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# Physics - Theses, Dissertations, and other Required Graduate Degree Essays

Receive updates for this collection## Nonlinear-optical and terahertz investigations of complex oxides

Ultrafast pulsed-laser sources offer many benefits for experimental condensed-matter physics. This thesis describes a variety of ways in which their high field strength and extreme time resolution aid in the study of complex oxide materials. The design, construction and characterization of a visible-pump, terahertz-probe spectrometer is described. This instrument is applied to the study of photoexcited carriers in Sr2CuO2Cl2, an undoped, insulating cuprate. Ultrafast laser pulses are used to excite photocarriers in the antiferromagnetic insulating lattice. The low-frequency dynamical conductivity of the resulting nonequilibrium state is then measured with time-domain terahertz spectroscopy. Photoconductivity appears promptly, followed by a non-exponential decay on picosecond timescales. In this first direct measurement of the low-energy conductivity after photoexcitation, the estimated peak mobility is just 0.3 cm2/V·s, much lower than the Hall mobility found by other workers in chemically doped systems with similar carrier concentrations. Possible physical mechanisms behind this discrepancy are discussed. The spectrometer is then used to determine the photoexcited carrier relaxation time in a proton-damaged GaAs photoconductive antenna. A frequency-domain model for antenna behaviour is verified, demonstrating that the response of an antenna fabricated from an arbitrary substrate can be used directly for material characterization. Calculations and modelling are presented which show how a resonantly-enhanced optical frequency comb can efficiently generate terahertz pulses while preserving the stabilized comb structure. This could be a useful tool for high-precision comb spectroscopy or coherent excitation of structural modes. Several issues concerning the realization of such a source are described. Finally, the nonlinear-optical response of the pyrochlore oxide Cd2Re2O7 is used to characterize the structural phase transition it undergoes at 200 K, which has an unusual tensor character. Optical second harmonic generation with polarization sensitivity is able to resolve an ambiguity in the low-temperature crystal structure, assigning the I¯4m2 space group while verifying an auxiliary condition on the structure that is implied by the order parameter symmetry. The temperature-dependence of the order parameter is consistent with thermal occupation of a Goldstone mode that results from the Eu order parameter symmetry. This methodology may be applied more widely in characterizing ordered states in matter.

## QCD topology and lattice perturbation theory from Monte Carlo simulations with improved staggered fermions

The staggered quark formulation is one of many ways to include fermions on the lattice. Dynamical simulations are now routinely done with improved staggered quark actions which are more efficient than other popular formalisms. In this thesis two research works on improved staggered fermions are presented. A systematic study of the staggered Dirac operator's spectral properties is first presented. It is a long standing belief that staggered fermions do not feel gauge field topology because of the lack of zero eigenvalues of the operator at finite lattice spacing. The existence of fermionic zero modes in topological nontrivial background gauge fields is required by the Atiyah-Singer index theorem. In this study we observe that eigenmodes with very small eigenvalue and large chirality appear if improved staggered operators are used. These small eigenmodes can be identified as the "zero modes" associated with the topology of the gauge fields. We have also compared the distribution of the remaining nonchiral modes with the predictions of Random Matrix Theory. Satisfactory agreement is obtained. In the second project perturbative expansions of Wilson loops are computed in full QCD from Monte Carlo simulations with improved staggered fermions. This approach provides a much simpler alternative to diagrammatic perturbation theory, and has previously been shown to be successful in reproducing the perturbation series in pure gauge theory. This method is applied here for the first time to unquenched QCD. Twisted boundary conditions are used to eliminate effects of zero momentum modes and to suppress tunneling between the degenerate Z3 vaccua. A new simulation algorithm, the rational hybrid Monte Carlo algorithm, with no finite step size error is also employed. This is the first time this algorithm has been used in a numerical application. Results are in excellent agreement with analytic perturbation theory; this provides an important cross-check of the perturbation theory input to a recent determination of the strong coupling am(MZ) by the HPQCD collaboration.

