Session J9: Superconductivity: Oxide Photoemission I and X-ray Scattering

A Proposed New Measurement of the Superconducting Gap in YBa$_{2}$Cu$_{3}$O$_{7}$

The superconducting energy gap of YBa$_{2}$Cu$_{3}$O$_{7}$ (YBCO) varies strongly with $\vec {k}$and from a sheet of the Fermi surface to another. The strong anisotropic superconducting gap in high Tc materials such as YBCO has led to conflicting d-wave and s-wave interpretations. We have utilized electronic wave functions from the ab-initio density functional calculation and the related electron-phonon interaction matrix elements for the calculation of the superconducting gap values of YBCO. For three pieces of the Fermi surfaces, the calculated superconducting gaps exhibit a strong anisotropy. In contrast, we have found that the superconducting gap on one sheet of the Fermi surface around S-point only shows a minor variation from about 18 meV to 25 meV. Especially, there is no node on this sheet of the Fermi surface. We propose a new test measurement of the superconducting gap of YBCO on this sheet of the Fermi surface around the S-point. This measurement is expected to shed light on the gap symmetry properties of high Tc superconductors. This work was funded in part by NSF (Award No. 0508245) and ONR (Grant No: N00014-05-1-0009).   

Dynamic Response Functions from Angle Resolved Photoemission Spectra

The linear response to an external probe as a function of energy and momentum is of great importance in elucidating the properties of complex materials. We introduce a formalism in the framework of diagrammatic k space approach with Random Phase Approximation (RPA), for calculating dynamic response functions using experimental single particle Green's function derived from ARPES spectra. Specifically we focus on using the single particle Green's function obtained from superconducting state of ARPES data in the High Tc cuprates to compute the dynamic spin susceptibility. We find good agreement between our results and the superconducting state neutron results, in particular the resonance at antiferromagnetic ordering wave vector, with its unusual `reverse magnon' dispersion. We anticipate, our formalism will also be useful in interpreting results from other spectroscopies such as optical and Raman responses.   

Self Energy Analysis of Photoemission spectral function from parent cuprates, Ca$_{2}$CuO$_{2}$Cl$_{2}$

Self energy $\Sigma $ (\textbf{k},$\omega )$ is the fundamental function that describes the effects of many-body interactions on an electron in a solid. But it is very difficult to extract the self energy from experimental data, especially for non-metallic materials. In this paper we developed a new and general method with which one can extract the self energy from angle-resolved photoemission spectroscopy (ARPES) data in the full \textbf{k}-$\omega $ space. We demonstrate the validity of this method by applying it to the ARPES data from Ca$_{2}$CuO$_{2}$Cl$_{2}$ (CCOC). We find the values for the imaginary part of the self energy is compatible with the value obtained by measuring the peak width.   

The hierarchy of multiple many-body interaction scales in high-temperature superconductors

Many-body interaction is key to novel properties of quantum matter. As an extreme example, the complexity due to charge, spin, and lattice interactions in high-T$_{c}$ superconductors makes it difficult to identify the essential microscopic ingredients for the basic model. Energy scales where these interactions are manifest usually provide important insights into the nature of the interactions. The energy-momentum dispersion relationship measured by angle-resolved photoemission spectroscopy (ARPES) provides an excellent tool for identifying these scales. To date, the focus of the discussion has been on the low energy anomaly near 0.03-0.09eV. Here we present improved experimental data from four families of high-T$_{c}$ superconductors that reveal a hierarchy of many-body interaction scales focused on: the low energy anomaly (``kink'') of 0.03-0.09eV, a high energy anomaly of 0.3-0.5eV, and an anomalous enhancement of the width of the LDA-based CuO$_{2}$ band extending to energies of approximately 2 eV.   

Energy scales revealed by ARPES study on four layered cuprate superconductor Ba$_{2}$Ca$_{3}$Cu$_{4}$O$_{8}$(O$_{\delta }$F$_{1-\delta })_{2}$

Recently discovered four layered cuprate superconductor Ba$_{2}$Ca$_{3}$Cu$_{4}$O$_{8}$(O$_{\delta }$F$_{1-\delta })_{2}$ posseses various properties that differ from the conventional cuprate High T$_{c}$ superconductors, making itself exceptional to the well known high T$_{c}$ superconductivity phase diagram. The understanding of this discrepancy will provide important implications for high T$_{c}$ theory. We study this material by Angular Resolved Photoemission Spectroscopy (ARPES) and find that the electronic band structure of this four layered system exhibits clear difference from the previously studied hole- or electron- doped cuprate superconductors such as Bi2212 or NCCO. The multiple energy scales associated with its electronic structures not only show the complexity of this system, but also provide hints to understand its uniqueness.   

