Conventional superconductivity in the type-II Dirac semimetal PdTe2

Abstract

The transition metal dichalcogenide PdTe2 was recently shown to be a unique system where a type II Dirac semimetallic phase and a superconducting phase co-exist. This observation has led to wide speculation on the possibility of the emergence of an unconventional topological superconducting phase in PdTe2. Here, through direct measurement of the superconducting energy gap by scanning tunneling spectroscopy (STS), and temperature and magnetic field evolution of the same, we show that the superconducting phase in PdTe2 is conventional in nature. The superconducting energy gap is measured to be 326 µeV at 0.38 K and it follows a temperature dependence that is well described within the framework of Bardeen-Cooper-Schriefer's (BCS) theory of conventional superconductivity. This is surprising because our quantum oscillation measurements confirm that at least one of the bands participating in transport has topologically non-trivial character.It is believed that a superconducting ground state realized on a material displaying topologically non-trivial character might lead to the discovery of the elusive phase of matter called topological superconductivity 1-3 where relativistic Majorana fermions could emerge as excitations. 4-10 In the past, attempts were made to realize such a phase of matter through a number of ways, like, doping a topological insulator, 11-13 applying pressure on topological systems, 14 fabricating proximity-coupled heterostructures of topological insulators and conventional superconductors, 10,15 making mesoscopically confined point contacts on topological crystalline insulator, 16 Dirac 17,18 or Weyl semimetals 19 etc. In such systems, while a superconducting phase could be achieved, clear manifestation of topological superconductivity remained an unattained goal. Most recently, with the discovery of a type-II Dirac semimetallic phase on the dichalcogenide PdTe 2 , 20-22 where a superconducting phase is known to emerge naturally, 23,24 there have been wide speculation that this superconducting phase might eventually emerge as a topological superconductor. Owing to the existence of a topological phase along with superconductivity, it warrants a thorough investigation of the superconducting phase of PdTe 2 through high-resolution spectroscopic tools.In this Letter, we report scanning tunneling microscopy and spectroscopy results performed on high quality single crystals of PdTe 2 in ultra-high vacuum and sub-kelvin temperatures to spectroscopically probe the nature of superconductivity. Through systematic temperature and magnetic field dependent experiments we found that the superconducting phase in PdTe 2 , despite it's proximity to a topologically non-trivial Dirac semimetallic phase, is conventional in nature where the superconducting order parameter shows a temperature dependence consistent with the prediction of the theory of Bardeen, Cooper and Schriefer (BCS) for conventional superconductors. 25 High quality single crystals of PdTe 2 were grown by a melt growth method. The starting elements, Pd...

Single full gap with mixed type-I and type-II superconductivity on surface of the type-II Dirac semimetal PdTe2 by point-contact spectroscopy

“…This crude model can roughly explain the disparate H c values between type-I and type-II SC in our PCS results. Our MPCS and SPCS spectra show a reproducible double peak structure with the absence of a zero-bias conductance peak and a perfect fitting with a single gap s-wave BTK model strongly supports a conventional swave paring gap in the type-II Dirac semimetal PdTe 2 as evidenced by other measurements [30,[34][35][36][37][38]. On the other hand, SC with a full gap is claimed for topological surface states to host Majorana zero modes in vortices for some iron-based superconductors [26][27][28].…”

“…A single gap swave BTK model can perfectly fit all experimental curves as shown by the black lines in Fig. 1 [37,38], but larger than the weakcoupling value estimated from bulk specific heat and penetration depth studies [34,35]. The discrepancy of gap size is suspected to be associated with the spin-polarized topological surface state, because both PCS and STM are more sensitive to the SC on surface rather than the bulk SC.…”

“…On the other hand, superconductivity in PdTe 2 with a transition temperature T c ∼ 1.7 K has been known for a long time, serving as a promising candidate for stoichiometric TSCs [33]. However, heat capacity, London penetration depth, tunneling junction and STM measurements all support a con-ventional fully-gapped s-wave superconductor for PdTe 2 [30,[34][35][36][37][38]. Moreover, recent electrical transport and heat capacity measurements reveal a puzzling discrepancy of its critical field for the superconducting state with 3000 Oe and 250 Oe, respectively.…”

Superconductivity under pressure in the Dirac semimetal PdTe2

“…These confirm conventional weak-coupling Bardeen-Cooper-Schrieffer (BCS) superconductivity, with a full gap in the bulk. At the same time zero-field scanning tunneling microscopy (STM) and spectroscopy (STS) experiments 8,18 lend further support for the absence of in-gap states, which seems to rule out topological superconductivity at the surface. Domi-arXiv:1902.01953v1 [cond-mat.supr-con] 5 Feb 2019 nant s-wave superconductivity was also concluded from tunneling spectroscopy experiments on side junctions 19 .…”

Two‐Dimensional Noble‐Metal Chalcogenides and Phosphochalcogenides

“…NMDCs gained a lot of attention as new members of the 2D family, since they showed a layer‐controllable metal‐to‐semiconductor transition, with few layers and the bulk phase featuring the topological properties of Dirac type‐II semimetals . The NMDCs also showed an outstanding performance as sensors, for example, for pressure or for specific molecular species .…”