for optimizing the optical and electronic properties of ZnO NRs. However, identifying the origin of such defects is a complicated matter, especially for NSs, where the information on anisotropy is usually lost due to the lack of coherent orientation. Thus, the aim of this thesis is towards the optimization of the lowtemperature solution-based synthesis of ZnO NRs for device applications. In this connection, we first started with investigating the effect of the precursor solution stirring durations on the deep level defects concentration and their spatial distribution along the ZnO NRs. Then, by choosing the optimal stirring time, we studied the influence of ZnO seeding layer precursor’s types, and its molar ratios on the density of interface defects. The findings of these investigations were used to demonstrate ZnO NRs-based heterojunction LEDs. The ability to tune the point defects along the NRs enabled us further to incorporate cobalt (Co) ions into the ZnO NRs crystal lattice, where these ions could occupy the vacancies or interstitial defects through substitutional or interstitial doping. Following this, high crystal quality vertically welloriented ZnO NRs have been demonstrated by incorporating a small amount of Co into the ZnO crystal lattice. Finally, the influence of Co ions incorporation on the reduction of core-defects (CDs) in ZnO NRs was systematically examined using electron paramagnetic resonance (EPR).

30

Han, Shou. „Magnetism in multiferroics and low dimensional metal-organic complexes“. Thesis, Queen Mary, University of London, 2016. http://qmro.qmul.ac.uk/xmlui/handle/123456789/23198.

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Multiferroics and magnetic metal-organic complexes are candidates for sophisticated applications in the future. In thisthesis, the magnetism in BiFeO3 (a multiferroic material with an incommensurate spin cycloidal structure), copper guanidiniam formate (a multiferroic metal-organic complex with a one-dimensional magnetic structure) and CP -RE-COT (a series of \zero-dimensional" single molecule magnets) are discussed. A radio-frequency plasma sputtering thin lm deposition system and a ferroelectric characterisation system were developed for the study of BiFeO3 epitaxial thin lms. A large leakage current was observed in BiFeO3 thin lms, which hindered the investigations on the ferroelectric properties and magnetoelectric coupling in them. An evidence of the spin cycloid in a BiFeO3 thin lm was observed by grazing-incidence small angle neutron scattering. The magnetism of a multiferroic metal-organic complex with a one-dimensional magnetic chain, [C(NH2)3][Cu(HCOO)3], was studied by magnetometry and muon spin spectroscopy. A spin-canted antiferromagnetic order and critical phenomenon in this material were investigated. It was shown that this material possessed an 3D Heisenberg long-range order below 4.6K. The one-dimensional magnetic chain was also studied by muon spin spectroscopy. The correlation length was measured with a eld dependence of H 1. Magnetisation relaxations of a series of single molecule magnets CP -RE-COT (COT = C8H8- CP = C5Me, which show "zero-dimensional" magnetism, were studied using an AC magnetometer and muon spin spectroscopy. Three possible relaxation pathways, including a quantum tunnelling process and two Orbach relaxation processes, were suggested by the relaxation behaviour. The suppression of the quantum tunnelling effect resulting from the entanglement of the ground states, which probably arises from the exchange interactions in CP -RE-COT, was also observed with a 1000 Oe applied magnetic eld. Data that were consistent with long-range magnetic ordering was observed in CP -Dy-COT, which would be the fi rst ever report of long-range magnetic order in a single ion magnet.

31

Casas, Brian Wesley. „Effects of disorder and low dimensionality on frozen dynamics in Ca3Co2-xMnxO6“. Scholar Commons, 2015. http://scholarcommons.usf.edu/etd/5657.

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Complex oxides represent an intersection of play grounds for the existence of exciting new fundamental physics and materials with potential technological implications. The realization of many exciting properties of these systems rely on the coupling of electronic, structural and magnetic degrees of freedom. Additionally, competing interactions within each type of coupling discussed previously lead to theoretically diverse ground states, which under the application of an external perturbation, can be tuned and probed. Ca3Co¬2-xMnxO6 represent a quasi-one dimensional Ising spin chain system oriented in an antiferromagnetic triangular lattice. The exotic behavior of the undoped compound Ca3Co2O6 has inspired work on continuing the fundamental understanding of frustrated magnetic systems. Through chemical doping of Manganese ions, the magnetic properties, namely the exotic spin glass like behavior can be enhanced for a modest doping range of x The effects of particle dimensionality were probed through the application of varied calcining conditions as to attempt to observe the altering of magnetic properties, mainly the out of equilibrium magnetization plateaus observed in Ca3Co1.75 Mn0.25O6. It appears that within the particle sizes studied the magnetic behavior is highly robust, even considering the inclusion of ionic disorder.

