Abstract

We report on experimental evidence for different electronic phases in the superconductor YBa2Cu4O8: simple metallic Cu–O chains and highly correlated CuO2 planes. YBa2Cu4O8 is a genuine untwinned compound; hence, we were able to determine the anisotropy of the resistivity along the a and b directions. Along the b direction (chain and plane conduction channels), a normal metallic temperature behavior of the chain dominates. For the a direction (only the plane conduction channel), the resistivity reveals unconventional behavior with a kink at 160 K, becoming linear at higher temperatures. Further, we present results of the dynamical (optical) conductivity of the CuO2 plane as a function of frequency and temperature. The frequency-dependence of the optical conductivity is consistent with a model of ferromagnetic polarons in an antiferromagnetic matrix. This is further confirmed by a depolarization experiment, which is sensitive to crystal regions with different spin polarizations. The temperature behavior of the thermal occupation of the ground state is in agreement with a real-space condensation.

Introduction

From the beginning [1] the high-Tc superconductors (HTSC) have been very challenging systems. Regarding their solid state chemistry, the defect structure with its wide variability in stoichiometry gives rise to homogeneity problems: the samples may show a chemical phase separation of different oxygenated regions. So, proper preparation of homogeneous samples is crucial. Concerning the physics, the normal as well as the superconducting state show unusual features. In addition, as we will show at least for YBa2Cu3O7 (123) or YBa2Cu4O8 (124), one has to take into account the coexistence of different physical phases.