Bulk transport paths through defects in floating zone and Al flux grown SmB6

Abstract

We investigate the roles of disorder on low-temperature transport in SmB 6 crystals grown by both the Al flux and floating zone methods. We used the inverted resistance method with Corbino geometry to investigate whether low-temperature variations in the standard resistance plateau arise from a surface or a bulk channel in floating zone samples. The results show significant sample-dependent residual bulk conduction, in contrast to smaller amounts of residual bulk conduction previously observed in Al flux grown samples with Sm vacancies. We consider hopping in an activated impurity band as a possible source for the observed bulk conduction, but it is unlikely that the large residual bulk conduction seen in floating zone samples is solely due to Sm vacancies. We therefore propose that one-dimensional defects, or dislocations, contribute as well. Using chemical etching, we find evidence for dislocations in both flux and floating zone samples, with higher dislocation density in floating zone samples than in Al flux grown samples. In addition to the possibility of transport through one-dimensional dislocations, we also discuss our results in the context of recent theoretical models of SmB 6 .

Surface excitations relaxation in the Kondo insulator Sm$_{1-x}$Gd$_{x}$B$_{6}$

“…However, when comparing skin depth results from both methods one must bear in mind that for ESR the local resistivity is the only relevant factor, while a non-local (global) character prevails in resistivity. It is therefore more sensitive to extrinsic effects, such as subsurface cracks, dislocations and any residual flux [24,25,47,77,78] while local ESR measurements are not strongly influenced by these extrinsic effects.…”

Visualizing the atomic-scale origin of metallic behavior in Kondo insulators

“…Additionally, many of the metallic properties were detected in floating zone–grown samples ( 5 , 6 , 9 – 11 ), which are known to have higher concentrations of Sm vacancies than samples grown with an aluminum flux ( 60 ). Floating-zone samples also contain a higher concentration of dislocations ( 31 ), which may similarly disrupt the Kondo screening cloud and thus further enhance the quantum oscillation amplitude beyond that expected from Sm vacancies alone. In contrast, the quantum oscillations completely disappear in flux-grown samples once embedded aluminum is removed ( 8 ).…”

“…More recently, the existence of local metallic puddles around Gd dopants in Sm 1− x Gd x B 6 was inferred from electron spin-resonance measurements ( 29 ). Meanwhile, an increased concentration of Sm vacancies in Sm 1− x B 6 was shown to globally inhibit the development of the hybridization gap ( 30 ), eventually leading to bulk conduction ( 12 , 31 ). All of these findings suggest that Sm-site defects manifest as Kondo holes in SmB 6 , yet their key signature—the accompanying charge oscillations relating to the parent metallic Fermi surface ( 22 )—remains undetected by any microscopic probe.…”

Extraordinary Bulk Insulating Behavior in the Strongly Correlated Materials FeSi and FeSb$_2$

“…This is surprising considering only metals or swave superconductors typically allow such high levels of the substitution before transitioning to a distinct ground state. Since the insulating bulk of SmB 6 is so robust to (zero-dimensional) point defects, there is growing evidence that higher-order (one-or two-dimensional) defects such as dislocations are the leading type of disorder important for bulk conduction, and those defects are unconventional due to their topologically non-trivial nature [39,40]. Given this unusual disorder and impurity response in bulk SmB 6 , it is also of interest to investigate other correlated insulators for similar properties.…”