1,327 research outputs found
Scalar and vector Keldysh models in the time domain
The exactly solvable Keldysh model of disordered electron system in a random
scattering field with extremely long correlation length is converted to the
time-dependent model with extremely long relaxation. The dynamical problem is
solved for the ensemble of two-level systems (TLS) with fluctuating well depths
having the discrete Z_2 symmetry. It is shown also that the symmetric TLS with
fluctuating barrier transparency may be described in terms of the planar
Keldysh model with dime-dependent random planar rotations in xy plane having
continuous SO(2) symmetry. The case of simultaneous fluctuations of the well
depth and barrier transparency is subject to non-abelian algebra. Application
of this model to description of dynamic fluctuations in quantum dots and
optical lattices is discussed.Comment: 6 pages, 5 eps figures. Extended version of the paper to be published
in JETP Lett 89 (2009
Quantum dots with two electrons: Singlet-triplet transitions
The magnetic character of the ground-state of two electrons on a double
quantum dot, connected in series to left and right single-channel leads, is
considered. By solving exactly for the spectrum of the two interacting
electrons, it is found that the coupling to the continuum of propagating states
on the leads, in conjunction with the electron-electron interactions, may
result in a delocalization of the bound state of the two electrons. This, in
turn, reduces significantly the range of the Coulomb interaction parameters
over which singlet-triplet transitions can be realized. It is also found that
the coupling to the leads favors the singlet ground-state.Comment: 8 pages, submitted to Phys. Rev.
Nonequilibrium theory of Coulomb blockade in open quantum dots
We develop a non-equilibrium theory to describe weak Coulomb blockade effects
in open quantum dots. Working within the bosonized description of electrons in
the point contacts, we expose deficiencies in earlier applications of this
method, and address them using a 1/N expansion in the inverse number of
channels. At leading order this yields the self-consistent potential for the
charging interaction. Coulomb blockade effects arise as quantum corrections to
transport at the next order. Our approach unifies the phase functional and
bosonization approaches to the problem, as well as providing a simple picture
for the conductance corrections in terms of renormalization of the dot's
elastic scattering matrix, which is obtained also by elementary perturbation
theory. For the case of ideal contacts, a symmetry argument immediately allows
us to conclude that interactions give no signature in the averaged conductance.
Non-equilibrium applications to the pumped current in a quantum pump are worked
out in detail.Comment: Published versio
Spin and Charge Correlations in Quantum Dots: An Exact Solution
The inclusion of charging and spin-exchange interactions within the Universal
Hamiltonian description of quantum dots is challenging as it leads to a
non-Abelian action. Here we present an {\it exact} analytical solution of the
probem, in particular, in the vicinity of the Stoner instabilty point. We
calculate several observables, including the tunneling density of states (TDOS)
and the spin susceptibility. Near the instability point the TDOS exhibits a
non-monotonous behavior as function of the tunneling energy, even at
temperatures higher than the exchange energy. Our approach is generalizable to
a broad set of observables, including the a.c. susceptibility and the
absorption spectrum for anisotropic spin interaction. Our results could be
tested in nearly ferromagnetic materials.Comment: JETPL class, 6 pages, 2 figure
Mesoscopic Tunneling Magnetoresistance
We study spin-dependent transport through
ferromagnet/normal-metal/ferromagnet double tunnel junctions in the mesoscopic
Coulomb blockade regime. A general transport equation allows us to calculate
the conductance in the absence or presence of spin-orbit interaction and for
arbitrary orientation of the lead magnetizations. The tunneling
magnetoresistance (TMR), defined at the Coulomb blockade conductance peaks, is
calculated and its probability distribution presented. We show that mesoscopic
fluctuations can lead to the optimal value of the TMR.Comment: 5 pages, 3 eps figures included using epsf.sty. Revised text and
improved notation, fig. 2 removed, explicit equations for the GSE case adde
Integrable model for interacting electrons in metallic grains
We find an integrable generalization of the BCS model with non-uniform
Coulomb and pairing interaction. The Hamiltonian is integrable by construction
since it is a functional of commuting operators; these operators, which
therefore are constants of motion of the model, contain the anisotropic Gaudin
Hamiltonians. The exact solution is obtained diagonalizing them by means of
Bethe Ansatz. Uniform pairing and Coulomb interaction are obtained as the
``isotropic limit'' of the Gaudin Hamiltonians. We discuss possible
applications of this model to a single grain and to a system of few interacting
grains.Comment: 4 pages, revtex. Revised version to be published in Phys. Rev. Let
Robot life: simulation and participation in the study of evolution and social behavior.
