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.

A searched for narrow-band-noise (NBN) modulations of the infrared transmission in blue bronze has been performed. No modulations were observed, giving an upper limits for NBN changes in the absorption coefficient of )2000/(/3.0.andlt;.cmNBN. The implication of these results on proposed CDW properties and NBN mechanisms are discussed. An infrared microscope with a capability of doing both reflectance and transmission measurements has been integrated into the previous electro-transmission system with tunable diode lasers. Electro-optic experiments were done using the microscope for the studies of the CDW states of K0.3MoO3 (blue bronze) and orthorhombic TaS3. The electro-reflectance signal for blue bronze has been evidenced for the first time. The infrared reflectance of K0.3MoO3 varied with position when a voltage greater than the CDW depinning threshold is applied. The spatial dependence of .R/R was slightly different than for ./, in that the magnitude of .R/R decreased and, for low voltages and frequencies, the signal became inverted near the contacts. Perhaps the differences might be associated with changes in the CDW properties on the surface. For blue bronze, the electro-reflectance signal was measured to be smaller than electro-transmittance signal by one order of magnitude for light polarized transverse to the chain direction, while the electro-reflectance signal for parallel polarized light was found to be a few times smaller than for transverse polarized light. The fits of the electro-reflectance spectrum showed that the changes in background dielectric constant were ~ 0.05 % and/or oscillator strength and/or frequency shifts of the phonons were ~ 0.05 % and ~ 0.005 cm-1 in the applied electric field. We also found that parallel polarized phonons are affected by CDW strain, and these changes dominate the electro-reflectance spectrum. We have examined the electro-reflectance spectra associated with CDW current investigation for light polarized parallel to the conducting chains for signs of expected current-induced intragap states, and conclude that the density of any such states is at most a few times less than expected. We have observed a large (~1%) change in infrared reflectance of orthorhombic TaS3, when its CDW is depinned. The change is concentrated near one current contact. Assuming that the change in reflectance is proportional to the degree of CDW polarization, we have studied the dynamics of CDW repolarization through position dependent measurements of the variation of the electro-reflectance with the frequency of square wave voltages applied to the sample, and have found that the response could be characterized as a damped harmonic oscillator with a distribution of relaxation (i.e. damping) times. The average relaxation time, which increases away from the contacts, varies with applied voltage as with p ~ 3/2, but the distribution of times broadens as the voltage approaches the depinning threshold. Very low resonant frequencies (~ 1 kHz) indicate a surprisingly large amount of inertia, which is observable in the time dependence of the change in reflectance as a polarity dependent delay of ~ 100 s.

Electrospinning is a method of producing nano structured material from a polymer solution or melt using high strength electric fields. It is a process that has yet to find extensive industrial application yet shows promise if obstacles such as low rate of production overcome perhaps by more complete theoretical modelling. This work examines the effects of adding an ionic salt to a solution of poly(vinyl alcohol) in water. The direct effect was an increase the charge density and electric current. It was found that an increase in charge density decreases the mass deposition rate and forms a thinner initial jet. When the sign of the charge on the polymer solution was changed from positive to negative the charge density increased and the initial jet diameter and mass deposition rate also decreased. It was proposed that a smaller radius of curvature is formed by the Taylor cone at higher charge densities resulting in a smaller “virtual orifice”. The extent of the bending instability was explored and it was found that adding ionic salt results in a decrease in the bending instability resulting in thicker fibres. Changing the sign of the charge on the polymer solution from positive to negative resulted in an increase in the bending instability and resulted in thinner fibres. The charge transfer mechanisms used in different electrospinning models are explored and some assumptions not explicitly stated are discussed. From this discussion a generalized equation describing the charge transport mechanisms is proposed.

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.

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