Pablo Burset

Transport in superlattices on single layer graphene.

Funding for the work described here was provided by MICINN-Spain via Grants No. FIS2009-08744 (L.B.) and No. FIS2008-04209 (P.B. and A.L.Y.), and by the NSF through Grant No. DMR-1005035 (H.A.F.)

Entanglement Detection from Conductance Measurements in Carbon Nanotube Cooper Pair Splitters

Spin-orbit interaction provides a spin filtering effect in carbon nanotube based Cooper pair splitters that allows us to determine spin correlators directly from current measurements. The spin filtering axes are tunable by a global external magnetic field. By a bending of the nanotube, the filtering axes on both sides of the Cooper pair splitter become sufficiently different that a test of entanglement of the injected Cooper pairs through a Bell-like inequality can be implemented. This implementation does not require noise measurements, supports imperfect splitting efficiency and disorder, and does not demand a full knowledge of the spin-orbit strength. Using a microscopic calculation we demonstrate that entanglement detection by violation of the Bell-like inequality is within the reach of current experimental setups.

Superconducting proximity effect in three-dimensional topological insulators in the presence of a magnetic field

The proximity induced pair potential in a topological insulator-superconductor hybrid features an interesting superposition of a conventional spin-singlet component from the superconductor and a spin-triplet one induced by the surface state of the topological insulator. This singlet-triplet superposition can be altered by the presence of a magnetic field. We study the interplay between topological order and superconducting correlations performing a symmetry analysis of the induced pair potential, using Green functions techniques to theoretically describe ballistic junctions between superconductors and topological insulators under magnetic fields. We relate a change in the conductance from a gapped profile into one with a zero-energy peak with the transition into a topologically nontrivial regime where the odd-frequency triplet pairing becomes the dominant component in the pair potential. The nontrivial regime, which provides a signature of odd-frequency triplet superconductivity, is reached for an out-of-plane effective magnetization with strength comparable to the chemical potential of the superconductor or for an in-plane one, parallel to the normal-superconductor interface, with strength of the order of the superconducting gap. Strikingly, in the latter case, a misalignment with the interface yields an asymmetry with the energy in the conductance unless the total contribution of the topological surface state is considered.

Odd-frequency triplet superconductivity at the helical edge of a topological insulator

Non-local pairing processes at the edge of a two-dimensional topological insulator in proximity to an

Microscopic theory of Cooper pair beam splitters based on carbon nanotubes

s

Selective focusing of electrons and holes in a graphene-based superconducting lens

-wave superconductor are usually suppressed by helicity. However, additional proximity of a ferromagnetic insulator can substantially influence the helical constraint and therefore open a new conduction channel by allowing for crossed Andreev reflection (CAR) processes. We show a one-to-one correspondence between CAR and the emergence of odd-frequency triplet superconductivity. Hence, non-local transport experiments that identify CAR in helical liquids yield smoking-gun evidence for unconventional superconductivity. Interestingly, we identify a setup -- composed of a superconductor flanked by two ferromagnetic insulators -- that allows us to favor CAR over electron cotunneling which is known to be a difficult but essential task to be able to measure CAR.

Transport signatures of superconducting hybrids with mixed singlet and chiral triplet states

This work was supported by MICINN-Spain via Grant No. FIS2008-04209, the EU FP7 project SE2ND (P.B. and A.L.Y.), and DIB from Universidad Nacional de Colombia, Project No. 12170 (W.J.H.)

Transport spectroscopy of induced superconductivity in the three-dimensional topological insulator HgTe

This work was supported by COLCIENCIAS, project 110152128235 (S.G. and W.J.H.), and MICINN-Spain via Grant No. FIS2008-04209 and EU project SE2ND

Quantum interference of edge supercurrents in a two-dimensional topological insulator

We propose a model for a superconductor where both spin-singlet and chiral triplet pairing amplitudes can coexist. By solving the Bogoliubov-de Gennes equations with a general pair potential that accounts for both spin states we study experimental signatures of normal metal and superconductor hybrids. The interplay between the spin-singlet and triplet correlations manifests in the appearance of two effective gaps. When the amplitude of the spin-triplet component is stronger than that of the spin-singlet, a topological phase transition into a non-trivial regime occurs. As a result, the normal metal-superconductor conductance evolves from a conventional gap profile onto an unconventional zero-bias peak. Additionally, in the topologically non-trivial phase, Andreev bound states formed at Josephson junctions present zero-energy modes; the number of those modes depends on the relative chirality of the junction. Finally, we present results for the current-phase relation and the temperature dependence of the Josephson critical current within both topological phases for several system parameters.

All-electrical generation and control of odd-frequency s-wave Cooper pairs in double quantum dots

A strained and undoped HgTe layer is a three-dimensional topological insulator, in which electronic transport occurs dominantly through its surface states. In this Letter, we present transport measurements on HgTe-based Josephson junctions with Nb as a superconductor. Although the Nb-HgTe interfaces have a low transparency, we observe a strong zero-bias anomaly in the differential resistance measurements. This anomaly originates from proximity-induced superconductivity in the HgTe surface states. In the most transparent junction, we observe periodic oscillations of the differential resistance as a function of an applied magnetic field, which correspond to a Fraunhofer-like pattern. This unambiguously shows that a precursor of the Josephson effect occurs in the topological surface states of HgTe.