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We study the electronic structure of electron-doped Pr$_{1.3-x}$La$_{0.7}$Ce$_{x}$CuO$_{4}$ (PLCCO ; $T_{c}$ = 27 K, x = 0.1) and hole-doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ (Bi2212 ; $T_{c}$ = 90 K) cuprate superconductors using x-ray absorption spectroscopy (XAS) and resonant photoemission spectroscopy (Res-PES). From Res-PES across the O K-edge and Cu L-edge, we identify the O 2p and Cu 3d partial density of states (PDOS) and their correlation satellites which originate in two-hole Auger final states. Using the Cini-Sawatzky method, analysis of the experimental O 2p PDOS shows an oxygen on-site Coulomb energy for PLCCO to be $U_{p}$ = 3.3$\pm$0.5 eV and for Bi2212, $U_{p}$ = 5.6$\pm$0.5 eV, while the copper on-site Coulomb correlation energy, $U_{d}$ = 6.5$\pm$0.5 eV for Bi2212. The expression for the Heisenberg exchange interaction $J$ in terms of the electronic parameters $U_{d}$, $U_{p}$, charge-transfer energy $\Delta$ and Cu-O hopping $t_{pd}$ obtained from a simple Cu$_2$O cluster model is used to carry out an optimization analysis consistent with $J$ known from scattering experiments. The analysis also provides the effective one band on-site Coulomb correlation energy $\tilde{U}$ and the effective hopping $\tilde{t}$. PLCCO and Bi2212 are shown to exhibit very similar values of $\tilde{U}$/$\tilde{t}$ $\sim$9-10, confirming the strongly correlated nature of the singlet ground state in the effective one-band model for both the materials., Comment: 13, pages, 11 figures

In a conventional framework, superconductivity is lost at a critical temperature (T_c) because, at higher temperatures, gluing bosons can no longer bind two electrons into a Cooper pair. In high-T_c cuprates, it is still unknown how superconductivity vanishes at T_c. We provide evidence that the so-called <~70-meV kink bosons that dress the quasi-particle excitations are playing a key role in the loss of superconductivity in a cuprate. We irradiated a 170-fs laser pulse on Bi2Sr2CaCu2O8+\delta and monitored the responses of the superconducting gap and dressed quasi-particles by time- and angle-resolved photoemission spectroscopy. We observe an ultrafast loss of superconducting gap near the d-wave node, or light-induced Fermi arcs, which is accompanied by spectral broadenings and weight redistributions occurring within the kink binding energy. We discuss that the underlying mechanism of the spectral broadening that induce the Fermi arc is the undressing of quasi-particles from the kink bosons. The loss mechanism is beyond the conventional framework, and can accept the unconventional phenomena such as the signatures of Cooper pairs remaining at temperatures above T_c.

We report on the Pt doping effect on surface and electronic structure in Ir$_{\mathrm{1-x}}$Pt$_{\mathrm{x}}$Te$_ {\mathrm{2}}$ by scanning tunneling microscopy (STM) and spectroscopy (STS). The surface prepared by cleavage at 4.2 K shows a triangular lattice of topmost Te atoms. The compounds that undergo structural transition have supermodulation with a fixed wave vector $q = \frac{2\pi}{5a_m}$ (where $a_m$ is the lattice constant in the monoclinic phase) despite the different Pt concentrations. The superconducting compounds show patch structures. The surface of the compound that exhibits neither the superconductivity nor the structural transition shows no superstructure. In all doped samples, the dopant is observed as a dark spot in STM images. The tunneling spectra near the dopant show the change in the local density of state at approximately -200 mV. Such microscopic effects of the dopant give us the keys for establishing a microscopic model of this material., Comment: 6 pages, 3 figures

We observed superconductivity ($T_{c}$ $\simeq$2-3 K) in Li$_{x}$RhB$_{y}$ intermetallics wherein $x$ and $y$ vary over a wide compositional range. The crystal structure consists of cubic unit-cell ($a$ $\simeq$ 12.1 \AA ) with centro-symmetric space group $Pn\bar{3}n$. A weak but positive pressure-induced increase of $T_{c}$ was observed. The correlations between the composition and each of the followings were followed over a wide range of $x$ and $y$: the unit-cell dimensions, $T_{c}$ , Sommerfeld coefficient $\gamma$, Debye temperature $\theta_{D}$, and critical fields $H$$_{c1}$ and $H$$_{c2}$. The thermal evolution of the electronic specific heat within the superconducting phase was observed to follow a quadratic-in-$T$ behavior. In addition, a paramagnetic Meissner Effect (PME) is manifested during a low-field-cooled magnetization cycle. This manifestation of quadratic-in-$T$ behavior and PME feature will be discussed., Comment: 6 pages, 8 figures, to appear in Materials Research Express (2014)

