Magnetotransport measurements of the surface states of samarium hexaboride using Corbino structures

Abstract

The recent conjecture of a topologically-protected surface state in SmB 6 and the verification of robust surface conduction below 4 K have prompted a large effort to understand the surface states.Conventional Hall transport measurements allow current to flow on all surfaces of a topological insulator, so such measurements are influenced by contributions from multiple surfaces of varying transport character. Instead, we study magnetotransport of SmB 6 using a Corbino geometry, which can directly measure the conductivity of a single, independent surface. Both (011) and (001) crystal surfaces show a strong negative magnetoresistance at all magnetic field angles measured.The (011) surface has a carrier mobility of 122 cm 2 /V·sec with a carrier density of 2.5×10 13 cm −2 , which are significantly smaller than indicated by Hall transport studies. This mobility value can explain a failure so far to observe Shubnikov-de Haas oscillations. Analysis of the angle-dependence of conductivity on the (011) surface suggests a combination of a field-dependent enhancement of the carrier density and a suppression of Kondo scattering from native oxide layer magnetic moments as the likely origin of the negative magnetoresistance. Our results also reveal a hysteretic behavior whose magnitude depends on the magnetic field sweep rate and temperature. Although this feature becomes smaller when the field sweep is slower, does not disappear or saturate during our slowest sweep-rate measurements, which is much slower than a typical magnetotransport trace.These observations cannot be explained by quantum interference corrections such as weak antilocalization, but are more likely due to an extrinsic magnetic effect such as the magnetocaloric effect or glassy ordering.

Reduction of the low-temperature bulk gap in samarium hexaboride under high magnetic fields

“…We conclude that the strong negative MR observed by Cooley et al is indeed due to the reduction of the bulk E A between the band edge and the Fermi energy, and is distinct from the smaller negative MR displayed by the surface states at even lower temperatures (< 2 K), as reported by older [35] and more recent studies [34,36,37].…”

“…[33] as a primarily geometric effect in which the MR is exaggerated at the crossover temperature by the change in the current path. We also note that none of the samples we measured up to 93 T at 1.4 K show any signs of SdH oscillations, though this is not surprising, considering the small surface mobilities reported for similar samples [7,34].…”

“…The temperature dependence of R s is known to be very weak, [34] justifying its treatment as temperature-independent in this calculation; nevertheless, such an error has been taken into account by the calculated error in R b (black error bar in the inset of Figure 3), which is then weighted in the fitting of E A . Figure 3 The raw resistance is also plotted (green squares) for reference.…”

“…al [10]. Meanwhile, the surface MR shows no indications of SdH oscillations up to 93 T, presumably due to the low mobilities of the surface states [34].…”

“…Motivated by the desire to separate the bulk MR behavior from the surface MR behavior we observed previously [34], as well as to detect 2D transport signatures of the surface states (e.g., Shubnikov-de Haas (SdH) oscillations), we have conducted temperature-dependent transport measurements up to 60 T from 1.4 K to 4.0 K, along with measurements up to 93 T at 1.4 K. The temperature dependence provides a quantitative measure of the activation energy's (E A ) dependence on the magnetic field, while clearly distinguishing it from the behavior of the surface states. We conclude that the strong negative MR observed by Cooley et al is indeed due to the reduction of the bulk E A between the band edge and the Fermi energy, and is distinct from the smaller negative MR displayed by the surface states at even lower temperatures (< 2 K), as reported by older [35] and more recent studies [34,36,37].…”