Diffuse x-ray and inelastic neutron scattering study of the spin Peierl's transition in N-methyl-N-ethyl-morpholinium bistetracyanoquinodimethane [MEM (TCNQ)2]

Abstract

X-ray and neutron scattering techniques have been used to investigate the spin Peierl's transition in $N$-methyl-$N$-ethyl-morpholinium bistetracyanoquinodimethane [MEM ${\mathrm{}\left(\mathrm{TCNQ}\right)\mathrm{}}_2$]. Both techniques reveal a doubling of the $c$-axis stacking of the TCNQ molecules below the transition. Diffuse x-ray scattering was observed well above the transition at the position of the low-temperature superlattice reflections. The geometry of the scattering showed that the critical scattering related to the structural changes at $T_c$ results from fluctuations of a mainly transverse nature. Neutron scattering measurements revealed no softening of longitudinal-acoustic phonons at the transition. It was impossible to determine the position of the transverse-acoustic phonons, because they have either very low energies or very large linewidths; therefore, we cannot define the fluctuations to be of a static or dynamic character.