Medimos a temperatura de transição supercondutora, TC , dos supercondutores amorfos La80Ga20, La80Al20 e La75Al15In10, em função da pressão hidrostática (até P~11Kbar). Nesses materiais o comportamento de Tc com pressão é similar ao do lantânio cristalino e aproximadamente independente de composição. / We have measured the superconducting transiction temperature, Tc , of amorphous La80Ga20, La80Al20 and La75Al15In10, as a function of hydrostatic pressure (up to P~11 kbar). In these materials the P-dependence of Tc is similar to that of crystalline La rougly independent of a alloy composition.

Thesis (Ph. D.)--University of California, San Diego, 1981. / Vita. Includes bibliographical references (leaves 87-94).

Thesis (Ph. D.)--University of California, San Diego, 1981. / Vita. Includes bibliographical references (leaves 87-94).

published_or_final_version / Physics / Master / Master of Philosophy

published_or_final_version / Physics / Master / Master of Philosophy

This thesis presents a theoretical analysis of upper critical magnetic fields in quasi-one-dimensional (Q1D), layered superconductors with highly anisotropic electron spectra. It is shown quantitatively how the temperature dependence and spacial orientation of the upper critical magnetic fields, Hc₂(T) can reveal important microscopic properties of such superconductors, including the nature of their pairing symmetry. The results obtained show that highly anisotropic, layered compounds can possess exotic superconducting properties such as: non-analytical angular dependence in the upper critical fields at low temperature, the rare spin-triplet Cooper pairing, and a novel quantum limit reentrant superconducting phase occurring in Q1D compounds under ultra-high magnetic fields. For this purpose, two unconventional superconductors are examined: the highly anisotropic Q1D organic superconductor (DMET)₂I₃, and the layered transition metal oxide superconductor Li₀.₉Mo₆O₁₇. In the first case, an angular dependence of Hc₂ that varies as ϴ³/² is predicted in (DMET)₂I₃ for small angles and low temperatures, in contrast to the well-established (Ginzburg-Landau) quadratic angular dependence near the transition temperature. For Li₀.₉Mo₆O₁₇, spin-triplet pairing is shown to be the most likely scenario, supported by theoretical analysis of the recent experimental data on Hc₂(T) when the field is aligned parallel to the most conducting axis. Furthermore, in Li₀.₉Mo₆O₁₇, a novel quantum limit (QL) superconducting phase is theoretically predicted as a consequence of dimensional crossover in ultra-high magnetic field. If confirmed experimentally, the QL phase would be the first example of existence of superconductivity in magnetic fields greater than 100 Tesla, and in addition would unequivocally confirm spin-triplet Cooper pairing in Li₀.₉Mo₆O₁₇.

No description available.

No description available.

This thesis describes the development of single crystal x-ray scattering applied to the study of charge stripes in some exotic transition-metal oxides. Charge stripes in the cuprates and manganites are strongly associated with high T(_c) superconductivity and colossal magnetoresistance, and can be characterised by the satellite reflections located around related Bragg peaks below the charge ordering transition temperature ( T(_co). The intensities of these extremely weak satellite reflections are between l0(^-4) and 10(^-6) less than those of Bragg reflections, thus very high brilliant rotating-anode x-ray and synchrotron radiation sources are required to study them. The structure factors and correlation lengths of the charge stripes can be directly obtained by measurements of the integrated intensities and peak widths of the charge ordering satellite reflections as a function of temperature. The charge stripes in single crystals of Bi(_0.24)Ca(_o.76)MnO(_3), Nd(_0.5)Sr(_0.5)MnO(_3) and La(_5/3)Sr(_1/3)NiO(_4) have been comprehensively characterised using both laboratory and synchrotron radiation x-ray scattering. The measurements on Bi(_0.24)Ca(_0.76)MnO(_3) directly demonstrate a strong relationship between the charge stripe ordering and the first-order structural phase transition, which is common in many perovskite manganites. Direct evidence for the existence of perpendicular charge-ordered domains in manganites was obtained for the first time by x-ray scattering in Nd(_0.5)Sr(_0.5)MnO(_3), and this result demonstrates that charge stripes have complex structures in the mesoscopic length scale. Scattering was also observed at the positions in reciprocal space associated with spin ordering into stripes. These showed evidence of different critical behaviour than the corresponding charge stripe satellites. The quasi two-dimensional feature of the charge stripes in La(_5/3)Sr(_1/3)NiO(_4) was successfully demonstrated by the critical exponents of the charge stripe melting and by the measurements of their m-plane and out-of-plane correlation lengths. Experimental evidence for the existence of electronic liquid crystal phases in La(_5/3)Sr(_1/3)NiO(_4) was obtained, in agreement with recent theoretical predictions

Thesis (BSc. (Hons))--Australian National University, 2002. / Available via the Australian National University Library Electronic Pre and Post Print Repository. Title from title screen (viewed Mar. 27, 2003). "A thesis submitted for the degree of Honours in theoretical physics at the Department of Physics and Theoretical Physics, The Faculty of Science, The Australian National University" "Honours thesis submitted November 2002" Includes bibliographical references.