32

Yee, Michael Manchun. „Scanning Tunneling Spectroscopy of Topological Insulators and Cuprate Superconductors“. Thesis, Harvard University, 2014. http://dissertations.umi.com/gsas.harvard:11584.

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Over the past twenty-five years, condensed matter physics has been developing materials with novel electronic characteristics for a wide range of future applications. Two research directions have shown particular promise: topological insulators, and high temperature copper based superconductors (cuprates). Topological insulators are a newly discovered class of materials that can be manipulated for spintronic or quantum computing devices. However there is a poor spectroscopic understanding of the current topological insulators and emerging topological insulator candidates. In cuprate superconductors, the challenge lies in raising the superconducting transition temperature to temperatures accessible in non-laboratory settings. This effort has been hampered by a poor understanding of the superconducting mechanism and its relationship with a mysterious pseudogap phase. In this thesis, I will describe experiments conducted on topological insulators and cuprate superconductors using scanning tunneling microscopy and spectroscopy, which provide nanoscale spectroscopic information in these materials.
Physics

33

Robinson, Neil Joe. „Pairing, paramagnetism and prethermalization in strongly correlated low-dimensional quantum systems“. Thesis, University of Oxford, 2014. http://ora.ox.ac.uk/objects/uuid:167d164c-e318-49b3-83ea-69b54ec531e0.

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Quasi-one-dimensional quantum models are ideal for theoretically exploring the physical phenomena associated with strong correlations. In this thesis we study three examples where strong correlations play an important role in the static or dynamic properties of the system. Firstly, we examine the behaviour of a doped fermionic two-leg ladder in which umklapp interactions are present. Such interactions arise at special band fillings and can be induced by the formation of charge density wave order in an array of two-leg ladders with long-range (three-dimensional) interactions. For the umklapp which arises from the half-filling of one of the bands, we show that the low-energy theory has a number of phases, including a strong coupling regime in which the dominant fluctuations are superconducting in nature. These superconducting fluctuations carry a finite wave vector – they are the one-dimensional analogue of Fulde-Ferrell-Larkin-Ovchinnikov superconductivity. In a second example, we consider a quantum spin model which captures the essential one-dimensional physics of CoNb₂O₆, a quasi-one-dimensional Ising ferromagnet. Motivated by high-resolution inelastic neutron scattering experiments, we calculate the dynamical structure in the paramagnetic phase and show that a small misalignment of the transverse field can lead to quasi-particle breakdown – a surprising broadening in the single particle mode observed in experiment. Finally, we study the out-of-equilibrium dynamics of a model with tuneable integrability breaking. When integrability is broken by the presence of weak interactions, we show that the system relaxes to a non-thermal state on intermediate time scales, the so-called “prethermalization plateau”. We describe the approximately stationary behaviour in this regime by constructing a generalised Gibbs ensemble with charges deformed to leading order in perturbation theory. Expectation values of these charges are time-independent, but interestingly the charges do not commute with the Hamiltonian to leading order in perturbation theory. Increasing the strength of the integrability breaking interactions leads to behaviour compatible with thermalisation. In each case we use a combination of perturbative analytical calculations and non-perturbative numerical computations to study the problem at hand.

34

Cardellino, Jeremy D. „Dynamics of Paramagnetic Spins: A Study of Spin Defects using Magnetic Resonance Force Microscopy“. The Ohio State University, 2015. http://rave.ohiolink.edu/etdc/view?acc_num=osu1448982765.

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35

Qiu, Lei. „Exploring 2D Metal-Insulator Transition in p-GaAs Quantum Well with High rs“. Case Western Reserve University School of Graduate Studies / OhioLINK, 2014. http://rave.ohiolink.edu/etdc/view?acc_num=case1386337954.

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36

Babonis, Gregory S. „Low Temperature Scanning Tunneling Microscope for Single Atom Manipulation“. Ohio University / OhioLINK, 2003. http://rave.ohiolink.edu/etdc/view?acc_num=ohiou1058475483.

