Optical Properties of Superconducting Nd_0.8Sr_0.2NiO₂ Nickelate

Abstract

The intensive search for alternative noncuprate high-transition-temperature (T c ) 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 T c 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 Nd 0.8 Sr 0.2 NiO 2 film measured using optical and terahertz spectroscopy at temperatures above and below the critical temperature T c ∼ 13 K. The normal-state electrodynamics of Nd 0.8 Sr 0.2 NiO 2 can be described by a scattering time at room temperature (τ ≃ 1.3 × 10 −14 s) and a plasma frequency ω p ≃ 5500 cm −1 in combination with an absorption band in the mid-infrared (MIR), characteristics of transition metal oxides, located around ω 0 ∼ 2500 cm −1 and with an amplitude ω p MIR of about 8000 cm −1 . The degree of electronic correlation can be estimated using the ratio ω p 2 /(ω p 2 + (ω p MIR ) 2 ). In the present system, the determined value of 0.32 ± 0.06 indicates a strong electron correlation in the NiO 2 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 T c , a superconducting energy gap 2Δ ∼ 3.3 meV can be extracted from the terahertz reflectivity using the Mattis−Bardeen model.