1,468 research outputs found
Possible field-tuned SIT in high-Tc superconductors: implications for pairing at high magnetic fields
The behavior of some high temperature superconductors (HTSC) such as and , at very high
magnetic field, is similar to that of thin films of amorphous InOx near the
magnetic field-tuned superconductor-insulator transition. Analyzing the InOx
data at high fields in terms of persisting local pairing amplitude, we argue by
analogy that local pairing amplitude also persists well into the dissipative
state of the HTSCs, the regime commonly denoted as the "normal state" in very
high magnetic field experiments.Comment: Revised figures and reference
Transport Properties of a spinon Fermi surface coupled to a U(1) gauge field
With the organic compound -(BEDT-TTF)-Cu(CN) in mind, we
consider a spin liquid system where a spinon Fermi surface is coupled to a U(1)
gauge field. Using the non-equilibrium Green's function formalism, we derive
the Quantum Boltzmann Equation (QBE) for this system. In this system, however,
one cannot a priori assume the existence of Landau quasiparticles. We show that
even without this assumption one can still derive a linearized equation for a
generalized distribution function. We show that the divergence of the effective
mass and of the finite temperature self-energy do not enter these transport
coefficients and thus they are well-defined. Moreover, using a variational
method, we calculate the temperature dependence of the spin resistivity and
thermal conductivity of this system.Comment: 12 page
Simulation of indivisible qubit channels in collision models
A sequence of controlled collisions between a quantum system and its
environment (composed of a set of quantum objects) naturally simulates (with
arbitrary precision) any Markovian quantum dynamics of the system under
consideration. In this paper we propose and study the problem of simulation of
an {\it arbitrary} quantum channel via collision models. We show that a
correlated environment is capable to simulate {\it non-Markovian} evolutions
leading to any indivisible qubit channel. In particular, we derive the
corresponding master equation generating a continuous time non-Markovian
dynamics implementing the universal NOT gate being an example of the most
non-Markovian quantum channels.Comment: 6 pages, 2 figures, submitted to JP
Simultaneous Measurements of the Torsional Oscillator Anomaly and Thermal Conductivity in Solid 4He
In these torsional oscillator experiments the samples of solid He were
characterized by measuring their thermal conducitvity. Polycrystalline samples
of helium of either high isotopic purity or natural concentration of He
were grown in an annular container by the blocked-capillary method and
investigated before and after annealing. No correlation has been found between
the magnitude of the low-temperature shift of the torsional oscillator
frequency and the amount of crystalline defects as measured by the thermal
conductivity. In samples with the natural He concentration a substantial
excess thermal conductivity over the usual dependence was observed below
120 mK.Comment: 4 pages, 3 figure
Signatures of exchange correlations in the thermopower of quantum dots
We use a many-body rate-equation approach to calculate the thermopower of a
quantum dot in the presence of an exchange interaction. At temperatures much
smaller than the single-particle level spacing, the known quantum jumps
(discontinuities) in the thermopower are split by the exchange interaction. The
origin and nature of the splitting are elucidated with a simple physical
argument based on the nature of the intermediate excited state in the
sequential tunneling approach. We show that this splitting is sensitive to the
number parity of electrons in the dot and the dot's ground-state spin. These
effects are suppressed when cotunneling dominates the electrical and thermal
conductances. We calculate the thermopower in the presence of elastic
cotunneling, and show that some signatures of exchange correlations should
still be observed with current experimental methods. In particular, we propose
a method to determine the strength of the exchange interaction from
measurements of the thermopower.Comment: 18 pages, 6 figures Revised figure 6, and changed discussion of
figure
Equally-distant partially-entangled alphabet states for quantum channels
Each Bell state has the property that by performing just local operations on
one qubit, the complete Bell basis can be generated. That is, states generated
by local operations are totally distinguishable. This remarkable property is
due to maximal quantum entanglement between the two particles. We present a set
of local unitary transformations that generate out of partially entangled
two-qubit state a set of four maximally distinguishable states that are
mutually equally distant. We discuss quantum dense coding based on these
alphabet states.Comment: 7 revtex pages, 2 eps figures, to appear in Phys. Rev. A 62, 1
November (2000
Shear viscosity of degenerate electron matter
We calculate the partial electron shear viscosity limited by
electron-electron collisions in a strongly degenerate electron gas taking into
account the Landau damping of transverse plasmons. The Landau damping strongly
suppresses in the domain of ultrarelativistic degenerate electrons
and modifies its %asymptotic temperature behavior. The efficiency of the
electron shear viscosity in the cores of white dwarfs and envelopes of neutron
stars is analyzed.Comment: 16 pages, 4 figures, accepted to Journal of Physics
Charge Ordering and Spin Dynamics in NaV2O5
We report high-resolution neutron inelastic scattering experiments on the
spin excitations of NaV2O5. Below Tc, two branches associated with distinct
energy gaps are identified. From the dispersion and intensity of the spin
excitation modes, we deduce the precise zig-zag charge distribution on the
ladder rungs and the corresponding charge order (about 0.6). We argue that the
spin gaps observed in the low-T phase of this compound are primarily due to the
charge transfer.Comment: 4 pages, 5 figures, to appear in Phys. Rev. Let
Heat transport of clean spin-ladders coupled to phonons: Umklapp scattering and drag
We study the low-temperature heat transport in clean two-leg spin ladder
compounds coupled to three-dimensional phonons. We argue that the very large
heat conductivities observed in such systems can be traced back to the
existence of approximate symmetries and corresponding weakly violated
conservation laws of the effective (gapful) low--energy model, namely
pseudo-momenta. Depending on the ratios of spin gaps and Debye energy and on
the temperature, the magnetic contribution to the heat conductivity can be
positive or negative, and exhibit an activated or anti-activated behavior. In
most regimes, the magnetic heat conductivity is dominated by the spin-phonon
drag: the excitations of the two subsystems have almost the same drift
velocity, and this allows for an estimate of the ratio of the magnetic and
phononic contributions to the heat conductivity.Comment: revised version, 8 pages, 3 figures, added appendi
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