VictorJuiz/thesis huggingface dataset:
Thermal conduction in the vortex state of unconventional superconductors
Keywords: Physics
Astronomy

When a magnetic field is applied to a type II superconductor, the field penetrates in the form of vortices wherein the superconducting order parameter vanishes over a length scale of 1 0 0 A or so. Each vortex supports localized electronic states. Above the upper critical field Hc2, the normal state is recovered and the electronic states are fully delocalized. A basic question is to ask how the electronic states evolve from being localized to delocalized as the intervortex distance is reduced (i.e. as the magnetic field is increased). For that purpose, we have performed systematic studies of the electronic thermal conductivity ne/ T through the vortex state at temperatures as low as 50 mK and fields as high as 13 Tesla. This was done in a variety of systems: VsSi, LuNi2 B2 C, NbSe2 and Tl2 Ba2 Cu0 6 +5 . The work performed on the phonon-mediated V3 SL LuNi2 B2C and NbSe2 led to many surprises. V3Si is an archetypal type-II superconductor and displays the expected activated behavior. It provides a solid basis for comparison. However, we found that the borocarbide superconductor LuNi2 B2C has a highly anisotropic gap of unprecedented magnitude (a factor of 1 0 ), while the layered compound NbSe2 is found to display multiband superconductivity. We also studied the overdoped cuprate superconductor Tl2 Ba2 Cu0 6 + < 5 where the upper critical field was low enough for us to reach the normal state. The Wiedemann-Franz law was tested and found to be satisfied with a 1 % experimental accuracy. This represents the first unambiguous evidence that the overdoped side of the phase diagram is a Fermi liquid and displays no sign of spin-charge separation and thus provides a solid basis for a comprehensive theory of the phase diagram of cuprate superconductors. In the superconducting state, we find that Tl2 Ba2CuO 6 + < 5 is the textbook example of a d-wave superconductor as it shows quantitative agreement with theory. In summary we obtained a range of behaviors in several different types of superconductors, from the test case of the conventional V3Si to the highly unconventional cuprates and through highly anisotropic and multi-band superconductors.
A study of the asymmetry in the photoproduction of D meson(*+) and D meson(*-) at photon energies of 40 to 160 GeV
Keywords: Physics

Evidence for an asymmetry in the photoprodurtion of D’+ and D'- at photon energies of 40-160 GeV was found in Fermilab experiment E51G. The experiment, studied high energy yp interactions using the Tagged Photon Spectrometer, consisting of a sophisticated recoil detector to study the target fragments, and a large acceptance forward spectrometer. The charged D* signal was detected in the decay channel D*+ —> D °~ +, with subsequent- decay of the D n via the decay modes D° - A - t t + . D ° - 7 v '- - + - + - - . £>° - A i V - - . or D° Using the unique capabilities of the recoil detectorto accurately indicate dilFracfive processes, the first direct measurement of the asymmetry (in the form of the ratio of anti-charmed to charmed D* mesons) for purely non-diffractive processes was found to be 1.3S ± 0.2S. The data also showed no indication of asymmetric production for purely diffractive events. The asymmetry in the non-diffractive component, of signal was interpreted as evidence for associated production of A* D*“ , which was consistent with a n earlier E516 report for the direct observation of associatively produced A(e)+. Finally, the results were seen as consistent with previously reported photoproduction experiments conducted at different photon energy ranges.
Magnetic ground state of the 2D S=1/2 square antiferromagnet copper formate tetrahydrate
Keywords: Physics

