Neutron scattering on mixed phonon-libron modes in solid H2

Solid hydrogen undergoes a first-order structural transformation from hcp (T>Tsub(lambda)) to fcc (T= 0.6 from Tsub(lambda) (x=1) = 3.14 K to Tsub(lambda) (x=0.6) = 0.47 K. This transformation is accompanied by a cooperative orientational ordering of the ortho (J=1) molecules giving rise to collective excitations in the J = 1 subspace which are called librons. They have been observed for β approximately equal to O by optical measurements but extension to non-zero wavevector by neutron spectroscopy resulted in a discrepancy to theoretical predictions. It could be explained by taking into account the phonon-libron coupling. For a more detailed investigation of this coupling, a new experiment has been done and some of its results are presented. (Auth.)