157 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
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
Carbon nanotubes adhesion and nanomechanical behavior from peeling force spectroscopy
Applications based on Single Walled Carbon Nanotube (SWNT) are good example
of the great need to continuously develop metrology methods in the field of
nanotechnology. Contact and interface properties are key parameters that
determine the efficiency of SWNT functionalized nanomaterials and nanodevices.
In this work we have taken advantage of a good control of the SWNT growth
processes at an atomic force microscope (AFM) tip apex and the use of a low
noise (1E-13 m/rtHz) AFM to investigate the mechanical behavior of a SWNT
touching a surface. By simultaneously recording static and dynamic properties
of SWNT, we show that the contact corresponds to a peeling geometry, and
extract quantities such as adhesion energy per unit length, curvature and
bending rigidity of the nanotube. A complete picture of the local shape of the
SWNT and its mechanical behavior is provided
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
Self-aligned nanoscale SQUID on a tip
A nanometer-sized superconducting quantum interference device (nanoSQUID) is
fabricated on the apex of a sharp quartz tip and integrated into a scanning
SQUID microscope. A simple self-aligned fabrication method results in
nanoSQUIDs with diameters down to 100 nm with no lithographic processing. An
aluminum nanoSQUID with an effective area of 0.034 m displays flux
sensitivity of 1.8 \mu_B/\mathrm{Hz}^{1/2}$ and high bandwidth, the SQUID on a tip is a highly
promising probe for nanoscale magnetic imaging and spectroscopy.Comment: 14 manuscript pages, 5 figure
Influence of training status and exercise modality on pulmonary O2 uptake kinetics in pre-pubertal girls
The limited available evidence suggests that endurance training does not influence the pulmonary oxygen uptake (V(O)(2)) kinetics of pre-pubertal children. We hypothesised that, in young trained swimmers, training status-related adaptations in the V(O)(2) and heart rate (HR) kinetics would be more evident during upper body (arm cranking) than during leg cycling exercise. Eight swim-trained (T; 11.4 +/- 0.7 years) and eight untrained (UT; 11.5 +/- 0.6 years) girls completed repeated bouts of constant work rate cycling and upper body exercise at 40% of the difference between the gas exchange threshold and peak V(O)(2). The phase II V(O)(2) time constant was significantly shorter in the trained girls during upper body exercise (T: 25 +/- 3 vs. UT: 37 +/- 6 s; P < 0.01), but no training status effect was evident in the cycle response (T: 25 +/- 5 vs. UT: 25 +/- 7 s). The V(O)(2) slow component amplitude was not affected by training status or exercise modality. The time constant of the HR response was significantly faster in trained girls during both cycle (T: 31 +/- 11 vs. UT: 47 +/- 9 s; P < 0.01) and upper body (T: 33 +/- 8 vs. UT: 43 +/- 4 s; P < 0.01) exercise. The time constants of the phase II V(O)(2)and HR response were not correlated regardless of training status or exercise modality. This study demonstrates for the first time that swim-training status influences upper body V(O)(2) kinetics in pre-pubertal children, but that cycle ergometry responses are insensitive to such differences
Microwave studies of the fractional Josephson effect in HgTe-based Josephson junctions
The rise of topological phases of matter is strongly connected to their
potential to host Majorana bound states, a powerful ingredient in the search
for a robust, topologically protected, quantum information processing. In order
to produce such states, a method of choice is to induce superconductivity in
topological insulators. The engineering of the interplay between
superconductivity and the electronic properties of a topological insulator is a
challenging task and it is consequently very important to understand the
physics of simple superconducting devices such as Josephson junctions, in which
new topological properties are expected to emerge. In this article, we review
recent experiments investigating topological superconductivity in topological
insulators, using microwave excitation and detection techniques. More
precisely, we have fabricated and studied topological Josephson junctions made
of HgTe weak links in contact with two Al or Nb contacts. In such devices, we
have observed two signatures of the fractional Josephson effect, which is
expected to emerge from topologically-protected gapless Andreev bound states.
We first recall the theoretical background on topological Josephson junctions,
then move to the experimental observations. Then, we assess the topological
origin of the observed features and conclude with an outlook towards more
advanced microwave spectroscopy experiments, currently under development.Comment: Lectures given at the San Sebastian Topological Matter School 2017,
published in "Topological Matter. Springer Series in Solid-State Sciences,
vol 190. Springer
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