The evolution of electronic structure of Bi$_2$Sr$_{2-x}$Bi$_{x}$CuO$_{6+\delta}$ revealed by ARPES

Bi$_{2}$Sr$_{2-x}$Ln$_{x}$CuO$_{6+\delta}$ (Ln is a trivalent element) is a good candidate to investigate the effects of charge doping and potential disorder to the properties of the high-Tc cuprates. High-quality single crystals of Bi$_2$Sr$_{2-x}$Bi$_x$CuO$_{6+\delta}$ (Bi-Bi2201) have been synthesized over a wide substitution range ($0 [Preview Abstract]

High-energy kink in high-temperature superconductors

Photoemission studies show the presence of a high energy anomaly in the observed band dispersion for two families of cuprate superconductors, Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{4+\delta }$and La$_{2-x}$Ba$_{x}$CuO$_{4}$. The anomaly, which occurs at a binding energy of approximately 340 meV, is found to be doping and momentum independent. The magnitude of the effect is momentum dependent. Scattering from short range or nearest neighbour spin excitations is found to supply an adequate description of the observed phenomena.   

Systematic ARPES study of n-doped cuprate superconductors

In contrast to the hole-doped high-temperature superconductors, for which the Cu2+ long-range antiferromagnetism (AF) is suppressed at low doping, the AF order is more robust and extends to higher doping in the case of the electron-doped superconductors RE2-xCexCuO4 (RE = Pr, Nd, Sm) and (Pr,La)2-xCexCuO4. Even though this long-range ordering is suppressed at optimal doping, neutron measurements and Hubbard model calculations suggest the persistence of short-range fluctuations. In order to investigate the impact of these fluctuations on the electronic structure of the electron-doped superconductors, we have performed systematic angular resolved photoemission spectroscopy measurements of optimally doped Pr2-xCexCuO4 and (Pr,La)2-xCexCuO4 samples. We present and discuss our recent results.   

{\it s}-wave-like excitation in the superconducting state of electron-doped cuprates with {\it d}-wave pairing

It has been quite controversial whether the superconducting (SC) pairing symmetry is {\it s}- or {\it d}-wave in electron-doped cuprates such as Nd$_{2-x}$Ce$_x$CuO$_4$ (NCCO) and Pr$_{2-x}$Ce$_x$CuO$_4$ (PCCO). In view that many experimental measurements to study the SC pairing symmetry only give direct information about the quasiparticle excitation gap and not the SC order parameter, we explore a physical mechanism to show the {\it s}-wave-like quasiparticle excitation under the {\it d}-wave SC pairing in electron-doped cuprates and intend to reconcile the contradictory experimental results. Our idea is based on the intrinsic Fermi surface (FS) evolution with doping as revealed by ARPES measurements on NCCO. It was found that at low doping only a small FS pocket appears around $(\pi,0)$ and with increasing doping a new FS pocket around $(\pi/2,\pi/2)$ will emerge. We argue that the FS pocket around $(\pi/2,\pi/2)$ has not yet formed until doping reaches about the optimal value $x=0.15$. Therefore in the underdoped regime, even if the SC order parameter is $d$-wave which vanishes along the diagonal line, the quasiparticle excitation gap is still finite and looks $s$-wave-like due to the absence of the FS across that line. This makes it possible, with $d$-wave SC pairing, to understand those experiments which evidenced the $s$-wave quasiparticle excitation.   

Spectral function analysis of an e-doped Hi-Tc superconductor near optimal doping, revisited

By comparison with the p-doped high transition temperature superconductors, their e-doped counterparts might give further insight into the unusual underlying physics. High resolution angle-resolved photoemission spectroscopy (ARPES) data of an e-doped cuprate near optimal doping is presented to further enrich previous comprehensive work on the subject \footnote{N.P. Armitage et al., PRB 68, 064517 (2003); H. Matsui et al., PRLetters 94, 047005 (2005)}. Spectral function analysis is also used to discuss band renormalizations. Other findings will be discussed as well.   

