25 research outputs found
Josephson supercurrent through a topological insulator surface state
Topological insulators are characterized by an insulating bulk with a finite
band gap and conducting edge or surface states, where charge carriers are
protected against backscattering. These states give rise to the quantum spin
Hall effect without an external magnetic field, where electrons with opposite
spins have opposite momentum at a given edge. The surface energy spectrum of a
threedimensional topological insulator is made up by an odd number of Dirac
cones with the spin locked to the momentum. The long-sought yet elusive
Majorana fermion is predicted to arise from a combination of a superconductor
and a topological insulator. An essential step in the hunt for this emergent
particle is the unequivocal observation of supercurrent in a topological phase.
Here, we present the first measurement of a Josephson supercurrent through a
topological insulator. Direct evidence for Josephson supercurrents in
superconductor (Nb) - topological insulator (Bi2Te3) - superconductor e-beam
fabricated junctions is provided by the observation of clear Shapiro steps
under microwave irradiation, and a Fraunhofer-type dependence of the critical
current on magnetic field. The dependence of the critical current on
temperature and length shows that the junctions are in the ballistic limit.
Shubnikov-de Haas oscillations in magnetic fields up to 30 T reveal a
topologically non-trivial two-dimensional surface state. We argue that the
ballistic Josephson current is hosted by this surface state despite the fact
that the normal state transport is dominated by diffusive bulk conductivity.
The lateral Nb-Bi2Te3-Nb junctions hence provide prospects for the realization
of devices supporting Majorana fermions
Explaining why simple liquids are quasi-universal
It has been known for a long time that many simple liquids have surprisingly
similar structure as quantified, e.g., by the radial distribution function. A
much more recent realization is that the dynamics are also very similar for a
number of systems with quite different pair potentials. Systems with such
non-trivial similarities are generally referred to as "quasi-universal". From
the fact that the exponentially repulsive pair potential has strong virial
potential-energy correlations in the low-temperature part of its thermodynamic
phase diagram, we here show that a liquid is quasi-universal if its pair
potential can be written approximately as a sum of exponential terms with
numerically large prefactors. Based on evidence from the literature we moreover
conjecture the converse, i.e., that quasi-universality only applies for systems
with this property
Direct observation of Josephson vortex cores
International audienceSuperconducting correlations may propagate between two superconductors separated by a tiny insulating or metallic barrier, allowing a dissipationless electric current to flow(1,2). In the presence of a magnetic field, the maximum supercurrent oscillates(3) and each oscillation corresponding to the entry of one Josephson vortex into the barrier(4). Josephson vortices are conceptual blocks of advanced quantum devices such as coherent terahertz generators(5) or qubits for quantum computing(6), in which on-demand generation and control is crucial. Here, we map superconducting correlations inside proximity Josephson junctions(7) using scanning tunnelling microscopy. Unexpectedly, we find that such Josephson vortices have real cores, in which the proximity gap is locally suppressed and the normal state recovered. By following the Josephson vortex formation and evolution we demonstrate that they originate from quantum interference of Andreev quasiparticles(8), and that the phase portraits of the two superconducting quantum condensates at edges of the junction decide their generation, shape, spatial extent and arrangement. Our observation opens a pathway towards the generation and control of Josephson vortices by applying supercurrents through the superconducting leads of the junctions, that is, by purely electrical means without any need for a magnetic field, which is a crucial step towards high-density on-chip integration of superconducting quantum devices
Evidence for an anomalous current–phase relation in topological insulator Josephson junctions
Chocolate Bars Based on Human Nutritional Requirements
International audienceKey Points * The nutritional value of chocolate bars should be based on the nutritional value of the low energy dense late Paleolithic human diet to help reduce mental ill health, obesity, and other postprandial insults. * Current chocolate bars have a high energy density (>2 kcal/g). * Cocoa can be sweetened by the addition of calorie-free Purefruit™ (Tate & Lyle) monk fruit ( Siraitia grosvenorii ) extract. PUREFRUIT™ is approximately 200 times sweeter than sugar and has exceptional stability. * The energetic cost of the assimilation of chocolate can be increased by increasing its protein and fibre content. * Self-assembled, water-filled, edible nanotubes that self-organise into a more complex structure, possibly a 3D network of nanocellulose, could be incorporated into chocolate bars to lower their energy density to <1.6 kcal/g. * Durethan® KU 2-2601 packaging film enables the water content of chocolate bars to be increased without reducing product shelf life. * Aquatic biotechnology can provide all the nutrients needed to make chocolate really nutritious
Quantum and Classical Magnetoresistance in Ambipolar Topological Insulator Transistors with Gate-tunable Bulk and Surface Conduction
Evidence of a one-dimensional thermodynamic phase diagram for simple glass-formers
Glass formers show motional processes over an extremely broad range of timescales, covering more than ten orders of magnitude, meaning that a full understanding of the glass transition needs to comprise this tremendous range in timescales. Here we report simultaneous dielectric and neutron spectroscopy investigations of three glass-forming liquids, probing in a single experiment the full range of dynamics. For two van der Waals liquids, we locate in the pressure-temperature phase diagram lines of identical dynamics of the molecules on both second and picosecond timescales. This confirms predictions of the isomorph theory and effectively reduces the phase diagram from two to one dimension. The implication is that dynamics on widely different timescales are governed by the same underlying mechanisms