5,288 research outputs found
Universal scaling in transport out of equilibrium through a single quantum dot using the noncrossing approximation
The universal scaling behavior is studied for nonequilibrium transport
through a quantum dot. To describe the dot we use the standard Anderson
impurity model and use the non-equilibrium non-crossing approximation in the
limit of infinite Coulomb repulsion. After solving de hamiltonian, we calculate
the conductance through the system as a function of temperature and bias
voltage in the Kondo and in the mixed valent regime. We obtain a good
scaling function in both regimes. In particular, in the mixed valent regime, we
find excellent agreement with recent experiments and previous theoretical
works.Comment: 6 pages, 5 figures, Accepted Physical Review
Quantum Transport Through a Stretched Spin--1 Molecule
We analyze the electronic transport through a model spin-1 molecule as a
function of temperature, magnetic field and bias voltage. We consider the
effect of magnetic anisotropy, which can be generated experimentally by
stretching the molecule. In the experimentally relevant regime the conductance
of the unstretched molecule reaches the unitary limit of the underscreened
spin- 1 Kondo effect at low temperatures. The magnetic anisotropy generates an
antiferromagnetic coupling between the remaining spin 1/2 and a singular
density of quasiparticles, producing a second Kondo effect and a reduced
conductance. The results explain recent measurements in spin-1 molecules
[Science 328 1370 (2010)].Comment: 5 pages, 3 figures, minor changes, accepted for publication in EP
Bohrification
New foundations for quantum logic and quantum spaces are constructed by
merging algebraic quantum theory and topos theory. Interpreting Bohr's
"doctrine of classical concepts" mathematically, given a quantum theory
described by a noncommutative C*-algebra A, we construct a topos T(A), which
contains the "Bohrification" B of A as an internal commutative C*-algebra. Then
B has a spectrum, a locale internal to T(A), the external description S(A) of
which we interpret as the "Bohrified" phase space of the physical system. As in
classical physics, the open subsets of S(A) correspond to (atomic)
propositions, so that the "Bohrified" quantum logic of A is given by the
Heyting algebra structure of S(A). The key difference between this logic and
its classical counterpart is that the former does not satisfy the law of the
excluded middle, and hence is intuitionistic. When A contains sufficiently many
projections (e.g. when A is a von Neumann algebra, or, more generally, a
Rickart C*-algebra), the intuitionistic quantum logic S(A) of A may also be
compared with the traditional quantum logic, i.e. the orthomodular lattice of
projections in A. This time, the main difference is that the former is
distributive (even when A is noncommutative), while the latter is not.
This chapter is a streamlined synthesis of 0709.4364, 0902.3201, 0905.2275.Comment: 44 pages; a chapter of the first author's PhD thesis, to appear in
"Deep Beauty" (ed. H. Halvorson
Nonequilibrium transport through magnetic vibrating molecules
We calculate the nonequilibrium conductance through a molecule or a quantum
dot in which the occupation of the relevant electronic level is coupled with
intensity to a phonon mode, and also to two conducting leads. The
system is described by the Anderson-Holstein Hamiltonian. We solve the problem
using the Keldysh formalism and the non-crossing approximation (NCA) for both,
the electron-electron and the electron-phonon interactions. We obtain a
moderate decrease of the Kondo temperature with for fixed
renormalized energy of the localized level . The meaning and value
of are discussed. The spectral density of localized electrons
shows in addition to the Kondo peak of width , satellites of this peak
shifted by multiples of the phonon frequency . The nonequilibrium
conductance as a function of bias voltage at small temperatures, also
displays peaks at multiples of in addition to the central dominant
Kondo peak near .Comment: 11 pages, 13 figures, accepted in Phys. Rev.
Impact of capacitance and tunneling asymmetries on Coulomb blockade edges and Kondo peaks in non-equilibrium transport through molecular quantum dots
We investigate theorerically the non-equilibrium transport through a
molecular quantum dot as a function of gate and bias voltage, taking into
account the typical situation in molecular electronics. In this respect, our
study includes asymmetries both in the capacitances and tunneling rates to the
source and drain electrodes, as well as an infinitely large charging energy on
the molecule. Our calculations are based on the out-of-equilibrium
Non-Crossing-Approximation (NCA), which is a reliable technique in the regime
under consideration. We find that Coulomb blockade edges and Kondo peaks
display strong renormalization in their width and intensity as a function of
these asymmetries, and that basic expectations from Coulomb blockade theory
must be taken with care in general, expecially when Kondo physics is at play.
In order to help comparison of theory to experiments, we also propose a simple
phenomenological model which reproduces semi-quantitatively the Coulomb
blockade edges that were numerically computed from the NCA in all regimes of
parameters.Comment: 9 pages, 8 figure
Replicas of the Kondo peak due to electron-vibration interaction in molecular transport properties
The low temperature properties of single level molecular quantum dots
including both, electron-electron and electron-vibration interactions, are
theoretically investigated. The calculated differential conductance in the
Kondo regime exhibits not only the zero bias anomaly but also side peaks
located at bias voltages which coincide with multiples of the energy of
vibronic mode . We obtain that the evolution with
temperature of the two main satellite conductance peaks follows the
corresponding one of the Kondo peak when , being the Kondo temperature, in agreement with recent transport measurements in
molecular junctions. However, we find that this is no longer valid when is of the order of a few times .Comment: 6 pages, 4 figures. Accepted for publication in Physical Review
Orbital Kondo spectroscopy in a double quantum dot system
We calculate the nonequilibrium conductance of a system of two capacitively
coupled quantum dots, each one connected to its own pair of conducting leads.
The system has been used recently to perform pseudospin spectroscopy by
controlling independently the voltages of the four leads. The pseudospin is
defined by the orbital occupation of one or the other dot. Starting from the
SU(4) symmetric point of spin and pseudospin degeneracy in the Kondo regime,
for an odd number of electrons in the system, we show how the conductance
through each dot varies as the symmetry is reduced to SU(2) by a pseudo-Zeeman
splitting, and as bias voltages are applied to any of the dots. We analize the
expected behavior of the system in general, and predict characteristic
fingerprint features of the SU(4) to SU(2) crossover that have not been
observed so far.Comment: 5 pages, 6 figures, submitted to Phys. Rev.
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