LOW-ENERGY AND LOW-TEMPERATURE PROPERTIES OF UNTWINNED YBA2CU4O8 - GAPS OR CONDENSATION

We report on experimental evidence of different electronic phases-in t he superconductor YBa2Cu4O8: simple metallic CuO chains and highly cor related 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 domina tes. For the a direction (only plane conduction channel), the resistiv ity reveals an unconventional behavior with a kink at 160 K, becoming linear at higher temperatures. Further, we present results of the dyna mical (optical) conductivity of the CuO2 plane as a function of freque ncy and temperature. The frequency dependence of the optical conductiv ity is consistent with a model of ferromagnetic polarons in an antifer romagnetic matrix. This is further confirmed by a depolarization exper iment which is sensitive to crystal regions with different spin polari zations. The temperature behavior of the thermal occupation of the gro und state is in agreement with a real-space condensation.