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37

Brunson, Jerilyn. „Hopping Conductivity and Charge Transport in Low Density Polyethylene“. DigitalCommons@USU, 2010. https://digitalcommons.usu.edu/etd/562.

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The properties and behaviors of charge transport mechanisms in highly insulating polymers are investigated by measuring conduction currents through thin film samples of low density polyethylene (LDPE). Measurements were obtained using a constant voltage method with copper electrodes inside a chamber adapted for measurements under vacuum and over a wide range of temperatures and applied fields. Field-dependent behaviors, including Poole-Frenkel conduction, space charge limited current (SCLC), and Schottky charge injection, were investigated at constant temperature. These field-dependent mechanisms were found to predict incorrect values of the dielectric constant and the field dependence of conductivity in LDPE was not found to be in agreement with SCLC predicted behavior. A model of thermally assisted hopping was a good fit at low applied fields and produced activation energies within the accepted range for LDPE. Low applied field measurements over the range of 213 K to 338 K were used to investigate two prominent hopping conduction mechanisms: thermally assisted hopping and variable range hopping. The observed temperature dependence of LDPE was found to be consistent with both thermally assisted hopping and variable range hopping. Activation energies determined for the range of temperatures were consistent with values reported in the literature for LDPE under similar conditions. A third aspect of charge transport behavior is a bulk response with time dependence. Conductivity behavior is examined in relation to transient current behavior, long time decay currents, and electrostatic discharge. Comparing charging and discharging cycles allowed qualitative separation of polarization and multiple trapping behaviors.

38

Lamouri, Abbas. „Low-energy sputtering of Teflon by oxygen ion bombardment“. Case Western Reserve University School of Graduate Studies / OhioLINK, 1991. http://rave.ohiolink.edu/etdc/view?acc_num=case1055777824.

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39

Diaconu, Andrei. „Ultra-low Temperature Measurements of London Penetration Depth in Iron Selenide Telluride Superconductors“. ScholarWorks@UNO, 2013. http://scholarworks.uno.edu/td/1731.

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The newly discovered iron based superconductors have captivated the attention of the scientific community due to the unusual mechanism behind their superconductivity and their promise as the next generation high temperature superconductors. After a century of superconductor research, the physical mechanism behind high temperature superconductivity is still not understood. These new materials bring renewed hope in elucidating the pairing mechanism responsible with high temperature superconductors and achieving the ultimate goal of the field, room temperature superconductivity. Consequently, a deeper understanding of the intriguing properties of iron based materials is essential. A great deal about the pairing mechanism of Cooper electron pairs can be inferred from the symmetry of their pairing wave function or order parameter. One of the most involved probes for studying the pairing symmetry is the London penetration depth. The low temperature behavior of London penetration depth in superconductors is directly related to the density of states and provides a powerful tool for investigating low-lying quasiparticle energy and, for this very reason, can give valuable hints on superconducting gap symmetry. The work presented focuses on investigating the pairing symmetry in the Fe1+y(Te1−xSex) system using a radio-frequency tunnel diode oscillator (TDO) technique for precise measurements of the temperature dependence of their in-plane penetration depth. The TDO technique, based on an original concept involving the use of planar inductors in an novel configuration, was implemented on a dilution refrigerator to investigate a significant number of single crystal samples, with nominal Se concentrations of 36%, 40%, 43% and 45% respectively, down to temperatures as low as 50 mK. A systematic study together with a comprehensive analysis regarding the order parameter symmetry in the Fe1+y(Te1−xSex) system is presented. In many cases we found that London penetration depth shows an upturn below at low temperatures, indicative of a paramagnetic-type contribution. Also the low-temperature behavior of penetration depth is best described by a quadratic power law with no systematic dependence on the Se concentration. Most importantly, in the limit of T → 0, in some samples we observed a narrow region of linear temperature dependence, suggestive of nodes in the superconducting gap of Fe1+y(Te1−xSex).

40

Chen, Liang-Yu. „Secondary ions sputtered by low energy ion bombardment of copper and aluminum surfaces“. Case Western Reserve University School of Graduate Studies / OhioLINK, 1995. http://rave.ohiolink.edu/etdc/view?acc_num=case1058535998.

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41

Gilbertson, Sarah Elizabeth. „Aerosol Gel production via controlled detonation of liquid precursors“. Thesis, Manhattan, Kan. : Kansas State University, 2008. http://hdl.handle.net/2097/1101.

