47 research outputs found
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 with the exponent
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 (Frhlich 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.