Charge stripes and local structural inhomogeneities in lanthanum (strontium, barium) copper oxide high-temperature superconductors

We have investigated the possibility that the doped charge distribution in the CuO₂ planes of the high temperature superconducting cuprates is microscopically inhomogeneous, possibly with a striped morphology. The existence and importance to the properties of charge stripes in superconducting samples is not fully established. No long-range ordered static stripes have been observed in superconducting samples which means that either such formations are not present, or they are short range ordered and not observable crystallographically.; Atomic pair distribution function local structural study has been carried out to investigate the nature of the doped-charge distribution within the CuO₂ planes of La_2−x(Sr,Ba) _xCuO₄ as a function of doping and temperature. The data were obtained by time-of-flight neutron powder diffraction experiments. Evidence is found for temperature dependent atomic-scale structural inhomogeneities at low temperature in the underdoped and optimally doped regions of the phase diagram, compatible with the presence of inhomogeneous charge distribution in the CuO₂ planes. Inhomogeneities are seen as a planar Cu-O bondlength distribution, accompanied by a significant degree of the CuO₂-octahedral tilt magnitude and directional disorder. The inhomogeneities disappear on crossing into the overdoped region at low temperature. They also disappear in underdoped samples at temperatures which correlate with spin and charge freezing in this system inferred from transport, NQR, and XANES measurements.; Recent theoretical calculations suggested that the doping in the La _2−xSr_xCuO ₄ system has predominantly apical character, perpendicular to the CuO ₂ planes, in contrast to the established understanding that doping has planar character. This new way of understanding the doping, if it is right, would result in a paradigm shift in the understanding of cuprate physics. The orbital character of doped holes in the LSCO system has been reexamined from the structural perspective. The structural results, in contrast to the above theoretical prediction, are in agreement with the planar-doping picture in the underdoped and optimally doped regime, and indicate that a small amount of doped charge may have apical character above the optimal doping.