Theses and Dissertations from UMD
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Item A Ring with a Spin : Superfluidity in a toroidal Bose-Einstein condensate(2011) Ramanathan, Anand Krishnan; Rolston, Steve L; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Superfluidity is a remarkable phenomenon. Superfluidity was initially characterized by flow without friction, first seen in liquid helium in 1938, and has been studied extensively since. Superfluidity is believed to be related to, but not identical to Bose-Einstein condensation, a statistical mechanical phenomena predicted by Albert Einstein in 1924 based on the statistics of Satyendra Nath Bose, where bosonic atoms make a phase transition to form a Bose-Einstein condensate (BEC), a gas which has macroscopic occupation of a single quantum state. Developments in laser cooling of neutral atoms and the subsequent realization of Bose-Einstein condensates in ultracold gases have opened a new window into the study of superfluidity and its relation to Bose-Einstein condensation. In our atomic sodium BEC experiment, we studied superfluidity and dissipationless flow in an all-optical toroidal trap, constructed using the combination of a horizontal ``sheet''-like beam and vertical ``ring''-like beam, which, like a circuit loop, allows flow around the ring. On inducing a single quantum of circulation in the condensate, the smoothness and uniformity of the toroidal BEC enabled the sustaining of a persistent current lasting 40 seconds, limited by the lifetime of the BEC due to background gas pressure. This success set the stage for further experiments studying superfluidity. In a first set of experiments, we studied the stability of the persistent current by inserting a barrier in the flow path of the ring. The superflow stopped abruptly at a barrier strength such that the local flow velocity at the barrier exceeded a critical velocity, which supported decay via the creation of a vortex-antivortex pair. Our precise control in inducing and arresting superflow in the BEC is a first step toward studying other aspects of superfluidity, such as the effect of temperature and dimensionality. This thesis discusses these experiments and also details partial-transfer absorption imaging, an imaging technique developed in the course of this work.Item Asymmetric Fluid Criticality(2011) Bertrand, Christopher Elliot; Anisimov, Mikhail A; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This work investigates features of critical phenomena in fluids. The canonical description of critical phenomena, inspired by the Ising model, fails to capture all features observed in fluid systems, specifically those associated with the density or compositional asymmetry of phase coexistence. A new theory of fluid criticality, known as "complete scaling", was recently introduced. Given its success in describing experimental results, complete scaling appears to supersede the previous theory of fluid criticality that was consistent with a renormalization group (RG) analysis of an asymmetric Landau-Ginzburg-Wilson (LGW) Hamiltonian. In this work, the complete scaling approach and the equation of state resulting from the RG analysis are shown to be consistent to order ε, where ε = 4 - d with d being the spatial dimensionality. This is accomplished by developing a complete scaling equation of state, and then defining a mapping between the complete scaling mixing-parameters and the coefficients of the asymmetric LGW Hamiltonian, thereby generalizing previous work [Phys. Rev. Lett. 97, 025703 (2006)] on mean-field equations of state. The seemingly different predictions of these approaches are shown to stem from an intrinsic ambiguity in the interpretation of the ε-expansion at fixed order. To first order in ε it is found that the asymmetric correction-to-scaling exponent θ5 predicted by the RG calculations can be fully absorbed into the 2β exponent of complete scaling. Complete scaling is then extended to spatially inhomogeneous fluids in the approximation η=0, where η is the anomalous dimension. This extension enables one to obtain a fluctuation-modified asymmetric interfacial density profile, which incorporates effects from both the asymmetry of fluid phase coexistence and the associated asymmetry of the correlation length. The derived asymmetric interfacial profile is used to calculate Tolman's length, the coefficient of the first curvature correction to the surface tension. The previously predicted divergence of Tolman's length at the critical point is confirmed and the amplitude of this divergence is found to depend nonuniversally on the asymmetry of the correlation length.Item QUANTUM MANY-BODY PHENOMENA IN ULTRA-COLD