I performed a series of NMR experiments to investigate the magnetic ground state properties of the 2D S = l/2 square antiferromagnet copper formate tetrahydrate. This material contains planes of copper atom s which are linked by formate groups and are arranged in a square lattice. I measured the NMR frequency shifts for the deuterium nuclei in the formate groups as a function of field orientation. At 10 K, the shifts depend on the orientation of the local spins at the Cu sites. The orientation of these spins is governed by the magnetic free energy which contains the anisotropic Heisenberg exchange, the Dzyaloshinskii-Moriya interaction and the anisotropic Zeeman coupling to the external field. I created a T = 0 classical model of the free energy and determined the orientation of the sublattice magnetizations for different strengths of the various interactions. By calculating the resulting NMR shifts and comparing them to the experimental data, I showed th a t both the Dzyaloshinskii-Moriya interaction and the anisotropic Zeeman coupling need to be included in the free energy in order to model the data successfully. This point has not been appreciated in the literature to date. Furthermore, I determined the various parameters needed to describe the magnetic behavior of copper formate tetrahydrate. The Heisenberg exchange coupling J is 4.0 ± 0 .4 meV or 4 7 ± 4 K and the anisotropies in this were found to be negligible. The average moment of the antiferromagnetically ordered spins is 0.571±0.004 (j,b and the two Dzyaloshinskii-Moriya vectors are D + = (—0.03 ± 0 .01,0.0 ,-0.18 ± 0.02) meV and D “ = (0.0,0.00 ± 0.06,0.0) meV.
Quantum and classical optics of dispersive and absorptive structured media
Keywords: Physics

This thesis presents a Hamiltonian formulation of the electromagnetic fields in structured (inhomogeneous) media of arbitrary dimensionality, with arbitrary material dispersion and absorption consistent with causality. The method is based on an identification of the photonic component of the polariton modes of the system. Although the medium degrees of freedom are introduced in an oscillator model, only the macroscopic response of the medium appears in the derived eigenvalue equation for the polaritons. For both the discrete transparent-regime spectrum and the continuous absorptive-regime spectrum, standard codes for photonic modes in nonabsorptive systems can easily be leveraged to calculate polariton modes. Two applications of the theory are presented: pulse propagation and spontaneous parametric down-conversion (SPDC). In the propagation study, the dynamics of the nonfluctuating part of a classical-like pulse are expressed in terms of a Schrodinger equation for a polariton e®ective ¯eld. The complex propagation parameters of that equation can be obtained from the same gener- alized dispersion surfaces typically used while neglecting absorption, without incurring additional computational complexity. As an example I characterize optical pulse propagation in an Au/MgF2 metallodielectric stack, using the empirical response function, and elucidate the various roles of Bragg scattering, interband absorption and field expulsion. Further, I derive the Beer coefficient in causal structured media. The SPDC calculation is rigorous, captures the full 3D physics, and properly incorporates linear dispersion. I obtain an expression for the down-converted state, quantify ii pair-production properties, and characterize the scaling behavior of the SPDC energy. Dispersion aspects the normalization of the polariton modes, and calculations of the down- conversion efficiency that neglect this can be o® by 100% or more for common media regardless of geometry if the pump is near the band edge. Furthermore, I derive a 3D three-wave group velocity walkoff factor; due to the interplay of a topological property with a symmetry property, I show that even if down-conversion is into a narrow forward cone, neglect of the transverse walkoff can lead to an overestimate of the SPDC energy by orders of magnitude.
Interband optical injection and control of electron spin populations and ballistic spin currents in bulk semiconductors
Keywords: Physics

This thesis theoretically studies in terband optical injection of spin current, carrier spin, current, and carrier population by one-photon absorption, two-photon absorption, and the interference of one- and two -photon absorption (“1 + 2 excitation) in cubic bulk semiconductors. Novel effects— “ 1 + 2 ” spin-current injection, “ 1 + 2 ” spin control, and one-photon pure spin-current injection — are proposed and studied , and theories of previously known effects— “ 1 + 2 ” current injection, “1 + 2 “ carrier-population control, and two -photon spin injection — are extended. Each of the effects is studied phenomenologically from the point of view of crystal symmetry to determine the polarization and crystal orientation dependence, especially for cubic materials. T h e focus of the thesis is on the optical injection, rather than on the subsequent scattering, transport, and relaxation of the non equilibrium carrier distributions. A microscopic expression for the injection rate of each effect is derived with the optical field treated asaperturbation. The effects are studied with simple analytical band models, perturbative in the Bloch wave vector k. “ 1 + 2 ” current injection and “ 1 + 2 ” spin -current injection, which are nonzero in isotropic materials, are evaluated using the isotropic, eight-b and Kane model. “ 1 + 2 ” population control, “ 1 + 2 “ spin control, and two-photon spin injection which require a lower symmetry model, are evaluated in the parabolic band approximation using a fourteen-band model. Each of these, and one-photon pure spin-current injection are further calculated numerically using the fourteen band k • p Hamiltonian . The calculation is nonperturbative in k , and hence shows the limit of validity of the simpler models. Strain is incorporated in to the fourteen -band calculation to show that one-pho ton pure spin -current injection can be increased with the application of strain. I t is shown that two-photon spin injection can yield a very high degree spin polarization , but only due to transitions that do not conserve angular momentum . Excitonic effects on “ 1 + 2 ”excitation are studied using the effective-mass theory of Wannier excitons and accounting for degenerate bands. I t is shown that excitonic effects cause a phase shift in the dependence of “1 + 2 ” current injection and “ 1 + 2 ” spin-current injection on the optical phases, and cause an enhancement of all four“ 1 + 2 ” effects.
Emergence of unconventional phases in quantum spin systems
Keywords: Physics

