Phys. Rev. Lett. 86, 2882–2885 (2001)Strong Anisotropy of Superexchange in the Copper-Oxygen Chains of La14-xCaxCu24O41Received 27 March 2000; published in the issue dated 26 March 2001 Electron spin resonance data of Cu2+ ions in La14-xCaxCu24O41 single crystals (x = 9,11,12) reveal a very large width of the resonance line in the paramagnetic state. This signals an unusually strong anisotropy of ∼10% of the isotropic Heisenberg superexchange in the Cu-O chains of this compound. The strong anisotropy can be explained by the specific geometry of two symmetrical 90° Cu-O-Cu bonds, which boosts the importance of orbital degrees of freedom. Our data show the apparent limitations of the applicability of an isotropic Heisenberg model to the low-dimensional cuprates. © 2001 The American Physical Society URL:
http://link.aps.org/doi/10.1103/PhysRevLett.86.2882
DOI:
10.1103/PhysRevLett.86.2882
PACS:
76.30.-v, 71.27.+a, 71.70.Gm, 75.40.Gb
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