Andreev Reflection in ans-Type Superconductor Proximized 3D Topological Insulator

Abstract

We investigate transport and shot noise in lateral N-TI-S contacts, where N is a normal metal, TI is a Bi-based three dimensional topological insulator (3D TI), and S is an s-type superconductor. In the normal state, the devices are in the elastic diffusive transport regime, as demonstrated by a nearly universal value of the shot noise Fano factor FN ≈ 1/3 in magnetic field and in a reference normal metal contact. In the absence of magnetic field, we identify the Andreev reflection (AR) regime, which gives rise to the effective charge doubling in shot noise measurements. Surprisingly, the Fano factor FAR ≈ 0.22 ± 0.02 is considerably reduced in the AR regime compared to FN, in contrast to previous AR experiments in normal metals and semiconductors. We suggest that this effect is related to a finite thermal conduction of the proximized, superconducting TI owing to a residual density of states at low energies.A surface state of a three-dimensional topological insulator (3D TI) is a unique example of a spin-orbit coupled and symmetry protected conductor 1 . Similar to graphene, in 3D TI the surface electronic states are massless Dirac fermions. Unlike in graphene, however, a single Dirac cone is lacking spin and valley degeneracies. That makes a 3D TI an intriguing candidate for the realization of a solid state two-dimensional topological superconductor 2 . As originally proposed by Fu and Kane 3 , p-wave like superconducting correlations are expected to emerge via proximity coupling the 3D TI to a conventional stype superconductor (S). This gives rise to symmetry protected Majorana zero modes bound at vortices or at the boundaries of various hybrid structures 4,5 . Emerging zero modes are predicted to have a strong impact on the low energy physics, modifying Andreev reflection (AR) at the interface with a normal metal 6 , affecting the edge conductance distribution 7 , noise 8 and thermal transport 9,10 .Proximity induced superconductivity has been demonstrated in 3D TIs based on Bi 11-17 and HgTe 18,19 with the reported values of the induced gap on the order of a few 100 µeV. Similar gap values were recently observed via Andreev spectroscopy 20 in quaternary BiSbTeSe compound, established as a 3D TI with negligible contribution of the bulk conduction 21,22 . In spite of these advances, the microscopic nature of the proximity induced gap in Bi-based 3D TIs remains largely unexplored. This particularly concerns the statistics of the transmission eigenvalue distribution in short junctions and the role of possible in-gap states. On this route, valuable information, often hidden in transport, can be obtained via measurements of the non-equilibrium current fluctuationsthe shot noise 23 . Relevant for the AR, prominent examples include unusual shot noise behavior at the interface between a normal metal and superconductors with other than s-type order parameter symmetry 24,25 and, more recently, shot noise detection of the thermal and charge transport via Majorana zero modes 9,10,26 .Here, we investigate the AR in latera...

Signatures of Topological Superconductivity in Bulk-Insulating Topological Insulator BiSbTe1.25Se1.75 in Proximity with Superconducting NbSe2

“…This distinction between superconductivity induced and e-e interaction induced differential conductance spectra is notable, particularly because previous experiments on superconducting proximity effects on bulk insulating TIs have been mired by strong differential conductance signals arising from e-e interactions rather than superconducting correlations. 33,34 We now discuss the low-field measurements presented in 4(b). Specifically, we notice several differential conductance ripples above the superconducting gap that persist and even appear to get enhanced with small magnetic fields B < 0.2T .…”

“…This may explain recent results on Nb/BSTS interfaces where electron-electron correlations rather than superconducting correlations appeared to dominate electrical transport. 33,34 Several things must therefore conspire for topological superconductivity to occur in bulk insulating TIs: i) The host TI should be able to support large surface carrier density whilst maintaining negligible bulk carrier density ii) The chemical identities of the TI and SC must be similar to create a homogeneous interface that allows strong wave-function overlap on either side iii) The Fermi surfaces on the SC and TI side should be well-matched to allow momentum conserved tunneling of quasiparticles. To this end, we fabricate van der Waals junctions between bulk-insulating topological insulator BiSbTe 1.25 Se 1.75 (BSTS) and twodimensional s-wave superconductor NbSe 2 and study their electrical transport properties.…”

Thermal conductance and nonequilibrium superconductivity in a diffusive NSN wire probed by shot noise

“…A correspondence between the shot noise and thermal conductance is a generic effect not limited to the Majorana case. A doubling of the shot noise in disordered NS junctions [16,17] is fundamentally related to the suppressed heat transfer in the S-lead [18,19], and can be useful to probe the sub-gap density of states in such structures [20,21]. In NSN NW-based devices the shot noise and thermal conductance are directly re-lated in the limit of charge neutral quasiparticle transport, that was demonstrated in a recent experiment set up in a trivial superconducting phase [22].…”

Current fluctuations in unconventional superconductor junctions with impurity scattering

“…We have demonstrated the utility of noise measurements for the identification of order parameters, even in the cases in which tunnel conductance can be ambiguous [62].…”