The superconductivity discovered in iron-pnictides is intimately related to a nematic ground state, where the C4 rotational symmetry is broken via the structural and magnetic transitions. We here study the nematicity in NaFeAs with the polarization dependent angle-resolved photoemission spectroscopy. A uniaxial strain was applied on the sample to overcome the twinning effect in the low temperature C2-symmetric state, and obtain a much simpler electronic structure than that of a twinned sample. We found the electronic structure undergoes an orbital-dependent reconstruction in the nematic state, primarily involving the dxy-and dyz-dominated bands. These bands strongly hybridize with each other, inducing a band splitting, while the dxz-dominated bands only exhibit an energy shift without any reconstruction. These findings suggest that the development of orbitaldependent spin polarization is likely the dominant force to drive the nematicity, while the ferroorbital ordering between dxz and dyz orbitals can only play a minor role here.

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...

Sr 2 IrO 4 was predicted to be a high-temperature superconductor upon electron doping since it highly resembles the cuprates in crystal structure, electronic structure, and magnetic coupling constants. Here, we report a scanning tunneling microscopy/spectroscopy (STM/STS) study of Sr 2 IrO 4 with surface electron doping by depositing potassium (K) atoms. We find that as the electron doping increases, the system gradually evolves from an insulating state to a normal metallic state, via a pseudogaplike phase, and a phase with a sharp, V-shaped low-energy gap with about 95% loss of density of state (DOS) at E F . At certain K coverage (0.5-0.6 monolayer), the magnitude of the low-energy gap is 25-30 meV, and it closes at around 50 K. Our observations show that the electron-doped Sr 2 IrO 4 remarkably resembles hole-doped cuprate superconductors.

Doubly charged sodiated and permethylated linear malto-oligosaccharides ({Glc}6-{Glc}9), branched N-linked glycans (high-mannose type GlcNAc2Man5-9, and complex asialo- and disialylated-biantennary glycans) were analyzed by tandem mass spectrometry using collisionally-activated dissociation (CAD) and "hot" electron capture dissociation (ECD) available in a custom-built ESI FTICR mass spectrometer. For linear permethylated malto-oligosaccharides, both CAD and "hot" ECD produced glycosidic cleavages (B, Y, C, and Z ions), cross-ring cleavages (A- and X-type), and internal cleavages (B/Y and C/Y type) to provide sequence and linkage information. For the branched N-linked glycans, CAD and "hot" ECD provided complementary structural information. CAD generated abundant B and Y fragment ions by glycosidic cleavages, whereas "hot" ECD produced dominant C and Z ions. A-type cross-ring cleavages were present in CAD spectra. Complementary A- and X-type cross-ring fragmentation pairs were generated by "hot" ECD, and these delineated the branching patterns and linkage positions. For example, 0, 4An and 3, 5An ions defined the linkage position of the major branch as the 6-position of the central core mannose residue. The internal fragments observed in CAD were more numerous and abundant than in "hot" ECD spectra. Since the triply charged (sodiated) molecular ion of the permethylated disialylated-biantennary N-linked glycan has relatively high abundance, it was isolated and fragmented in a "hot" ECD experiment and extensive fragment ions (glycosidic and complementary pairs of cross-ring cleavages) were generated to fully confirm the sequence, branching, and linkage assignments for this glycan.