642 research outputs found
Slow relaxation and sensitivity to disorder in trapped lattice fermions after a quench
We consider a system of non-interacting fermions in one dimension subject to
a single-particle potential consisting of (a) a strong optical lattice, (b) a
harmonic trap, and (c) uncorrelated on-site disorder. After a quench, in which
the center of the harmonic trap is displaced, we study the occupation function
of the fermions and the time-evolution of experimental observables.
Specifically, we present numerical and analytical results for the post-quench
occupation function of the fermions, and analyse the time-evolution of the
real-space density profile. Unsurprisingly for a non-interacting (and therefore
integrable) system, the infinite-time limit of the density profile is
non-thermal. However, due to Bragg-localization of the higher-energy
single-particle states, the approach to even this non-thermal state is
extremely slow. We quantify this statement, and show that it implies a
sensitivity to disorder parametrically stronger than that expected from
Anderson localization.Comment: 15 pages, 11 figure
Unbinding of giant vortices in states of competing order
Funding: EPSRC (UK) via Grants No. EP/I031014/1 and No. EP/H049584/1.We consider a two-dimensional system with two order parameters, one with O(2) symmetry and one with O(M), near a point in parameter space where they couple to become a single O(2+M) order. While the O(2) sector supports vortex excitations, these vortices must somehow disappear as the high symmetry point is approached. We develop a variational argument which shows that the size of the vortex cores diverges as 1/root Delta and the Berezinskii-Kosterlitz-Thouless transition temperature of the O(2) order vanishes as 1/1n(1/Delta), where Delta denotes the distance from the high-symmetry point. Our physical picture is confirmed by a renormalization group analysis which gives further logarithmic corrections, and demonstrates full symmetry restoration within the cores.Publisher PDFPeer reviewe
Entropy as a function of magnetisation for a 2D spin-ice model exhibiting a Kasteleyn transition
We present a combined analytical and numerical study of the entropy as a function of magnetization for an orientable 2D spin-ice model that exhibits a Kasteleyn transition. The model that we use is related to the well known six-vertex model but, as we show, our representation of it is more convenient for constructing approximate expressions for the entropy at fixed magnetization. We also discuss directions for further work, including the possibility of deforming our model into one exhibiting a quantum Kasteleyn transition.Instituto de FÃsica de LÃquidos y Sistemas Biológico
Spin dynamics from Majorana fermions
Using the Majorana fermion representation of spin-1/2 local moments, we show
how it is possible to directly read off the dynamic spin correlation and
susceptibility from the one-particle propagator of the Majorana fermion. We
illustrate our method by applying it to the spin dynamics of a non-equilibrium
quantum dot, computing the voltage-dependent spin relaxation rate and showing
that, at weak coupling, the fluctuation-dissipation relation for the spin of a
quantum dot is voltage-dependent. We confirm the voltage-dependent Curie
susceptibility recently found by Parcollet and Hooley [Phys. Rev. B {\bf 66},
085315 (2002)].Comment: Small modifications added to figure and tex
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