All Set for Majoranas When put in the proximity of a superconductor, topological insulators (TIs) are expected to support Majorana fermions, exotic particles that are their own antiparticles. For this to be realized, the interface between the TI and superconductor layers has to be atomically sharp but electronically transparent. Wang et al. (p. 52, published online 15 March) fabricated this heterostructure by growing a film of the TI material Bi2Se3 on the superconductor NbSe2 covered with a Bi bilayer. Scanning tunneling spectroscopy revealed a superconducting gap on the TI surface of the heterostructure with varying thickness of the Bi2Se3 film. This coexistence of superconductivity and topological order should now allow observation of exotic phenomena such as Majorana fermions. A thin layer of a topological insulator grown on the surface of a superconductor is shown to acquire a superconducting gap. Three-dimensional topological insulators (TIs) are characterized by their nontrivial surface states, in which electrons have their spin locked at a right angle to their momentum under the protection of time-reversal symmetry. The topologically ordered phase in TIs does not break any symmetry. The interplay between topological order and symmetry breaking, such as that observed in superconductivity, can lead to new quantum phenomena and devices. We fabricated a superconducting TI/superconductor heterostructure by growing dibismuth triselenide (Bi2Se3) thin films on superconductor niobium diselenide substrate. Using scanning tunneling microscopy and angle-resolved photoemission spectroscopy, we observed the superconducting gap at the Bi2Se3 surface in the regime of Bi2Se3 film thickness where topological surface states form. This observation lays the groundwork for experimentally realizing Majorana fermions in condensed matter physics.

/pdf/the-coexistence-of-superconductivity-and-topological-order-59btue84zw.pdf

The Coexistence of Superconductivity and Topological Order in the Bi2Se3 Thin Films

N concentration effects on structure and superconductivity of NbN thin films

https://www.onlinelibrary.wiley.com/doi/pdf/10.1002/pssa.202070014

Molecular Beam Epitaxy of Transition Metal Nitrides for Superconducting Device Applications

https://iopscience.iop.org/article/10.1088/1674-1056/28/6/067403/pdf

Topological superconductivity in a Bi2Te3/NbSe2 heterostructure: A review*

https://iopscience.iop.org/article/10.1088/1674-1056/28/5/058504/pdf

Photoinduced phase transitions in two-dimensional charge-density-wave 1T-TaS2*

2H- and -phase monolayer MoTe2 films on highly oriented pyrolytic graphite are studied using scanning tunneling microscopy and spectroscopy (STM/STS). The phase transition of MoTe2 can be controlled by a post-growth annealing process, and the intermediate state during the phase transition is directly observed by STM. For 2H-MoTe2, inversion domain boundaries are presented as bright lines at high sample bias, but as dark lines at lower sample bias. The dI/dV mappings reveal the distinct distributions of electronic states between domain boundaries and interiors of domains. It should be noted that a 2 × 2 periodic structure is clearly discernable inside the domains, where the STS measurement shows a small dip of size ~150 meV at the vicinity of the Fermi level, indicating that the 2 × 2 periodic structure may be an incommensurate charge density wave. Moreover, a 4 × 4 periodic structure appears in 2H-MoTe2 grown at a higher substrate temperature.

https://iopscience.iop.org/article/10.1088/0256-307X/35/6/066801/pdf

Charge Density Wave States in 2H-MoTe 2 Revealed by Scanning Tunneling Microscopy

HgTe (111) surface is comprehensively studied by scanning tunneling microscopy/spectroscopy (STS). In addition to the primitive (1 × 1) hexagonal lattice, six reconstructed surface structures are observed: (2 × 2), 2 × 1, 4 × 1, , and . The (2 × 2) reconstructed lattice maintains the primitive hexagonal symmetry, while the lattices of the other five reconstructions are rectangular. Moreover, the topographic features of the reconstruction are bias dependent, indicating that they have both topographic and electronic origins. The STSs obtained at different reconstructed surfaces show a universal dip feature with size ~100 mV, which may be attributed to the surface distortion. Our results reveal the atomic structure and complex reconstructions of the cleaved HgTe (111) surfaces, which paves the way to understand the rich properties of HgTe crystal.

https://iopscience.iop.org/article/10.1088/0256-307X/35/2/026802/pdf

Surface Structure and Reconstructions of HgTe (111) Surfaces

Atomically flat superconducting NbN thin films have been grown on the SrTiO3 (111) substrate by plasma-assisted molecular beam epitaxy for the first time. Using scanning tunneling microscopy and in situ reflection high energy electron diffraction, we investigate the surface structure of epitaxial NbN thin films on the SrTiO3 (111) substrate. The orientation [101¯]NbN//[112¯]STO is dominating at substrate temperature above 700 °C. The quality of the as-grown films can be further improved by annealing at elevated temperatures. The homogeneous spatial distribution of superconducting gaps and magnetic vortices further demonstrates the high quality of the NbN films.

Atomically flat superconducting NbN thin films grown on SrTiO3 (111) by plasma-assisted MBE

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Papers

Charge Density Wave States in 2H-MoTe 2 Revealed by Scanning Tunneling Microscopy

Surface Structure and Reconstructions of HgTe (111) Surfaces

Atomically flat superconducting NbN thin films grown on SrTiO3 (111) by plasma-assisted MBE