The concept of a topological Kondo insulator (TKI) has been brought forward as a new class of topological insulators in which non-trivial surface states reside in the bulk Kondo band gap at low temperature due to the strong spin-orbit coupling [1-3].
In contrast to other three-dimensional (3D) topological insulators (e.g. Bi2Se3), a TKI is truly insulating in the bulk [4]. Furthermore, strong electron correlations are present in the system, which may interact with the novel topological phase. Applying spin- and angle-resolved photoemission spectroscopy (SARPES) to the Kondo insulator SmB6, a promising TKI candidate, we reveal that the surface states of SmB6 are spin polarized, and the spin is locked to the crystal momentum. Counter-propagating states (i.e. at k and -k) have opposite spin polarizations protected by time-reversal symmetry. Together with the odd number of Fermi surfaces of surface states between the 4 time-reversal invariant momenta in the surface Brillouin zone [5], these findings prove, for the first time, that SmB6 can host non-trivial topological surface states in a full insulating gap in the bulk stemming from the Kondo effect. Hence our experimental results establish that SmB6 is the first realization of a 3D TKI. It can also serve as an ideal platform for the systematic study of the interplay between novel topological quantum states with emergent effects and competing order induced by strongly correlated electrons.
We report the magnetic and superconducting properties of locally noncentrosymmetric SrPtAs obtained by muon-spin-rotation/relaxation (muSR) measurements. Zero-field muSR reveals the occurrence of small spontaneous static magnetic fields with the onse
t of superconductivity. This finding suggests that the superconducting state of SrPtAs breaks time-reversal symmetry. The superfluid density as determined by transverse field muSR is nearly flat approaching T = 0 K proving the absence of extended nodes in the gap function. By symmetry, several superconducting states supporting time-reversal symmetry breaking in SrPtAs are allowed. Out of these, a dominantly d + id (chiral d-wave) order parameter is most consistent with our experimental data.
In a recent Letter Scanlon and Watson reported their first principles results on hydrogen in Cu2O. Their main conclusions are: (1) an interstitial H in Cu2O prefers to occupy the tetrahedral site, which is coordinated with four Cu cations, in all thr
ee charge states (+1, neutral, and -1); (2) H will bind with a Cu vacancy and form an electrically active H-VCu defect complex, which is amphoteric with (+/0) and (0/-)transition levels at Ev + 0.1 and Ev + 1.1 eV, respectively. However, these two conclusions contradict two generally observed behaviors of H in oxides: (i) cationic H usually binds with an O atom, forming a single O-H bond, while the anionic H usually binds with cations with multi-coordination; (ii) H usually passivates cation vacancies in oxides. In this Comment, we explicitly show that with charge state +1, H prefers to bind with a single O anion rather than with four Cu cations and that H-VCu does not induce any defect levels inside the band gap.
