Exploring the properties of layered materials as a function of thickness has largely been limited to semiconducting materials as thin layers of metallic materials tend to oxidize readily in atmosphere. This makes it challenging to further understand properties such as superconductivity and charge density waves as a function of layer thickness that are unique to metallic compounds. This dissertation discusses a set of materials that use the modulated elemental reactants technique to isolate 1 to 3 layers of VSe2 in a superlattice in order to understand the role of adjacent layers and VSe2 thickness on the charge density wave in VSe2. The modulated elemental reactants technique was performed on a custom built physical vapor deposition to prepare designed precursors that upon annealing will self assemble into the desired heterostructure. First, a series of (PbSe)1+δ(VSe2)n for n = 1 – 3 were synthesized to explore if the charge density wave enhancement in the isovalent (SnSe)1.15VSe2 was unique to this particular heterostructure. Electrical resistivity measurements show a large change in resistivity compared to room temperature resistivity for the n = 1 heterostructure. The overall change in resistivity was larger than what was observed in the analogous SnSe heterostructure. v A second study was conducted on (BiSe)1+δVSe2 to further understand the effect of charge transfer on the charge density wave of VSe2. It was reported that BiSe forms a distorted rocksalt layer with antiphase boundaries. The resulting electrical resistivity showed a severely dampened charge density wave when compared to both analogous SnSe and PbSe containing heterostructures but was similar to bulk. Finally, (SnSe2)1+δVSe2 was prepared to further isolate the VSe2 layers and explore interfacial effects on the charge density wave by switching from a distorted rocksalt structure to 1T-SnSe2. SnSe2 is semiconductor that is used to prevent adjacent VSe2 layers from coupling and thereby enhancing the quasi two-dimensionality of the VSe2 layer. Electrical characterization shows behavior similar to that of SnSe and PbSe containing heterostructures. However, structural characterization shows the presence of a SnSe impurity that is likely influencing the overall temperature dependent resistivity. This dissertation includes previously published and unpublished co-authored materials.

We perform microscopic mean-field studies of topological order in a two-dimensional topological superconductor in the Bogoliubov-de Gennes (BdG) formalism. By adopting a two-dimensional s-wave topological superconductivity (TSC) model on a minimal tight-binding system, we solve the BdG equations self-consistently to obtain not only the superconducting order parameter, but also the Hartree potential. By computing the Thouless, Kohmoto, Nightingale, and den Nijs (TKNN) number and investigating the bulk-boundary correspondence, we study the nature of Abelian and non-Abelian TSC in terms of self-consistent solutions to the BdG equations. In particular, we examine the effects of temperature and a single non-magnetic impurity deposited in the centre of the system and how they vary depending on topology. We find that the non-Abelian phase exhibits signs of unconventional superconductivity, and by examining the behaviour of this phase under both low and high Zeeman field conditions, we show that the magnitude of the Zeeman field largely dictates the susceptibility of the system to temperature. Furthermore, we investigate the possible interplay of charge density waves (CDW) and TSC. By self-consistently solving for the mean fields, we show that TSC and topological CDW are degenerate ground states---with the same excitation spectrum in the presence of surfaces---and thus can coexist in the Abelian phase. The effects of a non-magnetic impurity, which tends to pin the phase of charge density modulations, are examined in the context of topological CDW.

