Low-energy and low-temperature properties of untwinned YBa2Cu4O8: Gaps or condensation

Abstract

We report on experimental evidence of different electronic phases in the superconductor ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_4$ ${\mathrm{O}}_8$: simple metallic CuO chains and highly correlated ${\mathrm{CuO}}_2$ planes. ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_4$ ${\mathrm{O}}_8$ 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 plane conduction channel), the resistivity reveals an unconventional behavior with a kink at 160 K, becoming linear at higher temperatures. Further, we present results of the dynamical (optical) conductivity of the ${\mathrm{CuO}}_2$ 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.