3,293 research outputs found
Corner contribution to the entanglement entropy of strongly-interacting O(2) quantum critical systems in 2+1 dimensions
In a D=2+1 quantum critical system, the entanglement entropy across a
boundary with a corner contains a subleading logarithmic scaling term with a
universal coefficient. It has been conjectured that this coefficient is, to
leading order, proportional to the number of field components N in the
associated O(N) continuum field theory. Using density matrix
renormalization group calculations combined with the powerful numerical linked
cluster expansion technique, we confirm this scenario for the O(2)
Wilson-Fisher fixed point in a striking way, through direct calculation at the
quantum critical points of two very different microscopic models. The value of
this corner coefficient is, to within our numerical precision, twice the
coefficient of the Ising fixed point. Our results add to the growing body of
evidence that this universal term in the R\'enyi entanglement entropy reflects
the number of low-energy degrees of freedom in a system, even for strongly
interacting theories.Comment: 6 pages, 6 figure
Valence Bond Solids and Their Quantum Melting in Hard-Core Bosons on the Kagome Lattice
Using large scale quantum Monte Carlo simulations and dual vortex theory we
analyze the ground state phase diagram of hard-core bosons on the kagome
lattice with nearest neighbor repulsion. In contrast to the case of a
triangular lattice, no supersolid emerges for strong interactions. While a
uniform superfluid prevails at half-filling, two novel solid phases emerge at
densities and . These solids exhibit an only partial
ordering of the bosonic density, allowing for local resonances on a subset of
hexagons of the kagome lattice. We provide evidence for a weakly first-order
phase transition at the quantum melting point between these solid phases and
the superfluid.Comment: 4 pages, 7 figure
Classical and quantum anisotropic Heisenberg antiferromagnets
We study classical and quantum Heisenberg antiferromagnets with exchange
anisotropy of XXZ-type and crystal field single-ion terms of quadratic and
cubic form in a field. The magnets display a variety of phases, including the
spin-flop (or, in the quantum case, spin-liquid) and biconical (corresponding,
in the quantum lattice gas description, to supersolid) phases. Applying
ground-state considerations, Monte Carlo and density matrix renormalization
group methods, the impact of quantum effects and lattice dimension is analysed.
Interesting critical and multicritical behaviour may occur at quantum and
thermal phase transitions.Comment: 13 pages, 14 figures, conferenc
Supersolidity from defect-condensation in the extended boson Hubbard model
We study the ground state phase diagram of the hard-core extended boson
Hubbard model on the square lattice with both nearest- (nn) and
next-nearest-neighbor (nnn) hopping and repulsion, using Gutzwiller mean field
theory and quantum Monte Carlo simulations. We observe the formation of
supersolid states with checkerboard, striped, and quarter-filled crystal
structures, when the system is doped away from commensurate fillings. In the
striped supersolid phase, a strong anisotropy in the superfluid density is
obtained from the simulations; however, the transverse component remains
finite, indicating a true two-dimensional superflow. We find that upon doping,
the striped supersolid transitions directly into the supersolid with
quarter-filled crystal structure, via a first-order stripe melting transition.Comment: Revtex 4, 6 pages, 9 figure
Magnetotail structures in a simulated Earth's magnetosphere
The structure of the magnetotail is investigated in a laboratory simulated magnetosphere. Particular emphasis is placed on the region of distant magnetotail where the closed field line region of the plasma sheet terminates and the process of reconnection takes place. Our study builds upon the previous investigation of the magnetotail where the main results were based on the magnetic field measurements in the tail region of the simulated magnetosphere. In this paper, more elaborate measurements of plasma flow and electric field are presented. Besides these measurements, this region of distant magnetotail is also explored by high resolution imaging with a gated optical imager (GOI) and by digital image analysis. These images clearly reveal a Y-type magnetic neutral line for the northward 'interplanetary' field (IMF) and a usual X-type for the southward IMF that confirms our previous results deduced from the magnetic field measurements. In the neighborhood of these neutral points a strong component of dawn to dusk electric field (E(sub y)) and a counterstreaming plasma flow is also observed. Plasma flow is measured by using a double sided Faraday cup which is also used to measure the y-component of tail current (J(sub y)) at different locations. These measurements reveal that the tail current is not carried by ions as previously thought, rather it is carried by electrons alone
