828 research outputs found
Anomalous criticality near semimetal-to-superfluid quantum phase transition in a two-dimensional Dirac cone model
We analyze the scaling behavior at and near a quantum critical point
separating a semimetallic from a superfluid phase. To this end we compute the
renormalization group flow for a model of attractively interacting electrons
with a linear dispersion around a single Dirac point. We study both ground
state and finite temperature properties. In two dimensions, the electrons and
the order parameter fluctuations exhibit power-law scaling with anomalous
scaling dimensions. The quasi-particle weight and the Fermi velocity vanish at
the quantum critical point. The order parameter correlation length turns out to
be infinite everywhere in the semimetallic ground state.Comment: 8 pages, 2 figures, typos correcte
Some Low-Temperature Properties of a Generalized Hubbard Model with Correlated Hopping
In the present paper we study some correlation effects in a generalized
Hubbard model with correlated hopping within low-temperature region using a
generalized mean-field approximation. It is shown that in a series of cases the
model leads to consequences deviating essentially from those of the Hubbard
model. We consider the possibility of applying the result to interpret the
peculiarities of physical properties of systems with narrow energy bands.Comment: 2 pages, LaTex2e using Elsevier style, presented at LT22 Conference,
Helsinki, August 199
Cosmological Neutrino Entanglement and Quantum Pressure
Context: The widespread view that cosmological neutrinos, even if massive,
are well described since the decoupling redshift z~10^10 down to the present
epoch by an almost perfectly collisionless fluid of classical point particles
is re-examined. Aims: In view of the likely sub-eV rest mass of neutrinos, the
main effects due to their fermionic nature are studied. Methods: By numerical
means we calculate the accurate entropy, fugacity and pressure of cosmological
neutrinos in the Universe expansion. By solving the Schroedinger equation we
derive how and how fast semi-degenerate identical free fermions become
entangled. Results: We find that for sub-eV neutrinos the exchange degeneracy
has significantly increased during the relativistic to non-relativistic
transition epoch at z~10^4-10^5. At all times neutrinos become entangled in
less than 10^-6 s, much faster than any plausible decoherence time. The total
pressure is increased by quantum effect from 5% at high redshifts to 68% at low
redshifts with respect to a collisionless classical fluid. Conclusions: The
quantum overpressure has no dynamical consequences in the homogeneous regime at
high redshifts, but must be significant for neutrino clustering during the
non-linear structure formation epoch at low redshifts.Comment: 11 pages, 7 figures, accepted version to Astronomy & Astrophysics (no
change, correct wrong TeX rendering
Resistivity studies under hydrostatic pressure on a low-resistance variant of the quasi-2D organic superconductor kappa-(BEDT-TTF)2Cu[N(CN)2]Br: quest for intrinsic scattering contributions
Resistivity measurements have been performed on a low (LR)- and high
(HR)-resistance variant of the kappa-(BEDT-TTF)_2Cu[N(CN)_2]Br superconductor.
While the HR sample was synthesized following the standard procedure, the LR
crystal is a result of a somewhat modified synthesis route. According to their
residual resistivities and residual resistivity ratios, the LR crystal is of
distinctly superior quality. He-gas pressure was used to study the effect of
hydrostatic pressure on the different transport regimes for both variants. The
main results of these comparative investigations are (i) a significant part of
the inelastic-scattering contribution, which causes the anomalous rho(T)
maximum in standard HR crystals around 90 K, is sample dependent, i.e.
