Strong anisotropy of superexchange in the copper-oxygen chains of La14-xCaxCu24O41

Electron spin resonance data of Cu2+ ions in La14-xCaxCu24O41 single crysta ls (x = 9, 11, 12) reveal a very large width of the resonance line in the p aramagnetic state. This signals an unusually strong anisotropy of similar t o 10% of the isotropic Heisenberg superexchange in the Cu-O chains of this compound. The strong anisotropy can be explained by the specific geometry o f two symmetrical 90 degrees Cu-O-Cu bonds, which boosts the importance of orbital degrees of freedom. Our data show the apparent limitations of the a pplicability of an isotropic Heisenberg model to the low-dimensional cuprat es.