Two-dimensional Fermi surfaces in Kondo insulator SmB ₆

Abstract

In Kondo insulator samarium hexaboride SmB6, strong correlation and band hybridization lead to an insulating gap and a diverging resistance at low temperature. The resistance divergence ends at about 5 Kelvin, a behavior recently demonstrated to arise from the surface conductance. However, questions remain whether and where a topological surface state exists. Quantum oscillations have not been observed to map the Fermi surface. We solve the problem by resolving the Landau Level quantization and Fermi surface topology using torque magnetometry. The observed Fermi surface suggests a two dimensional surface state on the (101) plane. Furthermore, the tracking of the Landau Levels in the infinite magnetic field limit points to -1/2, which indicates a 2D Dirac electronic state.The recent development of topological insulators is a triumph of single electron band theory [1][2][3][4][5][6][7][8] . It is interesting to understand whether similar exotic states of matter can arise once strong electronic interaction comes into play. Kondo insulators, a strongly-correlated heavyfermion system, offer a good playground for the exploration of this question. In a Kondo insulator 9,10 , the hybridization between itinerant electrons and localized orbitals opens a gap and makes the material insulating. Once the sample temperature is cold enough, the electronic structure in the strongly correlated system can be mapped to a rather simple electronic state that resembles a normal topological insulator 11 . As a result, in the ground state of the Kondo insulator there exists a bulk insulating state and a conductive surface state. In samarium hexaboride (SmB 6 ), the existence of the surface state has been suggested by recent experimental observations of the surface conductance as well as a map of the hybridization gap 12-14 . However, a direct observation of the Fermi surface has not yet been achieved by transport measurements in Kondo insulators. In this letter we report the observation of quantum oscillations in Kondo insulator SmB 6 using torque magnetometry. The observed Fermi surface is shown to be two-dimensional (2D) and arises from the crystalline (101) surface, and the Landau Level index plot shows a Berry phase contributed -1/2 factor in the infinite field limit, which indicates that this Fermi surface encloses Dirac points, a characteristic property of topological insulators.The direct observation of quantum oscillations is an essential step in understanding the electronic state of the bulk and surfaces of Kondo insulator. Wolgast et al. have argued strongly that the great robustness and certain other properties of the low T surface conductivity of SmB 6 are best understood as a consequence of having TI surface states 12 . Nonetheless there is yet no direct evidence for this interpretation of the surface conduction. Such evidence should come from microscopic measurements of the electronic structure, as has been accomplished for the weakly correlated TI materials, such as Bi 2 Se 3 , Bi 2 Te 3 , and graphene [15][16][17][18][19...

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.…”

In-gap collective mode spectrum of the topological Kondo insulatorSmB6

“…As a putative strong topological insulator (TI), SmB 6 is expected to have a metallic crystal boundary protected against any source of backscattering that respects the time-reversal (TR) symmetry. Indeed, several transport [23][24][25][26][27][28] , quantum oscillation 29 and surface spectroscopy [30][31][32][33][34] experiments have directly probed this metallic boundary and generally provided evidence for its existance. At the same time, SmB 6 has been heralded as the first true TI with a fully insulating bulk 35 , unlike the original bismuth-based TIs whose bulks remain weakly conducting due to statistically unavoidable crystal defects [36][37][38] .…”

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

“…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.…”