Magnetotransport Measurements of the Surface States of Samarium Hexaboride using Corbino Structures

TL;DR

This study uses Corbino magnetotransport measurements to analyze the surface states of SmB$_6$, revealing a negative magnetoresistance, lower carrier mobility than Hall measurements, and complex magnetic effects influencing surface conduction.

Contribution

It introduces a Corbino geometry approach to isolate single-surface conductivity in SmB$_6$, providing new insights into its surface state properties and magnetic behavior.

Findings

Negative magnetoresistance observed on both surfaces

Carrier mobility significantly lower than Hall measurements suggest

Hysteretic behavior linked to magnetic effects, not quantum interference

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 $122\text{ cm}^2/\text{V}\cdot\text{sec}$ with a carrier density of $2.5\times10^{13} \text{ cm}^{-2}$, 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 anti-localization, but are more likely due to an extrinsic magnetic effect such as the magnetocaloric effect or glassy ordering.