This paper explores the case of using robots to simulate evolution, in particular the case of Hamilton's Law. The uses of robots raises several questions that this paper seeks to address. The first concerns the role of the robots in biological research: do they simulate something (life, evolution, sociality) or do they participate in something? The second question concerns the physicality of the robots: what difference does embodiment make to the role of the robot in these experiments. Thirdly, how do life, embodiment and social behavior relate in contemporary biology and why is it possible for robots to illuminate this relation? These questions are provoked by a strange similarity that has not been noted before: between the problem of simulation in philosophy of science, and Deleuze's reading of Plato on the relationship of ideas, copies and simulacra
Kondo effect in real quantum dots
Exchange interaction within a quantum dot strongly affects the transport
through it in the Kondo regime. In a striking difference with the results of
the conventional model, where this interaction is neglected, here the
temperature and magnetic field dependence of the conductance may become
non-monotonic: its initial increase follows by a drop when temperature and
magnetic field are lowered
Spin and e-e interactions in quantum dots: Leading order corrections to universality and temperature effects
We study the statistics of the spacing between Coulomb blockade conductance
peaks in quantum dots with large dimensionless conductance g. Our starting
point is the ``universal Hamiltonian''--valid in the g->oo limit--which
includes the charging energy, the single-electron energies (described by random
matrix theory), and the average exchange interaction. We then calculate the
magnitude of the most relevant finite g corrections, namely, the effect of
surface charge, the ``gate'' effect, and the fluctuation of the residual e-e
interaction. The resulting zero-temperature peak spacing distribution has
corrections of order Delta/sqrt(g). For typical values of the e-e interaction
(r_s ~ 1) and simple geometries, theory does indeed predict an asymmetric
distribution with a significant even/odd effect. The width of the distribution
is of order 0.3 Delta, and its dominant feature is a large peak for the odd
case, reminiscent of the delta-function in the g->oo limit. We consider finite
temperature effects next. Only after their inclusion is good agreement with the
experimental results obtained. Even relatively low temperature causes large
modifications in the peak spacing distribution: (a) its peak is dominated by
the even distribution at kT ~ 0.3 Delta (at lower T a double peak appears); (b)
it becomes more symmetric; (c) the even/odd effect is considerably weaker; (d)
the delta-function is completely washed-out; and (e) fluctuation of the
coupling to the leads becomes relevant. Experiments aimed at observing the T=0
peak spacing distribution should therefore be done at kT<0.1 Delta for typical
values of the e-e interaction.Comment: 15 pages, 4 figure
An efficient Fredholm method for calculation of highly excited states of billiards
A numerically efficient Fredholm formulation of the billiard problem is
presented. The standard solution in the framework of the boundary integral
method in terms of a search for roots of a secular determinant is reviewed
first. We next reformulate the singularity condition in terms of a flow in the
space of an auxiliary one-parameter family of eigenproblems and argue that the
eigenvalues and eigenfunctions are analytic functions within a certain domain.
Based on this analytic behavior we present a numerical algorithm to compute a
range of billiard eigenvalues and associated eigenvectors by only two
diagonalizations.Comment: 15 pages, 10 figures; included systematic study of accuracy with 2
new figures, movie to Fig. 4,
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