175 research outputs found
Phase Coherence and Andreev Reflection in Topological Insulator Devices
Topological insulators (TIs) have attracted immense interest because they
host helical surface states. Protected by time-reversal symmetry, they are
robust to non-magnetic disorder. When superconductivity is induced in these
helical states, they are predicted to emulate p-wave pairing symmetry, with
Majorana states bound to vortices. Majorana bound states possess non-Abelian
exchange statistics which can be probed through interferometry. Here, we take a
significant step towards Majorana interferometry by observing pronounced
Fabry-Perot oscillations in a TI sandwiched between a superconducting and
normal lead. For energies below the superconducting gap, we observe a doubling
in the frequency of the oscillations, arising from the additional phase
accumulated from Andreev reflection. When a magnetic field is applied
perpendicular to the TI surface, a number of very sharp and gate-tunable
conductance peaks appear at or near zero energy, which has consequences for
interpreting spectroscopic probes of Majorana fermions. Our results demonstrate
that TIs are a promising platform for exploring phase-coherent transport in a
solid-state system.Comment: 9 pages, 7 figure
Macroscopic Coherent Rectification in Andreev Interferometers
We investigate nonlinear transport through quantum coherent metallic
conductors contacted to superconducting components. We find that in certain
geometries, the presence of superconductivity generates a large, finite-average
rectification effect. Specializing to Andreev interferometers, we show that the
direction and magnitude of rectification can be controlled by a magnetic flux
tuning the superconducting phase difference at two contacts. In particular,
this results in the breakdown of an Onsager reciprocity relation at finite
bias. The rectification current is macroscopic in that it scales with the
linear conductance, and we find that it exceeds 5% of the linear current at
sub-gap biases of few tens of \mu eV's.Comment: 5 pages, 3 figure
Shot noise in the hybrid triple-quantum-dot interferometer coupled to superconductor and normal terminals
postprin
Nonlocal quantum heat engines made of hybrid superconducting devices
We discuss a quantum thermal machine that generates power from a thermally driven double quantum dot coupled to normal and superconducting reservoirs. Energy exchange between the dots is mediated by electron-electron interactions. We can distinguish three main mechanisms within the device operation modes. In the Andreev tunneling regime, energy flows in the presence of coherent superposition of zero- and two-particle states. Despite the intrinsic electron-hole symmetry of Andreev processes, we find that the heat engine efficiency increases with increasing coupling to the superconducting reservoir. The second mechanism occurs in the regime of quasiparticle transport. Here we obtain large efficiencies due to the presence of the superconducting gap and the strong energy dependence of the electronic density of states around the gap edges. Finally, in the third regime there exists a competition between Andreev processes and quasiparticle tunneling. Altogether, our results emphasize the importance of both pair tunneling and structured band spectrum for an accurate characterization of the heat engine properties in normal-superconducting coupled dot system
Thermoelectric effects in superconducting proximity structures
Attaching a superconductor in good contact with a normal metal makes rise to
a proximity effect where the superconducting correlations leak into the normal
metal. An additional contact close to the first one makes it possible to carry
a supercurrent through the metal. Forcing this supercurrent flow along with an
additional quasiparticle current from one or many normal-metal reservoirs makes
rise to many interesting effects. The supercurrent can be used to tune the
local energy distribution function of the electrons. This mechanism also leads
to finite thermoelectric effects even in the presence of electron-hole
symmetry. Here we review these effects and discuss to which extent the existing
observations of thermoelectric effects in metallic samples can be explained
through the use of the dirty-limit quasiclassical theory.Comment: 14 pages, 10 figures. 374th WE-Heraus seminar: Spin physics of
superconducting heterostructures, Bad Honnef, 200
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