ESR probing of quantum critical phenomena in doped S = 1/2 AF quantum spin chain

The results of high-frequency (60–315 GHz) electron spin resonance (ESR) studies of CuGeO3 single crystals containing 0.9% Mn impurity are reported. The quantitative ESR line shape analysis shows that the low-temperature (T < 40 K) magnetic susceptibility of Cu2+ chains diverges as χ ∼ 1/Tα with the critical exponent α = 0.81±0.03 and therefore indicates the onset of a quantum critical (QC) regime. A scenario in which the disorder caused by the Mn impurity in quantum spin chains of CuGeO3 may lead to the coexistence of the quantum critical regime and the spin-Peierls dimerization is discussed. For the quantitative description of the temperature dependences of the line width and g-factor, a model assuming a crossover from the high-temperature semiclassical Nagata and Tazuke limit to the low-temperature quantum case described by Oshikawa and Affleck’s theory is suggested.