740 research outputs found
Local and Nonlocal Continuum Limits of Ising-Type Energies for Spin Systems
We study, through a !-convergence procedure, the discrete to continuum limit of
Ising-type energies of the form F"(u) = −Pi,j c" i,juiuj , where u is a spin variable defined on a
portion of a cubic lattice "Zd 8 ⌦, ⌦ being a regular bounded open set, and valued in {−1, 1}. If the
constants c" i,j are nonnegative and satisfy suitable coercivity and decay assumptions, we show that all possible !-limits of surface scalings of the functionals F" are finite on BV (⌦; {±1}) and of the
formR Su'(x, ⌫u) dH11 d−1. If such decay assumptions are violated, we show that we may approximate
nonlocal functionals of the form
R
Su
'(⌫u) dHd−1+ K(x, y)g(u(x), u(y)) dxdy. We focus on the
approximation of two relevant examples: fractional perimeters and Ohta–Kawasaki-type energies. Eventually, we provide a general criterion for a ferromagnetic behavior of the energies F" even when
the constants c" i,j change sign. If such a criterion is satisfied, the ground states of F" are still the uniform states 1 and −1 and the continuum limit of the scaled energies is an integral surface energy of the form above
Low-Dimensional Spin Systems: Hidden Symmetries, Conformal Field Theories and Numerical Checks
We review here some general properties of antiferromagnetic Heisenberg spin
chains, emphasizing and discussing the role of hidden symmetries in the
classification of the various phases of the models. We present also some recent
results that have been obtained with a combined use of Conformal Field Theory
and of numerical Density Matrix Renormalization Group techniques.Comment: To be published in the proceedings of the XIII Conference on
"Symmetries in Physics", held in Bregenz (Voralberg, Austria), 21-24/7/2003.
Plain LaTeX2e, 4 EPS figure
Interplay between the edge-state magnetism and long-range Coulomb interaction in zigzag graphene nanoribbons: quantum Monte Carlo study
We perform projective quantum Monte Carlo simulations of zigzag graphene
nanoribbons within a realistic model with long-range Coulomb interactions.
Increasing the relative strength of nonlocal interactions with respect to the
on-site repulsion does not generate a phase transition but has a number of
nontrivial effects. At the single-particle level we observe a marked
enhancement of the Fermi velocity at the Dirac points. At the two-particle
level, spin- and charge-density-wave fluctuations compete. As a consequence,
the edge magnetic moment is reduced but the edge dispersion relation increases
in the sense that the single-particle gap at momentum
grows. We attribute this to nonlocal charge fluctuations which assist the spin
fluctuations to generate the aforementioned gap. In contrast, the net result of
the interaction-induced renormalization of different energy scales is a
constant spin-wave velocity of the edge modes. However, since the particle-hole
continuum is shifted to higher energies---due to the renormalization of the
Fermi velocity---Landau damping is reduced. As a result, a roughly linear
spin-wave-like mode at the edge spreads out through a larger part of the
Brillouin zone.Comment: 11 pages, 11 figures, comment about doped nanoribbon
Quantum criticalities in a two-leg antiferromagnetic S=1/2 ladder induced by a staggered magnetic field
We study a two-leg antiferromagnetic spin-1/2 ladder in the presence of a
staggered magnetic field. We consider two parameter regimes: strong (weak)
coupling along the legs and weak (strong) coupling along the rungs. In both
cases, the staggered field drives the Haldane spin-liquid phase of the ladder
towards a Gaussian quantum criticality. In a generalized spin ladder with a
non-Haldane, spontaneously dimerized phase, the staggered magnetic field
induces an Ising quantum critical regime. In the vicinity of the critical
lines, we derive low-energy effective field theories and use these descriptions
to determine the dynamical response functions, the staggered spin
susceptibility and the string order parameter.Comment: 29 pages of revtex, 10 figure
Does the XY Model have an integrable continuum limit?
The quantum field theory describing the massive O(2) nonlinear sigma-model is
investigated through two non-perturbative constructions: The form factor
bootstrap based on integrability and the lattice formulation as the XY model.
The S-matrix, the spin and current two-point functions, as well as the 4-point
coupling are computed and critically compared in both constructions. On the
bootstrap side a new parafermionic super selection sector is found; in the
lattice theory a recent prediction for the (logarithmic) decay of lattice
artifacts is probed.Comment: 69 pages, 18 figures. Equation (3.20) correcte
Fermionic SK-models with Hubbard interaction: Magnetism and electronic structure
Models with range-free frustrated Ising spin- and Hubbard interaction are
treated exactly by means of the discrete time slicing method. Critical and
tricritical points, correlations, and the fermion propagator, are derived as a
function of temperature T, chemical potential \mu, Hubbard coupling U, and spin
glass energy J. The phase diagram is obtained. Replica symmetry breaking
(RSB)-effects are evaluated up to four-step order (4RSB). The use of exact
relations together with the 4RSB-solutions allow to model exact solutions by
interpolation. For T=0, our numerical results provide strong evidence that the
exact density of states in the spin glass pseudogap regime obeys \rho(E)=const
|E-E_F| for energies close to the Fermi level. Rapid convergence of \rho'(E_F)
under increasing order of RSB is observed. The leading term resembles the
Efros-Shklovskii Coulomb pseudogap of localized disordered fermionic systems in
2D. Beyond half filling we obtain a quadratic dependence of the fermion filling
factor on the chemical potential. We find a half filling transition between a
phase for U>\mu, where the Fermi level lies inside the Hubbard gap, into a
phase where \mu(>U) is located at the center of the upper spin glass pseudogap
(SG-gap). For \mu>U the Hubbard gap combines with the lower one of two SG-gaps
(phase I), while for \mu<U it joins the sole SG-gap of the half-filling regime
(phase II). We predict scaling behaviour at the continuous half filling
transition. Implications of the half-filling transition between the deeper
insulating phase II and phase I for delocalization due to hopping processes in
itinerant model extensions are discussed and metal-insulator transition
scenarios described.Comment: 29 pages, 26 Figures, 4 jpeg- and 3 gif-Fig-files include
Quantum Field Theory for the Three-Body Constrained Lattice Bose Gas -- Part II: Application to the Many-Body Problem
We analyze the ground state phase diagram of attractive lattice bosons, which
are stabilized by a three-body onsite hardcore constraint. A salient feature of
this model is an Ising type transition from a conventional atomic superfluid to
a dimer superfluid with vanishing atomic condensate. The study builds on an
exact mapping of the constrained model to a theory of coupled bosons with
polynomial interactions, proposed in a related paper [11]. In this framework,
we focus by analytical means on aspects of the phase diagram which are
intimately connected to interactions, and are thus not accessible in a mean
field plus spin wave approach. First, we determine shifts in the mean field
phase border, which are most pronounced in the low density regime. Second, the
investigation of the strong coupling limit reveals the existence of a new
collective mode, which emerges as a consequence of enhanced symmetries in this
regime. Third, we show that the Ising type phase transition, driven first order
via the competition of long wavelength modes at generic fillings, terminates
into a true Ising quantum critical point in the vicinity of half filling.Comment: 22 pages, 5 figure
Critical properties of the double-frequency sine-Gordon model with applications
We study the properties of the double-frequency sine--Gordon model in the
vicinity of the Ising quantum phase transition displayed by this model. Using a
mapping onto a generalised lattice quantum Ashkin-Teller model, we obtain
critical and nearly-off-critical correlation functions of various operators. We
discuss applications of the double-sine-Gordon model to one-dimensional
physical systems, like spin chains in a staggered external field and
interacting electrons in a staggered potential.Comment: 51 pages, Latex fil
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