5,112 research outputs found
Bose-Einstein Condensates in Strongly Disordered Traps
A Bose-Einstein condensate in an external potential consisting of a
superposition of a harmonic and a random potential is considered theoretically.
From a semi-quantitative analysis we find the size, shape and excitation
energy as a function of the disorder strength. For positive scattering length
and sufficiently strong disorder the condensate decays into fragments each of
the size of the Larkin length . This state is stable over a large
range of particle numbers. The frequency of the breathing mode scales as
. For negative scattering length a condensate of size
may exist as a metastable state. These finding are generalized to anisotropic
traps
How many phases meet at the chiral critical point?
We explore the phase diagram of NJL-type models near the chiral critical
point allowing for phases with spatially inhomogeneous chiral condensates. In
the chiral limit it turns out that the region in the mean-field phase diagram
where those phases are energetically preferred very generically reaches out to
the chiral critical point. The preferred inhomogeneous ground state in this
vicinity possibly resembles a lattice of domain wall solitons. This raises the
question of their relevance for the phase diagram of QCD.Comment: 7 pages, 1 figure; v2: minor corrections, as published in PR
Recovering of superconductivity in exchange fields exceeding Pauli limiting field under spin-dependent quasiparticle distribution
We study theoretically the simultaneous influence of spin accumulation
potential and the Zeeman exchange field on singlet
superconductivity. It is shown that the pair-breaking effect of the Zeeman
field can be fully compensated by creation of the appropriate spin accumulation
potential in the superconductor. Moreover, superconductivity can be recovered
for exchange fields well exceeding the Pauli limiting field. It is proposed
that the effect can be experimentally realized on the basis of voltage biased
junction consisting of a thin superconducting film sandwiched between two half
metals.Comment: 4 pages, 1 figure, published versio
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
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
Suppression or enhancement of the Fulde-Ferrell-Larkin-Ovchinnikov order in a one-dimensional optical lattice with particle correlated tunnelling
We study through controlled numerical simulation the ground state properties
of spin-polarized strongly interacting fermi gas in an anisotropic optical
lattice, which is described by an effective one-dimensional general Hubbard
model with particle correlated hopping rate. We show that the
Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) type of state, while enhanced by a
negative correlated hopping rate, can be completely suppressed by positive
particle correlated hopping, yielding to an unusual magnetic phase even for
particles with on-site attractive interaction We also find several different
phase separation patterns for these atoms in an inhomogeneous harmonic trap,
depending on the correlated hopping rate
Two-bands effect on the superconducting fluctuating diamagnetism in MgB₂
The field dependence of the magnetization above the transition temperature Tc
in MgB₂ is shown to evidence a diamagnetic contribution consistent with
superconducting fluctuations reflecting both the σ and π bands. In
particular, the upturn field Hup in the magnetization curve, related to the
incipient effect of the magnetic field in quenching the fluctuating pairs,
displays a double structure, in correspondence to two correlation lengths. The
experimental findings are satisfactorily described by the extension to the
diamagnetism of a recent theory for paraconductivity, in the framework of a
zero-dimensional model for the fluctuating superconducting droplets above Tc
Transport properties of clean and disordered Josephson junction arrays
We investigate the influence of quantum fluctuations and weak disorder on the
vortex dynamics in a two-dimensional superconducting
Berezinskii-Kosterlitz-Thouless system. The temperature below which quantum
fluctuations dominate the vortex creep is determined, and the transport in this
quantum regime is described. The crossover from quantum to classical regime is
discussed and the quantum correction to the classical current-voltage relation
is determined. It is found that weak disorder can effectively reduce the
critical current as compared to that in the clean system.Comment: 4 pages, 2 figure
Localization and Interaction Effects in Strongly Underdoped La2-xSrxCuO4
The in-plane magnetoresistance (MR) in La2-xSrxCuO4 films with 0.03 < x <
0.05 has been studied in the temperature range 1.6 K to 100 K, and in magnetic
fields up to 14 T, parallel and perpendicular to the CuO2 planes. The behavior
of the MR is consistent with a predominant influence of interaction effects at
high temperatures, switching gradually to a regime dominated by spin scattering
at low T. Weak localization effects are absent. A positive orbital MR appears
close to the boundary between the antiferromagnetic and the spin-glass phase,
suggesting the onset of Maki-Thompson superconducting fluctuations deep inside
the insulating phase.Comment: 4 pages, 3 figures, to appear in Phys. Rev. Letter
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