5,171 research outputs found
On the ground state of metallic hydrogen
A proposed liquid ground state of metallic hydrogen at zero temperature is explored and a variational upper bound to the ground state energy is calculated. The possibility that the metallic hydrogen is a liquid around the metastable point (rs = 1.64) cannot be ruled out. This conclusion crucially hinges on the contribution to the energy arising from the third order in the electron-proton interaction which is shown here to be more significant in the liquid phase than in crystals
On the relevance of numerical simulations to booming sand
We have performed a simulation study of 3D cohesionless granular flows down
an inclined chute. We find that the oscillations observed in [L.E. Silbert,
Phys. Rev. Lett., 94, 098002 (2005)] near the angle of repose are harmonic
vibrations of the lowest normal mode. Their frequencies depend on the contact
stiffness as well as on the depth of the flow. Could these oscillations account
for the phenomena of "booming sand"? We estimate an effective contact stiffness
from the Hertz law, but this leads to frequencies several times higher than
observed. However, the Hertz law also predicts interpenetrations of a few
nanometers, indicating that the oscillations frequencies are governed by the
surface stiffness, which can be much lower than the bulk one. This is in
agreement with previous studies ascribing the ability to sing to the presence
of a soft coating on the grain surface.Comment: accepted for publication in Physical Review E http://pre.aps.org;
Physical Review E (2012) to be publishe
Effective mass in quasi two-dimensional systems
The effective mass of the quasiparticle excitations in quasi two-dimensional
systems is calculated analytically. It is shown that the effective mass
increases sharply when the density approaches the critical one of
metal-insulator transition. This suggests a Mott type of transition rather than
an Anderson like transition.Comment: 3 pages 3 figure
Magnetic superlattice and finite-energy Dirac points in graphene
We study the band structure of graphene's Dirac-Weyl quasi-particles in a one-dimensional magnetic superlattice formed by a periodic sequence of alternating magnetic barriers. The spectrum and the nature of the states strongly depend on the conserved longitudinal momentum and on the barrier width. At the center of the superlattice Brillouin zone we find new Dirac points at finite energies where the dispersion is highly anisotropic, in contrast to the dispersion close to the neutrality point which remains isotropic. This finding suggests the possibility of collimating Dirac-Weyl quasi-particles by tuning the doping
Frequency-dependent Thermal Response of the Charge System and Restricted Sum Rules in La(2-x)Sr(x)CuO(4)
By using new and previous measurements of the -plane conductivity
of LaSrCuO (LSCO) it is shown that
the spectral weight
obeys the same law which holds for a conventional
metal like gold, for 's below the plasma frequency. However
, which measures the "thermal response" of the charge system, in
LSCO exhibits a peculiar behavior which points towards correlation effects. In
terms of hopping models, is directly related to an energy scale
, smaller by one order of magnitude than the full bandwidth .Comment: 4 pages with 3 fig
Number distributions for fermions and fermionized bosons in periodic potentials
We compute the spatial population statistics for one-dimensional fermi-gases
and for bose-gases with hard core repulsions in periodic potentials. We show
how the statistics depend on the atomic density in the ground state of the
system, and we present calculations for the dynamical turn-on of the potential.Comment: 8 pages, 4 figures, submitted to Phys. Rev.
Tunable Charge and Spin Seebeck Effects in Magnetic Molecular Junctions
We study the charge and spin Seebeck effects in a spin-1 molecular junction
as a function of temperature (T), applied magnetic field (H), and magnetic
anisotropy (D) using Wilson's numerical renormalization group. A hard-axis
magnetic anisotropy produces a large enhancement of the charge Seebeck
coefficient Sc (\sim k_B/|e|) whose value only depends on the residual
interaction between quasiparticles in the low temperature Fermi-liquid regime.
In the underscreened spin-1 Kondo regime, the high sensitivity of the system to
magnetic fields makes it possible to observe a sizable value for the spin
Seebeck coefficient even for magnetic fields much smaller than the Kondo
temperature. Similar effects can be obtain in C60 junctions where the control
parameter is the gap between a singlet and a triplet molecular state.Comment: 5 pages, 4 figure
The role of quasi-momentum in the resonant dynamics of the atom-optics kicked rotor
We examine the effect of the initial atomic momentum distribution on the
dynamics of the atom-optical realisation of the quantum kicked rotor. The atoms
are kicked by a pulsed optical lattice, the periodicity of which implies that
quasi-momentum is conserved in the transport problem. We study and compare
experimentally and theoretically two resonant limits of the kicked rotor: in
the vicinity of the quantum resonances and in the semiclassical limit of
vanishing kicking period. It is found that for the same experimental
distribution of quasi-momenta, significant deviations from the kicked rotor
model are induced close to quantum resonance, while close to the classical
resonance (i.e. for small kicking period) the effect of the quasi-momentum
vanishes.Comment: 10 pages, 4 figures, to be published in J. Phys. A, Special Issue on
'Trends in Quantum Chaotic Scattering
Influence of Dimensionality on Thermoelectric Device Performance
The role of dimensionality on the electronic performance of thermoelectric
devices is clarified using the Landauer formalism, which shows that the
thermoelectric coefficients are related to the transmission, T(E), and how the
conducing channels, M(E), are distributed in energy. The Landauer formalism
applies from the ballistic to diffusive limits and provides a clear way to
compare performance in different dimensions. It also provides a physical
interpretation of the "transport distribution," a quantity that arises in the
Boltzmann transport equation approach. Quantitative comparison of
thermoelectric coefficients in one, two, and three dimension shows that the
channels may be utilized more effectively in lower-dimensions. To realize the
advantage of lower dimensionality, however, the packing density must be very
high, so the thicknesses of the quantum wells or wires must be small. The
potential benefits of engineering M(E) into a delta-function are also
investigated. When compared to a bulk semiconductor, we find the potential for
~50 % improvement in performance. The shape of M(E) improves as dimensionality
decreases, but lower dimensionality itself does not guarantee better
performance because it is controlled by both the shape and the magnitude of
M(E). The benefits of engineering the shape of M(E) appear to be modest, but
approaches to increase the magnitude of M(E) could pay large dividends.Comment: 23 pages, 5 figure
A causal look into the quantum Talbot effect
A well-known phenomenon in both optics and quantum mechanics is the so-called
Talbot effect. This near field interference effect arises when infinitely
periodic diffracting structures or gratings are illuminated by highly coherent
light or particle beams. Typical diffraction patterns known as quantum carpets
are then observed. Here the authors provide an insightful picture of this
nonlocal phenomenon as well as its classical limit in terms of Bohmian
mechanics, also showing the causal reasons and conditions that explain its
appearance. As an illustration, theoretical results obtained from diffraction
of thermal He atoms by both N-slit arrays and weak corrugated surfaces are
analyzed and discussed. Moreover, the authors also explain in terms of what
they call the Talbot-Beeby effect how realistic interaction potentials induce
shifts and distortions in the corresponding quantum carpets.Comment: 12 pages, 6 figure
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