Source: Dissertation Abstracts International, Volume: 35-07, Section: B, page: 3528. / Thesis (Ph.D.)--The Florida State University, 1974.

Source: Dissertation Abstracts International, Volume: 35-10, Section: B, page: 5055. / Thesis (Ph.D.)--The Florida State University, 1974.

Source: Dissertation Abstracts International, Volume: 39-11, Section: B, page: 5443. / Thesis (Ph.D.)--The Florida State University, 1978.

Source: Dissertation Abstracts International, Volume: 39-11, Section: B, page: 5444. / Thesis (Ph.D.)--The Florida State University, 1978.

Source: Dissertation Abstracts International, Volume: 39-11, Section: B, page: 5444. / Thesis (Ph.D.)--The Florida State University, 1978.

This dissertation details the use of 93Nb (γ = 10.405 MHz/T, I = 9/2) and 77Se (γ = 8.13 MHz/T, I = 1/2) nuclear magnetic resonance (NMR) to investigate the phase transitions of the layered transition metal dichalcogenide (TMD) niobium diselenide (2H-NbSe2). 2H-NbSe2 has a trigonal prismatic structure and exhibits a charge density wave (CDW) transition at TCDW = 33.5 K and a superconducting (SC) transition with Tc = 7.2 K. The present experiments were undertaken with the external field (H0) first parallel, and then perpendicular to the crystallographic axis (c-axis). Single crystals of 2H-NbSe2 were probed in the temperature range of 0.35 K – 100 K and at frequencies and fields ranging from 19 MHz – 135 MHz and 1.8 T – 17.5 T respectively to investigate the normal, CDW and superconducting states. A value of Tc = 7.04 K was measured in situ by cooling the sample below Tc and measuring detuning. A value of Tc = 6.2 K was measured in a sample grown from the same batch using a SQUID magnetometer. 93Nb NMR spectral line shape and Knight shift were used to detect the CDW phase. The full 93Nb spectrum was field-swept and observed to possess line broadening and asymmetry which gain intensity the further the respective transition is from the central line. Pre-transitional broadening was detected in both orientations, beginning at a temperature of 60 K for the central transition and as high as 80 K for the first lower-frequency satellite transition. The broadening continues below TCDW and ends below 15 K. These results are believed to be evidence of a discommensurate CDW phase. Spin-lattice relaxation rate (T1) measurements are used to directly probe the electronic density of states (DOS) and investigate the SC gap. The T1 data of both nuclei reveal Korringa behavior above Tc, no visible Hebel-Slichter peak just below Tc, and a linear crossover to further Korringa behavior for T << Tc in both orientations. Both orientations were accurately fit to a two-gap function and 1/T1T in the H0⊥c direction displays a two-step transition, suggesting two superconducting gaps are present. A field-dependence of T1 in the SC state was detected in both sample orientations. In both the H0∥c and H0⊥c directions, 〖1/T〗_1∝H due to the Zeeman contribution of the Volovik effect. The Doppler effect contribution from the vortex supercurrents was absent from the relaxation, which suggests 2H-NbSe2 is a two-gap s-wave superconductor. / A Dissertation submitted to the Program in Materials Science and Engineering in partial fulfillment of the requirements for the degree of Doctor of Philosophy. / Fall Semester 2017. / November 14, 2017. / Charge Density Wave, Condensed Matter Physics, NbSe2, NMR, Superconductivity, Transition Metal Dichalcogenide / Includes bibliographical references. / Arneil Reyes, Professor Co-Directing Dissertation; Irinel Chiorescu, Professor Co-Directing Dissertation; Adrian Barbu, University Representative; Eric Hellstrom, Committee Member; Theo Siegrist, Committee Member.

