Controlled finite momentum pairing and spatially varying order parameter in proximitized HgTe quantum wells

Abstract

Conventional s-wave superconductivity is understood to arise from singlet pairing of electrons with opposite Fermi momenta, forming Cooper pairs whose net momentum is zero [1]. Several recent studies have focused on structures where such conventional s-wave superconductors are coupled to systems with an unusual configuration of electronic spin and momentum at the Fermi surface. Under these conditions, the nature of the paired state can be modified and the system may even undergo a topological phase transition [2,3]. Here we present measurements and theoretical calculations of several HgTe quantum wells coupled to either aluminum or niobium superconductors and subject to a magnetic field in the plane of the quantum well. By studying the oscillatory response of Josephson interference to the magnitude of the in-plane magnetic field, we find that the induced pairing within the quantum well is spatially varying. Cooper pairs acquire a tunable momentum that grows with magnetic field strength, directly reflecting the response of the spin-dependent Fermi surfaces to the in-plane magnetic field. In addition, in the regime of high electron density, nodes in the induced superconductivity evolve with the electron density in agreement with our model based on the Hamiltonian of Bernevig, Hughes, and Zhang [4]. This agreement allows us to quantitatively extract the value ofg/vF , whereg is the effective g-factor and vF is the Fermi velocity. However, at low density our measurements do not agree with our model in detail. Our new understanding of the interplay between spin physics and superconductivity introduces a way to spatially engineer the order parameter, as well as a general framework within which to investigate electronic spin texture at the Fermi surface of materials.1 arXiv:1509.02940v1 [cond-mat.mes-hall] Sep 2015Below a critical temperature and magnetic field, certain materials undergo a phase transition to the superconducting state. Macroscopically identified through effects such as zero resistivity and the Meissner effect [5], superconductors may further be understood microscopically as arising due to pairing of electrons occupying opposite points on the Fermi surface and having opposite spin. Within a conventional setting this interaction results in Cooper pairs with zero net momentum. However, in certain materials the presence of both magnetic order and superconductivity can lead to intrinsically nonzero pairing momentum as the system enters the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state [6,7]. Studies of both CeCoIn 5 and κ-(BEDT-TTF) 2 Cu(NCS) 2 under large external magnetic fields found evidence for coupled magnetic order and superconductivity, although in each material the field strength needed was in excess of 10 T [8,9].Exotic superconductivity has recently come under additional investigation through the goal of combining s−wave superconductors with materials whose properties are rarely found among the conventional superconductors. For example, inducing the singlet pairing of an s-wave supercon...

Topological Superconductivity in a Planar Josephson Junction

“…Similar transitions have previously been studied in ferromagnetic Josephson junctions [7,[19][20][21]. Quite remarkably, our results suggest that such a first-order phase transition in the present setup is in fact a topological phase transition unique to the two-dimensional geometry.…”

“…At high temperatures, the critical current vanishes at the magnetic field of the underlying zero-temperature topological transition. This insight suggests that the experimental results presented by Hart et al [7] indicate an underlying topological phase transition in the ground state. This paper is organized as follows.…”

“…Advances in nanotechnology and the prospect of using Majorana states as building blocks of topological quantum computers have triggered intense experimental efforts to realize and characterize them in one-dimensional systems [3][4][5][6]. More recently, two-dimensional electron gases (2DEGs) with induced superconductivity [7][8][9][10] have emerged as a contender for topological superconductivity.…”

“…An alternative route towards realizing a scalable architecture is to pattern a network of one-dimensional channels into a proximitized 2DEG using gates [7,10]. While this approach offers great flexibility in designing networks, it may be cumbersome to drive many channels individually into a topological regime via local gates employing additional local probes.…”

“…Here, we pursue a different strategy to realize Majorana bound states in an effectively one-dimensional system, motivated by recent experiments on Josephson junctions in proximity-coupled 2DEGs [7][8][9][10]. Carriers with energies below the superconducting gap are trapped in the quasi-onedimensional junction region between two superconducting leads as depicted in Fig.…”