3,464 research outputs found
Geometry-induced memory effects in isolated quantum systems: Observations and applications
Memory effects can lead to history-dependent behavior of a system, and they
are ubiquitous in our daily life and have broad applications. Here we explore
possibilities of generating memory effects in simple isolated quantum systems.
By utilizing geometrical effects from a class of lattices supporting flat-bands
consisting of localized states, memory effects could be observed in ultracold
atoms in optical lattices. As the optical lattice continuously transforms from
a triangular lattice into a kagome lattice with a flat band, history-dependent
density distributions manifest quantum memory effects even in noninteracting
systems, including fermionic as well as bosonic systems in the proper ranges of
temperatures. Rapid growth in ultracold technology predicts a bright future for
quantum memory-effect systems, and here two prototypical applications of
geometry-induced quantum memory effects are proposed: An accelerometer
recording the mechanical change rate in a coupled system and a rate-controlled
memvalve where the rate of ramping the lattice potential acts as a control of
the remnant density in the lattice.Comment: 13 pages, 11 figures, update figures and references. We provided one
more application - quantum memory effects atomic memory (QMEAM
Quantification of the memory effect of steady-state currents from interaction-induced transport in quantum systems
Dynamics of a system in general depends on its initial state and how the
system is driven, but in many-body systems the memory is usually averaged out
during evolution. Here, interacting quantum systems without external
relaxations are shown to retain long-time memory effects in steady states. To
identify memory effects, we first show quasi-steady state currents form in
finite, isolated Bose and Fermi Hubbard models driven by interaction imbalance
and they become steady-state currents in the thermodynamic limit. By comparing
the steady state currents from different initial states or ramping rates of the
imbalance, long-time memory effects can be quantified. While the memory effects
of initial states are more ubiquitous, the memory effects of switching
protocols are mostly visible in interaction-induced transport in lattices. Our
simulations suggest the systems enter a regime governed by a generalized Fick's
law and memory effects lead to initial-state dependent diffusion coefficients.
We also identify conditions for enhancing memory effects and discuss possible
experimental implications.Comment: 11 pages, 11 figures, publish versio
Fermions with attractive interactions on optical lattices and implications for correlated systems
In this paper we address the behavior of the superfluid transition
temperature in the attractive Hubbard model. We study systematically the
effects of pairing fluctuations and address all filling fractions over the
entire range of attractive interaction strength. While the attractive Hubbard
model can be regarded as the generalization of BCS to Bose Einstein
condensation (BEC) crossover to a lattice, we find that the BEC limit of this
Hubbard model is very different from that of jellium, owing to the strong
inter-site repulsion between pairs, which becomes important near half filling
when the on-site attraction is strong. A central conclusion of our work is that
in a lattice, around half filling, the smooth evolution from the BCS to the BEC
limits is interrupted. For the attractive Hubbard model, vanishes when
the system approaches the bosonic regime with increasing interaction strength.
We suggest that the vanishing of at strong coupling strength may signal a
quantum critical transition to another form of superfluid not continuously
connected to a BCS-like phase. We present a simple variational ansatz for the
ground state in this more strongly coupled superfluid. We further generalize
the (s-wave) Hubbard model to d-wave pairing and address issues of potential
relevance to high temperature superconductors. For the d-wave case, we present
a phase diagram and show that here too, one observes a vanishing of when
the pairing onset temperature becomes sufficiently large. We suggest that
future experiments on ultracold fermions in optical lattices should not be
exclusively limited to the repulsive Hubbard model, but should address the
attractive model in order to elucidate features of high temperature
superconductivity.Comment: 10 pages, 5 figure
Hysteresis of noninteracting and spin-orbit coupled atomic Fermi gases with relaxation
Hysteresis can be found in driven many-body systems such as magnets and
superfluids. Rate-dependent hysteresis arises when a system is driven
periodically while relaxing towards equilibrium. A two-state paramagnet driven
by an oscillating magnetic field in the relaxation approximation clearly
demonstrates rate-dependent hysteresis. A noninteracting atomic Fermi gas in an
optical ring potential, when driven by a periodic artificial gauge field and
subjected to dissipation, is shown to exhibit hysteresis loops of atomic
current due to a competition of the driving time and the relaxation time. This
is in contrast to electronic systems exhibiting equilibrium persistent current
driven by magnetic flux due to rapid relaxation. Universal behavior of the
dissipated energy in one hysteresis loop is observed in both the magnetic and
atomic systems, showing linear and inverse-linear dependence on the relaxation
time in the strong and weak dissipation regimes. While interactions in general
invalidate the framework for rate-dependent hysteresis, an atomic Fermi gas
with artificial spin-orbit coupling exhibits hysteresis loops of atomic
currents. Cold-atoms in ring-shape potentials are thus promising in
