Physics Faculty PublicationsCopyright (c) 2015 University of Rhode Island All rights reserved.
http://digitalcommons.uri.edu/phys_facpubs
Recent documents in Physics Faculty Publicationsen-usThu, 03 Sep 2015 01:38:29 PDT3600Zero-Temperature Relaxation in Spin-Polarized Fermi Systems
http://digitalcommons.uri.edu/phys_facpubs/203
http://digitalcommons.uri.edu/phys_facpubs/203Tue, 01 Sep 2015 10:05:52 PDT
The effect of zero-temperature attenuation, which has been recently observed in spin dynamics of Fermi liquids, on various processes in helium and ferromagnetic systems is described. A brief review of theoretical and experimental data on zero-temperature attenuation in transverse spin dynamics of helium systems is followed by a discussion of coupling between longitudinal and transverse processes, the Castaing instability in 3He and 3He−4He mixtures, and applications to pure ferromagneticmetals.
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A. E. Meyerovich et al.Localization of Ultra-Cold Particles over Rough Substrates
http://digitalcommons.uri.edu/phys_facpubs/202
http://digitalcommons.uri.edu/phys_facpubs/202Tue, 01 Sep 2015 10:00:15 PDT
Localization and diffusion parameters are calculated for particles adsorbed over inhomogeneous substrates for discrete and quasicontinuous spectra of the adsorbed states. The results are expressed via the angular harmonics of the correlation function of surface roughness. The problem is solved analytically in the limiting cases of longwave particles and large correlation radii of surface inhomogeneities. Elsewhere, the problem is solved numerically for Gaussian correlation of inhomogeneities. Applications to electrons on helium films, mobile adsorbed hydrogen atoms and molecules, ultra-cold neutrons in gravitational or magnetic field, etc., are discussed.
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A. Stepaniants et al.Surface Roughness and Effective Stick-Slip Motion
http://digitalcommons.uri.edu/phys_facpubs/201
http://digitalcommons.uri.edu/phys_facpubs/201Mon, 31 Aug 2015 09:26:34 PDT
The effect of random surface roughness on hydrodynamics of viscous incompressible liquid is discussed. When the hydrodynamic decay length (the viscous wave penetration depth) is larger than the correlation radius (size) of random surface inhomogeneities, it is possible to replace a random rough surface by effective stick-slip boundary conditions on a flat surface with two constants: the stick-slip length and the renormalization of viscosity near the boundary. The stick-slip length and the renormalization coefficient are expressed explicitly via the correlation function of random surface inhomogeneities. The stick-slip length is always negative and the effective change of viscosity near the surface is positive signifying the effective average hampering of the hydrodynamic flows by the rough surface (stick rather than slip motion). A simple hydrodynamic model illustrates general hydrodynamic results. The effective boundary parameters are analyzed numerically for Gaussian, power-law and exponentially decaying correlators with various indices. The maximum on the frequency dependence of the dissipation allows one to extract the correlation radius (characteristic size) of the surface inhomogeneities directly from, for example, experiments with torsional quartz oscillators.
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I. V. Ponomarev et al.Kinetic Phenomena in Spin‐Polarized Quantum Systems
http://digitalcommons.uri.edu/phys_facpubs/200
http://digitalcommons.uri.edu/phys_facpubs/200Mon, 31 Aug 2015 09:15:29 PDT
I present a brief review of recent theoretical and experimental achievements concerning the kinetics of spin‐polarized quantum systems. Recently serious attention has been paid to generalized and more accurate schemes of derivation of kinetic equations for such systems. Within different approaches the exchange and non‐local effects, virial corrections and diagrammatic methods were studied in detail. The first calculations of transverse relaxation time responsible for attenuation of spin waves have also been performed. The new experimental results on transport phenomena are in reasonable agreement with theoretical predictions. The future studies of spin‐polarized quantum systems are hampered by the lack of adequate and concise description of the particle’s interaction with the walls such as boundary slip effects and magnetic relaxation. The latter is especially interesting and important at low temperatures because of possible magnetic ordering in the boundary ^{3}He layers.
