A field-gradient torque magnetometer has been designed and constructed which measures the magnetic moment of isotropic superconductors using a Quantum Design torque chip. The magnetometer fits onto the base of an Oxford Instruments Heliox probe; thus the temperature range is 300 mK to 20 K, in magnetic fields up to 15 T. The superconductors measured on the torque magnetometer in vacuum were NbTi wire, PbMo(_6)S(_8), and Pb(_0.75)Eu(_0.25)Mo(_6)S(_8) as a function of magnetic field, magnetic field- gradient and temperature. The utility of the torque magnetometer was demonstrated by determining the critical current density (J(_c)) and the upper critical field (B(_c2)(0)) of the samples. These superconducting properties were compared with the vibrating sample magnetometer (VSM) values, for the same samples. The change in the superconducting parameters and properties of Pb(_1-x)Eu(_x)Mo(_6)S(_8), for x = 0.0 & 0.25 were investigated. The samples were fabricated using two different heat treatment methods, which finished with hot isostatic pressing (HIP'ing) at 2000 bar and 800 C for 8 hours. To characterise the superconductors, ac resistivity, ac susceptibility, specific heat and magnetic measurements were carried out, as a function of magnetic field and temperature. From the magnetisation data, the GL parameter (k) and the Sommerfeld constant (y) were determined for the magnetic superconductor x = 0.25. For both samples, k was determined in the temperature range 6 K to T(_c), from the magnetisation data. For x = 0.0 & 0.25, the critical temperature (T(_c)), the upper critical field (B(_c2(0)), the critical current density (J(_c)) and the irreversibility fields (B(_irr)) were determined from the experimental data.

Solid solutions of general formula La1.5-xBa1.5+x-yAyCu3Oz (A=Ca, Sr) have been studied. These form over the complete composition range 0x0.5, 0y0.5, excluding compositions centred on the La-123 composition, LaBa2Cu3Oz. The solid solutions have properties which range from semiconducting (x=y=0) to superconducting, with Tc (max.) = 78K for the composition LaBa1.5Ca0.5Cu3O7.01, and were found to be tetragonal across their entire range. Rietveld refinement of X-ray powder data has shown that whereas Sr occupies the Ba site, Ca occupies the La site with concomitant displacement of La onto the Ba site. The magnetic behaviour and oxygen contents of these solid solutions are discussed. The variation of Tc with oxygen content has been studied for the composition LaBa1.5Ca0.5Cu3Oz; for quenched samples, Tc varies linearly with oxygen content, but slow cooled samples show a "Double Plateau" behaviour, and thus Tc depends critically upon the cooling regime. Analogues RE1.5-xBa1.5+x-yCayCu3Oz (RE = Nd, Pr, Sm, Eu, Gd and Dy) have also been studied; the extent of the solid solution decreases with decreasing size of rare earth. For Dy, no appreciable solid solubility was observed. For Nd and Pr, the solid solutions centre on the 336 composition La1.5Ba1.5Cu3Oz; for the smaller rare earths this composition cannot be prepared in air, and the solid solutions are centred on the orthorhombic, REBa2Cu3Oz phase. All were found to be superconducting except the Pr-based solid solutions; results suggest that these contain Pr4+ as well as Pr3+. In the solid solutions RE1+xBa2-xCu3Oz (x0, RE = Sm, Eu and Gd), critical temperatures exhibit a marked dependence on the size of the A-site cations.

Thesis advisor: Ziqiang Wang / The discovery of high-Tc superconductivity in cuprates, quantum Hall effect greatly challenge the single-electron understanding of condensed matter physics. In contrast to phonon-mediated BCS mechanism, the unconventional high-Tc superconductivity is widely believed to come from strongly electronic correlation. Strong electron-electron repulsion leads to the interplay among spin, charge, orbital and lattice degrees of freedom, resulting in high-temperature superconductivity, charge or spin density wave, Mott insulator, orbital order, nematicity etc. On the other hand, quantum Hall effect brings us the realization of the mathematical concept of topology in condensed matter. Topology has been widely explored in the topological insulator, topological superconductors, symmetry protected topological order etc. In this dissertation, we study theoretically the physics of electronic correlation and topology in various systems, including superconductivity in single layer CuO₂, electronic nematicity in FeSe, chiral spin density wave in honeycomb lattice and antiferromagnetic Chern insulator in 2D non-centrosymmetric systems. / Thesis (PhD) — Boston College, 2018. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

by Chao Wai-Kit. / Title also in Chinese. / Thesis (M.Ph.)--Chinese University of Hong Kong, 1989. / Bibliography: leaves [2-4] (second group)

Thesis (B.S.)--Massachusetts Institute of Technology, Dept. of Physics, 1979. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE. / Includes bibliographical references. / by Edward Ames Hildum. / B.S.

