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This report describes the development of a servo-controlled superconducting transition edge bolometer (STEB) intended for use as a sensitive broadband far infra-red (FIR), i. e. =1 mm to 100 μm wavelength, detector. A STEB uses a constant current biased superconducting film held on its superconducting transition, where it has a very high temperature coefficient of resistance, as a temperature sensing element for the absorbing substrate. Radiation absorbed by the substrate can be detected by measuring the change in voltage across the superconductor. The STEB's described in this thesis have in addition a resistive heater mounted on the substrate which is used as part of a servoregulating system maintaining the temperature of the bolometer at a fixed point on the superconducting transition. The potential advantages of this arrangement are higher frequency response, greater dynamic range, improved linearity and a STEB which can be more easily held on the superconducting transition. The STEB's described in this thesis used Sn films mounted on sapphire substrates as temperature sensors. Following a general introduction and review of previous work, the principles of operation of the servo system are described together with the theory of the system response, noise and the radiation performance of the detector. A detailed description is given of the construction of the bolometer, the bolometer optics, cryogenic apparatus and the servo circuitry. Optimisation of-the absorption of radiation by the bolometer substrate is studied using a transmission line analogue model. Experiments establishing the validity of the model are also included. The measured performance of the constructed STEB's is given and compared with theoretical predictions. The measured noise performance was found to be considerably better than predicted. Finally, suggestions are made for future improvements in detector-performance.

Hall probe magnetometry measurements have been used to investigate the magnetic state of various Pb microcrystals grown by electrodeposition, where careful control of deposition parameters has allowed the tuning of their architectures. The internal flux distribution when in the superconducting state is seen to be highly dependent on the size and geometry of the investigated samples. In nanowires single flux vortex states are seen to form, showing typical type II behaviour. As the diameter of the nanowires increases these vortex states are seen to bunch together, behaviour that begins to approach that of a type I superconductor. Measurements of Pb triangles with sloped sides show the formation of giant vortex states typical of type I behaviour. These results also highlight the importance of geometry on surface barriers and the corresponding effect these have on flux distribution. Investigation of an icosahedron shows how symmetry effects can determine the stability of different flux states. Experimental results are in good agreement with Ginzburg-Landau calculations.

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The third part of this thesis is on one hole problem of asymmetric T and T -- Jz model, which we found can be turned into a single body problem and is solvable. We get the spectrum of the model and calculate as a result the mass of the hole. This model may be related to the two species fermion coexisting system which may be realized in the optical lattice. / This thesis project is mainly inspired by the newly discovered superconductor NaxCoO2 · yH 2O. It has a layered structure with an almost separated single band out of the inner band electrons. It becomes the theoretical focus because it is the first layered strongly correlated system that shows superconductivity besides the cuprate superconductors. It may help people to understand the superconductivity in the cuprate system. In this thesis we mainly focus our attention on the strongly correlated models on triangular lattice. These models are expected to provide a theoretical background for the Nax CoO2 · yH2O system. We investigate the magnetic and superconductivity properties of these models on the triangular lattice. By the mean field calculation of different magnetic orders we got a phase diagram of the triangular lattice Hubbard model for different magnetic orders in the Hubbard model in the triangular lattice. To further investigate the superconductivity pairing symmetry of the triangular lattice, we use the standard variational Monte Carlo method to find which kind of pairing symmetry is stabilized in the triangular lattice T -- J -- V model. Our finding is that the extended- f wave pairing is most stable in the lattice. / To further understand the properties of the related model in the electron interacting system in the triangular lattice, we also solve the few particle problem in the triangular lattice. For two particles with the T -- U -- J -- V model in the triangular lattice, we got the exact solutions. For four particles of the Hubbard model in the triangular lattice we got the asymptotic solutions. We discuss the existing of the bound states in these solutions and the pairing symmetry of these bound states. For system with two-hole and one down-spin away from the all up spin background we also get a solution utilizing a large U expansion. We discussed the relation of this solution with the Nagaoka states. / Fan, Rui = 三角格子上磁性與超導的研究 / 樊睿. / "September 2007." / Adviser: Lin Hai Qing. / Source: Dissertation Abstracts International, Volume: 70-03, Section: B, page: 1719. / Thesis (Ph.D.)--Chinese University of Hong Kong, 2007. / Includes bibliographical references (p. 83-87). / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Electronic reproduction. [Ann Arbor, MI] : ProQuest Information and Learning, [200-] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstracts in English and Chinese. / School code: 1307. / Fan, Rui = San jiao ge zi shang ci xing yu chao dao de yan jiu / Fan Rui.

