Two-band superconductivity and transition temperature limited by thermal fluctuations in ambient pressure La3-xPrxNi2O7-d(x = 0.0, 0.15, 1.0) thin films

Recently, two research groups [1–3] reported on the observation of ambient pressure superconductivity in a few nanometres thick La3-xPrxNi2O7-d (x = 0.0, 0.15, 1.0) films with the T(c,onset)≅40 K and T(c,zero)≤14 K. Here I have analyzed the reported self-field critical current density, Jc(sf,T), and upper critical field, Bc2(T), for these films [1–3] and showed that the La3-xPrxNi2O7-d films exhibit a large in-plane London penetration depth, λab(0)=1.9-6.8 μm, and the Ginzburg-Landau parameter κc(0)=500-1000. Deduced λab(0) values are within uncertainty range for independently reported [2] λab(T=1.8 K)=(3.7±1.9) μm. Such large values of λab(0) explain a wide resistive transition inLa3-xPrxNi2O7-d films [1–3], because large λab(0) implies low superfluid density, ρs≡1/(λab)^2, and therefore large thermal fluctuations. Consequently, I calculated the phase fluctuation temperature, T(fluc), and found that the T(c,zero)<T(fluc). I also found that Jc(sf,T) and Bc2(T) data are nicely fitted to two-band gap models, from which the preference has been given to two-band (s+s)-wave model (for which the ratios of (2ΔL (0))/(kBT(c,L))≅3.6-4.0 and (2ΔS (0))/(kBT(c,S))=1.0-3.0 are for the larger and smaller bands, respectively). Besides I showed that bulk highly compressed Ruddlesden–Popper nickelates Lan+1NinO3n+1 (n = 2,3) and ambient pressure Lan+1NinO2n+2 (n = 5) thin film also demonstrate evidences for two-band superconductivity.