Session W1: ARPES in High Tc Superconductors

Laser ARPES, the sudden approximation, and quasiparticle-like peaks in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+d}$

A new low photon energy regime of angle resolved photoemission spectroscopy is accessed with lasers and used to study the superconductor Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+d}$. The low energy increases bulk-sensitivity, reduces background, and improves resolution. Crucial aspects of the data such as the dispersion, superconducting gaps, and the bosonic coupling kink are found to be robust to a possible breakdown of the sudden approximation. We observe spectral peaks which are sharp on the scale of their binding energy - the clearest evidence yet for quasiparticles in the normal state. The very sharp spectral peaks and high statistics enables detailed investigations of the temperature and energy dependences of the lineshapes, giving critical insights into the nature of the scattering mechanisms in these materials. We thank collaborations with J. D. Koralek, J.F. Douglas, N.C. Plumb, Z. Sun, A.V. Fedorov, M. Murnane, H. Kapteyn, S. Cundiff, Y. Aiura, K. Oka, and H. Eisaki   

What is the ``glue'' for high temperature superconductivity?

It has been 20 years since the high-temperature superconductivity (HTSC) was discovered in La$_{2-x} $Ba$_x$CuO$_4$, by Bednorz and M\"{u}ller. Since then, many different HTSC compounds, all containing copper-oxide planes, were synthesized and HTSC became one of the most studied problems in science. However, the mechanism of HTSC still remains unknown. Recent advancements in Angle-Resolved Photoemission Spectroscopy (ARPES) have enabled a direct probing of effects of interactions between electrons and different bosonic excitations in a system, raising the expectations that a ``pairing boson'' responsible for HTSC could finally be identified. Here, we will present the study of single-particle excitations in two different cuprates: Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$, a high-temperature superconductor with $T_C=91$ K and La$_{1.875}$Ba$_{0.125}$CuO$_4$, a system with suppressed superconductivity ($T_C\le 2.5$ K) due to the spin/charge ordering. The extracted self-energies and single-particle gaps will be compared and possible coupling mechanisms will be discussed. This work was supported by the DOE under contract number DE- AC02-98CH10886.   

Dispersion Anomalies and High Frequency Optical Conductivity in Cuprate Superconductors.

We argue that the shape of the dispersion along the nodal and antinodal directions in the cuprates can be understood as a consequence of the interaction of the electrons with collective spin excitations. In the normal state, the dispersion displays a crossover at an energy where the decay into spin fluctuations becomes relevant. In the superconducting state, the antinodal dispersion is strongly affected by the $(\pi,\pi)$ spin resonance and displays an $S-$shape whose magnitude scales with the resonance intensity. For nodal fermions, relevant spin excitations do not have resonance behavior, rather they are better characterized as a gapped continuum. As a consequence, the $S-$shape becomes a kink, and superconductivity does not affect the dispersion as strongly. We also analyzed recent infrared conductivity data in the normal state. We find that the high frequency behavior, which has been suggested as evidence for quantum critical scaling, is well described by the same interaction with overdamped collective modes. From explicit calculations, we find a frequency exponent for the modulus of the conductivity, and a phase angle, in good agreement with experiment.   

ARPES Investigation of Quasiparticle Renormalization in Cuprates

We investigated by Angle-Resolved-Photoemission-Spectroscopy the renormalization of the bands in Bi2212 as a function of doping, including underdoped (T$_{c}$ =85K), optimally doped (T$_{c}$ = 94K), and overdoped (T$_{c}$ = 65K) samples. We identified the sharp energy scale seen in the superconducting state with the B$_{1g}$ bond buckling mode, the out-of-phase vibrations of the in-plane oxygen. More recently, we compare doping dependent data to a theoretical calculation involving the identified$_{ }$modes, including both temperature and momentum dependence. This comparison brings insights to the doping induced spectral changes in the overdoped regime, in connection to some recent discussions. We will also present complimentary pressure dependent Raman scattering and X-Ray diffraction data showing how the B$_{1g}$ mode renormalizes with metallization of the insulating parent compound of Bi2212. The pressure variable allows a continuous assessment of the electron-phonon coupling lambda, based on both phonon frequency and lineshape, across the phase diagram. Finally, we discuss ways to test whether these mode couplings have a direct bearing on superconductivity.   

ARPES Study of Nodal Quasiparticles Using Low-Energy Tunable Photons

Low-energy quasiparticle excitations govern the thermodynamic properties of a superconductor both in the zero-field and vortex-mixed states. For a $d$-wave superconductor, nodal quasiparticles are crucial excitations starting from zero energy. So far, however, the nodal quasiparticle dynamics of high-Tc cuprates has been controversial. For example, it has been reported by an angle-resolved-photoemission (ARPES) experiment that the marginal-Fermi-liquid behavior persists into the superconducting state without appreciable change in the scattering rate, while microwave conductivity increases upon the superconducting transition. Here, we show a new ARPES result that solves the controversies with unprecedented momentum-resolution. Low-energy tunable photons have enabled us to resolve a small nodal bilayer splitting clearly, and to reveal the detailed temperature- and energy-dependence of the scattering rate, indicating the behaviors unique to the nodal quasiparticles. Due to the opening of the $d$-wave gap, the nodal scattering rate is remarkably suppressed, and shows a linear energy dependence. The difference in the energy-linear term between the bilayer-resolved scattering rates hints the nature of impurities involved. This work was done in collaboration with T. Yamasaki, T. Kamo, K. Yamazaki, H. Anzai, M. Arita, H. Namatame, M. Taniguchi, \textit{Grad.~Sch.~of Science and Hiroshima Synchrotron Radiation Center, Hiroshima Univ.}, A. Fujimori, \textit{Dept.~of Complexity Science and Engineering, Univ.~of Tokyo}, Z.-X. Shen, \textit{Dept.~of Physics, Applied Physics and SSRL, Stanford Univ.}, M. Ishikado, K. Fujita, and S. Uchida, \textit{Dept.~of Physics, Univ.~of Tokyo}.