Electrical transport properties of single-crystal CaB6, SrB6, and BaB6

The electrical resistivity and Hall effect of alkaline-earth-metal hexaboride single crystals are measured as a function of temperature, hydrostatic pressure, and magnetic field. The transport properties vary weakly with the external parameters and are modeled in terms of intrinsic variable-valence defects. These defects can stay either in (1) delocalized shallow levels or in (2) localized levels resonant with the conduction band, which can be neutral or negatively charged. Satisfactory agreement is obtained for electronic transport properties in a broad temperature and pressure range, although fitting the magnetoresistance is less straightforward and a combination of various mechanisms is needed to explain the field and temperature dependences.We acknowledge support from Grant No. MAT2012-38213-C02-01 from the Ministerio de Economía y Competividad of Spain. Additional support from Diputacion General de Aragon (DGA-CAMRADS) is also acknowledged. Work at Los Alamos was performed under the auspices of the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Science and Engineering. P.F.S.R. acknowledges a Director’s Postdoctoral Fellowship through the LANL LDRD program. P.S. acknowledges the support by the U.S. Department of Energy (BES) under Grant No. DE-FG02-98ER45707.Peer Reviewe

Magnetic and defect probes of the SmB6_6 surface state

The impact of non-magnetic and magnetic impurities on topological insulators is a central problem concerning their fundamental physics and possible novel spintronics and quantum computing applications. SmB$_6$, predicted to be a topological Kondo insulator, is considered a benchmark material. Using a spin-polarized tip in scanning tunneling spectroscopy destroys the signature peak of the topological surface state, revealing its spin texture. Further, combining local STS with macroscopic transport measurements on SmB$_6$ containing different substitutions enables us to investigate the effect of impurities. The surface states around impurities are locally suppressed with different length scales depending on their magnetic properties and, for sufficiently high impurity level, globally destroyed. Our study points directly to the topological nature of SmB$_6$, and unveils, microscopically and macroscopically, how impurities -- magnetic or non-magnetic -- affect topological surface states

Dynamical Bonding Driving Mixed Valency in a Metal Boride

Samarium hexaboride is an anomaly, having many exotic and seemingly mutually incompatible properties. It was proposed to be a mixed-valent semiconductor, and later - a topological Kondo insulator, and yet has a Fermi surface despite being an insulator. We propose a new and unified understanding of SmB$_6$ centered on the hitherto unrecognized dynamical bonding effect: the coexistence of two Sm-B bonding modes within SmB$_6$, corresponding to different oxidation states of the Sm. The mixed valency arises in SmB$_6$ from thermal population of these distinct minima enabled by motion of B. Our model simultaneously explains the thermal valence fluctuations, appearance of magnetic Fermi surface, excess entropy at low temperatures, pressure-induced phase transitions, and related features in Raman spectra and their unexpected dependence on temperature and boron isotope

Nodeless superconductivity in the noncentrosymmetric ThIrSi compound

The ThIrSi superconductor, with $T_c = 6.5$ K, is expected to show unusual features in view of its noncentrosymmetric structure and the presence of heavy elements featuring a sizable spin-orbit coupling. Here, we report a comprehensive study of its electronic properties by means of local-probe techniques: muon-spin rotation and relaxation ({\textmu}SR) and nuclear magnetic resonance (NMR). Both the superfluid density $\rho_\mathrm{sc}(T)$ (determined via transverse-field {\textmu}SR) and the spin-lattice relaxation rate $T_1^{-1}(T)$ (determined via NMR) suggest a nodeless superconductivity. Furthermore, the absence of spontaneous magnetic fields below $T_c$, as evinced from zero-field {\textmu}SR measurements, indicates a preserved time-reversal symmetry in the superconducting state of ThIrSi. Temperature-dependent upper critical fields as well as field-dependent superconducting muon-spin relaxations suggest the presence of multiple superconducting gaps in ThIrSi.Comment: 8 pages, 8 figure