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    Influence of helical spin structure on the magnetoresistance of an ideal topological insulator
    (IOP PUBLISHING LTD, 2017) Ozturk, T.; Field, R. L., III; Eo, Y. S.; Wolgast, S.; Sun, K.; Kurdak, C.
    In an ideal topological insulator, the helical spin structure of surface electrons suppresses backscattering and thus can enhance surface conductivity. In this study, we investigate the effect of perpendicular magnetic field on the spin structure of electrons at the Fermi energy and define a magnetic-field dependent topological enhancement factor using Boltzmann transport and calculate this factor for different disorder potentials, ranging from short-range disorder to screened Coulomb potential. Within the Boltzmann approximation, the topological enhancement factor reaches its maximum value of 4 for a short-range disorder at zero magnetic field and approaches a value of 1 at high magnetic fields. The topological enhancement factor becomes independent of the nature of the disorder potential at high magnetic fields.
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    Magnetotransport measurements of the surface states of samarium hexaboride using Corbino structures
    (AMER PHYSICAL SOC, 2015) Wolgast, S.; Eo, Y. S.; Oeztuerk, T.; Li, G.; Xiang, Z.; Tinsman, C.; Asaba, T.
    The recent conjecture of a topologically protected surface state in SmB6 and the verification of robust surface conduction below 4 K have prompted a large effort to understand surface states. Conventional Hall transport measurements allowcurrent 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 SmB6 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.s with a carrier density of 2.5 x 10(13) cm(-2), which are significantly lower than indicated by Hall transport studies. This mobility value can explain the 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, it 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.