Tuning of structure inversion asymmetry by the δ\delta-doping position in (001)-grown GaAs quantum wells

Structure and bulk inversion asymmetry in doped (001)-grown GaAs quantum wells is investigated by applying the magnetic field induced photogalvanic effect. We demonstrate that the structure inversion asymmetry (SIA) can be tailored by variation of the delta-doping layer position. Symmetrically-doped structures exhibit a substantial SIA due to impurity segregation during the growth process. Tuning the SIA by the delta-doping position we grow samples with almost equal degrees of structure and bulk inversion asymmetry.Comment: 4 pages 2 figure

Helicity sensitive terahertz radiation detection by dual-grating-gate high electron mobility transistors

We report on the observation of a radiation helicity sensitive photocurrent excited by terahertz (THz) radiation in dual-grating-gate (DGG) InAlAs/InGaAs/InAlAs/InP high electron mobility transistors (HEMT). For a circular polarization the current measured between source and drain contacts changes its sign with the inversion of the radiation helicity. For elliptically polarized radiation the total current is described by superposition of the Stokes parameters with different weights. Moreover, by variation of gate voltages applied to individual gratings the photocurrent can be defined either by the Stokes parameter defining the radiation helicity or those for linear polarization. We show that artificial non-centrosymmetric microperiodic structures with a two-dimensional electron system excited by THz radiation exhibit a dc photocurrent caused by the combined action of a spatially periodic in-plane potential and spatially modulated light. The results provide a proof of principle for the application of DGG HEMT for all-electric detection of the radiation's polarization state.Comment: 7 pages, 4 figure

Quantifying structure in networks

We investigate exponential families of random graph distributions as a framework for systematic quantification of structure in networks. In this paper we restrict ourselves to undirected unlabeled graphs. For these graphs, the counts of subgraphs with no more than k links are a sufficient statistics for the exponential families of graphs with interactions between at most k links. In this framework we investigate the dependencies between several observables commonly used to quantify structure in networks, such as the degree distribution, cluster and assortativity coefficients.Comment: 17 pages, 3 figure

Spin currents in diluted magnetic semiconductors (extended version)

Spin currents resulting in the zero-bias spin separation have been observed in unbiased diluted magnetic semiconductor structures (Cd,Mn)Te/(Cd,Mg)Te. The pure spin current generated due to the electron gas heating by terahertz radiation is converted into a net electric current by application of an external magnetic field. We demonstrate that polarization of the magnetic ion system enhances drastically the conversion due to the spin-dependent scattering by localized Mn(2+) ions and the giant Zeeman splitting.Comment: 6 pages, 4 figure