The pseudogap: friend or foe of highT_c?

Abstract

Although nineteen years have passed since the discovery of high temperature cuprate superconductivity [1], there is still no consensus on its physical origin. This is in large part because of a lack of understanding of the state of matter out of which the superconductivity arises. In optimally and underdoped materials, this state exhibits a pseudogap at temperatures large compared to the superconducting transition temperature [2,3]. Although discovered only three years after the pioneering work of Bednorz and Müller, the physical origin of this pseudogap behavior and whether it constitutes a distinct phase of matter is still shrouded in mystery. In the summer of 2004, a band of physicists gathered for five weeks at the Aspen Center for Physics to discuss the pseudogap. In this perspective, we would like to summarize some of the results presented there and discuss the importance of the pseudogap phase in the context of strongly correlated electron systems.

Damped spin excitations in a doped cuprate superconductor with orbital hybridization

“…To express these new terms, we introduce the following notation J 1 = 4t 2 U , J 2 = 4t 4 U 3 , J 1 = 4t 2 U , and J 2 = 4t 4 U 3 . Geometrically the following contributions correspond to different hopping path combinations including the cyclic ones, …”

“…As a consequence, the d z 2 states are less relevant and hence lead to a less pronounced zone-boundary dispersion. 2 -the expected theoretical value for t = t = 0. To obtain J 2 the spin-excitation dispersion is fitted with U/t and t /t as open parameters while keeping t = 0.43 eV, t = −t /2 and Z = 1.219 fixed.…”

“…A key prediction is thus that E ZB scales as (t /t) 2 with a prefactor that depends on (U/t) 2 . This effective Heisenberg model is in principle not applicable to doped and hence antiferromagnetically disordered cuprates.…”

“…Considerable research is being undertaken in the quest to reach consensus on the mechanism of high-temperature superconductivity [1] and the associated pseudogap phase [2] in copper-oxide materials (cuprates). The energy scales governing the physical properties of these layered materials therefore remain of great interest.…”

“…Empirically [11], the superconducting transition scales with the ratio t /t whereas Hubbard-type models predict the opposite trend [12,13]. As a resolution, a two-orbital model-in which hybridization of d z 2 and d x 2 −y 2 states suppresses T c and enhances t -has been put forward [14].…”