Anomalous 3D bulk AC conduction within the Kondo gap of SmB$_6$ single crystals

TL;DR

This study reveals significant bulk ac conduction within the Kondo gap of SmB$_6$ crystals, challenging the notion that the material's low-temperature conduction is solely due to surface states, and suggests complex in-gap bulk states.

Contribution

The paper provides direct optical evidence of bulk ac conduction within the Kondo gap of SmB$_6$, highlighting the importance of in-gap bulk states over surface states in low-temperature transport.

Findings

Both sample types show significant 3D bulk conduction within the Kondo gap.

Bulk ac conduction exceeds impurity band conduction by orders of magnitude.

Surface states, if present, have a sheet resistance R$/ox$ ≥ 1000 Ω.

Abstract

The Kondo insulator SmB$_6$ has long been known to display anomalous transport behavior at low temperatures, T$<5$ K. In this temperatures range, a plateau is observed in the dc resistivity, contrary to the exponential divergence expected for a gapped system. Recent theoretical calculations suggest that SmB$_6$ may be the first topological Kondo insulator (TKI) and propose that the residual conductivity is due to topological surface states which reside within the Kondo gap. Since the TKI prediction many experiments have claimed to observe high mobility surface states within a perfectly insulating hybridization gap. Here, we investigate the low energy optical conductivity within the hybridization gap of single crystals of SmB$_6$ via time domain terahertz spectroscopy. Samples grown by both optical floating zone and aluminum flux methods are investigated to probe for differences originating from sample growth techniques. We find that both samples display significant 3D bulk conduction originating within the Kondo gap. Although SmB$_6$ may be a bulk dc insulator, it shows significant bulk ac conduction that is many orders of magnitude larger than any known impurity band conduction. The nature of these in-gap states and their coupling with the low energy spin excitons of SmB$_6$ is discussed. Additionally, the well defined conduction path geometry of our optical experiments allows us to show that any surface states, which lie below our detection threshold if present, must have a sheet resistance of R$/\square \ge$ 1000 $\Omega$.