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(Ga,Fe)Sb is a promising ferromagnetic semiconductor for practical spintronic device applications because its Curie temperature $({T}_{\mathrm{C}})$ is above room temperature. However, the origin of ferromagnetism with high ${T}_{\mathrm{C}}$ remains to be elucidated. Here, we use soft x-ray angle-resolved photoemission spectroscopy (SX-ARPES) to investigate the valence-band (VB) structure of $(\mathrm{G}{\mathrm{a}}_{0.95},\mathrm{F}{\mathrm{e}}_{0.05})\mathrm{Sb}$ including the $\mathrm{Fe}\text{\ensuremath{-}}3d$ impurity band (IB), to unveil the mechanism of ferromagnetism in (Ga,Fe)Sb. We find that the VB dispersion in $(\mathrm{G}{\mathrm{a}}_{0.95},\mathrm{F}{\mathrm{e}}_{0.05})\mathrm{Sb}$ observed by SX-ARPES is similar to that of GaSb, indicating that the doped Fe atoms hardly affect the band dispersion. The $\mathrm{Fe}\text{\ensuremath{-}}3d$ resonant ARPES spectra demonstrate that the $\mathrm{Fe}\text{\ensuremath{-}}3d$ IB crosses the Fermi level $({E}_{\mathrm{F}})$ and hybridizes with the VB of GaSb. These observations indicate that the VB structure of $(\mathrm{G}{\mathrm{a}}_{0.95},\mathrm{F}{\mathrm{e}}_{0.05})\mathrm{Sb}$ is consistent with that of the IB model which is based on double-exchange interaction between the localized $3d$ electrons of the magnetic impurities.

The low energy electronic structure of LaFeAsO1-xHx (0.0 < x < 0.60), the system which exhibits two superconducting domes in its phase diagram, is investigated by utilizing the laser photoemission spectroscopy. From the precise temperature-dependent measurement of the spectra near the Fermi level, we find the suppression of the density of states with cooling, namely the pseudogap formation, for all doping range. The pseudogap in the low x range (i.e. the first superconducting dome regime) gets suppressed with increasing x, more or less similarly to the previous results in F-doped LaFeAsO system. On the other hand, the pseudogap behavior in the second superconducting dome regime at high-x becomes stronger with increasing the H-doping level. The systematic doping dependence shows that the pseudogap is enhanced toward the both ends of the phase diagram where the different types of antiferromagnetic order exist., 8 pages, 6 figures

The relationship between the superconducting gap and the pseudogap has been the subject of controversies. In order to clarify this issue, we have studied the superconducting gap and pseudogap of the $\text{high}\ensuremath{-}{T}_{c}$ superconductor ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$ ($x=0.10,14$) by angle-resolved photoemission spectroscopy (ARPES). Through the analysis of the ARPES spectra above and below ${T}_{c}$, we have identified a superconducting coherence peak even in the antinodal region on top of the pseudogap of a larger energy scale. The superconducting peak energy nearly follows the pure $d$-wave form. The $d$-wave order parameter ${\mathrm{\ensuremath{\Delta}}}_{0}$ [defined by $\mathrm{\ensuremath{\Delta}}(k)={\mathrm{\ensuremath{\Delta}}}_{0}(cos{k}_{x}a\ensuremath{-}cos{k}_{y}a)$ ] for $x=0.10$ and 0.14 are nearly the same, ${\mathrm{\ensuremath{\Delta}}}_{0}\ensuremath{\sim}12--14\phantom{\rule{0.28em}{0ex}}\mathrm{meV}$, leading to strong coupling $2{\mathrm{\ensuremath{\Delta}}}_{0}/{k}_{B}{T}_{c}\ensuremath{\sim}10$. The present result indicates that the pseudogap and the superconducting gap are distinct phenomena and can be described by the ``two-gap'' scenario.

We investigate the electronic reconstruction across the tetragonal-orthorhombic structural transition in FeSe by employing polarization-dependent angle-resolved photoemission spectroscopy on detwinned single crystals. Across the structural transition, the electronic structures around the $\mathrm{\ensuremath{\Gamma}}$ and $M$ points are modified from fourfold to twofold symmetry due to the lifting of degeneracy in ${d}_{xz}/{d}_{yz}$ orbitals. The ${d}_{xz}$ band shifts upward at the $\mathrm{\ensuremath{\Gamma}}$ point, while it moves downward at the $M$ point, suggesting that the electronic structure of orthorhombic FeSe is characterized by a momentum-dependent sign-changing orbital polarization. Due to this sign-changing orbital order, the elongated directions of the elliptical Fermi surfaces at the $\mathrm{\ensuremath{\Gamma}}$ and $M$ points are rotated by 90\ifmmode^\circ\else\textdegree\fi{} with respect to each other, which makes the nesting condition between these FSs imperfect. The present result, supported by calculations, indicates the possible suppression of the spin-fluctuation mediated superconductivity in the orthorhombic FeSe, as compared to the orbital-ordered state without sign change.

