New low activation energy processes in La2CuO4+d by elastic energy loss experiments

he anelastic relaxation spectrum of La2CuO4+? in the as prepared condition is measured from 200 K down to 1.3 K at frequencies included between 0.7 and 9 kHz. Two new relaxation processes are found with maxima at 4 and 70 K. The process at 70 K consists of two close peaks, with activation enthalpies of 0.11 and 0.13 eV. Possible mechanisms are thermally activated hopping of the holes doped by interstitial O, possibly bound to some lattice defect or impurity, or of atoms in off-centre positions. The peak at 4 K is, to our knowledge, by far the fastest anelastic relaxation process ever reported in an oxide superconductor. It is broader than a single relaxation process, but definitely sharper than the maxima which are typical of amorphous materials. The relaxation should again involve polarons or off-centre atoms, with a rate which is dominated by tunnelling.