No abstract

We report on the synthesis of large single crystals of a new FeSe layer superconductor Cs(0.8)(FeSe(0.98))(2). X-ray powder diffraction, neutron powder diffraction and magnetization measurements have been used to compare the crystal structure and the magnetic properties of Cs(0.8)(FeSe(0.98))(2) with those of the recently discovered potassium intercalated system K(x)Fe(2)Se(2). The new compound, Cs(0.8)(FeSe(0.98))(2), shows a slightly lower superconducting transition temperature (T(c) = 27.4 K) in comparison to 29.5 in (K(0.8)(FeSe(0.98))(2)). The volume of the crystal unit cell increases by replacing K by Cs-the c parameter grows from 14.1353(13) to 15.2846(11) Å. For the alkali metal intercalated layered compounds known so far, (K(0.8)Fe(2)Se(2) and Cs(0.8)(FeSe(0.98))(2)), the T(c) dependence on the anion height (distance between Fe layers and Se layers) was found to be analogous to those reported for As-containing Fe superconductors and Fe(Se(1 - x)Ch(x)), where Ch = Te, S.

A Weyl semimetal possesses spin-polarized band-crossings, called Weyl nodes, connected by topological surface arcs. The low-energy excitations near the crossing points behave the same as massless Weyl fermions, leading to exotic properties like chiral anomaly. To have the transport properties dominated by Weyl fermions, Weyl nodes need to locate nearly at the chemical potential and enclosed by pairs of individual Fermi surfaces with non-zero Fermi Chern numbers. Combining angle-resolved photoemission spectroscopy and first-principles calculation, here we show that TaP is a Weyl semimetal with only a single type of Weyl fermions, topologically distinguished from TaAs where two types of Weyl fermions contribute to the low-energy physical properties. The simple Weyl fermions in TaP are not only of fundamental interests but also of great potential for future applications. Fermi arcs on the Ta-terminated surface are observed, which appear in a different pattern from that on the As-termination in TaAs and NbAs.

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 SmB 6 are spin polarized. The spin is locked to the crystal momentum, fulfilling time reversal and crystal symmetries. Our results provide strong evidence that SmB 6 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.

We report on a detailed investigation of the electronic phase diagram of FeSe1-x under pressures up to 1.4 GPa by means of ac magnetization and muon-spin rotation. At a pressure 0.8 GPa the nonmagnetic and superconducting FeSe1-x enters a region where static magnetic order is realized above Tc and bulk superconductivity coexists and competes on short length scales with the magnetic order below Tc. For even higher pressures an enhancement of both the magnetic and the superconducting transition temperatures as well as of the corresponding order parameters is observed. These exceptional properties make FeSe1-x to be one of the most interesting superconducting systems investigated extensively at present.