We study unconventional superconductivity in exfoliated single crystals of a promising three-dimensional (3D) topological superconductor candidate, Nb-doped Bi
Se
through differential conductance ...spectroscopy and magneto-transport. The strong anisotropy of the critical field along the out-of-plane direction suggests that the thin exfoliated flakes are in the quasi-2D limit. Normal metal-superconductor (NS) contacts with either high or low transparencies made by depositing gold leads onto Nb-doped Bi
Se
flakes both show significant enhancement in zero bias conductance and coherence dips at the superconducting energy gap. Such behavior is inconsistent with conventional Blonder-Tinkham-Klapwijk theory. Instead, we discuss how our results are consistent with p-wave pairing symmetry, supporting the possibility of topological superconductivity in Nb-doped Bi
Se
. Finally, we observe signatures of multiple superconducting energy gaps, which could originate from multiple Fermi surfaces reported earlier in bulk crystals.
Combining topology and superconductivity provides a powerful tool for investigating fundamental physics as well as a route to fault-tolerant quantum computing. There is mounting evidence that the ...Fe-based superconductor FeTe0.55Se0.45 (FTS) may also be topologically nontrivial. Should the superconducting order be s±, then FTS could be a higher order topological superconductor with helical hinge zero modes (HHZMs). To test the presence of these modes, we have fabricated normal-metal/superconductor junctions on different surfaces via 2D atomic crystal heterostructures. As expected, junctions in contact with the hinge reveal a sharp zero bias anomaly that is absent when tunneling purely into the c-axis. Additionally, the shape and suppression with temperature are consistent with highly coherent modes along the hinge and are incongruous with other origins of zero bias anomalies. Additional measurements with soft-point contacts in bulk samples with various Fe interstitial contents demonstrate the intrinsic nature of the observed mode. Thus, we provide evidence that FTS is indeed a higher order topological superconductor.
Andreev bound states and their signatures Sauls, J. A.
Philosophical transactions of the Royal Society of London. Series A: Mathematical, physical, and engineering sciences,
08/2018, Letnik:
376, Številka:
2125
Journal Article
Recenzirano
Odprti dostop
Many of the properties of superconductors related to quantum coherence are revealed when the superconducting state is forced to vary in space in response to an external magnetic field, a proximity ...contact, an interface to a ferromagnet or to impurities embedded in the superconductor. Among the earliest examples is Andreev reflection of an electron into a retro-reflected hole at a normal-superconducting interface. In regions of strong inhomogeneity, multiple Andreev reflection leads to the formation of sub-gap states, Andreev bound states, with excitation energies below the superconducting gap. These states play a central role in our understanding of inhomogeneous superconductors. The discoveries of unconventional superconductivity in many classes of materials, advances in fabrication of superconducting/ferromagnetic hybrids and nanostructures for confining superfluid 3 He, combined with theoretical developments in topological quantum matter have dramatically expanded the significance of branch conversion scattering and Andreev bound state formation. This collection of articles highlights developments in inhomogeneous superconductivity, unconventional superconductivity and topological phases of superfluid 3 He, in which Andreev scattering and bound states underpin much of the physics of these systems. This article provides an introduction to the basic physics of Andreev scattering, bound-state formation and their signatures. The goal is both an introduction for interested readers who are not already experts in the field, and to highlight examples in which branch conversion scattering and Andreev bound states provide unique signatures in the transport properties of superconductors.
This article is part of the theme issue 'Andreev bound states'.
