Strong disorder renormalization group on fractal lattices: Heisenberg models and magnetoresistive effects in tight binding models

We use a numerical implementation of the strong disorder renormalization group (RG) method to study the low-energy fixed points of random Heisenberg and tight-binding models on different types of fractal lattices. For the Heisenberg model new types of infinite disorder and strong disorder fixed points are found. For the tight-binding model we add an orbital magnetic field and use both diagonal and off-diagonal disorder. For this model besides the gap spectra we study also the fraction of frozen sites, the correlation function, the persistent current and the two-terminal current. The lattices with an even number of sites around each elementary plaquette show a dominant $\phi_0=h/e$ periodicity. The lattices with an odd number of sites around each elementary plaquette show a dominant $\phi_0/2$ periodicity at vanishing diagonal disorder, with a positive weak localization-like magnetoconductance at infinite disorder fixed points. The magnetoconductance with both diagonal and off-diagonal disorder depends on the symmetry of the distribution of on-site energies.Comment: 19 pages, 20 figure

Complexation of DNA with positive spheres: phase diagram of charge inversion and reentrant condensation

The phase diagram of a water solution of DNA and oppositely charged spherical macroions is studied. DNA winds around spheres to form beads-on-a-string complexes resembling the chromatin 10 nm fiber. At small enough concentration of spheres these "artificial chromatin" complexes are negative, while at large enough concentrations of spheres the charge of DNA is inverted by the adsorbed spheres. Charges of complexes stabilize their solutions. In the plane of concentrations of DNA and spheres the phases with positive and negative complexes are separated by another phase, which contains the condensate of neutral DNA-spheres complexes. Thus when the concentration of spheres grows, DNA-spheres complexes experience condensation and resolubilization (or reentrant condensation). Phenomenological theory of the phase diagram of reentrant condensation and charge inversion is suggested. Parameters of this theory are calculated by microscopic theory. It is shown that an important part of the effect of a monovalent salt on the phase diagram can be described by the nontrivial renormalization of the effective linear charge density of DNA wound around a sphere, due to the Onsager-Manning condensation. We argue that our phenomenological phase diagram or reentrant condensation is generic to a large class of strongly asymmetric electrolytes. Possible implication of these results for the natural chromatin are discussed.Comment: Many corrections to text. SUbmitted to J. Chem. Phy

Energy-dependent relative charge transfer cross sections of Cs+ + Rb(5s, 5p)

Magneto optical trap recoil ion momentum spectroscopy is used to measure energy-dependent charge exchange cross sections in the Cs+ + Rb(5s, 5p) system over a range of projectile energies from 3.2 to 6.4 keV. The measurements are kinematically complete and yield cross sections that are differential in collision energy, scattering angle, and initial and final states

Anomalous Hall Effect of Calcium-doped Lanthanum Cobaltite Films

The Hall resistivity, magnetoresistance, and magnetization of La_{1-x}Ca_{x}CoO_{3} epitaxial films with x between 0.25 and 0.4 grown on lanthanum aluminate were measured in fields up to 7 T. The x=1/3 film, shows a reentrant metal insulator transition. Below 100 K, the x=1/3 and 0.4 films have significant coercivity which increases with decreasing temperature. At low temperature the Hall resistivity remains large and essentially field independent in these films, except for a sign change at the coercive field that is more abrupt than the switching of the magnetization. A unique magnetoresistance behavior accompanies this effect. These results are discussed in terms of a percolation picture and the mixed spin state model for this system. We propose that the low-temperature Hall effect is caused by spin-polarized carriers scattering off of orbital disorder in the spin-ordered clusters.Comment: REVTeX 4, 3 pages with 4 encapsulated postscript graphics. Submitted to MMM 2002 conference proceedings (J. Appl. Phys.

