Implications for the origin of the pseudogap:: Microwave spectroscopy of PrBa2Cu4O8

Using techniques of microwave spectroscopy at 13 GHz, we have observed antiferromagnetic resonance arising from the magnetically ordered cuprate planes of conducting PrBa2Cu4O8; the Cu Neel 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, BaCuO2, 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 PrBa2Cu4O8 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 BaCuO2. 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 CuO2 planes. These results imply that the existence of the pseudogap detected at 150 K in Pr124 by Backstrom [Phys. Rev. B. 61, 7049 (2000)] is unrelated to either carriers or spin fluctuations in the cuprate planes.