3,763 research outputs found
Effective action approach to strongly correlated fermion systems
We construct a new functional for the single particle Green's function, which
is a variant of the standard Baym Kadanoff functional.
The stability of the stationary solutions to the new functional is directly
related to aspects of the irreducible particle hole interaction through the
Bethe Salpeter equation.
A startling aspect of this functional is that it allows a simple and rigorous
derivation of both the standard and extended dynamical mean field (DMFT)
equations as stationary conditions. Though the DMFT equations were formerly
obtained only in the limit of infinite lattice coordination, the new functional
described in the work, presents a way of directly extending DMFT to finite
dimensional systems, both on a lattice and in a continuum. Instabilities of the
stationary solution at the bifurcation point of the functional, signal the
appearance of a zero mode at the Mott transition which then couples t o
physical quantities resulting in divergences at the transition.Comment: 9 page
Antiferromagnetism of ZnVO(PO and the dilution with Ti
We report static and dynamic properties of the antiferromagnetic compound
Zn(VO)(PO), and the consequences of non-magnetic Ti
doping at the V site. P nuclear magnetic resonance (NMR) spectra
and spin-lattice relaxation rate () consistently show the formation of
the long-range antiferromagnetic order below \,K. The critical
exponent estimated from the temperature dependence of the
sublattice magnetization measured by P NMR at 9.4\,MHz is consistent
with universality classes of three-dimensional spin models. The isotropic and
axial hyperfine couplings between the P nuclei and V spins are
Oe/ and Oe/, respectively. Magnetic susceptibility
data above 6.5\,K and heat capacity data above 4.5\,K are well described by
quantum Monte-Carlo simulations for the Heisenberg model on the square lattice
with \,K. This value of is consistent with the values obtained
from the NMR shift, and electron spin resonance (ESR) intensity
analysis. Doping ZnVO(PO with non-magnetic Ti leads to a
marginal increase in the value and the overall dilution of the spin
lattice. In contrast to the recent \textit{ab initio} results, we find neither
evidence for the monoclinic structural distortion nor signatures of the
magnetic one-dimensionality for doped samples with up to 15\% of Ti. The
N\'eel temperature decreases linearly with increasing the amount of
the non-magnetic dopant.Comment: 13 pages, 12 figures, 2 table
Spin Diffusion in Double-Exchange Manganites
The theoretical study of spin diffusion in double-exchange magnets by means
of dynamical mean-field theory is presented. We demonstrate that the
spin-diffusion coefficient becomes independent of the Hund's coupling JH in the
range of parameters JH*S >> W >> T, W being the bandwidth, relevant to colossal
magnetoresistive manganites in the metallic part of their phase diagram. Our
study reveals a close correspondence as well as some counterintuitive
differences between the results on Bethe and hypercubic lattices. Our results
are in accord with neutron scattering data and with previous theoretical work
for high temperatures.Comment: 4.0 pages, 3 figures, RevTeX 4, replaced with the published versio
Microscopic Model and Phase Diagrams of the Multiferroic Perovskite Manganites
Orthorhombically distorted perovskite manganites, RMnO3 with R being a
trivalent rare-earth ion, exhibit a variety of magnetic and electric phases
including multiferroic (i.e. concurrently magnetic and ferroelectric) phases
and fascinating magnetoelectric phenomena. We theoretically study the phase
diagram of RMnO3 by constructing a microscopic spin model, which includes not
only the superexchange interaction but also the single-ion anisotropy (SIA) and
the Dzyaloshinsky-Moriya interaction (DMI). Analysis of this model using the
Monte-Carlo method reproduces the experimental phase diagrams as functions of
the R-ion radius, which contain two different multiferroic states, i.e. the
ab-plane spin cycloid with ferroelectric polarization P//a and the bc-plane
spin cycloid with P//c. The orthorhombic lattice distortion or the
second-neighbor spin exchanges enhanced by this distortion exquisitely controls
the keen competition between these two phases through tuning the SIA and DMI
energies. This leads to a lattice-distortion-induced reorientation of P from a
