VictorJuiz/thesis huggingface dataset:
Direct evidence for LO phonon reabsorption by free holes in gallium arsenide quantum wells

The influence of an electron-hole plasma with a temperature close to that of the ambient lattice on the population lifetime of non-equilibrium LO phonons created by the relaxation of hot carriers is studied in a GaAs quantum well sample that was designed to allow one to clearly separate LO phonon emission from absorption processes. For the experiments a novel multiple-beam Raman scattering technique was developed that allows one to separately control the LO phonon generation and the electron-hole plasma injection process. The lifetime of non-equilibrium, small wavevector LO phonons is found to decrease from 4.5ps to 1.9 ps as the density of the plasma is increased to ~3.1011 cm-2 by adjusting the intensity of an infrared laser tuned close to the lowest subband gap of the quantum well. This result represents the first direct experimental evidence from the lattice perspective for longitudinal optical (LO) phonon reabsorption by free carriers.

Keywords: Physics
A study of leading neutrons in gamma-proton collisions at HERA

High energy (E>400 GeV) leading neutrons were observer for the first time in electron-proton (ep) collisions, in the data collected in 1993 with the ZEUS detector at the HERA collider. This was done using the test Forward Neutron Calorimeter (FNC). In this thesis, first, the ZEUS detector setup is described, including a detailed description of the FNC, its in-situ calibration and the determination of its acceptance. At an ep collider photon-proton physics (photoproduction) can also be studied using the almost real photons emitted by the electron beam. With the FNC, leading neutron physics was studied using the ZEUS photoproduction data sample, at an average yp center of mass energy of 180 GcV. The first results are presented here, including the inclusive neutron spectrum. The characteristics of event containing such leading neutrons in photoproduction are discussed and compared to the overall photoproduction sample. The One Pion Exchange model from hadron-hadron collisions was found to be consistent with the presented data. The total photon-pion cross section was measured using the FNC to tag an almost real pion beam. It was found to be Oyπ (90 GeV) = 92 ± 5 (stat.)± 28 (syst.) µb. The inclusive transverse momentum (pT) distributions of charged particles in photoproduction are also presented for non-diffractive events, the diffractive events in two bins of the diffracting mass, as well as for the non-diffractive leading neutron tagged events. The results are discussed and compared to the previous photoproduction and hadron-hadron data.

Keywords: Physics
Advisor: Martin, John F.
A non-linear field model of pattern formation with application to biological regulation

The study of complex physical systems promises inspiration, new conceptual approaches, and techniques for creating a theoretical foundation of biology. In particular, in the area of pattern formation, questions which are being explored from the physical point of view, such as the dynamics of growth, formation of pattern boundaries, occurrence of phase transitions, changes in symmetry, etc., may also be extended to the biological context. The multi-disciplinary seard1 is ongoing, not only for the solutions to dynamical equations, but even for appropriate basic models that capture the essence of pattern formation mechanisms. Useful insights into pattern formation problems in biological systems have been gained from the 'polar coordinate model' of positional information (French et al., 1976). On the other hand, the polar coordinate model lacks the formal structure to deal with global reorganization when an overall change in the size of the system occurs. To overcome this limitation and explore the insights offered into pattern formation by this established biological framework, we propose a morphogenetic field model of pattern formation, a physical model (Ginzburg Laundau theory) guided into particular form by biological phenomenology. We apply our model to pattern changes observed experimentally in Tetrahymena doublets, which show, in the conversion of two oral structures back to one, a surprising intermittent state with three oral structures. Our morphogenetic field model allows us to see 'why 3 is between 2 and 1', and to make detailed predictions on the location of pattern elements, novel cell configurations, and dynamical pathways of pattern formation. We conclude that the morphogenetic field approach has been very fruitful, in offering a 'simple' physical explanation for the enigmatic patterns of Tetrahymena, in suggesting new directions -both experimental and theoretical- for further studies of pattern formation, and in supporting the unification of the formalism, phenomenology and concepts of physical theory with the foundations of theory in biology.

Keywords: Physics
Advisor: Trainor, Lynn
Thermal conduction in the vortex state of unconventional superconductors

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.

Keywords: Physics
Astronomy
A study of the asymmetry in the photoproduction of D meson(*+) and D meson(*-) at photon energies of 40 to 160 GeV

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.

Keywords: Physics
Advisor: Luste, George
Magnetic ground state of the 2D S=1/2 square antiferromagnet copper formate tetrahydrate

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.

Keywords: Physics
Advisor: Jacobs, Allan
Quantum and classical optics of dispersive and absorptive structured media

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.

Keywords: Physics
Advisor: Sipe, John
Interband optical injection and control of electron spin populations and ballistic spin currents in bulk semiconductors

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.

Keywords: Physics
Advisor: Sipe, John
Emergence of unconventional phases in quantum spin systems

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.

Keywords: Physics
Advisor: Kim, Yong Baek
Direct measurements of line mixing, line broadening, and translational line shape in the nu(1) + nu(2) Q-branch of pure carbon dioxide

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.

Keywords: Physics
Advisor: Drummond, J.R.
A microscopic model for non-fermi-liquid behavior and charge carrier pairing in a purely repulsive two-dimensional electron system

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.

Keywords: Physics
Advisor: Sajeev, John
Search for leptoquarks in electron-proton collisions

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).

Keywords: Physics
Advisor: Bailey, David