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42

Goble, Nicholas James. „ELECTRONIC TRANSPORT AT SEMICONDUCTOR AND PEROVSKITE OXIDE INTERFACES“. Case Western Reserve University School of Graduate Studies / OhioLINK, 2016. http://rave.ohiolink.edu/etdc/view?acc_num=case1454002713.

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43

Peng, Han. „Spatial resolved electronic structure of low dimensional materials and data analysis“. Thesis, University of Oxford, 2018. http://ora.ox.ac.uk/objects/uuid:2f3503eb-93bf-48d6-b6fb-13409b925748.

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Two dimensional (2D) materials with interesting fundamental physics and potential applications attract tremendous efforts to study. The versatile properties of 2D materials can be further tailored by tuning the electronic structure with the layer-stacking arrangement, of which the main adjustable parameters include the thickness and the in-plane twist angle between layers. The Angle-Resolved Photoemission Spectroscopy (ARPES) has become a canonical tool to study the electronic structure of crystalline materials. The recent development of ARPES with sub-micrometre spatial resolution (micro-ARPES) has made it possible to study the electronic structure of materials with mesoscopic domains. In this thesis, we use micro-ARPES to investigate the spatially-resolved electronic structure of a series of few-layer materials: 1. We explore the electronic structure of the domains with different number of layers in few-layer graphene on copper substrate. We observe a layer- dependent substrate doping effect in which the Fermi surface of graphene shifts with the increase of number of layers, which is then explained by a multilayer effective capacitor model. 2. We systematically study the twist angle evolution of the energy band of twisted few-layer graphene over a wide range of twist angles (from 5° to 31°). We directly observe van Hove Singularities (vHSs) in twisted bilayer graphene with wide tunable energy range over 2 eV. In addition, the formation of multiple vHSs (at different binding energies) is observed in trilayer graphene. The large tuning range of vHS binding energy in twisted few-layer graphene provides a promising material base for optoelectrical applications with broad-band wavelength selectivity. 3. To better extract the energy band features from ARPES data, we propose a new method with a convolutional neural network (CNN) that achieves comparable or better results than traditional derivative based methods. Besides ARPES study, this thesis also includes the study of surface reconstruction for the layered material Bi2O2Se with the analysis of Scanning Tunnelling Microscopy (STM) images. To explain the origin of the pattern, we propose a tile model that produces the identical statistics with the experiment.

44

Lampen, Kelley Paula J. „Low Dimensionality Effects in Complex Magnetic Oxides“. Scholar Commons, 2015. http://scholarcommons.usf.edu/etd/5874.

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Complex magnetic oxides represent a unique intersection of immense technological importance and fascinating physical phenomena originating from interwoven structural, electronic and magnetic degrees of freedom. The resulting energetically close competing orders can be controllably selected through external fields. Competing interactions and disorder represent an additional opportunity to systematically manipulate the properties of pure magnetic systems, leading to frustration, glassiness, and other novel phenomena while finite sample dimension plays a similar role in systems with long-range cooperative effects or large correlation lengths. A rigorous understanding of these effects in strongly correlated oxides is key to manipulating their functionality and device performance, but remains a challenging task. In this dissertation, we examine a number of problems related to intrinsic and extrinsic low dimensionality, disorder, and competing interactions in magnetic oxides by applying a unique combination of standard magnetometry techniques and unconventional magnetocaloric effect and transverse susceptibility measurements. The influence of dimensionality and disorder on the nature and critical properties of phase transitions in manganites is illustrated in La0.7Ca0.3MnO3, in which both size reduction to the nanoscale and chemically-controlled quenched disorder are observed to induce a progressive weakening of the first-order nature of the transition, despite acting through the distinct mechanisms of surface effects and site dilution. In the second-order material La0.8Ca0.2MnO3, a strong magnetic field is found to drive the system toward its tricritical point as competition between exchange interactions in the inhomogeneous ground state is suppressed. In the presence of large phase separation stabilized by chemical disorder and long-range strain, dimensionality has a profound effect. With the systematic reduction of particle size in microscale-phase-separated (La, Pr, Ca)MnO3 we observe a disruption of the long-range glassy strains associated with the charge-ordered phase in the bulk, lowering the field and pressure threshold for charge-order melting and increasing the ferromagnetic volume fraction as particle size is decreased. The long-range charge-ordered phase becomes completely suppressed when the particle size falls below 100 nm. In contrast, low dimensionality in the geometrically frustrated pseudo-1D spin chain compound Ca3Co2O6 is intrinsic, arising from the crystal lattice. We establish a comprehensive phase diagram for this exotic system consistent with recent reports of an incommensurate ground state and identify new sub-features of the ferrimagnetic phase. When defects in the form of grain boundaries are incorporated into the system the low-temperature slow-dynamic state is weakened, and new crossover phenomena emerge in the spin relaxation behavior along with an increased distribution of relaxation times. The presence of both disorder and randomness leads to a spin-glass-like state, as observed in γFe2O3 hollow nanoparticles, where freezing of surface spins at low temperature generates an irreversible magnetization component and an associated exchange-biasing effect. Our results point to distinct dynamic behaviors on the inner and outer surfaces of the hollow structures. Overall, these studies yield new physical insights into the role of dimensionality and disorder in these complex oxide systems and highlight the sensitivity of their manifested magnetic ground states to extrinsic factors, leading in many cases to crossover behaviors where the balance between competing phases is altered, or to the emergence of entirely new magnetic phenomena.

