176 research outputs found
From Coulomb blockade to the Kondo regime in a Rashba dot
We investigate the electronic transport in a quantum wire with localized
Rashba interaction. The Rashba field forms quasi-bound states which couple to
the continuum states with an opposite spin direction. The presence of this
Rashba dot causes Fano-like antiresonances and dips in the wire's linear
conductance. The Fano lineshape arises from the interference between the direct
transmission channel along the wire and the hopping through the Rashba dot. Due
to the confinement, we predict the observation of large charging energies in
the local Rashba region which lead to Coulomb-blockade effects in the transport
properties of the wire. Importantly, the Kondo regime can be achieved with a
proper tuning of the Rashba interaction, giving rise to an oscillating linear
conductance for a fixed occupation of the Rashba dot.Comment: 6 pages, 3 figures; presentation improved, discussions extended.
Published versio
Orbital fluctuations and strong correlations in quantum dots
In this lecture note we focus our attention to quantum dot systems where
exotic strongly correlated behavior develops due to the presence of orbital or
charge degrees of freedom. After giving a concise overview of the theory of
transport and Kondo effect through a single electron transistor, we discuss how
SU(4) Kondo effect develops in dots having orbitally degenerate states and in
double dot systems, and then study the singlet-triplet transition in lateral
quantum dots. Charge fluctuations and Matveev's mapping to the two-channel
Kondo model in the vicinity of charge degeneracy point are also discussed.Comment: Lecture note to appear in Philosophical Magazin
Quantum Transport from the Perspective of Quantum Open Systems
By viewing the non-equilibrium transport setup as a quantum open system, we
propose a reduced-density-matrix based quantum transport formalism. At the
level of self-consistent Born approximation, it can precisely account for the
correlation between tunneling and the system internal many-body interaction,
leading to certain novel behavior such as the non-equilibrium Kondo effect. It
also opens a new way to construct time-dependent density functional theory for
transport through large-scale complex systems.Comment: 4 pages, 2 figures; the TDDFT scheme is explained in more detail in
this new versio
Josephson current through a molecular transistor in a dissipative environment
We study the Josephson coupling between two superconductors through a single
correlated molecular level, including Coulomb interaction on the level and
coupling to a bosonic environment. All calculations are done to the lowest,
i.e., the fourth, order in the tunneling coupling and we find a suppression of
the supercurrent due to the combined effect of the Coulomb interaction and the
coupling to environmental degrees of freedom. Both analytic and numerical
results are presented.Comment: 11 pages, 6 figures, to appear in Phys. Rev. B; v3: several misprints
corrected - in particular, sign inconsistencies throughout the paper should
be fixe
Theory for Decoupling in High-T_c Superconductors from an Analysis of the Layered XY Model with Frustration
The nature of decoupling in the mixed phase of extremely type-II layered
superconductors is studied theoretically through a duality transformation of
the layered XY model with frustration. In the limit of weak coupling, we
generally find that the Josephson effect is absent if and only if the phase
correlations within isolated layers are short range. In the case specific to
uniform frustration, we notably identify a decoupled pancake vortex liquid
phase that is bounded by first-order and second-order decoupling lines in the
magnetic field vs. temperature plane. These transitions potentially account for
the flux-lattice melting and for the flux-lattice depinning that is observed in
clean high-temperature superconductors.Comment: 11 pgs. of Plain TeX, 1 postscript fig., based on a talk given at the
VORTEX Euroconference held in Heraklion, Crete, Sept. 199
Quantum master equation scheme of time-dependent density functional theory to time-dependent transport in nano-electronic devices
In this work a practical scheme is developed for the first-principles study
of time-dependent quantum transport. The basic idea is to combine the transport
master-equation with the well-known time-dependent density functional theory.
The key ingredients of this paper include: (i) the partitioning-free initial
condition and the consideration of the time-dependent bias voltages which base
our treatment on the Runge-Gross existence theorem; (ii) the non-Markovian
master equation for the reduced (many-body) central system (i.e. the device);
and (iii) the construction of Kohn-Sham master equation for the reduced
single-particle density matrix, where a number of auxiliary functions are
introduced and their equations of motion (EOM) are established based on the
technique of spectral decomposition. As a result, starting with a well-defined
initial state, the time-dependent transport current can be calculated
simultaneously along the propagation of the Kohn-Sham master equation and the
EOM of the auxiliary functions.Comment: 9 pages, no figure
Probing spin and orbital Kondo effects with a mesoscopic interferometer
We investigate theoretically the transport properties of a closed
Aharonov-Bohm interferometer containing two quantum dots in the strong coupling
regime. We find two distinct physical scenarios depending on the strength of
the interdot Coulomb interaction. When the interdot Coulomb interaction is
negligible only spin fluctuations are important and each dot develops a Kondo
resonance at the Fermi level independently of the applied magnetic flux. The
transport is characterized by the interference of these two independent Kondo
resonances. On the contrary, for large interdot interaction, only one electron
can be accommodated onto the double dot system. In this situation, not only the
spin can fluctuate but also the orbital degree of freedom (the pseudo-spin). As
a result, we find different ground states depending on the value of the applied
flux. When (mod ) (, where is
applied flux, and the flux quantum) the electronic transport can
take place via simultaneous correlations in the spin and pseudo-spin sectors,
leading to the highly symmetric SU(4) Kondo state. Nevertheless, we find
situations with (mod ) where the pseudo-spin quantum number is
not conserved during tunneling events, giving rise to the common SU(2) Kondo
state with an enhanced Kondo temperature. We investigate the crossover between
both ground states and discuss possible experimental signatures of this physics
as a function of the applied magnetic flux.Comment: 12 pages, 3 figures; extended discussions, improved presentatio
Comment on "T-dependence of the magnetic penetration depth in unconventional superconductors at low temperatures: Can it be linear?"
We show that the clean superconductor with line of gap nodes is not in
conflict with the Nernst theorem. The answer to the question in the title of
the Schopohl-Dolgov paper in Phys. Rev. Lett. 80 (1998) 4761 (cond-mat/9802264)
is yes.Comment: Comment to the paper by Schopohl and Dolgov in Phys. Rev. Lett. 80
(1998) 4761 (cond-mat/9802264), RevTex file, 1 page, no figures, typos are
corrected, submitted to Phys. Rev. Let
Tunneling exponents in realistic quantum wires using the mean field approximation
It is demonstrated that the charge Tomonaga-Luttinger parameter of
quantum wires can be estimated accurately using the Hartree-Fock approximation
if carried out self consistently. The dependence of on the carrier
density distinguishes different regimes of importance of correlations
Resonant Josephson current through a quantum dot
We calculate the DC Josephson current through a semiconducting quantum dot
which is weakly coupled by tunnel barriers to two superconducting reservoirs. A
Breit-Wigner resonance in the conductance corresponds to a resonance in the
critical current, but with a different (non-lorentzian) lineshape.Comment: 5 pages including 1 figure; this paper was published in the
proceedings of SQUID'91; it is archived here because of its relevance to
cond-mat/011148
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