Structural and electronic properties of ultrathin nanocrystals of chalcogenide Bi2(Tex Se1-x)3 were studied. The nanocrystals were formed from the parent compound Bi2Te2Se on as-grown and thermally oxidized Si(100) substrates using Ar-assisted physical vapor deposition, resulting in well-faceted single crystals several quintuple layers thick and a few hundreds nanometers large. The chemical composition and structure of the nanocrystals were analyzed by energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, electron backscattering, and X-ray diffraction. The electron transport through nanocrystals connected to superconducting Nb electrodes demonstrated Josephson behavior, with the predominance of the topological channels [Stolyarov et al. Commun. Mater., 2020, 1, 38]. The present paper focuses on the effect of the growth conditions on the morphology, structural, and electronic properties of nanocrystals.

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Physical Vapor Deposition Features of Ultrathin Nanocrystals of Bi2(TexSe1-x)3.

Typically, topological superconductivity is reachable via proximity effect by a direct deposition of superconductor (S) on top of a topological insulator (TI) surface. Here, the double critical current in the Josephson junctions based on the topological insulator is observed in the fabricated planar Superconducting Quantum Interference Devicea. By measuring critical currents as a function of temperature and magnetic field, it is shown that the second critical current stems from the intrinsic superconductivity of the S–TI interface, which is supported by the modified Resistively Shunted Junction model and Transmission Electron Microscopy studies. This complex structure of the interface should be taken into account when the technological process involves Ar‐plasma cleaning.

Revealing Intrinsic Superconductivity of the Nb/BiSbTe2Se Interface

Josephson proximity junctions and devices employing topological insulators are promising candidates for realizing topological superconductivity and topologically protected quantum circuits. Here, the new type of oscillations of the critical Josephson current in the ballistic Nb-Bi2Te2.3Se0.7-Nb junctions subject to the magnetic fields is reported. The oscillations appear below ≈400 mK and have a very unusual sharp-peaked shape. Their ultra-short period ≈1 Oe, by orders of magnitude shorter than the expected periodicity due to fluxoid quantization in the device, corresponds to the extremely low energy scale ≈1 μ $\umu$ eV. It is established that the observed effect is due to the resonant transmission of Andreev quasiparticles via the peculiar energy levels forming near the S-TI interfaces. 

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1002/qute.202100124

Resonant Oscillations of Josephson Current in Nb‐Bi 2 Te 2.3 Se 0.7 ‐Nb Junctions

Local electronic properties of quasi-two-dimensional Pb(111) islands with screw dislocations of different types on their surfaces were experimentally studied by means of low-temperature scanning tu...

Observation of Hidden Parts of Dislocation Loops in Thin Pb Films by Means of Scanning Tunneling Spectroscopy

Abstract The influence of interface electronic structure is vital to control lower dimensional superconductivity and its applications to gated superconducting electronics, and superconducting layered heterostructures. Lower dimensional superconductors are typically synthesized on insulating substrates to reduce interfacial driven effects that destroy superconductivity and delocalize the confined wavefunction. Here, we demonstrate that the hybrid electronic structure formed at the interface between a lead film and a semiconducting and highly anisotropic black phosphorus substrate significantly renormalizes the superconductivity in the lead film. Using ultra-low temperature scanning tunneling microscopy and spectroscopy, we characterize the renormalization of lead’s quantum well states, its superconducting gap, and its vortex structure which show strong anisotropic characteristics. Density functional theory calculations confirm that the renormalization of superconductivity is driven by hybridization at the interface which modifies the confinement potential and imprints the anisotropic characteristics of the semiconductor substrate on selected regions of the Fermi surface of lead. Using an analytical model, we link the modulated superconductivity to an anisotropy that selectively tunes the superconducting order parameter in reciprocal space. These results illustrate that interfacial hybridization can be used to tune superconductivity in quantum technologies based on lower dimensional superconducting electronics. 

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Tuning lower dimensional superconductivity with hybridization at a superconducting-semiconducting interface

We report on structural and electronic properties of superconducting nanohybrids made of Pb grown in the ultrahigh vacuum on the atomically clean surface of single crystals of topological Bi2Te3. In situ scanning tunneling microscopy and spectroscopy demonstrated that the resulting network is composed of Pb-nanoislands dispersed on the surface and linked together by an amorphous atomic layer of Pb, which wets Bi2Te3. As a result, the superconducting state of the system is characterized by a thickness-dependent superconducting gap of Pb-islands and by a very unusual position-independent proximity gap between them. Furthermore, the data analysis and DFT calculations demonstrate that the Pb-wetting layer leads to significant modifications of both topological and trivial electronic states of Bi2Te3, which are responsible for the observed long-range proximity effect.

Sergey I. Bozhko

Papers

Superconducting Long-Range Proximity Effect through the Atomically Flat Interface of a Bi2Te3 Topological Insulator.