904 research outputs found
Inducing topological order in a honeycomb lattice
We explore the possibility of inducing a topological insulator phase in a
honeycomb lattice lacking spin-orbit interaction using a metallic (or Fermi
gas) environment. The lattice and the metallic environment interact through a
density-density interaction without particle tunneling, and integrating out the
metallic environment produces a honeycomb sheet with in-plane oscillating
long-ranged interactions. We find the ground state of the interacting system in
a variational mean-field method and show that the Fermi wave vector, kF, of the
metal determines which phase occurs in the honeycomb lattice sheet. This is
analogous to the Ruderman-Kittel-Kasuya-Yosida (RKKY) mechanism in which the
metal's kF determines the interaction profile as a function of the distance.
Tuning kF and the interaction strength may lead to a variety of ordered phases,
including a topological insulator and anomalous quantum-hall states with
complex next-nearest-neighbor hopping, as in the Haldane and the Kane-Mele
model. We estimate the required range of parameters needed for the topological
state and find that the Fermi vector of the metallic gate should be of the
order of 3Pi/8a (with a being the graphene lattice constant). The net coupling
between the layers, which includes screening in the metal, should be of the
order of the honeycomb lattice bandwidth. This configuration should be most
easily realized in a cold-atoms setting with two interacting Fermionic species.Comment: 7 pages; 2 figures; Version 2 - added references; added an appendix
about screenin
Parallels between the dynamics at the noise-perturbed onset of chaos in logistic maps and the dynamics of glass formation
We develop the characterization of the dynamics at the noise-perturbed edge
of chaos in logistic maps in terms of the quantities normally used to describe
glassy properties in structural glass formers. Following the recognition [Phys.
Lett. \textbf{A 328}, 467 (2004)] that the dynamics at this critical attractor
exhibits analogies with that observed in thermal systems close to
vitrification, we determine the modifications that take place with decreasing
noise amplitude in ensemble and time averaged correlations and in diffusivity.
We corroborate explicitly the occurrence of two-step relaxation, aging with its
characteristic scaling property, and subdiffusion and arrest for this system.
We also discuss features that appear to be specific of the map.Comment: Revised version with substantial improvements. Revtex, 8 pages, 11
figure
Continuous thermal melting of a two-dimensional Abrikosov vortex solid
We examine the question of thermal melting of the triangular Abrikosov vortex
solid in two-dimensional superconductors or neutral superfluids. We introduce a
model, which combines lowest Landau level (LLL) projection with the magnetic
Wannier basis to represent degenerate eigenstates in the LLL. Solving the model
numerically via large-scale Monte Carlo simulations, we find clear evidence for
a continuous melting transition, in perfect agreement with the
Kosterlitz-Thouless-Halperin-Nelson-Young theory and with recent experiments.Comment: 4 pages, 2 figures; published versio
Phase coherence and the Nernst effect at magic angles in organic conductors
A giant Nernst signal was recently observed for fields near crystallographic
directions in (TMTSF)PF. Such large Nernst signals are most naturally
associated with the motion of pancake vortices. We propose a model in which
phase coherence is destroyed throughout the sample except in planes closely
aligned with the applied field . A small tilt above or below the plane
changes the direction and density of the penetrating vortices and leads to a
Nernst signal that varies with the tilt angle of as observed. The
resistance notches at magic angles are understood in terms of flux-flow
dissipation from field-induced vortices.Comment: 4 pages, 4 figure
Topological defects in flat nanomagnets: the magnetostatic limit
We discuss elementary topological defects in soft magnetic nanoparticles in
the thin-film geometry. In the limit dominated by magnetostatic forces the
low-energy defects are vortices (winding number n = +1), cross ties (n = -1),
and edge defects with n = -1/2. We obtain topological constraints on the
possible composition of domain walls. The simplest domain wall in this regime
is composed of two -1/2 edge defects and a vortex, in accordance with
observations and numerics.Comment: 3 pages, eps figures. Proceedings of MMM 0
Lateral and normal forces between patterned substrates induced by nematic fluctuations
We consider a nematic liquid crystal confined by two parallel flat substrates
whose anchoring conditions vary periodically in one lateral direction. Within
the Gaussian approximation, we study the effective forces between the patterned
substrates induced by the thermal fluctuations of the nematic director. The
shear force oscillates as function of the lateral shift between the patterns on
the lower and the upper substrates. We compare the strength of this
fluctuation-induced lateral force with the lateral van der Waals force arising
from chemically structured adsorbed monolayers. The fluctuation-induced force
in normal direction is either repulsive or attractive, depending on the model
parameters.Comment: 9 pages, 9 figure
Accurate simulation estimates of cloud points of polydisperse fluids
We describe two distinct approaches to obtaining cloud point densities and
coexistence properties of polydisperse fluid mixtures by Monte Carlo simulation
within the grand canonical ensemble. The first method determines the chemical
potential distribution (with the polydisperse attribute)
under the constraint that the ensemble average of the particle density
distribution matches a prescribed parent form. Within the region
of phase coexistence (delineated by the cloud curve) this leads to a
distribution of the fluctuating overall particle density n, p(n), that
necessarily has unequal peak weights in order to satisfy a generalized lever
rule. A theoretical analysis shows that as a consequence, finite-size
corrections to estimates of coexistence properties are power laws in the system
size. The second method assigns such that an equal peak weight
criterion is satisfied for p(n)\mu(\sigma)$. We show how to ascertain the
requisite weight factor operationally. A theoretical analysis of the second
method suggests that it leads to finite-size corrections to estimates of
coexistence properties which are {\em exponentially small} in the system size.
The scaling predictions for both methods are tested via Monte Carlo simulations
of a novel polydisperse lattice gas model near its cloud curve, the results
showing excellent quantitative agreement with the theory.Comment: 8 pages, 6 figure
Quantum Lifshitz point in the infinite dimensional Hubbard model
We show that the Gutzwiller variational wave function is surprisingly
accurate for the computation of magnetic phase boundaries in the infinite
dimensional Hubbard model. This allows us to substantially extend known phase
diagrams. For both the half-hypercubic and the hypercubic lattice a large part
of the phase diagram is occupied by an incommensurate phase, intermediate
between the ferromagnetic and the paramagnetic phase. In case of the hypercubic
lattice the three phases join at a new quantum Lifshitz point at which the
order parameter is critical and the stiffness vanishes.Comment: 4 pages, 3 figure
Non-universal dynamics of dimer growing interfaces
A finite temperature version of body-centered solid-on-solid growth models
involving attachment and detachment of dimers is discussed in 1+1 dimensions.
The dynamic exponent of the growing interface is studied numerically via the
spectrum gap of the underlying evolution operator. The finite size scaling of
the latter is found to be affected by a standard surface tension term on which
the growth rates depend. This non-universal aspect is also corroborated by the
growth behavior observed in large scale simulations. By contrast, the
roughening exponent remains robust over wide temperature ranges.Comment: 11 pages, 7 figures. v2 with some slight correction
Asymmetry between the electron- and hole-doped Mott transition in the periodic Anderson model
We study the doping driven Mott metal-insulator transition (MIT) in the
periodic Anderson model set in the Mott-Hubbard regime. A striking asymmetry
for electron or hole driven transitions is found. The electron doped MIT at
larger U is similar to the one found in the single band Hubbard model, with a
first order character due to coexistence of solutions. The hole doped MIT, in
contrast, is second order and can be described as the delocalization of
Zhang-Rice singlets.Comment: 18 pages, 19 figure
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