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A. E. MeyerovichApplications and Identification of Surface Correlations
http://digitalcommons.uri.edu/phys_facpubs/199
http://digitalcommons.uri.edu/phys_facpubs/199Fri, 28 Aug 2015 08:47:30 PDT
We compare theoretical, experimental, and computational approaches to random rough surfaces. The aim is to produce rough surfaces with desirable correlations and to analyze the correlation functions extracted from the surface profiles. Physical applications include ultracold neutrons in a rough waveguide, lateral electronic transport, and scattering of longwave particles and waves. Results provide guidance on how to deal with experimental and computational data on rough surfaces. A supplemental goal is to optimize the neutron waveguide for GRANIT experiments. The measured correlators are identified by fitting functions or by direct spectral analysis. The results are used to compare the calculated observables with theoretical values. Because of fluctuations, the fitting procedures lead to inaccurate physical results even if the quality of the fit is very good unless one guesses the right shape of the fitting function. Reliable extraction of the correlation function from the measured surface profile seems virtually impossible without independent information on the structure of the correlation function. Direct spectral analysis of raw data rarely works better than the use of a "wrong" fitting function. Analysis of surfaces with a large correlation radius is hindered by the presence of domains and interdomain correlations.
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M. Escobar et al.Beams of Gravitationally Bound Ultracold Neutrons in Rough Waveguides
http://digitalcommons.uri.edu/phys_facpubs/198
http://digitalcommons.uri.edu/phys_facpubs/198Fri, 28 Aug 2015 08:28:55 PDT
We investigate the propagation of ultracold neutrons through a rough waveguide in conjunction with recent experiments in which the ultracold neutrons were beamed between a perfect mirror and a rough scatterer and absorber. The main goal is to find a way to resolve the lowest gravitationally quantized discrete states in the peV range. We compare the neutron count for various types of mirrors with Gaussian, power-law, and exponential correlation functions of surface inhomogeneities. The main conclusion is that all the information about inhomogeneities, including their amplitude, correlation radius, and the rate of decay of the correlation function, enter the exit neutron count via just a single constant Φ, which effectively renormalizes the amplitude of roughness. To observe well-defined quantum steps, one should have an experimental setup with Φ>40. For a wide variety of correlation functions, the constant Φ is proportional to the square of the amplitude of the surface roughness and is inversely proportional to the square root of the correlation radius. The strong dependence of Φ on roughness parameters and the shape of the correlation function opens a novel way for improving the resolution of gravitationally bound states by optimizing the roughness pattern without reverting to an undesirable strong roughness. We discuss how to optimize the scatterer and absorber by first generating numerically the desired roughness profile and then transferring it to the mirror. We also study the effect of beam preparation on the initial occupancies of gravitational states before the beam enters the waveguide. It turns out that there are simple ways to manipulate the beam in front of the waveguide that can help to resolve the gravitationally bound quantum states. Our results are in good agreement with available experimental data.
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M. Escobar et al.Bose-Einstein Condensation of Interacting Gases in Traps with and without Optical Lattice
http://digitalcommons.uri.edu/phys_facpubs/197
http://digitalcommons.uri.edu/phys_facpubs/197Fri, 28 Aug 2015 08:24:03 PDT
We compare effects of particle interaction on Bose-Einstein condensation in inhomogeneous traps with and without optical lattice inside. Interaction pushes normal particles away from the condensate droplet, which is located in the center of the trap, toward the periphery of the trap where the trapping potential is large. In the end, the remaining normal particles are squeezed to a quasi-two-dimensional (2D) shell around the condensate droplet, thus changing the effective dimensionality of the system. In the absence of the optical lattice, the index in the temperature dependence of the condensate density at the later stages of the process is close to 2 with a weak dependence on the number of trapped particles. In the presence of the lattice inside the trap, this simple picture breaks down and the index acquires a strong dependence on the number of particles inside the trap, gradually falling from a three-dimensional to a 2D value with an increase in the number of particles. This change in index is explained by the lattice-driven spread of the condensate droplet and the localization of the narrow-band particles by the trap potential.
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S. Chatterjee et al.Quantum Size Effect and Biased Diffusion of Gravitationally Bound Neutrons in a Rough Waveguide
http://digitalcommons.uri.edu/phys_facpubs/196
http://digitalcommons.uri.edu/phys_facpubs/196Fri, 28 Aug 2015 08:18:06 PDT
A comprehensive theory of gravitational quantum states of ultracold neutrons in a rough waveguide is presented. The theory covers recent experiments in which the ultracold neutrons were beamed between a mirror and a rough scatterer and absorber. The results are in very good agreement with experimental data. The analysis is based on a recently developed theory of quantum transport in waveguides with rough absorbing and scattering walls. The calculation is done using two methods: an exact transport equation and a simplified model of biased scattering-driven diffusion of neutrons between quantum states. Both sets of results are in excellent agreement with each other. The exit neutron count is sensitive to the amplitude and the correlation radius (lateral size) of surface inhomogeneities and to the overall time of flight (length of the waveguide). The results indicate that it is possible to choose the waveguide parameters in such a way so to observe the quantum size effect in neutron count—the quantum steps that correspond to individual quantum states—even in a weak roughness regime. Away from the obvious limiting cases, the results are not very sensitive to the ratio of the particle energy to the absorption threshold. The main unresolved issue, which is related to a complexity of required calculations for a “real” experimental cell, is the lack of accurate information on the occupation numbers of neutrons entering the waveguide. Our analysis indicates that the initial occupancies of all gravitational states are expected to be the same except for the smallest values of the waveguide width.