ATOMS IN OPTICAL LATTICES(2011) Hu, Anzi; Hu, Bei-Lok; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Two models are discussed here to illustrate the quantum many-body phenomena in mixtures of ultra-cold atoms in optical lattices. The first model describes a mixture of two species of bosonic atoms of equal masses in optical lattices and the second describes a mixture of heavy bosonic atoms and light fermionic atoms in optical lattices. For both models, we assume the trap is present and use parameters typical in experiment. For the first model, the discussion is aimed at providing a thorough description of the collective behavior of the binary mixture in various interaction regions, with emphasis on two many-body phenomena, pairing and anti-pairing, as a result of the inter-species interaction. The pairing leads to a new type of superfluid order, called the paired superfluid (PSF) and the anti-pairing leads to another type of superfluid order, called the counter-flow superfluid (CFSF). In addition, we discuss the coexistence of charge density wave order with the three superfluid orders in the strong interaction region. We use both Luttinger liquid theory and the time evolving block decimation (TEBD) method to study this model in one dimension. The discussion is organized in three parts: the phase diagram and the correlation functions; the noise correlation functions; and the transport properties. Two phase diagrams are constructed to map the different orders in the parameter space. The correlation functions, include noise correlations, are carefully examined for the determination of the orders and for possible detection methods. In the end, the transport properties of the PSF and CFSF orders are studied through the dipole oscillation induced by trap displacement. For the second model, examining a mixture of heavy bosons and light fermions, the discussion is oriented toward determining the thermal properties of the mixture for attractive inter-species interactions. This work is motivated by experiments creating artificial molecules through optical and magnetic control of ultra-cold atoms. We use the strong coupling (SC) expansion method to evaluate the density profile, the onsite inter-species correlations, the density fluctuations and the entropy per particle. Analytical expressions are derived for all the quantities above as well as the partition function. To benchmark the accuracy, the SC calculations are compared with inhomogeneous dynamical mean field theory (IDMFT) and Monte Carlo (MC) simulation. From the calculations, we find that 1) the efficiency of creating pre-formed molecules is significantly increased by confining the mixtures onto optical lattices; 2) the temperature of the mixtures in optical lattices can be reliably estimated through the density gradient and the density fluctuations.Item Reducing Decoherence in dc SQUID Phase Qubits(2010) Przybysz, Anthony Joseph; Wellstood, Frederick C.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis examines sources of dissipation and dephasing in a dc SQUID phase qubit. Coupling of the qubit to the bias lines and lossy dielectrics causes the qubit to lose quantum information through a process known generally as decoherence. Using knowledge of the possible sources of decoherence, a dc SQUID phase qubit is designed with parameters that should have made it resistant to dissipation and dephasing from those sources. Device PB9 was a dc SQUID with one small area 0.23 (μm)2 Josephson junction with a critical current of 130 nA, which was meant to be the qubit junction, and a larger area 5 (μm)2 junction with a critical current of 8.6 μA, which acted as part of an inductive isolation network. The qubit junction was shunted by a 1.5 pF low-loss interdigitated capacitor. The dc current bias line had an on-chip LC filter with a cutoff frequency of 180 MHz. The other control lines were also designed to minimize coupling of dissipative elements to the qubit. According to a theoretical model of the dissipation and dephasing, the qubit was expected to have an energy relaxation T1 ≤ 8.4 μs and dephasing time Tphi ~ 1 μs. Because of the relatively high Josephson inductance of the qubit junction, the device did not act perform like a conventional isolated single-junction phase qubit. Instead, the resonant modes that I observed were the normal modes of the entire SQUID. At 20 mK and a frequency of 4.047 GHz, the maximum energy relaxation time of the device was found to be 350 ± 70 ns, despite the optimized design. Through a study of T1 versus applied flux, T1 was found to depend on the strength of the coupling of the microwave drive line to the qubit. When the line was more coupled, T1 was shorter. This