1,533 research outputs found
Theory of Feshbach molecule formation in a dilute gas during a magnetic field ramp
Starting with coupled atom-molecule Boltzmann equations, we develop a
simplified model to understand molecule formation observed in recent
experiments. Our theory predicts several key features: (1) the effective
adiabatic rate constant is proportional to density; (2) in an adiabatic ramp,
the dependence of molecular fraction on magnetic field resembles an error
function whose width and centroid are related to the temperature; (3) the
molecular production efficiency is a universal function of the initial phase
space density, the specific form of which we derive for a classical gas. Our
predictions show qualitative agreement with the data from [Hodby et al, Phys.
Rev. Lett. {\bf{94}}, 120402 (2005)] without the use of adjustable parameters
Electron Drift Velocities In Gas Mixtures Of He, N2, And CO 2
An electron swarm experiment has been used to obtain electron drift velocities in the He:CO2:N2 mixtures 0:1:1, 3:1:2, and 3:1:1. The E/N range of 3 to 57 Td was studied with total gas pressure varied from 50 to 200 Torr. These particular mixtures have not been previously studied experimentally. Good agreement is observed between theoretical calculations and experimental data
Vortex line in a neutral finite-temperature superfluid Fermi gas
The structure of an isolated vortex in a dilute two-component neutral
superfluid Fermi gas is studied within the context of self-consistent
Bogoliubov-de Gennes theory. Various thermodynamic properties are calculated
and the shift in the critical temperature due to the presence of the vortex is
analyzed. The gapless excitations inside the vortex core are studied and a
scheme to detect these states and thus the presence of the vortex is examined.
The numerical results are compared with various analytical expressions when
appropriate.Comment: 8 pages, 6 embedded figure
Electron Drift Velocities In Xenon
The electron drift velocity has been measured in xenon over the range of reduced field strength 1 T
Superconductivity enhanced conductance fluctuations in few layer graphene nanoribbons
We investigate the mesoscopic disorder induced rms conductance variance
in a few layer graphene nanoribbon (FGNR) contacted by two
superconducting (S) Ti/Al contacts. By sweeping the back-gate voltage, we
observe pronounced conductance fluctuations superimposed on a linear background
of the two terminal conductance G. The linear gate-voltage induced response can
be modeled by a set of inter-layer and intra-layer capacitances.
depends on temperature T and source-drain voltage .
increases with decreasing T and . When lowering , a
pronounced cross-over at a voltage corresponding to the superconducting energy
gap is observed. For |V_{sd}|\ltequiv \Delta the fluctuations are
markedly enhanced. Expressed in the conductance variance of one
graphene-superconducutor (G-S) interface, values of 0.58 e^2/h are obtained at
the base temperature of 230 mK. The conductance variance in the sub-gap region
are larger by up to a factor of 1.4-1.8 compared to the normal state. The
observed strong enhancement is due to phase coherent charge transfer caused by
Andreev reflection at the nanoribbon-superconductor interface.Comment: 15 pages, 5 figure
Atom-molecule equilibration in a degenerate Fermi gas with resonant interactions
We present a nonequilibrium kinetic theory describing atom-molecule
population dynamics in a two-component Fermi gas with a Feshbach resonance. Key
collision integrals emerge that govern the relaxation of the atom-molecule
mixture to chemical and thermal equilibrium. Our focus is on the pseudogap
regime where molecules form above the superfluid transition temperature. In
this regime, we formulate a simple model for the atom-molecule population
dynamics. The model predicts the saturation of molecule formation that has been
observed in recent experiments, and indicates that a dramatic enhancement of
the atom-molecule conversion efficiency occurs at low temperatures.Comment: Updated manuscript on July 5, 2004. Four pages with three embedded
figure
Mesoscopic conductance fluctuations in InAs nanowire-based SNS junctions
We report a systematic experimental study of mesoscopic conductance
fluctuations in superconductor/normal/superconductor (SNS) devices
Nb/InAs-nanowire/Nb. These fluctuations far exceed their value in the normal
state and strongly depend on temperature even in the low-temperature regime.
This dependence is attributed to high sensitivity of perfectly conducting
channels to dephasing and the SNS fluctuations thus provide a sensitive probe
of dephasing in a regime where normal transport fails to detect it. Further,
the conductance fluctuations are strongly non-linear in bias voltage and reveal
sub-gap structure. The experimental findings are qualitatively explained in
terms of multiple Andreev reflections in chaotic quantum dots with imperfect
contacts.Comment: Manuscript and supplemen
Microscopic Structure of a Vortex Line in a Superfluid Fermi Gas
The microscopic properties of a single vortex in a dilute superfluid Fermi
gas at zero temperature are examined within the framework of self-consistent
Bogoliubov-de Gennes theory. Using only physical parameters as input, we study
the pair potential, the density, the energy, and the current distribution.
Comparison of the numerical results with analytical expressions clearly
indicates that the energy of the vortex is governed by the zero-temperature BCS
coherence length.Comment: 4 pages, 4 embedded figures. Added references. To be published in
Physical Review Letter
Zero-temperature phase diagram of binary boson-fermion mixtures
We calculate the phase diagram for dilute mixtures of bosons and fermions at
zero temperature. The linear stability conditions are derived and related to
the effective boson-induced interaction between the fermions. We show that in
equilibrium there are three possibilities: a) a single uniform phase, b) a
purely fermionic phase coexisting with a purely bosonic one and c) a purely
fermionic phase coexisting with a mixed phase.Comment: 8 pages, revtex, 3 postscript figures; NORDITA-1999/71 C
Theory of the optical absorption of light carrying orbital angular momentum by semiconductors
We develop a free-carrier theory of the optical absorption of light carrying
orbital angular momentum (twisted light) by bulk semiconductors. We obtain the
optical transition matrix elements for Bessel-mode twisted light and use them
to calculate the wave function of photo-excited electrons to first-order in the
vector potential of the laser. The associated net electric currents of first
and second-order on the field are obtained. It is shown that the magnetic field
produced at the center of the beam for the mode is of the order of a
millitesla, and could therefore be detected experimentally using, for example,
the technique of time-resolved Faraday rotation.Comment: Submitted to Phys. Rev. Lett. (23 Jan 2008
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