4,076 research outputs found
Induced superfluidity of imbalanced Fermi gases near unitarity
The induced intraspecies interactions among the majority species, mediated by
the minority species, is computed for a population-imbalanced two-component
Fermi gas. Although the Feshbach-resonance mediated interspecies interaction is
dominant for equal populations, leading to singlet s-wave pairing, we find that
in the strongly imbalanced regime the induced intraspecies interaction leads to
p-wave pairing and superfluidity of the majority species. Thus, we predict that
the observed spin-polaron Fermi liquid state in this regime is unstable to
p-wave superfluidity, in accordance with the results of Kohn and Luttinger,
below a temperature that, near unitarity, we find to be within current
experimental capabilities. Possible experimental signatures of the p-wave state
using radio-frequency spectroscopy as well as density-density correlations
after free expansion are presented.Comment: 15 pages, 13 figures, submitted to Phys. Rev.
Identity of electrons and ionization equilibrium
It is perhaps appropriate that, in a year marking the 90th anniversary of
Meghnad Saha seminal paper (1920), new developments should call fresh attention
to the problem of ionization equilibrium in gases. Ionization equilibrium is
considered in the simplest "physical" model for an electronic subsystem of
matter in a rarefied state, consisting of one localized electronic state in
each nucleus and delocalized electronic states considered as free ones. It is
shown that, despite the qualitative agreement, there is a significant
quantitative difference from the results of applying the Saha formula to the
degree of ionization. This is caused by the fact that the Saha formula
corresponds to the "chemical" model of matter.Comment: 9 pages, 2 figure
A bulk 2D Pauli Limited Superconductor
We present a nearly perfect Pauli-limited critical field phase diagram for
the anisotropic organic superconductor \-(ET)NH(SCN) when
the applied magnetic field is oriented parallel to the conducting layers. The
critical fields ({H_{c_2}) were found by use of penetration depth
measurements. Because {H_{c_2} is Pauli-limited, the size of the
superconducting energy gap can be calculated. The role of spin-orbit scattering
and many-body effects play a role in explaining our measurements.Comment: 4 pages, 5 figures. V5, corrections were made to the text, present
data was include
Zeeman effect in superconducting two-leg ladders: irrational magnetization plateaus and exceeding the Pauli limit
The effect of a parallel magnetic field on superconducting two-leg ladders is
investigated numerically. The magnetization curve displays an irrational
plateau at a magnetization equal to the hole density. Remarkably, its stability
is fundamentally connected to the existence of a well-known magnetic resonant
mode. Once the zero-field spin gap is suppressed by the field, pairs acquire a
finite momentum characteristic of a Fulde-Ferrell-Larkin-Ovchinnikov phase. In
addition, S^z=0 triplet superconducting correlations coexist with singlet ones
above the irrational plateau. This provides a simple mechanism in which the
Pauli limit is exceeded as suggested by recent experiments.Comment: 4 pages, 6 figure
Semiclassical theory of a quantum pump
In a quantum charge pump, the periodic variation of two parameters that
affect the phase of the electronic wavefunction causes the flow of a direct
current. The operating mechanism of a quantum pump is based on quantum
interference, the phases of interfering amplitudes being modulated by the
external parameters. In a ballistic quantum dot, there is a minimum time before
which quantum interference can not occur: the Ehrenfest time. Here we calculate
the current pumped through a ballistic quantum dot when the Ehrenfest time is
comparable to the mean dwell time. Remarkably, we find that the pumped current
has a component that is not suppressed if the Ehrenfest time is much larger
than the mean dwell time.Comment: 14 pages, 8 figures. Revised version, minor change
On the Lifshitz tail in the density of states of a superconductor with magnetic impurities
We argue that any superconductor with magnetic impurities is gapless due to a
Lifshitz tail in the density of states extending to zero energy. At low energy
the density of states remains finite. We show that fluctuations
in the impurity distribution produce regions of suppressed superconductivity,
which are responsible for the low energy density of states.Comment: 4 pages, uuencoded latex file + ps figure file
Angle-dependence of the Hall effect in HgBa2CaCu2O6 thin films
Superconducting compounds of the family Hg-Ba-Ca-Cu-O have been the subject
of intense study since the current record-holder for the highest critical
temperature of a superconductor belongs to this class of materials. Thin films
of the compound with two adjacent copper-oxide layers and a critical
temperature of about 120 K were prepared by a two-step process that consists of
the pulsed-laser deposition of precursor films and the subsequent annealing in
mercury-vapor atmosphere. Like some other high-temperature superconductors,
Hg-Ba-Ca-Cu-O exhibits a specific anomaly of the Hall effect, a double-sign
change of the Hall coefficient close to the superconducting transition. We have
investigated this phenomenon by measurements of the Hall effect at different
angles between the magnetic field direction and the crystallographic c-axis.
The results concerning the upper part of the transition, where the first sign
change occurs, are discussed in terms of the renormalized fluctuation model for
the Hall conductivity, adapted through the field rescaling procedure in order
to take into account the arbitrary orientation of the magnetic field.Comment: to be published in Phys. Rev.
High superconducting anisotropy and weak vortex pinning in Co doped LaFeAsO
Here, we present an electrical transport study in single crystals of
LaFeCoAsO ( K) under high magnetic fields. In
contrast to most of the previously reported Fe based superconductors, and
despite its relatively low , LaFeCoAsO shows a superconducting
anisotropy which is comparable to those seen for instance in the cuprates or
, where
is the effective mass anisotropy. Although, in the present case and as in all
Fe based superconductors, as . Under
the application of an external field, we also observe a remarkable broadening
of the superconducting transition particularly for fields applied along the
inter-planar direction. Both observations indicate that the low dimensionality
of LaFeCoAsO is likely to lead to a more complex vortex
phase-diagram when compared to the other Fe arsenides and consequently, to a
pronounced dissipation associated with the movement of vortices in a possible
vortex liquid phase. When compared to, for instance, F-doped compounds
pertaining to same family, we obtain rather small activation energies for the
motion of vortices. This suggests that the disorder introduced by doping
LaFeAsO with F is more effective in pinning the vortices than alloying it with
Co.Comment: 7 figures, 7 pages, Phys. Rev. B (in press
Quantum and classical resonant escapes of a strongly-driven Josephson junction
The properties of phase escape in a dc SQUID at 25 mK, which is well below
quantum-to-classical crossover temperature , in the presence of strong
resonant ac driving have been investigated. The SQUID contains two
Nb/Al-AlO/Nb tunnel junctions with Josephson inductance much larger than
the loop inductance so it can be viewed as a single junction having adjustable
critical current. We find that with increasing microwave power and at
certain frequencies and /2, the single primary peak in the
switching current distribution, \textrm{which is the result of macroscopic
quantum tunneling of the phase across the junction}, first shifts toward lower
bias current and then a resonant peak develops. These results are explained
by quantum resonant phase escape involving single and two photons with
microwave-suppressed potential barrier. As further increases, the primary
peak gradually disappears and the resonant peak grows into a single one while
shifting further to lower . At certain , a second resonant peak appears,
which can locate at very low depending on the value of . Analysis
based on the classical equation of motion shows that such resonant peak can
arise from the resonant escape of the phase particle with extremely large
oscillation amplitude resulting from bifurcation of the nonlinear system. Our
experimental result and theoretical analysis demonstrate that at ,
escape of the phase particle could be dominated by classical process, such as
dynamical bifurcation of nonlinear systems under strong ac driving.Comment: 10 pages, 9 figures, 1 tabl
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