4,267 research outputs found
Spin drag in an ultracold Fermi gas on the verge of a ferromagnetic instability
Recent experiments [Jo et al., Science 325, 1521 (2009)] have presented
evidence of ferromagnetic correlations in a two-component ultracold Fermi gas
with strong repulsive interactions. Motivated by these experiments we consider
spin drag, i.e., frictional drag due to scattering of particles with opposite
spin, in such systems. We show that when the ferromagnetic state is approached
from the normal side, the spin drag relaxation rate is strongly enhanced near
the critical point. We also determine the temperature dependence of the spin
diffusion constant. In a trapped gas the spin drag relaxation rate determines
the damping of the spin dipole mode, which therefore provides a precursor
signal of the ferromagnetic phase transition that may be used to experimentally
determine the proximity to the ferromagnetic phase.Comment: 4 pages, 3 fig
Accessing phonon polaritons in hyperbolic crystals by ARPES
Recently studied hyperbolic materials host unique phonon-polariton (PP)
modes. The ultra-short wavelengths of these modes, which can be much smaller
than those of conventional exciton-polaritons, are of high interest for extreme
sub-diffraction nanophotonics schemes. Polar hyperbolic materials such as
hexagonal boron nitride can be used to realize strong long-range coupling
between PP modes and extraneous charge degrees of freedom. The latter, in turn,
can be used to control and probe PP modes. Of special interest is coupling
between PP modes and plasmons in an adjacent graphene sheet, which opens the
door to accessing PP modes by angle-resolved photoemission spectroscopy
(ARPES). A rich structure in the graphene ARPES spectrum due to PP modes is
predicted, providing a new probe of PP modes and their coupling to graphene
plasmons
Probing the topological exciton condensate via Coulomb drag
The onset of exciton condensation in a topological insulator thin film was
recently predicted. We calculate the critical temperature for this transition,
taking into account screening effects. Furthermore, we show that the proximity
to this transition can be probed by measuring the Coulomb drag resistivity
between the surfaces of the thin film as a function of temperature. This
resistivity shows an upturn upon approaching the exciton-condensed state.Comment: 4 pages, 3 figure
Local density of states in metal - topological superconductor hybrid systems
We study by means of the recursive Green's function technique the local
density-of-states of (finite and semi-infinite) multi-band spin-orbit coupled
semiconducting nanowires in proximity to an s-wave superconductor and attached
to normal-metal electrodes. When the nanowire is coupled to a normal electrode,
the zero-energy peak, corresponding to the Majorana state in the topological
phase, broadens with increasing transmission between the wire and the leads,
eventually disappearing for ideal interfaces. Interestingly, for a finite
transmission a peak is present also in the normal electrode, even though it has
a smaller amplitude and broadens more rapidly with the strength of the
coupling. Unpaired Majorana states can survive close to a topological phase
transition even when the number of open channels (defined in the absence of
superconductivity) is even. We finally study the Andreev-bound-state spectrum
in superconductor-normal metal-superconductor junctions and find that in
multi-band nanowires the distinction between topologically trivial and
non-trivial systems based on the number of zero-energy crossings is preserved.Comment: 11 pages, 12 figures, published versio
Bound on the multiplicity of almost complete intersections
Let be a polynomial ring over a field of characteristic zero and let be a graded ideal of height which is minimally generated by
homogeneous polynomials. If where has degree
and has height , then the multiplicity of is
bounded above by .Comment: 7 pages; to appear in Communications in Algebr
Spin transport in a unitary Fermi gas close to the BCS transition
We consider spin transport in a two-component ultracold Fermi gas with
attractive interspecies interactions close to the BCS pairing transition. In
particular, we consider the spin-transport relaxation rate and the
spin-diffusion constant. Upon approaching the transition, the scattering
amplitude is enhanced by pairing fluctuations. However, as the system
approaches the transition, the spectral weight for excitations close to the
Fermi level is decreased by the formation of a pseudogap. To study the
consequence of these two competing effects, we determine the spin-transport
relaxation rate and the spin-diffusion constant using both a Boltzmann approach
and a diagrammatic approach. The former ignores pseudogap physics and finite
lifetime effects. In the latter, we incorporate the full pseudogap physics and
lifetime effects, but we ignore vertex corrections, so that we effectively
calculate single-particle relaxation rates instead of transport relaxation
rates. We find that there is qualitative agreement between these two approaches
although the results for the transport coefficients differ quantitatively.Comment: 9 pages, 10 figure
Quantum Breathing of an Impurity in a One-dimensional Bath of Interacting Bosons
By means of time-dependent density-matrix renormalization-group (TDMRG) we
are able to follow the real-time dynamics of a single impurity embedded in a
one-dimensional bath of interacting bosons. We focus on the impurity breathing
mode, which is found to be well-described by a single oscillation frequency and
a damping rate. If the impurity is very weakly coupled to the bath, a
Luttinger-liquid description is valid and the impurity suffers an
Abraham-Lorentz radiation-reaction friction. For a large portion of the
explored parameter space, the TDMRG results fall well beyond the
Luttinger-liquid paradigm.Comment: 10 pages, 7 figures, main text and supplementary material merged in a
single PRB style documen
Transport in strongly-coupled graphene-LaAlO3/SrTiO3 hybrid systems
We report on the transport properties of hybrid devices obtained by
depositing graphene on a LaAlO3/SrTiO3 oxide junction hosting a 4 nm-deep
two-dimensional electron system. At low graphene-oxide inter-layer bias the two
electron systems are electrically isolated, despite their small spatial
separation, and very efficient reciprocal gating is shown. A pronounced
rectifying behavior is observed for larger bias values and ascribed to the
interplay between electrostatic depletion and tunneling across the LaAlO3
barrier. The relevance of these results in the context of strongly-coupled
bilayer systems is discussed.Comment: 10 pages, 3 figure
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