Single-layer FeSe films with extremely expanded in-plane lattice constant of 3.99±0.02 Å are fabricated by epitaxially growing FeSe/Nb:SrTiO 3 /KTaO 3 heterostructures, and studied by in situ angle-resolved photoemission spectroscopy. Two elliptical electron pockets at the Brillion zone corner are resolved with negligible hybridization between them, indicating the symmetry of the low energy electronic structure remains intact as a free-standing single-layer FeSe, although it is on a substrate. The superconducting gap closes at a record high temperature of 70 K for the iron based superconductors. Intriguingly, the superconducting gap distribution is anisotropic but nodeless around the electron pockets, with minima at the crossings of the two pockets. Our results put strong constraints on the current theories, and support the coexistence of both even and odd parity spin-singlet pairing channels as classified by the lattice symmetry. [3][4][5]. For these systems, weak coupling theories based on spin-fluctuations predict a dwave pairing symmetry [6,7]. However, it is inconsistent with the isotropic superconducting gap observed by angle resolved photoemission spectroscopy (ARPES) [2,3,8,9], together with evidences for nodeless superconducting gap from specific heat [10], nuclear magnetic resonance [11], etc. On the other hand, the sign preserving s-wave pairing symmetry [12][13][14][15] could not account for the spin-resonance mode found in Rb x Fe 2−y Se 2 by inelastic neutron scattering [16], which suggests the sign change of the superconducting order parameter on different Fermi surface sections [17].To explain the sign changing isotropic gap in e-FeHTSs, several novel pairing scenarios were proposed. For example, it is argued in the bonding-antibonding s ± pairing scenario that with strong hybridization between electron pockets, the two reconstructed electron pockets can have different signs [18]. A further study suggested that this pairing likely coexists with the d-wave to form an s + id-wave pairing symmetry [19]. More recently, the importance of the parity of the 2-Fe unit cell has been emphasized [20], and it has been proposed that there are even and odd parity s-wave spin singlet pairing states, and the coexistence of both states gives a fully gapped state with varied signs in different Fermi surface sections [21,22]. The hybridization between the two electron pockets is not necessary in this scenario. So far, these scenarios could not be convincingly tested, since the detailed structure of the two electron pockets could not be resolved in all known e-FeHTSs.Two recent ARPES studies have found a gap in singlelayer FeSe/STO, which closes at 65 K and suggests a possible record high superconducting transition temperature (T c ) of 65 K for FeHTSs [4,5]; or at least, it is the pair-formation temperature record, if the superconducting transition there is a two dimensional Berezinskii-Kosterlitz-Thouless (BKT) type. Particularly, our previous ARPES study has found that the high T c in single-layer FeSe/STO is in...
ASnO 3 (A ¼ Ba, Sr) are unique perovskite oxides in that they have superior electron conductivity despite their wide optical band gaps. Using first-principles band structure calculations, we show that the small electron effective masses, thus, good electron conductivity of ASnO 3 can be attributed to the large size of Sn in this system that gives the conduction band edge with antibonding Sn and Os characters. Moreover, we show that ASnO 3 can be easily doped by La with shallow La A (þ/0) donor level. Our results, therefore, explain why the perovskite BaSnO 3 , SrSnO 3 , and their alloys are promising candidates for transparent conducting oxides. V C 2013 American Institute of Physics.
Recently, the concept of topological insulators has been generalized to topological semimetals, including three-dimensional (3D) Weyl semimetals, 3D Dirac semimetals, and 3D node-line semimetals. In particular, several compounds (e.g., certain three-dimensional graphene networks, Cu3PdN, Ca3P2) were discovered to be 3D node-line semimetals, in which the conduction and the valence bands cross at closed lines in the Brillouin zone. Except for the two-dimensional (2D) Dirac semimetal (e.g., in graphene), 2D topological semimetals are much less investigated. Here, we propose the new concept of a 2D node-line semimetal and suggest that this state could be realized in a new mixed lattice (we name it as HK lattice) composed by kagome and honeycomb lattices. We find that A3B2 (A is a group-IIB cation and B is a group-VA anion) compounds (such as Hg3As2) with the HK lattice are 2D node-line semimetals due to the band inversion between cation s orbital and anion pz orbital. In the presence of buckling or spin-orbit coupling, the 2D node-line semimetal state may turn into 2D Dirac semimetal state or 2D topological crystalline insulating state.
Optical measurements indicate an unconventional superconducting phase in the Bi/Ni bilayer that breaks time-reversal symmetry.
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