We study the low temperature electrical transport in gated BiSbTe1.25Se1.75/hexagonal-Boron Nitride van der Waals heterostructure devices. Our experiments indicate the presence of Rashba spin-split states confined to the sample surface. While such states have been observed previously in photo-emission spectroscopy and STM experiments, it has not been possible to unambiguously detect them by electrical means and their transport properties remain mostly unknown. We show that these states support high mobility conduction with Hall effect mobilities ∼2000 to 3000 cm2/V-s that are paradoxically much larger than the mobilities of the topological surface states ∼300 cm2/V-s at T = 2 K. The spin-split nature of these states is confirmed by magneto-resistance measurements that reveal multi-channel weak anti-localization. Our work shows that Rashba spin split states can be electrically accessed in Topological insulators paving the way for future spintronic applications.
The suggestion that non-reciprocal critical current (NRC) may be an intrinsic property of non-centrosymmetric superconductors has generated renewed theoretical and experimental interest motivated by an analogy with the non-reciprocal resistivity due to the magnetochiral effect in uniform materials with broken spatial and time-reversal symmetry. Theoretically it has been understood that terms linear in the Cooper pair momentum do not contribute to NRC, although the role of higher-order terms remains unclear. In this work we show that critical current non-reciprocity is a generic property of multilayered superconductor structures in the presence of magnetic field-generated diamagnetic currents. In the regime of an intermediate coupling between the layers, the Josephson vortices are predicted to form at high fields and currents. Experimentally, we report the observation of NRC in nanowires fabricated from InAs/Al heterostructures. The effect is independent of the crystallographic orientation of the wire, ruling out an intrinsic origin of NRC. Non-monotonic NRC evolution with magnetic field is consistent with the generation of diamagnetic currents and formation of the Josephson vortices. This extrinsic NRC mechanism can be used to design novel devices for superconducting circuits.
The combination of superconductivity and spin-momentum locking at the interface between an s-wave superconductor and a three-dimensional topological insulator (3D-TI) is predicted to generate exotic p-wave topological superconducting phases that can host Majorana fermions. However, large bulk conductivities of previously investigated 3D-TI samples and Fermi level mismatches between 3D bulk superconductors and 2D topological surface states have thwarted significant progress. Here we employ bulk insulating topological insulators in proximity with two-dimensional superconductor NbSe 2 assembled via Van der Waals epitaxy. Experimentally measured differential conductance yields unusual features including a double-gap spectrum, an intrinsic asymmetry that vanishes with small in-plane magnetic fields and differential conductance ripples at biases significantly larger than the superconducting gap. We explain our results on the basis of proximity induced superconductivity of topological 1 arXiv:1911.07208v1 [cond-mat.mes-hall] 17 Nov 2019 surface states, while also considering possibilities of topologically trivial superconductivity arising from Rashba-type surface states. Our work demonstrates the possibility of obtaining p-wave superconductors by proximity effects on bulk insulating TIs.Keywords: topological superconductivity, majorana fermions, proximity effect, topological insulator, 2D superconductor, van der Waals heterostructure Proximity effects between topological insulators and superconductors have attracted significant attention as potential sources of unconventional superconductivity. Right after the discovery of three dimensional topological insulators 1-5 it became evident that inducing superconductivity into two-dimensional surface states of 3D-TIs 2,6-8 could lead to p-wave superconductivity, [9][10][11][12] where defects in the form of edges or vortices host Majorana fermion states. In a different vein, it was also shown that inducing superconductivity into the 'bulk' of topological insulators could lead to 3D topological superconductors whose surfaces could host surface Andreev bound states: essentially two dimensional analogues of linearly dispersing Majorana fermions. 13-17 Both these directions have been pursued vigorously with encouraging results including purported demonstrations of surface Andreev bound states in Cu x Bi 2 Se 3 13,14 and similar materials, 18 proximity effects on 3D-TI/superconductor interfaces, 19-26 Majorana zero modes in vortex cores of Bi 2 Te 3 /NbSe 2 27,28 heterostructure, chiral 1D Majorana modes in Nb/quantum anomalous Hall insulator heterostructures. 29 Yet, a lot of these results remain ambiguous and can also be interpreted as consequences of more trivial effects. Experimental platforms that can manifest clear indications of topological superconductivity or Majorana fermions in TIs are therefore highly sought after. One of the primary sources of such ambiguity is the large bulk conductivity in first generation topological insulators like Bi 2 Se 3 . Most experiments on t...
Information processing devices operating in the quantum mechanical regime strongly rely on the quantum coherence of charge carriers. Studies of electronic dephasing in conventional metallic and semiconductor systems have not only paved the way towards high coherence quantum electronics, but also led to fundamental new insights in condensed matter physics. In this work, we perform a spatially resolved study of electronic dephasing in three dimensional topological insulators by exploiting an edge versus surface contacted measurement scheme. Unlike conventional two dimensional systems that are characterized by a single dephasing mechanism, we find that dephasing in our samples evolves from a variable-range-hopping type mechanism on the sample surface to a Nyquist type electron-electron interaction mechanism in the sub-surface layers. This is confirmed independently by the temperature and chemical potential dependence of the dephasing length, and gate dependent suppression/enhancement of the weak anti-localization effect. Our devices are fabricated using bulk insulating topological insulator BiSbTe 1.25 Se 1.75 capped with hexagonal-Boron Nitride in an inert environment, ruling out any extrinsic effects and confirming the topological surface state origin of our results. Our work introduces the idea of spatially resolved electronic dephasing and reveals a new regime of coherent transport in perhaps the most important topological insulator discovered so far. Our edge-vs-surface scheme may be applied to dephasing studies in a wide class of 2D materials.
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