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

Abstract

We use sub-Kelvin scanning tunneling spectroscopy to investigate the suppression of superconductivity in homogeneously disordered ultrathin MoC films. We observe that the superconducting state remains spatially homogeneous even on the films of 3 nm thickness. The vortex imaging suggests the global phase coherence in our films. Upon decreasing thickness, when the superconducting transition drops from 8.5 to 1.2 K, the superconducting energy gap  follows perfectly T c . All this is pointing to a two-stage fermionic scenario of the superconductor-insulator transition (SIT) via a metallic state as an alternative to the direct bosonic SIT scenario with a Cooper-pair insulating state evidenced by the last decade STM experiments. [7][8][9][10][11][12][13] bring evidences that upon increasing disorder decreases more slowly than T c , the variation of on a scale of the superconducting coherence length  increases, a pseudogap appears above T c in the tunneling spectra, the spectral coherence peaks are suppressed, and the vortex lattice is fading out. This phenomenology strongly supports the bosonic scenario and raises the question about the universality of the bosonic SIT [14]. On the other hand, potential evidence for the fermionic mechanism was provided even sooner by the tunneling experiments on planar junctions on amorphous Bi and PbBi/Ge films [15,16] indicating that the Cooper pair amplitude vanishes at SIT. But the unambiguousness of these results was challenged by the fact that the spatially averaged tunneling spectra were gapless. This gaplessness could be explained by a very inhomogeneous gap distribution due to phase fluctuations. Then, superconducting pair correlations might also exist in insulators close to SIT, contradicting the fermionic scenario. KeywordsHere, we present sub-Kelvin STM experiments on ultra thin superconducting MoC films with atomic spatial resolution. We demonstrate the spatial homogeneity of the superconducting state for film thicknesses down to 3 nm and the closing of the superconducting energy gap/order parameter as T c vanishes, in agreement with the fermionic scenario. Thus, we bring the first direct evidence on the local behavior of the superconducting order parameter in this class of the transition.The MoC films were prepared by the magnetron reactive sputtering from a Mo target in an argon-acetylene atmosphere onto a sapphire c-cut substrate. The details can be found elsewhere [17]. The preparation followed the procedure of Lee and Ketterson [18] who manufactured continuous MoC films down to 0.4 nm thickness showing

Correlating Decoherence in Transmon Qubits: Low Frequency Noise by Single Fluctuators

“…In the same measurement we still observed some correlation of the absolute uctuation strength with Γ Φ . Our explanation is Appendix G: Other decoherence sources Increased quasiparticle density due to pair-breaking by ingap states, as observed in Pb 0.9 Bi 0.1 junctions [54] or thin MoC lms [55,56], could increase the qubit's relaxation rate. To our knowledge, no evidence of excessive pair breaking due to in-gap states has been reported for material systems like our sample at 20 mK.…”

Local density of states and its mesoscopic fluctuations near the transition to a superconducting state in disordered systems

“…(iii) Disorder-averaged higher moments of the LDOS can be expressed in terms of higher-order irreducible correlation functions of the Q-field similarly to Eq. (9). Explicit examples of operators corresponding to the third and forth moments of the LDOS can be found in Ref.…”

Consistent two-lifetime model for spectral functions of superconductors

“…One might have the impression that the particle-hole asymmetry which causes the sum rule violation is an artifact of our generalization of the Green function (2) to the matrix form (3). That this is not the case can be seen by plotting the spectral function directly for Eq.…”

“…However, it is not obvious whether this type of theory is applicable to the cuprates. Moreover, even conventional superconductors may possess complicated phonon spectra and/or they can exhibit substantial elastic scattering, 3 both of which complicate the Eliashberg analysis. For all these reasons, it is desirable to have a simple and generic theory which does take finite quasiparticle lifetimes in superconductors into account.…”

“…2,4 However, it is well known that the density of states (or the socalled tomographic density of states in case of anisotropic superconductors 5 ) implied by such spectral functions exhibits a full spectral gap consistent with the Anderson theorem. On the other hand, experimentally, the gap is quite often only partial 3,5 and the tunneling density of states is better described by the phenomenological Dynes formula. 6,7 This means then that, in order to take the non-trivial density of states into account, also a second type of scattering processes -which are not subject to the Anderson theorem -have to be considered.…”