Applied Physics Seminar

It has been 25 years since high temperature superconductivity in the cuprates was discovered with Tc = 34K by Bednorz and Muller who doped the anti-ferromagnetic insulator, La2CuO4, with Ba to form La2-xBaxCuO4. YBa2Cu3O7-δ with Tc = 92K was discovered soon thereafter in 1987. By 1994, the highest cuprate superconducting temperature was increased to 138K in Hg0.8Tl0.2Ba2Ca2Cu3O8+δ. No further increases in Tc have occurred in the past 17 years despite intense experimental and theoretical effort. This is because the mechanism of superconductivity remains unknown, so that there has been no guidance from theory to search for improved systems. Indeed, even the normal state properties of these cuprates are quite anomalous, differing substantially from normal metals. Thus, a successful theory must explain far more than just the mechanism of superconductivity. The biggest puzzle in the normal-state properties of cuprates is the appearance of a gap in the density of states that occurs far above Tc. This is called the Pseudogap (PG). The PG is observed at temperatures up to 600K with a magnitude that goes to zero at doping of ≈ 0.19 holes per planar CuO2, whereas superconductivity occurs for dopings between ≈ 0.05 and ≈ 0.27 with the optimal Tc at ≈ 0.16. The PG is observed in all normal state measurements and its origin has been a major unsolved problem in materials science. In this talk, we explain the origin and doping dependence of the PG along with a broad spectrum of cuprate phenomenology using only simple counting arguments. We show that the level of ab initio quantum calculations that leads to accurate band gaps also leads to the formation of four-Cu-site plaquettes in the vicinity of the dopants comprised of orbitals out of the CuO2 planes. Counting the distribution of these plaquettes as a function of doping determines the superconducting phase diagram, the D-Wave superconducting gap symmetry, the origin of the PG and its doping dependence, and additional universal cuprate features such as the thermoelectricity, the neutron spin resonance, and dispersionless STM incommensurabilities. The success in explaining this diverse set of strange properties provides strong evidence in favor the importance of these inhomogeneous plaquettes in cuprates. Our theory also leads to a direct prediction on how to increase Tc by controlling the location of dopants in these materials. This suggests that even higher Tc s are possible in the cuprate class of superconductors.