174 research outputs found
Low-temperature transport through a quantum dot between two superconductor leads
We consider a quantum dot coupled to two BCS superconductors with same gap
energies . The transport properties are investigated by means of
infinite- noncrossing approximation. In equilibrium density of states, Kondo
effect shows up as two sharp peaks around the gap bounds. Application of a
finite voltage bias leads these peaks to split, leaving suppressed peaks near
the edges of energy gap of each lead. The clearest signatures of the Kondo
effect in transport are three peaks in the nonlinear differential conductance:
one around zero bias, another two at biases . This result is
consistent with recent experiment. We also predict that with decreasing
temperature, the differential conductances at biases anomalously
increase, while the linear conductance descends.Comment: replaced with revised versio
Symmetry and designability for lattice protein models
Native protein folds often have a high degree of symmetry. We study the
relationship between the symmetries of native proteins, and their
designabilities -- how many different sequences encode a given native
structure. Using a two-dimensional lattice protein model based on
hydrophobicity, we find that those native structures that are encoded by the
largest number of different sequences have high symmetry. However only certain
symmetries are enhanced, e.g. x/y-mirror symmetry and rotation, while
others are suppressed. If it takes a large number of mutations to destabilize
the native state of a protein, then, by definition, the state is highly
designable. Hence, our findings imply that insensitivity to mutation implies
high symmetry. It appears that the relationship between designability and
symmetry results because protein substructures are also designable. Native
protein folds may therefore be symmetric because they are composed of repeated
designable substructures.Comment: 13 pages, 10 figure
Inelastic resonant tunneling through single molecules and quantum dots: spectrum modification due to nonequilibrium effects
Resonant electron transport through a mesoscopic region (quantum dot or
single molecule) with electron-phonon interaction is considered at finite
voltage. In this case the standard Landauer-B\"uttiker approach cannot be
applied. Using the nonequilibrium Green function method we show that due to a
nonequilibrium distribution function of electrons in the mesoscopic region, the
inelastic scattering rate and spectral function of the dot become functions of
the voltage and have to be calculated self-consistently.Comment: 4 pages, 3 figure
Nonequilibrium Green's-Function Approach to the Suppression of Rectification at Metal--Mott-Insulator Interfaces
Suppression of rectification at metal--Mott-insulator interfaces, which is
previously shown by numerical solutions to the time-dependent Schr\"odinger
equation and experiments on real devices, is reinvestigated theoretically by
nonequilibrium Green's functions. The one-dimensional Hubbard model is used for
a Mott insulator. The effects of attached metallic electrodes are incorporated
into the self-energy. A scalar potential originating from work-function
differences and satisfying the Poisson equation is added to the model. For the
electron density, we decompose it into three parts. One is obtained by
integrating the local density of states over energy to the midpoint of the
electrodes' chemical potentials. The others, obtained by integrating lesser
Green's functions, are due to the couplings with the electrodes and correspond
to an inflow and an outflow of electrons. In Mott insulators, incoming
electrons and holes are extended over the whole system, avoiding further
accumulation of charge relative to the case without bias. This induces
collective charge transport and results in the suppression of rectification.Comment: 18 pages, Figs. 1(b), 2, and 8 replaced. Corrected typo
On the perturbative expansion of the magnetization in the out-of-equilibrium Kondo model
This paper is concerned with the out-of-equilibrium two-lead Kondo model,
considered as a model of a quantum dot in the Kondo regime. We revisit the
perturbative expansion of the dot's magnetization, and conclude that, even at
order 0 in the Kondo interactions, the magnetization is not given by the usual
equilibrium result. We use the Schwinger-Keldysh method to derive a Dyson
equation describing the steady state induced by the voltage between the two
leads, and thus present the correct procedure for calculating perturbative
expansions of steady-state properties of the system.Comment: Minor corrections forgotten in v
Resonant Photon-Assisted Tunneling Through a Double Quantum Dot: An Electron Pump From Spatial Rabi Oscillations
The time average of the fully nonlinear current through a double quantum dot,
subject to an arbitrary combination of ac and dc voltages, is calculated
exactly using the Keldysh nonequilibrium Green function technique. When driven
on resonance, the system functions as an efficient electron pump due to Rabi
oscillation between the dots. The pumping current is maximum when the coupling
to the leads equals the Rabi frequency.Comment: 6 pages, REVTEX 3.0, 3 postscript figure
Spin configurations of carbon nanotube in a nonuniform external potential
We study, theoretically, the ground state spin of a carbon nanotube in the
presence of an external potential. We find that when the external potential is
applied to a part of the nanotube, its variation changes the single electron
spectrum significantly. This, in combination with Coulomb repulsion and the
symmetry properties of a finite length armchair nanotube induces spin flips in
the ground state when the external potential is varied. We discuss the possible
application of our theory to recent measurements of Coulomb blocked peaks and
their dependence on a weak magnetic field in armchair carbon nanotubes.Comment: RevTeX, 5 pages + two figure
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