90 research outputs found
Non-Fermi liquid fixed point for an imbalanced gas of fermions in dimensions
We consider a gas of two species of fermions with population imbalance. Using
the renormalization group in dimensions, we show that for
spinless fermions and a fixed point appears at finite attractive
coupling where the quasiparticle residue vanishes, and identify this with the
transition to Larkin--Ovchinnikov--Fulde--Ferrell order (inhomogeneous
superconductivity). When the two species of fermions also carry spin degrees of
freedom we find a fixed point indicating a transition to spin density wave
order.Comment: 4 pages and 4 figure
Particle correlations in a fermi superfluid
We discuss correlations between particles of different momentum in a
superfluid fermi gas, accessible through noise measurements of absorption
images of the expanded gas. We include two elements missing from the simplest
treatment, based on the BCS wavefunction: the explicit use of a conserving
approximation satisfying particle number conservation, and the inclusion of the
contribution from Cooper pairs at finite momentum. We expect the latter to be a
significant issue in the strongly correlated state emerging in the BCS-BEC
crossover.Comment: Published versio
Relaxation, pre-thermalization and diffusion in a noisy Quantum Ising Chain
We study the dynamics of thermalization resulting from a time-dependent noise
in a Quantum Ising Chain subject to a sudden quench of the transverse magnetic
field. For weak noise the dynamics shows a pre-thermalized state at
intermediate time scales, eventually drifting towards an asymptotic infinite
temperature steady state characterized by diffusive behavior. By computing
analytically the density of kinks, as well as the transverse and longitudinal
magnetic field correlators, we characterize these two regimes, their
observability and their signatures in the various physical quantities.Comment: 5 pages, 2 figures. Accepted for publication in PRB Rapid
Communication
Quantum Brownian motion in a quasiperiodic potential
We consider a quantum particle subject to Ohmic dissipation, moving in a
bichromatic quasiperiodic potential. In a periodic potential the particle
undergoes a zero-temperature localization-delocalization transition as
dissipation strength is decreased. We show that the delocalized phase is absent
in the quasiperiodic case, even when the deviation from periodicity is
infinitesimal. Using the renormalization group, we determine how the effective
localization length depends on the dissipation. We show that {a similar problem
can emerge in} the strong-coupling limit of a mobile impurity moving in a
periodic lattice and immersed in a one-dimensional quantum gas.Comment: 5+6 pages, 1 figur
Geometry of quantum observables and thermodynamics of small systems
The concept of ergodicity---the convergence of the temporal averages of
observables to their ensemble averages---is the cornerstone of thermodynamics.
The transition from a predictable, integrable behavior to ergodicity is one of
the most difficult physical phenomena to treat; the celebrated KAM theorem is
the prime example. This Letter is founded on the observation that for many
classical and quantum observables, the sum of the ensemble variance of the
temporal average and the ensemble average of temporal variance remains constant
across the integrability-ergodicity transition.
We show that this property induces a particular geometry of quantum
observables---Frobenius (also known as Hilbert-Schmidt) one---that naturally
encodes all the phenomena associated with the emergence of ergodicity: the
Eigenstate Thermalization effect, the decrease in the inverse participation
ratio, and the disappearance of the integrals of motion. As an application, we
use this geometry to solve a known problem of optimization of the set of
conserved quantities---regardless of whether it comes from symmetries or from
finite-size effects---to be incorporated in an extended thermodynamical theory
of integrable, near-integrable, or mesoscopic systems
The phase diagram of 2D polar condensates in a magnetic field
Spin one condensates in the polar (antiferromagnetic) phase in two dimensions
are shown to undergo a transition of the Ising type, in addition to the
expected Kosterlitz--Thouless (KT) transition of half vortices, due to the
quadratic Zeeman effect. We establish the phase diagram in terms of temperature
and the strength of the Zeeman effect using Monte Carlo simulations. When the
Zeeman effect is sufficiently strong the Ising and KT transitions merge. For
very strong Zeeman field the remaining transition is of the familiar integer KT
type.Comment: 4 pages, 7 figure
Phase sensitive noise in quantum dots under periodic perturbation
We evaluate the ensemble averaged noise in a chaotic quantum dot subject to
DC bias and a periodic perturbation of frequency . The noise displays
cusps at bias that survive the average, even when the
period of the perturbation is far shorter than the dwell time in the dot. These
features are sensitive to the phase of the time-dependent scattering amplitudes
of electrons to pass through the system.Comment: Published version. Improved discussion, with a few small typos
correcte
Persistent currents in ferromagnetic condensates
Persistent currents in Bose condensates with a scalar order parameter are stabilized by the topology of the order parameter manifold. In condensates with multicomponent order parameters it is topologically possible for supercurrents to "unwind" without leaving the manifold. We study the energetics of this process in the case of ferromagnetic condensates using a long wavelength energy functional that includes both the superfluid and spin stiffnesses. Exploiting analogies to an elastic rod and rigid body motion, we show that the current carrying state in a 1D ring geometry transitions between a spin helix in the energy minima and a solitonlike configuration at the maxima. The relevance to recent experiments in ultracold atoms is briefly discussed
Critical velocity of a mobile impurity in one-dimensional quantum liquids
We study the notion of superfluid critical velocity in one spatial dimension.
It is shown that for heavy impurities with mass exceeding a critical mass
, the dispersion develops periodic metastable branches resulting
in dramatic changes of dynamics in the presence of an external driving force.
In contrast to smooth Bloch Oscillations for , a heavy impurity
climbs metastable branches until it reaches a branch termination point or
undergoes a random tunneling event, both leading to an abrupt change in
velocity and an energy loss. This is predicted to lead to a non-analytic
dependence of the impurity drift velocity on small forces.Comment: 5 pages, 2 figures; New version with Supplemental Material (3 pages,
6 figures); Accepted to PR
- …