61 research outputs found
Coulomb drag in double quantum wells with a perpendicular magnetic field
Momentum transfer due to electron-electron interaction (Coulomb drag) between
two quantum wells, separated by a distance , in the presence of a
perpendicular magnetic field, is studied at low temperatures. We find besides
the well known Shubnikov-de Haas oscillations, which also appear in the drag
effect, the momentum transfer is markedly enhanced by the magnetic field.Comment: 8 pages, Revtex, 4 Postscript figures are available upon request,
Accepted by Mod. Phys. Lett.
Positive current noise cross-correlations in capacitively coupled double quantum dots with ferromagnetic leads
We examine cross-correlations (CCs) in the tunneling currents through two
parallel interacting quantum dots coupled to four independent ferromagnetic
electrodes. We find that when either one of the two circuits is in the parallel
configuration with sufficiently strong polarization strength, a new mechanism
of dynamical spin blockade, i.e., a spin-dependent bunching of tunneling
events, governs transport through the system together with the inter-dot
Coulomb interaction, leading to a sign-reversal of the zero-frequency current
CC in the dynamical channel blockade regime, and to enhancement of positive
current CC in the dynamical channel anti-blockade regimes, in contrast to the
corresponding results for the case of paramagnetic leads.Comment: 9 pages, 3 figure
Vanishing spin-Hall current in a diffusive Rashba two-dimensional electron system: A quantum Boltzmann equation approach
We present a quantum Boltzmann equation analysis of the spin-Hall effect in a
diffusive Rashba two-dimensional electron system. Within the framework of the
self-consistent Born approximation, we consider the roles of disorder-induced
quasiclassical relaxation, collisional broadening of the quasiparticles, and
the intracollisional field effect in regard to spin-Hall dynamics. We present
an analytical proof that the spin-Hall current vanishes, independently of the
coupling strength, of the quasiparticle broadening, of temperature and of the
specific form of the isotropic scattering potential. A sum relation of the
collision terms in a helicity basis is also examined.Comment: final version, 11 pages, no figur
Robust negative differential conductance and enhanced shot noise in transport through a molecular transistor with vibration assistance
In this paper, we analyze vibration-assisted sequential tunneling (including
current-voltage characteristics and zero-frequency shot noise) through a
molecular quantum dot with two electronic orbitals asymmetrically coupled to
the internal vibration. We employ rate equations for the case of equilibrated
phonons, and strong Coulomb blockade. We find that a system with a strongly
phonon-coupled ground state orbital and weakly phonon-coupled excited state
orbital exhibits strong negative differential conductance; and it also shows
super-Poissonian current noise. We discuss in detail the reasons and conditions
for the appearance of negative differential conductance.Comment: 6 pages, 6 figures, and 1 tabl
Counting statistics of tunneling through a single molecule: effect of distortion and displacement of vibrational potential surface
We analyze the effects of a distortion of the nuclear potential of a
molecular quantum dot (QD), as well as a shift of its equilibrium position, on
nonequilibrium-vibration-assisted tunneling through the QD with a single level
() coupled to the vibrational mode. For this purpose, we derive an
explicit analytical expression for the Franck-Condon (FC) factor for a
displaced-distorted oscillator surface of the molecule and establish rate
equations in the joint electron-phonon representation to examine the
current-voltage characteristics and zero-frequency shot noise, and skewness as
well. Our numerical analyses shows that the distortion has two important
effects. The first one is that it breaks the symmetry between the excitation
spectra of the charge states, leading to asymmetric tunneling properties with
respect to and . Secondly, distortion (frequency
change of the oscillator) significantly changes the voltage-activated cascaded
transition mechanism, and consequently gives rise to a different nonequilibrium
vibrational distribution from that of the case without distortion. Taken in
conjunction with strongly modified FC factors due to distortion, this results
in some new transport features: the appearance of strong NDC even for a
single-level QD with symmetric tunnel couplings; a giant Fano factor even for a
molecule with an extremely weak electron-phonon interaction; and enhanced
skewness that can have a large negative value under certain conditions.Comment: 29 pages, 11 figures, published versio
Relativistic Green functions in a plane wave gravitational background
We consider a massive relativistic particle in the background of a
gravitational plane wave. The corresponding Green functions for both spinless
and spin 1/2 cases, previously computed by A. Barducci and R. Giachetti
\cite{Barducci3}, are reobtained here by alternative methods, as for example,
the Fock-Schwinger proper-time method and the algebraic method. In analogy to
the electromagnetic case, we show that for a gravitational plane wave
background a semiclassical approach is also sufficient to provide the exact
result, though the lagrangian involved is far from being a quadratic one.Comment: Last paper by Professor Arvind Narayan Vaidya, 18 pages, no figure
- …