1,897 research outputs found
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
Non local Andreev reflection in a carbon nanotube superconducting quantum interference device
We investigate a superconducting quantum interference device (SQUID) based on
carbon nanotubes in a fork geometry [J.-P. Cleuziou {\it et al.}, Nature
Nanotechnology {\bf 1}, 53 (2006)], involving tunneling of evanescent
quasiparticles through a superconductor over a distance comparable to the
superconducting coherence length, with therefore ``non local'' processes
generalizing non local Andreev reflection and elastic cotunneling. Non local
processes induce a reduction of the critical current and modify the
current-phase relation. We discuss arbitrary interface transparencies. Such
devices in fork geometries are candidates for probing the phase coherence of
crossed Andreev reflection.Comment: 13 pages, 8 figures, revised versio
Carbon nanotube-based quantum pump in the presence of superconducting lead
Parametric electron pump through superconductor-carbon-nanotube based
molecular devices was investigated. It is found that a dc current, which is
assisted by resonant Andreev reflection, can be pumped out from such molecular
device by a cyclic variation of two gate voltages near the nanotube. The pumped
current can be either positive or negative under different system parameters.
Due to the Andreev reflection, the pumped current has the double peak structure
around the resonant point. The ratio of pumped current of N-SWNT-S system to
that of N-SWNT-N system (I^{NS}/I^N) is found to approach four in the weak
pumping regime near the resonance when there is exactly one resonant level at
Fermi energy inside the energy gap. Numerical results confirm that in the weak
pumping regime the pumped current is proportional to the square of the pumping
amplitude V_p, but in the strong pumping regime the pumped current has the
linear relation with V_p. Our numerical results also predict that pumped
current can be obtained more easily by using zigzag tube than by using armchair
tube
Revealing the electronic structure of a carbon nanotube carrying a supercurrent
Carbon nanotubes (CNTs) are not intrinsically superconducting but they can
carry a supercurrent when connected to superconducting electrodes. This
supercurrent is mainly transmitted by discrete entangled electron-hole states
confined to the nanotube, called Andreev Bound States (ABS). These states are a
key concept in mesoscopic superconductivity as they provide a universal
description of Josephson-like effects in quantum-coherent nanostructures (e.g.
molecules, nanowires, magnetic or normal metallic layers) connected to
superconducting leads. We report here the first tunneling spectroscopy of
individually resolved ABS, in a nanotube-superconductor device. Analyzing the
evolution of the ABS spectrum with a gate voltage, we show that the ABS arise
from the discrete electronic levels of the molecule and that they reveal
detailed information about the energies of these levels, their relative spin
orientation and the coupling to the leads. Such measurements hence constitute a
powerful new spectroscopic technique capable of elucidating the electronic
structure of CNT-based devices, including those with well-coupled leads. This
is relevant for conventional applications (e.g. superconducting or normal
transistors, SQUIDs) and quantum information processing (e.g. entangled
electron pairs generation, ABS-based qubits). Finally, our device is a new type
of dc-measurable SQUID
First order quantum phase transition in the Kondo regime of a superconducting carbon nanotube quantum dot
We study a carbon nanotube quantum dot embedded into a SQUID loop in order to
investigate the competition of strong electron correlations with proximity
effect. Depending whether local pairing or local magnetism prevails, a
superconducting quantum dot will respectively exhibit positive or negative
supercurrent, referred to as a 0 or Josephson junction. In the regime of
strong Coulomb blockade, the 0 to transition is typically controlled by a
change in the discrete charge state of the dot, from even to odd. In contrast,
at larger tunneling amplitude the Kondo effect develops for an odd charge
(magnetic) dot in the normal state, and quenches magnetism. In this situation,
we find that a first order 0 to quantum phase transition can be triggered
at fixed valence when superconductivity is brought in, due to the competition
of the superconducting gap and the Kondo temperature. The SQUID geometry
together with the tunability of our device allows the exploration of the
associated phase diagram predicted by recent theories. We also report on the
observation of anharmonic behavior of the current-phase relation in the
transition regime, that we associate with the two different accessible
superconducting states. Our results ultimately reveal the spin singlet nature
of the Kondo ground state, which is the key process in allowing the stability
of the 0-phase far from the mixed valence regime.Comment: 10 pages, 6 figures in main text, 4 figures in appendi
Study of 0- phase transition in hybrid superconductor-InSb nanowire quantum dot devices
Hybrid superconductor-semiconducting nanowire devices provide an ideal
platform to investigating novel intragap bound states, such as the Andreev
bound states (ABSs), Yu-Shiba-Rusinov (YSR) states, and the Majorana bound
states. The competition between Kondo correlations and superconductivity in
Josephson quantum dot (QD) devices results in two different ground states and
the occurrence of a 0- quantum phase transition. Here we report on
transport measurements on hybrid superconductor-InSb nanowire QD devices with
different device geometries. We demonstrate a realization of continuous
gate-tunable ABSs with both 0-type levels and -type levels. This allow us
to manipulate the transition between 0 and junction and explore charge
transport and spectrum in the vicinity of the quantum phase transition regime.
Furthermore, we find a coexistence of 0-type ABS and -type ABS in the same
charge state. By measuring temperature and magnetic field evolution of the
ABSs, the different natures of the two sets of ABSs are verified, being
consistent with the scenario of phase transition between the singlet and
doublet ground state. Our study provides insights into Andreev transport
properties of hybrid superconductor-QD devices and sheds light on the crossover
behavior of the subgap spectrum in the vicinity of 0- transition
Transport Through Andreev Bound States in a Graphene Quantum Dot
Andreev reflection-where an electron in a normal metal backscatters off a
superconductor into a hole-forms the basis of low energy transport through
superconducting junctions. Andreev reflection in confined regions gives rise to
discrete Andreev bound states (ABS), which can carry a supercurrent and have
recently been proposed as the basis of qubits [1-3]. Although signatures of
Andreev reflection and bound states in conductance have been widely reported
[4], it has been difficult to directly probe individual ABS. Here, we report
transport measurements of sharp, gate-tunable ABS formed in a
superconductor-quantum dot (QD)-normal system, which incorporates graphene. The
QD exists in the graphene under the superconducting contact, due to a
work-function mismatch [5, 6]. The ABS form when the discrete QD levels are
proximity coupled to the superconducting contact. Due to the low density of
states of graphene and the sensitivity of the QD levels to an applied gate
voltage, the ABS spectra are narrow, can be tuned to zero energy via gate
voltage, and show a striking pattern in transport measurements.Comment: 25 Pages, included SO
Co-sputtered MoRe thin films for carbon nanotube growth-compatible superconducting coplanar resonators
Molybdenum rhenium alloy thin films can exhibit superconductivity up to
critical temperatures of . At the same time, the films are
highly stable in the high-temperature methane / hydrogen atmosphere typically
required to grow single wall carbon nanotubes. We characterize molybdenum
rhenium alloy films deposited via simultaneous sputtering from two sources,
with respect to their composition as function of sputter parameters and their
electronic dc as well as GHz properties at low temperature. Specific emphasis
is placed on the effect of the carbon nanotube growth conditions on the film.
Superconducting coplanar waveguide resonators are defined lithographically; we
demonstrate that the resonators remain functional when undergoing nanotube
growth conditions, and characterize their properties as function of
temperature. This paves the way for ultra-clean nanotube devices grown in situ
onto superconducting coplanar waveguide circuit elements.Comment: 8 pages, 6 figure
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