Transition metal dichalcogenides (TMD), which consist of layered structures of two-dimensional units stacked by relatively weak van der Waals interactions, have been actively studied because of their rich physical properties. In the so-called 1T-phase, chalcogens are octahedrally coordinated to the transition metal, forming a highly symmetric trigonal structure. In the case of NbTe2, this octahedral structure is distorted, resulting in the monoclinic symmetry (1T”-phase), which has been associated with a charge density wave [1]. Recently, superconductivity (SC) at 0.72 K with an unconventional field-temperature phase diagram has been reported in NbTe2 [2]. Moreover, NbTe2 was classified as a topological semimetal based on the symmetrical indicators [3]. Therefore, NbTe2 can be a good candidate of the topological superconductor.
In this study, we have examined superconducting and topological properties of the 1T”-NbTe2 both by experiments using single crystals and by first-principles calculations based on density functional theory.
Single crystals of NbTe2, as large as ~5 mm, were successfully grown by a chemical vapor transport technique using iodine as a transport agent. Resistivity measurements in NbTe2 single crystals down to 0.2 K were performed using the Quantum Design PPMS with a home-made adiabatic diamagnetization refrigerator (ADR). By applying magnetic fields in the x-, y-, and z-axis, the triaxial anisotropy of superconducting properties in NbTe2 was revealed for the first time. Unlike the previous report [2], the H-T phase diagram determined by the middle SC transitions was rather conventional, well fitted by the GL model. On the other hand, that determined by the on-set SC transitions was found to be anomalous, deviated from the GL model in the low temperature region (< 0.55 K). This suggests the existence of another superconducting component other than the bulk. Motivated by the experimental observations, we performed first-principles calculations to reveal the surface as well as bulk electronic structures. As a result, it was discovered that topological electronic states appeared on the surface. The possible two-dimensional topological SC at the surface is, therefore, likely to be the origin of the anomalous Tconset.
[1] C. Battaglia et al., Phys. Rev. B 72, 195114 (2005).
[2] Z. Xi et al., Chin. Phys. Lett. 36, 057402 (2019).
[3] F. Tang et al., Nature 566, 486 (2019).
This work was supported by JSPS KAKENHI (Grant Numbers 21H04652, 21K18181,21H05236,20H00337, and 21H05236) and JST CREST (Grant Number JPMJCR20B4).