Frustrated phase separation in the overdoped regime of "123" high-T_c superconductors

Recent inelastic neutron scattering measurements revealed the crystal-field (CF) spectra of the overdoped R_1-yCa_yBa ₂Cu₃O_≈7 (R = Ho, Er; 0 < y < 0.25) high-T_c superconductors to consist of two spectral components associated with the optimally doped and the overdoped domains. Increase of the Ca concentration does not affect the local charge density of domains, but changes the spectral weight of the components. In the sense of this "two-phase" picture, which was established earlier for the underdoped region, there is a smooth crossover between the under- and overdoped parts of the phase diagram. The charge origin of the observed superposition effect is confirmed by the systematic analysis of the concentration dependences of the CF parameters for both Ho- and Er-based compounds. Therefore, the local charge inhomogeneity in the CuO₂ planes of high-T_c cuprates is a characteristic feature of the doping process, which depends neither on the way to introduce doping nor on the doping level.