The role of angle-resolved photoemission in understanding the high temperature superconductors J.C. Campuzano a,b, * , A. Kaminski a,b , H. Fretwell a,b , J. Mesot a,b , T. Sato c , T. Takahashi c , M. Norman b , M. Randeria d , K. Kadowaki e , D. Hinks b a Department of Physics, University of Illinois at Chicago, Chicago, IL 60607, USA b Materials Science Division, Argonne National Laboratory, Argonne, IL 60439, USA c Tohoku University, Sendai, Japan d Tata Institute, Mumbai, India e Tsukuba University, Tsukuba, Japan Abstract The two-dimensional nature of the high temperature superconductors allows the determination of the energy±momentum relationship of electronic states by angle-resolved photoemission (ARPES). Furthermore, the shape of the ARPES spectra provides information on the many body interactions so prevalent in these materials. In this paper we review some results obtained by our group on the question of the existence of quasiparticles and their interactions. q 2000 Published by Elsevier Science Ltd. Keywords: C. Photoelectron spectroscopy; D. Superconductivity We have shown that the angle-resolved photoemission spectroscopy (ARPES) signal from the high temperature superconductors is given by [1], I k; v I 0 k; n; Af vAk; v 1 where k is the in-plane momentum, v is the energy of the initial state measured relative to the chemical potential, f v 1=expv=T  1 1 is the Fermi function and the one-particle spectral function Ak; v21=pIm G k; v 1 i0 1  is shown in Fig. 1a. The prefactor I 0 is propor- tional to the dipole matrix element uM fi u 2 and thus a function of k and of the incident photon energy Én and polarization A. The measurement of Ak; v provides a direct window into the many-body interactions. We believe this has resulted in a qualitative change in thinking about ARPES data and its analysis. Along with this have come a variety of new physics results which have shed very important new light on the high T c superconductors. Here we review our recent results concerning the existence of quasiparticles and their interactions. We ®rst examine the question of the existence of quasi- particles in the normal state. Curve (1) in Fig. 1b shows the lineshape at the antinode point of the Brillouin zone in the normal state for a Bi2212 T c  89 K sample. The line- width is very large, even in this optimally doped sample. Remembering that half the spectral function is cut off by the Fermi function, we see that the width of this peak is ,200 meV, or 2000 K, much larger than the temperature of the sample, and therefore intrinsic. It is very important to emphasize that the large linewidths observed in ARPES are not extrinsic, or artifacts of any analysis. As we will see next, when quasiparticles do exist (for T p T c  they are clearly seen in the experiment. We must therefore conclude that the normal state spectral function is extremely broad, and there are no well-de®ned quasiparticles above T c ! This anomalous normal state spectrum implies a breakdown of Fermi liquid theory, as suggested by numerous transport experiments. As the temperature is lowered below T c , the lineshape changes as shown in curve (5) in Fig. 1b, which may be understood as follows. For T , T c the SC gap opens up and spectral weight at k F shifts from v  0 (in the normal state) to either side of it, of which only the occupied side Journal of Physics and Chemistry of Solids 62 (2001) 35±39 0022-3697/00/$ - see front matter q 2000 Published by Elsevier Science Ltd. PII: S0022-3697(00)00097-4 www.elsevier.nl/locate/jpcs * Corresponding author. Department of Physics, University of Illinois at Chicago, 845 W. Taylor Street, Chicago, IL 60607, USA. E-mail address: jcc@uic.edu (J.C. Campuzano).