Dynamics and distribution of doped holes in theCuO2plane of slightly dopedY1−yCayBa2Cu3O6studied by Cu(1) NQR

Abstract

Cu(1) nuclear quadrupole resonance (NQR) in slightly doped ${\text{YBCO}}_{6}$: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 ${\text{CuO}}_{2}$ plane by ${\text{Ca}}^{2+}\ensuremath{\rightarrow}{\text{Y}}^{3+}$ substitution are distributed homogeneously in the ${\text{CuO}}_{2}$ 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(1) 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 ${\text{Ca}}^{2+}$ ions.