Anelastic relaxation evidence of very low activation energy processes in superconducting YBa₂Cu₃O_7-x: magnetic chain excitations?

Relaxation rates obtained from a wide range of acoustic loss measurements are explained by introducing three thermally activated processes controlled by activation energies of 0.06, 0.14 and 0.2 eV in superconducting YBa2Cu3O7-x. The ultrasonic attenuation peak magnitudes indicate that the interactions responsible originate in at least 10% of unit cells in the bulk of the material. An analysis of spin excitations on the copper-oxygen chains is presented that leads to a series of discrete excitation energies matching those obtained experimentally.

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