Intermediate energy structure of cuprates using Resonant Inelastic X-ray Scattering

We present a comprehensive study of the charge-transfer excitations in the 1-8 eV range using the burgeoning technique of resonant inelastic X-ray scattering (RIXS). Surprisingly, we find that the charge-transfer gap, distinct at around 2.25 eV in Mott insulating La$_{2}$CuO$_{4}$, is also discernible in the high-T$_{c}$ superconductor HgBa$_{2}$CuO$_{4+\delta }$. In addition, we are able to identify many distinct, weakly dispersive features above the charge-transfer gap of La$_{2}$CuO$_{4}$ [1-3] and the model high-Tc superconductor HgBa$_{2}$CuO$_{4+\delta }$ [1]. Detailed extension of this work in La$_{2}$CuO$_{4}$ reveals previously unresolved systematics in the vicinity of the charge-transfer gap, and a distinct dependence on scattering geometry of both the charge-transfer gap and the high-energy excitations. We interpret this scattering-geometry dependence as arising from the intrinsic symmetry selectivity of the RIXS/Raman process, and suggest that similar experiments can give definitive identification of excitation symmetry. [1] L. Lu et al., Phys. Rev. Lett. 95, 217003 (2005). [2] L. Lu et al. (to appear Phys. Rev. B 74; cond-mat/0607311) [3] J. N. Hancock et al. (in preparation).   

RIXS Spectra for Ladder Cuprate $Sr_{14}Cu_{24}O_{41}$

The ladder compound $Sr_{14}Cu_{24}O_{41}$ is of interest both as a quasi-one-dimensional analog of superconducting cupartes and as a superconductor in its own right when $Sr$ is substituted by $Ca$. In order to model recent resonant inelastic x-ray scattering (RIXS) spectra of this compound, we studied the simpler $SrCu_{2}O_{3}$ system in which the crystal structure contains very similar ladder planes. We approximated the LDA dispersion of $SrCu_{2}O_{3}$ by two different tight-binding models - either a copper only model with two bands or a copper plus planar oxygen model with seven bands. Due to the glide symmetry of the structure, the period of dispersion along the ladder is $4\pi$. Strong correlation effects were treated by assuming an anti-ferromagnetic ground state for both models. The resulting dispersion of the filled band at half filling matches the experimental ARPES spectra, and the RIXS spectra are in good agreement with experimental results. Work supported in part by the USDOE   

Inelastic X-ray scattering study of the bond stretching phonon mode in Bi$_2$Sr$_{2x}$Cu$_2$O$_{6+\delta}$

The phonon dispersions of the single layer high temperature superconductor Bi$_2$Sr$_{2-x}$Cu$_2$O$_{6+\delta}$ along the [$\xi$ 0 0] direction have been determined by inelastic x-ray scattering. The two highest longitudinal phonon branches, associated with the Cu-O bond stretching and out-of-plane oxygen vibration, are clearly resolved for the first time. The comparison with La$_{2}$Sr$_x$Cu$_2$O$_{0}$ will be discussed.   

Structures and properties of Ni$^{1+/2+}$ nickelates with infinite NiO$_{2}$ layers

Layered mixed valence Ni$^{1+/2+}$ nickelates possess similar crystal and electronic structures to Cu$^{2+/3+}$ high temperature superconducting cuprates. Only a few Ni$^{1+/2+}$ nickelates have been identified and they properties have not been reported so far. We present a first systematic study of Ln$_{n+1}$Ni$_{n}$O$_{2n+2}$, Ln = La or Nd, which structures could be described as an intergrowth of {\{}LnO$_{2}${\}} fluorite and infinite layer {\{}LaNiO$_{2}${\}}$_{n}$ blocks. The crystal structures of the new Ln$_{3}$Ni$_{2}$O$_{6,}$ Ln$_{4}$Ni$_{3}$O$_{8,}$ Ln$_{5}$Ni$_{4}$O$_{10}$ phases have been confirmed by X-ray and neutron powder diffraction. X-ray absorption spectroscopy data proves the 1+/2+ oxidation state and planar coordination of Ni atoms. Magnetic susceptibility data of Ln$_{n+1}$Ni$_{n}$O$_{2n+2}$ will be discussed.