The Weyl semimetal requires the breaking of either the time-reversal symmetry (TRS) or the lattice inversion symmetry. When the TRS and inversion symmetry coexist, a pair of degenerate Weyl points may exist, leading to the related Dirac semimetal phase. In other words, a Dirac semimetallic state can be regarded as two copies of Weyl semimetal states. In this dissertation, we study tellurium based compounds like the Weyl semimetal candidate MoTe2 and the Dirac semimetal candidate PtTe2 within the transition metal dichalcogenides family. Firstly, we report a systematic study on the Hall-effect of the semi-metallic state of bulk MoTe2, which was recently claimed to be a candidate for a novel type of Weyl semi-metallic state. The temperature (T) dependence of the carrier densities and of their mobilities, as estimated from a numerical analysis based on the isotropic two-carrier model, indicates that its exceedingly large and non-saturating magnetoresistance may be attributed to a near perfect compensation between the densities of electrons and holes at low temperatures. A sudden increase in hole density, with a concomitant rapid increase in the electron mobility below T ∼ 40 K, leads to comparable densities of electrons and holes at low temperatures suggesting a possible electronic phase-transition around this temperature. Secondly, the electronic structure of semi-metallic transition-metal dichalcogenides, such as WTe2 and orthorhombic γ−MoTe2, are claimed to contain pairs of Weyl points or linearly touching electron and hole pockets associated with a non-trivial Chern number. For this reason, these compounds were recently claimed to conform to a new class, deemed type-II, of Weyl semi-metallic systems. A series of angle resolved photoemission experiments (ARPES) claim a broad agreement with these predictions detecting, for example, topological Fermi arcs at the surface of these crystals. We synthesized single-crystals of semi-metallic MoTe2 through a Te flux method to validate these predictions through measurements of its bulk Fermi surface (FS) via quantum oscillatory phenomena. We find that the superconducting transition temperature of γ−MoTe2 depends on disorder as quantified by the ratio between the room- and low-temperature resistivities, suggesting the possibility of an unconventional superconducting pairing symmetry. Similarly to WTe2, the magnetoresistivity of γ−MoTe2 does not saturate at high magnetic fields and can easily surpass 106 %. Remarkably, the analysis of the de Haas-van Alphen (dHvA) signal superimposed onto the magnetic torque, indicates that the geometry of its FS is markedly distinct from the calculated one. The dHvA signal also reveals that the FS is affected by the Zeeman-effect precluding the extraction of the Berry-phase. A direct comparison between the previous ARPES studies and density-functional-theory (DFT) calculations reveals a disagreement in the position of the valence bands relative to the Fermi level εF . Here, we show that a shift of the DFT valence bands relative to εF , in order to match the ARPES observations, and of the DFT electron bands to explain some of the observed dHvA frequencies, leads to a good agreement between the calculations and the angular dependence of the FS cross-sectional areas observed experimentally. However, this relative displacement between electron- and hole-bands eliminates their crossings and, therefore, the Weyl type-II points predicted for γ−MoTe2. Finally, we investigate the electronic structure and transport properties in single crystals of the semi-metallic platinum ditelluride (PtTe2), recently claimed to be a novel type-II Dirac semimetal, via a methodology similar to that applied to γ−MoTe2, i.e. the temperature and angular dependence of the SdH and dHvA effects. Our high-quality PtTe2 crystal displays a large non-saturating magnetoresistance under magnetic field up to 61 T. The dHvA oscillation and SdH effect reveal several high and low frequencies suggesting a rather complex Fermi surface. We also find evidence for a non-trivial Berry phase. The crystal quality improved considerably under subsequent annealing at high-temperatures leading to the observation of linear in field magnetoresistivity. Combined with effective masses in the order of ∼ 0.1 free electron mass, these results further suggest that PtTe2 displays bulk Dirac-like bands. / A Dissertation submitted to the Department of Physics in partial fulfillment of the requirements for the degree of Doctor of Philosophy. / Fall Semester 2017. / October 16, 2017. / fermi surfaces, semimetals, topological materials / Includes bibliographical references. / Luis Balicas, Professor Co-Directing Dissertation; Nicholas Bonesteel, Professor Co-Directing Dissertation; Petru Andrei, University Representative; Peng Xiong, Committee Member; Horst Wahl, Committee Member.