Efficient cooperative spectrum sensing for three-hop cognitive wireless relay networks

This paper is concerned with cooperative spectrum sensing (CSS) mechanisms in three-hop cognitive wireless relay networks (CWRNs). The data transmission from a source to a destination is realised with the aid of two layers of cognitive radio (CR) users which are in the transmission coverage of two primary users. In this paper, we first propose a new CSS scheme for a layer of CR users to improve the spectrum sensing performance by exploiting both local decisions at the CR users and global decisions at the fusion centre. Particularly, we derive the probabilities of missed detection and false alarm for a practical scenario where all sensing, reporting, and backward channels suffer from Rayleigh fading. The derived expressions not only show that our proposed CSS achieves a better sensing performance than the conventional scheme but also characterise the effects of the fading channels on the sensing reliability. Furthermore, we propose a CSS scheme for two CR layers in a three-hop CWRN using binary XOR operator to help reduce one phase of sensing for a higher system throughput

Cooperative retransmission for wireless regenerative multirelay networks

This paper investigates retransmission (RT) mechanisms in wireless regenerative multirelay networks. Conventionally, the RT can be realized in a cooperative manner with the assistance of all available relays. However, this may result in high overall power consumption due to the RT of the same packets across the nodes, particularly when the number of relays is large. We propose a cooperative RT (CR) scheme based on relay cooperation (RC) and binary xor operations to significantly reduce the number of packets retransmitted to produce a more power-efficient system with nonoverlapped RTs. Significantly, we also derive the error probability of RT decisions at the source and relays and show that the proposed CR scheme improves the reliability of the RTs. Furthermore, by deriving the average number of packets to be retransmitted at the source and relays, we not only show that the proposed CR scheme reduces the number of RTs and removes overlapped retransmitted packets but determine the optimized number of relays used for the RT phase as well. Finally, simulation results are presented to demonstrate the validity of the analytical expressions

Robust online adaptive neural network control for the regulation of treadmill exercises

The paper proposes a robust online adaptive neural network control scheme for an automated treadmill system. The proposed control scheme is based on Feedback-Error Learning Approach (FELA), by using which the plant Jacobian calculation problem is avoided. Modification of the learning algorithm is proposed to solve the overtraining issue, guaranteeing to system stability and system convergence. As an adaptive neural network controller can adapt itself to deal with system uncertainties and external disturbances, this scheme is very suitable for treadmill exercise regulation when the model of the exerciser is unknown or inaccurate. In this study, exercise intensity (measured by heart rate) is regulated by simultaneously manipulating both treadmill speed and gradient in order to achieve fast tracking for which a single input multi output (SIMO) adaptive neural network controller has been designed. Real-time experiment result confirms that robust performance for nonlinear multivariable system under model uncertainties and unknown external disturbances can indeed be achieved. © 2011 IEEE

Negative Magnetoresistance in the Nearest-neighbor Hopping Conduction

We propose a size effect which leads to the negative magnetoresistance in granular metal-insulator materials in which the hopping between two nearest neighbor clusters is the main transport mechanism. We show that the hopping probability increases with magnetic field. This is originated from the level crossing in a few-electron cluster. Thus, the overlap of electronic states of two neighboring clusters increases, and the negative magnetoresistance is resulted.Comment: Latex file, no figur

Analysis and control of geometrically nonlinear responses of piezoelectric FG porous plates with graphene platelets reinforcement using B\'ezier extraction

In this study, we propose an effective numerical approach to analyse and control geometrically nonlinear responses for the functionally graded (FG) porous plates reinforced by graphene platelets (GPLs) integrated with piezoelectric layers. The basis idea is to use isogeometric analysis (IGA) based on the B\'ezier extraction and the $C^0$-type higher-order shear deformation theory ($C^0$-HSDT). By applying the B\'ezier extraction, the original Non-Uniform Rational B-Spline (NURBS) control meshes can be transformed into the B\'ezier elements which allow us to inherit the standard numerical procedure like the finite element method (FEM). The mechanical displacement field is approximated based on the $C^0$-HSDT whilst the electric potential is assumed to be a linear function through the thickness of each piezoelectric sublayer. The FG plate contains the internal pores and GPLs dispersed in the metal matrix either uniformly or non-uniformly according to various different patterns along the thickness of plate. In addition, to control dynamic responses, two piezoelectric layers are perfectly bonded on the top and bottom surfaces of the FG plate. The geometrically nonlinear equations are solved by the Newton-Raphson iterative procedure and the Newmark's time integration scheme. The influences of the porosity coefficients, weight fractions of GPLs as well as the external electrical voltage on the geometrically nonlinear behaviours of the plates with different porosity distributions and GPL dispersion patterns are evidently investigated through numerical examples. Then, a constant displacement and velocity feedback control approaches are adopted to active control the geometrically nonlinear static as well as the dynamic responses of the FG porous plates, where the effect of the structural damping is considered, based on a closed-loop control with piezoelectric sensors and actuators.Comment: 39 pages, 20 figure