In this thesis, we investigate strongly correlated phenomena in quantum spin systems. In the first part of this work, we study geometrically frustrated antiferromagnets (AFMs). Generalizing the SU(2) Heisenberg Hamiltonian to Sp(N) symmetry, we obtain, in the large-N limit, the mean-field phase diagrams for the planar pyrochlore and cubic AFMs. We then use gauge theories to consider fluctuation effects about their respective meanfield configurations. We find, in addition to conventional Néel states, a plethora of novel magnetically disordered phases: two kinds of spin liquids, Z2 in 2+1D and U(1) in 3+1D, and several valence bond solids such as two and three-dimensional plaquette and columnar singlet states. We use the same approach to study the diamond lattice AFM which possesses extended classical ground state degeneracy. We demonstrate that quantum and entropic fluctuations lift this degeneracy in different ways. In the second part of the thesis, we study ultracold spinor atoms confined in optical lattices. We first demonstrate the feasibility of experimental realization of rotor models using ultracold spin-one Bose atoms in a spin-dependent and disordered optical lattice. We show that the ground state of such disordered rotor models with quadrupolar interactions can exhibit biaxial nematic ordering in the disorder-averaged sense, and suggest an imaging experiment to detect the biaxial nematicity in such systems. Finally, using variational wavefunction methods, we study the Mott phases and superfluid-insulator transition of spin-three bosons in an optical lattice with an anisotropic two dimensional optical trap. We chart out the phase diagrams for Mott states with n = 1 and n = 2 ii atoms per lattice site. We show that the long-range dipolar interaction stabilizes a state characterized by antiferromagnetic chains made of ferromagnetically aligned spins. We also obtain the mean-field phase boundary for the superfluid-insulator transition, and show that inside the superfluid phase and near the superfluid-insulator phase boundary, the system undergoes a first order antiferromagnetic-ferromagnetic spin ordering transition.
Direct measurements of line mixing, line broadening, and translational line shape in the nu(1) + nu(2) Q-branch of pure carbon dioxide
Keywords: Physics

An infrared difference frequency spectrometer was improved such that the baseline spectra were constant to within 1 part in 1000 over a 1.3 cm-1 scan. Over a 0.5 second integration time, transmission through the gas cell could be measured with a frequency resolution of 1.5MHz and a signal-to-noise ratio in excess of 2000:1. Using this spectrometer, line mixing and high resolution line shape effects have been directly measured for the vt+v2 band of C0(2). At pressures lower than 11kPa, the spectra were analysed within the Rosenkranz line mixing approximation. Translational effects were encompassed using the hard collision model of Nelkin and Ghatak (1964), modified to include an asymmetry component due to line mixing. For each line between 0(2) and 0(30), the Rosenkranz first order line mixing coefficients, broadening coefficients, and narrowing parameters were measured. The line mixing coefficients were found to be qualitatively consistent with, but significantly different from, the mixing coefficients calculated by Strow et al. (1994) using an Exponential Power Gap (EPG) law. The broadening coefficients, estimated to be accurate to within 0.5% on average, were found to be consistent with the P- and R- branch broadening measurements made by other groups. The narrowing parameters were found to be strongly non-linear with pressure, a symptom of speed-dependent line shape effects unaccounted for by the line shape model used. In addition to the line parameters, the band shift and band strength were measured to 5% and 0.5% accuracy respectively. No other shift measurements of this band have been reported for comparison. However, the band strength was found to be 3% lower than the currently used value of Hitran96 and 2% higher than other recent measurements of this band. To analyse spectra at pressures greater than 11kPa, an empirical speed-dependent profile was formed from the sum of two hard collision profiles. Only with this profile could spectra up to 72kPa be fitted within error. The spectra displayed strong non-linear line mixing effects. In particular, intensity transfer between lines was directly measured and found to agree qualitatively with that predicted by an EPG scaling law calculation.
A microscopic model for non-fermi-liquid behavior and charge carrier pairing in a purely repulsive two-dimensional electron system
Keywords: Physics

