108 research outputs found
The BCS-like gap in superconductor SmFeAsO_0.85F_0.15
Since the discovery of superconductivity in the cuprates two decades ago, it
has been firmly established that the CuO_2 plane is consequential for high T_C
superconductivity and a host of other very unusual properties. A new family of
superconductors with the general composition of LaFeAsO_(1-x)F_x has recently
been discovered but with the conspicuous lacking of the CuO_2 planes, thus
raising the tantalizing questions of the different pairing mechanisms in these
oxypnictide superconductors. Intimately related to pairing in a superconductor
are the superconducting gap, its value, structure, and temperature dependence.
Here we report the observation of a single gap in the superconductor
SmFeAsO_0.85F_0.15 with T_C = 42 K as measured by Andreev spectroscopy. The gap
value of 2Delta = 13.34+/-0.3 meV gives 2Delta/k_BT_C = 3.68, close to the BCS
prediction of 3.53. The gap decreases with temperature and vanishes at T_C in a
manner consistent with the Bardeen-Cooper-Schrieffer (BCS) prediction but
dramatically different from that of the pseudogap behavior in the cuprate
superconductors. Our results clearly indicate a nodeless gap order parameter,
which is nearly isotropic in size across different sections of the Fermi
surface, and are not compatible with models involving antiferromagnetic
fluctuations, strong correlations, t-J model, and the like, originally designed
for cuprates.Comment: 8 pages, 3 figure
Why does a metal-superconductor junction have a resistance?
This is a tutorial article based on a lecture delivered in June 1999 at the
NATO Advanced Study Institute in Ankara. The phenomenon of Andreev reflection
is introduced as the electronic analogue of optical phase-conjugation. In the
optical problem, a disordered medium backed by a phase-conjugating mirror can
become completely transparent. Yet, a disordered metal connected to a
superconductor has the same resistance as in the normal state. The resolution
of this paradox teaches us a fundamental difference between phase conjugation
of light and electrons.Comment: 12 pages, 5 postscript figures [v2: all figures inline
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
Primordial magnetic fields at preheating
Using lattice techniques we investigate the generation of long range
cosmological magnetic fields during a cold electroweak transition. We will show
how magnetic fields arise, during bubble collisions, in the form of magnetic
strings. We conjecture that these magnetic strings originate from the alignment
of magnetic dipoles associated with EW sphaleron-like configurations. We also
discuss the early thermalisation of photons and the turbulent behaviour of the
scalar fields after tachyonic preheating.Comment: 7 pages. Talk presented at Lattice200
Gate-tuned normal and superconducting transport at the surface of a topological insulator
Three-dimensional topological insulators are characterized by the presence of
a bandgap in their bulk and gapless Dirac fermions at their surfaces. New
physical phenomena originating from the presence of the Dirac fermions are
predicted to occur, and to be experimentally accessible via transport
measurements in suitably designed electronic devices. Here we study transport
through superconducting junctions fabricated on thin Bi2Se3 single crystals,
equipped with a gate electrode. In the presence of perpendicular magnetic field
B, sweeping the gate voltage enables us to observe the filling of the Dirac
fermion Landau levels, whose character evolves continuously from electron- to
hole-like. When B=0, a supercurrent appears, whose magnitude can be gate tuned,
and is minimum at the charge neutrality point determined from the Landau level
filling. Our results demonstrate how gated nano-electronic devices give control
over normal and superconducting transport of Dirac fermions at an individual
surface of a three-dimensional topological insulator.Comment: 28 pages, 5 figure
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
Shot noise in mesoscopic systems
This is a review of shot noise, the time-dependent fluctuations in the
electrical current due to the discreteness of the electron charge, in small
conductors. The shot-noise power can be smaller than that of a Poisson process
as a result of correlations in the electron transmission imposed by the Pauli
principle. This suppression takes on simple universal values in a symmetric
double-barrier junction (suppression factor 1/2), a disordered metal (factor
1/3), and a chaotic cavity (factor 1/4). Loss of phase coherence has no effect
on this shot-noise suppression, while thermalization of the electrons due to
electron-electron scattering increases the shot noise slightly. Sub-Poissonian
shot noise has been observed experimentally. So far unobserved phenomena
involve the interplay of shot noise with the Aharonov-Bohm effect, Andreev
reflection, and the fractional quantum Hall effect.Comment: 37 pages, Latex, 10 figures (eps). To be published in "Mesoscopic
Electron Transport," edited by L. P. Kouwenhoven, G. Schoen, and L. L. Sohn,
NATO ASI Series E (Kluwer Academic Publishing, Dordrecht
Ge/Si nanowire mesoscopic Josephson junctions
The controlled growth of nanowires (NWs) with dimensions comparable to the
Fermi wavelengths of the charge carriers allows fundamental investigations of
quantum confinement phenomena. Here, we present studies of proximity-induced
superconductivity in undoped Ge/Si core/shell NW heterostructures contacted by
superconducting leads. By using a top gate electrode to modulate the carrier
density in the NW, the critical supercurrent can be tuned from zero to greater
than 100 nA. Furthermore, discrete sub-bands form in the NW due to confinement
in the radial direction, which results in stepwise increases in the critical
current as a function of gate voltage. Transport measurements on these
superconductor-NW-superconductor devices reveal high-order (n = 25) resonant
multiple Andreev reflections, indicating that the NW channel is smooth and the
charge transport is highly coherent. The ability to create and control coherent
superconducting ordered states in semiconductor-superconductor hybrid
nanostructures allows for new opportunities in the study of fundamental
low-dimensional superconductivity
TRY plant trait database - enhanced coverage and open access
Plant traits-the morphological, anatomical, physiological, biochemical and phenological characteristics of plants-determine how plants respond to environmental factors, affect other trophic levels, and influence ecosystem properties and their benefits and detriments to people. Plant trait data thus represent the basis for a vast area of research spanning from evolutionary biology, community and functional ecology, to biodiversity conservation, ecosystem and landscape management, restoration, biogeography and earth system modelling. Since its foundation in 2007, the TRY database of plant traits has grown continuously. It now provides unprecedented data coverage under an open access data policy and is the main plant trait database used by the research community worldwide. Increasingly, the TRY database also supports new frontiers of trait-based plant research, including the identification of data gaps and the subsequent mobilization or measurement of new data. To support this development, in this article we evaluate the extent of the trait data compiled in TRY and analyse emerging patterns of data coverage and representativeness. Best species coverage is achieved for categorical traits-almost complete coverage for 'plant growth form'. However, most traits relevant for ecology and vegetation modelling are characterized by continuous intraspecific variation and trait-environmental relationships. These traits have to be measured on individual plants in their respective environment. Despite unprecedented data coverage, we observe a humbling lack of completeness and representativeness of these continuous traits in many aspects. We, therefore, conclude that reducing data gaps and biases in the TRY database remains a key challenge and requires a coordinated approach to data mobilization and trait measurements. This can only be achieved in collaboration with other initiatives
Dynamic expression of tyrosine hydroxylase mRNA and protein in neurons of the striatum and amygdala of mice, and experimental evidence of their multiple embryonic origin
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