Seminar: Equal-spin Cooper pairs and odd-frequency pairing at the quantum spin Hall helical edge

  • Date:
  • Location: Ångströmlaboratoriet, Lägerhyddsvägen 1 The seminar room in the Villa
  • Lecturer: Pablo Burset, Aalto University
  • Contact person: Jorge Cayao
  • Seminarium

The key challenge in superconducting spintronics is the non-equilibrium and on-demand generation of equal-spin Cooper pairs in a viable fashion, desirably avoiding the complicated manipulation of magnetic components. In this talk, I will show how the helical edge states of a quantum spin Hall insulator (QSHI) provide a robust platform for creating equal-spin Cooper pairs.

Helicity dramatically affects the transport properties of hybrid junctions composed of normal and superconducting electrodes. In such hybrid junctions, the microscopic transport processes coupling two different normal leads across a superconducting one are the quantum tunnelling of electrons, known as electron cotunneling (EC), and the nonlocal (or crossed) Andreev reflections (CAR). However, helicity demands that nonlocal Andreev processes can only convert electrons into holes with the same spin. Therefore, CAR at the helical edge of a quantum spin Hall insulator must involve equal-spin triplet Cooper pairs. Interestingly, the interplay of helicity and proximity-induced superconducting order gives rise to a superposition of spin-singlet and triplet Cooper pairs at the QSHI edge. The additional proximity of a ferromagnetic insulator can substantially influence the helical constraint and therefore open a new conduction channel by allowing for CAR.

I will show a one-to-one correspondence between CAR and the emergence of odd-frequency, equal-spin triplet pairing [1]. I will then propose a setup composed of a superconductor flanked by two ferromagnetic insulators (FSF junction), that favours CAR over EC and thus provides a signature of odd-frequency, equal-spin Cooper pairs in the nonlocal conductance. Next, I will consider a SFS junction, connecting two leads at different temperatures, on top of the helical edge of a QSHI [2]. In this setup, the nonlocal current becomes dominated by CAR processes, with almost complete suppression of the EC contribution. This is only possible due to a unique interference effect for CAR processes. Such a purely nonlocal Andreev thermoelectric effect injects spin-polarized Cooper pairs into the superconductors, generating a supercurrent that can be switched on/off by tuning their relative phase. Finally, I will describe how a similar interference effect between Andreev processes takes place in an all-electrical NSS junction, with N a normal electrode. In this setup, the coexistence of even and odd-frequency pairings with equal magnitude results in an asymmetry in the conductance that can be measured as an Andreev-dominated thermoelectric current [3].

[1] F. Crépin, P. Burset, B. Trauzettel, Phys. Rev. B 92, 100507(R) (2015)

[2] F. Keidel, S.-Y. Hwang, B. Trauzettel, B. Sothmann, P. Burset, arXiv:1907.00965 (2019)

[3] Y. Tanaka, F. Keidel, B. Trauzettel, P. Burset, in preparation..