Evidence for a spin pseudogap in the normal state of superconducting Mo₃Sb₇

Using muon spin relaxation (μSR) and inelastic neutron scattering(INS) we have investigated the normal state of the superconductor Mo ₃Sb₇ and the reference compound Ru₃Sn ₇. The μSR experiments on Ru₃Sn₇ reveal static and relatively slow dynamic relaxations, which are ascribed to a random static nuclear dipole field and thermally activated muon motion, respectively. INS experiments on Ru₃Sn₇, on the other hand, reveal three phononic excitations at 11, 18 and 23meV, substantiating the assertion of Einstein and Debye oscillations derived from the specific heat and electrical resistivity data. The distinct difference in the μSR as well as INS spectra between Ru₃Sn₇ and Mo₃Sb₇ provides strong evidence for a magnetic/electronic nature of the phase transition at T^* = 50K in the Mo-based compound. On the basis of the μSR and INS data, the energy spin pseudogap of 150(10)K was estimated. The observed weak magnetism in the dynamic susceptibility χ^″(Q,ω) and residual longitudinal field relaxation at 5K imply a static ordering or quantum fluctuations.