Structural evidence of charge density waves in HMTTF-TCNQ (hexamethylene-tetrathiofulvalenium- tetracyanoquinodimethane)

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Abstract
X-ray diffuse scattering from the 1-D conductor HMTTF-TCNQ in the high temperature phase (T > 49 K) reveals one 1-D scattering at the wave-vector 0.36 b* in the chain direction, assigned to a 2 kF anomaly in contrast to the case of TTF-TCNQ where both 2 kF and 4 kF precursors have been reported. Furthermore, this 2 kF scattering was found to have only a longitudinal polarization while in TTF-TCNQ and TSeF-TCNQ a 2 k F transverse component is also observed. Around 49 K, the charge density waves couple three dimensionaly producing satellite reflections corresponding to a modulation period of 2.38 a x 2.78 b x xc. No further change of this modulation period was observed down to 10 K