Comprehensive surface magnetotransport study of SmB6

Abstract

After the theoretical prediction that SmB6 is a topological Kondo insulator, there has been an explosion of studies on the SmB6 surface. However, there is not yet an agreement on even the most basic quantities such as the surface carrier density and mobility. In this paper, we carefully revisit Corbino disk magnetotransport studies to find those surface transport parameters. We first show that subsurface cracks exist in the SmB6 crystals, arising both from surface preparation and during the crystal growth. We provide evidence that these hidden subsurface cracks are additional conduction channels, and the large disagreement between earlier surface SmB6 studies may originate from previous interpretations not taking this extra conduction path into account. We provide an update of a more reliable magnetotransport data than the previous one (Phys. Rev. B 92, 115110) and find that the orders-of-magnitude large disagreements in carrier density and mobility come from the surface preparation and the transport geometry rather than the intrinsic sample quality. From this magnetotransport study, we find an updated estimate of the carrier density and mobility of 2.71×10 13 (1/cm 2 ) and 104.5 (cm 2 /V·sec), respectively. We compare our results with other studies of the SmB6 surface. By this comparison, we provide insight into the disagreements and agreements of the previously reported angle-resolved photoemission spectroscopy, scanning tunneling microscopy, and magnetotorque quantum oscillations measurements.

Topological surface conduction in Kondo insulator YbB$_{12}$

“…which is attributed to the bulk insulating channel. On further lowering the temperature, ρ(T ) becomes weakly T -dependent and shows a resistivity plateau, as observed in SmB 6 [6][7][8]. Although ρ(T ) above 30 K well coincides for all samples, the magnitude of the resistivity plateau is strongly suppressed with reducing t. These results indicate that the resistivity plateau is attributed to the surface metallic states.…”

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.…”

“…With a simple cubic structure (space group P m3m), SmB 6 has all the properties required of a cubic topological Kondo insulator (TKI), such as the Γ 8 quartet crystal field ground state and an odd number of band inversion at the X point in the Brillouin zone [14,17,23]. The hybridization between the d conduction electrons and the Sm-4f electrons opens a gap at the Fermi energy, and the system becomes a good insulator at low temperatures [24,25]. In consequence, the bulk carriers contribute less to the transport upon lowering the temperature, and a plateau is observed in resistivity at low temperatures, which is related to sur-face states dominating the conductivity [15,[25][26][27][28]].…”

“…The hybridization between the d conduction electrons and the Sm-4f electrons opens a gap at the Fermi energy, and the system becomes a good insulator at low temperatures [24,25]. In consequence, the bulk carriers contribute less to the transport upon lowering the temperature, and a plateau is observed in resistivity at low temperatures, which is related to sur-face states dominating the conductivity [15,[25][26][27][28]]. An additional energy scale inside of the hybridization gap in scanning tunneling spectroscopy has been linked to these surface states [29][30][31].…”

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

“…At low temperatures, measurements of mobility in SmB 6 are not straightforward. Experimental reports-ARPES, dHvA oscillations, and transport-disagree on the order of magnitude of the mobility, which ranges from about 10 cm 2 /V s in transport [59] to about 1000 cm 2 /V s in quantum oscillations [28]; surface preparation can even affect the extracted mobility [60]. A recent study also showed that the two proposed topological surface channels would have very different mobilities [60], and accounting for a disorderbased channel could be an additional challenge.…”