33 research outputs found
Flow and critical velocity of an imbalanced Fermi gas through an optical potential
Optical lattices offer the possibility to investigate the superfluid
properties of both Bose condensates and Fermionic superfluid gases. When a
population imbalance is present in a Fermi mixture, this leads to frustration
of the pairing, and the superfluid properties will be affected. In this
contribution, the influence of imbalance on the flow of a Fermi superfluid
through an optical lattice is investigated. The flow through the lattice is
analysed by taking into account coupling between neighbouring layers of the
optical lattice up to second order in the interlayer tunneling amplitude for
single atoms. The critical velocity of flow through the lattice is shown to
decrease monotonically to zero as the imbalance is increased to 100%.
Closed-form analytical expressions are given for the tunneling contribution to
the action and for the critical velocity as a function of the binding energy of
pairs in the (quasi) two-dimensional Fermi superfluid and as a function of the
imbalance.Comment: 8 pages, 1 figure, contribution for the QFS 2007 conferenc
Homogeneous Fermion Superfluid with Unequal Spin Populations
For decades, the conventional view is that an s-wave BCS superfluid can not
support uniform spin polarization due to a gap in the quasiparticle
excitation spectrum. We show that this is an artifact of the dismissal of
quasiparticle interactions in the conventional approach at the
outset. Such interactions can cause triplet fluctuations in the ground state
and hence non-zero spin polarization at "magnetic field" . The
resulting ground state is a pairing state of quasiparticles on the ``BCS
vacuum". For sufficiently large , the spin polarization of at unitarity
has the simple form . Our study is motivated by the recent
experiments at Rice which found evidence of a homogenous superfluid state with
uniform spin polarization.Comment: 4 pages, 3 figure
Superconductive proximity effect in interacting disordered conductors
We present a general theory of the superconductive proximity effect in
disordered normal--superconducting (N-S) structures, based on the recently
developed Keldysh action approach. In the case of the absence of interaction in
the normal conductor we reproduce known results for the Andreev conductance G_A
at arbitrary relation between the interface resistance R_T and the diffusive
resistance R_D. In two-dimensional N-S systems, electron-electron interaction
in the Cooper channel of normal conductor is shown to strongly affect the value
of G_A as well as its dependence on temperature, voltage and magnetic field. In
particular, an unusual maximum of G_A as a function of temperature and/or
magnetic field is predicted for some range of parameters R_D and R_T. The
Keldysh action approach makes it possible to calculate the full statistics of
charge transfer in such structures. As an application of this method, we
calculate the noise power of an N-S contact as a function of voltage,
temperature, magnetic field and frequency for arbitrary Cooper repulsion in the
normal metal and arbitrary values of the ratio R_D/R_T.Comment: RevTeX, 28 pages, 18 PostScript figures; added and updated reference
Theory of charge transport in diffusive normal metal / unconventional singlet superconductor contacts
We analyze the transport properties of contacts between unconventional
superconductor and normal diffusive metal in the framework of the extended
circuit theory. We obtain a general boundary condition for the Keldysh-Nambu
Green's functions at the interface that is valid for arbitrary transparencies
of the interface. This allows us to investigate the voltage-dependent
conductance (conductance spectrum) of a diffusive normal metal (DN)/
unconventional singlet superconductor junction in both ballistic and diffusive
cases. For d-wave superconductor, we calculate conductance spectra numerically
for different orientations of the junctions, resistances, Thouless energies in
DN, and transparencies of the interface. We demonstrate that conductance
spectra exhibit a variety of features including a -shaped gap-like
structure, zero bias conductance peak (ZBCP) and zero bias conductance dip
(ZBCD). We show that two distinct mechanisms: (i) coherent Andreev reflection
(CAR) in DN and (ii) formation of midgap Andreev bound state (MABS) at the
interface of d-wave superconductors, are responsible for ZBCP, their relative
importance being dependent on the angle between the interface normal
and the crystal axis of d-wave superconductors. For , the ZBCP is due
to CAR in the junctions of low transparency with small Thouless energies, this
is similar to the case of diffusive normal metal / insulator /s-wave
superconductor junctions. With increase of from zero to , the
MABS contribution to ZBCP becomes more prominent and the effect of CAR is
gradually suppressed. Such complex spectral features shall be observable in
conductance spectra of realistic high- junctions at very low temperature
Theory of charge transport in diffusive normal metal / conventional superconductor point contacts
Tunneling conductance in diffusive normal metal / insulator / s-wave
superconductor (DN/I/S) junctions is calculated for various situations by
changing the magnitudes of the resistance and Thouless energy in DN and the
transparency of the insulating barrier. The generalized boundary condition
introduced by Yu. Nazarov [Superlattices and Microstructures 25 1221 (1999)] is
applied, where the ballistic theory by Blonder Tinkham and Klapwijk (BTK) and
the diffusive theory by Volkov Zaitsev and Klapwijk based on the boundary
condition of Kupriyanov and Lukichev (KL) are naturally reproduced. It is shown
that the proximity effect can enhance (reduce) the tunneling conductance for
junctions with a low (high) transparency. A wide variety of dependencies of
tunneling conductance on voltage bias is demonstrated including a -shaped
gap like structure, a zero bias conductance peak (ZBCP) and a zero bias
conductance dip (ZBCD)
Magnetization and dimerization profiles of the cut two-leg spin ladder and spin-1 chain
The physical properties of the edge states of the cut two-leg spin ladder are
investigated by means of the bosonization approach. By carefully treating
boundary conditions, we derive the existence of spin-1/2 edge states in the
spin ladder with a ferromagnetic rung exchange and for the open spin-1
Heisenberg chain. In contrast, such states are absent in the antiferromagnetic
rung coupling case. The approach, based on a mapping onto decoupled
semi-infinite off-critical Ising models, allows us to compute several physical
quantities of interest. In particular, we determine the magnetization and
dimerization profiles of the cut two-leg spin ladder and of the open
biquadratic spin-1 chain in the vicinity of the SU(2) WZNW critical point.Comment: RevTeX 4, no figure, 26 page
Para-infectious brain injury in COVID-19 persists at follow-up despite attenuated cytokine and autoantibody responses
To understand neurological complications of COVID-19 better both acutely and for recovery, we measured markers of brain injury, inflammatory mediators, and autoantibodies in 203 hospitalised participants; 111 with acute sera (1–11 days post-admission) and 92 convalescent sera (56 with COVID-19-associated neurological diagnoses). Here we show that compared to 60 uninfected controls, tTau, GFAP, NfL, and UCH-L1 are increased with COVID-19 infection at acute timepoints and NfL and GFAP are significantly higher in participants with neurological complications. Inflammatory mediators (IL-6, IL-12p40, HGF, M-CSF, CCL2, and IL-1RA) are associated with both altered consciousness and markers of brain injury. Autoantibodies are more common in COVID-19 than controls and some (including against MYL7, UCH-L1, and GRIN3B) are more frequent with altered consciousness. Additionally, convalescent participants with neurological complications show elevated GFAP and NfL, unrelated to attenuated systemic inflammatory mediators and to autoantibody responses. Overall, neurological complications of COVID-19 are associated with evidence of neuroglial injury in both acute and late disease and these correlate with dysregulated innate and adaptive immune responses acutely
Care of the adult cancer patient at the end of life: ESMO Clinical Practice Guidelines.
ESMO guideline on best clinical practice in the management of patients at end-of-lif