9,775 research outputs found
Theory of the striped superconductor
We define a distinct phase of matter, a "pair density wave" (PDW), in which
the superconducting order parameter varies periodically as a function of
position such that when averaged over the center of mass position, all
components of vanish identically. Specifically, we study the simplest,
unidirectional PDW, the "striped superconductor," which we argue may be at the
heart of a number of spectacular experimental anomalies that have been observed
in the failed high temperature superconductor, La BaCuO. We
present a solvable microscopic model with strong electron-electron interactions
which supports a PDW groundstate. We also discuss, at the level of Landau
theory, the nature of the coupling between the PDW and other order parameters,
and the origins and some consequences of the unusual sensitivity of this state
to quenched disorder.Comment: 16 pages, 3 figures, 1 table; Journal ref. adde
Attractive Fermi gases with unequal spin populations in highly elongated traps
We investigate two-component attractive Fermi gases with imbalanced spin
populations in trapped one dimensional configurations. The ground state
properties are determined within local density approximation, starting from the
exact Bethe-ansatz equations for the homogeneous case. We predict that the
atoms are distributed according to a two-shell structure: a partially polarized
phase in the center of the trap and either a fully paired or a fully polarized
phase in the wings. The partially polarized core is expected to be a superfluid
of the FFLO type. The size of the cloud as well as the critical spin
polarization needed to suppress the fully paired shell, are calculated as a
function of the coupling strength.Comment: Final accepted versio
Fermi-liquid effects in the Fulde-Ferrell-Larkin-Ovchinnikov state of two-dimensional d-wave superconductors
We study the effects of Fermi-liquid interactions on quasi-two-dimensional
d-wave superconductors in a magnetic field. The phase diagram of the
superconducting state, including the periodic Fulde-Ferrell-Larkin-Ovchinnikov
(FFLO) state in high magnetic fields, is discussed for different strengths of
quasiparticle many-body interactions within Landau's theory of Fermi liquids.
Decreasing the Fermi-liquid parameter causes the magnetic spin
susceptibility to increase, which in turn leads to a reduction of the FFLO
phase. It is shown that a negative results in a first-order phase
transition from the normal to the uniform superconducting state in a finite
temperature interval. Finally, we discuss the thermodynamic implications of a
first-order phase transition for CeCoIn.Comment: published version; removed direct comparison with experiment for the
upper critical field, as required by the referee
Profiles of near-resonant population-imbalanced trapped Fermi gases
We investigate the density profiles of a partially polarized trapped Fermi
gas in the BCS-BEC crossover region using mean field theory within the local
density approximation. Within this approximation the gas is phase separated
into concentric shells. We describe how the structure of these shells depends
upon the polarization and the interaction strength. A Comparison with
experiments yields insight into the possibility of a polarized superfluid
phase.Comment: 4 pages, 5 Figures, Published versio
Finite-momentum Bose-Einstein condensates in shaken 2D square optical lattices
We consider ultracold bosons in a 2D square optical lattice described by the
Bose-Hubbard model. In addition, an external time-dependent sinusoidal force is
applied to the system, which shakes the lattice along one of the diagonals. The
effect of the shaking is to renormalize the nearest-neighbor hopping
coefficients, which can be arbitrarily reduced, can vanish, or can even change
sign, depending on the shaking parameter. It is therefore necessary to account
for higher-order hopping terms, which are renormalized differently by the
shaking, and introduce anisotropy into the problem. We show that the
competition between these different hopping terms leads to finite-momentum
condensates, with a momentum that may be tuned via the strength of the shaking.
We calculate the boundaries between the Mott-insulator and the different
superfluid phases, and present the time-of-flight images expected to be
observed experimentally. Our results open up new possibilities for the
realization of bosonic analogs of the FFLO phase describing inhomogeneous
superconductivity.Comment: 7 pages, 7 figure
Two-dimensional chiral crystals in the NJL model
We investigate the phase structure of the Nambu--Jona-Lasinio model at zero
temperature, allowing for a two-dimensional spatial dependence of the chiral
condensate. Applying the mean-field approximation, we consider various periodic
structures with rectangular and hexagonal geometries, and minimize the
corresponding free energy. We find that these two-dimensional chiral crystals
are favored over homogeneous phases in a certain window in the region where the
phase transition would take place when the analysis was restricted to
homogeneous condensates. It turns out, however, that in this regime they are
disfavored against a phase with a one-dimensional modulation of the chiral
condensate. On the other hand, we find that square and hexagonal lattices
eventually get favored at higher chemical potentials. Although stretching the
limits of the model to some extent, this would support predictions from
quarkyonic-matter studies.Comment: 12 pages, 6 figures. v2: added figure, small modifications, matches
published versio
Finite-Size Scaling Critical Behavior of Randomly Pinned Spin-Density Waves
We have performed Monte Carlo studies of the 3D model with random
uniaxial anisotropy, which is a model for randomly pinned spin-density waves.
We study simple cubic lattices, using values in the
range 16 to 64, and with random anisotropy strengths of = 1, 2, 3, 6
and . There is a well-defined finite temperature critical point, ,
for each these values of . We present results for the angle-averaged
magnetic structure factor, at for . We also use
finite-size scaling analysis to study scaling functions for the critical
behavior of the specific heat, the magnetization and the longitudinal magnetic
susceptibility. Good data collapse of the scaling functions over a wide range
of is seen for = 6 and . For our finite values of the scaled magnetization function increases with below , and
appears to approach an -independent limit for large . This suggests that
the system is ferromagnetic below .Comment: 21 pages in single column format, 20 .eps files, revised and
expanded, errors corrected, submitted to PR
Macroscopic quantum tunneling in "small" Josephson junctions in magnetic field
We study the phenomenon of macroscopic quantum tunneling (MQT) in small
Josephson junctions (JJ) with an externally applied magnetic field. The latter
results in the appearance of the Fraunhofer type modulation of the current
density along the barrier. The problem of MQT for a point-like JJ is reduced to
the motion of the quantum particle in the washboard potential. In the case of a
finite size JJ under consideration, this problem corresponds to a MQT in
potential which itself, besides the phase, depends on space variables. Finally,
the general expression for the crossover temperature T_0 between thermally
activated and macroscopic quantum tunneling regimes and the escaping time
tau_esc have been calculated
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