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Register a new userWeak Antilocalization and Linear Magnetoresistance in The Surface State of SmB6
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Strongly correlated Kondo insulator SmB6 is known for its peculiar low temperature residual conduction, which has recently been demonstrated to arise from a robust metallic surface state, as predicted by the theory of topological Kondo insulator (TKI). Photoemission, quantum oscillation and magnetic doping experiments have provided evidence for the Dirac-like dispersion and topological protection. Questions arise as whether signatures of spin-momentum locking and electron interaction could be resolved in transport measurements. Here we report metallic conduction of surface state down to mK temperatures with saturation behaviors suggestive of Kondo effect. We observe in the surface state the weak-antilocalization (WAL) effect that is in agreement with a spin-momentum locked metallic surface. At larger perpendicular magnetic fields, the surface state exhibits an unusual linear magnetoresistance similar to those found in Bi-based topological insulators and in graphene. (Correspondence to: [email protected])
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Recently, the resistance saturation at low temperature in Kondo insulator SmB6, a long-standing puzzle in condensed matter physics, was proposed to originate from topological surface state. Here,we systematically studied the magnetoresistance of SmB6 at low temperature up to 55 Tesla. Both temperature- and angular-dependent magnetoresistances show a similar crossover behavior below 5 K. Furthermore, the angular-dependent magnetoresistance on different crystal face confirms a two-dimensional surface state as the origin of magnetoresistances crossover below 5K. Based on two-channels model consisting of both surface and bulk states, the field-dependence of bulk gap with critical magnetic field (Hc) of 196 T is extracted from our temperature-dependent resistance under different magnetic fields. Our results give a consistent picture to understand the low-temperature transport behavior in SmB6, consistent with topological Kondo insulator scenario.
In this study, we report the first results of the high-pressure Hall coefficient (RH) measurements in the putative topological Kondo insulator SmB6 up to 37 GPa. Below 10 GPa, our data reveal that RH(T) exhibits a prominent peak upon cooling below 20 K. Remarkably, the temperature at which surface conduction dominates coincides with the temperature of the peak in RH(T). The temperature dependent resistance and Hall coefficient can be well fitted by a two-channel model with contributions from the metallic surface and the thermally activated bulk states. When the bulk of SmB6 becomes metallic and magnetic at ~ 10 GPa, both the RH(T) peak and the resistance plateau disappear simultaneously. Our results indicate that the RH(T) peak is a fingerprint to diagnose the presence of a metallic surface state in SmB6. The high-pressure magnetic state of SmB6 is robust to 180 GPa, and no evidence of superconductivity is observed in the metallic phase.
SmB6 is a mixed valence Kondo insulator that exhibits a sharp increase in resistance following an activated behavior that levels off and saturates below 4K. This behavior can be explained by the proposal of SmB6 representing a new state of matter, a Topological Kondo insulator, in which a Kondo gap is developed and topologically protected surface conduction dominates low-temperature transport. Exploiting its non-linear dynamics, a tunable SmB6 oscillator device was recently demonstrated, where a small DC current generates large oscillating voltages at frequencies from a few Hz to hundreds of MHz. This behavior was explained by a theoretical model describing the thermal and electronic dynamics of coupled surface and bulk states. However, a crucial aspect of this model, the predicted temperature oscillation in the surface state, hasnt been experimentally observed to date. This is largely due to the technical difficulty of detecting an oscillating temperature of the very thin surface state. Here we report direct measurements of the time-dependent surface state temperature in SmB6 with a RuO micro-thermometer. Our results agree quantitatively with the theoretically simulated temperature waveform, and hence support the validity of the oscillator model, which will provide accurate theoretical guidance for developing future SmB6oscillators at higher frequencies.
The proximity effect at the interface between a topological insulator (TI) and a superconductor is predicted to give rise to chiral topological superconductivity and Majorana fermion excitations. In most TIs studied to date, however, the conducting bulk states have overwhelmed the transport properties and precluded the investigation of the interplay of the topological surface state and Cooper pairs. Here, we demonstrate the superconducting proximity effect in the surface state of SmB6 thin films which display bulk insulation at low temperatures. The Fermi velocity in the surface state deduced from the proximity effect is found to be as large as 10^5 m/s, in good agreement with the value obtained from a separate transport measurement. We show that high transparency between the TI and a superconductor is crucial for the proximity effect. The finding here opens the door to investigation of exotic quantum phenomena using all-thin-film multilayers with high-transparency interfaces.
Samarium hexaboride crystallizes in a simple cubic structure (space group #221, Pm-3m), but its properties are far from being straightforward. Initially classified as a Kondo insulator born out of its intriguing intermediate valence ground state, SmB6 has been recently predicted to be a strongly correlated topological insulator. The subsequent experimental discovery of surface states has revived the interest in SmB6, and our purpose here is to review the extensive and in many aspects perplexing experimental record of this material. We will discuss both surface and bulk properties of SmB6 with an emphasis on the role of crystal growth and sample preparation. We will also highlight the remaining mysteries and open questions in the field.
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