We report on the scanning tunneling spectroscopy experiments on single crystals of IrTe$_{2}$. A structural supermodulation and a local density-of-states (LDOS) modulation with a wave vector of $q$ = 1/5$\times$$2\pi /a_{0}$ ($a_{0}$ is the lattice constant in the $ab$-plane) have been observed at 4.2K where the sample is in the monoclinic phase. %We cannot find an energy gap emerging reproducibly.% on the region where the supermodulation resides. As synchronized with the supermodulation, the LDOS spatially modulates within two energy ranges (below -200 meV and around -100 meV). We further investigated the effect of the local perturbations including the antiphase boundaries and the twin boundaries on the LDOS. These perturbations also modify the LDOS below -200 meV and around -100 meV, even though the lattice distortions induced by these perturbations appear to be different from those by the supermodulation. Our results indicating several microscopic structural effects on the LDOS seem to offer crucial keys for the establishment of the microscopic model describing the parent state., Comment: 5 pages, 4 figures, to be published in Phys. Rev. B

We present scanning tunneling microscopy and spectroscopy studies around an individual excess Fe atom, working as a local perturbation, in the parent material of the iron-chalcogenide superconductor Fe$_{1+\delta}$Te. Spectroscopic imaging reveals a novel isosceles triangular electronic structure around the excess Fe atoms. Its spatial symmetry reects the underlying bicollinear antiferromagnetic spin state and the structural monoclinic symmetry. These findings provide important clues to understand the role of the excess Fe atoms, which complicate the understanding of the phenomena occurring in iron-chalcogenide materials., Comment: 6 pages, 5 figures, to be published in Phys. Rev. B

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.

Low temperature specific heat has been investigated in a novel ternary superconductor La_{3}Pd_{4}Ge_{4} with an U_{3}Ni_{4}Si_{4}-type structure consisting of the alternating BaAl_{4} (ThCr_{2}Si_{2})- and AlB$_{2}$-type layers. A comparative study with the related ThCr_{2}Si_{2}-type superconductor LaPd_{2}Ge_{2}, one of the layers in La_{3}Pd_{4}Ge_{4}, is also presented. From the normal state specific heat, the Sommerfeld coefficient $\gamma_{n} = 27.0$ mJ/mol K^2 and the Debye temperature $\Theta_{\rm D}$ = 256 K are derived for the La_{3}Pd_{4}Ge_{4}, while those for the LaPd_{2}Ge_{2} are $\gamma_{n} =8.26$ mJ/mol K^2 and $\Theta_{\rm D}$ = 291 K. The La_{3}Pd_{4}Ge_{4} has moderately high electronic density of state at the Fermi level. Electronic contribution on the specific heat, $C_{\rm el}$, in each compound is well described by the BCS behavior, suggesting that both of the La_{3}Pd_{4}Ge_{4} and the LaPd_{2}Ge_{2} have fully opened isotropic gap in the superconducting state.

We have performed a detailed study of Cu $2p$ core-level spectra in single layer La$_{2-x}$Sr$_{x}$CuO$_{4}$, La doped Bi$_2$Sr$_{1.6}$La$_{0.4}$CuO$_{6+\delta}$ (Bi2201) and bilayer Bi$_2$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ (Bi2212) high-temperature superconductors by using hard x-ray photoemission (HX-PES). We identify the Cu$^{2+}$ derived (i) the Zhang-Rice singlet (ZRS) feature, (ii) the $d^{n+1}\underline{L}$ (ligand screened) feature, (iii) the $d^{n}$ satellite feature, as well as the hole-doping derived high binding energy feature in the main peak. In Bi-based cuprates, intensities of the $d^{n}$ satellite features seem to be strongly enhanced compared to La$_{2-x}$Sr$_{x}$CuO$_{4}$. From x-ray photon energy dependent measurements, it is shown that the increased intensity in the satellite region is associated with Bi $4s$ core-level spectral intensity. The corrected $d^{n}$ satellite intensity is independent of the doping content or number of Cu-O layers. Our results suggest a correlation of the relative intensity of ZRS feature and hole-doping induced high binding energy spectral changes in the main peak with superconductivity., Comment: 6 pages, 3 figures

Hard X-ray Photoemission spectroscopy (PES) of copper core electronic states, with a probing depth of $\sim$60 \AA, is used to show that the Zhang-Rice singlet feature is present in La$_2$CuO$_4$ but is absent in Nd$_2$CuO$_4$. Hole- and electron doping in La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) and Nd$_{2-x}$Ce$_x$CuO$_4$ (NCCO) result in new well-screened features which are missing in soft X-ray PES. Impurity Anderson model calculations establish metallic screening as its origin, which is strongly suppressed within 15 $\text{\AA}$ of the surface. Complemented with X-ray absorption spectroscopy, the small chemical-potential shift in core levels ($\sim0.2$ eV) are shown to be consistent with modifications of valence and conduction band states spanning the band gap ($\sim1$ eV) upon hole- and electron-doping in LSCO and NCCO., Comment: 4 pages, 4 figures

