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Register a new user4$f$ crystal field ground state of the strongly correlated topological insulator SmB$_6$
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We investigated the crystal-electric field ground state of the 4$f$ manifold in the strongly correlated topological insulator SmB$_6$ using core level non-resonant inelastic x-ray scattering (NIXS). The directional dependence of the scattering function that arises from higher multipole transitions establishes unambiguously that the $Gamma_8$ quartet state of the Sm $f^5$ $J$=$5/2$ configuration governs the ground-state symmetry and hence the topological properties of SmB$_6$. Our findings contradict the results of density functional calculations reported so far.
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Several technical issues and challenges are identified and investigated for the planar tunneling spectroscopy of the topological Kondo insulator SmB$_6$. Contrasting behaviors of the tunnel junctions prepared in two different ways are analyzed and explained in detail. The conventional approach based on an AlO$_text{x}$ tunnel barrier results in unsatisfactory results due to the inter-diffusion between SmB$_6$ and deposited Al. On the contrary, plasma oxidation of SmB$_6$ 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 {it 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 SmB$_6$ and also other correlated topological materials are remarked.
Samarium hexaboride is a topological Kondo insulator, with metallic surface states manifesting from its insulating band structure. Since the insulating bulk itself is driven by strong correlations, both the bulk and surface host compelling magnetic and electronic phenomena. We employed X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) at the Sm M$_{4,5}$ edges to measure surface and bulk magnetic properties of Sm$^{2+}$ and Sm$^{3+}$ within SmB$_6$. We observed anti-alignment to the applied field of the Sm$^{3+}$ magnetic dipole moment below $T = 75$ K and of the total orbital moment of samarium below 30 K. The induced Sm$^{3+}$ moment at the cleaved surface at 8 K and 6 T implies 1.5% of the total Sm as magnetized Sm$^{3+}$. The field dependence of the Sm$^{3+}$ XMCD dichorism at 8 K is diamagnetic and approximately linear. The bulk magnetization at 2 K is however driven by Sm$^{2+}$ Van Vleck susceptibility as well as 1% paramagnetic impurities with $mu_{rm Eff} = 5.2(1)~mu_{rm B}$. This indicates diamagnetic Sm$^{3+}$ is compensated within the bulk. The XAS and XMCD spectra are weakly affected by Sm vacancies and carbon doping while XAS is strongly affected by polishing.
We have succeeded in establishing the crystal-field ground state of CeRu2Al10, an orthorhombic intermetallic compound recently identified as a Kondo insulator. Using polarization dependent soft x-ray absorption spectroscopy at the Ce M4,5 edges, together with input from inelastic neutron and magnetic susceptibility experiments, we were able to determine unambiguously the orbital occupation of the 4f shell and to explain quantitatively both the measured magnetic moment along the easy a axis and the small ordered moment along the c-axis. The results provide not only a platform for a realistic modeling of the spin and charge gap of CeRu2Al10, but demonstrate also the potential of soft x-ray absorption spectroscopy to obtain information not easily accessible by neutron techniques for the study of Kondo insulators in general.
The temperature and thickness dependencies of the in-plane anisotropic magnetoresistance (AMR) of SmB$_6$ thin films are reported. We find that the AMR changes sign from negative ($rho_{||}<rho_{perp}$) at high temperatures to positive ($rho_{||}>rho_{perp}$) at low temperatures. The temperature, T$_s$, at which this sign change occurs, decreases with increasing film thickness $t$ and T$_s$ vanishes for $t$ $>$ 30 nm. We interpret our results in the framework of a competition between two components: a negative bulk contribution and a positive surface AMR.
The true topological nature of the Kondo insulator SmB$_6$ remains to be unveiled. Our previous tunneling study not only found evidence for the existence of surface Dirac fermions, but it also uncovered that they inherently interact with the spin excitons, collective excitations in the bulk. We have extended such a spectroscopic investigation into crystals containing a Sm deficiency. The bulk hybridization gap is found to be insensitive to the deficiency up to 1% studied here, but the surface states in Sm-deficient crystals exhibit quite different temperature evolutions from those in stoichiometric ones. We attribute this to the topological surface states remaining incoherent down to the lowest measurement temperature due to their continued interaction with the spin excitons that remain uncondensed. This result shows that the detailed topological nature of SmB$_6$ could vary drastically in the presence of disorder in the lattice. This sensitiveness to disorder is seemingly contradictory to the celebrated topological protection, but it can be understood as being due to the intimate interplay between strong correlations and topological effects.
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