This thesis examines the properties of a new microscopic model for non-Fermi-liquid behavior and d-wave pairing in a strongly correlated quasi-two-dimensional (2D) electron gas with purely repulsive interactions. Themicroscopic Hamiltonian for this system involves a nearest neighbor electron hopping matrix element t, an on-site Coulom b repulsion U , and a nearest neighbor Coulomb repulsion V. We suggest that nearest neighbor Coulomb repulsion on the energy scale of t stabilizes a state in which electrons undergo a “somersault” in their internal spin-space (spin-flux). Spin-flux is a new form of spontaneous symmetry breaking in a strongly correlated electron system in which the Hamiltonian acquires a term with the symmetry of spin-orbit coupling at the mean-field level. Spin-flux modifies the single quasi-particle dispersion relations from that of a conventional antiferro-magnet (AFM ). When this spin-1/2 A FM insulator is doped, the charge carriers are mobile, charged, bosonic meron-vortex solitons accompanied by unoccupied states deep inside the Mott-Hubbard charge-transfer gap. This model provides a unified microscopic basis for (i) non-Fermi-liquid transport properties for low and intermediate doping, (ii) mid-infrared optical absorption, (iii) destruction of A FM long range order with doping, (iv) angled resolved spectroscopy (ARPES), (v) d-wave preformed charge carrier pairs, an d (vi) a transition from the non-Fermi liquid state to a conventional Fermi-liquid metal for large doping. The approximations used to study the 2D spin-flux Hubbard model are the unrestricted Hartree-Fock Approximation and the Configuration Interaction (C l) Method. In ID, the C l approximation leads to excellent agreement with the exact Bethe Ansatz solution of the Hubbard model, as well as a clear demonstration of the spin-charge separation: the charge of the doping hole is carried by charged bosonic domain-walls, while the spin of the doping hole is carried by neutral fermionic domain-walls. In 2D, the C l method suggests a precursor to the spin-charge separation: addition of a single hole leads to the appearance of a charge-carrying vortex (the meron-vortex) bound to a spin-carrying antivortex.
Search for leptoquarks in electron-proton collisions
Keywords: Physics

Leptoquarks carry lepton number as well as baryon number and could provide an explanation for the observed symmetry between the quarks and leptons. Leptoquarks coupling to first-generation quarks and leptons were searched for in electron-protoncollisions at HERA using the ZEUS detector. In a sample of e~p —> v X events corresponding to an integrated luminosity of 27.5 nb- 1, no leptoquark candidates were found. Limits on leptoquark couplings are derived for leptoquark masses ranging from 50 G e V /c 2 to 2 2 5 G e V /c 2. Scalar (vector) isosinglet leptoquarks coupling to a left-h a n d e d electron and a u-quark (d -quark) with electroweak coupling strength are ruled out a t the 95% confidence level for masses below 154 G e V /c 2 (108 G e V /c2).
Inclusive rare B decays using effective field theories
Keywords: Physics