We have studied the charge density wave (CDW) repolarization dynamics in blue bronze (K0.3MoO3) by applying symmetric bipolar square-wave voltages of different frequencies to the sample and measuring the changes in infrared transmittance, proportional to CDW strain. The frequency dependence of the electro-transmittance was fit to a modified harmonic oscillator response and the evolution of the parameters as functions of voltage, position, and temperature are discussed. We found that resonance frequencies decrease with distance from the current contacts, indicating that the resulting delays are intrinsic to the CDW with the strain effectively flowing from the contact. For a fixed position, the average relaxation time for most samples has a voltage dependence given by τ0 ∼ V −p, with 1 < p < 2. The temperature dependence of the fitting parameters shows that the dynamics are governed by both the force on the CDW and the CDW current: for a given force and position, both the relaxation and delay times are inversely proportional to the CDW current as temperature is varied. The long delay times (∼ 100 μs) for large CDW currents suggest that the strain response involves the motion of macroscopic objects, presumably CDW phase dislocation lines. We have done frequency domain simulations to study charge-density-wave (CDW) polarization dynamics when symmetric bipolar square current pulses of different frequencies and amplitudes are applied to the sample, using parameters appropriate for NbSe3 at T = 90 K. The frequency dependence of the strain at one fixed position was fit to the same modified harmonic oscillator response and the behavior of the parameters as functions of current and position are discussed. Delay times increase nonlinearly with distance from the current contacts again, indicating that these are intrinsic to the CDWwith the strain effectively flowing from the contact. For a fixed position and high currents the relaxation time increases with decreasing current, but for low currents its behavior is strongly dependent on the distance between the current contact and the sample ends. This fact clearly shows the effect of the phase-slip process needed in the current conversion process at the contacts. The relaxation and delay times computed (∼ 1 μs) are much shorter than observed in blue bronze (> 100 μs), as expected because NbSe3 is metallic whereas K0.3MoO3 is semiconducting. While our simulated results bear a qualitative resemblance with those obtained in blue bronze, we can not make a quantitative comparison with the K0.3MoO3 results since the CDW in our simulations is current driven, whereas the electro-optic experiment was voltage driven. Different theoretical models predict that for voltages near the threshold Von, quantities such as the dynamic phase velocity correlation length and CDW velocity vary as ξ ∼ |V/Von − 1| −ν and v ∼ |V/Von − 1|ξ with ν ∼ 1/2 and ζ = 5/6. Additionally, a weakly divergent behavior for the diffusion constant D ∼ |V/Von − 1|−2ν+ζ is expected. Motivated by these premises and the fact that no convincing experimental evidence is known, we carried out measurements of the parameters that govern the CDW repolarization dynamic for voltages near threshold. We found that for most temperatures considered the relaxation time still increases for voltages as small as 1.06Von indicating that the CDW is still in the plastic and presumably in the noncritical limit. However, at one temperature we found that the relaxation time saturates with no indication of critical behavior, giving a new upper limit to the critical regime, of |V/Von − 1| < 0.06.

Thesis advisor: Stephen D. Wilson / This dissertation investigates the cubic type II superconductor, Ca₃Ir₄Sn₁₃, discovered by Remeika and the coauthors more than 30 years ago. It was originally discovered be to a superconductor and later suggested to host ferromagnetic spin fluctuations, which lead to a peak-like anomaly in thermodynamic and transport measurements. Later detailed x-ray single crystal structural refinement associated the peak-like anomaly in transport and magnetization measurements with a charge density wave phase transition at the same temperature. The potential charge density wave phase transition T* can be suppressed either by pressure or chemical potential through substitution on the Ca and Ir site such that a temperature-pressure/composition phase diagram can be constructed. Upon investigating magnetism in this compound, polarized neutron scattering and μSR data from our group and other researchers did not reveal any magnetic order or magnetic spin fluctuations at the time scale of μSR . However, through the partial substitution of Ir by Rh, we realized a structural quantum critical point at ambient pressure with 30% of Ir substituted by Rh--providing the research community a valuable material's platform for studying the interplay between 3D charge density wave order and superconductivity. On the other hand, our surprising discovery of the weak HHL (L=odd) type of super-lattice peaks from neutron scattering led us to a tentative model of a distorted Ca sublattice in this material. The similarity of the lattice instabilites of the Remeika compound and A15 superconductors are discussed, which may give us more insight into its role in the formation of the superconducting phase. / Thesis (PhD) — Boston College, 2015. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

