Unconventional Fermi surface in an insulating state

Abstract

Insulators occur in more than one guise, a recent finding was a class of topological insulators, which host a conducting surface juxtaposed with an insulating bulk. Here we report the observation of an unusual insulating state with an electrically insulating bulk that simultaneously yields bulk quantum oscillations with characteristics of an unconventional Fermi liquid. We present quantum oscillation measurements of magnetic torque in high purity single crystals of the Kondo insulator SmB 6 , which reveal quantum oscillation frequencies characteristic of a large three-dimensional conduction electron Fermi surface similar to the metallic rare earth hexaborides such as PrB 6 and LaB 6 . The quantum oscillation amplitude strongly increases at low temperatures, appearing strikingly at variance with conventional metallic behaviour.Kondo insulators, a class of materials positioned close to the border between insulating and metallic behaviour, provide fertile ground for unusual physics [1,2,3,4,5,6,7,8,9,10,11,12,13,14]. This class of strongly correlated materials is thought to be characterised by a 1 arXiv:1507.01129v1 [cond-mat.str-el] 4 Jul 2015

Magnetoresistance Anomaly in Topological Kondo Insulator SmB6 Nanowires with Strong Surface Magnetism

“…A recent quantum oscillation experiment reveals a 3D bulk Fermi surface in TKI SmB 6 , instead of 2D one as expected 19, 20. Surface magnetic ordering is also detected in the latest hysteretic magnetoresistance (MR) measurement of SmB 6 crystals below 600 mK 21.…”

Bulk Fermi Surface of Charge-Neutral Excitations inSmB6or Not: A Heat-Transport Study

“…Especially, the recent quantum oscillation measurement claims multiple Fermi seas in the bulk of an insulating SmB 6 sample [20], in direct contradiction with transport measurements. As the electrical transport only measures the charge degrees of freedom and magnetization focuses more on spins, it is possible that the bulk of the sample may have some gapless and charge-neutral degrees of freedom.…”

“…At low temperatures, however, bulk excitations vanishes because of the energy gap, and surface signals become dominant. The existence of the metallic surface states is now well established and observed in a number of experiments [18][19][20][21][22][23][24][25][26][27][28][29][30][31].Although electric transport measurements so far show that the bulk of SmB 6 has no gapless charge carriers (namely a finite charge-gap), there exist other experiments suggesting possible gapless excitations in the bulk. Especially, the recent quantum oscillation measurement claims multiple Fermi seas in the bulk of an insulating SmB 6 sample [20], in direct contradiction with transport measurements.…”

Planar tunneling spectroscopy of the topological Kondo insulator SmB6

“…Also, it is known that the polar nature of the (001) surface, where the topological surface states are predicted to exist, causes various complex issues including surface reconstruction 37 and time-dependent evolution 41 . Quantum oscillation measurements have shown the existence of surface bands and their possible topological nature 42 but the nature of the bulk insulating state inferred from such measurements is currently under debate 43 . From the fundamental physics point of view, the complexity has to do with the strong correlations in these materials underlying their topological origin.…”