Fermionic scenario for the destruction of superconductivity in ultrathin MoC films evidenced by STM measurements

    There are two possible scenarios of the superconductor-insulator (SI) transition. The fermionic model predicts transition to the insulating state through a bad metallic state. At the first transition between superconductor and metal (SM) the fluctuations drive the amplitude of the order parameter Δ to zero. Later, at MI transition the fermions localize. The bosonic model assumes a direct SI transition, where phase incoherent superconducting areas can exist even in the (Bose) insulating state. The last decade STM experiments strongly suggest that the bosonic picture represents an universal mechanism of the SI transition. STM and STS studies on ultrathin MoC films provide evidence that, in contrast to TiN, InOx and NbN, where the bosonic scenario of SIT is found, in the ultrathin MoC films the superconducting state is very homogeneous for all the thicknesses down to 3 nm where the superconducting transition is suppressed from bulk Tc = 8.5 K to 1.3 K (see Fig.1). Finally, the superconducting energy gap or order parameter terminates, Δ → 0 as the bulk superconductivity ceases with Tc → 0(see Fig. 2). All these observations point to the fermionic route of the SIT confirming that there are at least two different scenarios of SIT that can be realized depending on the physical parameters of the systems.

M. Žemlička, P. Neilinger, M. Trgala, M. Rehák, D. Manca, M. Grajcar, P. Szabó, P. Samuely, Š. Gaži, U. Hübner, V. M. Vinokur, E. Iľichev:
Finite quasiparticle lifetime in disordered superconductors,
Phys. Rev. B 92, 224506 (2015).

P. Szabó, T. Samuely, V. Hašková, J. Kačmarčík, M. Žemlička, M. Grajcar, J. G. Rodrigo, and P. Samuely:
Fermionic scenario for the destruction of superconductivity in ultrathin MoC films evidenced by STM measurements,
Phys. Rev. B 93, 014505 (2016).

Figure 1: STM surface topography and locally measured tunneling spectra for 10, 5 and 3 nm MoC films (a), (b) and (c), resp. Top: STM surface topographies at 500 mK, zoom in 5 nm film shows atomic structure. Middle: 100 STS spectra along ∼ 200 nm line on the surface of the respective film taken at 450 mK. Bottom: Temperature dependence of a typical tunneling spectrum at indicated temperatures. Inset of (a) and (b): Temperature dependence of the gap determined from fits to Dynes formula (points) and BCS-like Δ(T) dependence (line).

Figure 2: Concluding results. (a) Typical tunneling conductances of the 3, 5, 10 and 30 nm thin MoC films at T = 450 mK – solid lines. The symbols are fits to the thermally smeared Dynes formula. b) Film thickness dependence of the superconducting energy gap (red solid symbols – right scale) and of the critical temperature determined from tunneling experiment (open circles – left scale). (c) Film thickness dependence of the Dynes smearing parameter normalized to the gap - Γ/Δ at T = 450 mK (open circles - left scale) and of the superconducting coupling strength 2Δ/kBTc (red solid diamonds – right scale).