extrinsic in nature, (ii) the abrupt change in rho(T) at T* approx. 40 K from a
strongly temperature-dependent behavior at T > T* to an only weakly T-dependent
rho(T) at T < T* is unaffected by this scattering contribution and thus marks
an independent property, most likely a second-order phase transition, (iii)
both variants reveal a rho(T) proportional to AT^2 dependence at low
temperatures, i.e. for T_c < T < T_0, although with strongly sample-dependent
coefficients A and upper bounds for the T^2 behavior measured by T_0. The
latter result is inconsistent with the T^2 dependence originating from coherent
Fermi-liquid excitations.Comment: 8 pages, 6 figure
Anomalous scaling of fermions and order parameter fluctuations at quantum criticality
We analyze the quantum phase transition between a semimetal and a superfluid
in a model of attractively interacting fermions with a linear dispersion. The
quantum critical properties of this model cannot be treated by the Hertz-Millis
approach since integrating out the fermions leads to a singular Landau-Ginzburg
order parameter functional. We therefore derive and solve coupled
renormalization group equations for the fermionic degrees of freedom and the
bosonic order parameter fluctuations. In two spatial dimensions, fermions and
bosons acquire anomalous scaling dimensions at the quantum critical point,
associated with non-Fermi liquid behavior and non-Gaussian order parameter
fluctuations.Comment: 8 pages, 9 figures, highlighted differences to Gross-Neveu model,
updated version as publishe
Superconductivity from correlated hopping
We consider a chain described by a next-nearest-neighbor hopping combined
with a nearest-neighbor spin flip. In two dimensions this three-body term
arises from a mapping of the three-band Hubbard model for CuO planes to a
generalized model and for large O-O hopping favors resonance-valence-bond
superconductivity of predominantly -wave symmetry. Solving the ground state
and low-energy excitations by analytical and numerical methods we find that the
chain is a Luther-Emery liquid with correlation exponent , where is the particle density.Comment: 10 pages, RevTeX 3.0 + 2 PostScript figs. Accepted for publication in
Phys.Rev.
Magnetic Properties of the t-J Model in the Dynamical Mean-Field Theory
We present a theory for the spin correlation function of the t-J model in the
framework of the dynamical mean-field theory. Using this mapping between the
lattice and a local model we are able to obtain an intuitive expression for the
non-local spin susceptibility, with the corresponding local correlation
function as input. The latter is calculated by means of local Goldstone
diagrams following closely the procedures developed and successfully applied
for the (single impurity) Anderson model.We present a systematic study of the
magnetic susceptibility and compare our results with those of a Hubbard model
at large U. Similarities and differences are pointed out and the magnetic phase
diagram of the t-J model is discussed.Comment: 28 pages LaTeX, postscript figures as compressed and uuencoded file
included fil
Robustness of a local Fermi Liquid against Ferromagnetism and Phase Separation
We study the properties of Fermi Liquids with the microscopic constraint of a
local self-energy. In this case the forward scattering sum-rule imposes strong
limitations on the Fermi-Liquid parameters, which rule out any Pomeranchek
instabilities. For both attractive and repulsive interactions, ferromagnetism
and phase separation are suppressed. Superconductivity is possible in an s-wave
channel only. We also study the approach to the metal-insulator transition, and
find a Wilson ratio approaching 2. This ratio and other properties of
Sr_{1-x}La_xTiO_3 are all consistent with the local Fermi Liquid scenario.Comment: 4 pages (twocolumn format), can compile with or without epsf.sty
latex style file -- Postscript files: fig1.ps and fig2.p
Propagation of a hole on a Neel background
We analyze the motion of a single hole on a N\'eel background, neglecting
spin fluctuations. Brinkman and Rice studied this problem on a cubic lattice,
introducing the retraceable-path approximation for the hole Green's function,
exact in a one-dimensional lattice. Metzner et al. showed that the
approximationalso becomes exact in the infinite-dimensional limit. We introduce
a new approach to this problem by resumming the Nagaoka expansion of the
propagator in terms of non-retraceable skeleton-paths dressed by
retraceable-path insertions. This resummation opens the way to an almost
quantitative solution of the problemin all dimensions and, in particular sheds
new light on the question of the position of the band-edges. We studied the
motion of the hole on a double chain and a square lattice, for which deviations
from the retraceable-path approximation are expected to be most pronounced. The
density of states is mostly adequately accounted for by the
retra\-ce\-able-path approximation. Our band-edge determination points towards
an absence of band tails extending to the Nagaoka energy in the spectrums of
the double chain and the square lattice. We also evaluated the spectral density
and the self-energy, exhibiting k-dependence due to finite dimensionality. We
find good agreement with recent numerical results obtained by Sorella et al.
with the Lanczos spectra decoding method. The method we employ enables us to
identify the hole paths which are responsible for the various features present
in the density of states and the spectral density.Comment: 26 pages,Revte
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