La dynamique de spins dans les supraconducteurs à haute température critique est discutée à la lumière de la diffusion inélastique de neutrons. Ces mesures montrent l'existence de fortes fluctuations magnétiques maximales autour du vecteur d'onde antiferromagnétique, $(\pi/a,\pi/a)$. tant dans l'état supraconducteur que dans l'état normal. Le rôle de ces fluctuations magnétiques est nécessairement important pour le mécanisme de la supraconductivité à haute température critique. L'état supraconducteur est en particulier caractérisé par une excitation collective magnétique, "pic de résonance", dont l'énergie est proportionnelle à la température critique supraconductrice.

Spin and charge density fluctuations are important excitations in the strongly correlated systems, especially in the recently discovered high temperature superconductors. Several different theories on high temperature superconductors have been proposed based on spin fluctuations. However, experiments have also shown the existence of strong charge fluctuations. It is, therefore, desirable to investigate the physical consequences of the coexistence of strong spin and charge density fluctuations. As a first step toward a full understanding of both spin and charge excitations, a self-consistent theory is established. In this self-consistent theory, there are three important quantities, the spin susceptibility, the charge susceptibility, and the phonon Green's function. These three quantities are coupled together by the electron-phonon and phonon-spin fluctuation interactions. The phonon-spin fluctuation interaction is derived by making use of the spin-orbital coupling. For a strongly correlated system, the spin and charge density excitations have to be considered self-consistently. They are intimately related. The effects of antiparamagnons on phonons are also investigated. Antiparamagnons can have dramatic effects on phononic properties. It is found that new modes are formed in the presence of antiferromagnetic spin fluctuations. The de Haas-van Alphen effect in marginal and nearly antiferromagnetic Fermi liquids is studied. It is found that the de Haas-van Alphen frequency is unaffected by the anomalous response functions of the marginal and nearly antiferromagnetic Fermi liquids due to the absence of real parts of self-energies on the imaginary frequency axis. / Graduation date: 1993

First principles calculations provide a powerful tool for sorting out the interplay of chemical composition and structure with the physical properties of materials. In this dissertation, I discuss the physical properties and their microscopic basis within this framework for following illustrative examples. (i) The Zintl phase hydrides, where I find H is anionic and the formation of covalent sp2 bonds in the Al/Ga/Al-Si planes, which is a highly unusual bonding configuration for these elements. (ii) PbTe, which shows strong coupling between the longitudinal acoustic and transverse optic modes that may explain its low thermal conductivity. (iii) The double perovskites BiPbZnNbO6 and BiSrZnNbO6, where introducing size disorder at A-site prevents the BO6 octahedra from tiling and enhances the polar behavior. (iv) FeSe, which shares the salient electronic and magnetic features of other Fe superconductors and cannot be described as a conventional electron phonon superconductor. (v) NbFe2, which is near a magnetic quantum critical point and shows strong competition between various magnetic orderings that may explain its unusual non-Fermi liquid behavior at very low temperatures. (vi) The nickel analogues of Fe superconductors LaNiPO and BaNi2As2, where I show that superconductivity is of conventional electron-phonon type in contrast to the Fe-based superconductors. (vii) Noncentrosymmetric LaNiC2, which I find is a conventional electron-phonon superconductor with intermediate coupling.

L'ancrage des vortex est utilisé comme une sonde pour l'identification du type de désordre et son effet sur la supraconductivité dans la famille 122 des supraconducteurs à base de fer. Une nouvelle technique d'analyse obtenue d'images de décoration de Bitter prenant en compte l'interaction de chaque vortex avec ses voisins, a permis d'obtenir l'énergie et la force de piégeage dans Ba(Fe1-xCox)2As2, dans le régime de bas champ magnétique. La corrélation avec des mesures de courant critique Jc a montré que le piégeage des vortex dans ce composé est due à l'hétérogénéité des propriétés supraconductrices sur une échelle de 20-100 nm. Application de la même méthode d'analyse pour les vortex dans le BaFe2(As1-xPx)2 a montré que l'énergie et la force d'ancrage dépendent du dopage x. Les mesures de Jc et de la distribution des forces de piégeage ont montré que la distance moyenne entre différents centres de piégeage est de l'ordre de 90 nm et que cette distance augmente quand on augmente le conteneur en P. La combinaison de ces résultats avec les mesures de Jc mène à la conclusion que l'ancrage fort des lignes de flux due à l'hétérogénéité des propriétés supraconductrice à l'échelle de nm est à l'origine de la constante observé à des champ faibles dans les courbes de Jc ainsi que la diminution en loi de puissance qui suit. On traite également la contribution d'ancrage faible collectif à Jc, qui se manifeste à des champs magnétiques plus importants, de l'ordre de 1 T. Cette contribution a été analysée en terme de la diffusion des quasiparticules et de la fluctuation spatiale du libre parcours moyen. Afin de tester l'hypothèse avancé ci-dessus, l'irradiation aux électrons d'énergie 2.5 MeV, sur les composés dopé au Co, Ni et P de la famille 122 a été réalisé à des différentes doses pour plusieurs dopage de ces matériaux. Ce type d'irradiation introduit des défauts ponctuels de taille atomique dans le matériau. La température critique Tc de tous les matériaux étudiés diminue après irradiation de façon similaire. Une claire augmentation de la contribution d'ancrage faible collectif à Jc dans le composé dopé au Co a été observée. De plus, cette contribution qui, avant irradiation, était absente dans tous les dopages du composé au P, apparait après irradiation. Les défauts ponctuels de taille atomique, diffuseur des quasiparticules, dans les supraconducteurs à base de fer sont donc à l'origine de la contribution d'ancrage faible collectif à Jc .