We study superconducting FeSe (Tc = 9 K) exhibiting the tetragonal-orthorhombic structural transition (Ts = 90 K) without any antiferromagnetic ordering, by utilizing angle-resolved photoemission spectroscopy. In the detwinned orthorhombic state, the energy position of the dyz orbital band at the Brillouin zone corner is 50 meV higher than that of dxz, indicating the orbital order similar to NaFeAs and BaFe2As2 families. Evidence of orbital order also appears in the hole bands at the Brillouin zone center. Precisely measured temperature dependence using strain-free samples shows that the onset of the orbital ordering (To) occurs very close to Ts, thus suggesting that the electronic nematicity above Ts is considerably weaker in FeSe compared to BaFe2As2 family., Comment: 6 pages, 4 figures

Using time-resolved photoemission spectroscopy, we have investigated the nonequilibrium electronic structures of a ${\mathrm{VO}}_{2}$ thin film upon photoexcitation. We employed the high-harmonic generation method, which was crucial in obtaining the results. Irradiation by a 170-fs optical pulse at a fluence above $\ensuremath{\sim}$6 mJ/cm${}^{2}$ rapidly converts the insulating ${\mathrm{VO}}_{2}$ thin film into a metallic state, and we show that the transition is accompanied by a spectral-weight redistribution over a 1-eV scale. This observation provides direct spectroscopic evidence of an ultrafast insulator-to-metal transition. The transient metallic state has a unique spectrum that deviates from the static spectrum of the rutile metal phase. We also observe an anomalous spreading of spectral weight up to $\ensuremath{\sim}$0.4 eV above the Fermi level as soon as the transient metallic state emerges. The temporal evolution of the spectral weight near the Fermi level exhibits an ultrafast increase and a subsequent slower increase over $\ensuremath{\sim}$3 ps. We suggest that a broadening mechanism related to the excitation of phonon modes is responsible for the spreading of the spectral weight and that the slower increase of the spectral weight is associated with the expansion of the metallic region after photoinduced nucleation. In addition, the initial nucleation of the metallic state appears to be spatially inhomogeneous even near the surface. The results of this study highlight the uniqueness of the nonequilibrium metallic state in comparison to the equilibrium rutile metallic state from an electronic-state perspective.

The nature of the pseudogap in high transition temperature (high-Tc) superconducting cuprates has been a major issue in condensed matter physics. It is still unclear whether the high-Tc superconductivity can be universally associated with the pseudogap formation. Here we provide direct evidence of the existence of the pseudogap phase via angle-resolved photoemission spectroscopy in another family of high-Tc superconductor, iron-pnictides. Our results reveal a composition dependent pseudogap formation in the multi-band electronic structure of BaFe2(As1-xPx)2. The pseudogap develops well above the magnetostructural transition for low x, persists above the nonmagnetic superconducting dome for optimal x and is destroyed for x ~ 0.6, thus showing a notable similarity with cuprates. In addition, the pseudogap formation is accompanied by inequivalent energy shifts in xz/yz orbitals of iron atoms, indicative of a peculiar iron orbital ordering which breaks the four-fold rotational symmetry., Comment: 14 pages, 10 figures

In Fe-based superconductors, electron doping is often realized by the substitution of transition-metal atoms for Fe. In order to investigate how the electronic structure of the parent compound is influenced by Zn substitution, which supplies nominally four extra electrons per substituted atom but is expected to induce the strongest impurity potential among the transition-metal atoms, we have performed an angle-resolved photoemission spectroscopy measurement on Ba(Fe${}_{1\ensuremath{-}x}$Zn${}_{x}$)${}_{2}$As${}_{2}$ (Zn-122). In Zn-122, the temperature dependence of the resistivity shows a kink around $T\ensuremath{\sim}$ 135 K, indicating antiferromagnetic order below the N$\stackrel{\ifmmode \acute{}\else \'{}\fi{}}{\mathrm{e}}$el temperature of ${T}_{\mathrm{N}}\ensuremath{\sim}$ 135 K. In fact, folded Fermi surfaces (FSs) similar to those of the parent compound have been observed below ${T}_{\mathrm{N}}$. The hole and electron FS volumes are, therefore, different from those expected from the rigid-band model. The results can be understood if all the extra electrons occupy the Zn 3$d$ state $\ensuremath{\sim}$10 eV below the Fermi level and do not participate in the formation of the FSs.