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Recent studies of the transition metal dichalcogenide niobium diselenide have led to debate in the scientific community regarding the mechanism of the charge density wave (CDW) instability in this material. Moreover, whether or not CDW boosts or competes with superconductivity (SC) is still unknown, as there are experimental measurements which supports both scenarios. Motivated by these measurements we study the interplay of charge density modulations and superconductivity in the context of the Bogoliubov de-Gennes (BdG) equations formulated on a tight-binding lattice. As the BdG equations require large numerical demand, software which utilizes parallel algorithms have been developed to solve these equations directly and numerically. Calculations were performed on a large-scale Beowulf-class PC cluster at the University of Saskatchewan.<p> We first study the effects of inhomogeneity on nanoscale superconductors due to the presence of surfaces or a single impurity deposited in the sample. It is illustrated that CDW can coexist with SC in a finite-size s-wave superconductor. Our calculations show that a weak impurity potential can lead to significant suppression of the superconducting order parameter, more so than a strong impurity. In particular, in a nanoscale d-wave superconductor with strong electron-phonon coupling, the scattering by a weakly attractive impurity can nearly kill superconductivity over the entire sample.<p> Calculations for periodic systems also show that CDW can coexist with s-wave superconductivity. In order to identify the cause of the CDW instability, the BdG equations have been generalized to include the next-nearest neighbour hopping integral. It is shown that the CDW state is strongly affected by the magnitude of the next-nearest neighbour hopping, while superconductivity is not. The difference between the CDW and SC states is a result of the anomalous, or off-diagonal, coupling between particle and hole components of quasiparticle excitations. The Fermi surface is changed as next-nearest neighbour hopping is varied; in particular, the perfect nesting and coincidence of the nesting vectors and the vectors connecting van Hove singularities (vHs) for zero next-nearest neighbor hopping is destroyed, and vHs move away from the Fermi energy. It is found that within our one-band tight-binding model with isotropic s-wave superconductivity, CDW and SC can coexist only for vanishing nearest neighbor hopping and for non-zero hopping, the homogeneous SC state always has the lowest ground-state energy. Furthermore, we find in our model that as the magnitude of the next-nearest neighbor hopping parameter increases, the main cause of the divergence in the dielectric response accompanying the CDW transition changes from nesting to the vHs mechanism proposed by Rice and Scott. It is still an open question as to the origin of CDW and its interplay with SC in multiple-band, anisotropic superconductors such as niobium diselenide, for which fundamental theory is lacking. The work presented in this thesis demonstrates the possible coexistence of charge density waves and superconductivity, and provides insight into the mechanism of electronic instability causing charge density waves.

Recent studies of the transition metal dichalcogenide niobium diselenide have led to debate in the scientific community regarding the mechanism of the charge density wave (CDW) instability in this material. Moreover, whether or not CDW boosts or competes with superconductivity (SC) is still unknown, as there are experimental measurements which supports both scenarios. Motivated by these measurements we study the interplay of charge density modulations and superconductivity in the context of the Bogoliubov de-Gennes (BdG) equations formulated on a tight-binding lattice. As the BdG equations require large numerical demand, software which utilizes parallel algorithms have been developed to solve these equations directly and numerically. Calculations were performed on a large-scale Beowulf-class PC cluster at the University of Saskatchewan.<p> We first study the effects of inhomogeneity on nanoscale superconductors due to the presence of surfaces or a single impurity deposited in the sample. It is illustrated that CDW can coexist with SC in a finite-size s-wave superconductor. Our calculations show that a weak impurity potential can lead to significant suppression of the superconducting order parameter, more so than a strong impurity. In particular, in a nanoscale d-wave superconductor with strong electron-phonon coupling, the scattering by a weakly attractive impurity can nearly kill superconductivity over the entire sample.<p> Calculations for periodic systems also show that CDW can coexist with s-wave superconductivity. In order to identify the cause of the CDW instability, the BdG equations have been generalized to include the next-nearest neighbour hopping integral. It is shown that the CDW state is strongly affected by the magnitude of the next-nearest neighbour hopping, while superconductivity is not. The difference between the CDW and SC states is a result of the anomalous, or off-diagonal, coupling between particle and hole components of quasiparticle excitations. The Fermi surface is changed as next-nearest neighbour hopping is varied; in particular, the perfect nesting and coincidence of the nesting vectors and the vectors connecting van Hove singularities (vHs) for zero next-nearest neighbor hopping is destroyed, and vHs move away from the Fermi energy. It is found that within our one-band tight-binding model with isotropic s-wave superconductivity, CDW and SC can coexist only for vanishing nearest neighbor hopping and for non-zero hopping, the homogeneous SC state always has the lowest ground-state energy. Furthermore, we find in our model that as the magnitude of the next-nearest neighbor hopping parameter increases, the main cause of the divergence in the dielectric response accompanying the CDW transition changes from nesting to the vHs mechanism proposed by Rice and Scott. It is still an open question as to the origin of CDW and its interplay with SC in multiple-band, anisotropic superconductors such as niobium diselenide, for which fundamental theory is lacking. The work presented in this thesis demonstrates the possible coexistence of charge density waves and superconductivity, and provides insight into the mechanism of electronic instability causing charge density waves.

