926 research outputs found
Dynamic projection on Feshbach molecules: a probe of pairing and phase fluctuations
We describe and justify a simple model for the dynamics associated with rapid
sweeps across a Feshbach resonance, from the atomic to the molecular side, in
an ultra cold Fermi system. The model allows us to relate the observed molecule
momentum distribution, including its dependence on the sweep rate, to
equilibrium properties of the initial state. For initial state near resonance,
we find that phase fluctuations sharply reduce the observed condensate
fraction. Moreover, for very fast sweeps and low temperatures, we predict a
surprising nonmonotonic dependence of the molecule condensate fraction on
detuning, that is a direct signature of quantum phase fluctuations. The
dependence of the total molecule number on sweep rate is found to be a
sensitive probe of pairing in the initial state, whether condensed or not.
Hence it can be utilized to establish the presence of a phase fluctuation
induced `psuedogap' phase in these systems.Comment: Added reference
Anomalous Zeeman response in coexisting phase of superconductivity and spin-density wave as a probe of extended -wave pairing structure in ferro-pnictide
In several members of the ferro-pnictides, spin density wave (SDW) order
coexists with superconductivity over a range of dopings. In this letter we
study the anomalous magnetic Zeeman response of this coexistence state and show
that it can be used to confirm the extended s-wave gap structure as well as
structure of superconducting (SC) gap in coexisting phase. On increasing the
field, a strongly anisotropic reduction of SC gap is found. The anisotropy is
directly connected to the gap structure of superconducting phase. The signature
of this effect in quasiparticle interference measured by STM, as well as heat
transport in magnetic field is discussed. For the compounds with the nodal SC
gap we show that the nodes are removed upon formation of SDW. Interestingly the
size of the generated gap in the originally nodal areas is anisotropic in the
position of the nodes over the Fermi surface in direct connection with the form
of SC pairing.Comment: 5 pages, 2 figure
Anomalous Symmetry Fractionalization and Surface Topological Order
In addition to possessing fractional statistics, anyon excitations of a 2D
topologically ordered state can realize symmetry in distinct ways , leading to
a variety of symmetry enriched topological (SET) phases. While the symmetry
fractionalization must be consistent with the fusion and braiding rules of the
anyons, not all ostensibly consistent symmetry fractionalizations can be
realized in 2D systems. Instead, certain `anomalous' SETs can only occur on the
surface of a 3D symmetry protected topological (SPT) phase. In this paper we
describe a procedure for determining whether an SET of a discrete, onsite,
unitary symmetry group is anomalous or not. The basic idea is to gauge the
symmetry and expose the anomaly as an obstruction to a consistent topological
theory combining both the original anyons and the gauge fluxes. Utilizing a
result of Etingof, Nikshych, and Ostrik, we point out that a class of
obstructions are captured by the fourth cohomology group ,
which also precisely labels the set of 3D SPT phases, with symmetry group .
We thus establish a general bulk-boundary correspondence between the anomalous
SET and the 3d bulk SPT whose surface termination realizes it. We illustrate
this idea using the chiral spin liquid () topological order with a
reduced symmetry , which can
act on the semion quasiparticle in an anomalous way. We construct exactly
solved 3d SPT models realizing the anomalous surface terminations, and
demonstrate that they are non-trivial by computing three loop braiding
statistics. Possible extensions to anti-unitary symmetries are also discussed.Comment: 18 pages + 3 pages appendices. v2: added section on identifying the
SPT order from nontrivial loop statistics in the 3D bulk with projective
semion surface stat
Relay-Assisted User Scheduling in Wireless Networks with Hybrid-ARQ
This paper studies the problem of relay-assisted user scheduling for downlink
wireless transmission. The base station or access point employs hybrid
automatic-repeat-request (HARQ) with the assistance of a set of fixed relays to
serve a set of mobile users. By minimizing a cost function of the queue lengths
at the base station and the number of retransmissions of the head-of-line
packet for each user, the base station can schedule an appropriate user in each
time slot and an appropriate transmitter to serve it. It is shown that a
priority-index policy is optimal for a linear cost function with packets
arriving according to a Poisson process and for an increasing convex cost
function where packets must be drained from the queues at the base station.Comment: 14 pages, 5 figures, submitted to the IEEE Transactions on Vehicular
Technology in October 2008, revised in March 2009 and May 200
Two dimensional anisotropic non Fermi-liquid phase of coupled Luttinger liquids
We show using bosonization techniques, that strong forward scattering
interactions between one dimensional spinless Luttinger liquids (LL) can
stabilize a phase where charge-density wave, superconducting and transverse
single particle hopping perturbations are irrelevant. This new phase retains
its LL like properties in the directions of the chains, but with relations
between exponents modified by the transverse interactions, whereas, it is a
perfect insulator in the transverse direction. The mechanism that stabilizes
this phase are strong transverse charge density wave fluctuations at
incommensurate wavevector, which frustrates crystal formation by preventing
lock-in of the in-chain density waves.Comment: (4 pages, 2 figures
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