We study interference patterns of a magnetically doped topological insulator Bi(2-x)Fe(x)Te(3+d) by using Fourier transform scanning tunneling spectroscopy and observe several new scattering channels. A comparison with angle-resolved photoemission spectroscopy allows us to unambiguously ascertain the momentum-space origin of distinct dispersing channels along high-symmetry directions and identify those originating from time-reversal symmetry breaking. Our analysis also reveals that the surface state survives far above the energy where angle-resolved photoemission spectroscopy finds the onset of continuum bulk bands.

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We study unconventional superconductivity in exfoliated single crystals of a promising three-dimensional (3D) topological superconductor candidate, Nb-doped Bi 2 Se 3 through differential conductance spectroscopy and magnetotransport. 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 2 Se 3 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 2 Se 3 . Finally, we observe signatures of multiple superconducting energy gaps, which could originate from multiple Fermi surfaces reported earlier in bulk crystals.

We study Fabry-Perot interference in hybrid devices, each consisting of a mesoscopic superconducting disk deposited on the surface of a three-dimensional topological insulator. Such structures are hypothesized to contain protected zero modes known as Majorana fermions bound to vortices. The interference manifests as periodic conductance oscillations of magnitude ∼ 0.1 e 2 /h. These oscillations show no strong dependence on bulk carrier density or sample thickness, suggesting that they result from phase coherent transport in surface states. However, the Fabry-Perot interference can be tuned by both top and back gates, implying strong electrostatic coupling between the top and bottom surfaces of topological insulator.The wave-like nature of electrons can manifest in mesoscopic devices through interference effects. For example, if electrons traverse through a sufficiently clean sample between two leads with a finite probability of reflection at the sample-lead interface, then periodic conductance oscillations can occur as incident electrons interfere constructively or destructively with reflected carriers. Such Fabry-Perot interference has been observed in carbon nanotubes [1], semiconducting nanowires [2], and graphene [3]. In all cases, special care during sample preparation must be taken to ensure quasi-ballistic transport.Phase coherent transport in topological insulators (TIs) attracts attention for two reasons. First, at the surface of a TI there are gapless states that behave as helical Dirac electrons [4], with the electron spin constrained to be perpendicular to momentum. This spin-momentum locking generates a nontrivial Berry phase of π as an electron moves in a complete circle around the Brillouin zone, which suppresses backscattering and protects the surface states from nonmagnetic disorder. Second, if superconductivity is induced in the surface states of TIs, the resulting system will resemble a spinless p x + ip y superconductor, with protected zero energy modes known as Majorana fermions bound to vortices [5]. These Majorana fermions possess non-Abelian exchange statistics [6,7] and can be detected through interferometry [8][9][10]. The complex geometry of the proposed interferometers as well as possible complications from bulk states encourage further studies of phase coherent transport in hybrid TIsuperconductor devices.Here, we report on transport studies of dual gated hybrid devices consisting of a superconducting disk deposited on the surface of a 3D TI. We find clear signatures of gate-tuned Fabry-Perot oscillations. The magnitude of the oscillations is essentially the same for lightly-and heavily-doped devices, implying that they originate from surface states of the TI rather than the bulk. However, the oscillations can be tuned by applying a bias either to the top or back gate, suggesting the Fermi levels of the top and bottom surfaces are locked. The resilience of the Fabry-Perot oscillations after multiple fabrication steps and over lengths of at least 800 nm as well as the option of using ...

Proximity-induced superconductivity in three dimensional (3D) topological insulators forms a new quantum phase of matter and accommodates exotic quasiparticles such as Majorana bound states. One of the biggest drawbacks of the commonly studied 3D topological insulators is the presence of conducting bulk that obscures both surface states and low energy bound states. Introducing superconductivity in topological Kondo insulators such as SmB6, however, is promising due to their true insulating bulk at low temperatures. In this work, we develop an unconventional Josephson junction by coupling superconducting Nb leads to the surface states of a SmB6 crystal. We observe a robust critical current at low temperatures that responds to the application of an out-of-plane magnetic field with significant deviations from usual Fraunhofer patterns. The appearance of Shaphiro steps under microwave radiation gives further evidence of a Josephson effect. Moreover, we explore the effects of Kondo breakdown in our devices, such as ferromagnetism at the surface and anomalous temperature dependence of supercurrent. Particularly, the magnetic diffraction patterns show an anomalous hysteresis with the field sweep direction suggesting the coexistence of magnetism with superconductivity at the SmB6 surface. The experimental work will advance the current understanding of topologically nontrivial superconductors and emergent states associated with such unconventional superconducting phases.