Superconducting energy scales and anomalous dissipative conductivity in thin films of molybdenum nitride

Abstract

We report investigations of molybdenum nitride (MoN) thin films with different thickness and disorder and with superconducting transition temperature 9.89 K ≥ Tc ≥ 2.78 K. Using terahertz frequency-domain spectroscopy we explore the normal and superconducting charge carrier dynamics for frequencies covering the range from 3 to 38 cm −1 (0.1 to 1.1 THz). The superconducting energy scales, i.e. the critical temperature Tc, the pairing energy ∆, and the superfluid stiffness J, and the superfluid density ns can be well described within the Bardeen-Cooper-Schrieffer theory for conventional superconductors. At the same time, we find an anomalously large dissipative conductivity, which cannot be explained by thermally excited quasiparticles, but rather by a temperature-dependent normal-conducting fraction, persisting deep into the superconducting state. Our results on this disordered system constrain the regime, where discernible effects stemming from the disorder-induced superconductor-insulator transition possibly become relevant, to MoN films with a transition temperature lower than at least 2.78 K.

Third harmonic generation from collective modes in disordered superconductors

“…In the regime 1/(2 τ ) 1 the full response including all fluctuations remains peaked around , i.e., half of the spectral gap, while in the strong disorder limit 1/(2 τ ) 1 charge/phase modes tend to contribute to a wide spectral range below . The finite contribution of charge/phase modes to the nonlinear current is reminiscent of the same result already established for the linear current [30][31][32], that has been invoked to explain the anomalous subgap absorption in strongly disordered films of conventional superconductors [40][41][42][43][44]. In the case of cuprate superconductors, which belong to the low-disorder limit, our results suggest that the paramagnetic BCS response gives the largest contribution, even though all collective modes cooperate to the THG enhancement below T c .…”

“…As a consequence, such an effective filamentary structure is less phase rigid than a uniformly suppressed SC background [37], and carries on a finite effective dipole moment [30,31,38], explaining the suppression of superfluid density and the extra finite-frequency absorption. The emergence of subgap absorption in strongly disordered thins films has been observed experimentally in several systems [40][41][42][43][44], showing that a substantial deviation from the MB paradigm can also be used experimentally to estimate the level of disorder in the sample.…”

Electromagnetic properties of impure superconductors with pair-breaking processes

“…Our presentation concentrates on various qualitative aspects of the optical conductivity for all disorder strengths from the clean up to the dirty limit and for all relative ratios Γ/Γ s . In Section 4 we analyze the recent anomalous optical data on thin MoN films 20 and we demonstrate that they can be reasonably fitted by the theory for the Dynes superconductors.…”

Magnetic Properties in Mn-Doped δ-MoN: A Systematic Density Functional Theory Study

“…Transition metal nitrides, by virtue of their unique electronic structure and exact nature of bonding (mixed metallic, ionic, and covalent bonding), possess excellent physical properties, such as high hardness [ 1 , 2 ], high melting point [ 3 ], and high electronic conductivity [ 4 , 5 ], which have made them attract tremendous attentions in recent years. In order to enhance the performance of this class of materials, much effort has been devoted to investigating the possibility of improving the energetic [ 6 , 7 , 8 ], mechanical [ 9 , 10 ], and electrical [ 11 ] properties by doping foreign atoms.…”