Spin transitions in time-dependent regular and random magnetic fields

Journals published by the American Physical Society can be found at http://journals.aps.org/We study the transition between Zeeman levels of an arbitrary spin placed into a regular time-dependent magnetic field and a random field with the Gaussian distribution. One component of the regular field changes its sign at some moment of time, whereas another component does not change substantially. The noise is assumed to be fast. In this assumption we find analytically the ensemble average of the spin density matrix and its fluctuations

Spin Hall and spin-diagonal conductivity in the presence of Rashba and Dresselhaus spin-orbit coupling

Journals published by the American Physical Society can be found at http://journals.aps.org/We investigate the spin-current linear response conductivity tensor to an electric field in a paramagnetic two-dimensional electron gas with both Rashba and Dresselhaus spin-orbit coupling in the weak scattering regime within the Born approximation. In the usual sample parameters, and for cross-sectional size smaller than the spin-coherence length, the spin-Hall conductivity depends only on the sign of the difference in magnitude of the Rashba and Dresselhaus coupling except within a narrow window where both coupling strengths are equal. We also find that a spin current is generated in the direction of the driving field for a non-zero Dresselhaus term. Possible experimental setups for its detection, taking into account the finite mobility and typical parameters of current samples, are discussed

Topology by Design in Magnetic nano-Materials: Artificial Spin Ice

Artificial Spin Ices are two dimensional arrays of magnetic, interacting nano-structures whose geometry can be chosen at will, and whose elementary degrees of freedom can be characterized directly. They were introduced at first to study frustration in a controllable setting, to mimic the behavior of spin ice rare earth pyrochlores, but at more useful temperature and field ranges and with direct characterization, and to provide practical implementation to celebrated, exactly solvable models of statistical mechanics previously devised to gain an understanding of degenerate ensembles with residual entropy. With the evolution of nano--fabrication and of experimental protocols it is now possible to characterize the material in real-time, real-space, and to realize virtually any geometry, for direct control over the collective dynamics. This has recently opened a path toward the deliberate design of novel, exotic states, not found in natural materials, and often characterized by topological properties. Without any pretense of exhaustiveness, we will provide an introduction to the material, the early works, and then, by reporting on more recent results, we will proceed to describe the new direction, which includes the design of desired topological states and their implications to kinetics.Comment: 29 pages, 13 figures, 116 references, Book Chapte

Towards a matrix mechanics framework for dynamic protein network

Protein–protein interaction networks are currently visualized by software generated interaction webs based upon static experimental data. Current state is limited to static, mostly non-compartmental network and non time resolved protein interactions. A satisfactory mathematical foundation for particle interactions within a viscous liquid state (situation within the cytoplasm) does not exist nor do current computer programs enable building dynamic interaction networks for time resolved interactions. Building mathematical foundation for intracellular protein interactions can be achieved in two increments (a) trigger and capture the dynamic molecular changes for a select subset of proteins using several model systems and high throughput time resolved proteomics and, (b) use this information to build the mathematical foundation and computational algorithm for a compartmentalized and dynamic protein interaction network. Such a foundation is expected to provide benefit in at least two spheres: (a) understanding physiology enabling explanation of phenomenon such as incomplete penetrance in genetic disorders and (b) enabling several fold increase in biopharmaceutical production using impure starting materials

Properties of Graphene: A Theoretical Perspective

In this review, we provide an in-depth description of the physics of monolayer and bilayer graphene from a theorist's perspective. We discuss the physical properties of graphene in an external magnetic field, reflecting the chiral nature of the quasiparticles near the Dirac point with a Landau level at zero energy. We address the unique integer quantum Hall effects, the role of electron correlations, and the recent observation of the fractional quantum Hall effect in the monolayer graphene. The quantum Hall effect in bilayer graphene is fundamentally different from that of a monolayer, reflecting the unique band structure of this system. The theory of transport in the absence of an external magnetic field is discussed in detail, along with the role of disorder studied in various theoretical models. We highlight the differences and similarities between monolayer and bilayer graphene, and focus on thermodynamic properties such as the compressibility, the plasmon spectra, the weak localization correction, quantum Hall effect, and optical properties. Confinement of electrons in graphene is nontrivial due to Klein tunneling. We review various theoretical and experimental studies of quantum confined structures made from graphene. The band structure of graphene nanoribbons and the role of the sublattice symmetry, edge geometry and the size of the nanoribbon on the electronic and magnetic properties are very active areas of research, and a detailed review of these topics is presented. Also, the effects of substrate interactions, adsorbed atoms, lattice defects and doping on the band structure of finite-sized graphene systems are discussed. We also include a brief description of graphane -- gapped material obtained from graphene by attaching hydrogen atoms to each carbon atom in the lattice.Comment: 189 pages. submitted in Advances in Physic

