Pairing symmetry is a fundamental property that characterizes a superconductor. For the iron-based high-temperature superconductors, an s(±)-wave pairing symmetry has received increasing experimental and theoretical support. More specifically, the superconducting order parameter is an isotropic s-wave type around a particular Fermi surface, but it has opposite signs between the hole Fermi surfaces at the zone centre and the electron Fermi surfaces at the zone corners. Here we report the low-energy electronic structure of the newly discovered superconductors, A(x)Fe(2)Se(2) (A=K,Cs) with a superconducting transition temperature (Tc) of about 30 K. We found A(x)Fe(2)Se(2) (A=K,Cs) is the most heavily electron-doped among all iron-based superconductors. Large electron Fermi surfaces are observed around the zone corners, with an almost isotropic superconducting gap of ~10.3 meV, whereas there is no hole Fermi surface near the zone centre, which demonstrates that interband scattering or Fermi surface nesting is not a necessary ingredient for the unconventional superconductivity in iron-based superconductors. Thus, the sign change in the s(±) pairing symmetry driven by the interband scattering as suggested in many weak coupling theories becomes conceptually irrelevant in describing the superconducting state here. A more conventional s-wave pairing is probably a better description.

Strong spin–orbit coupling fosters exotic electronic states such as topological insulators and superconductors, but the combination of strong spin–orbit and strong electron–electron interactions is just beginning to be understood. Central to this emerging area are the 5d transition metal iridium oxides. Here, in the pyrochlore iridate Pr2Ir2O7, we identify a non-trivial state with a single-point Fermi node protected by cubic and time-reversal symmetries, using a combination of angle-resolved photoemission spectroscopy and first-principles calculations. Owing to its quadratic dispersion, the unique coincidence of four degenerate states at the Fermi energy, and strong Coulomb interactions, non-Fermi liquid behaviour is predicted, for which we observe some evidence. Our discovery implies that Pr2Ir2O7 is a parent state that can be manipulated to produce other strongly correlated topological phases, such as topological Mott insulator, Weyl semimetal, and quantum spin and anomalous Hall states.

Inducing magnetism into topological insulators is intriguing for utilizing exotic phenomena such as the quantum anomalous Hall effect (QAHE) for technological applications. While most studies have focused on doping magnetic impurities to open a gap at the surface-state Dirac point, many undesirable effects have been reported to appear in some cases that makes it difficult to determine whether the gap opening is due to the time-reversal symmetry breaking or not. Furthermore, the realization of the QAHE has been limited to low temperatures. Here we have succeeded in generating a massive Dirac cone in a MnBi2Se4/Bi2Se3 heterostructure, which was fabricated by self-assembling a MnBi2Se4 layer on top of the Bi2Se3 surface as a result of the codeposition of Mn and Se. Our experimental results, supported by relativistic ab initio calculations, demonstrate that the fabricated MnBi2Se4/Bi2Se3 heterostructure shows ferromagnetism up to room temperature and a clear Dirac cone gap opening of ∼100 meV without any other significant changes in the rest of the band structure. It can be considered as a result of the direct interaction of the surface Dirac cone and the magnetic layer rather than a magnetic proximity effect. This spontaneously formed self-assembled heterostructure with a massive Dirac spectrum, characterized by a nontrivial Chern number C = −1, has a potential to realize the QAHE at significantly higher temperatures than reported up to now and can serve as a platform for developing future “topotronics” devices.

We have reexamined the valence-band (VB) and core-level electronic structure of NiO by means of hard and soft x-ray photoemission spectroscopies. The spectral weight of the lowest energy state was found to be enhanced in the bulk sensitive Ni 2p core-level spectrum. A configuration-interaction model including a bound state screening has shown agreement with the core-level spectrum and off- and on-resonance VB spectra. These results identify the lowest energy states in the core-level and VB spectra as the Zhang-Rice (ZR) doublet bound states, consistent with the spin-fermion model and recent ab initio calculations within dynamical mean-field theory. The results indicate that the ZR character first ionization (the lowest hole-addition) states are responsible for transport properties in NiO and doped NiO.

Both electrical conductivity σ and Seebeck coefficient S are functions of carrier concentration being correlated with each other, and the value of power factor S2σ is generally limited to less than 0.01 W m−1 K−2. Here we report that, under the temperature gradient applied simultaneously to both parallel and perpendicular directions of measurement, a metallic copper selenide, Cu2Se, shows two sign reversals and colossal values of S exceeding ±2 mV K−1 in a narrow temperature range, 340 K < T < 400 K, where a structure phase transition takes place. The metallic behavior of σ possessing larger magnitude exceeding 600 S cm−1 leads to a colossal value of S2σ = 2.3 W m–1 K–2. The small thermal conductivity less than 2 W m−1 K−1 results in a huge dimensionless figure of merit exceeding 400. This unusual behavior is brought about by the self-tuning carrier concentration effect in the low-temperature phase assisted by the high-temperature phase.

