Two‐Band Superconductivity and Transition Temperature Limited by Thermal Fluctuations in Ruddlesden–Popper Nickelates at Ambient and High Pressures

Abstract Recently, two research groups reported the observation of ambient‐pressure superconductivity in few‐nanometers‐thick La 3‐x Pr x Ni 2 O 7‐δ (x = 0.0, 0.15, and 1.0) films with the onset transition temperature and zero resistance temperature . In this study, the reported self‐field critical current density and upper critical field for these films are analyzed. The La 3‐x Pr x Ni 2 O 7‐δ films are find to exhibit a large in‐plane London penetration depth, , and the Ginzburg–Landau parameter . The deduced values are within the uncertainty range for the independently reported . Such large values of explain the wide resistive transition in La 3‐x Pr x Ni 2 O 7‐δ films, because a large implies low superfluid density () and therefore large thermal fluctuations. Consequently, it is find that the calculated phase fluctuation temperature satisfies the inequality . It is also find that the data of and fit well to those of two‐gap models, of which the two‐band s ‐wave model (for which the ratios of the superconducting energy gap Δ(0) to the critical temperature and for the larger and smaller bands, respectively) is preferred. In addition, it is shown that bulk, highly compressed Ruddlesden–Popper nickelates La n+1 Ni n O 3n+1 ( n = 2,3) and ambient‐pressure La n+1 Ni n O 2n+2 ( n = 5) thin film demonstrate evidence of two‐band superconductivity.