Surface superconductivity in the Weyl semimetal MoTe ₂ detected by point contact spectroscopy

Abstract

MoTe2 is a Weyl semimetal, which exhibits unique non-saturating magnetoresistance and strongly reinforced superconductivity under pressure. Here, we demonstrate that a novel mesoscopic superconductivity at ambient pressure arises on the surface of MoTe2 with a critical temperature up to 5 K significantly exceeding the bulk Tc = 0.1 K. We measured the derivatives of I-V curves for hetero-contacts of MoTe 2 with Ag or Cu, homo-contacts of MoTe2 as well as "soft" point contacts (PCs). Large number of these hetero-contacts exhibit a dV/dI dependence, which is characteristic for Andreev reflection. It allows us to determine the superconducting gap Δ. The average gap values are 2Δ=1.30±0.15 meV with a 2Δ/k BTc ratio of 3.7± 0.4, which slightly exceeds the standard BCS value of 3.52. Furthermore, the temperature dependence of the gap follows a BCS-like behavior, which points to a nodeless superconducting order parameter with some strong-coupling renormalization. Remarkably, the observation of a "gapless-like" single minimum in the dV/dI of "soft" PCs may indicate a topological superconducting state of the MoTe2 surface as these contacts probe mainly the interface and avoid additional pressure effect. Therefore, MoTe2 might be a suitable material to study new forms of topological superconductivity. ResultsPoint contact spectroscopy with "hard" tips. Figures 1 and 2 show the dV/dI curves for typical MoTe2-Ag hetero-contact (hereinafter -"hard" contact) in dependence on their temperature and in magnetic field. About a dozen of "hard" contacts with either Ag or Cu (of total 20 "hard" contacts) show similar a dV/dI dependence with a characteristic AR structure, i.e., a double-minimum around zero-bias.

Yanson point-contact spectroscopy of Weyl semimetal WTe 2

“…Let us compare the results for WTe2 with our previous PC measurements in the sister compound MoTe2 [Naid18]. Contrary to the present measurements, the SC features in dV/dI of MoTe2-Ag PCs were much more pronounced and they even showed Andreev-like structure that enabled us to obtain the SC gap value and its temperature and magnetic field dependence.…”

“…Contrary to the present measurements, the SC features in dV/dI of MoTe2-Ag PCs were much more pronounced and they even showed Andreev-like structure that enabled us to obtain the SC gap value and its temperature and magnetic field dependence. On the other hand, we measured more detailed EPI spectra in the case of WTe2 with good reproducibility, while EPI spectra in MoTe2 were rare and showed only a broad maximum between 15-20 mV [Naid18]. At the same time, the calculated phonon density of states in MoTe2 has a rich structure [Tse] and, on the whole, has some similarity to that of WTe2 [Lu] below 20 meV.…”

“…In our recent paper [Naid18], we have investigated MoTe2 by point-contact (PC) spectroscopy. We have discovered a tremendous enhancement of the SC critical temperature up to 5K in MoTe2 at the surface seen by the surface sensitive PC measurements.…”

Applications of point-contact spectroscopy in the study of topological materials

“…另外, 某些形态的Bi本 身可能是超导体 [132] , 而点接触作为一种局部测量手段 有可能探测到夹杂在Bi样品中的超导团簇的信号 [130] . [19,134,135] . 然而, 在实现针尖诱导 超导的拓扑半金属材料中, 有些在很高的压力作用下 也没有出现超导, 比如WC, TaAs以及TaAs 2 等 [118] ; 有 些在高压下可以转变为超导体, 但是出现超导的临界 压力一般比较高, 比如TaP在30 GPa出现超导电性 [136] , [119] .…”

Equilibrium current in a Weyl-semimetal - superconductor heterostructure

“…One prominent type of such heterostructures is the Josephson junction (SNS-heterostructure), which has been extensively studied theoretically exploring the influence of various types of superconducting pairing mechanisms [21][22][23][24][25][26][27][28][29][30][31][32][33][34][35], and has also been realized experimentally [19,[36][37][38][39][40]. Other examples of similar heterostructures are NS-type [17,20,[41][42][43][44][45][46][47][48][49][50][51], and NSN-type [52][53][54][55][56] heterostructures.…”