Abstract

We present a series of electron spin resonance (ESR) and infrared (IR) transmission experiments in antiferromagnetic lightly hole doped YBa2Cu3O6 in search for the effect of a spatially inhomogeneous ground state on the magnetic and electric properties. Crystal compositions were Ca_xGd_yY_(1-x-y)Ba_2Cu_3O_6 with x=0, 0.008, 0.02 and 0.03; Gd is an ESR probe that does not effect the electronic structure. We show that holes are not preferentially localized at low temperatures in the vicinity of Ca dopants. We mapped the antiferromagnetic domain structure as a function of hole concentration, temperature and magnetic fields up to 8 T. We find a hole doping induced rotation of the magnetic easy axis from collateral to diagonal with respect to the tetragonal CuO2 lattice. We argue that at low temperatures the AF magnetization is pinned to a static modulation or a phase separated network of the hole density. The dominantly fourfold symmetry of pinning suggests that the hole density network has fourfold symmetry and is not an array of stripes. We discuss the possibility of AF domain wall fluctuations at higher temperatures. We measured IR transmission under magnetic field in search of an anisotropy of the conductivity at finite frequencies. A small zero frequency anisotropy has been interpreted by Ando et al. as arising from the rotation of stripes in a magnetic field. We find no anisotropy and conclude that while the antiferromagnetic domain structure is weakly pinned, the network of holes is rigid and is not affected by magnetic fields.