Ultrafast terahertz spectroscopy study of Kondo insulating thin film SmB$_{6}$: evidence for an emergent surface state

TL;DR

This study uses ultrafast terahertz spectroscopy to reveal the emergence of a surface state in SmB6 thin films below 20K, providing insights into Kondo insulator surface phenomena and low-energy electrodynamics.

Contribution

It provides the first ultrafast spectroscopic evidence for a surface state in SmB6 emerging below 20K, distinct from the hybridization gap onset.

Findings

Surface state appears below 20K with an effective mass of 0.1me.

Conductivity dynamics suggest an additional low-energy channel.

Hybridization gap energy estimated at 17 meV.

Abstract

We utilize terahertz time domain spectroscopy to investigate thin films of the heavy fermion compound SmB6, a prototype Kondo insulator. Temperature dependent terahertz (THz) conductivity measurements reveal a rapid decrease in the Drude weight and carrier scattering rate at ~T*=20 K, well below the hybridization gap onset temperature (100 K). Moreover, a low-temperature conductivity plateau (below 20K) indicates the emergence of a surface state with an effective electron mass of 0.1me. Conductivity dynamics following optical excitation are also measured and interpreted using Rothwarf-Taylor (R-T) phenomenology, yielding a hybridization gap energy of 17 meV. However, R-T modeling of the conductivity dynamics reveals a deviation from the expected thermally excited quasiparticle density at temperatures below 20K, indicative of another channel opening up in the low energy electrodynamics. Taken together, these results suggest the onset of a surface state well below the crossover temperature (100K) after long-range coherence of the f-electron Kondo lattice is established.