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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

We have performed a temperature dependent angle-resolved photoemission spectroscopy (ARPES) study of the tri-layer high-$T_c$ cuprate superconductor (HTSC) Bi$_2$Sr$_2$Ca$_2$Cu$_3$O$_{10+\delta}$ (Bi2223), and have shown that the \textquotedblleft effective\textquotedblright superconducting (SC) gap $\Delta_{\rm{sc}}$ defined at the end point of the Fermi arc and the $T_c$ (= 110 K) approximately satisfies the weak-coupling BCS-relationship 2$\Delta_{\rm{sc}}$ = 4.3$k_{\rm{B}}T_c$. Combining this result with previous ARPES results on single- and double-layer cuprates, we show that the relationship between 2$\Delta_{\rm{sc}}$ = 4.3$k_{\rm{B}}T_c$ holds for various HTSCs. Furthermore, at $T$ $\sim$ $T_c$, the quasi-patricle width at the end point of the Fermi arc is found to coincide with $\Delta_{\rm{sc}}$, consistent with the context of Planckian dissipation.

The disorder dependence of the oxygen isotope effect in optimally doped ${\text{Bi}}_{2}{\text{Sr}}_{1.6}{\text{Ln}}_{0.4}{\text{CuO}}_{6+\ensuremath{\delta}}$ superconductors which is introduced into cation sites in the planes adjacent to ${\text{CuO}}_{2}$ planes is studied. The degree of out-of-plane disorder is controlled by substituting Ln ions with different ionic radii from La to Gd. This type of disorder does not explicitly alter the nodal electronic structure or the underlying Fermi surface but markedly enhances the oxygen isotope exponent at the superconducting transition temperature $({\ensuremath{\alpha}}_{{T}_{c}})$. The enhancement in ${\ensuremath{\alpha}}_{{T}_{c}}$ is well explained by the pair-breaking model, suggesting the growth of a pseudogap state around the antinodal region due to the out-of-plane disorder.

We explore the electronic structure in the heavily overdoped regime of the single-layer cuprate superconductor ${\mathrm{Bi}}_{1.74}{\mathrm{Pb}}_{0.38}{\mathrm{Sr}}_{1.88}\mathrm{Cu}{\mathrm{O}}_{6+\ensuremath{\delta}}$. We found that the nodal quasiparticle behavior is dominated mostly by phonons, while the antinodal quasiparticle line shape is dominated by spin fluctuations. Moreover, while long range spin fluctuations diminish at very high doping, the local magnetic fluctuations still dominate the quasiparticle dispersion, and the system exhibits a strange metal behavior in the entire overdoped regime.

The filling-controlled metal-insulator transition (MIT) in a two-dimensional Mott-Hubbard system ${\mathrm{La}}_{1.17\ensuremath{-}x}{\mathrm{Pb}}_{x}{\mathrm{VS}}_{3.17}$ has been studied by photoemission spectroscopy. With Pb substitution x, chemical potential $\ensuremath{\mu}$ abruptly jumps by $\ensuremath{\sim}0.07\mathrm{eV}$ between $x=0.15$ and 0.17, indicating that a charge gap is opened at $x\ensuremath{\simeq}0.16$ in agreement with the Mott insulating state of the ${d}^{2}$ configuration. When holes or electrons are doped into the Mott insulator of $x\ensuremath{\simeq}0.16,$ the gap is filled and the photoemission spectral weight at $\ensuremath{\mu},$ $\ensuremath{\varrho}(\ensuremath{\mu}),$ gradually increases in a similar way to the electronic specific-heat coefficient, although the spectral weight remains depressed around $\ensuremath{\mu}$ compared to that expected for a normal metal, showing a pseudogap behavior in the metallic samples. The observed behavior of $\ensuremath{\varrho}(\ensuremath{\mu})\ensuremath{\rightarrow}0$ for $\stackrel{\ensuremath{\rightarrow}}{x}0.16$ is contrasted with the usual picture that the electron effective mass of the Fermi-liquid system is enhanced towards the metal-insulator boundary. With increasing temperature, the gap or the pseudogap is rapidly filled up, and the spectra at $T=300\mathrm{K}$ appears to be almost those of a normal metal. Near the metal-insulator boundary, the spectra around $\ensuremath{\mu}$ are consistent with the formation of a Coulomb gap, suggesting the influence of long-range Coulomb interaction under the structural disorder intrinsic to this system.

Valence-band and conduction-band electronic structure of the transparent p-type semiconductor (LaO)CuS and the Ca and Ni co-doped system (La 1 - x Ca x O)Cu 1 - x Ni x S (x≤0.10) has been investigated by means of photoemission and inverse-photoemission spectroscopies. It is confirmed that the valence bands of (LaO)CuS are composed of the bonding and antibonding states of the Cu 3d (t 2 g ) and S 3p orbitals as well as the nearly localized Cu 3d (e g ), and the O 2p states. With increasing x up to x=0.03, the structure due to the Cu 3d (eg) states shifts away from the Fermi level (E F ) and the energy separation between the valence-band maximum and conduction-band minimum decreases rapidly, followed by an appearance of new density of states (DOS) derived from the Ni 3d and S 3p states just above E F of (LaO)CuS. For x≤0.03, the new DOS shows up with x, while the energy position of the Cu 3d (eg)-derived structure no longer shifts. The finite DOS at E F has been observed as a clear structure for x=0.10. These results are not understood in terms of a rigid-band model. We propose a different band model for (La 1 - x Ca x O)Cu 1 - x Ni x S taking into account the Ni 3d and S 3p-derived new DOS.

We have studied the electronic structures of ${\mathrm{Yb}}_{2}{\mathrm{Co}}_{3}{X}_{9}$ $(X=\mathrm{Ga}$ and Al) compounds by high-resolution photoemission spectroscopy. The isostructural series of compounds ${\mathrm{Yb}}_{2}{T}_{3}{X}_{9}$ $(T=\mathrm{Co},\mathrm{Rh},\mathrm{Ir};$ $X=\mathrm{Ga},\mathrm{Al})$ are known to be antiferromagnetic Kondo lattice for $X=\mathrm{Al},$ while they show mixed-valent behavior for $X=\mathrm{Ga}.$ In the measurements at 14 K, we observed a sharp divalent Yb $4f$ peak just below ${E}_{F}$ for ${\mathrm{Yb}}_{2}{\mathrm{Co}}_{3}{\mathrm{Ga}}_{9}$ while no $4f$-related signal could be clearly identified just below ${E}_{F}$ for $X=\mathrm{Al}.$ Both the position and the width of the peak observed for $X=\mathrm{Ga}$ are comparable to ${k}_{B}{T}_{K},$ where ${T}_{K}$ is the Kondo temperature, and the intensity of the peak shows a strong temperature dependence. The result is generally consistent with the behavior of the Kondo peak predicted by the single-impurity Anderson model. The origin of the difference between the mixed-valent Ga-based compounds and the Kondo-lattice Al-based compounds is attributed both to the difference in the $4f$ energy position and the $4f$-ligand hybridization strength. The line shape of the Kondo peak on the higher binding-energy side is, however, significantly different from that predicted by the calculation, which we attribute to the excitation of low-energy electron-hole pairs across the Fermi level neglected in the noncrossing approximation treatment of the single-impurity Anderson model.