Optical conductivity signatures of strong correlations and multiband superconductivity in infinite-layer nickelates

Since the discovery of superconductivity in infinite-layer nickelates, there have been extensive efforts to unravel their electronic structure and pairing mechanism. In particular, understanding how the electronic structure evolves with doping is essential for clarifying theoretical models of superconductivity in nickelates. Here we present studies of the optical conductivity of Nd1-xSrxNiO2 thin films spanning the full phase diagram 0.025 < x < 0.30 using spectroscopic ellipsometry. The data are consistent with a two-band Drude model, which allows the decomposition of the intraband response into distinct contributions. One is from a “narrow” Drude term which we associate with electron bands, and the other a “broad” Drude term linked to the hole band with strong correlations. Increasing Sr doping leads to an expansion of the hole band spectral weight, and a corresponding reduction in the electron band, indicative of the multiband electronic structure and a doping-dependent reconstruction of the Fermi surface. Both doping and temperature-dependent optical spectra display significant spectral weight transfer from high to low energy, a hallmark of strong electronic correlations. In the superconducting state at optimal doping (x = 0.15), both electron and hole bands contribute to the superconducting condensate, signifying multiband superconductivity.

💡 Research Summary

In this work the authors investigate the charge dynamics of infinite‑layer nickelate superconductors Nd₁₋ₓSrₓNiO₂ (NSNO) across the entire superconducting dome (0.025 ≤ x ≤ 0.30) using broadband spectroscopic ellipsometry. Thin films (~5 nm) were grown on LSAT substrates, which are optically transparent and provide excellent epitaxy, allowing reliable extraction of the in‑plane optical conductivity σ₁(ω) from 0.1 eV to 5 eV.

The measured σ₁(ω) displays pronounced Drude‑like low‑energy response together with several interband transitions at ≈ 0.4, 1.4, 2.5, 3.5 and 4.0 eV. The high‑energy features at 3.5 eV and 4.0 eV are assigned to Ni 3d → La 4f and O 2p → La 4f excitations, respectively, confirming that the on‑site Coulomb repulsion U is smaller than the charge‑transfer energy Δ, i.e., the system lies in a Mott‑Hubbard rather than a charge‑transfer regime.

A key observation is a systematic spectral‑weight transfer (SWT) from energies above ≈ 2.5 eV to lower energies as Sr content increases. An isosbestic point at ~2.5 eV marks the boundary where the lost high‑energy weight reappears in the Drude region. This broad‑range SWT is a hallmark of strongly correlated electron systems and mirrors the behavior of doped cuprates. By integrating σ₁(ω) up to a cutoff Ω, the effective electron number N_eff(Ω) is obtained; N_eff(2.5 eV) grows roughly linearly with x in the underdoped regime, exceeds the nominal hole count because of self‑doping, and saturates in the overdoped region where the Hall coefficient changes sign.

To capture the multiband nature of NSNO, the low‑energy conductivity (≤ 1.5 eV) is fitted with a two‑Drude plus Lorentz model. Two distinct Drude components are required: a narrow Drude (small scattering rate, smaller plasma frequency) and a broad Drude (large scattering rate, larger plasma frequency). The narrow component is attributed to electron pockets derived from Nd 5d xy and 5d 3z²‑r² orbitals, while the broad component originates from the Ni 3dₓ²₋ᵧ² hole pocket. With increasing Sr doping the plasma‑frequency squared (ωₚ²) of the broad Drude grows substantially, whereas that of the narrow Drude diminishes, indicating that holes are added to the Ni‑derived band while the electron pockets shrink. The scattering rate of the hole band decreases up to x ≈ 0.20, reflecting enhanced quasiparticle coherence, but rises again in the heavily overdoped regime, likely due to disorder. The electron‑band scattering rate shows only modest variation, consistent with a more incoherent background.

Temperature‑dependent measurements on the optimally doped sample (x = 0.15, T_c,onset ≈ 22 K) reveal that, in the normal state, the hole‑band Drude weight increases while the electron‑band Drude weight decreases as temperature is lowered, consistent with Hall‑effect trends. The ratio N_eff(150 K)/N_eff(300 K) exceeds unity at all photon energies, confirming continued SWT toward low energy upon cooling—a signature of strong correlations and kinetic‑energy gain of carriers.

Below T_c the Drude weights of both bands drop sharply and their scattering rates collapse, demonstrating that carriers from both the hole and electron pockets condense into the superconducting superfluid. This provides direct spectroscopic evidence for multiband superconductivity in infinite‑layer nickelates, in contrast to the predominantly single‑band nature of cuprates.

In the discussion the authors emphasize that the observed doping‑induced enhancement of coherent Drude response together with high‑energy SWT mirrors cuprate phenomenology, yet occurs within a Mott‑Hubbard framework. The persistence of electron pockets even beyond the superconducting dome argues against models that predict complete depletion of the electron band before superconductivity sets in. Instead, the data support scenarios where Hund’s coupling, Kondo‑like hybridization, or inter‑orbital correlations play a decisive role.

Overall, the paper establishes broadband optical spectroscopy as a powerful probe of band‑specific carrier dynamics in NSNO, quantifies the redistribution of spectral weight with doping and temperature, and demonstrates that both Ni‑derived holes and Nd‑derived electrons actively participate in the superconducting condensate. These findings provide essential experimental constraints for theoretical models of nickelate superconductivity and highlight the unique combination of strong electronic correlations and multiband physics in this emerging family of high‑T_c materials.