923 research outputs found
Zero-Temperature Properties of the Quantum Dimer Model on the Triangular Lattice
Using exact diagonalizations and Green's function Monte Carlo simulations, we
have studied the zero-temperature properties of the quantum dimer model on the
triangular lattice on clusters with up to 588 sites. A detailed comparison of
the properties in different topological sectors as a function of the cluster
size and for different cluster shapes has allowed us to identify different
phases, to show explicitly the presence of topological degeneracy in a phase
close to the Rokhsar-Kivelson point, and to understand finite-size effects
inside this phase. The nature of the various phases has been further
investigated by calculating dimer-dimer correlation functions. The present
results confirm and complement the phase diagram proposed by Moessner and
Sondhi on the basis of finite-temperature simulations [Phys. Rev. Lett. {\bf
86}, 1881 (2001)].Comment: 10 pages, 16 figure
Electron-phonon coupling and phonon self-energy in MgB: do we really understand MgB Raman spectra ?
We consider a model Hamiltonian fitted on the ab-initio band structure to
describe the electron-phonon coupling between the electronic bands and
the phonon E mode in MgB. The model allows for analytical
calculations and numerical treatments using very large k-point grids. We
calculate the phonon self-energy of the E mode along two high symmetry
directions in the Brillouin zone. We demonstrate that the contribution of the
bands to the Raman linewidth of the E mode via the
electron-phonon coupling is zero. As a consequence the large resonance seen in
Raman experiments cannot be interpreted as originated from the mode at
. We examine in details the effects of Fermi surface singularities in
the phonon spectrum and linewidth and we determine the magnitude of finite
temperature effects in the the phonon self-energy. From our findings we suggest
several possible effects which might be responsible for the MgB Raman
spectra.Comment: 10 pages, 9 figure
Violation of Ioffe-Regel condition but saturation of resistivity of the high Tc cuprates
We demonstrate that the resistivity data of a number of high Tc cuprates, in
particular La(2-x)SrxCuO4, are consistent with resistivity saturation, although
the Ioffe-Regel condition is strongly violated. By using the f-sum rule
together with calculations of the kinetic energy in the t-J model, we show that
the saturation resistivity is unusually large. This is related to the strong
reduction of the kinetic energy due to strong correlation effects. The
fulfilment of the Ioffe-Regel condition for conventional transition metal
compounds is found to be somewhat accidental.Comment: 4 pages, RevTeX, 2 eps figures, additional material available at
http://www.mpi-stuttgart.mpg.de/andersen/saturation
Evidence for a GABAergic system in rodent and human testis: Local GABA production and GABA receptors
The major neurotransmitter of the central nervous system, gamma-aminobutyric acid (GABA), exerts its actions through GABA(A), GABA(B) and GABA(C) receptors. GABA and GABA receptors are, however, also present in several non-neural tissues, including the endocrine organs pituitary, pancreas and testis. In the case of the rat testis, GABA appears to be linked to the regulation of steroid synthesis by Leydig cells via GABA(A) receptors, but neither testicular sources of GABA, nor the precise nature of testicular GABA receptors are fully known. We examined these points in rat, mouse, hamster and human testicular samples. RT-PCR followed by sequencing showed that the GABA-synthesizing enzymes glutamate decarboxylase (GAD) 65 and/or GAD67, as well as the vesicular GABA transporter vesicular inhibitory amino acid transporter (VIAAT/VGAT) are expressed. Testicular GAD in the rat was shown to be functionally active by using a GAD assay, and Western blot analysis confirmed the presence of GAD65 and GAD67. Interstitial cells, most of which are Leydig cells according to their location and morphological characteristics, showed positive immunoreaction for GAD and VIAAT/VGAT proteins. In addition, several GABA(A) receptor subunits (alpha1-3, beta1-3, gamma1-3), as well as GABAB receptor subunits R1 and R2, were detected by RT-PCR. Western blot analysis confirmed the results for GABA(A) receptor subunits beta2/3 in the rat, and immunohistochemistry identified interstitial Leydig cells to possess immunoreactive GABA(A) receptor subunits beta2/3 and alpha1. The presence of GABA(A) receptor subunit alpha1 mRNA in interstitial cells of the rat testis was further shown after laser microdissection followed by RT-PCR analysis. In summary, these results describe molecular details of the components of an intratesticular GABAergic system expressed in the endocrine compartment of rodent and human testes. While the physiological significance of this peripheral neuroendocrine system conserved throughout species remains to be elucidated, its mere presence in humans suggests the possibility that clinically used drugs might be able to interfere with testicular function. Copyright (C) 2003 S. Karger AG, Basel
Hole-depletion of ladders in SrCuO induced by correlation effects
The hole distribution in SrCuO is studied by low
temperature polarization dependent O K Near-Edge X-ray Absorption Fine
Structure measurements and state of the art electronic structure calculations
that include core-hole and correlation effects in a mean-field approach.
