Enhancement of the finite-frequency superfluid response in the pseudogap regime of strongly disordered superconducting films

Abstract

The persistence of a soft gap in the density of states above the superconducting transition temperature Tc, the pseudogap, has long been thought to be a hallmark of unconventional high-temperature superconductors. However, in the last few years this paradigm has been strongly revised by increasing experimental evidence for the emergence of a pseudogap state in strongly-disordered conventional superconductors. Nonetheless, the nature of this state, probed primarily through scanning tunneling spectroscopy (STS) measurements, remains partly elusive. Here we show that the dynamic response above Tc, obtained from the complex ac conductivity, is highly modified in the pseudogap regime of strongly disordered NbN films. Below the pseudogap temperature, T*, the superfluid stiffness acquires a strong frequency dependence associated with a marked slowing down of critical fluctuations. When translated into the length-scale of fluctuations, our results suggest a scenario of thermal phase fluctuations between superconducting domains in a strongly disordered s-wave superconductor.

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

“…Indeed, thin films of TiN and NbN with only marginal disorder and no structural inhomogeneity do not show a finite σ D contribution in the superconducting state [36,45]. Note, that this inhomogeneity does not necessarily require structural inhomogeneities, but may result from homogeneous disorder leading to an electronically inhomogeneous state as it was shown directly [5,6] and indirectly [12,13,15] for similar thin films of TiN and NbN. In addition [32], an inhomogeneous superfluid density, as it may result from either structural or electronic inhomogeneity, causes anomalous tails in the temperature dependence of the dc resistivity that themselves cannot be attributed to superconducting or Berezinskii-Kosterlitz-Thouless type fluctuations.…”

“…1, we indeed observe such resistive tails for all samples, which cannot be captured by fluctuations. Given that this tail is present irrespective of thickness calls for an intrinsic structural inhomogeneity rather than emergent electronic inhomogeneity usually relevant only at strong disorder near the superconductor-insulator transition [5,6,12,13,15]. The ALD growth of MoN thin films generally initiates with the growth of a 1-2 nm layer of Mo 2 N before MoN growth sets in.…”

“…Here, the system undergoes a transition from a coherent many-body ground state composed of delocalized Cooper pairs and thermally activated quasiparticle states (a superconductor) to a ground state where the quasiparticle states are incoherent and localized (an insulator) presumably without an intermediate ground state of incoherent but delocalized quasiparticle states (a metal) [2,3]. This transition from superconductor to insulator (SIT) has become both a paradigm for a quantum phase transition [4] tuned by a non-thermal control parameter such as disorder or structural granularity and a rich host of intriguing phenomena such as a spatially dependent energy gap [5,6], a notable peak in the magnetoresistance [7,8], scaling [9] and vortex-charge-duality behavior [10], enhanced fluctuations at the resistive transition [11,12], and a peculiar gapped density of states above T c [13][14][15][16]. Owing to their unique properties such as a small electron diffusion coefficient and a low charge carrier density, extremely thin disordered superconductors also play a key role in advanced applications such as superconducting nanowire single photon detectors (SNSPDs) [17][18][19].…”

Dynamic and structural stability of cubic vanadium nitride

“…We obtain ω p = 6.4 eV/ from a Drude-Lorentz fit to the VN dielectric function obtained using variable-angle spectroscopic ellipsometry and confirmed by first-principles DFT bandstructure calculations [15,74], which yield ω p = 6.5 eV/ . Using the Fermi velocity v F = 2.09 × 10 6 m/s [75,76] for NbN, which is isoelectronic with VN, yields λ 300 K = 7 nm, approximately 17× the measured lattice parameter, a o = 0.4130 nm. A linear extrapolation of ρ(T ) results near 300 K shows that λ = a o at T ∼ 10 000 K, greatly exceeding the VN melting point, T m = 2350 K [77].…”

Dynamical Conductivity across the Disorder-Tuned Superconductor-Insulator Transition

“…Recent conductivity measurements at frequencies well within the superconducting gap (0-20 GHz) [15][16][17][18][19][20] have observed low-frequency features that cannot be accounted for by pair-breaking mechanisms. A theoretical understanding of the low-frequency dynamical conductivity is vital for understanding the role of fluctuations and for guiding future experiments that probe the SIT.…”