demonstrating rate-dependent hysteresis and its associated phenomena.Comment: 11 pages, 5 figure
Quantum phase transitions in superconductor--quantum-dot--superconductor Josephson structures with attractive intradot interaction
We theoretically study the superconducting proximity effect in a quantum dot
coupled to two superconducting leads when the intradot interaction between
electrons is made attractive. Because of the superconducting proximity effect,
the electronic states for the embedded quantum dot are either spin-polarized
states with an odd occupation number or BCS-like states with an even occupation
number. We show that in the presence of an external magnetic field, the system
can exhibit quantum phase transitions of fermion parity associated with the
occupation number. In this work, we adopt a self-consistent theoretical method
to extend our considerations beyond the so-called superconducting atomic limit
in which the superconducting gap for the leads is assumed to be the largest
energy scale. The method enables us to numerically investigate the electronic
structure of the dot as results of the attractive interaction. For energy phase
diagrams in the regime away from the atomic limit, we find a reentrant behavior
where a BCS-like phase of the dot exists in an intermediate range of the
hybridization strength between the quantum dot and the leads. We also consider
Josephson current phase relations and identify a number of examples showing
phase transitions that may offer important switching effects
Ground State Description of a Single Vortex in an Atomic Fermi gas: From BCS to Bose-Einstein Condensation
We use a Bogoliubov-de Gennes (BdG) formulation to describe a single vortex
in a neutral fermionic gas. It is presumed that the attractive pairing
interaction can be arbitrarily tuned to exhibit a crossover from BCS to
Bose-Einstein condensation. Our starting point is the BCS-Leggett mean field
ground state for which a BdG approach is microscopically justified. At strong
coupling, we demonstrate that this approach is analytically equivalent to the
Gross-Pitaevskii description of vortices in true bosonic systems. We analyze
the sizable density depletion found for the unitary regime and relate it to the
presence of unoccupied (positive energy) quasi-bound states at the core center.Comment: 4 pages 3 figure
Finite temperature effects in trapped Fermi gases with population imbalance
We study the finite temperature behavior of trapped Fermi gases as they
undergo BCS-Bose Einstein condensation (BEC) crossover, in the presence of a
population imbalance. Our results, in qualitative agreement with recent
experiments, show how the superfluid phase transition is directly reflected in
the particle density profiles. We demonstrate that at and in the
near-BEC and unitary regimes, the polarization is excluded from the superfluid
core. Nevertheless a substantial polarization fraction is carried by a normal
region of the trap having strong pair correlations, which we associate with
noncondensed pairs or the ``pseudogap phase''
Single-plane-wave Larkin-Ovchinnikov-Fulde-Ferrell state in BCS--Bose-Einstein condensation crossover
We study the single-plane-wave Larkin-Ovchinnikov-Fulde-Ferrell (LOFF) states
for BCS--Bose-Einstein condensation (BEC) crossover at general temperatures
. Because we include the important effects of noncondensed pairs, our phase diagrams are different from those reported in earlier work. We
find that generalized LOFF phases may be the ground state for a wide range of
(weak through moderately strong) interactions, including the unitary regime.
However, these LOFF phases are readily destroyed by non-zero .Comment: 4 pages, 3 figures, some discussions are revise
Theory of Superfluids with Population Imbalance: Finite Temperature and BCS-BEC Crossover Effects
In this paper we present a very general theoretical framework for addressing
fermionic superfluids over the entire range of BCS to Bose Einstein
condensation (BEC) crossover in the presence of population imbalance or spin
polarization. Our emphasis is on providing a theory which reduces to the
standard zero temperature mean field theories in the literature, but
necessarily includes pairing fluctuation effects at non-zero temperature within
a consistent framework. Physically, these effects are associated with the
presence of pre-formed pairs (or a fermionic pseudogap) in the normal phase,
and pair excitations of the condensate, in the superfluid phase. We show how
this finite theory of fermionic pair condensates bears many similarities to
the condensation of point bosons. In the process we examine three different
types of condensate: the usual breached pair or Sarma phase and both the one
and two plane wave Larkin- Ovchinnikov, Fulde-Ferrell (LOFF) states. The last
of these has been discussed in the literature albeit only within a
Landau-Ginzburg formalism, generally valid near . Here we show how to
arrive at the two plane wave LOFF state in the ground state as well as at
general temperature .Comment: 15 pages, 5 figure
Intermediate temperature superfluidity in an atomic Fermi gas with population imbalance
We derive the underlying finite temperature theory which describes Fermi gas
superfluidity with population imbalance in a homogeneous system. We compute the
pair formation temperature and superfluid transition temperature and
superfluid density in a manner consistent with the standard ground state
equations, and thereby present a complete phase diagram. Finite temperature
stabilizes superfluidity, as manifested by two solutions for , or by low
instabilities. At unitarity the polarized state is an ``intermediate
temperature superfluid".Comment: Replace with the published versio
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