8,545 research outputs found
Phase separation in systems with charge ordering
A simple model of charge ordering is considered. It is shown explicitly that
at any deviation from half-filling () the system is unstable with
respect to phase separation into charge ordered regions with and
metallic regions with smaller electron or hole density. Possible structure of
this phase-separated state (metallic droplets in a charge-ordered matrix)is
discussed. The model is extended to account for the strong Hund-rule onsite
coupling and the weaker intersite antiferromagnetic exchange. An analysis of
this extended model allows us to determine the magnetic structure of the
phase-separated state and to reveal the characteristic features of manganites
and other substances with charge ordering.Comment: 9 pages, revte
Double-exchange model: phase separation versus canted spins
We study the competition between different possible ground states of the
double-exchange model with strong ferromagnetic exchange interaction between
itinerant electrons and local spins. Both for classical and quantum treatment
of the local spins the homogeneous canted state is shown to be unstable against
a phase separation. The conditions for the phase separation into the mixture of
the antiferromagnetic and ferromagnetic/canted states are given. We also
discuss another possible realization of the phase-separated state:
ferromagnetic polarons embedded into an antiferromagnetic surrounding. The
general picture of a percolated state, which emerges from these considerations,
is discussed and compared with results of recent experiments on doped
manganaties.Comment: 10 pages, revtex, modified text and 2 new figure
Universal low-energy properties of three two-dimensional particles
Universal low-energy properties are studied for three identical bosons
confined in two dimensions. The short-range pair-wise interaction in the
low-energy limit is described by means of the boundary condition model. The
wave function is expanded in a set of eigenfunctions on the hypersphere and the
system of hyper-radial equations is used to obtain analytical and numerical
results. Within the framework of this method, exact analytical expressions are
derived for the eigenpotentials and the coupling terms of hyper-radial
equations. The derivation of the coupling terms is generally applicable to a
variety of three-body problems provided the interaction is described by the
boundary condition model. The asymptotic form of the total wave function at a
small and a large hyper-radius is studied and the universal logarithmic
dependence in the vicinity of the triple-collision point is
derived. Precise three-body binding energies and the scattering length
are calculated.Comment: 30 pages with 13 figure
Adsorption and two-body recombination of atomic hydrogen on He-He mixture films
We present the first systematic measurement of the binding energy of
hydrogen atoms to the surface of saturated He-He mixture films.
is found to decrease almost linearly from 1.14(1) K down to 0.39(1) K, when the
population of the ground surface state of He grows from zero to
cm, yielding the value K cm
for the mean-field parameter of H-He interaction in 2D. The experiments
were carried out with overall He concentrations ranging from 0.1 ppm to 5 %
as well as with commercial and isotopically purified He at temperatures
70...400 mK. Measuring by ESR the rate constants and for
second-order recombination of hydrogen atoms in hyperfine states and we
find the ratio to be independent of the He content and to
grow with temperature.Comment: 4 pages, 4 figures, all zipped in a sigle file. Submitted to Phys.
Rev. Let
Small-scale phase separation in doped anisotropic antiferromagnets
We analyze the possibility of the nanoscale phase separation manifesting
itself in the formation of ferromagnetic (FM) polarons (FM droplets) in the
general situation of doped anisotropic three- and two-dimensional
antiferromagnets. In these cases, we calculate the shape of the most
energetically favorable droplets. We show that the binding energy and the
volume of a FM droplet in the three-dimensional (3D) case depend only upon two
universal parameters and , where and are effective
antiferromagnetic (AFM) exchange and hopping integrals, respectively. In the
two-dimensional (2D) case, these parameters have the form and . The most favorable shape of a
ferromagnetic droplet corresponds to an ellipse in the 2D case and to an
ellipsoid in the 3D case.Comment: 6 pages, 1 figure, RevTe
Condensation and vortex formation in Bose-gas upon cooling
The mechanism for the transition of a Bose gas to the superfluid state via
thermal fluctuations is considered. It is shown that in the process of external
cooling some critical fluctuations (instantons) are formed above the critical
temperature. The probability of the instanton formation is calculated in the
three and two-dimensional cases. It is found that this probability increases as
the system approaches the transition temperature. It is shown that the
evolution of an individual instanton is impossible without the formation of
vortices in its superfluid part
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