Optical Properties of Superconducting Nd0.8Sr0.2NiO2 Nickelate

The intensive search for alternative noncuprate high-transition-temperature (Tc) superconductors has taken a positive turn recently with the discovery of superconductivity in infinite-layer nickelates. This discovery is expected to be the basis for disentangling the puzzle behind the physics of high Tc values in oxides. In the unsolved quest for the physical conditions necessary for inducing superconductivity, we report on a broad-band optical study of a Nd0.8Sr0.2NiO2 film measured using optical and terahertz spectroscopy at temperatures above and below the critical temperature Tc ∼ 13 K. The normal-state electrodynamics of Nd0.8Sr0.2NiO2 can be described by a scattering time at room temperature (τ ≃ 1.3 × 10-14 s) and a plasma frequency ωp ≃ 5500 cm-1 in combination with an absorption band in the mid-infrared (MIR), characteristics of transition metal oxides, located around ω0 ∼ 2500 cm-1 and with an amplitude ωpMIR of about 8000 cm-1. The degree of electronic correlation can be estimated using the ratio ωp2/(ωp2 + (ωpMIR)2). In the present system, the determined value of 0.32 ± 0.06 indicates a strong electron correlation in the NiO2 plane with similar strength as cuprates. From 300 to 20 K, we observe a spectral weight transfer between the Drude and MIR band, together with a strong increase in the Drude scattering time, in agreement with DC resistivity measurements. Below Tc, a superconducting energy gap 2Δ ∼ 3.3 meV can be extracted from the terahertz reflectivity using the Mattis-Bardeen model.