90 research outputs found
Spin tunneling through an indirect barrier
Spin-dependent tunneling through an indirect bandgap barrier like the
GaAs/AlAs/GaAs heterostructure along [001] direction is studied by the
tight-binding method. The tunneling is characterized by the proportionality of
the Dresselhaus Hamiltonians at and points in the barrier and by
Fano resonances. The present results suggest that large spin polarization can
be obtained for energy windows that exceed significantly the spin splitting. We
also formulate two conditions that are necessary for the existence of energy
windows with large polarization.Comment: 19 pages, 7 figure
Effect of the Coulomb repulsion on the {\it ac} transport through a quantum dot
We calculate in a linear response the admittance of a quantum dot out of
equilibrium. The interaction between two electrons with opposite spins
simultaneously residing on the resonant level is modeled by an Anderson
Hamiltonian. The electron correlations lead to the appearence of a new feature
in the frequency dependence of the conductance. For certain parameter values
there are two crossover frequencies between a capacitive and an inductive
behavior of the imaginary part of the admittance. The experimental implications
of the obtained results are briefly discussed.Comment: 13 pages, REVTEX 3.0, 2 .ps figures from [email protected],
NUB-308
Charging effects in the ac conductance of a double barrier resonant tunneling structure
There have been many studies of the linear response ac conductance of a
double barrier resonant tunneling structure (DBRTS). While these studies are
important, they fail to self-consistently include the effect of time dependent
charge density in the well. In this paper, we calculate the ac conductance by
including the effect of time dependent charge density in the well in a
self-consistent manner. The charge density in the well contributes to both the
flow of displacement currents and the time dependent potential in the well. We
find that including these effects can make a significant difference to the ac
conductance and the total ac current is not equal to the average of
non-selfconsitently calculated conduction currents in the two contacts, an
often made assumption. This is illustrated by comparing the results obtained
with and without the effect of the time dependent charge density included
properly
What is novel in quantum transport for mesoscopics?
The understanding of mesoscopic transport has now attained an ultimate
simplicity. Indeed, orthodox quantum kinetics would seem to say little about
mesoscopics that has not been revealed - nearly effortlessly - by more popular
means. Such is far from the case, however. The fact that kinetic theory remains
very much in charge is best appreciated through the physics of a quantum point
contact. While discretization of its conductance is viewed as the exclusive
result of coherent, single-electron-wave transmission, this does not begin to
address the paramount feature of all metallic conduction: dissipation. A
perfect quantum point contact still has finite resistance, so its ballistic
carriers must dissipate the energy gained from the applied field. How do they
manage that? The key is in standard many-body quantum theory, and its
conservation principles.Comment: 10 pp, 3 figs. Invited talk at 50th Golden Jubilee DAE Symposium,
BARC, Mumbai, 200
Ballistic transport is dissipative: the why and how
In the ballistic limit, the Landauer conductance steps of a mesoscopic
quantum wire have been explained by coherent and dissipationless transmission
of individual electrons across a one-dimensional barrier. This leaves untouched
the central issue of conduction: a quantum wire, albeit ballistic, has finite
resistance and so must dissipate energy. Exactly HOW does the quantum wire shed
its excess electrical energy? We show that the answer is provided, uniquely, by
many-body quantum kinetics. Not only does this inevitably lead to universal
quantization of the conductance, in spite of dissipation; it fully resolves a
baffling experimental result in quantum-point-contact noise. The underlying
physics rests crucially upon the action of the conservation laws in these open
metallic systems.Comment: Invited Viewpoint articl
Quantum Dynamical Echoes in the Spin 'Diffusion' in Mesoscopic Systems
The evolution of local spin polarization in finite systems involves
interference phenomena that give rise to {\bf quantum dynamical echoes }and
non-ergodic behavior. We predict the conditions to observe these echoes by
exploiting the NMR sequences devised by Zhang et al. [Phys. Rev. Lett. {\bf %
69}, 2149 (1992)], which uses a rare C as {\bf local probe }for a
dipolar coupled H spin system. The non-ideality of this probe when testing
mesoscopic systems is carefully analyzed revealing the origin of various
striking experimental features.Comment: 4 pages, Revtex, 3 Figures available upon reques
Microscopic theory of quantum-transport phenomena in mesoscopic systems: A Monte Carlo approach
A theoretical investigation of quantum-transport phenomena in mesoscopic
systems is presented. In particular, a generalization to ``open systems'' of
the well-known semiconductor Bloch equations is proposed. The presence of
spatial boundary conditions manifest itself through self-energy corrections and
additional source terms in the kinetic equations, whose form is suitable for a
solution via a generalized Monte Carlo simulation. The proposed approach is
applied to the study of quantum-transport phenomena in double-barrier
structures as well as in superlattices, showing a strong interplay between
phase coherence and relaxation.Comment: to appear in Phys. Rev. Let
Fluctuations in the transmission properties of a quantum dot with interface roughness and impurities
Non Linear Current Response of a Many-Level Tunneling System: Higher Harmonics Generation
The fully nonlinear response of a many-level tunneling system to a strong
alternating field of high frequency is studied in terms of the
Schwinger-Keldysh nonequilibrium Green functions. The nonlinear time dependent
tunneling current is calculated exactly and its resonance structure is
elucidated. In particular, it is shown that under certain reasonable conditions
on the physical parameters, the Fourier component is sharply peaked at
, where is the spacing between
two levels. This frequency multiplication results from the highly nonlinear
process of photon absorption (or emission) by the tunneling system. It is
also conjectured that this effect (which so far is studied mainly in the
context of nonlinear optics) might be experimentally feasible.Comment: 28 pages, LaTex, 7 figures are available upon request from
[email protected], submitted to Phys.Rev.
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