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College Park, Maryland      June 6 - 10 , 2004

TP54: Spin singlet formation in MgTi2O4: evidence of a helical dimerization pattern

M. Schmidt (ISIS, Rutherford Appleton Laboratory; Department of Physics and Astronomy, Michigan State University, E. Lansing, MI, 48824), W. Ratcliff II (Department of Physics and Astronomy, Rutgers University, Piscataway, New Jersey 08854, USA), P. G. Radaelli (ISIS, Rutherford Appleton Laboratory; Department of Physics and Astronomy, University College London, London, WC1E 6BT, United Kingdom), K. Refson (ISIS, Rutherford Appleton Laboratory), N. M. Harrison (Department of Chemistry, Imperial College London, London, SW7 2AY, United Kingdom; CCLRC, Daresbury Laboratory, Warrington, WA4 4AD, United Kingdom), S.W. Cheong (Department of Physics and Astronomy, Rutgers University, Piscataway, New Jersey 08854, USA)

The transition metal spinel MgTi2O4 was studied using a combination of neutron and x-ray powder diffraction, electrical resistivity and magnetic susceptibility measurements. MgTi2O4 undergoes a metal-insulator transition on cooling below T(M-I) = 260K and a step-like reduction of the magnetic susceptibility below T(M-I). The temperature dependence of transport properties suggests an onset of a magnetic singlet state. Using high-resolution synchrotron and neutron powder diffraction, we have solved the low-temperature crystal structure of the oxide. MgTi2O4 has tetragonal crystal structure below 260K (S.G: P41 21 2, a=6.02201(1), c=8.48482(2) at 200K) which is found to contain dimers with short Ti-Ti distances (the locations of the spin singlets). The shortest Ti-Ti bonds alternate with longest bonds to form helices running along the tetragonal c axis. Additional band structure calculations based on hybrid exchange density functional theory show that, at low temperatures, MgTi2O4 is an orbitally ordered band insulator.

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