<p> This Dissertation presents the magneto-optical properties of monolayer (ML) transition metal dichalcogenide (TMDC) materials using our several magneto-optical setups that were developed at UB. In this Dissertation, we discuss a magneto-photoluminescence (PL) setup, a broadband magneto-FTIR setup, and a two-color spectroscopy setup in detail. We also discuss the double modulation technique, which we use in two-color spectroscopy. </p><p> The primary results of this work include magneto-PL measurements of ML WSe₂ on YIG. We pump these materials with circularly polarized light and analyze with a circular polarizer. We reported a 30% polarization and 10 nm peak shift in a localized state with an applied magnetic field. We see a polarization up to T = 80 K. By changing the magnetic field from &ndash;7 Tesla to +7 Tesla, localized impurity-bound exciton states show strong polarization under optical excitation of opposite helicity. Right circularly polarized PL peaks are shifted to lower energies and their PL become stronger than left circularly polarized PL peaks. This is opposite for left circularly polarized peaks. They shift to higher energies (shorter wavelengths) and become weaker than right circularly polarized peaks. We also found that localized states show more polarization than free exciton and trion peaks on YIG substrate. </p><p> We also investigated Kerr rotation and Kerr ellipticity properties of ML MoS₂ and ML WSe₂ on YIG with our new broadband magneto&mdash;FTIR optical setup. Samples and substrate do not show any Kerr ellipticity features when exposed to a changing magnetic field. All samples show strong magnetic field dependent Kerr rotation signal but we found that ML MoS₂ by itself does not show any magnetic field dependent Kerr rotation signal. We found that there are two broad peaks in the YIG and ML WSe₂ on YIG Kerr rotation spectrum. YIG&rsquo;s two broad peak centers are located at around 1800 cm^&ndash;1 and 2300 cm^&ndash;1 and ML WSe₂ on YIG peak centers are located at around 1900 cm^&ndash;1 and 2500 cm^&ndash;1. For both samples, these peak intensities are linear with the magnetic field and they are symmetric with respect to B = 0 T. ML WSe₂ on YIG peaks are shifted to higher energies with respect to YIG peak. We also report that the center of the peaks has no shift with a magnetic field. </p><p> With our two-color spectroscopy setup, we have tested Imamoglu&rsquo;s theory that predicts a splitting of dark 2p states at B = 0 Tesla. A circularly polarized laser and a linearly polarized IR laser were used together to excite electrons to dark states. We used red or green laser and CO or CO₂ IR laser together in our experimental setup. Samples are ML MoS₂ on sapphire and ML WS₂ on Si/SiO₂. Within a sensitivity of 10 &micro;rad, we did not see any splitting at B = 0 Tesla on any samples.</p><p>

<p>The topological consequences of time reversal symmetry breaking in two dimensional electronic systems have been a focus of interest since the discovery of the quantum Hall effects. Similarly interesting phenomena arise from breaking inversion symmetry in three dimensional systems. For example, in Dirac and Weyl semimetals the inversion symmetry breaking allows for non-trivial topological states that contain symmetry-protected pairs of chiral gapless fermions. This thesis presents our work on the linear and nonlinear electromagnetic responses in topological semimetals using both a semiclassical Boltzmann equation approach and a full quantum mechanical approach. In the linear response, we find a ``gyrotropic magnetic effect" (GME) where the current density $j</p><p>B$ in a clean metal is induced by a slowly-varying magnetic field. It is shown that the experimental implications and microscopic origin of GME are both very different from the chiral magnetic effect (CME). We develop a systematic way to study general nonlinear electromagnetic responses in the low-frequency limit using a Floquet approach and we use it to study the circular photogalvanic effect (CPGE) and second-harmonic generation (SHG). Moreover, we derive a semiclassical formula for magnetoresistance in the weak field regime, which includes both the Berry curvature and the orbital magnetic moment. Our semiclassical result may explain the recent experimental observations on topological semimetals. In the end, we present our work on the Hall conductivity of insulators in a static inhomogeneous electric field and we discuss its relation to Hall viscosity.

In this dissertation I present inelastic and elastic light scattering studies of collective states emerging from interactions in electron systems confined to two dimensions. These studies span the first, second and third Landau levels. I report for the first time, high energy excitations of composite fermions in the quantum fluid at v = 1/3. The high energies discovered represent excitations across multiple composite fermion energy levels, demonstrating the topological robustness of the fractional quantum Hall state at v = 1/3. This study sets the ground work for similar measurements of states in the second Landau level, such as those at v = 5/2. I present the first light scattering studies of low energy excitations of quantum fluids in the second Landau level. The study of low energy excitations of the quantum fluid at 3 ≥ v ≥ 5/2 reveals a rapid loss of spin polarization for v ≤ 3, as monitored by the intensity of the spin wave excitation at the Zeeman energy. The emergence of a continuum of low-lying excitations for v ≤ 3 reveals competing quantum phases in the second Landau level with intriguing roles of spin degrees of freedom and phase inhomogeneity. The first light scattering studies of the electron systems in the third Landau level are reported here. Measurements of low energy excitations and their spin degrees of freedom reveal contrasting behavior of states in the second and third Landau levels. I discuss these measurements in the context of the charge density wave phases, that are believed, by some, to dominate the third Landau level, and suggest ways of verifying this belief using light scattering. Distinct behavior in the dispersion of the spin wave at v = 3 is measured for the first time. The study may highlight differences in the first and second Landau levels that are manifested through the electron wavefunctions. In addition to intra-Landau level measurements, inter-Landau level studies are also reported. The results of which reveal roles of spin degrees of freedom and many body interactions in odd denominator integer quantum Hall states.