6,842 research outputs found
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
Cold Attractive Spin Polarized Fermi Lattice Gases and the Doped Positive U Hubbard Model
Experiments on polarized fermion gases performed by trapping ultracold atoms
in optical lattices, allow the study of an attractive Hubbard model for which
the strength of the on site interaction is tuned by means of a Feshbach
resonance. Using a well-known particle-hole transformation we discuss how
results obtained for this system can be reinterpreted in the context of a doped
repulsive Hubbard model. In particular we show that the
Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state corresponds to the striped state
of the two-dimensional doped positive U Hubbard model. We then use the results
of numerical studies of the striped state to relate the periodicity of the FFLO
state to the spin polarization. We also comment on the relationship of the
superconducting phase of the doped 2D repulsive Hubbard model to
a d-wave spin density wave state for the attractive case.Comment: 4 pages, 2 figure
Exotic superfluid states of lattice fermions in elongated traps
We present real-space dynamical mean-field theory calculations for
attractively interacting fermions in three-dimensional lattices with elongated
traps. The critical polarization is found to be 0.8, regardless of the trap
elongation. Below the critical polarization, we find unconventional superfluid
structures where the polarized superfluid and
Fulde-Ferrell-Larkin-Ovchinnikov-type states emerge across the entire core
region
Resonant pairing between Fermions with unequal masses
We study the pairing between Fermions of different masses, especially at the
unitary limit. At equal populations, the thermodynamic properties are identical
with the equal mass case provided an appropriate rescaling is made. At unequal
populations, for sufficiently light majority species, the system does not phase
separate. For sufficiently heavy majority species, the phase separated normal
phase have a density larger than that of the superfluid. For atoms in harmonic
traps, the density profiles for unequal mass Fermions can be drastically
different from their equal-mass counterparts.Comment: 10 pages, 4 figure
Quantitative Probe of Pairing Correlations in a Cold Fermionic Atom Gas
A quantitative measure of the pairing correlations present in a cold gas of
fermionic atoms can be obtained by studying the dependence of RF spectra on
hyperfine state populations. This proposal follows from a sum rule that relates
the total interaction energy of the gas to RF spectrum line positions. We argue
that this indicator of pairing correlations provides information comparable to
that available from the spin-susceptibility and NMR measurements common in
condensed-matter systems.Comment: 5 pages, 1 figur
Interplay between phase defects and spin polarization in the specific heat of the spin density wave compound (TMTTF)_2Br in a magnetic field
Equilibrium heat relaxation experiments provide evidence that the ground
state of the commensurate spin density wave (SDW) compound (TMTTF)Br after
the application of a sufficient magnetic field is different from the
conventional ground state. The experiments are interpreted on the basis of the
local model of strong pinning as the deconfinement of soliton-antisoliton pairs
triggered by the Zeeman coupling to spin degrees of freedom, resulting in a
magnetic field induced density wave glass for the spin carrying phase
configuration.Comment: 4 pages, 5 figure
Superfluid stability in BEC-BCS crossover
We consider a dilute atomic gas of two species of fermions with unequal
concentrations under a Feshbach resonance. We find that the system can have
distinct properties due to the unbound fermions. The uniform state is stable
only when either (a) beyond a critical coupling strength, where it is a gapless
superfluid, or (b) when the coupling strength is sufficiently weak, where it is
a normal Fermi gas mixture. Phase transition(s) must therefore occur when the
resonance is crossed.Comment: 4 pages, 4 figure
Vortex-assisted photon counts and their magnetic field dependence in single-photon detectors
We argue that photon counts in a superconducting nanowire single-photon
detector (SNSPD) are caused by the transition from a current-biased metastable
superconducting state to the normal state. Such a transition is triggered by
vortices crossing the thin film superconducting strip from one edge to another
due to the Lorentz force. Detector counts in SNSPDs may be caused by three
processes: (a) a single incident photon with energy sufficient to break enough
Cooper pairs to create a normal-state belt across the entire width of the strip
(direct photon count), (b) thermally induced single-vortex crossing in the
absence of photons (dark count), which at high bias currents releases the
energy sufficient to trigger the transition to the normal state in a belt
across the whole width of the strip, and (c) a single incident photon with
insufficient energy to create a normal-state belt but initiating a subsequent
single-vortex crossing, which provides the rest of the energy needed to create
the normal-state belt (vortex-assisted single photon count). We derive the
current dependence of the rate of vortex-assisted photon counts. The resulting
photon count rate has a plateau at high currents close to the critical current
and drops as a power-law with high exponent at lower currents. While the
magnetic field perpendicular to the film plane does not affect the formation of
hot spots by photons, it causes the rate of vortex crossings (with or without
photons) to increase. We show that by applying a magnetic field one may
characterize the energy barrier for vortex crossings and identify the origin of
dark counts and vortex-assisted photon counts.Comment: 9 pages, 8 figures [v3: added extensive discussion of boundary
condition of Fokker-Planck equation and magnitude of vortex crossing rate
Interplay between disorder, quantum and thermal fluctuations in ferromagnetic alloys: The case of UCu2Si(2-x)Ge(x)
We consider, theoretically and experimentally, the effects of structural
disorder, quantum and thermal fluctuations in the magnetic and transport
properties of certain ferromagnetic alloys.We study the particular case of
UCu2Si(2-x)Ge(x). The low temperature resistivity, rho(T,x), exhibits Fermi
liquid (FL) behavior as a function of temperature T for all values of x, which
can be interpreted as a result of the magnetic scattering of the conduction
electrons from the localized U spins. The residual resistivity, rho(0,x),
follows the behavior of a disordered binary alloy. The observed non-monotonic
dependence of the Curie temperature, Tc(x), with x can be explained within a
model of localized spins interacting with an electronic bath whose transport
properties cross-over from ballistic to diffusive regimes. Our results clearly
show that the Curie temperature of certain alloys can be enhanced due to the
interplay between quantum and thermal fluctuations with disorder.Comment: 4 pages, 3 figures, to appear in Phys. Rev. Let
Local electronic structure near oxygen dopants in BSCCO-2212: a window on the high-Tc pair mechanism?
The cuprate material BSCCO-2212 is believed to be doped by a combination of
cation switching and excess oxygen. The interstitial oxygen dopants are of
particular interest because scanning tunnelling microscopy (STM) experiments
have shown that they are positively correlated with the local value of the
superconducting gap, and calculations suggest that the fundamental attraction
between electrons is modulated locally. In this work, we use density functional
theory to try to ascertain which locations in the crystal are energetically
most favorable for the O dopant atoms, and how the surrounding cage of atoms
deforms. Our results provide support for the identification of STM resonances
at -1eV with dopant interstitial O atoms, and show how the local electronic
structure is modified nearby.Comment: 5 pages, 3 figure
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