10 research outputs found
Green's-function theory of the Heisenberg ferromagnet in a magnetic field
We present a second-order Green's-function theory of the one- and
two-dimensional S=1/2 ferromagnet in a magnetic field based on a decoupling of
three-spin operator products, where vertex parameters are introduced and
determined by exact relations. The transverse and longitudinal spin correlation
functions and thermodynamic properties (magnetization, isothermal magnetic
susceptibility, specific heat) are calculated self-consistently at arbitrary
temperatures and fields. In addition, exact diagonalizations on finite lattices
and, in the one-dimensional case, exact calculations by the Bethe-ansatz method
for the quantum transfer matrix are performed. A good agreement of the
Green's-function theory with the exact data, with recent quantum Monte Carlo
results, and with the spin polarization of a quantum Hall ferromagnet
is obtained. The field dependences of the position and height of the maximum in
the temperature dependence of the susceptibility are found to fit well to power
laws, which are critically analyzed in relation to the recently discussed
behavior in Landau's theory. As revealed by the spin correlation functions and
the specific heat at low fields, our theory provides an improved description of
magnetic short-range order as compared with the random phase approximation. In
one dimension and at very low fields, two maxima in the temperature dependence
of the specific heat are found. The Bethe-ansatz data for the field dependences
of the position and height of the low-temperature maximum are described by
power laws. At higher fields in one and two dimensions, the temperature of the
specific heat maximum linearly increases with the field.Comment: 9 pages, 9 figure
Spin singlet small bipolarons in Nb-doped BaTiO3
The magnetic susceptibility and electrical resistivity of n-type
BaTi{1-x}Nb{x}O3 have been measured over a wide temperature range. It is found
that, for 0 < x < 0.2, dopant electrons form immobile spin singlet small
bipolarons with binding energy around 110 meV. For x = 0.2, a maximum in the
electrical resistivity around 15 K indicates a crossover from band to hopping
transport of the charge carriers, a phenomenon expected but rarely observed in
real polaronic systems.Comment: 5 pages, 4 figure
Giant enhancement of anisotropy by electron-phonon interaction
Anisotropic electron-phonon interaction is shown to lead to the anisotropic
polaron effect. The resulting anisotropy of the polaron band is an exponential
function of the electron-phonon coupling and might be as big as . This
also makes anisotropy very sensitive to small changes of coupling and implies
wide variations of anisotropy among compounds of similar structure. The isotope
effect on mass anisotropy is predicted. Polaron masses are obtained by an exact
Quantum Monte Carlo method. Implications for high-temperature superconductors
are briefly discussed.Comment: 5 pages, 4 figure
Thermodynamics of low dimensional spin-1/2 Heisenberg ferromagnets in an external magnetic field within Green function formalism
The thermodynamics of low dimensional spin-1/2 Heisenberg ferromagnets (HFM)
in an external magnetic field is investigated within a second-order two-time
Green function formalism in the wide temperature and field range. A crucial
point of the proposed scheme is a proper account of the analytical properties
for the approximate transverse commutator Green function obtained as a result
of the decoupling procedure. A good quantitative description of the correlation
functions, magnetization, susceptibility, and heat capacity of the HFM on a
chain, square and triangular lattices is found for both infinite and
finite-sized systems. The dependences of the thermodynamic functions of 2D HFM
on the cluster size are studied. The obtained results agree well with the
corresponding data found by Bethe ansatz, exact diagonalization, high
temperature series expansions, and quantum Monte Carlo simulations.Comment: 11 pages, 14 figure
Mobile small polaron
Extending the Froehlich polaron problem to a discrete ionic lattice we study
a polaronic state with a small radius of the wave function but a large size of
the lattice distortion. We calculate the energy dispersion and the effective
mass of the polaron with the 1/\lambda perturbation theory and with the exact
Monte Carlo method in the nonadiabatic and adiabatic regimes, respectively. The
``small'' Froehlich polaron is found to be lighter than the small Holstein
polaron by one or more orders of magnitude.Comment: 4 pages, 4 figures, published versio
Thermal variational principle and gauge fields
A Feynman-Jensen version of the thermal variational principle is applied to
hot gauge fields, Abelian as well as non-Abelian: scalar electrodynamics
(without scalar self-coupling) and the gluon plasma. The perturbatively known
self-energies are shown to derive by variation from a free quadratic
(''Gaussian'') trial Lagrangian. Independence of the covariant gauge fixing
parameter is reached (within the order studied) after a reformulation of
the partition function such that it depends on only even powers of the gauge
field. Also static properties (Debye screening) are reproduced this way. But
because of the present need to expand the variational functional, the method
falls short of its potential nonperturbative power.Comment: 36 pages, LaTeX, no figures. Updated version: new title, section on
static properties and some references adde