In this thesis we will discuss several properties of rare decays of B mesons. First we discuss properties of the inclusive radiative decay B —► X 3(5)7, where X(5) stands for any hadronic state containing an s quark. We extend previous studies of this decay, which included perturbative corrections to order a 3 and nonperturbative contributions up to order (AQCD/mb )2 and calculate the O(AQCD/mb)3 contributions to this decay. The values of the nonperturbative parameters entering at this order are unknown, leading to uncertainties in the standard model prediction of this decay. We estimate the size of these nonperturbative uncertainties by varying these parameters in the range suggested by dimensional analysis. We also estimate uncertainties arising from a cut on the photon energy which is required experimentally. Another decay mode investigated is B —> X(5) l+l~. We study the O(AQCD/mb)3 contributions to the leptonic invariant mass spectrum, the forward-backward asymmetry and hadronic invariant mass moments and estimate the resulting uncertainties. We calculate how the size of these uncertainties depend on the value of an experimental cut that has to be applied to eliminate the large background from other B decays. A model independent way to determinate the CKM matrix element |Vub| from the dilepton invariant mass spectrum of the inclusive decay B -> X (u) l +v is presented next. We show that cuts required to eliminate the charm background still allow for a theoretically clean way to determine the CKM matrix element |K,6|- We also discuss the utility of the B —> X s£+£~ decay rate above the V>(2S) resonance to reduce the resulting uncertainties. Finally, we introduce a novel effective theory valid for highly energetic particles. In decays where the phase space is sufficiently restricted such that final state particles have very high energies compared to their mass, the perturbative as well as nonperturbative series diverge. The effective theory presented allows to sum perturbative Sudakov logarithms in a framework that also incorporates the nonperturbative physics in such limits of phase space.
Quantization of a classical model with symmetry
Keywords: Physics

A theory of quantization is given, for a model with symmetry, as the inverse of dequantization. Dequanrization is defined as tin’ process of extracting the classical mechanics of a model from its quantal counterpart. It is shown that classical dynamics is embedded in quantum dynamics as a constrained subsystem. Thus, classical mechanics is obtained from quantum mechanics without taking limits of large-dimensional representations or letting h —► 0. The embedding shows that, under appropriate conditions, a quantal density can be identified with a classical state. The results are consistent with the conjecture that an algebraic model is quantized by constructing a unitary irrep of its spectrum generating algebra. Quantization of operators not in this algebra is determined by compatibility with dequantization: this method resolves the Dirac problem in a way that avoids the ambiguities of canonical quantization. Geometric quantization is used to express the vector coherent state inducing construction as a theory of quantization. The result provides physically useful quantum models for systems with an algebraic structure and intrinsic degrees of freedom. The results are illustrated by application to the S i (3) model of a rotor. An algebraic model of a rigid rotor is presented, anti the techniques of vector coherent state quantization are applied to give a quantization, including intrinsic degrees of freedom, that is compatible with the geometric structure of the corresponding classical model. An explicit dequantization of the asymmetric top, leading to Euler’s equations, illustrates the consistency of this quantization.
The dissociation energy of hydrogen and of deuterium
Keywords: Physics

The ab initio computation and experimental determination of the dissociation energy (Do) of the hydrogen and deuterium molecules has long been an important problem in molecular spectroscopy. Kolos, Monkhorst, and Szalewicz (1986) have calculated D0 with an accuracy of± 0.1 cm-1 while including fine interactions such as relativistic effects, nonadiabatic mixing, and radiative shifts. These terms introduce corrections as small as 0.2 cm-I, and experimental precision must exceed this to test the calculation in all of its aspects. In our experiment, a jet of H2 (or Di) was probed with coherent and tunable radiation at - 84 nm, near the second dissociation limit which leads to the products H(n=2) + H(ls). Molecules in their ground state were exited into the highest vibronic levels of the electronic states B, B', and C, and into the dissociation continuum by tuning the radiation across the dissociation threshold. Metastable H(2s) photofragments were detected by applying a delayed electric field to mix the 2s with 2p states, thereby quenching H(2s) atoms to yield fluorescence at the Lyman-a wavelength. The onset of quenched fluorescence marked the second dissociation limit; from this energy, the atomic 2s-ls interval was subtracted, thus yielding 0 0• We obtained the values D0(Hi) = 36 118.11 ± 0.08 cm·l and D0(Di) = 36 748.38 ± 0.07 cm·l, which are in excellent agreement with the most recent theoretical calculations. Thus, a long-standing discrepancy between experiment and theory has been resolved, and the ab initio calculation of D0 appears to be successful. Finally, from our D0 values, the dissociation energies of the molecular ions were deduced, and they also confirm the latest (1991) ab initio values of D0(H!) and D0(D2).