to c in agreement with the experiments. We also discuss spin structures in the
A-type antiferromagnetic state, those in the cycloidal spin states, origin and
nature of the sinusoidal collinear spin state, and many other issues.Comment: 23 pages, 19 figures. Recalculated results after correcting errors in
the assignment of Dzyaloshinsky-Moriya vector
Self-Consistent Second Order Perturbation Theory for the Hubbard Model in Two Dimensions
We apply self-consistent second order perturbation theory (SCSOPT) with
respect to the on-site repulsive interaction U to study the Hubbard model in
two dimensions. We investigate single particle properties of the model over the
entire doping range at zero temperature. It is shown that as doping decreases
toward half-filling -mass enhancement factor increases, while k-mass
enhancement factor decreases. The increase in -mass enhancement factor
is larger than the decrease in k-mass enhancement factor, so that total-mass is
larger than that in the non-interacting case. When particle number density per
unit cell n is given by 0.64<n<1.0 interaction enhances anisotropy of the Fermi
surface, whereas at lower densities n<0.64 interaction suppresses anisotropy of
it. Due to the decrease in k-mass enhancement factor the density of states
(DOS) at the Fermi level is suppressed. It is possible to understand the
results within the framework of the weak coupling Fermi liquid theory.Comment: 8 pages, 12 embedded EPS figures, to appear in J. Phys. Soc. Jpn.
Vol. 68-3 (1999
Robustness of the van Hove scenario for high-Tc superconductors
The pinning of the Fermi level to the van Hove singularity and the formation
of flat bands in the two-dimensional t-t' Hubbard model is investigated by the
renormalization group technique. The "van-Hove" scenario of non-Fermi-liquid
behavior for high-Tc compounds can take place in a broad enough range of the
hole concentrations. The results are in qualitative agreement with the recent
ARPES data on La2CuO4.Comment: 4 pages, LaTeX, 3 figure
Optical spectra of the heavy fermion uniaxial ferromagnet UGe
We report a detailed study of UGe single crystals using infrared
reflectivity and spectroscopic ellipsometry. The optical conductivity suggests
the presence of a low frequency interband transition and a narrow free-carrier
response with strong frequency dependence of the scattering rate and effective
mass. We observe sharp changes in the low frequency mass and scattering rate
below the upper ferromagnetic transition . The characteristic
changes are exhibited most strongly at an energy scale of around 12 meV (100
cm). They recover their unrenormalized value above and for 40 meV. In contrast no sign of an anomaly is seen at the lower transition
temperature of unknown nature 30 K, observed in transport and
thermodynamic experiments. In the ferromagnetic state we find signatures of a
strong coupling to the longitudinal magnetic excitations that have been
proposed to mediate unconventional superconductivity in this compound
3D Spinodal Decomposition in the Inertial Regime
We simulate late-stage coarsening of a 3D symmetric binary fluid using a
lattice Boltzmann method. With reduced lengths and times l and t respectively
(scales set by viscosity, density and surface tension) our data sets cover 1 <
l
100 we find clear evidence of Furukawa's inertial scaling (l ~ t^{2/3}),
although the crossover from the viscous regime (l ~ t) is very broad. Though it
cannot be ruled out, we find no indication that Re is self-limiting (l ~
t^{1/2}) as proposed by M. Grant and K. R. Elder [Phys. Rev. Lett. 82, 14
(1999)].Comment: 4 pages, 3 eps figures, RevTex, minor changes to bring in line with
published version. Mobility values added to Table
Kondo lattice model at half-filling
The single- and two-channel Kondo lattice model consisting of localized spins
interacting antiferromagnetically with the itinerent electrons, are studied
using dynamical mean field theory. As an impurity solver for the effective
single impurity Anderson model we used the exact diagonalization (ED) method.
Using ED allowed us to perform calculations for low temperatures and couplings
of arbitrary large strength. Our results for the single-channel case confirm
and extend the recent investigations. In the two-channel case we find a
symmetry breaking phase transition with increasing coupling strength.Comment: 11 pages, 5 figure
- …