45

Fang, Jieping. „New Methods to Create Multielectron Bubbles in Liquid Helium“. Thesis, Harvard University, 2012. http://dissertations.umi.com/gsas.harvard:10673.

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An equilibrium multielectron bubble (MEB) in liquid helium is a fascinating object with a spherical two-dimensional electron gas on its surface. After it was first observed a few decades ago, a plethora of physical properties of MEBs, for example, a tunable surface electron density, have been predicted. In this thesis, we will discuss two new methods to create MEBs in liquid helium. Before the discussion, the way to generate a large number of electrons in a low temperature system will be discussed, including thermionic emission and field emission in helium. In the first new method to make MEBs, we used a dome-shaped cell filled with superfluid helium in which an MEB was created and confined at the dome. The lifetime of the MEB was substantially longer than the previously reported observations of MEBs. In the second method, MEBs were extracted from the vapor sheath around an electrically heated tungsten filament submerged in liquid helium, either by a high electric field (up to 15 kV/cm) or by a sudden increase of a negative pressure in liquid helium. High-speed photography was used to capture the MEB's motion. A method to determine the number of electrons was developed by monitoring the oscillations of the MEBs. Finally, an electromagnetic trap was designed to localize the MEBs created using the second method, which was important for future studies of the properties of MEBs.
Physics

46

Dargel, Piet. „Spectral functions of low-dimensional quantum systems“. Doctoral thesis, Niedersächsische Staats- und Universitätsbibliothek Göttingen, 2012. http://hdl.handle.net/11858/00-1735-0000-000D-F1A3-6.

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47

Wheeler, Elisa Maria da Silva. „Neutron scattering from low-dimensional quantum magnets“. Thesis, University of Oxford, 2007. http://ora.ox.ac.uk/objects/uuid:a8411774-4a3e-4fc3-80a1-d7e8612cba71.

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Neutron scattering measurements were used to investigate the magnetic and crystal structure and magnetic excitations of three compounds characterized as low-dimensional quantum magnets. The materials are frustrated systems with low spin quantum number. The first was a powder sample of AgNiO₂. The Ni ions form a triangular lattice antiferromagnet in which, according to the published crystal structure, both the orbital order and magnetic couplings are frustrated. However, it is shown here that there was a small distortion of the crystal structure at 365 K, which is proposed to result from charge disproportionation and this relieves the orbital frustration. The magnetic structure was investigated and, below 20 K, the triangular lattice of electron-rich Ni sites was observed to order into antiferromagnetic stripes. Investigations of the magnetic excitations showed that the main dispersions were within the triangular plane, indicating a strong two-dimensionality. The dispersion was larger along the stripes than between the stripes of collinear spins. The second material investigated was CoNb₂O₆, a quasi Ising-like ferromagnet. It was studied with a magnetic field applied transverse to the Ising direction. The magnetic field introduced quantum fluctuations which drove a phase transition at a field comparable to the main exchange interaction. The phase diagram of the magnetic order was mapped outs and a transition from an ordered phase to a paramagnetic phase was identified at high field. This low-temperature high-field phase transition was further investigated by inelastic neutron scattering measurements to observe the change in the energy gap and magnetic excitation spectrum on either side of the transition. The spectrum had two components in the ordered phase and had sharp magnon modes in the paramagnetic phase. The third material was the spin-half layered antiferromagnet CuSb₂O₆. It has a square lattice of Cu²⁺ ions in which the main interaction is across only one diagonal of the square. The magnetic structure was studied by neutron scattering with a field applied along the direction of the zero-field ordered moment. A spin-flop was observed at low field and there was evidence for a high-field transition. The magnetic excitation spectrum was unusual in that it had an intense resonance at 13 meV at the magnetic Brillouin zone boundary.