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PrOs₄Sb₁₂ is the first known heavy fermion superconductor containing Pr. Many experiments show results indicating unconventional superconductivity in this system. Most notably, PrOs₄Sb₁₂ exhibits two superconducting transitions. The upper transition occurs at Tc₁ ≈ 1.89 K and the lower transition occurs at Tc₂ ≈ 1.72 K. There are many features of this system that are not understood and do not have a unifi ed consensus among researchers. Many issues remain unanswered, such as the origin of superconductivity, symmetry of the superconducting energy gap, and the nature of the two superconducting transitions. We present specific heat data for a single PrOs₄Sb₁₂ crystal. The data show two superconducting transitions. Variance in the lower superconducting transition (among various samples) suggests multiple superconducting phases. Power law dependence of the low temperature specific heat is indicative of asymmetric energy gap. Cuprates are unconventional high temperature superconductors. The theory on origin and characteristics of superconductivity in these systems is still a debated issue. Experiments indicate presence of a phase in these systems that occurs above the superconducting transition temperature which exhibits some common characteristics with the superconducting state. An important issue is the nature of this so-called "pseudogap" phase and its relationship to the superconducting state. We have developed an experimental apparatus and procedure for measuring the specific heat of a high temperature superconductor and demonstrated it by measuring the heat capacity of a YBCO high-Tc superconductor sample.

The problem of the evolution from BCS theory with cooperative Cooper pairing to the formation and condensation of composite bosons has attracted considerable attention for the past several decades. It has gained renewed impetus in the mid-eighties with the discovery of the high-Tc superconductors, which have a coherence length comparable to the interparticle spacing. More recently, this subject has spurred a great deal of research activity in connection with experiments involving dilute atomic gases of fermionic atoms. The initial objective of this work will be to use functional integral techniques to analyze the low-temperature BCS-to-BEC evolution of d-wave superconductors within the saddle point (mean field) approximation for a continuum model. Then, the same mathematical formalism will be applied to the problem of the BCS-to-BEC evolution of fully spin-polarized p-wave Fermi gases in two dimensions. We find that a quantum phase transition occurs for both systems as they are driven from the BCS-like regime of weakly interacting fermionic pairs to the opposite BEC-like regime of strongly interacting bosonic molecules. This is in contrast to the smooth crossover predicted and observed in systems that exhibit s-wave pairing symmetry. We calculate several spectroscopic and thermodynamic properties that signal the occurrence of this phase transition, and suggest some possible experimental realizations. Finally, fluctuations about the saddle point solution are included in the calculations, and the effects of such correction are analyzed in the low (T~0) and high (T~Tc) temperature limits. We conclude that, at high temperatures, the bosonic degrees of freedom that arise from two-particle bound states become essential to describe the strong coupling limit, as the saddle point approximation alone becomes unreliable.

Since the early 1960s, Abrikosov-Gorkov theory has been used to describe superconductors with paramagnetic impurities. Interestingly, the density of states resulting from the theoretical framework has to date only been known approximately, as a numeric solution of a complex polynomial. An analytical solution to the theory was discovered and applied to extract the spin polarization from the tunneling conductance of superconducting aluminium with 3-dimensional (3-D) amorphous (a-) gadoliniumxsilicon1-x (GdxSi1-x) as a counter electrode (Al/Al2O3/a-GdxSi1-x planar tunnel junction measured at T = 25 mK and H less than or equal to 3.0 T) in the quantum critical regime (QCR). The analytical solution is valid in the whole regime of Abrikosov-Gorkov theory independent of the presence of an energy gap. Applying the spin polarized Abrikosov-Gorkov theory to describe aluminium gives a larger spin polarization in GdxSi1-x than the spin polarized Bardeen-Cooper-Schrieffer (BCS) theory. The purpose of this study is to extract polarization at various applied magnetic fields, but no specific relationship between the two could be determined. Results obtained shows a transition from a superconductor with a gap to a gapless superconductor in varying external magnetic fields was observed. To improve understanding of GdxSi1-x near the metal-insulator transition (MIT) and compare it with prior work, the initial experimental attempts to investigate the transport property of GdxSi1-x near the MIT in the 2-dimensional limit are presented. A low temperature ultra high vacuum quench condensation system was used to make thin films of GdxSi1-x and in-situ measurements were performed. The transport properties for different values of x and thicknesses were measured for T = 4.2 K to ~10 K. In addition to other possible causes, the uncertainty in the electron impact emission spectroscopy (EIES) appeared to be a major reason behind the observed error in x when gadolinium and silicon are co-evaporated. The problems faced during the co-evaporation are also discussed.

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

A superconducting microwave cavity has been designed to test advanced materials for use in the accelerating structures contained within linear colliders. The electromagnetic design of this cavity produces surface magnetic fields on the sample wafer exceeding the critical limit of Niobium. The ability of this cavity to push up to 4 times the critical field provides, for the first time, a short sample method to reproducibly test these thin films to their ultimate limit. In order for this Wafer Test cavity to function appropriately, the large sapphire at the heart of the cavity must have specific inherent qualities. A second cavity was constructed to test these parameters: dielectric constant, loss tangent, and heat capacity. Several tests were performed and consistent values were obtained. The consequences of these measurements were then applied to the Wafer Cavity, and its performance was evaluated for different power inputs. The Q_0 of the cavity could be as low as 10^7 because of the sapphire heating, therefore removing the ability to measure nano-resistances. However, with additional measurements in a less complex environment, such as the Wafer Test Cavity, the Q_0 could be higher than 10^9.