452 research outputs found
The effect of pressure on statics, dynamics and stability of multielectron bubbles
The effect of pressure and negative pressure on the modes of oscillation of a
multi-electron bubble in liquid helium is calculated. Already at low pressures
of the order of 10-100 mbar, these effects are found to significantly modify
the frequencies of oscillation of the bubble. Stabilization of the bubble is
shown to occur in the presence of a small negative pressure, which expands the
bubble radius. Above a threshold negative pressure, the bubble is unstable.Comment: 4 pages, 2 figures, accepted for publication in Physical Review
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X-Ray Study of Pressure-Collapsed Fullerite
X-ray-diffraction studies are described for a new phase of carbon called collapsed fullerite (CF) that was produced by application of high pressure to fullerite (C). At 20 GPa there is an irreversible transition to a phase that has neither the (111) Bragg peak of diamond nor any of the Bragg peaks associated with the fcc phase of C. The spectrum of CF is flat and featureless in the range of study.Engineering and Applied Science
Atomic Deuterium Adsorbed on the Surface of Liquid Helium
We investigate deuterium atoms adsorbed on the surface of liquid helium in
equilibrium with a vapor of atoms of the same species. These atoms are studied
by a sensitive optical method based on spectroscopy at a wavelength of 122 nm,
exciting the 1S-2P transition. We present a direct measurement of the
adsorption energy of deuterium atoms on helium and show evidence for the
existence of resonantly enhanced recombination of atoms residing on the surface
to molecules.Comment: 6 pages 4 figure
Pathways to metallic hydrogen
The traditional pathway that researchers have used in the goal of producing atomic metallic hydrogen is to compress samples with megabar pressures at low temperature. A number of phases have been observed in solid hydrogen and its isotopes, but all are in the insulating phase. The results of experiment and theory for this pathway are reviewed. In recent years a new pathway has become the focus of this challenge of producing metallic hydrogen, namely a path along the melting line. It has been predicted that the hydrogen melt line will have a peak and with increasing pressure the melt line may descend to zero Kelvin so that high pressure metallic hydrogen may be a quantum liquid. Even at lower pressures hydrogen may melt from a molecular solid to an atomic liquid. Earlier attempts to observe the peak in the melting line were thwarted by diffusion of hydrogen into the pressure cell components and other problems. In the second part of this paper we present a detailed description of our recent successful demonstration of a peak in the melting line of hydrogen
On Multistep Bose-Einstein Condensation in Anisotropic Traps
Multistep Bose-Einstein condensation of an ideal Bose gas in anisotropic
harmonic atom traps is studied. In the presence of strong anisotropy realized
by the different trap frequency in each direction, finite size effect dictates
a series of dimensional crossovers into lower-dimensional excitations. Two-step
condensation and the dynamical reduction of the effective dimension can appear
in three separate steps. When the multistep behavior occurs, the occupation
number of atoms excited in each dimension is shown to behave similarly as a
function of the temperature.Comment: 26 pages, 7 figures, revised version, to appear in Jour. Phys.
Excitation-assisted inelastic processes in trapped Bose-Einstein condensates
We find that inelastic collisional processes in Bose-Einstein condensates
induce local variations of the mean-field interparticle interaction and are
accompanied by the creation/annihilation of elementary excitation. The physical
picture is demonstrated for the case of three body recombination in a trapped
condensate. For a high trap barrier the production of high energy trapped
single particle excitations results in a strong increase of the loss rate of
atoms from the condensate.Comment: 4 pages, no figure
The COVID-19 Impact on Health Administration Education: Understanding Student Perspectives on the Transition from In-Person to Remote Course Instruction
COVID-19 has infected millions of Americans. To combat the spread of the virus, state and local officials instituted social distancing guidelines that forced schools to shutter campuses and transition from in-person to remote learning. In this study, we examined health administration (HA) student perspectives on the transition from in-person to remote learning. We sought to understand how schools attempted to manage student concerns, how adaptations to remote learning were implemented, and what influences the transition had on student stress and anxiety. We used a mixed-methods study design that included (1) a survey of undergraduate and graduate students from six geographically diverse HA programs, and (2) a focus group with 6–10 students from each program. Our survey response rate was 52% (n =215). We found that students experienced five phases following the transition: grief, loss of engagement, fatigue, coping, and resilience. Focus groups also revealed stress and anxiety, as well as communication from leaders, as important themes. We present integrated survey and focus group findings, and supplement with exemplary quotes where applicable. We conclude by discussing a number of insights provided by HA students that may help guide program leadership and HA faculty who are teaching future remote courses
Limits to Sympathetic Evaporative Cooling of a Two-Component Fermi Gas
We find a limit cycle in a quasi-equilibrium model of evaporative cooling of
a two-component fermion gas. The existence of such a limit cycle represents an
obstruction to reaching the quantum ground state evaporatively. We show that
evaporatively the \beta\mu ~ 1. We speculate that one may be able to cool an
atomic fermi gas further by photoassociating dimers near the bottom of the
fermi sea.Comment: Submitted to Phys. Rev
Adsorption of para-Hydrogen on Krypton pre-plated graphite
Adsorption of para-Hydrogen on the surface of graphite pre-plated with a
single layer of atomic krypton is studied thoretically by means of Path
Integral Ground State Monte Carlo simulations. We compute energetics and
density profiles of para-hydrogen, and determine the structure of the adsorbed
film for various coverages. Results show that there are two thermodynamically
stable monolayer phases of para-hydrogen, both solid. One is commensurate with
the krypton layer, the other is incommensurate. No evidence is seen of a
thermodynamically stable liquid phase, at zero temperature. These results are
qualitatively similar to what is seen for for para-hydrogen on bare graphite.
Quantum exchanges of hydrogen molecules are suppressed in this system.Comment: 12 pages, 6 figures, to appear in the proceedings of "Advances in
Computational Many-Body Physics", Banff, Alberta (Canada), January 13-16 200
Collective oscillations of an interacting trapped Fermi gas
We calculate the effects of two-body interactions on the low frequency
oscillations of a normal Fermi gas confined in a harmonic trap. The mean field
contribution to the collective frequencies is evaluated in the collisionless
regime using a sum rule approach. We also discuss the transition between the
collisionless and hydrodynamic regime with special emphasis to the spin dipole
mode in which two atomic clouds occupying different spin states oscillate in
opposite phase. The spin dipole mode is predicted to be overdamped in the
hydrodynamic regime. The relaxation time is calculated as a function of
temperature and the effects of Fermi statistics are explicitly pointed out.Comment: 4 pages, 1 figure include
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