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We have performed high-resolution angle-resolved photoemission spectroscopy on hole-doped high- T c cuprate superconductors (HTSCs) to study many-body interaction and the universality of low-energy excitation gap. In Bi 2 Sr 2 CaCu 2 O 8 (Bi2212), we observed a kink in the dispersion in the off-nodal region in the superconducting state, which remarkably weakens on impurity substitution. We also find that the appearance of the kink in the off-nodal region is a common feature of Bi2212 and La 1.85 Sr 0.15 CuO 4 (LSCO), while the energy scale is remarkably different between two compounds (70 and 20 meV). We discuss universality of the kink in dispersion in the hole-doped HTSCs in terms of the coupling of electrons with spin fluctuations.

We report angle-resolved photoemission spectroscopy on optimally doped high- T c superconductor La 1.85 Sr 0.15 CuO 4 ( T c = 37 K) to elucidate the character of low-energy excitations in the superconducting state. We find a clear evidence for the emergence of a sharp superconducting quasiparticle peak below T c together with opening of a superconducting gap at off-nodal region, while no signature of a quasiparticle peak is observed at (π, 0). From the comparison of spectral lineshape and energy dispersion between the nodal and the off-nodal region, we conclude that there are two characteristic energy scales (70 and 20 meV) in the superconducting state of optimally doped La 1.85 Sr 0.15 CuO 4 .

We report high-resolution angle-resolved photoemission spectroscopy on electron-doped cuprate Nd2-xCexCuO4 to study the quasiparticle (QP) state in the vicinity of the Fermi level. Measurements of momentum dependence have revealed that the mass-renormaliz

Systematic angle-resolved photoemission spectroscopy (ARPES) has been performed on Bi2Sr2Can-1CunO2n+4 (n = 1-3). For n = 2 and 3, the dispersion kink becomes pronounced on approaching (pi, 0) below T-c, while a kink appears only around the nodal directio

Despite years of intensive research on copper oxide superconductors with high transition temperatures (T-c), the driving force of superconductivity has not yet been clarified. Angle-resolved photoemission spectroscopy (ARPES)(1-3) measurements have uncove

High-resolution angle-resolved photoemission spectroscopy (ARPES) has been performed on Nb1−xTixXc2 (Xc = S, Se, Te) to elucidate the change in the band structure as a function of x and Xc. We found that spectral intensity near E F of S- and Se-based compounds is remarkably suppressed, suggesting the strong scattering of electrons by the substitution of transition metals. In contrast, Te-based compound shows a clear Fermi edge. We discuss the origin of metal–semiconductor transition in terms of the strength of hybridization between chalcogen p band and transition metal d band.

We have performed high-resolution angle-resolved photoemission spectroscopy (ARPES) with synchrotron radiation on layered transition-metal dichalcogenide 1 T -VSe 2 to study the mechanism of the three-dimensional charge-density-wave (CDW) transition. We found that the Fermi surface around the M(L) and K(H) points shows considerable wiggling along the wave vector normal to the layer ( k z ) despite the apparently two-dimensional crystal structure. ARPES spectra below the CDW transition temperature show a signature of pseudogap opening at a specific k z -region on the Fermi surface, demonstrating that the CDW transition of 1 T -VSe 2 is driven by three-dimensional Fermi-surface nesting.

We have performed systematic high-resolution angle-resolved photoemission spectroscopy (ARPES) on layered transition-metal dichalcogenides Nb1−xTixXc2 (Xc = S, Se, Te) to study the mechanism of metal–semiconductor transition as a function of x and Xc. We found a pseudogap near EF in the S- and Se-based compounds while a clear Fermi edge is observed in the Te-based compound. The existence of the pseudogap indicates the strong scattering of electrons by the random potential caused by substitution, which prevents formation of the charge density wave and at the same time decreases the metallic conductivity. The metallic character of Nb1−xTixTe2 is explained in terms of the relatively strong Te 5p–Nb 4d (Ti 3d) hybridization near EF.

