We have performed numerical calculations of the diagonal and off-diagonal (Hall) components of the conductivity tensor for a system of noninteracting electrons in two dimensions at high magnetic fields (lowest Landau level), in the presence of a random potential. We have considered five different potentials, with varying correlation length, with and without electron-hole symmetry. Our results are consistent, within statistical uncertainties, with a conductivity tensor at the critical energy with both components equal to 0.5e 2 /h. This suggests that the conductivity tensor at the lowest-Landau-level integer quantum Hall transition is universal, independent of the details for short-range random potentials. PACS numbers: 71.50.+t, 71.30.+h, 71.55.Jv In recent years, many investigations have helped provide evidence that the quantum Hall effect (QHE), for both the integer [1] and the fractional case [2], can be understood in terms of critical energies E c at the quantum Hall steps, at which continuous r=0 quantum phase transitions occur. In the case of the integer QHE, this transition is from a state with a Landau band partially populated with (localized) electrons, to a state with a filled Landau band partially depleted by localized holes. For the fractional QHE, the transition is between different (Laughlin) ground states [3,4] with localized quasiparticles and quasiholes, carrying fractional charges, related to the fractional Landau-level occupancy. Experiments [5,6] and numerical [7-9] methods appear to have converged on a universal exponent of the diverging correlation length, at least for the integer QHE in the lowest Landau level.Since the transitions are characterized by steps in the Hall conductivity o xy , it forms the natural coupling constant, and has been treated as such in the field theoretic treatments of the problem [10,11]. Numerical finite-size scaling studies [12,13] for the lowest Landau level are also consistent with such an interpretation. This naturally raises the question of what the value of the coupling constant is at the transition, and whether the value for either or both components of the conductivity tensor is universal, independent of the microscopic details of the problem. Indeed such a universality has been suggested in a series of recent papers [11,14]. Lee, Kivelson, and Zhang [11] have proposed that all these transitions belong to the same universality class, with values of the conductivity tensor simply related to each other in terms of the quasiparticle charge and the Landau-level filling.In this Letter, we use numerical methods to examine the issue of the universality of the critical diagonal (

We study the interplay between a novel vortex-loop unbinding in finite magnetic field at T=T_V and flux-line lattice (FLL) melting at T=T_M in type-II superconductors. The FLL melts due to nucleation of vortex loops parallel to c-axis, connected to flux lines. For moderate anisotropy, phase coherence along c-axis is lost at T_V > T_M due to an ab-plane vortex-loop unbinding with loops located close to thermal FLL fluctuations. For large anisotropy, phase coherence along c-axis is lost at T_V < T_M due to nucleation of ab-plane vortex-loops uncorrelated to flux lines.Comment: 4 pages, 3 postscript figure

We investigate the charge-order transition at zero temperature in a two-leg Hubbard ladder with additional nearest-neighbor Coulomb repulsion V using the Density Matrix Renormalization Group technique. We consider electron densities between quarter and half filling. For quarter filling and U = 8t, we find evidence for a continuous phase transition between a homogeneous state at small V and a broken-symmetry state with "checkerboard" [wavevector Q = (π, π)] charge order at large V . This transition to a checkerboard charge-ordered state remains present at all larger fillings, but becomes discontinuous at sufficiently large filling. We discuss the influence of U/t on the transition and estimate the position of the tricritical points.

Superconducting properties of coupled Hubbard planes are studied by quantum Monte Carlo simulations. It is found that the interlayer coupling supports the pairing mechanism but it does not lead to oE-diagonal long-range order in the purely electronic system. In addition, we have performed Monte Carlo studies based on a recently proposed variational wave function. Yielding identical results, the variational wave function overcomes the sign problem and allows us to analyze ground-state properties of strongly correlated many-body problems for system sizes and complexities not accessible otherwise.

We explore the behavior of the nearest-neighbor Ising spin glass with discrete bonds (± J) on the face-centered cubic lattice using both Monte Carlo simulations and high-temperature expansions. A phase transition to a spin glass phase is found in zero magnetic field (H) at a nonzero temperature (T = Tc), with critical exponents close to that for the simple cubic lattice. In the (H, T)-plane, the contours of constant spin glass susceptibility show an upturn towards higher H at the lowest T, consistent with the possibility of a de Almeida-Thouless line of phase transitions in a field. Further, the upturn is similar to those on the simple cubic lattice and less pronounced than on the four-dimensional hypercubic lattice, despite the higher coordination number of the f.c.c. lattice, suggesting that the upturn is a dimension-related property.