Spin fluctuations and carrier mass enhancement in pyrochlore Y2-xBixRu2O7 -: art. no. 060402

We present: magnetic susceptibility and heat capacity data for pyrochlore Y2-xBixRu2O7 (0 <= x <= 2). With decreasing x, the ground state of the system evolves from paramagnetic (PM) metal to spin-glass (SG)-like metal, and to an antiferromagnetic (AF) Mott insulator. The low-temperature heat capacity in the metallic phases can be represented as gamma T+beta T-3 + delta T(3)lnT, with significant enhancement of both gamma and delta as the SG-like phase is approached in the PM phase. The delta T(3)lnT term signals spin fluctuations in the nearly localized regime, possibly enhanced by the strong geometrical frustration. The heat capacity of the integer-spin (S = 1) insulator Y2Ru2O7 suggests the presence of gapped spin excitations in the cubic AF phase.