Spectral weight redistribution in (LaNiO3)n/(LaMnO3)2 superlattices from optical spectroscopy

TL;DR

This study investigates how optical properties and spectral weight distribution change in LaNiO3/LaMnO3 superlattices as LaNiO3 layer thickness varies, revealing an insulator-metal transition driven by interfacial charge transfer.

Contribution

It provides new insights into the spectral weight redistribution and the role of interfacial charge transfer in driving the insulator-metal transition in these superlattices.

Findings

Spectral weight shifts from high to low frequency with increasing LaNiO3 thickness.

The optical conductivity is not a simple sum of LaMnO3 and LaNiO3 contributions.

Interfacial charge transfer induces a valence change leading to the insulator-metal transition.

Abstract

We have studied the optical properties of four (LaNiO$_3$)$_n$/(LaMnO$_3$)$_2$ superlattices (SL) ($n$=2, 3, 4, 5) on SrTiO$_3$ substrates. We have measured the reflectivity at temperatures from 20 K to 400 K, and extracted the optical conductivity through a fitting procedure based on a Kramers-Kronig consistent Lorentz-Drude model. With increasing LaNiO$_3$ thickness, the SLs undergo an insulator-to-metal transition (IMT) that is accompanied by the transfer of spectral weight from high to low frequency. The presence of a broad mid-infrared band, however, shows that the optical conductivity of the (LaNiO$_3$)$_n$/(LaMnO$_3$)$_2$ SLs is not a linear combination of the LaMnO$_3$ and LaNiO$_3$ conductivities. Our observations suggest that interfacial charge transfer leads to an IMT due to a change in valence at the Mn and Ni sites.