[en] In this dissertation I discuss a variety of anisotropic or quasi-one dimensional materials. NbSe³, TTT²I³, and (SN)_x. The possible ground states, superconductivity or charge density waves, of such materials are discussed. In the case of NbSe³ and TTT²I³ the roles of impurities and/or disorder in stabilizing a given ground state has been studied. The ground states were probed via a variety of transport measurements: resistivity and thermopower among others

[en] The critical field, H/sub c₂/ of Eu ₁₂Mo₆S₈, has been measured as a function of pressure and temperature using pressures up to 18 kbar, temperatures from 1.5 K to T/sub c/(approx. 10K) and magnetic fields up to 17 T. An extremely anomolous dependence of H/sub c₂/ on pressure from 13 kbar to 18 kbar has been found. A strongly pressure dependent RKKY interaction producing a compensation field at the Mo site is believed responsible for this unusual behavior

[en] The theory of the interplay between superconductivity and itinerant ferromagnetism based on the single conduction band model in the mean field approximation is summarized. This theory predicts that superconductivity and itinerant ferromagnetism cannot co-exist, that the higher temperature magnetic phase can transform into the low temperature superconducting state, and that a superconducting state will not transform into a ferromagnetic state at a temperature below the superconducting transition. For the magnetic superconductor Y₉Co₇ we have reviewed the experimental results and in particular, we have shown that pressure suppresses magnetism resulting in a higher superconducting transition temperature and have concluded that Y₉Co₇ is an itinerant magnet. Pressure also sharpens the superconducting transition and increases the upper critical field, signifying that the ferromagnetic correlations and superconducting fluctuations co-exist but very spatially. For pressures greater than 6 kbar, the magnetoresistance is always positive, further indicating the suppression of magnetism by high pressure. Several microscopic experiments and some improvement in theory are suggested

[en] We have investigated the 9- and 10-GHz microwave properties of bulk samples of strontium and barium substituted La₂CuO₄ and YBa₂Cu₃O₇ and one YBa₂Cu₃O₇ MBE film on SrTiO₃. Anomalous microwave absorption is seen especially for poorly compacted bulk ceramic samples. Transitions were generally broadened near T/sub c/ and the residual losses were high. The real part of the surface impedance could be fit below T/sub c/ to the empirical equation with an added term linear in temperature. The microwave response of the film was masked by paraelectric absorption and acoustic resonances in the substrate

[en] We report measurements performed on a granular YBa_2.1Cu_3.4O_7-x film which are consistent with a transition to the zero-resistance state that can be described as a two-dimensional transition in terms of the Kosterlitz-Thouless formalism. A photoresponse associated with this transition is interpreted in terms of a photon-assisted vortex-antivortex unbinding process

[en] We have measured some of the superconducting properties of (SNBr/sub 0.4/)/sub x/. The critical temperature is found to be enhanced over that of (SN)/sub x/ by approximately 20%. The transition region is greatly narrowed. The perpendicular upper critical field is linear with temperature over a wide domain, attaining a value of about 400 Oe at absolute zero, and totally lacking the upward curvature shown by (SN)/sub x/. The parallel field is not greatly changed. On the basis of these and other measurements, it is shown that (SNBr/sub 0.4/)/sub x/ is a ''three-dimensional'' superconductor due to improved coupling between the (SN)/sub x/ fibers. The parallel field appears to be paramagnetically limited. We believe that spin-orbit scattering is significant, despite the small atomic numbers involved, because of severe disorder and boundary effects

[en] Preparation and characterization were conducted on high T/sub c/ single phase samples of La/sub 2-x/M/sub x/CuO₄ (where M=Sr, Ba, and Ca and x = 0.1, 0.2 or 0.3) and Ba₂YCu₃O₇. Transmission electron microscopy of the Ba₂YCu₃O₇ samples reveals twinning defects in the [110] direction which disappear when the sample is heated above 400⁰C. The dc susceptibility, magnetization, and magnetoresistance are reported. Many of the critical superconducting and normal state parameters have been extracted from the data

[en] The resistivity of NbSe₃ doped with 5% Ta has been measured from room temperature down to 0.5 K and the results compared with similar measurements on pure NbSe₃. The pure sample remains normal to the lowest temperature (0.5 K), whereas the doped sample has a sharp transition to the superconducting state with Tsub(c) = 1.5 +- 0.2 K. Measurements of the critical magnetic field indicate that the Ta doped samples are homogeneous, anisotropic three dimensional superconductors. (author)

[en] We have measured the resistance and critical fields of a series of alloys of Ti and Ta substituted for Nb in NbSe₃. For concentrations of 5 at. % Ta and 0.1 at. % Ti we find that the charge-density-wave transitions are greatly smeared or suppressed and that the alloys become bulk anisotropic superconductors. Our results are discussed in terms of the competition between the charge-density waves and superconductivity for states at the Fermi surface

[en] A new Mo--N phase has been prepared by rf reactive sputtering. The structure, as determined by x-ray analyses matches well with the hexagonal Mo²C structure. The films were produced by reactively sputtering Mo in a gaseous N² atmosphere in an attempt to prepare thin films of the predicted high T_c superconductor MoN in the B1 phase. By further nitriding these Mo²N films with NH³ at 780 ₀C for 24 h, they appeared more stoichiometric as determined by their superconducting properties. These films are superconducting T_c roughly-equal7 K and have high critical magnetic fields H_c2(0) roughly-equal7.5 T. Varying both the temperature of the substrate between 500 and 900 ₀C, and the power density did not effect the phase of the Mo--N compound