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...

The type II Dirac semimetal PdTe2 is unique in the family of topological parent materials because it displays a superconducting ground state below 1.7 K. Despite wide speculations on the possibility of an unconventional topological superconducting phase, tunneling and heat capacity measurements revealed that the superconducting phase of PdTe2 follows predictions of the microscopic theory of Bardeen, Cooper and Shriefer (BCS) for conventional superconductors. The superconducting phase in PdTe2 is further interesting because it also displays properties that are characteristics of type-I superconductors and are generally unexpected for binary compounds. Here, from scanning tunneling spectroscopic measurements we show that the surface of PdTe2 displays intrinsic electronic inhomegenities in the normal state which leads to a mixed type I and type II superconducting behaviour along with a spatial distribution of critical fields in the superconducting state. Understanding of the origin of such inhomogeneities may be important for understanding the topological properties of PdTe2 in the normal state.PdTe 2 has been known to be a superconductor for almost six decades. 1-4 But, due to a very low critical temperature (∼ 1.7 K), the details of the superconducting phase of PdTe 2 did not receive much attention. The superconducting properties of PdTe 2 attracted renewed attention of the community following recent discovery of complex topological features in the band structure of PdTe 2 in it's non-superconducting normal state 5-7 . This discovery naturally led to the question of the possibility of a topological character of the superconducting phase of PdTe 2 . 6 This question is extremely important because a single material showing both topological character and superconductivity in it's parental stoichiometric phase is thought to be the best candidate to show topological superconductivity [8][9][10][11][12][13][14][15][16][17][18] . Immediately after the discovery of the topological nature of PdTe 2 , 5,6 transport and magnetization experiments revealed a peculiar type-I like superconducting phase but with multiple critical fields (H c ). 19,20 The possible type-I nature of superconductivity in PdTe 2 is unique because only elemental metals are known to show type-I superconductivity and superconductivity in binary compounds and alloys are generally seen to be of type II with perhaps only one exception of TaSi 2 21 . The observation of multiple critical fields in PdTe 2 was attributed to the possibility of an anisotropic superconducting order parameter and to an enhanced critical field of the surface sheath (H s c ) of PdTe 2 . 19,20 However, the measured H s c turned out to be around 3000 G which is one order of magnitude higher than H c ∼ 250 G. This deviates from the expected value for a standard Saint-James-deGennes (SJdG) surface critical field which, in principle, should be less than 1.7 H c . 22-24 This * Electronic address: anshusirohi@iisermohali.ac.in † Electronic address: goutam@iisermohali.ac.in mismatch of the me...

A Weyl semimetal is a topologically non-trivial phase of matter that hosts mass-less Weyl fermions, the particles that remained elusive for more than 80 years since their theoretical discovery. The Weyl semimetals exhibit unique transport properties and remarkably high surface spin polarization. Here we show that a mesoscopic superconducting phase with critical temperature Tc=7 K can be realized by forming metallic point contacts with silver (Ag) on single crystals of TaAs, while neither Ag nor TaAs are superconductors. Andreev reflection spectroscopy of such point contacts reveals a superconducting gap of 1.2 meV that coexists with a high transport spin polarization of 60% indicating a highly spin-polarized supercurrent flowing through the point contacts on TaAs. Therefore, apart from the discovery of a novel mesoscopic superconducting phase, our results also show that the point contacts on Weyl semimetals are potentially important for applications in spintronics.

Discovery of exotic phases of matter from the topologically non-trivial systems not only makes the research on topological materials more interesting but also enriches our understanding of the fascinating physics of such materials. Pb 0.6 Sn 0.4 Te was recently shown to be a topological crystalline insulator. Here we show that by forming a mesoscopic point-contact using a normal nonsuperconducting elemental metal on the surface of Pb 0.6 Sn 0.4 Te a novel superconducting phase is created locally in a confined region under the point-contact. This happens while the bulk of the sample remains to be non-superconducting and the superconducting phase emerges as a nanodroplet under the point-contact. The superconducting phase shows a high transition temperature T c that varies for different point-contacts and falls in a range between 3.7 K and 6.5 K. Therefore, this Letter presents the discovery of a new superconducting phase on the surface of a topological crystalline insulator and the discovery is expected to shed light on the mechanism of induced superconductivity in topologically non-trivial systems in general.

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