Low dispersion of frequencies of the muon precession in samples of the high-temperature superconductors YBa2Cu3Ox and EuBa2Cu3Ox

The rate of relaxation of the muon spin (μSR) has been measured in single-crystal nonoriented grains of YBa2Cu3O6.89 and in the ceramics EuBa2Cu3O6.84 to determine the penetration depth λ of the magnetic field. The measurements have been carried out under cooling in a magnetic field of 2.5 kG in the temperature range 10 < T < 100 K. The experimental value of the mean-square deviation of the frequencies of the muon precession left-angle Δω2 right-angle 1/2(T→0) is ≅ 2.5 μsec-1, which corresponds to a depth of penetration of the field perpendicular to the CuO2 planes λab(0) ≅ 1,600 angstrom. The resulting value of the mean-square deviation of the frequencies is significantly smaller than the values of left-angle Δω2 right-angle 1/2 ∝ 1/λ2 corresponding to the penetration depth λab (0) = 1,300--1,400 angstrom obtained in most studies of the μSR. The low value of left-angle Δω2 right-angle 1/2 may be due to pinning of open-quotes vortex spotsclose quotes in superconducting planes of CuO2. The result obtained in the present study casts doubt on the validity of the traditional view of the penetration depth λμSR measured in a μSR experiment as a lower limit on λ(λ≥λμSR) in highly anisotropic high-temperature superconductors. 10 refs., 2 figs., 1 tab