6,659 research outputs found
Universal phase diagram of a strongly interacting Fermi gas with unbalanced spin populations
We present a theoretical interpretation of a recent experiment presented in
ref. \cite{Zwierlein06} on the density profile of Fermi gases with unbalanced
spin populations. We show that in the regime of strong interaction, the
boundaries of the three phases observed in \cite{Zwierlein06} can be
characterized by two dimensionless numbers and .
Using a combination of a variational treatment and a study of the experimental
results, we infer rather precise bounds for these two parameters.Comment: 4 pages, 2 figure
Pairing states of a polarized Fermi gas trapped in a one-dimensional optical lattice
We study the properties of a one-dimensional (1D) gas of fermions trapped in
a lattice by means of the density matrix renormalization group method, focusing
on the case of unequal spin populations, and strong attractive interaction. In
the low density regime, the system phase-separates into a well defined
superconducting core and a fully polarized metallic cloud surrounding it. We
argue that the superconducting phase corresponds to a 1D analogue of the
Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state, with a quasi-condensate of
tightly bound bosonic pairs with a finite center-of-mass momentum that scales
linearly with the magnetization. In the large density limit, the system allows
for four phases: in the core, we either find a Fock state of localized pairs or
a metallic shell with free spin-down fermions moving in a fully filled
background of spin-up fermions. As the magnetization increases, the Fock state
disappears to give room for a metallic phase, with a partially polarized
superconducting FFLO shell and a fully polarized metallic cloud surrounding the
core.Comment: 4 pages, 5 fig
Spectral properties of a partially spin-polarized one-dimensional Hubbard/Luttinger superfluid
We calculate the excitation spectra of a spin-polarized Hubbard chain away
from half-filling, using a high-precision momentum-resolved time-dependent
Density Matrix Renormalization Group method. Focusing on the U<0 case, we
present in some detail the single-fermion, pair, density and spin spectra, and
discuss how spin-charge separation is altered for this system. The pair spectra
show a quasi-condensate at a nonzero momentum proportional to the polarization,
as expected for this Fulde-Ferrel-Larkin-Ovchinnikov-like superfluid.Comment: 4 pages, 3 low resolution color fig
Emergent Antiferromagnetism in D-wave Superconductor with Strong Paramagnetic Pair-Breaking
It is theoretically shown that, in the four-fold symmetric d-wave
superconducting phase, a paramagnetic pair-breaking (PPB) enhanced sufficiently
by increasing the applied magnetic field induces not only the
Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconducting state but also an
incommensurate antiferromagnetic (AFM) order with Q-vector parallel to a gap
node. This AFM ordering tends to occur only below H_{c2} at low temperatures,
i.e., in the presence of a nonvanishing superconducting energy gap
rather than in the normal phase. Through a detailed study on the resulting AFM
order and its interplay with the FFLO spatial modulation of , it is
argued that the strange high field and low temperature (HFLT) superconducting
phase of CeCoIn_5 is a coexisting phase of the FFLO and incommensurate AFM
orders, and that this PPB mechanism of an AFM ordering is also the origin of
the AFM quantum critical fluctuation which has occurred close to H_{c2}(0) in
several unconventional superconductors including CeCoIn_5.Comment: 22 pages, 12 figures.2 references and related comnments are
added.Accepted for publication in Phys. Rev.
Magnetic field dependence of the superconducting gap node topology in non-centrosymmetric CePtSi
The non-centrosymmetric superconductor CePtSi is believed to have a line
node in the energy gap arising from coexistence of s-wave and p-wave pairing.
We show that a weak c-axis magnetic field will remove this line node, since it
has no topological stability against time-reversal symmetry breaking
perturbations. Conversely a field in the plane is shown to remove the
line node on some regions of the Fermi surface, while bifurcating the line node
in other directions, resulting in two 'boomerang'-like shapes. These line node
topological changes are predicted to be observable experimentally in the low
temperature heat capacity.Comment: 4 pages, 3 figure
Upper critical field from normal state fluctuations in BiSrCuO
The in-plane magnetoresistance of an epitaxial BiSrCuO
thin film was systematically investigated as a function of doping, above .
The orbital magnetoconductance is used to extract the crossover field line
in the fluctuation regime. This field is found in good agreement
with the upper critical field obtained from resistivity data below , and
exhibits a similar upward curvature, thus pointing toward the existence of a
critical correlation length. The consequences regarding the nature of the
resistive transition are discussed
Controlling the Sign of Magnetoconductance in Andreev Quantum Dots
We construct a theory of coherent transport through a ballistic quantum dot
coupled to a superconductor. We show that the leading-order quantum correction
to the two-terminal conductance of these Andreev quantum dots may change sign
depending on (i) the number of channels carried by the normal leads or (ii) the
magnetic flux threading the dot. In contrast, spin-orbit interaction may affect
the magnitude of the correction, but not always its sign. Experimental
signatures of the effect include a non-monotonic magnetoconductance curve and a
transition from an insulator-like to a metal-like temperature dependence of the
conductance. Our results are applicable to ballistic or disordered dots.Comment: Final version (4pages 3figs)- improved presentation and fig 3, and
updated reference
Conventional and charge six superfluids from melting hexagonal Fulde-Ferrell-Larkin-Ovchinnikov phases in two dimensions
We consider defect mediated melting of Fulde-Ferrell-Larkin-Ovchinnikov
(FFLO) and pair density wave (PDW) phases in two dimensions. Examining
mean-field ground states in which the spatial oscillations of the FFLO/PDW
superfluid order parameter exhibit hexagonal lattice symmetry, we find that
thermal melting leads to a variety of novel phases. We find that a spatially
homogeneous charge six superfluid can arise from melting a hexagonal
vortex-anitvortex lattice FFLO/PDW phase. The charge six superfluid has an
order parameter corresponding to a bound state of six fermions. We further find
that a hexagonal vortex-free FFLO/PDW phase can melt to yield a conventional
(charge two) homogeneous superfluid. A key role is played by topological
defects that combine fractional vortices of the superfluid order and fractional
dislocations of the lattice order.Comment: 8 pages, 3 figure
Coherent oscillations in a superconducting multi-level quantum system
We have observed coherent time evolution of states in a multi-level quantum
system, formed by a current-biased dc SQUID. The manipulation of the quantum
states is achieved by resonant microwave pulses of flux. The number of quantum
states participating in the coherent oscillations increases with increasing
microwave power. Quantum measurement is performed by a nanosecond flux pulse
which projects the final state onto one of two different voltage states of the
dc SQUID, which can be read out
Long-time behavior of the momentum distribution during the sudden expansion of a spin-imbalanced Fermi gas in one dimension
We study the sudden expansion of spin-imbalanced ultracold lattice fermions
with attractive interactions in one dimension after turning off the
longitudinal confining potential. We show that the momentum distribution
functions of majority and minority fermions approach stationary values quickly
due to a quantum distillation mechanism that results in a spatial separation of
pairs and majority fermions. As a consequence, Fulde-Ferrell-Larkin-Ovchinnikov
(FFLO) correlations are lost during the expansion. Furthermore, we argue that
the shape of the stationary momentum distribution functions can be understood
by relating them to the integrals of motion in this integrable quantum system.
We discuss our results in the context of proposals to observe FFLO
correlations, related to recent experiments by Liao et al., Nature 467, 567
(2010).Comment: 8 pages including supplementary material, 9 eps figures, revised
version as published, some text moved to the supplemental materia
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