175 research outputs found
Interplay of Kondo effect and strong spin-orbit coupling in multi-hole ultraclean carbon nanotubes
We report on cotunneling spectroscopy magnetoconductance measurements of
multi-hole ultraclean carbon nanotube quantum dots in the SU(4) Kondo regime
with strong spin-orbit coupling. Successive shells show a gradual weakening of
the Kondo effect with respect to the spin-orbital splittings, leading to an
evolution from SU(4) to SU(2) symmetry with a suppressed conductance at half
shell filling. The extracted energy level spectrum, overally consistent with
negligible disorder in the nanotube, shows in the half filled case large
renormalizations due to Coulombian effects.Comment: 5 pages, 4 figures, 1 supplementary fil
Gate-tuned high frequency response of carbon nanotube Josephson junctions
Carbon nanotube (CNT) Josephson junctions in the open quantum dot limit
exhibit superconducting switching currents which can be controlled with a gate
electrode. Shapiro voltage steps can be observed under radiofrequency current
excitations, with a damping of the phase dynamics that strongly depends on the
gate voltage. These measurements are described by a standard RCSJ model showing
that the switching currents from the superconducting to the normal state are
close to the critical current of the junction. The effective dynamical
capacitance of the nanotube junction is found to be strongly gate-dependent,
suggesting a diffusive contact of the nanotube.Comment: 14 pages, 8 figure
Blocking transport resonances via Kondo entanglement in quantum dots
Many-body entanglement is at the heart of the Kondo effect, which has its
hallmark in quantum dots as a zero-bias conductance peak at low temperatures.
It signals the emergence of a conducting singlet state formed by a localized
dot degree of freedom and conduction electrons. Carbon nanotubes offer the
possibility to study the emergence of the Kondo entanglement by tuning
many-body correlations with a gate voltage. Here we quantitatively show an
undiscovered side of
Kondo correlations, which counterintuitively tend to block conduction
channels: inelastic cotunneling lines in the magnetospectrum of a carbon
nanotube strikingly disappear when tuning the gate voltage. Considering the
global \SUT\ \SUT\ symmetry of a carbon nanotube coupled to leads,
we find that only resonances involving flips of the Kramers pseudospins,
associated to this symmetry, are observed at temperatures and voltages below
the corresponding Kondo scale. Our results demonstrate the robust formation of
entangled many-body states with no net pseudospin.Comment: 9 pages, 4 figure
Kondo effect with non collinear polarized leads: a numerical renormalization group analysis
The Kondo effect in quantum dots attached to ferromagnetic leads with general
polarization directions is studied combining poor man scaling and Wilson's
numerical renormalization group methods. We show that polarized electrodes will
lead in general to a splitting of the Kondo resonance in the quantum dot
density of states except for a small range of angles close to the antiparallel
case. We also show that an external magnetic field is able to compensate this
splitting and restore the unitary limit. Finally, we study the electronic
transport through the device in various limiting cases.Comment: 6 pages, 4 figures, final versio
Coupling carbon nanotube mechanics to a superconducting circuit
The quantum behaviour of mechanical resonators is a new and emerging field
driven by recent experiments reaching the quantum ground state. The high
frequency, small mass, and large quality-factor of carbon nanotube resonators
make them attractive for quantum nanomechanical applications. A common element
in experiments achieving the resonator ground state is a second quantum system,
such as coherent photons or superconducting device, coupled to the resonators
motion. For nanotubes, however, this is a challenge due to their small size.
Here, we couple a carbon nanoelectromechanical (NEMS) device to a
superconducting circuit. Suspended carbon nanotubes act as both superconducting
junctions and moving elements in a Superconducting Quantum Interference Device
(SQUID). We observe a strong modulation of the flux through the SQUID from
displacements of the nanotube. Incorporating this SQUID into superconducting
resonators and qubits should enable the detection and manipulation of nanotube
mechanical quantum states at the single-phonon level
Charge localization and reentrant superconductivity in a quasi-ballistic InAs nanowire coupled to superconductors
A semiconductor nanowire with strong spin-orbit coupling in proximity to a superconductor is predicted to display Majorana edge states emerging under a properly oriented magnetic field. The experimental investigation of these exotic states requires assessing the one-dimensional (1D) character of the nanowire and understanding the superconducting proximity effect in the presence of a magnetic field. Here, we explore the quasi-ballistic 1D transport regime of an InAs nanowire with Ta contacts. Fine-tuned by means of local gates, the observed plateaus of approximately quantized conductance hide the presence of a localized electron, giving rise to a lurking Coulomb blockade effect and Kondo physics. When Ta becomes superconducting, this local charge causes an unusual, reentrant magnetic field dependence of the supercurrent, which we ascribe to a 0 - p transition. Our results underline the relevant role of unintentional charge localization in the few-channel regime where helical subbands and Majorana quasi-particles are expected to ariseWe acknowledge financial support from the Agence Nationale de la Recherche (TOPONANO project) and from the EU (ERC grant no. 280043). R.A. acknowledges financial support from the Spanish Ministry of Economy and Competitiveness (grant FIS2015-64654-P). R.Ž. acknowledges support from the Slovenian Research Agency (ARRS) under Program P1-
0044 and J1-725
Josephson and Andreev transport through quantum dots
In this article we review the state of the art on the transport properties of
quantum dot systems connected to superconducting and normal electrodes. The
review is mainly focused on the theoretical achievements although a summary of
the most relevant experimental results is also given. A large part of the
discussion is devoted to the single level Anderson type models generalized to
include superconductivity in the leads, which already contains most of the
interesting physical phenomena. Particular attention is paid to the competition
between pairing and Kondo correlations, the emergence of \pi-junction behavior,
the interplay of Andreev and resonant tunneling, and the important role of
Andreev bound states which characterized the spectral properties of most of
these systems. We give technical details on the several different analytical
and numerical methods which have been developed for describing these
properties. We further discuss the recent theoretical efforts devoted to extend
this analysis to more complex situations like multidot, multilevel or
multiterminal configurations in which novel phenomena is expected to emerge.
These include control of the localized spin states by a Josephson current and
also the possibility of creating entangled electron pairs by means of non-local
Andreev processes.Comment: 39 pages, 54 figures, corresponds to a review article as submitted to
Advances in Physic
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