We study electronic transport through a quantum point contact, where the interaction between the electrons is approximated by a contact potential. Our numerical approach is based on the nonequilibrium Green-function technique which is evaluated at the Hartree-Fock level. We show that this approach allows us to reproduce relevant features of the so-called "0.7 anomaly" observed in the conductance at low temperatures, including the characteristic features in recent shot-noise measurements. This is consistent with a spin-splitting interpretation of the process, and indicates that the 0.7 anomaly should also be observable in transport experiments with ultracold fermionic atoms.
We perform a comparative study of the quantum and classical transport probabilities of low-energy quasiparticles ballistically traversing normal and Andreev two-dimensional open cavities with a Sinai-billiard shape. We focus on the dependence of the transport on the strength of an applied magnetic field B. With increasing field strength the classical dynamics changes from mixed to regular phase space. Averaging out the quantum fluctuations, we find an excellent agreement between the quantum and classical transport coefficients in the complete range of field strengths. This allows an overall description of the nonmonotonic behavior of the average magnetoconductance in terms of the corresponding classical trajectories, thus, establishing a basic tool useful in the design and analysis of experiments.
Abstract. -We outline a generic ratchet mechanism for creating directed spin-polarized currents in ac-driven double well or double dot structures by employing resonant spin transfer through the system engineered by local external magnetic fields. We show its applicability to semiconductor nanostructures by considering coherent transport through two coupled lateral quantum dots, where the energy levels of the two dots exhibit opposite Zeeman spin splitting. We perform numerical quantum mechanical calculations for the I-V characteristics of this system in the nonlinear regime, which requires a self-consistent treatment of the charge redistribution due to the applied finite bias. We show that this setting enables nonzero averaged net spin currents in the absence of net charge transport.Introduction. -The field of semiconductor spintronics has seen rapid progress lately, yet there are still many obstacles on the way from fundamental research to operating spin-based devices [1]. The creation of spin polarized currents is one basic requirement for the realization of semiconductor spintronics systems that share the prospect of being able to outperform conventional electronics. Due to a better controlability and faster processing times it is favorable to generate those currents by electrical means, e.g. by the variation of (contact) voltages. Promising classes of devices include spin pumps [2][3][4][5], spin rectification [6] and spin ratchets [7][8][9][10][11][12]. These proposals share the common idea to generate directed spin currents, e.g. mediated by spin-orbit interaction, upon time variation of external potentials. Here we focus on spin ratchets, a generalization of the particle quantum ratchet mechanism [13][14][15]. In such systems with broken spatial symmetry, pure spin currents are generated by means of an ac-driving with no net average bias. This idea has been put forward for both, nonlinearly driven coherent conductors [7][8][9], as well as conductors in the dissipative regime, where Brownian particle motion is converted into directed spin currents [10][11][12]. While a net spin current could be shown to exist for the different settings, its magnitude is difficult to predict and an optimization towards larger spin currents is often not evident.Here we propose another, generic, spin ratchet mecha-
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