Electronic Structure of Sr_2FeMoO_6
  We have analysed the unusual electronic structure of Sr_2FeMoO_6 combining ab-initio and model Hamiltonian approaches. Our results indicate that there are strong enhancements of the intraatomic exchange strength at the Mo site as well as the antiferromagnetic coupling strength between Fe and Mo sites. We discuss the possibility of a negative effective Coulomb correlation strength (U_{eff}) at the Mo site due to these renormalised interaction strengths. 
D.D. Sarma, Priya Mahadevan, T. Saha-Dasgupta, Sugata Ray and Ashwani   Kumar (Solid State and Structural Chemistry Unit, In...