We report on the electronic structure and Fermi surfaces of the transition-metal dichalcogenide 1T-VS 2 in the low-temperature charge-density-wave ͑CDW͒ ordered phase. Using soft x-ray angle-resolved photoemission spectroscopy ͑ARPES͒, we investigate the in-plane and out-of-plane vanadium-and sulfur-derived band dispersions and identify k z dispersions in this layered system. Core-level photoemission and x-ray absorption spectroscopy show that vanadium electrons are in the d 1 configuration while 2p-3d resonant ARPES shows only 3d-derived dispersive bands near the Fermi level. Comparison of energy-and angle-dependent data with band-structure calculations reveals renormalization of the 3d bands, but no lower Hubbard band, a signature of the rather weak electron-electron correlations in VS 2 . High-resolution temperature-dependent low-energy ARPES measurements show the opening of an energy gap at the Fermi level that is attributed to the condensation of the CDW phase. The results indicate a CDW transition in the absence of nesting for 1T-VS 2

Electronic structures of the quantum critical superconductor β-YbAlB4 and its polymorph α-YbAlB4 are investigated by using bulk-sensitive hard x-ray photoemission spectroscopy. From the Yb 3d core level spectra, the values of the Yb valence are estimated to be ∼2.73 and ∼2.75 for α- and β-YbAlB4, respectively, thus providing clear evidence for valence fluctuations. The valence band spectra of these compounds also show Yb2+ peaks at the Fermi level. These observations establish an unambiguous case of a strong mixed valence at quantum criticality for the first time among heavy fermion systems, calling for a novel scheme for a quantum critical model beyond the conventional Doniach picture in β-YbAlB4.

Time-resolved hard x-ray photoelectron spectroscopy (trHAXPES) is established using the x-ray free-electron laser SACLA. The technique extends timeresolved photoemission into the hard x-ray regime and, as a core-level spectroscopy, combines element and atomic-site specificity and sensitivity to the chemical environment with femtosecond time resolution and bulk (sub-surface) sensitivity. The viability of trHAXPES using 8 keV x-ray free-electron-laser radiation is demonstrated by a systematic investigation of probe and pump pulseinduced vacuum space-charge effects on the V 1s emission of VO 2 and the Ti 1s emission of SrTiO 3 . The time and excitation energy dependencies of the measured spectral shifts and broadenings are compared to the results of N-body numerical simulations and simple analytic (mean-field) models. Good agreement between the experimental and calculated results is obtained. In particular, the 9 Present address: characteristic temporal evolution of the pump pulse-induced spectral shift is shown to provide an effective means to determine the temporal overlap of pump and probe pulses. trHAXPES opens a new avenue in the study of ultrafast atomic-site specific electron and chemical dynamics in materials and at buried interfaces.Keywords: time-resolved photoelectron spectroscopy, x-ray free-electron laser, space-charge effects IntroductionSub-picosecond time-resolved solid-state photoemission spectroscopy has recently emerged as a powerful novel technique for studying the electronic properties of condensed matter. The power of the technique is that it provides direct access to the electronic structure dynamics in materials and at their surfaces on the time scales of the underlying elementary electronic and lattice processes, such as electron-electron scattering, electron screening and thermalization, coherent phonon vibrations, electron-phonon and phonon-phonon coupling, as well as substrate-adsorbate charge transfer or the buildup of surface photovoltages. Time-resolved photoemission spectroscopy generally combines frequency-domain information with subpicosecond time resolution through a pump-probe scheme in which typically an infrared (IR) pump pulse is used to excite the system whose dynamics is then probed at different time delays by detecting the photoelectrons emitted by ultrashort pulses in the ultraviolet (UV) to soft x-ray regime. When probe pulses in the UV to extreme ultraviolet (XUV) range are used and angular resolution is added, the technique is referred to as time-and angle-resolved photoemission spectroscopy (trARPES) and provides direct information on the momentum-resolved dynamics of valence electrons, including the temporal evolution of electronic populations, band structures, Fermi surfaces, and energy gaps [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19]. When the probe pulses have higher photon energies, in the extreme ultraviolet to soft x-ray range, the technique becomes time-resolved x-ray photoemission spectroscopy (trXPS), with element specificity, sensitivi...