## Magnetization dynamics in ultrathin magnetic films

Ultrathin magnetic multilayer structures are prepared by Molecular Beam Epitaxy (MBE) on GaAs(001) substrate. Growth is monitored and characterized by Reflection High Energy Electron Diffraction (RHEED), Auger Electron Spectroscopy (AES), X-Ray Photoelectron Spectroscopy (XPS) and Scanning Tunneling Microscopy (STM). The interface of Fe/GaAs system and its influence on the static magnetic properties in magnetic ultrathin films were studied by means of FMR and Mossbauer spectroscopy. Studies were performed on high quality single crystalline magnetic films. It will be shown that in ultrathin magnetic films magnetic properties are governed by a combination of interface and bulk effects. Static and dynamic magnetic properties of magnetic single and double layers were studied using Ferromagnetic Resonance (FMR) and Time Resolved Magneto-optic Kerr Effect (TRMOKE) in a wide range of microwave frequencies. Magnetic damping was studied in Au/Fe/GaAs structures as a function of the thickness of magnetic Fe layer and as a function of the capping Au layer. Spin currents generated by spin pump/spin sink effect structures were extensively investigated in magnetic doublelayer. Pure spin current driven dynamics in magnetic doublelayers was observed using TRMOKE technique. The spin diffusion and spin accumulation analysis was applied to explain spin momentum propagation in the normal metal. Spin diffusion length in Au was measured via direct observation of magnetic moment transfer. Spin pump/spin sink theory was tested for antiparallel driving of two magnetic layers of the magnetic doublelayer structure. This extreme condition was achieved via patterning sample into a coplanar waveguide. Results are in good agreement with simulations based on theory, which includes spin pump/spin sink model together with diffusive transport of accumulated spin momentum through the non-magnetic layer. Au/Fe/Au/Fe/GaAs magnetic double layer structure was used to study spin pump/spin sink effect in case of non-collinear orientation of magnetic moments. It will be shown that only component of spin momentum which is transverse to static magnetization contributes to non-local dynamics.

## Positron-neutrino correlation measurements in the beta decay of magneto-optically trapped 38mK atoms

This thesis describes the measurement of the angular correlation between the positron and the neutrino emitted in the beta decay of the isomer 38mK. This is a superallowed transition between nuclear states of the same spin and parity (0+), which is known to result primarily from the vector component of the weak interaction. The angular correlation involves two parameters. In the Standard Model of the weak interaction these have the values a=1 and b=0. Any meaningful deviation from this result can be interpreted as evidence for the existence of a scalar component in the weak interaction. The fundamentally new method that was used involved selectively confining neutral atoms of the isomer in a magneto-optical trap located between two detectors, one to measure the energy and direction of the positron and the other to detect the 38Ar nuclei that recoil with a momentum pR=-(pe+pv). The 38mK atoms were produced using the TRIUMF/ISAC facility. The trap provided a pure, cold, compact source essential to avoid distortion of the recoil momenta. For those events in which the positron was detected, the recoil momenta were deduced by measuring the time of flight from the trap to the recoil detector. About 500,000 positron-recoil coincident events were recorded. When the analysis, based on detailed Monte Carlo simulations, was restricted to positrons with kinetic energy >2.5 MeV, it showed that the angular correlation could be characterized by a "reduced" correlation parameter ã=0.9988±0.0028(stat)±0.0034(syst) (68% CL) where ã=a/(1+0.1503b). This measurement is consistent with the Standard Model and is 33% more restrictive than the only comparable previous measurement for such a transition. In the most general form, the strength of a possible scalar interaction can be specified in terms of two complex numbers, L and R, which define, respectively, the coupling to left- and right-handed neutrinos. This experiment did not usefully restrict the value of Re(L) (or b). Other experiments do provide rather strict limits on Re(L). If these are combined with the result of the present experiment one obtains the most restrictive direct limits available on Re(R), Im(R) and Im(L).