Contrary to all previous analysis, based on semi-empirical models, we show that
correlations and antiferromagnetic ordering favor the strong chain
hole-attraction. For the remaining small number of holes accommodated on
ladders, leg-sites are preferred to rung-sites. The small hole affinity of
rung-sites explains naturally the 1D - 2D cross-over in the phase diagram of
(La,Y,Sr,Ca)CuOComment: 6 pages, 8 figure
Effects of phase transitions in devices actuated by the electromagnetic vacuum force
We study the influence of the electromagnetic vacuum force on the behaviour
of a model device based on materials, like germanium tellurides, that undergo
fast and reversible metal-insulator transitions on passing from the crystalline
to the amorphous phase. The calculations are performed at finite temperature
and fully accounting for the behaviour of the material dielectric functions.
The results show that the transition can be exploited to extend the distance
and energy ranges under which the device can be operated without undergoing
stiction phenomena. We discuss the approximation involved in adopting the
Casimir expression in simulating nano- and micro- devices at finite
temperature
Phonon dispersion and lifetimes in MgB2
We measure phonon dispersion and linewidth in a single crystal of MgB_2 along
the Gamma-A, Gamma-M and A-L directions using inelastic X-Ray scattering. We
use Density Functional Theory to compute the effect of both electron-phonon
coupling and anharmonicity on the linewidth, obtaining excellent agreement with
experiment. Anomalous broadening of the E_2g phonon mode is found all along
Gamma-A. The dominant contribution to the linewidth is always the
electron-phonon coupling.Comment: 4 pages, 3 figure
Metals with Small Electron Mean-Free Path: Saturation versus Escalation of Resistivity
Resistivity of metals is commonly observed either to 'escalate' beyond the
Ioffe-Regel limit (mean free path l equal to lattice constant a) or to
'saturate' at this point. It is argued that neither behavior is
well-understood, and that 'escalation' is not necessarily more mysterious than
'saturation.'Comment: 3 pages with 3 embedded figures. This article is intended for the
Zacchary Fisk festschrift, which will be published in Physica
Green Function Monte Carlo with Stochastic Reconfiguration: an effective remedy for the sign problem disease
A recent technique, proposed to alleviate the ``sign problem disease'', is
discussed in details. As well known the ground state of a given Hamiltonian
can be obtained by applying the imaginary time propagator to a
given trial state for large imaginary time and sampling
statistically the propagated state . However
the so called ``sign problem'' may appear in the simulation and such
statistical propagation would be practically impossible without employing some
approximation such as the well known ``fixed node'' approximation (FN). This
method allows to improve the FN dynamic with a systematic correction scheme.
This is possible by the simple requirement that, after a short imaginary time
propagation via the FN dynamic, a number of correlation functions can be
further constrained to be {\em exact} by small perturbation of the FN
propagated state, which is free of the sign problem. By iterating this scheme
the Monte Carlo average sign, which is almost zero when there is sign problem,
remains stable and finite even for large . The proposed algorithm is
tested against the exact diagonalization results available on finite lattice.
It is also shown in few test cases that the dependence of the results upon the
few parameters entering the stochastic technique can be very easily controlled,
unless for exceptional cases.Comment: 44 pages, RevTeX + 5 encaplulated postscript figure
Stripes and spin-incommensurabilities are favored by lattice anisotropies
Structural distortions in cuprate materials give a natural origin for
anisotropies in electron properties. We study a modified one-band t-J model in
which we allow for different hoppings and antiferromagnetic couplings in the
two spatial directions ( and ). Incommensurate peaks
in the spin structure factor show up only in the presence of a lattice
anisotropy, whereas charge correlations, indicating enhanced fluctuations at
incommensurate wave vectors, are almost unaffected with respect to the
isotropic case.Comment: accepted for publication on Physical Review Letters, one color figur
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