was evidence that the microwave line was overcoupled to the qubit, and was limiting the lifetime of the excited state T1. Through a study of the spectroscopic coherence time T2*, which measured the effects of low-frequency inhomogeneous broadening and higher frequency dephasing from noise, I discovered that device PB9 has several sweet spots. In particular, the presence of a sweet spot with respect to critical current fluctuations allowed me to identify critical current noise as a major source of broadening and dephasing in the qubit. From the spectroscopy I estimated the 1/f critical current noise power density at 1 Hz was and the 1/f flux noise power spectral density at 1 Hz was . Both of these values were quite high, possibly due to switching of the device between measurements.Item High Frequency Electrical Transport Properties of Carbon Nanotubes(2010) Cobas, Enrique Darío; Fuhrer, Michael S; Takeuchi, Ichiro; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Carbon nanotubes (CNTs) have extraordinary electronic properties owing to the unique band structure of graphene and their one-dimensional nature. Their small size and correspondingly small capacitances make them candidates for novel high-frequency devices with cut-off frequencies approaching one terahertz, but their high individual impedance hampers measurements of their high-frequency transport properties. In this dissertation, I describe the fabrication of carbon nanotube Schottky diodes on high-frequency compatible substrates and the measurement of their rectification at frequencies up to 40GHz as a method of examining the high-frequency transport of individual CNTs despite their high impedance. The frequency dependence of the rectified signal is then used to extract the Schottky junction capacitance as a function of applied bias and ambient doping and to look for resonances which might be a signature of a room-temperature Luttinger Liquid.Item Materials for large-area electronics: characterization of pentacene and graphene thin films by ac transport, Raman spectroscopy, and optics(2010) Lenski, Daniel Roy; Fuhrer, Michael S; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation explores techniques for fabricating and characterizing two classes of novel materials which may be useful for large-area electronics applications: organic semiconductors and graphene. Organic semiconductors show promise for large-area electronics because of their low cost, compatibility with a variety of substrates, and relative ease of fabricating and patterning thin-film transistors (TFTs). Nearly all published work has focused on the dc electronic transport properties of these materials, rather than their ac behavior, which could be affected by their polycrystalline, granular structure. To address this, I have constructed a model of organic TFTs based on lossy transmission lines, and determined the relationship between the film conductivity and the overall device behavior for a bottom-contacted TFT. I apply this transmission-line framework to interpret my experiments on pentacene TFTs designed in a special long-channel geometry to hasten the onset of high-frequency effects. The experiments reveal an intrinsic frequency-dependent conductivity of polycrystalline pentacene, which can be understood within the context of the universal dielectric response model of ac conduction in disordered solids. The results are important for establishing practical limits on pentacene's ac performance. Graphene is a two-dimensional crystalline form of carbon, with a remarkably simple structure. It is a gapless semiconductor with an extremely high mobility and very high optical transparency, attracting great interest both for its possible uses as a replacement for silicon and as a transparent conducting material. I have synthesized large-area films of graphene via atmospheric-pressure chemical vapor deposition (CVD) on copper substrates, adapting a low-pressure CVD method previously reported to produce exclusively monolayer graphene films. I have transferred the graphene films to insulating SiO2, and characterized them using optical transparency, Raman spectroscopy, and atomic-force microscopy, observing significant differences from the measured properties of widely studied mechanically-exfoliated graphene. I analyze the strengths and weaknesses of these three techniques for distinguishing films of different layer number, and relate them to uncertainties in the known properties of one- and few-layer graphene. I conclude that atmospheric-pressure CVD of graphene on copper produces significant areas of multilayer, rotationally-misoriented graphene, in a significant departure from results on low-pressure CVD of