## OMVPE growth and characterization of carbon doped InAs

Carbon-doped InAs samples grown by organometallic vapor phase epitaxy were studied by Raman and IR spectroscopy. Local vibrational modes (LVMs) related to isolated substitutional carbon acceptors, carbon acceptor-hydrogen complexes, and dicarbon centers were detected in samples doped with two isotopes of carbon. Energies of the observed carbon-hydrogen modes are in close agreement with carbon acceptor-hydrogen modes in GaAs and InP, and are consistent with hydrogen occupying a bond-center position in the complex. No sign of substitutional carbon donors was observed. N-type conductivity of carbon-doped InAs was explained by the presence of dicarbon centers that are believed to be deep donors. The stretch mode of this complex was detected at 1832 cm-1 by Raman spectroscopy in as-grown and annealed samples. Annealing measurements performed on heavily carbon-doped InAs samples confirmed that the n-type conductivity observed for this material is due to the formation of dicarbon defects. The structural, electrical, and optical properties of n-type carbon-doped InAs samples annealed at temperatures of 400°C and higher were studied using Raman and infrared spectroscopy, and X-ray diffraction. Based on the expected energy level of dicarbon donors in GaAs, we predicted that the di- carbon defects in InAs should introduce a resonant level close to or slightly above the conduction band minimum.

## Symmetries, interactions and phase transitions on graphene honeycomb lattice

Graphene, a monolayer of graphite, opened a new frontier in physics with reduced dimen- sionality. Due to the Dirac nature of the quasi particles it exhibits interesting experimental phenomena. It is believed that electron-electron interac- tions also play important role in graphene. We derive here a generalized theory of short ranged interactions consistent with the various discrete symmetries present on the lattice. Restrictions on the theory imposed by the atomic limit are also discussed. Within the framework of this model we calculated the beta functions governing the renormalization flow of the couplings to sub-leading order in 1/N. Our calculations show that charge den- sity wave and anti-ferromagnetic quantum critical points are in the Gross-Neveu universality class even beyond mean-field level. Thereafter we use the extended Hubbard model to extract the phase diagram. It shows that the semimetallic ground state is stabilized once we include corrections to sub-leading order in 1/N.

## Commissioning of the ATLAS liquid argon calorimeters

ATLAS, a multi-purpose detector built at the LHC at CERN, requires an extensive commissioning campaign to be ready for proton-proton collisions. In this work, we focus on the commissioning of the liquid Argon (LAr) calorimeters, with emphasis on commissioning with cosmic rays. First we outline one phase of the commissioning work, which involves testing of the front-end electronics of the two endcap calorimeters. We then describe two cosmic ray generators as input to a Monte-Carlo simulation of cosmic rays in ATLAS, and compare their results. Finally, we explain a technique developed for this work which uses information from the Tile calorimeters to predict the timing of cosmic rays within the LAr calorimeters, because cosmic rays occur randomly in time whereas the electronics are clocked at 1/40.08 MHz. The results from this analysis tool are compared to default tools, using both simulated and real cosmic ray data in the calorimeters.

## Gaussian fluctuations of lipid bilayer vesicles: a numerical study

Fluid-phase phospholipid vesicles are abundant in biological cells and play an essential role in cellular function. Artificial phospholipid “liposomes” can be synthesized in the lab. Observed vesicle shapes are governed by the elastic energy of the enclosing membrane. Contributions to this energy come from bilayer bending and from the so-called area-difference-elasticity (ADE) energy associated with the relative stretching/compression of membrane monolayers generated by the requirement of vesicle closure. We construct a shape-energy functional representing these elements and identify observable vesicle shapes as local energy minima at fixed values of membrane area and vesicle volume. As control parameters vary, low-energy observable shape classes may be organized into a “phase diagram” in analogy with thermodynamics. At any temperature $T>0,$ thermal excitations generate shape fluctuations. Such fluctuations are generally small at room temperature but become enhanced close to the instability thresholds. In this thesis, we calculate numerically vesicle shapes and Gaussian-level shape fluctuations. Our focus is on axisymmetric shapes of spherical topology with volumes significantly less than the maximum allowed by given membrane area. For each minimum-energy shape, we show how to calculate the full spectrum of thermal shape fluctuations available at strictly fixed membrane area and volume. Euclidean modes, not associated with shape change, are systematically removed. The hard geometric constraints lead to thermally-induced shifts in the average shape, which would not occur in a standard Gaussian analysis without constraints. We show how to calculate these shifts but conclude that they are generally negligible at laboratory temperatures. Results are illustrated by the case study of a stable prolate vesicle, for which energy eigenvalues, shape eigenmodes, and static correlations are all given. A final chapter develops specific applications: (a) Constructing the phase diagram by analyzing energy levels; (b) Finding instability boundaries by tracking soft modes; (c) Illustration of characteristic spectra and eigenmodes for representative shape classes; (d) Calculation of mean-square shape fluctuations and two-point correlation functions. In particular, we confirm the expected ``stiffness" of narrow-necked shapes.