Thesis. 1978. B.S.--Massachusetts Institute of Technology. Dept. of Physics. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE. / Includes bibliographical references. / by Philip R. Kesten. / B.S.

A magneto-optical apparatus has been designed to investigate flux and current distributions in various forms of high-temperature superconductor after the application of external magnetic fields, transport currents, and both in sequence. It is concluded that flux motion depends significantly on the superconductor characteristic flux pinning strength and geometry. The flux profiles at the surface of an isolated thin film strip, and thin films in x-array and shunted-x-array arrangements have been studied in detail. It has been found that the theoretical flux profile, characterised by a logarithmic singularity at its edges and a vertical slope at the flux front, become rounded when the sample thickness is taken into account. Experiment has confirmed that the current density reaches its critical value in the flux-penetrated region while there is a continuous non-zero current density in the central flux-free region. Also magnetic interactions exist between the strips. Band-shaped and dome-shaped flux distributions caused by geometrical barrier at different weak pinning strengths were observed in Bi2Sr2CaCu2O8+x single crystals between 20 K and 70 K. The geometrical barrier also governs the transport current leading to a square-shaped distribution near the crystal edges at 20 K. Cracks and defects in Bi-2223/Ag tapes were visualised without destruction of the tape. The inhomogeneous flux distribution indicated that defects, including cracks, in superconducting filaments limit the current-carrying capacity of the tape. In addition, at 70 K a fraction of the transport current has been found to flow in the silver sheath. An investigation of the influence of the twin planes on vortex motion in a YBa2Cu3O7-d single crystal has shown that after a twin barrier is initially overcome, the twin planes, regardless of their directions, act as an easy-flow channel for vortex penetration at low temperatures as the pinning strength in the twin planes is weaker than that in the untwinned region, whereas, they act as a barrier at high temperatures where the pinning strength in the twin planes is stronger. Furthermore, vortex penetration along the twin planes is a factor of four greater than that for the untwinned region at 25 K.

Graduation date: 1998

Recent superconducting thin films studies have attempted to create pure metal layer films of Niobium and Titanium with the same properties of superconducting NbTi wire used in industry. These studies have all reported depression of the superconducting properties of the pure metal films which has been attributed to the proximity effect. The purpose of this research project was to construct several NbTi films composed of alloyed layers to overcome the proximity depression of superconducting properties. These films are unique in that they are the only films with both alloyed Nb/Ti superconducting and normal layers reported in the literature. Films with several different compositions and bilayer geometries were designed, constructed and their superconducting properties characterized. The films were created by the RF sputtering of alloyed targets at ambient temperatures. Characterization of the composition of the films was performed by microprobe analysis at two different electron beam voltages. A simulation of the electron beam excitation volume of the microprobe was performed to determine the difference in the two analyses and to determine whether the substrate would be found upon microprobe examination. X-ray diffraction was utilized to determine the bilayer spacing and to give a qualitative understanding of the alignment of the film microstructures. The critical temperature and upper critical magnetic field were measured to determine the superconducting properties and the extent of the proximity effect in the films. The microprobe analysis found most films to be very pure alloys except for films 6001, 6002, 6003(1) in which there were between 1-8 wt% of impurities. All films contained compositional variations on the order of 10 wt% from the design values. X-ray diffraction indicated agreement with the designed bilayer spacing in all films but 6003(2), 6005(1), and 6005(2) which had bilayer periods larger than originally designed. The proximity effect was not observed in any of the film's superconducting properties examined. T[subscript]c and H[subscript]c��� properties for the films without impurities had properties equal to that of bulk Nb/47wt% Ti. Films 6001, 6003(1) had depressed T[subscript]c and H[subscript]c��� values which were attributed to their impurity contamination. The low Ti composition in many of the films points to the inaccuracy of the deposition parameters when the films were first processed. The lack of T[subscript]c and H[subscript]c��� depression normally seen in other film studies with bilayer periods between 10-30 nm demonstrates that alloyed layers should be used to overcome the proximity effect in multilayer thin film superconductor studies. Overcoming the proximity effect should translate into a better understanding of flux pinning mechanisms in the material and increased superconducting critical currents in these films. / Graduation date: 1999

Graduation date: 1997