On self-protecting singlets in cuprate superconductors

The basal area (Cu-Cu grid) of the cuprate superconductors not only tends to shrink on hole doping, as expected from single electron quantum chemistry, but exhibits also an electronically incompressible "hump'' around optimum doping n_opt = 0.16. The hump collapses near critical doping n_crit = 0.19. We analyze the origin of the hump in terms of a classical liquid of interacting incompressible particles in a container with antiferromagnetic walls. Oxygen holes interacting with the wall form singlets, protect themselves against other holes by an incompressible "spin fence'', and thus interact also with the lattice. Occupation of the CuO_2 lattice with holes must therefore follow a non-double-occupant constraint also for the oxygen cage enclosing the copper hole. Closest packing of self-protecting singlets is found to occur around critical doping; closest packing of paired self-protecting singlets around optimum doping. These singlet-states are bosonic, but are not magnetic polarons.Comment: reviewed version, 7 pages, 10 figure

Plane dimpling and Cu 4s hybridization in YBa_2Cu_3O_x

Oxygen doping dimples the CuO_2 planes of YBa_2Cu_3O_{6.8-6.92} by displacing copper normal to the planes and further towards Ba. The correlated oxygen displacements, however, are constrained to the in-plane axes. This displacement pattern is discussed in terms of doping dependent Cu 4s - 3d hybridizations.Comment: 2 pages, submitted to Physica B (Proc. LT22

Nonlinear Doping of Cuprate Superconductors -- The case of Bi_2Sr_{2-x}La_xCuO_{6+\delta}

We analyze the hole doping mechanism in Bi_2Sr_{2-x}La_xCuO_{6+\delta} (BSLCO). The singular optimum around x=0.35 is found to be connected with a feedback between the doped CuO_2 layers and its dopant reactant [La^{3+}/Bi^{3+}-O_\delta] locking the number of doped holes preferentially on to the universal optimum n_opt=0.16.Comment: Typos correcte

Intraplanar couplings in the CuO_2 lattice of cuprate superconductors

We have investigated the doping dependencies of the basal areas in single-layer high-T_c cuprates La_{2-x}Sr_xCuO_4 and HgBa_2CuO_x, as well as in two-layer Y_{1-y}Ca_yBa_2Cu_3O_x and HgBa_2CaCu_2O_x. The basal areas not only tend to shrink on hole doping, as expected from single electron quantum chemistry, but exhibit also a "bulge'' around optimum doping. We attribute the "bulge'' to the effects of the strongly correlated quantum liquid on the CuO_2 lattice, rendering it nearly incompressible around optimum doping, but highly compressible in the weakly overdoped regime. Inhomogenous doping cannot account for this anomaly in the electronic compressibility of the CuO_2 lattice.Comment: 2 pages, 2 figures, to appear in Physica C (Proc. M2S-HTSC-VII

The dimpling in the CuO_2 planes of YBa_2Cu_3O_x (x=6.806-6.984, T=20-300 K) measured by yttrium EXAFS

The dimpling of the CuO_2 planes (spacing between the O2,3 and Cu2 layers) in YBa_2Cu_3O_x has been measured as a function of oxygen concentration and temperature by yttrium x-ray extended-fine-structure spectroscopy (EXAFS). The relative variations of the dimpling with doping (x=6.806-6.984) and temperature (20-300 K) are weak (within 0.05 AA), and arise mainly from displacements of the Cu2 atoms off the O2,3 plane towards Ba. The dimpling appears to be connected with the transition from the underdoped to the overdoped regimes at x=6.95, and with a characteristic temperature in the normal state, T*=150 K.Comment: 6 pages, 2 ps figs, LaTEX, Elsevier Elsart styl