14,750 research outputs found
Localized reversible nanoscale phase separation in Pr_0.63Ca_0.37MnO_3 single crystal using a scanning tunneling microscope tip
We report the destabilization of the charge ordered insulating (COI) state in
a localized region of Pr_0.63Ca_0.37MnO_3 single crystal by current injection
using a scanning tunneling microscope tip. This leads to controlled phase
separation and formation of localized metallic nanoislands in the COI matrix
which have been detected by local tunneling conductance mapping. The metallic
regions thus created persist even after reducing the injected current to lower
values. The original conductance state can be restored by injecting a current
of similar magnitude but of opposite polarity. We thus achieve reversible
nanoscale phase separation that gives rise to the possibility to "write, read,
and erase" nanosized conducting regions in an insulating matrix with high
spatial resolution.Comment: 8 pages, 4 figures, Appl. Phys. Lett (accepted for publication
Black Hole Geometries in Noncommutative String Theory
We obtain a generalized Schwarzschild (GS-) and a generalized
Reissner-Nordstrom (GRN-) black hole geometries in (3+1)-dimensions, in a
noncommutative string theory. In particular, we consider an effective theory of
gravity on a curved -brane in presence of an electromagnetic (EM-) field.
Two different length scales, inherent in its noncommutative counter-part, are
exploited to obtain a theory of effective gravity coupled to an U(1)
noncommutative gauge theory to all orders in . It is shown that the
GRN-black hole geometry, in the Planckian regime, reduces to the GS-black hole.
However in the classical regime it may be seen to govern both
Reissner-Nordstrom and Schwarzschild geometries independently. The emerging
notion of 2D black holes evident in the frame-work are analyzed. It is argued
that the -string in the theory may be described by the near horizon 2D black
hole geometry, in the gravity decoupling limit. Finally, our analysis explains
the nature of the effective force derived from the nonlinear EM-field and
accounts for the Hawking radiation phenomenon in the formalism.Comment: 30 pages, 2 figure
An Algorithm to Generate Classical Solutions for String Effective Action
It is shown explicitly, that a number of solutions for the background field
equations of the string effective action in space-time dimension D can be
generated from any known lower dimensional solution, when background fields
have only time dependence. An application of the result to the two dimensional
charged black hole is presented. The case of background with more general
coordinate dependence is also discussed.Comment: 12 page
Study of supersolidity in the two-dimensional Hubbard-Holstein model
We derive an effective Hamiltonian for the two-dimensional Hubbard-Holstein
model in the regimes of strong electron-electron and strong electron-phonon
interactions by using a nonperturbative approach. In the parameter region where
the system manifests the existence of a correlated singlet phase, the effective
Hamiltonian transforms to a Hamiltonian for hard-core-bosons
on a checkerboard lattice. We employ quantum Monte Carlo simulations, involving
stochastic-series-expansion technique, to obtain the ground state phase
diagram. At filling , as the strength of off-site repulsion increases, the
system undergoes a first-order transition from a superfluid to a diagonal
striped solid with ordering wavevector or
. Unlike the one-dimensional situation, our results in the
two-dimensional case reveal a supersolid phase (corresponding to the diagonal
striped solid) around filling and at large off-site repulsions.
Furthermore, for small off-site repulsions, we witness a valence bond solid at
one-fourth filling and tiny phase-separated regions at slightly higher
fillings.Comment: Accepted in EPJ
The Generalised Raychaudhuri Equations : Examples
Specific examples of the generalized Raychaudhuri Equations for the evolution
of deformations along families of dimensional surfaces embedded in a
background dimensional spacetime are discussed. These include string
worldsheets embedded in four dimensional spacetimes and two dimensional
timelike hypersurfaces in a three dimensional curved background. The issue of
focussing of families of surfaces is introduced and analysed in some detail.Comment: 8 pages (Revtex, Twocolumn format). Corrected(see section on string
worldsheets), reorganised and shortened slightl
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