arXiv subjects
Publications and source records attributed to J. Röhler.
The bulge in the doping dependence of the basal plane area of cuprate superconductors is shown to be an effect of the particular inhomogenous electronic structure created by the dense packing of paired self-protected singlets (PSPS) in CuO_2 lattices.
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.
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.
We discuss the static deformations of the doped CuO_2 lattices in YBa_2Cu_3O_x. Intrinsic lattice effects driven by the strong correlations of the electron system are separated from extrinsic chemical and bandstructure effects. c-axis displacements of the planar copper atoms seem to be a generic property of the metallic CuO_2 lattices.
Oxygen doping in metallic YBa_2Cu_3O_x induces quadrupolar "alpha-ortho'', and breathing "beta-ortho" deformations of the CuO_2 planes. Breathing beta-ortho deformations favour hybridizations of the pd sigma Cu3d_x^2-y^2 - O2p_x,y with the pd pi Cu3d_x,z, 3d_y,z-O2p_z bands relaxing the confinement of the carriers in the overdoped regime, x>6.95.
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.
The dimpling in the CuO_2 planes of overdoped Y_{1-y}Ca_yBa_2Cu_3 O_{6.96-6.98} (y=0.02-0.2) has been measured by x-ray absorption-fine- structure spectroscopy (Y-K EXAFS). A step-like decrease around 12% Ca indicates a percolation threshold for distorted sites of 5 cells, and thus phase segregation. We conclude the charge carriers added by substitution of Y{3+} by Ca{2+} to be trapped at the Ca sites and their nn environment.
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.
In the pressure range 5-12 GPa the fractional valence of cerium is independent of pressure because of the coexistence of two or more structural phases. The observation of a single crystallographic structure in this pressure range arises from a non-equilibrium situation induced by the initial conditions of the high pressure experiment.
A structural phase transformation in the CuO_2 planes of YBa_2Cu_3O_x has been observed at the onset of the overdoped regime, x=6.95. We have measured as a function of x the dimpling in the CuO_2 planes by EXAFS, and the O2,3 in-phase (A_1g) mode by Raman scattering. The data show for x>6.95 anomalously large static displacements of the Cu2 atoms off the O2,3 layer, and a gap in the distribution of the O2,3 in-phase Raman shifts. We conclude the structure of overdoped YBa_2Cu_3O_x to be a martensitic form of the structure most favourable for the superconductivity, and possibly at the origin of the split superconducting transition in the overdoped regime.
We have measured the extended x-ray absorption-fine-structure (EXAFS) at the Y-K edge of YBa_2Cu_3O_x for x=6.801, 6.947, 6.968, 6.984 at T=20-300 K. Across the underdoped-overdoped transition the Y-Cu2 pairs exhibit harmonic vibrations which freeze out in the superconducting phase, whereas the Y-O2,3 pairs exhibit strong anharmonicities with a singularity at T_c. In the underdoped regime the average spacing O2,3-Cu2 increases with x due to displacements of Cu2 towards Ba, but the average position of O2,3 along c is almost unaffected. Optimum doping is a notable point in the x-T phase diagram, also concerning the lattice degrees of freedom. Here the O2,3-Cu2 spacing (buckling) is largest, and in the superconducting phase the Cu2 position along the c-axis is independent on temperature. In the overdoped regime increasing x displaces the Cu2 along c further towards Ba, but the O2,3 layer starts to shift in the same direction, thus decreasing the O2,3-Cu2 spacing.
Metal-oxygen complexes containing Cu,- Tl-, Hg-, Bi- and Pb-cations are electronically active in superconducting copper-oxides by stabilizing single phases with enhanced $T_c$, whereas other metal-oxygen complexes deteriorate copper-oxide superconductivity. Cu, Tl, Hg, Bi, Pb in their actual oxidation states are closed shell $d^{10}$ or inert $s^2$ pair ions. Their electronic configurations have a strong tendency to polarize the oxygen environment. The closed shell $d$ ions with low lying $nd^{10}\leftrightarrow nd^9(n+1)s$ excitations form linear complexes through $d_{z^2}-s$ hybridization polarizing the apical oxygens. Comparatively low $nd^9(n+1)s$ excitation energies distinguish $\rm Cu^{1+,3+}, Tl^{3+}, Hg^{2+}$ from other closed shell $d^{10}$ ions deteriorating copper-oxide superconductivity, {\it e.g.} $\rm Zn^{2+}$.