The spin-double refraction in two-dimensional electron gas

We briefly review the phenomenon of the spin-double refraction that originates at an interface separating a two-dimensional electron gas with Rashba spin-orbit coupling from a one without. We demonstrate how this phenomenon in semiconductor heterostructures can produce and control a spin-polarized current without ferromagnetic leads

Stochastic dynamics for a single vibrational mode in molecular junctions

We propose a very accurate computational scheme for the dynamics of a classical oscillator coupled to a molecular junction driven by a finite bias, including the finite mass effect. We focus on two minimal models for the molecular junction: Anderson-Holstein (AH) and two-site Su-Schrieffer-Heeger (SSH) models. As concerns the oscillator dynamics, we are able to recover a Langevin equation confirming what found by other authors with different approaches and assessing that quantum effects come from the electronic subsystem only. Solving numerically the stochastic equation, we study the position and velocity distribution probabilities of the oscillator and the electronic transport properties at arbitrary values of electron-oscillator interaction, gate and bias voltages. The range of validity of the adiabatic approximation is established in a systematic way by analyzing the behaviour of the kinetic energy of the oscillator. Due to the dynamical fluctuations, at intermediate bias voltages, the velocity distributions deviate from a gaussian shape and the average kinetic energy shows a non monotonic behaviour. In this same regime of parameters, the dynamical effects favour the conduction far from electronic resonances where small currents are observed in the infinite mass approximation. These effects are enhanced in the two-site SSH model due to the presence of the intermolecular hopping t. Remarkably, for sufficiently large hopping with respect to tunneling on the molecule, small interaction strengths and at intermediate bias (non gaussian regime), we point out a correspondence between the minima of the kinetic energy and the maxima of the dynamical conductance.Comment: 19 pages, 16 figures, submitted to Physical Review

Ballistic transport in one-dimensional loops with Rashba and Dresselhaus spin-orbit coupling

We discuss the combined effect of Rashba and Dresselhaus spin-orbit interactions in polygonal loops formed by quantum wires, when the electron are injected in a node and collected at the opposite one. The conditions that allow perfect localization are found. Furthermore, we investigate the suppression of the Al'tshuler--Aronov--Spivak oscillations that appear, in presence of a magnetic flux, when the electrons are injected and collected at the same node. Finally, we point out that a recent realization of a ballistic spin interferometer can be used to obtain a reliable estimate of the magnitude ratio of the two spin-orbit interactions.\bigskipComment: 6 figure

Ground state features of the Frohlich model

Following the ideas behind the Feynman approach, a variational wave function is proposed for the Fr\"ohlich model. It is shown that it provides, for any value of the electron-phonon coupling constant, an estimate of the polaron ground state energy better than the Feynman method based on path integrals. The mean number of phonons, the average electronic kinetic and interaction energies, the ground state spectral weight and the electron-lattice correlation function are calculated and successfully compared with the best available results.Comment: 6 figure

Probing nonlinear mechanical effects through electronic currents: the case of a nanomechanical resonator acting as electronic transistor

We study a general model describing a self-detecting single electron transistor realized by a suspended carbon nanotube actuated by a nearby antenna. The main features of the device, recently observed in a number of experiments, are accurately reproduced. When the device is in a low current-carrying state, a peak in the current signals a mechanical resonance. On the contrary, a dip in the current is found in high current-carrying states. In the nonlinear vibration regime of the resonator, we are able to reproduce quantitatively the characteristic asymmetric shape of the current-frequency curves. We show that the nonlinear effects coming out at high values of the antenna amplitude are related to the effective nonlinear force induced by the electronic flow. The interplay between electronic and mechanical degrees of freedom is understood in terms of an unifying model including in an intrinsic way the nonlinear effects driven by the external probe.Comment: 7 pages, 3 figures, submitted to Physical Review

Effects of electron coupling to intra- and inter-molecular vibrational modes on the transport properties of single crystal organic semiconductors

Electron coupling to intra- and inter-molecular vibrational modes is investigated in models appropriate to single crystal organic semiconductors, such as oligoacenes. Focus is on spectral and transport properties of these systems beyond perturbative approaches. The interplay between different couplings strongly affects the temperature band renormalization that is the result of a subtle equilibrium between opposite tendencies: band narrowing due to interaction with local modes, band widening due to electron coupling to non local modes. The model provides an accurate description of the mobility as function of temperature: indeed, it has the correct order of magnitude, at low temperatures, it scales as a power-law $T^{-\delta}$ with the exponent $\delta$ larger than unity, and, at high temperatures, shows an hopping behavior with a small activation energy.Comment: 3 Figures, Submitte

Noise-assisted Thouless pump in elastically deformable molecular junctions

We study a Thouless pump realized with an elastically \textit{deformable quantum dot} whose center of mass follows a non-linear stochastic dynamics. The interplay of noise, non-linear effects, dissipation and interaction with an external time-dependent driving on the pumped charge is fully analyzed. The results show that the quantum pumping mechanism not only is not destroyed by the force fluctuations, but it becomes stronger when the forcing signal frequency is tuned close to the resonance of the vibrational mode. The robustness of the quantum pump with temperature is also investigated and an exponential decay of the pumped charge is found when the coupling to the vibrational mode is present. Implications of our results for nano-electromechanical systems are also discussed.Comment: 2 Appendices and figures adde

Behavior of quantum entropies in polaronic systems

Quantum entropies and state distances are analyzed in polaronic systems with short range (Holstein model) and long range (Fr$\ddot{o}$hlich model) electron-phonon coupling. These quantities are extracted by a variational wave function which describes very accurately polaron systems with arbitrary size in all the relevant parameter regimes. With the use of quantum information tools, the crossover region from weak to strong coupling regime can be characterized with high precision. Then, the linear entropy is found to be very sensitive to the range of the electron-phonon coupling and the adiabatic ratio. Finally, the entanglement entropy is studied as a function of the system size pointing out that it not bounded, but scales as the logarithm of the size either for weak electron-phonon coupling or for short range interaction. This behavior is ascribed to the peculiar coupling induced by the single electron itinerant dynamics on the phonon subsystem.Comment: 4 figures, to be published in Phys. Rev.