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Petersen, Greg M. „Anderson Localization in Low-Dimensional Systems with Long-Range Correlated Disorder“. Ohio University / OhioLINK, 2013. http://rave.ohiolink.edu/etdc/view?acc_num=ohiou1365762218.

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49

Mendoza, Arenas Juan José. „Spin and energy transport in boundary-driven low-dimensional open quantum systems“. Thesis, University of Oxford, 2014. http://ora.ox.ac.uk/objects/uuid:44b89c4d-e9eb-4136-a540-c80bcabeb6f6.

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In spite of being the subject of intense research, several key but complex questions on the nonequilibrium physics of correlated quantum systems remain controversial. For example, the nature of particle and energy transport in different interacting regimes, the relevance of integrability and the impact of environmental coupling are still under active debate. These problems can now be approached numerically, due to the development of powerful algorithms which allow the efficient simulation of the dynamics of correlated systems. In the present thesis we study numerically and analytically the transport properties of low-dimensional quantum systems. In particular, we consider the steady-state spin and energy conduction through XXZ boundary-driven spin-1/2 chains. In the first part, we analyse the transport through chains with only coherent processes in the bulk. For spin transport induced by a magnetisation imbalance between the boundaries, previously identified ballistic, diffusive and negative differential conductivity regimes are reproduced. We provide a comprehensive explanation of the latter. The energy conduction induced by this driving scheme features the same properties as spin transport. For thermally-driven chains, we discuss the nature of energy transport and the emergence of local thermal states when the integrability of the Hamiltonian is broken. In the second part of the thesis we analyse the effect of bulk incoherent effects on the transport properties previously discussed. First we find that for weak particle-particle interactions, pure dephasing degrades spin and energy conduction. In contrast, for strong interactions dephasing induces a significant transport enhancement. We identify the underlying mechanism and discuss its generality. Finally, motivated by the lattice structure of several organic conductors, we study the interplay between coherent and incoherent processes in systems of weakly-coupled chains. We find an enhancement effect due to incoherent interchain hopping, stronger than that by dephasing, which increases with the chain length and relates to superdiffusive transport.

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Coak, Matthew. „Quantum tuning and emergent phases in charge and spin ordered materials“. Thesis, University of Cambridge, 2018. https://www.repository.cam.ac.uk/handle/1810/280284.

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A major area of interest in condensed matter physics over the past decades has been the emergence of new states of matter from strongly correlated electron systems. A few limited examples would be the emergence of unconventional superconductivity in the high-T$_c$ superconductors and heavy-fermion systems, the appearance of the skyrmion magnetic vortex state in MnSi and magnetically mediated superconductivity in UGe$_2$. While detailed studies of many of the emergent phases have been made, there are still many gaps in understanding of the underlying states and mechanisms that allow them to form. This work aims to add to knowledge of the basic physics behind such states, and the changes within them as they are tuned to approach new phases. The cubic perovskite material SrTiO$_3$ has been studied for many decades and is well-documented to be an incipient ferroelectric, theorised to exist in the absence of any tuning in the proximity of a ferroelectric quantum critical point. This work presents the first high-precision dielectric measurements under hydrostatic pressure carried out on a quantum critical ferroelectric, leading to a full pressure-temperature phase diagram for SrTiO$_3$. The influence of quantum critical fluctuations is seen to diminish as the system is tuned away from the quantum critical point and a novel low temperature phase is shown to be emergent from it. The Néel Temperature of the two-dimensional antiferromagnet FePS$_3$ was found to increase linearly with applied hydrostatic pressure. Evidence of an insulator-metal transition is also presented, and an unexplained upturn in the resistivity at low temperatures in the metallic phase.

Dissertationen zum Thema „Physics, Low Temperature|Physics, Condensed Matter“

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