The search for proximity-induced superconductivity in topological materials has generated widespread interest in the condensed matter physics community. The superconducting states inheriting ...nontrivial topology at interfaces are expected to exhibit exotic phenomena such as topological superconductivity and Majorana zero modes, which hold promise for applications in quantum computation. However, a practical realization of such hybrid structures based on topological semimetals and superconductors has hitherto been limited. Here, we report the strong proximity-induced superconductivity in type-II Weyl semimetal WTe2, in a van der Waals hybrid structure obtained by mechanically transferring NbSe2 onto various thicknesses of WTe2. When the WTe2 thickness (t WTe2 ) reaches 21 nm, the superconducting transition occurs around the critical temperature (T c) of NbSe2 with a gap amplitude (Δp) of 0.38 meV and an unexpected ultralong proximity length (l p) up to 7 μm. With the thicker 42 nm WTe2 layer, however, the proximity effect yields T c ≈ 1.2 K, Δp = 0.07 meV, and a short l p of less than 1 μm. Our theoretical calculations, based on the Bogoliubov–de Gennes equations in the clean limit, predict that the induced superconducting gap is a sizable fraction of the NbSe2 superconducting one when t WTe2 is less than 30 nm and then decreases quickly as t WTe2 increases. This agrees qualitatively well with the experiments. Such observations form a basis in the search for superconducting phases in topological semimetals.
Abstract
In the framework of Bogoliubov–de Gennes equation, we theoretically study the transport properties in normal-superconducting junctions based on semi-Dirac materials (SDMs). Owing to the ...intrinsic anisotropy of SDMs, the configuration of Andreev reflection (AR) and the differential conductance are strongly orientation-dependent. For the transport along the linear dispersion direction, the differential conductance exhibits a clear crossover from retro AR to specular AR with increasing the bias-voltage, and the differential conductance oscillates with the interfacial barrier strength without a decaying profile. Conversely, for the transport along the quadratic dispersion direction, the boundary between the retro AR and specular AR becomes ambiguous when the orientation angle increases, and the differential conductance decays with increasing the momentum mismatch or the interfacial barrier strength. We illustrate the pseudo-spin textures to reveal the underling physics behind the anisotropic coherent transport properties. These results enrich the understanding of the superconducting coherent transport in SDMs.
We study the influence of magnetic field on the transport properties of ferromagnetic metal (FM)/ferromagnetic insulator (FI)/s-wave superconductor (s-wave SC) junctions and FM/FM/s-wave SC junctions ...formed on the surfaces of three dimensional topological insulators (3DTI), where the magnetic field is applied perpendicular to the s-wave SC surfaces. For a broad range of the magnetic field strength, the conductance is investigated. Unlike the preceding studies, we apply the magnetic field to the superconductor surfaces of the junctions instead of the magnetization induced by the ferromagnets adjacent to their surfaces. We determine the dependence of conductance on the magnetic field for the cases of parallel and antiparallel magnetizations and reveal that the magnetoresistance sensitively depends on the magnitude of the magnetic field for the biases below the renormalized gap. The results obtained in this work will find applications in superconductor spintronics.
•We consider a ferromagnet/ferromagnet/s-wave superconductor junctions formed on the surface of a topological insulator.•The magnetic field is applied perpendicular to the s-wave superconductor surfaces.•We study the influence of magnetic field on the conductance and the magnetoresistance for such a model.•We use Dirac-Bogoliubov-de Gennes equation for treating the conductance and the magnetoresistance.•The magnetoresistance (MR) for bias voltage below the renormalized superconductor gap is sensitively dependent on magnetic field.
With the recent discovery of the quantum anomalous Hall insulator (QAHI), which exhibits the conductive quantum Hall edge states without external magnetic field, it becomes possible to create a ...topological superconductor (SC) by introducing superconductivity into these edge states. In this case, 2 distinct topological superconducting phases with 1 or 2 chiral Majorana edge modes were theoretically predicted, characterized by Chern numbers (𝓝) of 1 and 2, respectively. We present spectroscopic evidence from Andreev reflection experiments for the presence of chiral Majorana modes in an Nb/(Cr0.12Bi0.26Sb0.62)₂Te₃ heterostructure with distinct signatures attributed to 2 different topological superconducting phases. The results are in qualitatively good agreement with the theoretical predictions.
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