5,900 research outputs found
Driven Disordered Polymorphic Solids: Phases and Phase Transitions, Dynamical Coexistence and Peak Effect Anomalies
We study a model for the depinning and driven steady state phases of a solid
tuned across a polymorphic phase transition between ground states of triangular
and square symmetry. These include pinned states which may have dominantly
triangular or square correlations, a plastically flowing liquid-like phase, a
moving phase with hexatic correlations, flowing triangular and square states
and a dynamic coexistence regime characterized by the complex interconversion
of locally square and triangular regions. We locate these phases in a dynamical
phase diagram. We demonstrate that the apparent power-law orientational
correlations we obtain in our moving hexatic phase arise from circularly
averaging an orientational correlation function with qualitatively different
behaviour in the longitudinal (drive) and transverse directions. The
intermediate coexistence regime exhibits several novel properties, including
substantial enhancement in the current noise, an unusual power-law spectrum of
current fluctuations and striking metastability effects. This noise arises from
the fluctuations of the interface separating locally square and triangular
ordered regions. We demonstrate the breakdown of effective ``shaking
temperature'' treatments in the coexistence regime by showing that such shaking
temperatures are non-monotonic functions of the drive in this regime. Finally
we discuss the relevance of these simulations to the anomalous behaviour seen
in the peak effect regime of vortex lines in the disordered mixed phase of
type-II superconductors. We propose that this anomalous behavior is directly
linked to the behavior exhibited in our simulations in the dynamical
coexistence regime, thus suggesting a possible solution to the problem of the
origin of peak effect anomalies.Comment: 22 pages, double column, higher quality figures available from
author
Magnetoelastic Effects in Iron Telluride
Iron telluride doped lightly with selenium is known to undergo a first order
magneto-structural transition before turning superconducting at higher doping.
We study the effects of magneto-elastic couplings on this transition using
symmetry considerations. We find that the magnetic order parameters are coupled
to the uniform monoclinic strain of the unit cell with one iron per cell, as
well as to the phonons at high symmetry points of the Brillouin zone. In the
magnetic phase the former gives rise to monoclinic distortion while the latter
induces dimerization of the ferromagnetic iron chains due to alternate
lengthening and shortening of the nearest-neighbour iron-iron bonds. We compare
this system with the iron arsenides and propose a microscopic magneto-elastic
Hamiltonian which is relevant for all the iron based superconductors. We argue
that this describes electron-lattice coupling in a system where
electron-electron interaction is crucial.Comment: 5 pages, 2 figure
Driven Disordered Periodic Media with an Underlying Structural Phase Transition
We investigate the driven states of a two-dimensional crystal whose ground
state can be tuned through a square-triangular transition. The depinning of
such a system from a quenched random background potential occurs via a complex
sequence of dynamical states, which include plastic flow states, hexatics,
dynamically stabilized triangle and square phases and intermediate regimes of
phase coexistence. These results are relevant to transport experiments in the
mixed phase of several superconductors which exhibit such structural
transitions as well as to driven colloidal systems whose interactions can be
tuned via surface modifications.Comment: Two-column, 4 pages, figures include
Quantum Hall Bilayer as Pseudospin Magnet
We revisit the physics of electron gas bilayers in the quantum Hall regime
[Nature, 432 (2004) 691; Science, 305 (2004) 950], where transport and
tunneling measurements provided evidence of a superfluid phase being present in
the system. Previously, this behavior was explained by the possible formation
of a BEC of excitons in the half-filled electron bilayers, where empty states
play the role of holes. We discuss the fundamental difficulties with this
scenario, and propose an alternative approach based on a treatment of the
system as a pseudospin magnet. We show that the experimentally observed
tunneling peak can be linked to the XY ferromagnet (FM) to Ising
antiferromagnet (AFM) phase transition of the S=1/2 XXZ pseudospin model,
driven by the change in total electron density. This transition is accompanied
by a qualitative change in the nature of the low energy spin wave dispersion
from a gapless linear mode in the XY-FM phase to a gapped, quadratic mode in
the Ising-AFM phase.Comment: 5 pages, 4 figures; corrected and close to printed versio
Unconventional scanning tunneling conductance spectra for graphene
We compute the tunneling conductance of graphene as measured by a scanning
tunneling microscope (STM) with a normal/superconducting tip. We demonstrate
that for undoped graphene with zero Fermi energy, the first derivative of the
tunneling conductance with respect to the applied voltage is proportional to
the density of states of the STM tip. We also show that the shape of the STM
spectra for graphene doped with impurities depends qualitatively on the
position of the impurity atom in the graphene matrix and relate this
unconventional phenomenon to the pseudopsin symmetry of the Dirac
quasiparticles in graphene. We suggest experiments to test our theory.Comment: 6 pages, 3 figure
Spin Supersolid in Anisotropic Spin-One Heisenberg Chain
We consider an S=1 Heisenberg chain with strong exchange (Delta) and
single--ion uniaxial anisotropy (D) in a magnetic field (B) along the symmetry
axis. The low energy spectrum is described by an effective S=1/2 XXZ model that
acts on two different low energy sectors for a given window of fields. The
vacuum of each sector exhibits Ising-like antiferromagnetic ordering that
coexists with the finite spin stiffness obtained from the exact solution of the
effective XXZ model. In this way, we demonstrate the existence of a spin
supersolid phase. We also compute the full Delta-B quantum phase diagram by
means of a quantum Monte Carlo simulation.Comment: 4+ pages, 2 fig
Probing Disordered Substrates by Imaging the Adsorbate in its Fluid Phase
Several recent imaging experiments access the equilibrium density profiles of
interacting particles confined to a two-dimensional substrate. When these
particles are in a fluid phase, we show that such data yields precise
information regarding substrate disorder as reflected in one-point functions
and two-point correlations of the fluid. Using Monte Carlo simulations and
replica generalizations of liquid state theories, we extract unusual two-point
correlations of time-averaged density inhomogeneities induced by disorder.
Distribution functions such as these have not hitherto been measured but should
be experimentally accessible.Comment: 10 pages revtex 4 figure
Anomalous structural and mechanical properties of solids confined in quasi one dimensional strips
We show using computer simulations and mean field theory that a system of
particles in two dimensions, when confined laterally by a pair of parallel hard
walls within a quasi one dimensional channel, possesses several anomalous
structural and mechanical properties not observed in the bulk. Depending on the
density and the distance between the walls , the system shows
structural characteristics analogous to a weakly modulated liquid, a strongly
modulated smectic, a triangular solid or a buckled phase. At fixed , a
change in leads to many reentrant discontinuous transitions involving
changes in the number of layers parallel to the confining walls depending
crucially on the commensurability of inter-layer spacing with . The solid
shows resistance to elongation but not to shear. When strained beyond the
elastic limit it fails undergoing plastic deformation but surprisingly, as the
strain is reversed, the material recovers completely and returns to its
original undeformed state. We obtain the phase diagram from mean field theory
and finite size simulations and discuss the effect of fluctuations.Comment: 14 pages, 13 figures; revised version, accepted in J. Chem. Phy
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