X-Ray Scattering Study of Spin-Lattice Dimerization in a Quasi One-Dimensional Heisenberg Antiferromagnet

Abstract

X-ray measurements on the quasi one-dimensional Heisenberg antiferromagnet TTF-(tetrathiafulvalenium-)Cu${\mathrm{S}}_4$ ${\mathrm{C}}_4$ ${\left(\mathrm{C}{\mathrm{F}}_3\right)}_4$ reveal a new crystallographic phase below $T_0=11\mathrm{}$ K in which spin-½ TT${\mathrm{F}}^{+}$ units are paired. The temperature dependence of the order parameter below 11 K is accurately given by the BCS gap equation. Unexpectedly, the pairing is found along a diagonal direction in the ${\overrightarrow{\mathrm{b}}}_F-{\overrightarrow{\mathrm{c}}}_F$ plane rather than along the molecular stacking axis ${\overrightarrow{\mathrm{c}}}_F$. Furthermore, strong diffuse scattering dominates the x-ray cross section at the dimerization superlattice positions for temperatures as high as 225 K.