"Hunting for helical boundary states and Majoranas in Superconductor-Topological Insulator-Superconductor hybrid junctions"

Who: Arturo Tagliacozzo ( Universita' di Napoli "Federico II", Italy )

Place: Donostia International Physics Center (DIPC). Paseo Manuel de Lardizabal, 4, Donostia

Date: Thursday, 12 February 2015, 12:00

Hunting for helical boundary states and Majoranas in
Superconductor-Topological Insulator-Superconductor hybrid junctions
  A. Tagliacozzo 1,2
 1 CNR-SPIN  & Dipartimento di Fisica, UniversitÓ di Napoli Federico II,
  Monte S. Angelo, via Cintia I-80126 Napoli, Italy  
2 Istituto Nazionale Fisica Nucleare, Laboratori Nazionali di Frascati, Italy


 Superconductor-3D Topological Insulator -Superconductor  ( S-3D TI-S) hybrid devices can exploit the topologically protected boundary states of the TI for the Josephson coupling .  However, unveiling the presence of the helical boundary states in these structures with transport measurements appears to be a hard task.
 Superconductive proximity induced by  an even parity superconductor in flakes of 3D TI ( e.g.Bi2Se3 , Bi2Te3) provides p-wave superconducting pairing in the helical edge state, a requirement for the existence of Majorana states  bound at a   ?-Josephson Junction.
Proximity induced by high-critical-temperature cuprate superconductors (HTc), besides offering a wider and more robust range of stability conditions due to the large gap values,  displays novel functionalities for the design of devices, thanks to their order parameter anisotropy.
We propose a ?-rf-SQUID device based on a HTc superconducting  tricrystal structure, with appropriate orientations of the crystal lattices,  to build a topologically non trivial Josephson superconducting ring. The  overall phase ? acquired by the order parameter when circulating along the ring supports  a zero energy Majorana Bound State(MBS) in the TI  ring. Although we prove that the ring geometry guarantees total protection of the doubly degenerate ground state when the ring is isolated, charge poisoning  is expected to invalidate the fermion parity, implying an average zero measured flux. A zero flux measure for the  ?-ring is the signature of the presence of the MBS in the topologically non trivial device.

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