1,515 research outputs found
Strong electron-lattice coupling as the mechanism behind charge density wave transformations in transition-metal dichalcogenides
We consider single band of conduction electrons interacting with
displacements of the transitional ions. In the classical regime strong enough
coupling transforms the harmonic elastic energy for an ion to the one of the
well with two deep minima, so that the system is described in terms of Ising
spins. Inter-site interactions order spins at lower tempratures. Extention to
the quantum regime is discussed. Below the CDW-transition the energy spectrum
of electrons remains metallic because the structural vector Q and the FS sizes
are not related. Large values of the CDW gap seen in the tunneling experiments
correspond to the energy of the minima in the electron-ion two-well complex.
The gap is defined through the density of states (DOS) inside the electronic
bands below the CDW transition. We focus mainly on electronic properties of
transition-metal dichalcogenides.Comment: new references added; accepted for publication in Physical Review B.
arXiv admin note: substantial text overlap with arXiv:1110.043
Pion Superfluidity and Meson Properties at Finite Isospin Density
We investigate pion superfluidity and its effect on meson properties and
equation of state at finite temperature and isospin and baryon densities in the
frame of standard flavor SU(2) NJL model. In mean field approximation to quarks
and random phase approximation to mesons, the critical isospin chemical
potential for pion superfluidity is exactly the pion mass in the vacuum, and
corresponding to the isospin symmetry spontaneous breaking, there is in the
pion superfluidity phase a Goldstone mode which is the linear combination of
the normal sigma and charged pion modes. We calculate numerically the gap
equations for the chiral and pion condensates, the phase diagrams, the meson
spectra, and the equation of state, and compare them with that obtained in
other effective models. The competitions between pion superfluidity and color
superconductivity at finite baryon density and between pion and kaon
superfluidity at finite strangeness density in flavor SU(3) NJL model are
briefly discussed.Comment: Updated version: (1)typos corrected; (2)an algebra error in Eq.(87)
corrected; (3)Fig.(17) renewed according to Eq.(87). We thank Prof.Masayuki
Matsuzaki for pointing out the error in Eq.(87
Production of the X(3872) at the Tevatron and the LHC
We predict the differential cross sections for production of the X(3872) at
the Tevatron and the Large Hadron Collider from both prompt QCD mechanisms and
from decays of b hadrons. The prompt cross section is calculated using the
NRQCD factorization formula. Simplifying assumptions are used to reduce the
nonperturbative parameters to a single NRQCD matrix element that is determined
from an estimate of the prompt cross section at the Tevatron. For X(3872) with
transverse momenta greater than about 4 GeV, the predicted cross section is
insensitive to the simplifying assumptions. We also discuss critically a recent
analysis that concluded that the prompt production rate at the Tevatron is too
large by orders of magnitude for the X(3872) to be a weakly-bound charm-meson
molecule. We point out that if charm-meson rescattering is properly taken into
account, the upper bound is increased by orders of magnitude and is compatible
with the observed production rate at the Tevatron.Comment: 29 pages, 5 figure
Universality of the single-particle spectra of cuprate superconductors
All the available data for the dispersion and linewidth of the
single-particle spectra above the superconducting gap and the pseudogap in
metallic cuprates for any doping has universal features. The linewidth is
linear in energy below a scale and constant above. The cusp in the
linewidth at mandates, due to causality, a "waterfall", i.e., a
vertical feature in the dispersion. These features are predicted by a recent
microscopic theory. We find that all data can be quantitatively fitted by the
theory with a coupling constant and an upper cutoff at
which vary by less than 50% among the different cuprates and for varying
dopings. The microscopic theory also gives these values to within factors of
O(2).Comment: 4 pages, 4 figures; accepted by Phys. Rev. Let
Response functions of cold neutron matter: density fluctuations
We compute the finite temperature density response function of
nonrelativistic cold fermions with an isotropic condensate. The pair-breaking
contribution to the response function is evaluated in the limit of small
three-momentum transfers q within an effective theory which exploits series
expansion in powers of small q/p_F, where p_F is the Fermi momentum. The
leading order O(q^2) contribution is universal and depends only on two
