158 research outputs found
Magnetic field resistant quantum interferences in bismuth nanowires based Josephson junctions
We investigate proximity induced superconductivity in micrometer-long bismuth
nanowires con- nected to superconducting electrodes with a high critical field.
At low temperature we measure a supercurrent that persists in magnetic fields
as high as the critical field of the electrodes (above 11 T). The critical
current is also strongly modulated by the magnetic field. In certain samples we
find regular, rapid SQUID-like periodic oscillations occurring up to high
fields. Other samples ex- hibit less periodic but full modulations of the
critical current on Tesla field scales, with field-caused extinctions of the
supercurrent. These findings indicate the existence of low dimensionally, phase
coherent, interfering conducting regions through the samples, with a subtle
interplay between orbital and spin contributions. We relate these surprising
results to the electronic properties of the surface states of bismuth, strong
Rashba spin-orbit coupling, large effective g factors, and their effect on the
induced superconducting correlations.Comment: 5 page
Superconductivity in Ropes of Single-Walled Carbon Nanotubes
We report measurements on ropes of Single Walled Carbon Nanotubes (SWNT) in
low-resistance contact to non-superconducting (normal) metallic pads, at low
voltage and at temperatures down to 70 mK. In one sample, we find a two order
of magnitude resistance drop below 0.55 K, which is destroyed by a magnetic
field of the order of 1T, or by a d.c. current greater than 2.5 microA. These
features strongly suggest the existence of superconductivity in ropes of SWNT.Comment: Accepted for publication in Phys. Rev. Let
Ballistic- and quantum-conductor carbon nanotubes: A reference experiment put to the test
We have performed electrical transport experiments on individual carbon nanotubes (CNTs) in situ in a transmission electron microscope using the liquid-metal contact method (LMC method), which consists of immersing a CNT placed on the apex of a metallic tip into a drop of liquid mercury (Hg). In the literature, this method has been mostly employed without visualization (ex situ) to show the ballistic- and quantum-conductance properties of different kinds of CNTs. We show that on the one hand the in situ LMC method is well suited to create low-resistance contacts with the CNTs but on the other hand the ballistic and quantum conductance measured by the ex situ LMC method is likely to give false positives for three reasons: (a) the CNTs are likely to be removed from the tip surface through contact with the Hg, (b) occurring Hg-tip surface nanocontacts are likely to be mistaken for quantum-conductor CNTs, and (c) occurring Hg nanomenisci are likely to be mistaken for ballistic-conductor CNTs. These findings have strong consequences for the interpretation of previously reported works
Theory of superconductivity of carbon nanotubes and graphene
We present a new mechanism of carbon nanotube superconductivity that
originates from edge states which are specific to graphene. Using on-site and
boundary deformation potentials which do not cause bulk superconductivity, we
obtain an appreciable transition temperature for the edge state. As a
consequence, a metallic zigzag carbon nanotube having open boundaries can be
regarded as a natural superconductor/normal metal/superconductor junction
system, in which superconducting states are developed locally at both ends of
the nanotube and a normal metal exists in the middle. In this case, a signal of
the edge state superconductivity appears as the Josephson current which is
sensitive to the length of a nanotube and the position of the Fermi energy.
Such a dependence distinguishs edge state superconductivity from bulk
superconductivity.Comment: 5 pages, 2 figure
Fano effect of a strongly interacting quantum dot in contact with superconductor
The physics of a system consisting of an Aharonov Bohm (AB) interferometer
containing a single level interacting quantum dot (QD) on one of its arms, and
attached to normal (N) and superconducting (S) leads is studied and elucidated.
Here the focus is directed mainly on N-AB-S junctions but the theory is capable
of studying S-AB-S junctions as well. The interesting physics comes into play
under the conditions that both the Kondo effect in the QD and the the Fano
effect are equally important.It is found the conductance of the junction is
suppressed as the Fano effect becomes more dominant.Comment: 4 pages, Talk to be given at the NATO Conference MQO, Bled, Slovenia
7-10 September 200
Andreev current in finite sized carbon nanotubes
We investigate the effect of interactions on Andreev current at a
normal-superconducting junction when the normal phase is a Luttinger liquid
with repulsive interactions. In particular, we study the system of a finite
sized carbon nanotube placed between one metallic and one superconducting lead.
We show that interactions and finite size effects give rise to significant
deviations from the standard picture of Andreev current at a normal-
superconductor junction in the nearly perfect Andreev limit.Comment: 4 pages, 2 figure
Disorder, pseudospins, and backscattering in carbon nanotubes
We address the effects of disorder on the conducting properties of metal and
semiconducting carbon nanotubes. Experimentally, the mean free path is found to
be much larger in metallic tubes than in doped semiconducting tubes. We show
that this result can be understood theoretically if the disorder potential is
long-ranged. The effects of a pseudospin index that describes the internal
sublattice structure of the states lead to a suppression of scattering in
metallic tubes, but not in semiconducting tubes. This conclusion is supported
by tight-binding calculations.Comment: four page
Kondo physics in carbon nanotubes
The connection of electrical leads to wire-like molecules is a logical step
in the development of molecular electronics, but also allows studies of
fundamental physics. For example, metallic carbon nanotubes are quantum wires
that have been found to act as one-dimensional quantum dots, Luttinger-liquids,
proximity-induced superconductors and ballistic and diffusive one-dimensional
metals. Here we report that electrically-contacted single-wall nanotubes can
serve as powerful probes of Kondo physics, demonstrating the universality of
the Kondo effect. Arising in the prototypical case from the interaction between
a localized impurity magnetic moment and delocalized electrons in a metallic
host, the Kondo effect has been used to explain enhanced low-temperature
scattering from magnetic impurities in metals, and also occurs in transport
through semiconductor quantum dots. The far higher tunability of dots (in our
case, nanotubes) compared with atomic impurities renders new classes of
Kondo-like effects accessible. Our nanotube devices differ from previous
systems in which Kondo effects have been observed, in that they are
one-dimensional quantum dots with three-dimensional metal (gold) reservoirs.
This allows us to observe Kondo resonances for very large electron number (N)
in the dot, and approaching the unitary limit (where the transmission reaches
its maximum possible value). Moreover, we detect a previously unobserved Kondo
effect, occurring for even values of N in a magnetic field.Comment: 7 pages, pdf onl
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