Orientador: Rafael Zadorosny / Banca: Eudes Borges de Araujo / Banca: Fernando Rogerio de Paula / Banca: Daniel Henrique Nogueira Dias / Banca: Rubens de Andrade Junior / Resumo: Um dos fatores ainda pouco tratados na literatura a respeito do sistema supercondutor BSCCO é o processo de dopagem e/ou substituição química da fase 2223. Isto se deve às dificuldades de síntese dessa fase. Neste ínterim, no presente trabalho foram feitas análises da influência do tratamento térmico nas propriedades estruturais, elétricas e morfológicas de amostras sem substituição química (BPSCCO) e com substituições de La nos sítios de Sr e de Gd nos sítios de Sr e Ca. Estas foram preparadas de acordo com as composições nominais Bi 1,60 Pb 0,40 Sr 2,00- x La x Ca 2,00 Cu 3,00 O 10+δ, x Gd x Ca 2,00 Cu 3,00 O 10+δ, x Gd x Cu 3,00 O 10+δ, para x=0, 0,25, 0,50, 0,75 e 1,00 mol/g; Bi 1,60 Pb 0,40 Sr 2,00- para x= 0,25 e 0,50 mol/g e Bi 1,60 Pb 0,40 Sr 2,00 Ca 2,00- para x= 0,25 e 0,50 mol/g. Foram efetuadas, ainda, análises térmicas dos exemplares produzidos e a influência dos vários elementos substituintes na obtenção da fase 2223 foi investigada. Os resultados indicaram que as substituições realizadas provocam a degradação das propriedades de supercondução tais como a diminuição da T c e J c . Além disso verificou-se o favorecimento da formação da fase 2212 e de fases secundárias quando comparadas à 2223 nas amostras com substituição química. Por outro lado, entre tais amostras, as que apresentaram melhoras nas propriedades estruturais, elétricas e morfológicas (aumento de T c e melhor conectividade entre os grãos) foram aquelas tratadas em maiores temperaturas. Isso indica que amostras com substituição química podem apresentar boas características supercondutoras se receberem tratamentos térmicos adequados / Abstract: One of the issues that is not well studied in the literature is the doping process and/or the chemical substitution in BSCCO-2223. The reasons for that is related to the difficulties to synthesize such phase. Thus, in the present study we carried out analysis of the influence of the heat treatment in samples with and without (BPSCCO) chemical substitutions. The elements used in the substitutions were La in Sr sites and Gd in Sr and Ca sites. The analyzes were focused on the structural, electrical and morphological properties of the materials. The samples were prepared according to the nominal composition Bi 1.60 Pb 0.40 Sr 2.00-x La x Ca 2.00 Cu 3.00 O 10+δ, with x = 0, 0.25, 0.50, 0.75 and 1.00 mol/g, Bi 1.60 Pb 0.40 Sr 2.00-x Gd x Ca 2.00 Cu 3.00 O 10+δ with x = 0.25 and 0.50 mol/g and Bi 1.60 Pb 0.40 Sr 2.00 Ca 2.00-x Gd x Cu 3.00 O 10+δ, with x = 0.25 and 0.50 mol/g. We also carried out thermal analyzes of the samples, for which the influence of the various substitutions in the obtainment of the 2223 phase were studied. The results indicated that the substitutions allow the deterioration of the superconducting properties, such as a decrease in the T c and J c, as well as the formation of 2212 and secondary phases when compared to the 2223 phase. On the other hand, processes with different heat treatments lead to improvements in the structural, electrical and morphological properties (increase of T c and better connectivity between the grains). This indicates that samples with chemical substitution can have good superconducting characteristics if they receive the adequate heat treatment / Doutor

