3,037 research outputs found
Pairing based cooling of Fermi gases
We propose a pairing-based method for cooling an atomic Fermi gas. A three
component (labels 1, 2, 3) mixture of Fermions is considered where the
components 1 and 2 interact and, for instance, form pairs whereas the component
3 is in the normal state. For cooling, the components 2 and 3 are coupled by an
electromagnetic field. Since the quasiparticle distributions in the paired and
in the normal states are different, the coupling leads to cooling of the normal
state even when initially (notation ).
The cooling efficiency is given by the pairing energy and by the linewidth of
the coupling field. No superfluidity is required: any type of pairing, or other
phenomenon that produces a suitable spectral density, is sufficient. In
principle, the paired state could be cooled as well but this requires
. The method has a conceptual analogy to cooling based on
superconductor -- normal metal (SN) tunneling junctions. Main differences arise
from the exact momentum conservation in the case of the field-matter coupling
vs. non-conservation of momentum in the solid state tunneling process.
Moreover, the role of processes that relax the energy conservation requirement
in the tunneling, e.g. thermal fluctuations of an external reservoir, is now
played by the linewidth of the field. The proposed method should be
experimentally feasible due to its close connection to RF-spectroscopy of
ultracold gases which is already in use.Comment: Journal version 4 pages, 4 figure
Cannon-Thurston maps for pared manifolds of bounded geometry
Let Nh ∈, H(M, P) be a hyperbolic structure of bounded geometry on a pared manifold such that each component of ∂0M = ∂M - P is incompressible. We show that the limit set of Nh is locally connected by constructing a natural Cannon-Thurston map. This provides a unified treatment, an alternate proof and a generalization of results due to Cannon and Thurston, Minsky, Bowditch, Klarreich and the author
Signatures of superfluidity for Feshbach-resonant Fermi gases
We consider atomic Fermi gases where Feshbach resonances can be used to
continuously tune the system from weak to strong interaction regime, allowing
to scan the whole BCS-BEC crossover. We show how a probing field transferring
atoms out of the superfluid can be used to detect the onset of the superfluid
transition in the high- and BCS regimes. The number of transferred atoms,
as a function of the energy given by the probing field, peaks at the gap
energy. The shape of the peak is asymmetric due to the single particle
excitation gap. Since the excitation gap includes also a pseudogap
contribution, the asymmetry alone is not a signature of superfluidity.
Incoherent nature of the non-condensed pairs leads to broadening of the peak.
The pseudogap and therefore the broadening decay below the critical
temperature, causing a drastic increase in the asymmetry. This provides a
signature of the transition.Comment: Revised version, accepted to Phys. Rev. Letters. Figures changed,
explanations adde
On discreteness of commensurators
We begin by showing that commensurators of Zariski dense subgroups of isometry groups of symmetric spaces of non-compact type are discrete provided that the limit set on the Furstenberg boundary is not invariant under the action of a (virtual) simple factor. In particular for rank one or simple Lie groups, Zariski dense subgroups with non-empty domain of discontinuity have discrete commensurators. This generalizes a Theorem of Greenberg for Kleinian groups. We then prove that for all finitely generated, Zariski dense, infinite covolume discrete subgroups of Isom(H3), commensurators are discrete. Together these prove discreteness of commensurators for all known examples of finitely presented, Zariski dense, infinite covolume discrete subgroups of Isom(X) for X an irreducible symmetric space of non-compact type
Mapping class groups and interpolating complexes: rank
Abstract. A family of interpolating graphs C(S, ξ) of complexity ξ is constructed for a surface S and -2 ≤ ξ ≤ ξ(S). For ξ = -2,-1, ξ (S) -1 these specialize to graphs quasi-isometric to the marking graph, the pants graph and the curve graph respectively. We generalize the notion of a hierarchy and Theorems of Brock-Farb and Behrstock-Minsky to show that the rank of C(S, ξ) is rξ, the largest number of disjoint copies of subsurfaces of complexity greater than ξ that may be embedded in S. The interpolating graphs C(S, ξ) interpolate between the pants graph and the curve graph
Polarons and Molecules in a Two-Dimensional Fermi Gas
We study an impurity atom in a two-dimensional Fermi gas using variational
wave functions for (i) an impurity dressed by particle-hole excitations
(polaron) and (ii) a dimer consisting of the impurity and a majority atom. In
contrast to three dimensions, where similar calculations predict a sharp
transition to a dimer state with increasing interspecies attraction, we show
that the polaron ansatz always gives a lower energy. However, the exact
solution for a heavy impurity reveals that both a two-body bound state and
distortions of the Fermi sea are crucial. This reflects the importance of
particle-hole pairs in lower dimensions and makes simple variational
calculations unreliable. We show that the energy of an impurity gives important
information about its dressing cloud, for which both ans\"atze give inaccurate
results.Comment: 5 pages, 2 figures, minor change
Dielectric function of the semiconductor hole liquid: Full frequency and wave vector dependence
We study the dielectric function of the homogeneous semiconductor hole liquid
of p-doped bulk III-V zinc-blende semiconductors within random phase
approximation. The single-particle physics of the hole system is modeled by
Luttinger's four-band Hamiltonian in its spherical approximation. Regarding the
Coulomb-interacting hole liquid, the full dependence of the zero-temperature
dielectric function on wave vector and frequency is explored. The imaginary
part of the dielectric function is analytically obtained in terms of
complicated but fully elementary expressions, while in the result for the real
part nonelementary one-dimensional integrations remain to be performed. The
correctness of these two independent calculations is checked via Kramers-Kronig
relations.
The mass difference between heavy and light holes, along with variations in
the background dielectric constant, leads to dramatic alternations in the
plasmon excitation pattern, and generically, two plasmon branches can be
identified. These findings are the result of the evaluation of the full
dielectric function and are not accessible via a high-frequency expansion. In
the static limit a beating of Friedel oscillations between the Fermi wave
numbers of heavy and light holes occurs.Comment: 16 pages, 11 figures included. Update: Minor additions and
adjustments, published versio
Multilayer Thermionic Refrigerator and Generator
A new method of refrigeration is proposed. Cooling is obtained by thermionic
emission of electrons over periodic barriers in a multilayer geometry. These
could be either Schottky barriers between metals and semiconductors or else
barriers in a semiconductor superlattice. The same device is an efficient power
generator. A complete theory is provided.Comment: 17 pages with 5 postscript figures, submitted to J. Appl. Phy
Dielectric function of the semiconductor hole gas
We study the dielectric function of the homogeneous hole gas in p-doped
zinc-blende III-V bulk semiconductors within random phase approximation with
the valence band being modeled by Luttinger's Hamiltonian in the spherical
approximation. In the static limit we find a beating of Friedel oscillations
between the two Fermi momenta for heavy and light holes, while at large
frequencies dramatic corrections to the plasmon dispersion occur.Comment: 4 pages, 1 figure included. Version to appear in Europhys. Let
Canted Magnetization Texture in Ferromagnetic Tunnel Junctions
We study the formation of inhomogeneous magnetization texture in the vicinity
of a tunnel junction between two ferromagnetic wires nominally in the
antiparallel configuration and its influence on the magnetoresistance of such a
device. The texture, dependent on magnetization rigidity and crystalline
anisotropy energy in the ferromagnet, appears upon an increase of ferromagnetic
inter-wire coupling above a critical value and it varies with an external
magnetic field.Comment: 5 pages, 4 figure
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