## Spontaneous atomic ordering in MOVPE grown GaAsSb

Spontaneous atomic ordering of semiconductor alloys is of great practical and fundamental interest. Atomic ordering of III-V alloys such as InGaP has been extensively studied experimentally and theoretically. In this thesis, we investigate a little-studied, atomic-ordering phenomenon, the so-called CuAu structure in the III-V material GaAsSb, grown by the technique of metalorganic vapor-phase epitaxy(MOVPE). Despite being first observed in 1986 in this material, there is as yet no detailed microscopic model for its formation mechanism. A key part of the thesis involves the study of surfactant effects on the ordering process in GaAsSb. Surfactants are elements which modify the growth surface without incorporation in the bulk. Nevertheless, they influence the incorporation of the bulk elements. We first explored the surfactant behavior of Bi on GaAs in order to understand how Bi incorporates at the surface and in the bulk in a related III-V material. For GaAs(001), Bi surface layers are stable at temperatures below 500C but rapidly desorb at temperatures of 550C and higher. Bi coverages of over 1ML induce the formation of Bi islands, whose sizes increase with increasing Bi exposure. Bulk incorporation of Bi remains essentially zero at typical MOVPE growth temperatures. In the case of GaAsSb alloys, Bi surfactant was found to induce CuAu ordering, with no measurable Bi incorporation in the bulk. High resolution TEM was used to study the detailed microstructural features for ordered and disordered samples. The domain sizes of the ordered regions are from 5nm to 20nm under all growth conditions. In contrast to orderings in other alloys such as InGaP, CuAu ordering had no observable effect on the bandgap. CuAu ordering in GaAsSb was studied in a function of growth conditions, including Bi surfactant concentration, growth temperature, growth rate, and substrate miscut. All of these experiments confirm that bulk CuAu ordering is a surface driven, rather than bulk process. It is unlikely that the ordering mechanism is similar to the dimer-induced strain models that have been successfully used to explain CuPt ordering in InGaP. We propose a simple model based on alternating incorporation of group V adatoms at step edges.

## Polarized Parton Distributions Measured at the HERMES Experiment

The HERMES experiment at DESY, Germany was designed to carry out precision measurements of the proton spin structure using polarized deep-inelastic scattering. The experiment utilizes the 27.5 GeV electron or positron beam of the HERA accelerator which is longitudinally polarized at HERMES, in combination with a polarized internal gas target. For this work, data on longitudinally polarized hydrogen and deuterium targets were used to determine cross section asymmetries with respect to the alignment of the target and beam polarizations. Inclusive asymmetries on the proton and the deuteron, where only the scattered electronlpositron is detected, were measured with high precision. In semi-inclusive deep-inelastic scattering, at least one final-state hadron is detected in coincidence. Semi-inclusive asymmetries of pions on the proton and pion and kaon asymmetries on the deuteron were measured for the first time by the HERMES collaboration. The measured asymmetries include detector effects and effects of higher-order processes in quantum electrodynamics. A new unfolding procedure that takes into account the correlations between kinematic bins was implemented to correct for these effects. The polarized parton densities of the up, down, and sea flavours were obtained from the unfolded inclusive and semi-inclusive asymmetries in a probabilistic analysis based on leading-order quantum chromodynarnics. In the case of the up quark and the down quark, the polarized densities were determined to be positive and negative, respectively. The polarized densities of the sea flavours, decomposed for the first time into the densities of the anti-up, anti-down, and strange quarks, were found to be compatible with zero. Moments of the polarized parton densities were computed. The Bj~rken sum rule was verified and the total spin carried by the quark spins was determined to be (38.0 f 8.0) %. This latter result is larger than earlier measurements but still smaller than - 60 % predicted in relativistic models of the proton.