Dynamic Inhomogeneity, Pairing and Superconductivity in Cuprates

Abstract

In cuprates there is a significant body of evidence for the existence of electronically driven dynamic inhomogeneity which might arise from the existence of polarons, bipolarons or other charged objects such as stripes, whose presence may be an essential ingredient for high-temperature superconductivity. In this review we examine the experimental evidence for such objects, defining the length, time and energy scales of the relevant elementary excitations. The dynamics of the objects below and above T_c are examined in detail with femtosecond spectroscopy and compared with magnetic and other measurements. The dynamically inhomogeneous state is described theoretically by considering an interaction between electrons, spins and the lattice. By symmetry, only electrons in degenerate states can couple to the lattice and spins to give an anisotropic, d-wave symmetry interaction. The proposed interaction acts on a mesoscopic length-scale, taking into account the interplay of Coulomb repulsion between particles and anisotropic elastic strain, and leads to the formation of bipolaron pairs and stripes. The predicted symmetry breaking associated with pairing and stripe formation are observed in numerous experiments. The phenomenology associated with the co-existence of pairs and clusters (stripes) is found to apply to many different experiments ranging from femtosecond dynamics to transport measurements. Excitations of the system are described quite well in terms of a two-level system, although we find that a complete description may require a more complicated energy landscape due to presence of mesoscopic objects such as stripes or clusters. The formation of the superconducting state can be understood quantitatively to be the result of the establishment of phase coherence percolation across pairs and stripes.