Quantum critical point in CuGeO 3 doped with magnetic impurities

Using high frequency (up to 450 GHz) ESR and low temperature specific heat measurements we find that insertion of 1% Fe and 2% Co damps spin-Peierls and Neel transitions and for T <30K may give rise to onset of a quantum critical behaviour characteristic for a Griffiths phase. Most of the available data for doped inorganic spin-Peierls compound CuGeO 3 correspond to the limit of weak disorder when density of states have a pseudo-gap [1,2]. In the oppposite case of a strong disorder the situation is expected to change dramatically: the ground state of CuGeO 3 should be a Griffiths phase (GP) which thermodynamic properties are controlled by relatively rare spin clusters correlated more strongly than average [3,4]. In this case density of states becomes gapless and diverges at ǫ=0: ρ(ǫ)∝|ǫ| −α [1]. As a consequence the temperature dependences of magnetic susceptibility χ and magnetic contribution to specific heat c m for Cu 2+ spin-Peierls chains acquire the forms [3,4] χ ∝ T −α ; c m ∝ T 1−α (1) where α<1. As long as T =0 becomes singular or critical point, the aforemen-tioned situation may be described in terms of quantum critical (QC) behaviour [3].