486 research outputs found
Kondo lattice on the edge of a two-dimensional topological insulator
We revisit the problem of a single quantum impurity on the edge of a
two-dimensional time-reversal invariant topological insulator and show that the
zero temperature phase diagram contains a large local moment region for
antiferromagnetic Kondo coupling which was missed by previous poor man's
scaling treatments. The combination of an exact solution at the so-called
decoupling point and a renormalization group analysis \`a la
Anderson-Yuval-Hamann allows us to access the regime of strong
electron-electron interactions on the edge and strong Kondo coupling. We apply
similar methods to the problem of a regular one-dimensional array of quantum
impurities interacting with the edge liquid. When the edge electrons are at
half-filling with respect to the impurity lattice, the system remains gapless
unless the Luttinger parameter of the edge is less than 1/2, in which case
two-particle backscattering effects drive the system to a gapped phase with
long-range Ising antiferromagnetic order. This is in marked contrast with the
gapped disordered ground state of the ordinary half-filled one-dimensional
Kondo lattice.Comment: 18 pages, 3 figures; fixed typos, updated reference
Universal crossing probability in anisotropic systems
Scale-invariant universal crossing probabilities are studied for critical
anisotropic systems in two dimensions. For weakly anisotropic standard
percolation in a rectangular-shaped system, Cardy's exact formula is
generalized using a length-rescaling procedure. For strongly anisotropic
systems in 1+1 dimensions, exact results are obtained for the random walk with
absorbing boundary conditions, which can be considered as a linearized
mean-field approximation for directed percolation. The bond and site directed
percolation problem is itself studied numerically via Monte Carlo simulations
on the diagonal square lattice with either free or periodic boundary
conditions. A scale-invariant critical crossing probability is still obtained,
which is a universal function of the effective aspect ratio r_eff=c r where
r=L/t^z, z is the dynamical exponent and c is a non-universal amplitude.Comment: 7 pages, 4 figure
"The Ising model on spherical lattices: dimer versus Monte Carlo approach"
We study, using dimer and Monte Carlo approaches, the critical properties and
finite size effects of the Ising model on honeycomb lattices folded on the
tetrahedron. We show that the main critical exponents are not affected by the
presence of conical singularities. The finite size scaling of the position of
the maxima of the specific heat does not match, however, with the scaling of
the correlation length, and the thermodynamic limit is attained faster on the
spherical surface than in corresponding lattices on the torus.Comment: 25 pages + 6 figures not included. Latex file. FTUAM 93-2
Local functional models of critical correlations in thin-films
Recent work on local functional theories of critical inhomogeneous fluids and
Ising-like magnets has shown them to be a potentially exact, or near exact,
description of universal finite-size effects associated with the excess
free-energy and scaling of one-point functions in critical thin films. This
approach is extended to predict the two-point correlation function G in
critical thin-films with symmetric surface fields in arbitrary dimension d. In
d=2 we show there is exact agreement with the predictions of conformal
invariance for the complete spectrum of correlation lengths as well as the
detailed position dependence of the asymptotic decay of G. In d=3 and d>=4 we
present new numerical predictions for the universal finite-size correlation
length and scaling functions determining the structure of G across the
thin-film. Highly accurate analytical closed form expressions for these
universal properties are derived in arbitrary dimension.Comment: 4 pages, 1 postscript figure. Submitted to Phys Rev Let
Local renormalization method for random systems
In this paper, we introduce a real-space renormalization transformation for
random spin systems on 2D lattices. The general method is formulated for random
systems and results from merging two well known real space renormalization
techniques, namely the strong disorder renormalization technique (SDRT) and the
contractor renormalization (CORE). We analyze the performance of the method on
the 2D random transverse field Ising model (RTFIM).Comment: 12 pages, 13 figures. Submitted to the Special Issue on "Quantum
Information and Many-Body Theory", New Journal of Physics. Editors: M.B.
