Dynamics and distribution of doped holes in the CuO_2 plane of slightly doped Y_(1-x)Ca_yBa_2Cu_3Q_6 studied by Cu(1) NQR

摘要

Cu(1) nuclear quadrupole resonance (NQR) in slightly doped YBCO_6:Ca compounds allows us to study the incidence of doped holes on the antiferromagnetic state. Distributions of transverse (1/T_2) and longitudinal (1/T_1) relaxation rates of the NQR are found at low temperature, which allows us to determine a fraction of doped holes which are localized. We conclude that the holes doped in the CuO_2 plane by Ca~(2+)→Y~(3+) substitution are distributed homogeneously in the CuO_2 plane above 70 K and move freely in the plane. We establish that the reduction in hole mobility from metallic to variable-range hopping induces the differentiation of Cu(l) nuclei. At lower T the holes' motion slows down and we estimate that the holes localize finally in restricted regions (4-6 lattice constants) in the Coulomb potential of the Ca~(2+) ions.