395 research outputs found
Elastic constants from microscopic strain fluctuations
Fluctuations of the instantaneous local Lagrangian strain
, measured with respect to a static ``reference''
lattice, are used to obtain accurate estimates of the elastic constants of
model solids from atomistic computer simulations. The measured strains are
systematically coarse- grained by averaging them within subsystems (of size
) of a system (of total size ) in the canonical ensemble. Using a
simple finite size scaling theory we predict the behaviour of the fluctuations
as a function of and extract elastic
constants of the system {\em in the thermodynamic limit} at nonzero
temperature. Our method is simple to implement, efficient and general enough to
be able to handle a wide class of model systems including those with singular
potentials without any essential modification. We illustrate the technique by
computing isothermal elastic constants of the ``soft'' and the hard disk
triangular solids in two dimensions from molecular dynamics and Monte Carlo
simulations. We compare our results with those from earlier simulations and
density functional theory.Comment: 24 pages REVTEX, 10 .ps figures, version accepted for publication in
Physical Review
Effective interactions and melting of a one dimensional defect lattice within a two-dimensional confined colloidal solid
We report Monte Carlo studies of a two-dimensional soft colloidal crystal
confined in a strip geometry by parallel walls. The wall-particle interaction
has corrugations along the length of the strip. Compressing the crystal by
decreasing the distance between the walls induces a structural transition
characterized by the sudden appearance of a one-dimensional array of extended
defects each of which span several lattice parameters, a "soliton staircase".
We obtain the effective interaction between these defects. A Lindemann
criterion shows that the reduction of dimensionality causes a finite periodic
chain of these defects to readily melt as the temperature is raised. We discuss
possible experimental realizations and speculate on potential applications.Comment: 4 pages 5 embedded figure
The Observation of Formation and Annihilation of Solitons and Standing Strain Wave Superstructures in a Two-Dimensional Colloidal Crystal
Confining a colloidal crystal within a long narrow channel produced by two
parallel walls can be used to impose a meso-scale superstructure of a
predominantly mechanical elastic character [Chui et al., EPL 2008, 83, 58004].
When the crystal is compressed in the direction perpendicular to the walls, we
obtain a structural transition when the number of rows of particles parallel to
the walls decreases by one. All the particles of this vanishing row are
distributed throughout the crystal. If the confining walls are structured (say
with a corrugation along the length of the walls), then these extra particles
are distributed neither uniformly nor randomly; rather, defect structures are
created along the boundaries resembling "soliton staircases", inducing a
non-uniform strain pattern within the crystal. Here we study the conditions of
stability, formation and annihilation of these solitons using a coarse grained
description of the dynamics. The processes are shown by comparing superimposed
configurations as well as molecular animations obtained from our simulations.
Also the corresponding normal and shear stresses during the transformation are
calculated. A study of these dynamical processes should be useful for
controlling strain wave superstructures in the self-assembly of various nano-
and meso scaled particles.Comment: 26 pages, 6 figure
Dynamical transitions of a driven Ising interface
We study the structure of an interface in a three-dimensional Ising system created by an external nonuniform field H(r,t). H changes sign over a two-dimensional plane of arbitrary orientation. When the field is pulled with velocity ve, [i.e., H(r,t)=H(r-vt)], the interface undergoes several dynamical transitions. For low velocities it is pinned by the field profile and moves along with it, the distribution of local slopes undergoing a series of commensurate-incommensurate transitions. For large vthe interface depins and grows with Kardar-Parisi-Zhang exponents
Position dependent photodetector from large area reduced graphene oxide thin films
We fabricated large area infrared photodetector devices from thin film of
chemically reduced graphene oxide (RGO) sheets and studied their photoresponse
as a function of laser position. We found that the photocurrent either
increases, decreases or remain almost zero depending upon the position of the
laser spot with respect to the electrodes. The position sensitive photoresponse
is explained by Schottky barrier modulation at the RGO film-electrode
interface. The time response of the photocurrent is dramatically slower than
single sheet of graphene possibly due to disorder from the chemically synthesis
and interconnecting sheets
Manifestation of geometric frustration on magnetic and thermodynamic properties of pyrochlores (X=Ti, Zr)
We present here magnetization, specific heat and Raman studies on
single-crystalline specimens of the first pyrochlore member of
the rare-earth titanate series. Its analogous compound in the
rare-earth zirconate series is also investigated in the polycrystalline form.
The Sm spins in remain unordered down to at least T = 0.5 K. The
absence of magnetic ordering is attributed to very small values of exchange
() and dipolar interaction ()
between the spins in this pyrochlore. In contrast, the pyrochlore
is characterized by a relatively large value of Sm-Sm spin
exchange (); however, long-range ordering of the
spins is not established at least down to T = 0.67 K, due to
frustration of the spins on the pyrochlore lattice. The ground state
of ions in both pyrochlores is a well-isolated Kramer's doublet. The
higher-lying crystal field excitations are observed in the low-frequency region
of the Raman spectra of the two compounds recorded at T = 10 K. At higher
temperatures, the magnetic susceptibility of shows a broad
maximum at T = 140 K while that of changes monotonically. Whereas
is a promising candidate for investigating spin-fluctuations on a
frustrated lattice as indicated by our data, the properties of
seem to conform to a conventional scenario where geometrical frustration of the
spin exclude their long-range ordering.Comment: 24 pages, 6 figures, Accepted for publication in Phys. Rev.
Anomalous Raman scattering from phonons and electrons of superconducting FeSe
We report interesting anomalies in the temperature dependent Raman spectra of
FeSe measured from 3K to 300K in the spectral range from 60 to 1800
cm and determine their origin using complementary first-principles
density functional calculations. A phonon mode near 100 cm exhibits a
sharp increase by 5% in frequency below a temperature T ( 100
K) attributed to strong spin-phonon coupling and onset of short-range
antiferromagnetic order. In addition, two high frequency modes are observed at
1350 cm and 1600 cm, attributed to electronic Raman scattering
from ()to / -orbitals of Fe.Comment: 19 pages, 4 figures, 1 tabl
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