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.

We introduce a new resistance measurement method that is useful in characterizing materials with both surface and bulk conduction, such as three-dimensional topological insulators. The transport geometry for this new resistance measurement configuration consists of one current lead as a closed loop that fully encloses the other current lead on the surface, and two voltage leads that are both placed outside the loop. We show that in the limit where the transport is dominated by the surface conductivity of the material, the four-terminal resistance measured from such a transport geometry is proportional to σ b /σ 2 s , where σ b and σ s are the bulk and surface conductivities of the material, respectively. We call this new type of measurement inverted resistance measurement, as the resistance scales inversely with the bulk resistivity. We discuss possible implementations of this new method by performing numerical calculations on different geometries and introduce strategies to extract the bulk and surface conductivities. We also demonstrate inverted resistance measurements on SmB 6 , a topological Kondo insulator, using both single-sided and coaxiallyaligned double-sided Corbino disk transport geometries. Using this new method, we are able to measure the bulk conductivity, even at low temperatures, where the bulk conduction is much smaller than the surface conduction in this material.

We report an optically induced reduction of the f-d hybridization in the prototypical heavyfermion compound YbRh2Si2. We use femtosecond time-and angle-resolved photoemission spectroscopy to monitor changes of spectral weight and binding energies of the Yb 4f and Rh 4d states before the lattice temperature increases after pumping. Overall, the f-d hybridization decreases smoothly with increasing electronic temperature up to ∼ 250 K but changes slope at ∼ 100 K. This temperature scale coincides with the onset of coherent Kondo scattering and with thermally populating the first excited crystal electrical field level. Extending previous photoemission studies, we observe a persistent f-d hybridization up to at least ∼ 250 K, which is far larger than the coherence temperature defined by transport but in agreement with the temperature dependence of the noninteger Yb valence. Our data underlines the distinction of probes accessing spin and charge degrees of freedom in strongly correlated systems characterized by spin-charge separation.

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.

Several technical issues and challenges are identified and investigated for the planar tunneling spectroscopy of the topological Kondo insulator SmB6. Contrasting behaviors of the tunnel junctions prepared in two different ways are analyzed and explained in detail. The conventional approach based on an AlOx tunnel barrier results in unsatisfactory results due to the inter-diffusion between SmB6 and deposited Al. On the contrary, plasma oxidation of SmB6 crystals produces high-quality tunnel barriers on both (001) and (011) surfaces. Resultant conductance spectra are highly reproducible with clear signatures for the predicted surface Dirac fermions and the bulk hybridization gap as well. The surface states are identified to reside on two or one distinguishable Dirac cone(s) on the (001) and (011) surface, respectively, in good agreement with the recent literature. However, their topological protection is found to be limited within the low energy region due to their inevitable interaction with the bulk excitations, called spin excitons, consistent with a recent theoretical prediction. Implications of our findings on other physical properties in SmB6 and also other correlated topological materials are remarked.