Mixed type I and type II superconductivity due to intrinsic electronic inhomogeneities in the type II Dirac semimetal PdTe₂

Abstract

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...

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].…”

“…While type-I SC for PdTe 2 can been established by the sudden drop at 130 Oe in ZBC for contacts M7-M16, we notice that other ZBC curves M1-M6 show a smooth evolution into the normal state at a critical field H c2 around 600 Oe. The continuous transition for M1-M6 with a much larger critical field than the bulk H c1 ∼ 130 Oe suggests a local type-II superconductivity at these contacts, consistent with the STM and resistivity measurements for current along c axis [31,37]. A corresponding conductance curve G(V ) at 0.3 K has an obvious dip structure at higher bias voltages as in Fig.…”

“…The MPCS zero-bias conductance as a function of field displays a sudden drop at 130 Oe as in Fig. 3(c), strongly supporting a type-I superconductor for PdTe 2 as reported by magnetization, heat capacity and STM measurements [33,34,37]. In contrast, the SPCS conductance curves for PdTe 2 as in Fig.…”

“…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.…”

“…It is difficult to be explained by either a general Saint-James-de Gennes (SJdG) surface critical field or filamentary superconductivity [33,34]. One possible scenario proposed by Siroshi et al is that bulk PdTe 2 is a typical type-I superconductor and there exists a mixing of type-I and type-II superconductivity on its surface [33,34,37]. More experiments are thus desired to elucidate its exact superconducting nature for the type-II Dirac semimetal PdTe 2 .…”