37 research outputs found
Colossal Magnetoresistance using the Small Polaron Picture with Finite Bandwidth Effects
We present a small polaron picture and show that finite bandwidth effects are
important to understand colossal magnetoresistance. Besides the polaron size
parameter, we show that there is another parameter (adiabaticity parameter)
that is relevant to studying magnetoresistance. We find that for fixed values
of the polaron size parameter an increase in the adiabaticity parameter
increases the magnetoresistance. The magnetic transition is studied within a
mean field approach. We point out important oversights in the literature. We
find that for the reported values of the bandwidth (based on band structure
calculations) and for experimentally determined values of activation energy and
Debye frequency, the calculated values of the magnetoresistance compare
favorably with experimental ones. We calculate the optical conductivity too and
find that there is reasonable agreement with experiment.Comment: 11 pages, 4 figures, Accepted in Physical Review
Mixed Charge-Spin Response Functions of an Arbitrarily Polarized Electron Gas
In this paper, using different approaches we demonstrate the equality of the
two mixed charge-spin response functions of a spin-polarized electron gas when
orbital effects are negligible. Within a generalized STLS approximation we show
that the two mixed responses are equal. We also present arguments for the
equality of the two dynamic responses by considering a symmetry of the
effective screened interaction between two opposite spin electrons.
Furthermore, using the reflection symmetry of the system and the fact that the
hamiltonian is real we prove rigorously that the two responses coincide
identically.Comment: 4 page
Many-player entangled state solutions in game theory problems
We propose a non-classical multi-player entangled state which eliminates the
need for communication, yet can solve problems (that require coordination)
better than classical approaches. For the entangled state, we propose a slater
determinant of all allowed states of a filled band in a condensed matter system
-- the integer quantum Hall state at filling factor 1. Such a state gives the
best solution (i.e., best Nash equilibrium) for some classical stochastic
problems where classical solutions are far from ideal
Small Magnetic Polaron Picture of Colossal Magnetoresistance in Manganites
We present a small-but-sizeable magnetic polaron picture where transport at
high temperatures is activated while at low temperatures it is band-like. We
show that both double exchange and finite bandwidth effects are important to
understand colossal magnetoresistance as well as the coincidence of the
metal-insulator and the ferromagnetic transitions in manganites. The magnetic
transition is explained using band-like motion of the polarons.Comment: 4 pages, 2 figures, corrected forma
Coexistence of long-range orders in a Bose-Holstein model
Exploring supersolidity in naturally occurring and artificially designed
systems has been and will continue to be an area of immense interest. Here, we
study how superfluid and charge-density-wave (CDW) states cooperate or compete
in a minimal model for hard-core-bosons (HCBs) coupled locally to optical
phonons: a two-dimensional Bose-Holstein model. Our study is restricted to the
parameter regimes of strong HCB-phonon coupling and non-adiabaticity. We use
Quantum Monte Carlo simulation (involving stochastic-series-expansion
technique) to study phase transitions and to investigate whether we have
homogeneous or phase-separated coexistence. The effective Hamiltonian involves,
besides a nearest-neighbor hopping and a nearest-neighbor repulsion, sizeable
double-hopping terms (obtained from second-order perturbation). At densities
not far from half-filling, in the parameter regime where the double-hopping
terms are non-negligible (negligible) compared to the nearest-neighbor hopping,
we get checkerboard-supersolidity (phase separation) with CDW being
characterized by ordering wavevector .Comment: Revised versio
Supersolidity in a Bose-Holstein model
We derive an effective d-dimensional Hamiltonian for a system of
hard-core-bosons coupled to optical phonons in a lattice. At non-half-fillings,
a superfluid-supersolid transition occurs at intermediate boson-phonon
couplings, while at strong-couplings the system phase separates. We demonstrate
explicitly that the presence of next-nearest-neighbor hopping and
nearest-neighbor repulsion leads to supersolidity. Thus we present a
microscopic mechanism for the homogeneous coexistence of charge-density-wave
and superfluid orders
Quantum phase transition in Bose-Holstein model in two dimensions
We derive an effective d-dimensional Hamiltonian for a system of
hard-core-bosons coupled to optical phonons in a lattice. Away from
half-filling, we show that the presence of next-nearest-neighbor hopping in the
effective Hamiltonian leads to a superfluid-to-supersolid transition at
intermediate boson-phonon (b-p) couplings, while at strong-couplings the system
phase separates. However, at half-filling and at a critical b-p coupling (as in
the xxz-model), the system undergoes a superfluid-to-charge-density-wave
transition without any signature of supersolidity. Our analyses is based on
extensive calculations of the structure factor, the superfluid fraction, the
Bose-Einstein condensate fraction, and the system energy at various fillings.
We present a phase diagram for this system and compare it to that of the
xxz-model. We also demonstrate explicitly that the next-nearest-neighbor
hopping (in the absence of nearest-neighbor hopping) in the effective
Hamiltonian leads only to a single transition, i.e., a first-order
superfluid-to-supersolid transition
Analytic treatment of a trading market model
We mathematically analyze a simple market model where trading at each point
in time involves only two agents with the sum of their money being conserved
and with neither parties resulting with negative money after the interaction
process. The exchange involves random re-distribution among the two players of
a fixed fraction of their total money. We obtain a simple integral nonlinear
equation for the money distribution. We find that the zero savings and finite
savings cases belong to different universality classes. While the zero savings
case can be solved analytically, the finite savings solution is obtained by
numerically solving the integral equation. We find remarkable agreement with
results obtained by other researchers using sophisticated numerical techniques.Comment: 2 pages, RevTeX4, 1 ps figure, to be published in Physica Scripta T:
Proc. Vol. `Unconventional Applications of Statistical Physics', March, 200
A distribution function analysis of wealth distribution
We develop a general framework to analyze the distribution functions of
wealth and income. Within this framework we study wealth distribution in a
society by using a model which turns on two-party trading for poor people while
for rich people interaction with wealthy entities (huge reservoir) is relevant.
At equilibrium, the interaction with wealthy entities gives a power law
(Pareto-like) behavior in the wealth distribution while the two party
interaction gives a distribution similar to that reported earlier.Comment: 6 pages and 5 PS figure
Study of the ferromagnetic-insulator phase in manganites
Understanding the coexistence of ferromagnetism and insulating behavior in
manganites is an unsolved problem. We propose a localized-band model involving
effective intermediate-range electron-electron (electron-hole) repulsion
(attraction) generated by cooperative electron-phonon interaction. Double
exchange mechanism, involving holes virtually hopping to nearest neighbors and
back, produces magnetic polarons in an antiferromagnetic environment; when
these magnetic polarons coalesce and percolate the system, we get a
ferromagnetic insulator. Ferromagnetism gets more pronounced when the holes
(doping) increases or when the ratio hopping/polaronic-energy dominates over
superexchange-coupling/hopping.Comment: 11 figure