We have performed a high-resolution angle-resolved photoemission spectroscopy (ARPES) on high- T c superconductors (HTSCs) Bi 2 Sr 2 Ca n -1 Cu n O 2 n +4 ( n =1–3) to study the systematics of the electronic structure and the origin of many-body interactions responsible for superconductivity. We find that a large hole-like Fermi surface and d-wave superconducting gap are generic features of Bi-based HTSCs. For n =2 and 3, a sudden change in the energy dispersion, so-called “kink”, becomes pronounced on approaching (π,0) in the superconducting state, while a kink appears only around the nodal direction in the normal state. The kink around (π,0) disappears at T c . For n =1, the kink shows no significant temperature dependence even across T c . This could suggest that the coupling of electrons with Q =(π,π) magnetic mode is dominant in the superconducting state for multi-layered cuprates, while the interaction at the normal state and that of single-layered cuprates have a different origin.

We have performed ultrahigh-resolution angle-resolved photoemission spectroscopy of (Bi,Pb)2Sr2CuO6 by using a newly developed xenon-plasma light source to clarify the origin of the pseudogap (PG). We determined the comprehensive momentum and temperature dependences of the superconducting (SC) gap and the PG, and revealed a smooth evolution of the PG from the SC gap. We also found a linear scaling behavior of the characteristic PG temperature with the SC gap size regardless of the momentum location. These experimental results strongly suggest that the observed PG is caused by the precursor pairing.

We report results of angle-resolved photoemission spectroscopy on a heavily electron-doped BaFe2−xCoxAs2. By using synchrotron-radiation light, we have investigated the band dispersion near the Fermi level ( E F ) of BaFe2−xCoxAs2 (x = 0.3) for different wave vectors perpendicular to the crystal surface ( k z ) . We find that observed holelike band does not cross E F around Γ or Z point of the Brillouin zone, unlike the optimally- or under-doped samples. The holelike band also shows a finite k z dispersion of about 20 meV. These results indicate that the coexistence of hole and electron pockets connected by the antiferromagnetic wave vector plays an important role to the mechanism of superconductivity.

We have performed high-resolution angle-resolved photoemission spectroscopy of an electron-doped iron-based superconductor BaFe 1.85 Co 0.15 As 2 , and experimentally determined the Fermi-surface, the band structure and the superconducting gap. We found an isotropic s -wave-like superconducting gap on a hole pocket centered at the Γ point and on electron pockets at the M point. The superconducting gap magnitude is significantly enhanced on the Fermi-surfaces connected by the antiferromagnetic wave vector, suggesting the importance of an interband interaction on the pairing mechanism.

We studied the impurity effect on the electronic structure near the Fermi level in Bi 2 Sr 2 CaCu 2 O 8+ δ by high-resolution angle-resolved photoemission spectroscopy to clarify the origin of dispersion kink in high- T c superconductors. We found that the dispersion kink at off-nodal region is significantly suppressed by Zn or Ni impurity while that at the node is less affected, suggesting that electrons are strongly coupled to the spin excitations at the off-nodal region.

We have performed high-resolution angle-resolved photoemission spectroscopy (ARPES) on Co-, Ni-, and Zn-substituted Bi 2 Sr 2 CaCu 2 O 8+ δ (Bi2212) to study the impurity effect on the electronic structure. The shape of Fermi surface and the symmetry of superconducting gap are unchanged by impurities, while the superconducting coherent peak around (π, 0) is remarkably suppressed. The temperature dependence of the coherent-peak intensity shows a good correspondence to the reported superfluid density. The intensity of coherent peak scales well with the T c in pristine, Co–, and Ni–Bi2212, but not in Zn–Bi2212, suggesting that non-magnetic Zn atoms destroy the superconductivity more strongly than the magnetic impurities.

We have performed high-resolution angle-resolved photoemission spectroscopy (ARPES) of Co-, Ni-, and Zn-substituted Bi2Sr2CaCu2O8+δ (Bi2212) to study the impurity effect on the electronic structure. In all compounds, we observed a remarkable suppression of the intensity of coherent peak around ( π , 0) point at the superconducting state, consistent with the decrease in the superfluid density. By quantitatively analyzing ARPES lineshape, we found that the weight of the coherent peak well scales with Tc among substituted and unsubstituted Bi-family high-Tc superconductors exept for Zn-substituted Bi2212.