Charge dynamics and its critical behavior are investigated near the metal-insulator transition of layered-nickelate ${R}_{2\ensuremath{-}x}$Sr${}_{x}$NiO${}_{4}$ ($R=\mathrm{Nd}$, Eu). The polarized x-ray absorption spectroscopy experiment clearly shows the multiorbital nature which enables the ${x}^{2}\ensuremath{-}{y}^{2}$-orbital-based checkerboard-type charge ordering or correlation to persist up to the critical doping region ($x\ensuremath{\sim}1$). In the barely metallic region proximate to the charge-ordered insulating phase, the nominal carrier density estimated from the Hall coefficient markedly decreases in accord with development of the pseudogap structure in the optical conductivity spectrum, while the effective mass is least enhanced. The present findings combined with the results of recent angle-resolved photoemission spectroscopy show that the pseudogap in the metal-insulator critical state evolves due to the checkerboard-type charge correlation to extinguish the coherent-motion carriers in a characteristic momentum (k)-dependent manner with lowering temperature.

The three-dimensional Fermi-surface structure of hole-doped metallic layered nickelate Eu${}_{2\ensuremath{-}x}$Sr${}_{x}$NiO${}_{4}$ ($x=1.1$), an important counterpart to the isostructural superconducting cuprate La${}_{2\ensuremath{-}x}$Sr${}_{x}$CuO${}_{4}$, is investigated by energy-dependent soft-x-ray angle-resolved photoemission spectroscopy. In addition to a large cylindrical hole Fermi surface analogous to the cuprates, we observe a $\ensuremath{\Gamma}$-centered $3{z}^{2}\ensuremath{-}{r}^{2}$-derived small electron pocket. This finding demonstrates that in the layered nickelate the $3{z}^{2}\ensuremath{-}{r}^{2}$ band resides close to the ${x}^{2}\ensuremath{-}{y}^{2}$ one in energy. The resultant multiband feature with varying orbital character as revealed may strongly work against the emergence of the high-temperature superconductivity.

Time-resolved photoelectron spectroscopy (trPES) can directly detect transient electronic structure, thus bringing out its promising potential to clarify nonequilibrium processes arising in condensed matters. Here we report the result of core-level trPES on 1$T$-TaS${}_{2}$, realized by developing a high-intensity 60-eV laser obtained by high-order harmonic generation. Ta$4f$ core-level trPES offers the transient amplitude of the charge-density wave (CDW), via the site-selective and real-time observation of Ta electrons. The present result indicates an ultrafast photoinduced melting and recovery of CDW amplitude, followed by a peculiar long-life oscillation (i.e., collective amplitudon excitation) accompanying the transfer of electrons among adjacent Ta atoms. core-level trPES offers a broad range of opportunities for investigating the ultrafast atom-specific electron dynamics in photorelated phenomena of interest.

Low-energy electronic structure of optimally doped ${\text{YBa}}_{2}{\text{Cu}}_{3}{\text{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ is investigated using laser-excited angle-resolved photoemission spectroscopy. The surface state and the CuO chain band that usually overlap the ${\text{CuO}}_{2}$ plane derived bands are not detected, thus enabling a clear observation of the bulk superconducting state. The observed bilayer splitting of the Fermi surface is $\ensuremath{\sim}0.08\text{ }{\text{\AA{}}}^{\ensuremath{-}1}$ along $(0,0)\ensuremath{-}(\ensuremath{\pi},\ensuremath{\pi})$ direction, significantly larger than ${\text{Bi}}_{2}{\text{Sr}}_{2}{\text{CaCu}}_{2}{\text{O}}_{8+\ensuremath{\delta}}$. The kink structure of the band dispersion reflecting the renormalization effect at $\ensuremath{\sim}60\text{ }\text{meV}$ shows up similarly as in other hole-doped cuprates. The momentum dependence of the superconducting gap shows ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$-wave-like amplitude but exhibits a nonzero minimum of $\ensuremath{\sim}12\text{ }\text{meV}$ along the $(0,0)\ensuremath{-}(\ensuremath{\pi},\ensuremath{\pi})$ direction. Possible origins of such an unexpected ``nodeless'' gap behavior are discussed.

We have performed a temperature-dependent angle-integrated laser photoemission study of iron oxypnictide superconductors LaFeAsO:F and LaFePO:F exhibiting critical-transition temperatures (${T}_{c}$'s) of 26 and 5 K, respectively. We find that high-${T}_{c}$ LaFeAsO:F exhibits a temperature-dependent pseudogaplike feature extending over $\ensuremath{\sim}0.1\text{ }\text{eV}$ about the Fermi level at 250 K, whereas such a feature is absent in low-${T}_{c}$ LaFePO:F. We also find $\ensuremath{\sim}20\text{-meV}$ pseudogaplike features and signatures of superconducting gaps in both LaFeAsO:F and LaFePO:F. We discuss the possible origins of the unusual pseudogaplike features through comparison with the high-${T}_{c}$ cuprates.