archives@tulane.edu / The discovery of topological semimetals has attracted enormous interest since they not only possess many unusual exotic properties, but also offer a fertile ground for searching for new fermions in the low energy spectrum. The first established example of a topological state of matter is the quantum Hall effect, which supports a gapless edge state protected by topological invariance. Later the concept of topology has been extended to describe electronic band structure of solid state materials and this effort leads to discoveries of many new topological quantum states, such as Dirac cone state in graphene, quantum spin Hall insulator states in semiconductor quantum wells, 3D topological insulators, etc. The recently discovered Dirac/Weyl semimetals can be viewed as a 3D analog of graphene. This thesis work aims to discover new Dirac/Weyl semimetals through single crystal synthesis and characterization. This thesis is organized as follows: In chapter 1, I will first briefly review several basic concepts of topological properties and introduce a few prototype topological semimetals related to my thesis work. Since one important part of my thesis work involves single crystal growth of topological semimetals, I will introduce the crystal growth methods used in my research in chapter 2. In chapters 3, 4 and 5, I will present my experimental discoveries of new topological semimetals, including YSn2, CaSn3 and TbPtBi. I will not only show property characterization of these material, but also discuss their underlying physics. For YSn2, my work reveals that its slightly distorted square lattice of Sn generates multiple topologically non-trivial bands, one of which likely hosts nodal line and tunable Weyl semimetal state induced by the Rashba spin-orbit coupling (SOC) and proper external magnetic field. The quasiparticles described as relativistic fermions from these bands are manifested by nearly zero mass, and non-trivial Berry phases probed in de Haas–van Alphen (dHvA) oscillations. The dHvA study also reveals YSn2 has a complex Fermi surface (FS), consisting of several 3D and one 2D pocket. Our first principle calculations show the point-like 3D pocket at Y point on the Brillouin zone boundary hosts the possible Weyl state. Our findings establish YSn2 as a new interesting platform for observing novel topological phases and studying their underlying physics. In the study of CaSn3, we not only found it possesses non-trivial band topology, but also discovered its intrinsic superconductivity at 1.178 K. Its topological fermion properties, including the nearly zero quasi-particle mass and the non-trivial Berry phase accumulated in cyclotron motions, were revealed from the dHvA quantum oscillation studies of this material. Our findings make CaSn3 a promising candidate for exploring new exotic states arising from the interplay between non-trivial band topology and superconductivity, e.g., topological superconductivity. For the Half-Heusler compound TbPtBi, we have studied its field-induced Weyl semimetal state. We have observed remarkable transport signatures of its Weyl state, including the chiral anomaly, intrinsic anomalous Hall effect (AHE), and in-plane Hall effect. Moreover, we found TbPtBi exhibits a much larger AHE than the previously reported field-induced Weyl semimetal state in GdPtBi. The distinct aspect of TbPtBi is that Tb ions carry greater magnetic moments than Gd ions in GdPtBi (9.0B/Tb vs.7.0B/Gd). We find that such a moment increase in TbPtBi drastically enhances its AHE, with its anomalous Hall angle reaching as large as 0.50-0.76 in its antiferromagnetic (AFM) state. This finding not only strongly supports that the Zeeman effect due to the large exchange field from 4f electrons plays a critical role in creating the field-included Weyl state, but also provides clear evidence for the theoretical prediction that the intrinsic anomalous Hall conductivity is proportional to the separation of the Weyl points with opposite chirality. / 1 / Yanglin Zhu

Bulk and thin film samples of Bi₂₋[subscript x]Pb[subscript x]Sr₂Ca₂Cu₃O[subscript y] compounds were prepared by suitable methods. The thin film and bulk samples were exposed to argon gas at temperatures of about 2000 °C and pressures of about 300 atm. in a ballistic compressor (BC) and then cooled at a rate of about 10⁵ °C/sec. The samples before and after this treatment were examined and compared using a transmission electron microscope (TEM), a scanning electron microscope (SEM), an energy dispersive spectrometer (EDS) , and an x-ray diffractometer (XRD). Resistance and magnetic susceptibility measurements were made to determine changes in superconducting temperature Tc. Resistance and AC susceptibility measurements of bulk and thin film samples demonstrated that the Tc increased 3- 6 K after the BC treatment, and step-shaped curves were often observed in the resistance versus temperature graphs. The x-ray diffraction study revealed that the relative intensities of lines of the two high Tc phases clearly increased, and the relative intensities of lines of the low Tc phase decreased after the BC treatment. Scanning electron micrographs of the exposed samples showed that surface melting had occurred, and the BC treatment affected the surface of samples to a depth of about 10 μm. Energy dispersive spectral analysis showed that oxygen loss occurred in the exposure process. Transmission electron diffraction patterns showed that the crystal structure of the Bi-based compounds have an incommensurate modulation along the b-axis with different periods of 25.4 Å, 38.7 Å and 72.6 Å. After the BC treatment, the lattice parameters of superconductors did not change, but the relative intensities of spots changed, the modulation of 72.6 Å disappeared, and the density of twist boundaries increased. These observations suggest that the treatment changed the density of structural defects and the atomic arrangement. These studies tentatively indicate that the increase in structural defects was caused by oxygen loss. A very small oxygen loss increases the fraction of copper in CU‴ oxidation state, and thus increases the density of hole carriers in the Cu-O planes. This may be the reason why the Tc of Bi-based cuprate superconductors increased after the BC treatment.