Topological Kondo insulators have been proposed 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 strong spin–orbit coupling. In contrast to other three-dimensional topological insulators, a topological Kondo insulator is truly bulk insulating. Furthermore, strong electron correlations are present in the system, which may interact with the novel topological phase. By applying spin- and angle-resolved photoemission spectroscopy, here we show that the surface states of SmB₆ are spin polarized. The spin is locked to the crystal momentum, fulfilling time reversal and crystal symmetries. Our results provide strong evidence that SmB₆ can host topological surface states in a bulk insulating gap stemming from the Kondo effect, which can serve as an ideal platform for investigating of the interplay between novel topological quantum states with emergent effects and competing orders induced by strongly correlated electrons.

Recent theoretical calculations and experimental results suggest that the strongly correlated material SmB${}_6$ may be a realization of a topological Kondo insulator. We have performed an angle-resolved photoemission spectroscopy study on SmB${}_6$ in order to elucidate elements of the electronic structure relevant to the possible occurrence of a topological Kondo insulator state. The obtained electronic structure in the whole three-dimensional momentum space reveals one electron-like $5d$ bulk band centered at the $X$ point of the bulk Brillouin zone that is hybridized with strongly correlated $f$ electrons, as well as the opening of a Kondo band gap ($\Delta$ ${}_B$ $\sim$ 20 meV) at low temperature. In addition, we observe electron-like bands forming three Fermi surfaces at the center $\overline{\Gamma }$ point and boundary $\overline{X}$ point of the surface Brillouin zone. These bands are not expected from calculations of the bulk electronic structure, and their observed dispersion characteristics are consistent with surface states. Our results suggest that the unusual low-temperature transport behavior of SmB${}_6$ is likely to be related to the pronounced surface states sitting inside the band hybridization gap and/or the presence of a topological Kondo insulating state.

We used angle-resolved photoemission spectroscopy and thermoelectric power to study the poorly explored, highly overdoped side of the phase diagram of Ba(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$ high-temperature superconductor. Our data demonstrate that several Lifshitz transitions—topological changes of the Fermi surface—occur for large $x$. The central hole barrel changes to ellipsoids that are centered at $Z$ at $x\sim 0.11$ and subsequently disappear around $x\sim 0.2$; changes in thermoelectric power occur at similar $x$ values. $T_c$ decreases and goes to zero around $x\sim 0.15$—between the two Lifshitz transitions. Beyond $x=0.2$ the central pocket becomes electron-like and superconductivity does not exist. Our observations reveal the importance of the underlying Fermiology in electron-doped iron arsenides. We speculate that a likely necessary condition for superconductivity in these materials is the presence of the central hole pockets rather than nesting between central and corner pockets.

The compound ${\mathrm{BaMn}}_2{\mathrm{As}}_2$ with the tetragonal ${\mathrm{ThCr}}_2{\mathrm{Si}}_2$ structure is a local-moment antiferromagnetic insulator with a Néel temperature $T_{\mathrm{N}}=625\mathrm{K}$ and a large ordered moment $\mu =3.9{\mu }_{\mathrm{B}}/\mathrm{Mn}$. We demonstrate that this compound can be driven metallic by partial substitution of Ba by K while retaining the same crystal and antiferromagnetic structures together with nearly the same high $T_{\mathrm{N}}$ and large $\mu$. ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{Mn}}_2{\mathrm{As}}_2$ is thus the first metallic ${\mathrm{ThCr}}_2{\mathrm{Si}}_2$-type $M\mathrm{As}$-based system containing local $3d$ transition metal $M$ magnetic moments, with consequences for the ongoing debate about the local-moment versus itinerant pictures of the FeAs-based superconductors and parent compounds. The ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{Mn}}_2{\mathrm{As}}_2$ class of compounds also forms a bridge between the layered iron pnictides and cuprates and may be useful to test theories of high $T_{\mathrm{c}}$ superconductivity.

A facile, efficient and environmentally-friendly protocol for the synthesis of xanthenes by graphene oxide based nanocomposite (GO-CuFe₂O₄) has been developed by one-pot condensation route. The nanocomposite was designed by decorating copper ferrite nanoparticles on graphene oxide (GO) surface via a solution combustion route without the use of template. The as-synthesized GO-CuFe₂O₄ composite was comprehensively characterized by XRD, FTIR, Raman, SEM, EDX, HRTEM with EDS mapping, XPS, N₂ adsorption-desorption and ICP-OES techniques. This nanocomposite was then used in an operationally simple, cost effective, efficient and environmentally benign synthesis of 14H-dibenzo xanthene under solvent free condition. The present approach offers several advantages such as short reaction times, high yields, easy purification, a cleaner reaction, ease of recovery and reusability of the catalyst by a magnetic field. Based upon various controlled reaction results, a possible mechanism for xanthene synthesis over GO-CuFe₂O₄ catalyst was proposed. The superior catalytic activity of the GO-CuFe₂O₄ nanocomposite can be attributed to the synergistic interaction between GO and CuFe₂O₄ nanoparticles, high surface area and presence of small sized CuFe₂O₄ NPs. This versatile GO-CuFe₂O₄ nanocomposite synthesized via combustion method holds great promise for applications in wide range of industrially important catalytic reactions.