Implications for the origin of the pseudogap: Microwave spectroscopy of Pr Ba2 Cu4 O8

Using techniques of microwave spectroscopy at 13 GHz, we have observed antiferromagnetic resonance arising from the magnetically ordered cuprate planes of conducting Pr Ba2 Cu4 O8; the Cu Néel temperature is approximately 80 K. In addition, we have detected a Cu electron spin resonance which is due either to Cu in chain fragments or more likely to Cu in a minority impurity phase, BaCu O2, a phase also detected by x-ray diffraction. Although this sample was conducting to the lowest temperatures, no indication of even granular superconductivity was detected. The c-axis parameter of Pr Ba2 Cu4 O8 is dependent on processing and varies more than in other Ln124 materials; we propose that the lack of superconductivity is a consequence of a defect, namely, substitution of a variable content of smaller Pr on the Ba site. Such substitution also results in the formation of the minority phase BaCu O2. Our observation of cuprate-plane antiferromagnetism coupled with evidence that the cuprate planes are insulating (provided by Horii [Phys. Rev. B 6, 6327 (2000)]) indicates that any conductivity is associated with the CuO chains, not with the Cu O2 planes. These results imply that the existence of the pseudogap detected at 150 K in Pr124 by Bäckström [Phys. Rev. B. 61, 7049 (2000)] is unrelated to either carriers or spin fluctuations in the cuprate planes.