Temperature Control of Fimbriation Circuit Switch in Uropathogenic Escherichia coli: Quantitative Analysis via Automated Model Abstraction

Uropathogenic Escherichia coli (UPEC) represent the predominant cause of urinary tract infections (UTIs). A key UPEC molecular virulence mechanism is type 1 fimbriae, whose expression is controlled by the orientation of an invertible chromosomal DNA element—the fim switch. Temperature has been shown to act as a major regulator of fim switching behavior and is overall an important indicator as well as functional feature of many urologic diseases, including UPEC host-pathogen interaction dynamics. Given this panoptic physiological role of temperature during UTI progression and notable empirical challenges to its direct in vivo studies, in silico modeling of corresponding biochemical and biophysical mechanisms essential to UPEC pathogenicity may significantly aid our understanding of the underlying disease processes. However, rigorous computational analysis of biological systems, such as fim switch temperature control circuit, has hereto presented a notoriously demanding problem due to both the substantial complexity of the gene regulatory networks involved as well as their often characteristically discrete and stochastic dynamics. To address these issues, we have developed an approach that enables automated multiscale abstraction of biological system descriptions based on reaction kinetics. Implemented as a computational tool, this method has allowed us to efficiently analyze the modular organization and behavior of the E. coli fimbriation switch circuit at different temperature settings, thus facilitating new insights into this mode of UPEC molecular virulence regulation. In particular, our results suggest that, with respect to its role in shutting down fimbriae expression, the primary function of FimB recombinase may be to effect a controlled down-regulation (rather than increase) of the ON-to-OFF fim switching rate via temperature-dependent suppression of competing dynamics mediated by recombinase FimE. Our computational analysis further implies that this down-regulation mechanism could be particularly significant inside the host environment, thus potentially contributing further understanding toward the development of novel therapeutic approaches to UPEC-caused UTIs

Disorder effects in the anomalous Hall effect induced by Berry curvature RID B-5617-2009

Journals published by the American Physical Society can be found at http://journals.aps.org/We describe the charge transport in ferromagnets with spin-orbit coupled Bloch bands by combining the wave-packet evolution equations with the classical Bolzmann equation. This approach can be justified in the limit of smooth disorder potential. Besides the skew scattering contribution, we demonstrate how other effects of disorder appear which are closely linked to the Berry curvature of the Bloch states associated with the wave packet. We show that, although being of the same order of magnitude as the clean limit contribution, generally disorder corrections depend differently on various parameters and can lead to the sign reversal of the Hall current as the function of the chemical potential in systems with a nonconstant Berry curvature in momentum space. Earlier conclusions on the effects of disorder on the anomalous Hall effect depended stricly on the lack of momentum dependence of the Berry curvature in the models studied and generalizations of their findings to other systems with more complicated band structures were unjustified

Sub-picosecond temporal resolution of anomalous Hall currents in GaAs

Abstract The anomalous Hall (AH) and spin Hall effects are important tools for the generation, control, and detection of spin and spin-polarized currents in solids and, thus, hold promises for future spintronic applications. Despite tremendous work on these effects, their ultrafast dynamic response is still not well explored. Here, we induce ultrafast AH currents in a magnetically-biased semiconductor by optical femtosecond excitation at room temperature. The currents’ dynamics are studied by detecting the simultaneously emitted THz radiation. We show that the temporal shape of the AH currents can be extracted by comparing its THz radiation to the THz radiation emitted from optically induced currents whose temporal shape is well known. We observe a complex temporal shape of the AH currents suggesting that different microscopic origins contribute to the current dynamics. This is further confirmed by photon energy dependent measurements revealing a current inversion at low optical excitation intensities. Our work is a first step towards full time resolution of AH and spin Hall currents and helps to better understand the underlying microscopic origins, being a prerequisite for ultrafast spintronic applications using such currents