Dynamical structure factors and excitation modes of the bilayer Heisenberg model
Using quantum Monte Carlo simulations along with higher-order spin-wave
theory, bond-operator and strong-coupling expansions, we analyse the dynamical
spin structure factor of the spin-half Heisenberg model on the square-lattice
bilayer. We identify distinct contributions from the low-energy Goldstone modes
in the magnetically ordered phase and the gapped triplon modes in the quantum
disordered phase. In the antisymmetric (with respect to layer inversion)
channel, the dynamical spin structure factor exhibits a continuous evolution of
spectral features across the quantum phase transition, connecting the two types
of modes. Instead, in the symmetric channel we find a depletion of the spectral
weight when moving from the ordered to the disordered phase. While the
dynamical spin structure factor does not exhibit a well-defined distinct
contribution from the amplitude (or Higgs) mode in the ordered phase, we
identify an only marginally-damped amplitude mode in the dynamical singlet
structure factor, obtained from interlayer bond correlations, in the vicinity
of the quantum critical point. These findings provide quantitative information
in direct relation to possible neutron or light scattering experiments in a
fundamental two-dimensional quantum-critical spin system.Comment: 19 pages, 15 figure
Field-Induced Magnetic Order in Quantum Spin Liquids
We study magnetic field-induced three-dimensional ordering transitions in
low-dimensional quantum spin liquids, such as weakly coupled, antiferromagnetic
spin-1/2 Heisenberg dimers and ladders. Using stochastic series expansion
quantum Monte Carlo simulations, thermodynamic response functions are obtained
down to ultra-low temperatures. We extract the critical scaling exponents which
dictate the power-law dependence of the transition temperature on the applied
magnetic field. These are compared with recent experiments on candidate
materials and with predictions for the Bose-Einstein condensation of magnons
obtained in mean-field theory.Comment: RevTex, 4 pages with 5 figure
Modeling association between DNA copy number and gene expression with constrained piecewise linear regression splines
DNA copy number and mRNA expression are widely used data types in cancer
studies, which combined provide more insight than separately. Whereas in
existing literature the form of the relationship between these two types of
markers is fixed a priori, in this paper we model their association. We employ
piecewise linear regression splines (PLRS), which combine good interpretation
with sufficient flexibility to identify any plausible type of relationship. The
specification of the model leads to estimation and model selection in a
constrained, nonstandard setting. We provide methodology for testing the effect
of DNA on mRNA and choosing the appropriate model. Furthermore, we present a
novel approach to obtain reliable confidence bands for constrained PLRS, which
incorporates model uncertainty. The procedures are applied to colorectal and
breast cancer data. Common assumptions are found to be potentially misleading
for biologically relevant genes. More flexible models may bring more insight in
the interaction between the two markers.Comment: Published in at http://dx.doi.org/10.1214/12-AOAS605 the Annals of
Applied Statistics (http://www.imstat.org/aoas/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Supersolid Order from Disorder: Hard-Core Bosons on the Triangular Lattice
We study the interplay of Mott localization, geometric frustration, and
superfluidity for hard-core bosons with nearest-neighbor repulsion on the
triangular lattice. For this model at half-filling, we demonstrate that
superfluidity survives for arbitrarily large repulsion, and that diagonal solid
order emerges in the strongly correlated regime from an order-by-disorder
mechanism. This is thus an unusual example of a stable supersolid phase of
hard-core lattice bosons at a commensurate filling.Comment: 4 pages, 2 figures; finite-size scaling discussion adde
- …