Plenio, J. Eiser
Droplet shapes on structured substrates and conformal invariance
We consider the finite-size scaling of equilibrium droplet shapes for fluid
adsorption (at bulk two-phase co-existence) on heterogeneous substrates and
also in wedge geometries in which only a finite domain of the
substrate is completely wet. For three-dimensional systems with short-ranged
forces we use renormalization group ideas to establish that both the shape of
the droplet height and the height-height correlations can be understood from
the conformal invariance of an appropriate operator. This allows us to predict
the explicit scaling form of the droplet height for a number of different
domain shapes. For systems with long-ranged forces, conformal invariance is not
obeyed but the droplet shape is still shown to exhibit strong scaling
behaviour. We argue that droplet formation in heterogeneous wedge geometries
also shows a number of different scaling regimes depending on the range of the
forces. The conformal invariance of the wedge droplet shape for short-ranged
forces is shown explicitly.Comment: 20 pages, 7 figures. (Submitted to J.Phys.:Cond.Mat.
Finite-size scaling and conformal anomaly of the Ising model in curved space
We study the finite-size scaling of the free energy of the Ising model on
lattices with the topology of the tetrahedron and the octahedron. Our
construction allows to perform changes in the length scale of the model without
altering the distribution of the curvature in the space. We show that the
subleading contribution to the free energy follows a logarithmic dependence, in
agreement with the conformal field theory prediction. The conformal anomaly is
given by the sum of the contributions computed at each of the conical
singularities of the space, except when perfect order of the spins is precluded
by frustration in the model.Comment: 4 pages, 4 Postscript figure
Quasi-2D Confinement of a BEC in a Combined Optical and Magnetic Potential
We have added an optical potential to a conventional Time-averaged Orbiting
Potential (TOP) trap to create a highly anisotropic hybrid trap for ultracold
atoms. Axial confinement is provided by the optical potential; the maximum
frequency currently obtainable in this direction is 2.2 kHz for rubidium. The
radial confinement is independently controlled by the magnetic trap and can be
a factor of 700 times smaller than in the axial direction. This large
anisotropy is more than sufficient to confine condensates with ~10^5 atoms in a
Quasi-2D (Q2D) regime, and we have verified this by measuring a change in the
free expansion of the condensate; our results agree with a variational model.Comment: 11 pages, 10 figur
Bekenstein entropy bound for weakly-coupled field theories on a 3-sphere
We calculate the high temperature partition functions for SU(Nc) or U(Nc)
gauge theories in the deconfined phase on S^1 x S^3, with scalars, vectors,
and/or fermions in an arbitrary representation, at zero 't Hooft coupling and
large Nc, using analytical methods. We compare these with numerical results
which are also valid in the low temperature limit and show that the Bekenstein
entropy bound resulting from the partition functions for theories with any
amount of massless scalar, fermionic, and/or vector matter is always satisfied
when the zero-point contribution is included, while the theory is sufficiently
far from a phase transition. We further consider the effect of adding massive
scalar or fermionic matter and show that the Bekenstein bound is satisfied when
the Casimir energy is regularized under the constraint that it vanishes in the
large mass limit. These calculations can be generalized straightforwardly for
the case of a different number of spatial dimensions.Comment: 32 pages, 12 figures. v2: Clarifications added. JHEP versio
Logarithmic Corrections for Spin Glasses, Percolation and Lee-Yang Singularities in Six Dimensions
We study analytically the logarithmic corrections to the critical exponents
of the critical behavior of correlation length, susceptibility and specific
heat for the temperature and the finite-size scaling behavior, for a generic
theory at its upper critical dimension (six). We have also computed
the leading correction to scaling as a function of the lattice size. We
distinguish the obtained formulas to the following special cases: percolation,
Lee-Yang (LY) singularities and -component spin glasses. We have compared
our results for the Ising spin glass case with numerical simulations finding a
very good agreement. Finally, and using the results obtained for the Lee-Yang
singularities in six dimensions, we have computed the logarithmic corrections
to the singular part of the free energy for lattice animals in eight
dimensions.Comment: 18 pages. We have extended the computation to lattice animals in
eight dimensions. To be published in Journal of Physics
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