We report bulk-sensitive hard X-ray ($h\nu$ = 5.95 KeV) core level photoemission spectroscopy (PES) of single crystal V$_{1.98}$Cr$_{0.02}$O$_{3}$ and the high-$T_c$ cuprate Bi$_2$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ (Bi2212). V$_{1.98}$Cr$_{0.02}$O$_{3}$ exhibits low binding energy "satellites" to the V $2p$ "main lines" in the metallic phase, which are suppressed in the antiferromagnetic insulator phase. In contrast, the Cu $2p$ spectra of Bi2212 do not show temperature dependent features, but a comparison with soft X-ray PES indicates a large increase in the $2p^5 3d^9$ "satellites" or $3d^9$ weight in the bulk. Cluster model calculations, including full multiplet structure and a screening channel derived from the coherent band at the Fermi energy, give very satisfactory agreement with experiments.

The metastable phase of an yttrium sesquicarbide compound, which was vary recently reported as a new superconductive phase with a Tc value of 18 K, was prepared under high pressure from the nominal composition of Y2C3 and Y2C2.9B0.1 in order to investigate this phase. We have succeeded in reproduction, and in the transport measurement for this 18-K phase. It is considered that the reason for the higher Tc value than the 11.5 K reported previously is not the small amount of B as the impurity element from the BN crucible. The Bc2 value at 0 K estimated from the resistivity data for the 18-K phase seems to be over 30 T. It means that the Tc and Bc2 values of the 18-K phase are as high as those of the general A15 compounds., Comment: 6 pages, 4 figures

We study the normal state electronic excitations probed by angle resolved photoemission spectroscopy (ARPES) in Bi2201 and Bi2212. Our main goal is to establish explicit criteria for determining the Fermi surface from ARPES data on strongly interacting systems where sharply defined quasiparticles do not exist and the dispersion is very weak in parts of the Brillouin zone. Additional complications arise from strong matrix element variations within the zone. We present detailed results as a function of incident photon energy, and show simple experimental tests to distinguish between an intensity drop due to matrix element effects and spectral weight loss due to a Fermi crossing. We reiterate the use of polarization selection rules in disentangling the effect of umklapps due to the BiO superlattice in Bi2212. We conclude that, despite all the complications, the Fermi surface can be determined unambiguously: it is a single large hole barrel centered about (pi,pi) in both materials., Comment: Expanded discussion of symmetrization method in Section 5, figures remain the same

Photoemission spectra of Bi2Sr2CaCu2O8 reveal that the high energy feature near (pi,0), the "hump", scales with the superconducting gap and persists above Tc in the pseudogap phase. As the doping decreases, the dispersion of the hump increasingly reflects the wavevector (pi,pi) characteristic of the undoped insulator, despite the presence of a large Fermi surface. This can be understood from the interaction of the electrons with a collective mode, supported by our observation that the doping dependence of the resonance observed by neutron scattering is the same as that inferred from our data., Comment: 4 pages (revtex), 5 figures (eps)

Recent improvements in momentum resolution by a factor of 32 lead to qualitatively new ARPES results on the spectra of Bi2Sr2CaCu2O8 (Bi2212) along the (pi,pi) direction, where there is a node in the superconducting gap. With improved resolution, we now see the intrinsic lineshape, which indicates the presence of true quasiparticles at the Fermi momentum in the superconducting state, and lack thereof in the normal state. The region of momentum space probed here is relevant for charge transport, motivating a comparison of our results to conductivity measurements by infrared reflectivity., Comment: revised paper with new figures

Comparing ARPES measurements on Bi2212 with penetration depth data, we show that a description of the nodal excitations of the d-wave superconducting state in terms of non-interacting quasiparticles is inadequate, and we estimate the magnitude and doping dependence of the Landau interaction parameter which renormalizes the linear T contribution to the superfluid density. Furthermore, although consistent with d-wave symmetry, the gap with underdoping cannot be fit by the simple coskx-cosky form, which suggests an increasing importance of long range interactions as the insulator is approached., Comment: 4 pages, 3 eps figs, manuscript and Fig. 3 significantly revised

In contrast with the recent photoemission observations by Shen et al. of anomalous temperature-induced momentum-dependent spectral weight transfer in Bi_2Sr_2CaCu_2O_{8+delta}, we find that in the same material, in spite of the unusual spectral lineshape change between superconducting and normal states, the integrated spectral weight displays minimum variation as a function of temperature, which is in agreement with the sum rule of angle-resolved photoemission: the integrated spectral weight is proportional to the momentum density, n(k)., Comment: 3 pages, revtex, 4 postscript figures

