Random Network Models and Quantum Phase Transitions in Two Dimensions

An overview of the random network model invented by Chalker and Coddington, and its generalizations, is provided. After a short introduction into the physics of the Integer Quantum Hall Effect, which historically has been the motivation for introducing the network model, the percolation model for electrons in spatial dimension 2 in a strong perpendicular magnetic field and a spatially correlated random potential is described. Based on this, the network model is established, using the concepts of percolating probability amplitude and tunneling. Its localization properties and its behavior at the critical point are discussed including a short survey on the statistics of energy levels and wave function amplitudes. Magneto-transport is reviewed with emphasis on some new results on conductance distributions. Generalizations are performed by establishing equivalent Hamiltonians. In particular, the significance of mappings to the Dirac model and the two dimensional Ising model are discussed. A description of renormalization group treatments is given. The classification of two dimensional random systems according to their symmetries is outlined. This provides access to the complete set of quantum phase transitions like the thermal Hall transition and the spin quantum Hall transition in two dimension. The supersymmetric effective field theory for the critical properties of network models is formulated. The network model is extended to higher dimensions including remarks on the chiral metal phase at the surface of a multi-layer quantum Hall system.Comment: 176 pages, final version, references correcte

DIRECTION OF ISOMETRIC BALLISTIC FORCE IS RELATED TO ANTAGONISTIC MUSCULE DISCHARGE

The purpose of this study was to investigate the relationship between electromyographic (EMG) activities of human thigh muscles (vastus medialis, vastus lateralis, rectus femoris, biceps femoris and semitendinosus) and the direction of knee extension force during ramp and ballistic contractions. Four subjects exerted isometric knee extension forces at a target force level of 40% of maximum voluntary contraction (MVC) at various speeds. Variations of EMG amplitudes of all thigh muscles during ballistic contraction were much larger than those during ramp contractions. Only biceps femoris was related to the direction of force. These results suggest that EMG activity of biceps femoris muscle is an important factor for deciding direction of isometric ballistic knee extension force

Mesoscopic Hall effect driven by chiral spin order

A Hall effect due to spin chirality in mesoscopic systems is predicted. We consider a 4-terminal Hall system including local spins with geometry of a vortex domain wall, where strong spin chirality appears near the center of vortex. The Fermi energy of the conduction electrons is assumed to be comparable to the exchange coupling energy where the adiabatic approximation ceases to be valid. Our results show a Hall effect where a voltage drop and a spin current arise in the transverse direction. The similarity between this Hall effect and the conventional spin Hall effect in systems with spin-orbit interaction is pointed out.Comment: 4 pages, 4 figure

Comparison of two linearization schemes for the nonlinear bending problem of a beam pinned at both ends

The nonlinear bending problem of a constant cross-section simply supported beam pinned at both ends and subject to a uniformly distributed load qðxÞ is analyzed in detail. The numerical integration of the two-point boundary value problem (BVP) derived for the nonlinear Timoshenko beam is tackled through two different linearization schemes, the multi-step transversal linearization (MTrL) and the multi-step tangential linearization (MTnL), proposed by Viswanath and Roy (2007). The fundamentals of these linearization techniques are to replace the nonlinear part of the governing ODEs through a set of conditionally linearized ODE systems at the nodal grid points along the neutral axis, ensuring the intersection between the solution manifolds (transversally in the MTrL and tangentially in the MTnL). In this paper, the solution values are determined at grid points by means of a centered finite differences method with multipoint linear constraints (Keller, 1969), and a simple iterative strategy. The analytical solution for this kind of bending problem, including the extensional effects, can be worked out by integration of the governing two-point BVP equations (Monleón et al., 2008). Finally, the comparison of analytical and numerical results shows the better ability of MTnL with the proposed iterative strategy to reproduce the theoretical behavior of the beam for each load step, because the restraint of equating derivatives in MTnL leads to further closeness between solution paths of the governing ODEs and the linearized ones, in comparison with MTrL. This result is opposed to the conclusion reached in Viswanath and Roy (2007), where the relative errors produced by MTrL are said to be smaller than the MTnL ones for the simply supported beam and the tip-loaded cantilever beam problems. 2009 Elsevier Ltd. All rights reserved.Merli Gisbert, R.; Lazaro, C.; Monleón Cremades, S.; Domingo Cabo, A. (2010). Comparison of two linearization schemes for the nonlinear bending problem of a beam pinned at both ends. International Journal of Solids and Structures. 47(6):865-874. doi:10.1016/j.ijsolstr.2009.12.001S86587447

The Anderson transition: time reversal symmetry and universality

We report a finite size scaling study of the Anderson transition. Different scaling functions and different values for the critical exponent have been found, consistent with the existence of the orthogonal and unitary universality classes which occur in the field theory description of the transition. The critical conductance distribution at the Anderson transition has also been investigated and different distributions for the orthogonal and unitary classes obtained.Comment: To appear in Physical Review Letters. Latex 4 pages with 4 figure

Anderson transition in three-dimensional disordered systems with symplectic symmetry

The Anderson transition in a 3D system with symplectic symmetry is investigated numerically. From a one-parameter scaling analysis the critical exponent $\nu$ of the localization length is extracted and estimated to be $\nu = 1.3 \pm 0.2$. The level statistics at the critical point are also analyzed and shown to be scale independent. The form of the energy level spacing distribution $P(s)$ at the critical point is found to be different from that for the orthogonal ensemble suggesting that the breaking of spin rotation symmetry is relevant at the critical point.Comment: 4 pages, revtex, to appear in Physical Review Letters. 3 figures available on request either by fax or normal mail from [email protected] or [email protected]

Free-space optical communication employing subcarrier modulation and spatial diversity in atmospheric turbulence channel

An expression for the bit error rate of a multiple subcarrier intensity-modulated atmospheric optical communication system employing spatial diversity is derived. Spatial diversity is used to mitigate scintillation caused by atmospheric turbulence, which is assumed to obey lognormal distribution. Optimal but complex maximum ratio, equal gain combining (EGC) and relatively simple selection combining spatial diversity techniques in a clear atmosphere are considered. Each subcarrier is modulated using binary phase shift keying. Laser irradiance is subsequently modulated by a subcarrier signal, and a direct detection PIN receiver is employed (i.e. intensity modulation/direction detection). At a subcarrier level, coherent demodulation is used to extract the transmitted data/information. The performance of on–off-keying is also presented and compared with the subcarrier intensity modulation under the same atmospheric conditions

Multifractal properties of critical eigenstates in two-dimensional systems with symplectic symmetry

The multifractal properties of electronic eigenstates at the metal-insulator transition of a two-dimensional disordered tight-binding model with spin-orbit interaction are investigated numerically. The correlation dimensions of the spectral measure $\widetilde{D}_{2}$ and of the fractal eigenstate $D_{2}$ are calculated and shown to be related by $D_{2}=2\widetilde{D}_{2}$. The exponent $\eta=0.35\pm 0.05$ describing the energy correlations of the critical eigenstates is found to satisfy the relation $\eta=2-D_{2}$.Comment: 6 pages RevTeX; 3 uuencoded, gzipped ps-figures to appear in J. Phys. Condensed Matte