125Te spin-lattice relaxation in a candidate to Weyl semimetals WTe2

The tungsten ditelluride WTe2 was suggested to belong to the Weyl semimetal family. We studied 125Te spin-lattice relaxation and NMR spectra in a WTe2 single crystal within a large range from 28 K up to room temperature. Measurements were carried out on a Bruker Avance 500 NMR pulse spectrometer for the crystalline c axis directed in parallel and perpendicular to the magnetic field. Relaxation proved to be single-exponential. The relaxation time varied depending on the sample position in magnetic field and frequency offset. The relaxation rate increased about linearly with temperature below 70 K, however the dependence became nearly quadratic at higher temperatures. The relaxation rate within the total temperature range was fitted using a theoretical model developed in Ref. [41] for Weyl semimetals and assuming the decrease of the chemical potential with increasing temperature. The results obtained for 125Te spin-lattice relaxation evidence in favor of the topological nontriviality of the WTe2 semimetal. The 125Te NMR spectra agreed with the occurrence of nonequivalent tellurium sites and varied insignificantly with temperature. © 2020 The Author(s)The authors acknowledge the financial support from RFBR , grants 19-57-52001 , 19-29-03004 , and 19-07-00028 . The work was partly supported by Minobrnauki of Russia (“Spin” No. AAAA-A18-118020290104-2 ) and Russian Government (contract No. 02.A03.21.0006 ). Measurements were partly carried out using the equipment of the Research park of St. Petersburg State University

NMR studies of 3D topological insulators over a large temperature range

Results of NMR measurements of the Bi2Te3 and Bi2Se3 3D topological insulators within the temperature range 11-293 K are presented. Two lines were demonstrated in the 125Te and 77Se spectra over the whole temperature range for single crystals in the c^B0 and cB0 orientations as well as for Bi2Te3 and Bi2Se3 powders. The observed lines correlated to two non-equivalent positions of chalcogen atoms in the crystalline structure of investigated bismuth chalcogenides. The thermal activated character of charge carriers was found for the Bi2Te3 single crystal and powder. The activation energy was evaluated. It was shown for the Bi2Se3 powder that thermally activated charge carriers did not play the main role in conductivity. The nontrivial behaviour of the 125Te and 77Se NMR spectra for Bi2Te3 and Bi2Se3 single crystals in the orientation cB0 was found at low temperatures which could not be explained by the orientation dependence of the line shapes and shifts in the topologically trivial semiconductors without structural phase transitions. © Published under licence by IOP Publishing Ltd.The studies were supported by RFBR, grant 19-57-52001. NMR and XRD measurements were carried

Porous silicon templates for superconducting devices

The use of porous silicon (PS) templates in the field of superconducting nanoelectronics is reviewed.We focus on the influence of the morphology of the pores (porosity, average pore diameter) on the superconducting properties of ultrathin films deposited on these templates. We describe and discuss some basic and advanced properties of the obtained nanostructured superconductors. In particular, we show that, due to the extremely reduced dimensions of PS templates, the formation of commensurate vortex structures can be realized at low temperatures and at matching fields as high as μ0H1≈ 1 T. We also show that with this fabrication procedure, we can obtain networks of one-dimensional superconducting nanowires, which exhibit features typical of quantum phase slip (QPS) phenomena. This creates preconditions for the development and implementation of new highly sensitive radiation detectors, magnetometers, QPS qubits, QPS transistors, and quantum current standards