In this paper, we review some of our ARPES results on the superconducting and pseudo gaps in Bi2Sr2CaCu2O8+x. We find that optimally and overdoped samples exhibit a d-wave gap, which closes at the same temperature, Tc, for all k points. In underdoped samples, a leading edge gap is found up to a temperature T* > Tc. We find that T* scales with the maximum low temperature gap, increasing as the doping is reduced. The momentum dependence of the pseudogap is similar to that of the superconducting gap; however, the pseudogap closes at different temperatures for different k points., Comment: 7 pages, revtex, 7 postscript figures, SNS97 proceedings

The Fermi surface is a central concept in the theory of metals. Even though the optimally doped high temperature superconductors exhibit an anomalous normal state, angle resolved photoemission spectroscopy (ARPES) has revealed a large Fermi surface despite the absence of well-defined quasiparticles and the necessity of working at finite temperatures. However, the even more unusual behavior in the underdoped high temperature superconductors, which show a pseudogap above Tc, requires us to carefully re-examine this concept. Here, we present the first results on how the Fermi surface is destroyed as a function of temperature in underdoped Bi2212 using ARPES. We find the remarkable effect that different k points become gapped at different temperatures. This leads to a break up of the Fermi surface at a temperature T* into disconnected Fermi arcs which shrink with decreasing T, eventually collapsing to the point nodes of the dx2-y2 superconducting ground state below Tc. This novel behavior, where the Fermi surface does not form a continuous contour in momentum space, is unprecedented in that it occurs in the absence of long range order. Moreover, although the d-wave superconducting gap below Tc smoothly evolves into the pseudogap above Tc, the gaps at different k points are not related to one another above Tc the same way as they are below, implying an intimate, but non-trivial relation, between the two., Comment: 4 pages, revtex, 5 eps figures, minor changes (note, a and c should be interchanged in upper right panel of old Fig. 2), accepted for Nature

Photoemission spectra of Bi_2Sr_2CaCu_2O_{8+delta} below T_c show two features near the (pi,0) point of the zone: a sharp peak at low energy and a higher binding energy hump. We find that the sharp peak persists at low energy even as one moves towards (0,0), while the broad hump shows significant dispersion which correlates well with the normal state dispersion. We argue that these features are naturally explained by the mixing of electrons with a bosonic mode which appears only below T_c, and speculate that the latter may be related to the resonance seen in recent neutron data., Comment: 4 pages, revtex, 4 postscript figures

We show, by use of angle-resolved photoemission spectroscopy, that underdoped Bi_2Sr_2CaCu_2O_{8+\delta} appears to have a large Fermi surface centered at (\pi,\pi), even for samples with a T_c as low as 15 K. No clear evidence of a Fermi surface pocket around (\pi/2,\pi/2) has been found. These conclusions are based on a determination of the minimum gap locus in the pseudogap regime T_c < T < T^*, which is found to coincide with the locus of gapless excitations in momentum space (Fermi surface) determined above T^*. These results suggest that the pseudogap is more likely of precursor pairing rather than magnetic origin., Comment: 4 pages, revtex, 4 postscript color figures

We report measurements of the momentum dependence of the superconducting gap in Bi_2Sr_2CaCu_2O_{8+x} (Bi2212) with angle-resolved photoemission spectroscopy using a dense sampling of the Brillouin zone in the vicinity of the Fermi surface. In the Y quadrant of the zone, where there are no complications from ghost bands caused by the superlattice, we find a gap function consistent within error bars to the form cos(k_x)-cos(k_y) expected for a d-wave order parameter. Similar results are found in the X quadrant with the photon polarization chosen to enhance main band emission over that due to ghost bands., Comment: 3 pages, revtex, 4 postscript figures

Particle-hole (p-h) mixing is a fundamental consequence of the existence of a pair condensate. We present direct experimental evidence for p-h mixing in the angle-resolved photoemission (ARPES) spectra in the superconducting state of Bi_2Sr_2CaCu_2O_{8+\delta}. In addition to its pedagogical importance, this establishes unambiguously that the gap observed in ARPES is associated with superconductivity., Comment: 3 pages, revtex, 4 postscript figures

From a detailed study, including polarization dependence, of the normal state angle-resolved photoemission spectra for Bi$_2$Sr$_2$CaCu$_2$O$_8$, we find only one CuO$_2$ band related feature. All other spectral features can be ascribed either to umklapps from the superlattice or to ``shadow bands''. Even though the dispersion of the peaks looks like band theory, the lineshape is anomalously broad and no evidence is found for bilayer splitting. We argue that the ``dip feature'' in the spectrum below $T_c$ arises not from bilayer splitting, but rather from many body effects., Comment: 4 pages, revtex, 3 uuencoded postscript figures