Transport gap in SmB ₆ protected against disorder

Abstract

The inverted resistance method was used in this study to extend the bulk resistivity ofSmB6to a regime where the surface conduction overwhelms the bulk. Remarkably, regardless of the large off-stoichiometric growth conditions (inducing disorder by samarium vacancies, boron interstitials, etc.), the bulk resistivity shows an intrinsic thermally activated behavior that changes ∼7–10 orders of magnitude, suggesting thatSmB6is an ideal insulator that is immune to disorder.

Physical properties of SmxB6 single crystals

“…Although the bulk becomes metallic at low T for lower x, as shown in Ref. 39 with Corbino disk measurements, the existence of surface states appears not to be affected. Moderate disorder does in general not change the main topological Z 2 invariants as observed in Bi alloys 68 .…”

“…Some reports claim a strong decrease of the resistivity on doping, 34,37 which is not supported by other groups. 38,39 It is important to note that SmB 6 crystals studied in Refs. 34 and 37 were grown using the floating zone method whereas flux grown crystals were used in Refs.…”

“…as a function of the inverse temperature 1/T . All samples show a thermally activated behavior ρ b ∝ exp(E A /k B T ), with an almost identical activation energy of 3.9 meV (corresponding to the hybridization gap), from 30 K down to 4 K. The bulk resistivity rises by more than 5 orders of magnitude before it starts to saturate below 4 K. This point corresponds to the transition from bulk-dominated to surface-dominated regime in which other methods like inverted resistance measurements are needed to separate the bulk and surface resistivities 39. This method yields at low T a mysterious high bulk resistivity plateau, which possibly arises from extended defect conduction 39.…”

Fragility of the Kondo insulating gap against disorder: Relevance to recent puzzles in topological Kondo insulators

“…However, the origin of the experimental observations are also unclear, demonstrating extensive sample dependence or influence from the specific experimental design, indicating the possible role of extrinsic effects [16][17][18]. Moreover, very recent observations strongly suggest bulk conduction instead [19,20] and advocate the essential role of disorder [19,[21][22][23].…”

Magnetic dichroism in the Kondo insulator SmB6

“…[1][2][3] Evidence continues to grow in support of the claim that SmB 6 is a topological Kondo insulator, with an insulating bulk at low temperatures and a topologically protected metallic surface, though there is not universal agreement. [3][4][5][6][7][8][9] In light of the potential topological aspects of SmB 6 , great attention has been focused on its surface phenomena. [7,[10][11][12][13] The strongly-correlated nature of the insulating state implies that topological surface states should also be strongly correlated, with potentially exotic implications.…”