graphene on copper.Item Visualization of the Vortex Lattice Dynamics in Superfluid Helium(2010) Gaff, Kristina Teresa; Lathrop, Daniel P; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We study the lattice structure and dynamics of the quantized vortices in superfluid helium-4 using a new rotating experiment. This setup includes control of the entire apparatus from the rotating frame, installation of a new EMCCD camera that allows for imaging of nanoscale tracer particles, and the development and implementation of a new isolation cell, which permits investigation into new phenomena such as differential rotation in helium-II. We have observed the vortex lattice dynamics in the (r, &phi) plane (i.e. transverse to the vortices) and present here the first real-time visualization of Tkachenko waves in helium-II from this cross section. Additionally, we present evidence of differential rotation with distinct Stewartson layer boundaries, possible Kelvin-Helmholtz instabilities, and the formation and propagation of superfluid collective vortex eddies. We show that the angular velocity is a function of radius and may be driven by the geometry of the isolation cell. We also document the observation and analysis of gravity-capillary surface waves that demonstrate an interaction between the liquid helium free surface and the bulk of the fluid.Item Broadband In-plane Relative Permittivity Characterization of Ruddlesden-Popper Sr(n+1)Ti(n)O(3n+1) Thin Films(2010) Orloff, Nathan Daniel; Takeuchi, Ichiro; Booth, James C.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We present a broadband on-wafer measurement technique for the characterization of the in-plane complex relative permittivity of a thin-film test wafer and a companion substrate test wafer from 100 Hz to 40 GHz, and potentially to 110 GHz. From 100 Hz to 300 MHz, the approach uses an ensemble of interdigitated capacitors with different interdigitated active lengths l = (0.100 mm, 0.325 mm, 0.875 mm, 1.835 mm, 2.9 mm) fabricated on both test wafers. Within this regime, from 100 Hz to 1 MHz, the measurements were performed with an inductance-capacitance-resistance meter. From 1 MHz to 300 MHz, the scattering parameters of the set of interdigitated capacitors were measured with a radio frequency vector network analyzer. In the high frequency regime, 300 MHz to 40 GHz, we measure scattering parameters of a set of coplanar waveguides of active lengths l = (0.420 mm, 1.270 mm, 2.155 mm, 3.22 mm, 3.993 mm, 5.933 mm) fabricated on the test wafers. We extracted the capacitance and conductance of the interdigitated capacitors and coplanar waveguides on the test wafers for the appropriate frequency regimes. We then obtained a mapping function from 2D finite element simulations that relates the change in capacitance of the thin-film test wafer relative to the companion substrate test wafer to the real part of the in-plane relative permittivity. The imaginary part of the in-plane relative permittivity was obtained from the real part of the in-plane relative permittivity and the in-plane loss tangent. We applied this broadband dielectric spectroscopy technique to explore the frequency-dependent relative permittivity of unstrained Ruddlesden-Popper series Srn+1TinO3n+1(n=1, 2, 3) thin films as a function of temperature and dc electric field. At room temperature, the in-plane relative permittivities (K11) obtained for Srn+1TinO3n+1(n=1, 2, 3) were 42 plus/minus 3, 54 plus/minus3, and 77 plus/minus2, respectively, and were independent of frequency. At low temperatures, K11 increased with a behavior consistent with an incipient ferroelectric, and paraelectric behavior developed in Sr4Ti3O10(n=3). In 2004, J. H. Haeni, et al. showed that SrTiO3 (n = infinity) on DyScO3 (110) undergoes a ferroelectric to paraelectric phase transition around room temperature. As a means to understand the origins of the loss and tunability in strained SrTiO3 (n = infinity), we performed our broadband dielectric spectroscopy technique on epitaxial thin-films of Ruddlesden-Popper series Srn+1TinO3n+1(n=2, 3, 4, 5, 6) on the rare-earth scandate substrates, DyScO3 (110) and GdScO3 (110). For these thin films, DyScO3 (110) and GdScO3 (110) corresponded to biaxial tensile strain of approximately 1% and 1.7%, respectively. The thin films were 50 nm thick on DyScO3 (110) and 25 nm thick on GdScO3 (110), which ensured uniform strain throughout the film. We report the dependence of the critical temperature, tunability, and loss tangent on series number and strain at 1 MHz. We also examined the broadband frequency dependent dielectric properties of these thin films as a function of temperature, electric field, series number and strain.