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Register a new userInvestigation of high-temperature bulk transport characteristics and skew scattering in samarium hexaboride
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A well-known feature in transport data of the topological Kondo insulator SmB$_6$ is the sign change in the Hall coefficient at 65 K. Carriers in SmB$_6$ are known to be negative, but above 65 K, the Hall sign suggests that the carriers are positive. Here, we extend Hall measurements up to 400 K and observe that the Hall coefficient changes back to the correct (negative) sign at about 305 K. We interpret the anomalous sign of the Hall coefficient in the context of skew scattering arising from the strong correlations between the $f$ and $d$ electrons. At energy scales where the gap is closed, the number of $d$ electrons in resonance with the $f$ electrons at the Fermi energy varies. When a large proportion of $d$ and $f$ electrons are in resonance, skew scattering is dominant, leading to the observation of the positive sign, but when fewer are in resonance, conventional scattering mechanisms dominate instead.
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We present a new model to explain the difference between the transport and spectroscopy gaps in samarium hexaboride (SmB$_6$), which has been a mystery for some time. We propose that SmB$_6$ can be modeled as an intrinsic semiconductor with a depletion length that diverges at cryogenic temperatures. In this model, we find a self-consistent solution to Poissons equation in the bulk, with boundary conditions based on Fermi energy pinning due to surface charges. The solution yields band bending in the bulk; this explains the difference between the two gaps because spectroscopic methods measure the gap near the surface, while transport measures the average over the bulk. We also connect the model to transport parameters, including the Hall coefficient and thermopower, using semiclassical transport theory. The divergence of the depletion length additionally explains the 10-12 K feature in data for these parameters, demonstrating a crossover from bulk dominated transport above this temperature to surface-dominated transport below this temperature. We find good agreement between our model and a collection of transport data from 4-40 K. This model can also be generalized to materials with similar band structure.
SmB$_6$ exhibits a small (15-20 meV) bandgap at low temperatures due to hybridized $d$ and $f$ electrons, a tiny (3 meV) transport activation energy $(E_{A})$ above 4 K, and surface states accessible to transport below 2 K. We study its magnetoresistance in 60-T pulsed fields between 1.5 K and 4 K. The response of the nearly $T$-independent surface states (which show no Shubnikov-de Haas oscillations) is distinct from that of the activated bulk. $E_{A}$ shrinks by 50% under fields up to 60 T. Data up to 93 T suggest that this trend continues beyond 100 T, in contrast with previous explanations. It rules out emerging theories to explain observed exotic magnetic quantum oscillations.
Recent theoretical and experimental studies suggest that SmB$_6$ is the first topological Kondo insulator: A material in which the interaction between localized and itinerant electrons renders the bulk insulating at low temperature, while topological surface states leave the surface metallic. While this would elegantly explain the materials puzzling conductivity, we find the experimentally observed candidates for both predicted topological surface states to be of trivial character instead: The surface state at $bar{Gamma}$ is very heavy and shallow with a mere $sim 2$ meV binding energy. It exhibits large Rashba splitting which excludes a topological nature. We further demonstrate that the other metallic surface state, located at $bar{X}$, is not an independent in-gap state as supposed previously, but part of a massive band with much higher binding energy (1.7 eV). We show that it remains metallic down to 1 K due to reduced hybridization with the energy-shifted surface 4$f$ level.
Strongly correlated electron systems show many exotic properties such as unconventional superconductity, quantum criticality, and Kondo insulating behavior. In addition, the Kondo insulator SmB6 has been predicted theoretically to be a 3D topological insulator with a metallic surface state. We report here transport measurements on doped SmB6, which show that ~3% magnetic and non-magnetic dopants in SmB6 exhibit clearly contrasting behavior, evidence that the metallic surface state is only destroyed when time reversal symmetry is broken. We find as well a quantum percolation limit of impurity concentration which transform the topological insulator into a conventional band insulator by forming impurity band. Our careful thickness dependence results show that SmB6 is the first demonstatrated perfect 3D topological insulator with virtually zero residual bulk conductivity.
Impurities and defects in Kondo insulators can have an unusual impact on dynamics that blends with effects of intrinsic electron correlations. Such crystal imperfections are difficult to avoid, and their consequences are incompletely understood. Here we study magnetic impurities in Kondo insulators via perturbation theory of the s-d Kondo impurity model adapted to small bandgap insulators. The calculated magnetization and specific heat agree with recent thermodynamic measurements in samarium hexaboride (SmB$_6$). This qualitative agreement supports the physical picture of multi-channel Kondo screening of local moments by electrons and holes involving both intrinsic and impurity bands. Specific heat is thermally activated in zero field by Kondo screening through sub-gap impurity bands and exhibits a characteristic upturn as the temperature is decreased. In contrast, magnetization obtains a dominant quantum correction from partial screening by virtual particle-hole pairs in intrinsic bands. We point out that magnetic impurities could impact de Haas-van Alphen quantum oscillations in SmB$_6$, through the effects of Landau quantization in intrinsic bands on the Kondo screening of impurity moments.
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