We investigate the doping dependent low energy, low temperature ($T$ = 5 K) properties of nodal quasiparticles in the d-wave superconductor Bi$_{2.1}$Sr$_{1.9}$CaCu$_2$O$_{8+\delta}$ (Bi2212). By utilizing ultrahigh resolution laser-excited angle-resolved photoemission spectroscopy, we obtain precise band dispersions near $E_{F}$, mean free paths and scattering rates ($\Gamma$) of quasiparticles. For optimally and overdoped, we obtain very sharp quasiparticle peaks of 8 meV and 6 meV full-width at half-maximum, respectively, in accord with terahertz conductivity. For all doping levels, we find the energy-dependence of $\Gamma \sim |\omega |$, while $\Gamma$($\omega =0$) shows a monotonic increase from overdoping to underdoping. The doping dependence suggests the role of electronic inhomogeneity on the nodal quasiparticle scattering at low temperature (5 K $\lsim 0.07T_{\rm c}$), pronounced in the underdoped region.

We have investigated the temperature and doping variations of optical and transport properties in the electron-doped high-${T}_{\mathrm{c}}$ cuprate crystals ${\mathrm{Nd}}_{2\ensuremath{-}x}{\mathrm{Ce}}_{x}{\mathrm{CuO}}_{4}$ $(0l~xl~0.15).$ In the optical spectra of underdoped crystals $(xl0.15),$ a notable pseudogap is observed at low temperatures. A Drude-like response evolves concomitantly with pseudogap formation. Both the magnitude $({\ensuremath{\Delta}}_{\mathrm{PG}})$ and onset temperature ${(T}^{*})$ of the pseudogap decrease with electron doping, while holding the relation that ${\ensuremath{\Delta}}_{\mathrm{PG}}\ensuremath{\approx}{10k}_{\mathrm{B}}{T}^{*}.$ The ${\ensuremath{\Delta}}_{\mathrm{PG}}$ is comparable to the magnitude of the pseudogap at around $(\ensuremath{\pi}/2,\ensuremath{\pi}/2)$ in the photoemission spectra reported by Armitage et al. [Phys. Rev. Lett. 88, 257001 (2002)], which indicates that the pseudogap appearing in the optical spectra is identical to that discerned by the photoemission spectroscopy. The scattering rate spectra $1/\ensuremath{\tau}(\ensuremath{\omega})$ of the $x=0.10--0.15$ crystals show a kink structure at around 0.07 eV, which can be ascribed not to the pseudogap but to the electron-phonon coupling. In accordance with the evolution of the Drude response, the in-plane resistivity begins to decrease rapidly at around ${T}^{*}$ in the underdoped region. The out-of-plane resistivity shows an even more distinct decrease below ${T}^{*}.$ This is because the interplane charge transport is governed by electronic states at around $(\ensuremath{\pi},0),$ where the quasiparticle spectral weight is accumulated in the case of the electron-doped system. This is contrary to the hole-doped case with the pseudogap around this point. The origin of the pseudogap has been ascribed to the antiferromagnetic spin correlation, which is consistent with the evolution of a two-magnon band in the ${B}_{1\mathrm{g}}$ Raman spectra below ${T}^{*}.$ The pseudogap phenomenon in the electron-doped cuprate has been argued comparatively with that of the hole-doped cuprate.

The evolution of spin- and orbital-ordered states has been investigated for a series of insulating perovskites $R{\mathrm{MnO}}_{3}$ $(R=\mathrm{L}\mathrm{a},\mathrm{P}\mathrm{r},\mathrm{N}\mathrm{d},\dots{}).$ $R{\mathrm{MnO}}_{3}$ with a large ${\mathrm{GdFeO}}_{3}$-type distortion is regarded as a frustrated spin system having ferromagnetic nearest-neighbor and antiferromagnetic (AF) next-nearest-neighbor (NNN) interactions within a ${\mathrm{MnO}}_{2}$ plane. The staggered orbital order associated with the ${\mathrm{GdFeO}}_{3}$-type distortion induces the anisotropic NNN interaction, and yields unique sinusoidal and up-up-down-down AF ordered states in the distorted perovskites with ${e}_{g}^{1}$ configuration.

We have investigated the charge-spin-ordering transition in ${\mathrm{La}}_{1.5}{\mathrm{Sr}}_{0.5}{\mathrm{NiO}}_{4}$ by measurements of Raman and optical conductivity spectra. The checkerboard-type charge ordering below $\ensuremath{\sim}400\mathrm{K}$ can be probed by activated phonon ${(A}_{1g}$ and ${E}_{u})$ and two-magnon ${(B}_{1g})$ modes in the infrared and Raman spectra as well as by formation of pseudogaps in the optical conductivity spectra. Besides, another ${B}_{2g}$ Raman mode emerges below 150 K together with reduction of the optical gap, which can be attributed to another ordering process, i.e., the stripe formation.