Bi2Rh3Se2 have been known as a charge-density-wave (CDW) superconductor, where the superconducting critical temperature Tc and the CDW phase transition are about 0.7 K and 250 K, respectively. Since there has no definite proof that the anomaly at around 250 K comes from charge-density-wave, we wished to provide another evidence to study whether the superconductor had the properties of CDW by electric resistivity measurements applied different pressures. Bi2Rh3Se2 was prepared by using the solid state reaction method and heating in the quartz tube. After the sample was synthesized, the quality was identified by XRD, MPMS, and specific heat probe. With the confirmation of the above-mentioned measurements, we can determine the sample¡¦s quality is good. Furthermore, temperature-dependent resistivity (2-340 K) under pressure (up to 22.23 kbar) on the ternary superconductor Bi2Rh3Se2 are performed to study the possible coexistence of CDW and superconductivity. Interestingly, the resistive anomaly occurred at Ts~250 K, is shifted to higher temperature with increasing pressure. This experimental finding is not consistent with a traditional CDW transition. Moreover, the temperature-dependent Transmission Electron Microscopy (TEM) electron diffraction is evident a structural phase transition from space group ¡§C1 2/m 1¡¨ (Ts > 250 K) to ¡§P1 2/m 1¡¨ (Ts < 250 K). Finally, We do the Co doping to make sure the effects of chemical pressure on this phase transition. The results are opposite to imposed by physical pressure that the transition is shift to lower temperature with more Co inside the sample.

Made available in DSpace on 2014-06-11T19:25:32Z (GMT). No. of bitstreams: 0 Previous issue date: 2003-02-18Bitstream added on 2014-06-13T20:27:50Z : No. of bitstreams: 1 souza_vpm_me_rcla.pdf: 535132 bytes, checksum: d3bce6e40a2c0ef0a0bc8cc227fe99dd (MD5) / Neste trabalho, apresentamos um estudo experimental das propriedades elétricas não lineares de polímeros conjugados, como por exemplo, curvas I(V) não lineares para baixos valores de campo elétrico aplicado e existência de um campo elétrico de threshold. Investigamos o comportamento elétrico não linear em pastilhas prensadas de poli(3-metiltiofeno) (P3MT) oxidadas, obtidas através do processo de síntese eletroquímica. Das medidas elétricas realizadas, na faixa de temperatura (~9 K - ~297 K), os resultados experimentais obtidos: curvas I(V), condutividade versus freqüência e constante dielétrica versus freqüência, foram comparados com os diversos modelos teóricos existentes na literatura. Os dados obtidos em toda a faixa de temperatura mencionada mostraram boa concordância com a teoria de tunelamento para CDW (charge density wave deppining) proposta por J. Bardeen. / In this work, we present an experimental study of the non-linear electrical properties in conjugated polymers, as for example, non-linear I(V) curves at low electric fields and existence of a threshold electric field. We investigated the non linear electrical behavior of pressed pellets of poly(3-methilthiopene) (P3MT), which were obtained through electrochemical synthesis oxidized. From the electrical measurements, in the temperature range (~9K - ~297K), the experimental results: I(V) curves, conductivity versus frequency and dielectric constant versus frequency, were compared with the several theoretical models discussed in the literature. The data obtained in the whole temperature range have shown good agreement on the tunneling theory for CDW systems (charge density wave depinning) proposed by J. Bardeen.