1,036 research outputs found
Doping dependence of thermopower and thermoelectricity in strongly correlated systems
The search for semiconductors with high thermoelectric figure of merit has
been greatly aided by theoretical modeling of electron and phonon transport,
both in bulk materials and in nanocomposites. Recent experiments have studied
thermoelectric transport in ``strongly correlated'' materials derived by doping
Mott insulators, whose insulating behavior without doping results from
electron-electron repulsion, rather than from band structure as in
semiconductors. Here a unified theory of electrical and thermal transport in
the atomic and ``Heikes'' limit is applied to understand recent transport
experiments on sodium cobaltate and other doped Mott insulators at room
temperature and above. For optimal electron filling, a broad class of
narrow-bandwidth correlated materials are shown to have power factors (the
electronic portion of the thermoelectric figure of merit) as high at and above
room temperature as in the best semiconductors.Comment: 4 pages, 4 figure
Vortex Lattice Transitions in Cyclic Spinor Condensates
We study the energetics of vortices and vortex lattices produced by rotation in the cyclic phase of F=2 spinor condensates. In addition to the familiar triangular lattice predicted by Tkachenko for 4He, many more complex lattices appear in this system as a result of the spin degree of freedom. In particular, we predict a magnetic-field-driven transition from a triangular lattice to a honeycomb lattice. Other transitions and lattice geometries are driven at constant field by changes in the temperature-dependent ratio of charge and spin stiffnesses, including a transition through an aperiodic vortex structure. Finally, we compute the renormalization of the ratio of the spin and charge stiffnesses from thermal fluctuations using a nonlinear sigma model analysis
Dynamics after a sweep through a quantum critical point
The coherent quantum evolution of a one-dimensional many-particle system
after sweeping the Hamiltonian through a critical point is studied using a
generalized quantum Ising model containing both integrable and non-integrable
regimes. It is known from previous work that universal power laws appear in
such quantities as the mean number of excitations created by the sweep. Several
other phenomena are found that are not reflected by such averages: there are
two scaling regimes of the entanglement entropy and a relaxation that is
power-law rather than exponential. The final state of evolution after the
quench is not well characterized by any effective temperature, and the
Loschmidt echo converges algebraically to a constant for long times, with
cusplike singularities in the integrable case that are dynamically broadened by
nonintegrable perturbations.Comment: 4 pages, 4 figure
Branching ratios in low-energy deuteron-induced reactions
We consider (d,p) and (d,n) reactions on light nuclei at low energies. A simple estimate using the second-order distorted-wave Born approximation shows that Coulomb-induced predissociation of the deuteron influences the relative rate by less than 10%. This disagrees with a previous explanation of experiments involving 6Li targets and invalidates speculations about such effects in "cold fusion" experiments
Towards a statistical theory of transport by strongly-interacting lattice fermions
We present a study of electric transport at high temperature in a model of
strongly interacting spinless fermions without disorder. We use exact
diagonalization to study the statistics of the energy eigenvalues, eigenstates,
and the matrix elements of the current. These suggest that our nonrandom
Hamiltonian behaves like a member of a certain ensemble of Gaussian random
matrices. We calculate the conductivity and examine its
behavior, both in finite size samples and as extrapolated to the thermodynamic
limit. We find that has a prominent non-divergent singularity
at reflecting a power-law long-time tail in the current
autocorrelation function that arises from nonlinear couplings between the
long-wavelength diffusive modes of the energy and particle number
Nernst effect in the vortex-liquid regime of a type-II superconductor
We measure the transverse thermoelectric coefficient in
simulations of type-II superconductors in the vortex liquid regime, using the
time-dependent Ginzburg-Landau (TDGL) equation with thermal noise. Our results
are in reasonably good quantitative agreement with experimental data on cuprate
samples, suggesting that this simple model of superconducting fluctuations
contains much of the physics behind the large Nernst effect observed in these
materials.Comment: 6 pages. Expanded version of text. New Fig.
Studies of orbital parameters and pulse profile of the accreting millisecond pulsar XTE J1807-294
The accreting millisecond pulsar XTE J1807-294 was observed by XMM-Newton on
March 22, 2003 after its discovery on February 21, 2003 by RXTE. The source was
detected in its bright phase with an observed average count rate of 33.3 cts/s
in the EPIC-pn camera in the 0.5-10 keV energy band (3.7 mCrab). Using the
earlier established best-fit orbital period of 40.0741+/-0.0005 minutes from
RXTE observations and considering a circular binary orbit as first
approximation, we derived a value of 4.8+/-0.1 lt-ms for the projected orbital
radius of the binary system and an epoch of the orbital phase of MJD
52720.67415(16). The barycentric mean spin period of the pulsar was derived as
5.2459427+/-0.0000004 ms. The pulsar's spin-pulse profile showed a prominent
(1.5 ms FWHM) pulse, with energy and orbital phase dependence in the amplitude
and shape. The measured pulsed fraction in four energy bands was found to be
3.1+/-0.2 % (0.5-3.0 keV), 5.4+/-0.4 % (3.0-6.0 keV), 5.1+/-0.7 % (6.0-10.0
keV) and 3.7+/-0.2 % (0.5-10.0 keV), respectively. Studies of spin-profiles
with orbital phase and energy showed significant increase in its pulsed
fraction during the second observed orbit of the neutron star, gradually
declining in the subsequent two orbits, which was associated with sudden but
marginal increase in mass accretion. From our investigations of orbital
parameters and estimation of other properties of this compact binary system, we
conclude that XTE J1807-294 is very likely a candidate for a millisecond radio
pulsar.Comment: 4 pages, 4 figures, Accepted for publication in Astronomy and
Astrophysics letter
Understanding the nature of electronic effective mass in double-doped SrTiO
We present an approach to tune the effective mass in an oxide semiconductor
by a double doping mechanism. We demonstrate this in a model oxide system
SrLaTiO, where we can tune the effective mass ranging
from 6--20 as a function of filling or carrier concentration and
the scattering mechanism, which are dependent on the chosen lanthanum and
oxygen vacancy concentrations. The effective mass values were calculated from
the Boltzmann transport equation using the measured transport properties of
thin films of SrLaTiO. Our method, which shows that
the effective mass decreases with carrier concentration, provides a means for
understanding the nature of transport processes in oxides, which typically have
large effective mass and low electron mobility, contrary to the tradional high
mobility semiconductors.Comment: 5 pages with 4 figure
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