Seminar By Steffen Wirth, Max Planck Institute, Germany
Title: Rare earth hexaborides: Materials of diverse physics
Host: Nicholas Bonesteel
Abstract: The hexaborides of cubic CaB6 structure type exhibit an astonishing wealth of physical phenomena primarily depending on the rare earth (or alkaline earth) put on the Ca site. As just two examples, LaB6 is used as a cathode material because of its low work function while YB6 is a superconductor.
Recently, the Kondo insulator SmB6 has been at the focus of intense research, mainly because it was proposed to possess topologically protected nontrivial surface states inside the bulk hybridization gap. Experimentally, hybridization between localized 4f and conduction band states, which results in a screening of the 4f local moments by the conduction electrons at sufficiently low temperatures, is well established . Yet, the observation of many basic properties is still controversial, which is in part due to the reconstruction/disorder of the cleaved surfaces of SmB6. By conducting scanning tunneling microscopy and spectroscopy (STS), we are able to perform local measurements on well identified surfaces [1,2] and confirm the Kondo picture. The spectroscopic response to impurities and magnetic fields allows to distinguish between dominating bulk and surface contributions to these states. The surface contributions possess a strong temperature dependence below about 7 K, i.e. the temperature at which the well-known plateau in the resistance of SmB6 sets in . These findings are consistent with a suppression of the Kondo interaction at the surface of SmB6. Moreover, magnetic impurities are shown to suppress the surface state on a much larger length scale compared to non-magnetic impurities. These measurements are complemented by STS performed with magnetic tips . Based on our high resolution data, we provide detailed insight into the band structure of SmB6.
EuB6 is a magnetic semimetal for which the formation of polarons at low temperatures has been discussed as the underlying cause of its colossal magnetoresistance. Utilizing STS we visualize this polaron formation . In addition, the analysis of Friedel oscillations around defects at the surface provides some insight into the complex band structure of this material.
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