From bad metal to Kondo insulator: temperature evolution of the optical properties of SmB$_{6}$

TL;DR

This study investigates the temperature-dependent optical properties of SmB$_{6}$, revealing a new infrared phonon mode and insights into the formation of the Kondo insulating state through spectral weight analysis.

Contribution

It provides detailed bulk-sensitive optical measurements of SmB$_{6}$, identifying a new phonon mode and elucidating the spectral weight redistribution during Kondo insulator formation.

Findings

Detection of a new infrared active phonon mode involving Sm ions.

Rapid decrease in free charge density below 200 K.

Spectral weight transfer indicating large Coulomb interactions in Kondo insulator formation.

Abstract

The recent rekindling of interest in the mixed valent Kondo insulator SmB$_{6}$ as candidate for a first correlated topological insulator has resulted in a wealth of new experimental observations. In particular, angle-resolved photoemission experiments have provided completely new insights into the formation of the low temperature Kondo insulating state starting from the high temperature correlated metal. Here, we report detailed temperature and energy dependent measurements of the optical constants of SmB$_6$ in order to provide a detailed study from the point of view of a bulk sensitive spectroscopic probe. We detect a previously unobserved infrared active optical phonon mode, involving the movement of the Sm ions against the boron cages. The changes taking place in the free carrier response with temperature and their connection to changes in optical transitions between different bands are discussed. We find that the free charge density starts to decrease rapidly below approximately 200 K. Below 60 K a small amount of spectral weight begins to accumulate in low lying interband transitions, indicating the formation of the Kondo insulating state; however, the total integrated spectral weight in our experimental window ($\sim 4.35$ eV) decreases. This indicates the involvement of a large Coulomb interaction ($>$ 5 eV) in the formation of the Kondo insulator.