High Frequency EPR Spectroscopy of Germanium Cuprate. I. The Case of   Doping with Cobalt
  Resonant magnetoabsorption spectra for CuGeO3 single crystals containing 2% of Co impurity are studied in the frequency range 60-360 GHz in magnetic field up to 16 T at temperatures 2-60 K. It is found that together with the resonance on Cu2+ chains a new absorption line related with Co2+ ions appears in EPR spectrum. The full set of spectroscopic data (g-factors, linewidths, integrated intensities) have been determined for both resonances. The obtained results exhibit considerable deviations from the universal scenario of doping for CuGeO3 and discussed in the frames of the models of the quantum critical behavior and the three-dimensional antiferromagnet with the reduced by disorder Neel temperature. The Oshikawa and Affleck theory for EPR in 1D S=1/2 AF spin chain is compared with experiment. 
S.V. Demishev, A.V. Semeno, N.E. Sluchanko, N.A.Samarin, A.A. Pronin,   Y. Inagaki, S. Okubo, H. Ohta, Y. Oshima, L.I. Leonyuk