Unconventional proximity-induced superconductivity in bilayer systems

Abstract

We study the proximity-induced superconducting state in a general bilayer -conventional s-wave superconductor hybrid structure. For the bilayer we include a general parabolic dispersion, Rashba spin-orbit coupling, and finite layer tunneling as well as the possibility to apply a bias potential and a magnetic Zeeman field, in order to address experimentally relevant bilayer systems, ranging from topological insulator thin films to generic double quantum well systems. By extracting the proximity-induced anomalous Green's function in the bilayer we show on a very rich structure for the superconducting pairing, including different spin states and odd-frequency pairing. Equal-spin spin-triplet px ± ipy-wave pairing is induced in both layers in the presence of a finite spin-orbit coupling and opposite-spin spin-triplet s-wave pairing with odd-frequency dependence appears for an applied magnetic Zeeman field. Finite interlayer pairing is also generally present in the bilayer. The interlayer pairing can be either even or odd in the layer index, with a complete reciprocity between parity in frequency and in layer index. We also find that a bilayer offers the possibility of sign reversal of the superconducting order parameters, both between the two layers and between multiple Fermi surfaces.

Pair symmetry conversion in driven multiband superconductors

“…To better understand how the drive affects the even-ω pair amplitudes, in Figs 5(c)-5(f) we show both the symmetry preserving (green/dashed) and symmetry reversing (red/solid) corrections to the even-ω intraband pair amplitudes appearing in Figs 4(a) and 4(b), calculated using Eqs (22) and (23). Consistent with the results in Sec III B, we find that, in general, both contributions are nonzero.…”

“…Plotted over this range, the three curves are essentially indistinguishable; however, there a slight differences which we highlight in (c)-(f). In (c)-(f) we show the symmetry preserving (green/dashed) and symmetry reversing (red/solid) contributions to the plots appearing in (a) and (b) calculated using Eqs (22) and (23). In (c) and (e) we show the corrections to the even-ω intraband pairing in band-a at times T = 0 and T = π/2Ω0, respectively; in (d) and (f) we show the corrections to the even-ω intraband pairing in band-b at times T = 0 and T = π/2Ω0, respectively.…”

Specular Andreev reflection in thin films of topological insulators

“…The superconducting correlations can be induced in the TI thin film via the proximity effect to the S electrode with desired properties. Recently, proximity-induced superconductivity was experimentally demonstrated in a Bi 2 Se 3 thin film by growing the TI on a conventional NbSe 2 superconductor [16,17], and it was also addressed theoretically [18]. To describe the superconducting correlations between electrons and holes with the wave functions u and v, we use the following Dirac-Bogoliubov-de Gennes (DBdG) equation [19] …”

Experimentally observable signatures of odd-frequency pairing in multiband superconductors

“…Alternatively, the band hybridization can be interpreted as a coupling process in real space, with the a-and belectrons living to the left and the right of a junction or in different layers. 32,51 In this picture the Josephson coupling would instead be a two-particle tunneling term (not included here). We start by calculating the spin-singlet s-wave interband anomalous Green's functions F 12 and F 21 , which express the pairing correlations of two electrons belonging to different bands.…”