EPR evidence of onset of the quantum critical point in CuGeO3:Fe

Abstract

This chapter studies the influence of doping by iron impurity on spin-Peierls state in CuGeO3. EPR measurements for the frequency/temperature domain 60-450 GHz/1.8-300 K show that insertion of 1% of Fe completely destroy both spin-Peierls and antiferromagnetic orders. Damping of long-range magnetic order is accompanied by onset at T <20K of power asymptotic for magnetic susceptibility ϰ∝T-α, with the index α=0.35. This effect is characteristic to the limit of strong disorder for doped CuGeO3 and may reflect onset of a quantum critical point in CuGeO3:Fe. Discovery of inorganic spin-Peierls compound CuGeOs opened an opportunity to study influence of doping and disorder on the spin-Peierls state. Numerous experiments and theoretical studies have been carried out in this field up to now. However, from the theoretical point of view, most of the available data correspond to the limit of weak disorder when density of states have a pseudogap, i.e. a spin-Peierls gap filled by disorder-induced states. In this case the expected temperature-concentration T-x phase diagram consists of uniform state, spin-Peierls state, antiferromagnetic state and the region, where antiferromagnetic and spin-Peierls orders coexist. This structure of T-x phase diagram was observed experimentally for Zn, Si, Ni, Co, Mg and Mn impurities.