In 1986, high-Tc superconductivity was discovered in a class of materials known as the cuprates [86B1]. For the first time, superconductivity was achieved at temperatures above those thought to be enabled through the electron-phonon interaction. In short order materials were identified with transition temperatures, Tc, above liquid nitrogen temperatures [87W1]. These discoveries prompted some major challenges for condensed matter physics research, namely, how do you explain the high-transition temperatures and how do we develop new theories that calculate the properties of strongly correlated materials. An excellent review of ARPES studies of the cuprates has been provided by Damascelli and coworkers [03D1]. While there has been very little study of surface-related phenomena in the cuprates, we include a discussion here because in that the cuprate materials are essentially two dimensional; they have proven to be perfect materials for study by photoemission. The 2D property plus new developments in the technology associated with ARPES pushed the photoemission technique to the very forefront in the study of condensed matter systems.
In the area of high-Tc superconductivity with the transition temperatures sufficiently high that the superconducting gap was visible in the ARPES spectra, it became possible to demonstrate the order parameter in the gap function, both in the superconducting phase and in the so-called pseudogap phase that exists in the underdoped region of the phase diagram in the normal state. The comparison of gap measurements along the copper-oxygen bond direction (A) and the zone diagonal (B) provided a clear demonstration of a superconducting order parameter having d-wave symmetry described by
where Δ0 represents the gap measured in the copper-oxygen bond direction at the zone boundary. This function is clearly very different from the isotropic s-wave symmetry associated with phonon-mediated superconductivity. Subsequent measurements found evidence of a gap, the so-called pseudogap, showing an angular dependence in the underdoped regime, even above Tc [96D2, 96M1]. There are now several studies that indicate that while in the overdoped regime the Fermi surface represents a full Fermi surface with area equal to 1 + p with p the number of doped holes, in the underdoped or pseudogap regime the Fermi surface is not simply a truncated full Fermi surface but rather deviates form that shape indicative of the presence of smaller pockets with area proportional to p (Figs. 125.1, 125.2, 125.3, 125.4, 125.5, 125.6, and 125.7).
There have also been a number of studies that have targeted the self-energy effects in these materials. Studies of the imaginary part of the self-energy, ImΣ, as a function of binding energy and temperature [99V2, 06V1] confirmed a so-called marginal Fermi liquid picture [89V1] whereby ImΣ scaled linearly with ω and T in the normal state. These studies have also revealed a mass renormalization characteristic of the coupling to some form of boson. From its very first observation, i.e., Fig. 90.2 [99V2], through to the present time, the origin of the renormalization has been heavily disputed between on the one hand coupling to spin excitations [01J1] or to phonons [01L2]. Aside from structure in the measured dispersion in the vicinity of 60–70 meV binding energy, other structure has also been identified in the region of 10 meV [09R1, 10P1] binding energy and also at the higher binding energy of 300 meV [07V1]. Lifetime effects have also been investigated in pump-probe studies or two-photon photoemission (Figs. 125.8, 125.9, 125.10, 125.11, 125.12, 125.13, 125.14, 125.15, and 125.16).