V. Marigliano Ramaglia

Spin polarization of electrons with Rashba double-refraction

We demonstrate how the Rashba spin?orbit coupling in semiconductor heterostructures can produce and control a spin-polarized current without ferromagnetic leads. The key idea is to use spin-double refraction of an electronic beam with a nonzero incidence angle. A region where the spin?orbit coupling is present separates the source and the drain without spin?orbit coupling. We show how the transmission and the beam spin polarization critically depend on the incidence angle. The transmission halves when the incidence angle is greater than a limit angle and a significant spin polarization appears. On increasing the spin?orbit coupling one can obtain the modulation of the intensity and of the spin polarization of the output electronic current when the input current is unpolarized. Our analysis shows the possibility of realizing a spin-field-effect transistor based on the propagation of only one mode with the region with spin?orbit coupling, whereas the original Datta and Das device (1990 Appl.?Phys.?Lett. 56 665) uses the spin precession that originates from the interference between two modes with orthogonal spin.

Modeling of strain effects in manganite films

Thickness dependence and strain effects in films of La 1 - x A x MnO 3 perovskites are analyzed in the colossal magnetoresistance regime. The calculations are based on a generalization of a variational approach previously proposed for the study of manganite bulk. It is found that a reduction in the thickness of the film causes a decrease of critical temperature and magnetization, and an increase of resistivity at low temperatures. The strain is introduced through the modifications of in-plane and out-of-plane electron hopping amplitudes due to substrate-induced distortions of the film unit cell. The strain effects on the transition temperature and transport properties are in good agreement with experimental data only if the dependence of the hopping matrix elements on the Mn-O-Mn bond angle is properly taken into account. Finally variations of the electron-phonon coupling linked to the presence of strain turn out to be important in influencing the balance of coexisting phases in the film.

Rashba quantum wire : exact solution and ballistic transport

The effect of Rashba spin-orbit interaction in quantum wires with hard-wall boundaries is discussed. The exact wavefunction and eigenvalue equation are worked out, pointing out the mixing between the spin and spatial parts. The spectral properties are also studied within perturbation theory with respect to the strength of the spin-orbit interaction and diagonalization procedure. A comparison is made with the results of a simple model, the two-band model, that takes account only of the first two sub-bands of the wire. Finally, the transport properties within the ballistic regime are analytically calculated for the two-band model and through a tight-binding Green function for the entire system. Single and double interfaces separating regions with different strengths of spin-orbit interaction are analysed by injecting carriers into the first and the second sub-band. It is shown that in the case of a single interface the spin polarization in the Rashba region is different from zero, and in the case of two interfaces the spin polarization shows oscillations due to spin-selective bound states.

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: the Anderson-Holstein and two-site Su-Schrieffer-Heeger (SSH) models. As concerns the oscillator dynamics, we are able to recover a Langevin equation confirming what has been found by other authors with different approaches and indicating 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 and gate and bias voltages. The range of validity of the adiabatic approximation is established in a systematic way by analyzing the behavior 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 nonmonotonic behavior. In this same regime of parameters, the dynamical effects favor 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

Spectral, optical, and transport properties of the adiabatic anisotropic Holstein model: Application to slightly doped organic semiconductors

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

. 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.

Effects of electron-phonon coupling near and within the insulating Mott phase

Spectral, optical, and transport properties of an anisotropic three-dimensional Holstein model are studied within the adiabatic approximation. The parameter regime is appropriate for organic semiconductors used in single-crystal-based field-effect transistors. Different approaches have been used to solve the model: the self-consistent Born approximation valid for weak electron-phonon coupling, the coherent potential approximation exact for infinite dimensions, and numerical diagonalization for finite lattices. With increasing temperature, the width of the spectral functions gets larger and larger, making the approximation of a quasiparticle less accurate. On the contrary, their peak positions are never strongly renormalized in comparison with the bare ones. As expected, the density of states is characterized by an exponential tail corresponding to localized states at low temperature. For weak electron-lattice coupling, the optical conductivity follows a Drude behavior, while for intermediate electron-lattice coupling, a temperature-dependent peak is present at low frequency. For high temperatures and low particle densities, the mobility always exhibits a power-law behavior as a function of temperature. With decreasing particle density, at low temperature, the mobility shows a transition from metallic to insulating behavior. Results are discussed in connection with available experimental data.

Electronic transport within a quasi-two-dimensional model for rubrene single-crystal field effect transistors

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.

Effects of electron coupling to intramolecular and intermolecular vibrational modes on the transport properties of single-crystal organic semiconductors

The role of the electron-phonon interaction in the Holstein-Hubbard model is investigated in the metallic phase close to the Mott transition and in the insulating Mott phase. The model is studied by means of a variational slave boson technique. At half-filling, mean-field static quantities are in good agreement with the results obtained by numerical techniques. By taking into account gaussian fluctuations, an analytic expression of the spectral density is derived in the Mott insulating phase showing that an increase of the electron-phonon coupling leads to a sensitive reduction of the Mott gap through a reduced effective repulsion. The relation of the results with recent experimental observations in strongly correlated systems is discussed.

Infrared absorption of the charge-ordering phase: Lattice effects

Spectral and transport properties of the quasi-two-dimensional adiabatic Su-Schrieffer-Heeger model are studied, adjusting the parameters in order to model rubrene single-crystal field effect transistors with small but finite density of injected charge carriers. We show that, with increasing temperature