We have performed high-resolution angle-resolved photoemission spectroscopy of ${\text{YBa}}_{2}{\text{Cu}}_{3}{\text{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ with various hole concentrations from under-to-optimal doping. We have determined the momentum dependence of bulk superconducting gap as a function of hole concentration and found the enhancement of antinodal gap, the deviation from simple ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ wave, and the reduction of superconducting coherence weight in the underdoped region. We also found the universality of the doping dependence of the SC-gap size and symmetry in bilayered cuprates. These results suggest the importance of the long-range pairing interactions in the underdoped region.

We have performed ultrahigh energy- and momentum-resolved angle-resolved photoemission spectroscopy of impurity-doped ${\text{Bi}}_{2}{\text{Sr}}_{2}{\text{CaCu}}_{2}{\text{O}}_{8}$. We found that the point node of the superconducting gap near the $(\ensuremath{\pi}/2,\ensuremath{\pi}/2)$ point in a pristine sample gradually evolves into a small gapless Fermi arc upon Zn or Ni doping. The shape of impurity-induced density of states inside the superconducting gap shows a marked difference between Zn and Ni doping, demonstrating that the low-energy excitation properties are strongly sensitive to the character of impurities.

High-resolution angle-resolved photoemission spectroscopy has been performed on Zn-substituted ${\mathrm{La}}_{1.85}{\mathrm{Sr}}_{0.15}\mathrm{Cu}{\mathrm{O}}_{4}$ to study the impurity effect on the electronic structure near the Fermi level. Upon Zn substitution, the kink in the band dispersion at $20\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ and the coherence peak in the off-nodal region in the superconducting state are significantly weakened and, at the same time, additional electronic states systematically evolve inside the superconducting gap. The suppression of the kink with Zn substitution shows a good correspondence to the low-energy incommensurate magnetic excitations.

The doping dependence of electronic states in an electron-doped high-temperature superconductor (HTSC) Nd2-xCexCuO4 was studied by high-resolution angle-resolved photoemission spectroscopy. We observed that the high-energy pseudogap around the hot spot sh

We have performed high-resolution angle-resolved photoemission spectroscopy on $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ (Y123; $\ensuremath{\delta}=0.06$; ${T}_{\mathrm{c}}=92\phantom{\rule{0.3em}{0ex}}\mathrm{K}$). By accurately determining the Fermi surface and energy band dispersion, we solve long-standing controversial issues as to the anomalous electronic states of Y-based high-${T}_{\mathrm{c}}$ cuprates. We unambiguously identified surface-bilayer-derived bonding and antibonding bands, together with their bulk counterparts. The surface bands are highly overdoped (hole concentration $x=0.29$), showing no evidence for the gap opening or the dispersion anomaly in the antinodal region, while the bulk bands show a clear ${d}_{{x}^{2}\text{\ensuremath{-}}{y}^{2}}$-wave superconducting gap and the Bogoliubov quasiparticlelike behavior with a characteristic energy scale of $50--60\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ indicative of a strong electron-boson coupling in the superconducting state. All these results suggest that the metallic and superconducting states coexist at the adjacent bilayer of Y123 surface.

We have performed systematic angle-resolved photoemission spectroscopy (ARPES) on single-layered cuprate superconductor Bi2Sr2CuO6 to elucidate the origin of shadow band. We found that the shadow band is exactly the c(2x2) replica of the main band irrespective of the carrier concentration and its intensity is invariable with respect to temperature, doping, and substitution constituents of block layers. This result rules out the possibility of antiferromagnetic correlation and supports the structural origin of shadow band. ARPES experiments on optimally doped La1.85Sr0.15CuO4 also clarified the existence of the c(2x2) shadow band, demonstrating that the shadow band is not a unique feature of Bi-based cuprates. We conclude that the shadow band is related to the orthorhombic distortion at the crystal surface.

