From bad metal to Kondo insulator: temperature evolution of the optical properties of SmB6

A Tytarenko,E Van Heumen,Y K Huang,S Johnston,K Nakatsukasa

doi:10.1088/1367-2630/18/12/123003

A Tytarenko, E Van Heumen + Show 3 more

Open Access

https://doi.org/10.1088/1367-2630/18/12/123003

Copy DOI

Abstract

The recent rekindling of interest in the mixed valent Kondo insulator SmB6 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 SmB6 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 (∼4.35 eV) decreases. This indicates the involvement of a large Coulomb interaction (> 5 eV) in the formation of the Kondo insulator.

Highlights

The enormous interest in SmB6 as the first realization of a correlated topological insulator, has resulted in a wealth of new information on its electronic properties and the formation of the Kondo insulating state
We suggest that strong correlation effects, associated with the effective Coulomb interaction U, are responsible for the transfer of spectral weight to very high energies that has far not been observed in mixed valent Kondo insulators
The total spectral weight decreases, but we observe a small increase in spectral weight in all of the interband transitions as the Kondo insulating state forms In the SOM we show that this result is robust against substantial shifts of the experimental reflectivity data and corresponding changes in the spectral weight analysis

Summary

Introduction

The enormous interest in SmB6 as the first realization of a correlated topological insulator (or in this case more precisely a topological Kondo insulator [1, 2]), has resulted in a wealth of new information on its electronic properties and the formation of the Kondo insulating state. As the temperature below which surface states are expected to become detectable is below the lowest temperature achievable in our setup, we instead focus on the changes in the optical spectra related to the formation of the Kondo insulating state. The interband transitions can be related to LSDA+U calculations [30], where Sm has been taken to have both a 2+ and 3+ valence, providing an indirect signature of the mixed valence state of Sm. We observe for the first time an infrared mode at 19.4 meV that we associate with the motion of Sm ions against the B6 cages. We suggest that strong correlation effects, associated with the effective Coulomb interaction U, are responsible for the transfer of spectral weight to very high energies that has far not been observed in mixed valent Kondo insulators

Methods

Results

Conclusion