7,305 research outputs found
Pairing gap and in-gap excitations in trapped fermionic superfluids
We consider trapped atomic Fermi gases with Feshbach-resonance enhanced
interactions in pseudogap and superfluid temperatures. We calculate the
spectrum of RF(or laser)-excitations for transitions that transfer atoms out of
the superfluid state. The spectrum displays the pairing gap and also the
contribution of unpaired atoms, i.e. in-gap excitations. The results support
the conclusion that a superfluid, where pairing is a many-body effect, was
observed in recent experiments on RF spectroscopy of the pairing gap.Comment: Journal versio
Strongly interacting Fermi gases with density imbalance
We consider density-imbalanced Fermi gases of atoms in the strongly
interacting, i.e. unitarity, regime. The Bogoliubov-deGennes equations for a
trapped superfluid are solved. They take into account the finite size of the
system, as well as give rise to both phase separation and FFLO type
oscillations in the order parameter. We show how radio-frequency spectroscopy
reflects the phase separation, and can provide direct evidence of the FFLO-type
oscillations via observing the nodes of the order parameter.Comment: Added one reference. Published in PR
Pairing based cooling of Fermi gases
We propose a pairing-based method for cooling an atomic Fermi gas. A three
component (labels 1, 2, 3) mixture of Fermions is considered where the
components 1 and 2 interact and, for instance, form pairs whereas the component
3 is in the normal state. For cooling, the components 2 and 3 are coupled by an
electromagnetic field. Since the quasiparticle distributions in the paired and
in the normal states are different, the coupling leads to cooling of the normal
state even when initially (notation ).
The cooling efficiency is given by the pairing energy and by the linewidth of
the coupling field. No superfluidity is required: any type of pairing, or other
phenomenon that produces a suitable spectral density, is sufficient. In
principle, the paired state could be cooled as well but this requires
. The method has a conceptual analogy to cooling based on
superconductor -- normal metal (SN) tunneling junctions. Main differences arise
from the exact momentum conservation in the case of the field-matter coupling
vs. non-conservation of momentum in the solid state tunneling process.
Moreover, the role of processes that relax the energy conservation requirement
in the tunneling, e.g. thermal fluctuations of an external reservoir, is now
played by the linewidth of the field. The proposed method should be
experimentally feasible due to its close connection to RF-spectroscopy of
ultracold gases which is already in use.Comment: Journal version 4 pages, 4 figure
Signatures of superfluidity for Feshbach-resonant Fermi gases
We consider atomic Fermi gases where Feshbach resonances can be used to
continuously tune the system from weak to strong interaction regime, allowing
to scan the whole BCS-BEC crossover. We show how a probing field transferring
atoms out of the superfluid can be used to detect the onset of the superfluid
transition in the high- and BCS regimes. The number of transferred atoms,
as a function of the energy given by the probing field, peaks at the gap
energy. The shape of the peak is asymmetric due to the single particle
excitation gap. Since the excitation gap includes also a pseudogap
contribution, the asymmetry alone is not a signature of superfluidity.
Incoherent nature of the non-condensed pairs leads to broadening of the peak.
The pseudogap and therefore the broadening decay below the critical
temperature, causing a drastic increase in the asymmetry. This provides a
signature of the transition.Comment: Revised version, accepted to Phys. Rev. Letters. Figures changed,
explanations adde
Harnack's Inequality for Parabolic De Giorgi Classes in Metric Spaces
In this paper we study problems related to parabolic partial differential
equations in metric measure spaces equipped with a doubling measure and
supporting a Poincare' inequality. We give a definition of parabolic De Giorgi
classes and compare this notion with that of parabolic quasiminimizers. The
main result, after proving the local boundedness, is a scale and location
invariant Harnack inequality for functions belonging to parabolic De Giorgi
classes. In particular, the results hold true for parabolic quasiminimizers
Effect of a thin AlO_x layer on transition-edge sensor properties
We have studied the physics of transition-edge sensor (TES) devices with an
insulating AlOx layer on top of the device to allow implementation of more
complex detector geometries. By comparing devices with and without the
insulating film, we have observed significant additional noise apparently
caused by the insulator layer. In addition, AlOx was found to be a relatively
good thermal conductor. This adds an unforeseen internal thermal feature to the
system.Comment: 6 pages, 5 figures, Low Temperature Detectors 14 conferenc
Readout of solid-state charge qubits using a single-electron pump
A major difficulty in realizing a solid-state quantum computer is the
reliable measurement of the states of the quantum registers. In this paper, we
propose an efficient readout scheme making use of the resonant tunneling of a
ballistic electron produced by a single electron pump. We treat the measurement
interaction in detail by modeling the full spatial configuration, and show that
for pumped electrons with suitably chosen energy the transmission coefficient
is very sensitive to the qubit state. We further show that by using a short
sequence of pumping events, coupled with a simple feedback control procedure,
the qubit can be measured with high accuracy.Comment: 5 pages, revtex4, 4 eps figures. v2: published versio
Riesz potentials and nonlinear parabolic equations
The spatial gradient of solutions to nonlinear degenerate parabolic equations
can be pointwise estimated by the caloric Riesz potential of the right hand
side datum, exactly as in the case of the heat equation. Heat kernels type
estimates persist in the nonlinear cas
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