We performed high-resolution angle-resolved photoemission spectroscopy on Nd1.87Ce0.13CuO4, which is located at the boundary of the antiferromagnetic (AF) and the superconducting phase. We observed that the quasiparticle (QP) effective mass around (pi,0)

We performed high-resolution angle-resolved photoemission spectroscopy on electron-doped high-Tc superconductor Pr0.89LaCe0.11CuO4 to study the anisotropy of the superconducting gap. The observed momentum dependence is basically consistent with the dx2-y2-wave symmetry, but obviously deviates from the monotonic dx2-y2 gap function. The maximum gap is observed not at the zone boundary, but at the hot spot where the antiferromagnetic spin fluctuation strongly couples to the electrons on the Fermi surface. The present experimental results unambiguously indicate the spin-mediated pairing mechanism in electron-doped high-Tc superconductors., 4 pages, 4 figures

High-resolution angle-resolved photoemission spectroscopy (ARPES) has been performed on a layered transition-metal dichalcogenide (TMDC) $1T\ensuremath{-}{\mathrm{VSe}}_{2}$ to study the $(4\ifmmode\times\else\texttimes\fi{}4)$ charge-density wave (CDW) mechanism. We observed a partial Fermi-surface (FS) nesting on the electronlike FS centered at the M (L) point. The spectral weight near ${E}_{F}$ is considerably suppressed below the transition temperature ${(T}_{c}=110\mathrm{K})$ around the nested portion, while a negligible spectral change is observed even across ${T}_{c}$ in other portions of FS. This suggests that the CDW transition in $1T\ensuremath{-}{\mathrm{VSe}}_{2}$ is caused by the three-dimensional FS nesting. Implications are discussed in relation to the physical properties of $1T\ensuremath{-}{\mathrm{VSe}}_{2}$ as well as the ARPES results of other TMDC's.

To study the superconducting-gap symmetry in electron-doped HTSC, we performed a high-resolution ARPES on Pr0.89LaCe0.11CuO4 (optimally-doped, Tc = 26 K). Obtained momentum dependence of the superconducting gap is basically consistent with the d x 2 - y 2 -wave symmetry, but obviously deviates from the monotonic form of the d x 2 - y 2 gap function. The maximum superconducting gap is not located at the zone boundary as expected from a simple d x 2 - y 2 -wave, but observed at the so-called hot spot where the antiferromagnetic spin fluctuation strongly couples to the electrons on the Fermi surface. This ARPES result suggests that the spin-correlation plays an essential role in the pairing mechanism of electron-doped HTSC.

We report angle-resolved photoemission results on high- T c superconductor La 1.85 Sr 0.15 CuO 4 . A sharp Bogoliubov quasiparticle peak and a dispersion kink at 20 meV are observed in the superconducting state at off-nodal region. Present experimental result suggests a coupling between electrons and the incommensurate magnetic resonance.

We have performed systematic and comprehensive high-resolution angle-resolved photoemission (ARPES) measurements on Bi-system high-Tc superconductors to study the quasi-particles related to and/or responsible for the occurrence of the superconductivity. We have experimentally determined the full energy dispersion and the coherence factors of the Bogoliubov quasi-particles, which show a good quantitative agreement with the prediction from the BCS theory. This proves the basic validity of the BCS theory in the broad sense to describe the high-Tc superconductivity. We have experimentally identified two different bosonic modes in Bi-system high-Tc superconductors. One produces a 'small' kink in the limited momentum region around the nodal cut and another is related to a 'large' kink which exists in a relatively wide momentum region with the stronger magnitude closer to the (π, 0) point. The observed momentum and temperature dependence of the kinks as well as the impurity effect show that the large kink is of magnetic origin and closely related to the high-Tc superconductivity.

Core-level and valence-bond electronic structures of Bi2Sr2Ca1 xYxCu2Oy (x=0, 0.4, and 0.7) have been studied by X-ray and UV photoemission spectroscopy. A Fermi edge is observed for x=0.4 as well as for x=0. It is found that the density of states at the Fermi level and the intensity of a 1.2-eV peak are reduced but the binding energies of Bi 5d are not changed by substitution of Y for Ca. The origin of the states and the structure is discussed in terms of 02p bands mainly in the Cu O layers.