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R. Adhikari et al.Gravitational Quantum States of Neutrons in a Rough Waveguide
http://digitalcommons.uri.edu/phys_facpubs/195
http://digitalcommons.uri.edu/phys_facpubs/195Fri, 28 Aug 2015 08:02:04 PDT
A theory of gravitational quantum states of ultracold neutrons in waveguides with absorbing/scattering walls is presented. The theory covers recent experiments in which the ultracold neutrons were beamed between a mirror and a rough scatterer/absorber. The analysis is based on a recently developed theory of quantum transport along random rough walls which is modified in order to include leaky (absorbing) interfaces and, more importantly, the low-amplitude high-aperture roughness. The calculations are focused on a regime where the direct transitions into the continuous spectrum above the absorption threshold dominate the depletion of neutrons from the gravitational states and are more efficient than the processes involving the intermediate states. The theoretical results for the neutron count are sensitive to the correlation radius (lateral size) of surface inhomogeneities and to the ratio of the particle energy to the absorption threshold in the weak-roughness limit. The main impediment for observation of the higher gravitational states is the “overhang” of the particle wave functions which can be overcome only by using scatterers with strong roughness. In general, strong roughness with high amplitude is preferable if one wants just to detect the individual gravitational states, while strong-roughness experiments with small amplitude and high aperture are preferable for the quantitative analysis of the data. We also discuss ways to further improve the accuracy of calculations and to optimize the experimental regime.
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A. E. Meyerovich et al.Alkali-Metal Gases in Optical Lattices: Possible New Type of Quantum Crystals
http://digitalcommons.uri.edu/phys_facpubs/194
http://digitalcommons.uri.edu/phys_facpubs/194Fri, 28 Aug 2015 07:39:56 PDT
Similarities between alkali-metal gases in optical lattices with noninteger occupation of the lattice sites and quantum crystals are explored. The analogy with the vacancy liquid (VL) provides an alternative explanation to the Mott transition for the recent experiment on the phase transition in the lattice. The VL can undergo Bose-Einstein condensation (BEC) with Tc within experimental reach. Direct and vacancy-assisted mechanisms of the band motion for hyperfine impurities are discussed. A large concentration of vacancies can result in the spatial decomposition of the system into pure hyperfine components. Below the vacancy condensation the impurity component resembles 3He in 3He–He II mixtures.
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A. E. MeyerovichPrecessing Domains and the Castaing Instability in Confined Alkali-Metal Gases
http://digitalcommons.uri.edu/phys_facpubs/193
http://digitalcommons.uri.edu/phys_facpubs/193Fri, 28 Aug 2015 07:23:18 PDT
We explore the analogy between two-component quantum alkali-metal gases and spin-polarized helium systems. Recent experiments in trapped gases are put into the frame of the existing theory for the Castaing instability in transverse channels and the formation of homogeneous precessing domains in spin-polarized systems. Analogous effects have already been observed in spin-polarized 3He and 3He−4He mixture systems. The threshold effect of the confining potential on the instability is analyzed. Additional experimental possibilities for the observation of a transverse instability in a trap are discussed.
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A. Kuklov et al.Interference of Bulk and Boundary Scattering in Ultrathin Quantized Systems
http://digitalcommons.uri.edu/phys_facpubs/192
http://digitalcommons.uri.edu/phys_facpubs/192Fri, 28 Aug 2015 07:12:30 PDT
Interference of bulk and surface scattering processes is analyzed for ultrathin systems with quantized motion in the direction perpendicular to the walls. The effective relaxation time for scattering by random bulk and surface inhomogeneities is calculated, when possible analytically, beyond the Matthiessen's rule. The applications to the size effect, weak localization, transport along the walls, and boundary slip are discussed.
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A. E. Meyerovich et al.Localization and Diffusion in Quasi-2D Helium and Hydrogen Systems with Corrugated Boundaries
http://digitalcommons.uri.edu/phys_facpubs/191
http://digitalcommons.uri.edu/phys_facpubs/191Fri, 28 Aug 2015 07:07:49 PDT
Localization length and diffusion coefficient are calculated for quantized quasi-2D low-temperature systems with randomly corrugated boundaries or substrates. Applications include electrons on helium or hydrogen surfaces, quasiparticles in capillaries and ultrathin films, surface states, ultracold neutrons in gravitational traps, etc.
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A. E. Meyerovich et al.