fundamental scales, the Fermi energy and the pairing gap. The particle-hole
Landau Fermi-liquid interaction contributes first at the next-to-leading-order
O(q^4). The scattering contribution to the polarization tensor is
nonperturbative (in the above sense) and is evaluated numerically. The spectral
functions of density fluctuations are constructed and the relevance of the q^2
scaling for the pair-breaking neutrino emission from neutron stars is
discussed.Comment: v2: 11 pages, 4 figures, matches published version
Neutral weak currents in nucleon superfluid Fermi liquids: Larkin-Migdal and Leggett approaches
Neutrino emission in processes of breaking and formation of nucleon Cooper
pairs is calculated in the framework of the Larkin-Migdal and the Leggett
approaches to the description of superfluid Fermi liquids at finite
temperatures. We explain peculiarities of both approaches and explicitly
demonstrate that they lead to the same expression for the emissivity in pair
breaking and formation processes.Comment: 24 pages, 3 figure
Pion condensation in the two--flavor chiral quark model at finite baryochemical potential
Pion condensation is studied at one--loop level and nonzero baryochemical
potential in the framework of two flavor constituent quark model using the
one--loop level optimized perturbation theory for the resummation of the
perturbative series. A Landau type of analysis is presented for the
investigation of the phase boundary between the pion condensed/non-condensed
phases. The statement that the condensation starts at \muI = m_{\pi} is
slightly modified by one--loop corrections. The second order critical surface
is determined and analysed in the \muI-\muB-T space. The \muI dependence of
the one--loop level charged pion pole masses is also studied.Comment: 12 pages, 5 figures, submitted to PR
A description of the f2(1270), rho3(1690), f4(2050), rho5(2350) and f6(2510) resonances as multi-rho(770) states
In a previous work regarding the interaction of two resonances,
the () resonance was obtained dynamically as a
two- molecule with a very strong binding energy, 135~MeV per
particle. In the present work we use the interaction in spin 2 and
isospin 0 channel to show that the resonances (),
(), () and ()
are basically molecules of increasing number of particles. We use
the fixed center approximation of the Faddeev equations to write the multi-body
interaction in terms of the two-body scattering amplitudes. We find the masses
of the states very close to the experimental values and we get an increasing
value of the binding energy per as the number of mesons is
increased.Comment: 17 pages, 6 figure
Diquark and Pion Condensation in Random Matrix Models for two-color QCD
We introduce a random matrix model with the symmetries of QCD with two colors
at nonzero isospin and baryon chemical potentials and temperature. We analyze
its phase diagram and find phases with condensation of pion and diquark states
in addition to the phases with spontaneously broken chiral symmetries. In the
limit of small chemical potentials and quark masses, we reproduce the mean
field results obtained from chiral Lagrangians. As in the case of QCD with
three colors, the presence of two chemical potentials breaks the flavor
symmetry and leads to phases that are characterized by different behaviors of
the chiral condensates for each flavor. In particular, the phase diagram we
obtain is similar to QCD with three colors and three flavors of quarks of equal
masses at zero baryon chemical potential and nonzero isospin and strange
chemical potentials. A tricritical point of the superfluid transitions found in
lattice calculations and from an analysis in terms of chiral Lagrangians does
not appear in the random matrix model. Remarkably, at fixed isospin chemical
potential, for the regions outside of the superfluid phases, the phase diagram
in the temperature - baryon chemical potential plane for two colors and three
colors are qualitatively the same.Comment: 19 pages, 7 figures, RevTeX
Possibility of s-wave pion condensates in neutron stars revisited
We examine possibilities of pion condensation with zero momentum (s-wave
condensation) in neutron stars by using the pion-nucleus optical potential U
and the relativistic mean field (RMF) models. We use low-density
phenomenological optical potentials parameterized to fit deeply bound pionic
atoms or pion-nucleus elastic scatterings. Proton fraction (Y_p) and electron
chemical potential (mu_e) in neutron star matter are evaluated in RMF models.
We find that the s-wave pion condensation hardly takes place in neutron stars
and especially has no chance if hyperons appear in neutron star matter and/or
b_1 parameter in U has density dependence.Comment: 4 pages, 3 figures, REVTe
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