Inelastic neutron-scattering study of free proton dynamics in gamma-MnO2

Inelastic neutron scattering measurements of gamma-MnO2 at 30, 100 and 200 K give S(Q, omega) maps of intensity over large energy and momentum transfer ranges (from 0 to 1 eV and from 0 to 40 Angstrom(-1), respectively). They reveal a ridge of intensity due to recoil of free particles with effective mass of approximate to 1 amu. The width of the recoil line is one order of magnitude greater than that anticipated from the kinetic momentum distribution of an isolated gas of protons at the temperature of the sample. Several models are considered: ideal gas obeying Maxwell-Boltzmann or Fermi statistics, multiple scattering, phonon wings, protons trapped in shallow potentials and quantum correlations between free and bound protons. None of them is satisfactory. Finally, broadening by zero-point motions of the lattice provides a satisfactory theoretical framework to represent the shape and intensity of the recoil spectra. New aspects of proton dynamics in gamma-MnO2 are highlighted.