Role of electron correlation and disorder on the electronic structure of layered nickelate (La$_{0.5}$Sr$_{0.5}$)$_2$NiO$_4$

TL;DR

This study combines spectroscopy and theoretical calculations to explore how electron correlation and disorder influence the electronic structure of layered nickelate (La$_{0.5}$Sr$_{0.5}$)$_2$NiO$_4$, revealing strong correlations and disorder effects leading to metallic yet insulating behavior.

Contribution

It provides a comprehensive analysis of electron correlation and disorder effects in layered nickelate using combined experimental and theoretical approaches, highlighting the role of these factors in its electronic properties.

Findings

Strong electron correlation evidenced by mass enhancement factor ~3.

Disorder contributes to Anderson insulating behavior despite metallic spectral features.

Spectroscopic data and DFT+DMFT calculations agree on the renormalization of $e_g$ bands.

Abstract

We investigate the role of electron correlation and disorder on the electronic structure of layered nickelate (La$_{0.5}$Sr$_{0.5}$)$_2$NiO$_4$ using core level and valence band photoemission spectroscopy in conjunction with density functional theory (DFT) and dynamical mean field theory (DMFT) calculations. Sr 3$d$ and La 4$d$ core level spectra exhibit multiple features associated with photoemission final state effects. An increase of unscreened features in the Sr 3$d$ and La 4$d$ core level spectra with lowering temperature suggests the reduction in density of states (DOS) at the Fermi level, $E_F$. Valence band spectra collected using different photon energies reveal finite intensity at $E_F$ and overall spectra are well captured by DFT+DMFT. Strong renormalization of partially filled $e_g$ bands in DFT+DMFT result indicates strong correlation in this system. Mass enhancement factor, $m^*/m_{\text{DFT}} \sim$ 3, agrees well with values obtained from specific heat measurements. High resolution spectra in the vicinity of $E_F$ show monotonically decreasing spectral intensity with lowering temperature, which evolves to exhibit a Fermi cut-off at low temperatures indicating metallic character in contrast to insulating transport, suggesting Anderson insulating state. $|E-E_F|^{1/2}$ dependence of the spectral DOS and square root temperature dependence of spectral DOS at $E_F$ evidences the role of disorder in the electronic structure of (La$_{0.5}$Sr$_{0.5}$)$_2$NiO$_4$.