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Register a new userNonmonotonous classical magneto-conductivity of a two-dimensional electron gas in a disordered array of obstacles
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Magnetotransport measurements in combination with molecular dynamics (MD) simulations on two-dimensional disordered Lorentz gases in the classical regime are reported. In quantitative agreement between experiment and simulation, the magnetoconductivity displays a pronounced peak as a function of perpendicular magnetic field $B$ which cannot be explained in the framework of existing kinetic theories. We show that this peak is linked to the onset of a directed motion of the electrons along the contour of the disordered obstacle matrix when the cyclotron radius becomes smaller than the size of the obstacles. This directed motion leads to transient superdiffusive motion and strong scaling corrections in the vicinity of the insulator-to-conductor transitions of the Lorentz gas.
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We propose the weak localization of magnons in a disordered two-dimensional antiferromagnet. We derive the longitudinal thermal conductivity $kappa_{xx}$ for magnons of a disordered Heisenberg antiferromagnet in the linear-response theory with the linear-spin-wave approximation. We show that the back scattering of magnons is enhanced critically by the particle-particle-type multiple impurity scattering. This back scattering causes a logarithmic suppression of $kappa_{xx}$ with the length scale in two dimensions. We also argue a possible effect of inelastic scattering on the temperature dependence of $kappa_{xx}$. This weak localization is useful to control turning the magnon thermal current on and off.
The conductivity of an electron gas can be alternatively calculated either from the current--current or from the density--density correlation function. Here, we compare these two frequently used formulations of the Kubo formula for the two--dimensional Dirac electron gas by direct evaluations for several special cases. Assuming the presence of weak disorder we investigate perturbatively both formulas at and away from the Dirac point. While to zeroth order in the disorder amplitude both formulations give identical results, with some very strong assumptions though, they show significant discrepancies already in first order. At half filling we evaluate all second order diagrams. Virtually none of the topologically identical diagrams yield the same corrections for both formulations. We conclude that a direct comparison of conductivities of disordered system calculated in both formulas is not possible.
We present a numerical study of the spin Hall effect in a two-dimensional hole gas (2DHG) system in the presence of disorder. We find that the spin Hall conductance (SHC), extrapolated to the thermodynamic limit, remains finite in a wide range of disorder strengths for a closed system on torus. But there is no intrinsic spin Hall accumulation as induced by an external electric field once the disorder is turned on. The latter is examined by performing a Laughlins Gedanken gauge experiment numerically with the adiabatical insertion of a flux quantum in a belt-shaped sample, in which the absence of level crossing is found under the disorder effect. Without disorder, on the other hand, energy levels do cross each other, which results in an oscillating spin-density-modulation at the sample boundary after the insertion of one flux quantum in the belt-shaped system. But the corresponding net spin transfer is only about one order of magnitude smaller than what is expected from the bulk SHC. These apparently contradictory results can be attributed to the violation of the spin conservation law in such a system. We also briefly address the dissipative Fermi surface contribution to spin polarization, which may be relevant to experimental measurements.
Donors in silicon can now be positioned with an accuracy of about one lattice constant, making it possible in principle to form donor arrays for quantum computation or quantum simulation applications. However the multi-valley character of the silicon conduction band combines with central cell corrections to the donor state Hamiltonian to translate atomic scale imperfections in donor placement into strongly disordered inter-donor hybridization. We present a simple model that is able to account accurately for central-cell corrections, and use it to assess the impact of donor-placement disorder on donor array properties in both itinerant and localized limits.
In the present paper we describe the properties induced by disorder on an ultracold gas of Bosonic atoms loaded into a two-dimensional optical lattice with global confinement ensured by a parabolic potential. Our analysis is centered on the spatial distribution of the various phases, focusing particularly on the superfluid properties of the system as a function of external parameters and disorder amplitude. In particular, it is shown how disorder can suppress superfluidity, while partially preserving the system coherence.
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