Anomalous thermal expansion behavior in (TMTTF)₂PF₆and (TMTSF)₂PF₆around 100 K: Indications for a strong spin-phonon interaction?

Abstract

High-resolution measurements of the coefficient of thermal expansion along the b axis, α b , on the quasi-1D Spin-Peierls and spin-density-wave systems (TMTTF) 2 PF 6 and (TMTSF) 2 PF 6 , respectively, reveal an anomalous a(T) contribution around 100 K. This feature, which is most strongly pronounced in the SP system, is reminiscent of the magnetic contribution observed in low-dimensional spin systems with a strong spin-lattice interaction such as CuGeO 3 . Remarkably enough, the position of the α b anomalies for both compounds almost coincide with the temperatures where the thermal conductivity shows a distinct minimum [T. Lorenz et al., Nature 418 (2002) 614]. Our observation indicates a significant role of the spin-lattice interaction for these systems.