103 research outputs found
Mn-doped II-VI quantum dots: artificial molecular magnets
The notion of artifical atom relies on the capability to change the number of
carriers one by one in semiconductor quantum dots, and the resulting changes in
their electronic structure. Organic molecules with transition metal atoms that
have a net magnetic moment and display hysteretic behaviour are known as single
molecule magnets (SMM). The fabrication of CdTe quantum dots chemically doped
with a controlled number of Mn atoms and with a number of carriers controlled
either electrically or optically paves the way towards a new concept in
nanomagnetism: the artificial single molecule magnet. Here we study the
magnetic properties of a Mn-doped CdTe quantum dot for different charge states
and show to what extent they behave like a single molecule magnet.Comment: Conference article presented at QD2006, Chamonix, May 200
Electron-Photon interaction in resonant tunneling diodes
We develope a model to describe the transmission coefficient and tunneling
current in the presence of photon-electron coupling in a resonant diode. Our
model takes into account multiphoton processes as well as the transitions
between electronic states with different wave numbers. This is crutial to
explain the experimental features observed in the tunneling current through a
double barrier which cannot be reproduced with more simplified established
models. According to our results, what experiments show in the current density
are quantum photon-assisted features coming from multiphoton transitions which
are not related with sample heating.Comment: 8 pages,2 Postscript Figure
AC Josephson effect in finite-length nanowire junctions with Majorana modes
It has been predicted that superconducting junctions made with topological
nanowires hosting Majorana bound states (MBS) exhibit an anomalous
4\pi-periodic Josephson effect. Finding an experimental setup with these
unconventional properties poses, however, a serious challenge: for
finite-length wires, the equilibrium supercurrents are always 2\pi-periodic as
anticrossings of states with the same fermionic parity are possible. We show,
however, that the anomaly survives in the transient regime of the ac Josephson
effect. Transients are moreover protected against decay by quasiparticle
poisoning as a consequence of the quantum Zeno effect, which fixes the parity
of Majorana qubits. The resulting long-lived ac Josephson transients may be
effectively used to detect MBS.Comment: 9 pages, 4 figures, published version (with supplementary material
SU(4) Kondo Effect in Carbon Nanotubes
We investigate theoretically the non-equilibrium transport properties of
carbon nanotube quantum dots. Owing to the two-dimensional band structure of
graphene, a double orbital degeneracy plays the role of a pseudo-spin, which is
entangled with the spin. Quantum fluctuations between these four degrees of
freedom result in an SU(4) Kondo effect at low temperatures. This exotic Kondo
effect manifests as a four-peak splitting in the non-linear conductance when an
axial magnetic field is applied.Comment: 5 pages, 4 figure
Transport spectroscopy of NS nanowire junctions with Majorana fermions
We investigate transport though normal-superconductor nanowire junctions in
the presence of spin-orbit coupling and magnetic field. As the Zeeman field
crosses the critical bulk value B_c of the topological transition, a Majorana
bound state (MBS) is formed, giving rise to a sharp zero-bias anomaly (ZBA) in
the tunneling differential conductance. We identify novel features beyond this
picture in wires with inhomogeneous depletion, like the appearance of two MBSs
inside a long depleted region for B<B_c. The resulting ZBA is in most cases
weakly split and may coexist with Andreev bound states near zero energy. The
ZBA may appear without evidence of a topological gap closing. This latter
aspect is more evident in the multiband case and stems from a smooth pinch-off
barrier. Most of these features are in qualitative agreement with recent
experiments [Mourik et al, Science 336, 1003 (2012)]. We also discuss the rich
phenomenology of the problem in other regimes which remain experimentally
unexplored.Comment: 10 pages, 12 figures. Published version, supplementary material
include
Kondo effect in AC transport through Quantum dots
A theory of the Kondo effect in quantum dots at zero temperature in the
presence of arbitrary intense AC potentials is presented.We generalize the
Friedel-Langreth sum rule to take care of charge conservation and propose a
consistent procedure to study a time dependent Anderson Hamiltonian. The effect
of the AC potential on both the quantum dot density of states and the linear
conductance shows the importance of using a theory which describes intradot
finite interaction and nonperturbative effects.Comment: 11 RevTex pages, 4 Postscript figures, submmited to PR
Shiba states and zero-bias anomalies in the hybrid normal-superconductor Anderson model
We determine the phase diagram of an Anderson impurity in contact with
superconducting and normal-state leads for arbitrary ratio of the gap
to the Kondo temperature . We observe a considerable effect of even very
weak coupling to the normal lead that is usually considered as a non-perturbing
tunneling probe. The numerical renormalization group results are analyzed in
the context of relevant experimental scenarios such as parity crossing
(doublet-singlet) quantum phase transitions induced by a gap reduction as well
as novel Kondo features induced by the normal lead. We point out the important
role of finite temperatures and magnetic fields. Overall, we find a very rich
behavior of spectral functions with zero-bias anomalies which can emerge
irrespective of whether the ground state is a doublet or a singlet. Our
findings are pertinent to the tunnelling-spectroscopy experiments aiming at
detecting Majorana modes in nanowires.Comment: Final version, 18 pages, 17 figure
Dynamic current susceptibility as a probe of Majorana bound states in nanowire-based Josephson junctions
We theoretically study a Josephson junction based on a semiconducting
nanowire subject to a time-dependent flux bias. We establish a general density
matrix approach for the dynamical response of the Majorana junction and
calculate the resulting flux-dependent susceptibility using both microscopic
and effective low-energy descriptions for the nanowire. We find that the
diagonal component of the susceptibility, associated with the dynamics of the
Majorana states populations, dominates over the standard Kubo contribution for
a wide range of experimentally relevant parameters. The diagonal term, thus far
unexplored in the context of Majorana physics, allows to probe accurately the
presence of Majorana bound states in the junction.Comment: 5 pages, 3 figures, 15 pages of supplemental materia
Josephson current in carbon nanotubes with spin-orbit interaction
We demonstrate that curvature-induced spin-orbit (SO) coupling induces a
transition in the Josephson current through a carbon nanotube quantum
dot coupled to superconducting leads. In the non-interacting regime, the
transition can be tuned by applying parallel magnetic field near the critical
field where orbital states become degenerate. Moreover, the interplay between
charging and SO effects in the Coulomb Blockade and cotunneling regimes leads
to a rich phase diagram with well-defined (analytical) boundaries in parameter
space. Finally, the 0 phase always prevails in the Kondo regime. Our
calculations are relevant in view of recent experimental advances in transport
through ultra-clean carbon nanotubes.Comment: 4 (main text) + 10 (appendices) pages, 3 figure
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