Magnetoelastic coupling in the spin-Peierls ground state of hydrogenated and deuterated (TMTTF)2PF6salts

Abstract

We report an ultrasonic study of the magneto-elastic coupling of the hydrogenated and deuterated (TMTTF)2PF6 organic salts. For both salts the temperature dependence of the longitudinal velocity along the c * axis displays a monotonic stiffening of the C33 compressibility modulus upon cooling. Below the characteristic temperature scale 40 K the modulus stiffening becomes markedly enhanced, in concomitance with the reduction of spin degrees of freedom previously seen in magnetic measurements as low dimensional precursors of the spin-Peierls transition. The magneto-elastic coupling appears to be much weaker in the hydrogenated salt due to the highly inhomogeneous elastic behavior induced by the proximity of the charge ordering transition to the spin-Peierls phase. For the deuterated salt, an important anomaly in the ultrasound velocity is observed below the spin-Peierls transition temperature TSP in agreement with scaling of the elastic deformation with the spin-Peierls order parameter. In spite of the weakly inhomogeneous character of the spin-Peierls phase transition, the magnetic field dependence of TSP is well captured with the mean-field prediction for the lattice distorted Heisenberg spin chain.

Charge, spin, and lattice effects in the spin-Peierls ground state of MEM(TCNQ)2

“…Although this type of stiffening anomaly on equivalent elastic moduli was observed for inorganic [21] and organic [19] SP compounds, it cannot explain the important softening seen at T SP from Figs. 3-4.…”

“…As the temperature is decreased below 25 K in zero field, ∆V L /V L increases until it shows a local maximum near 18 K, followed by a dip and then an increase that continues down to 2 K where it saturates. In constrast with the situation found in other inorganic [21] and organic [19] materials for an equivalent elastic modulus, the dip indicates that this particular mode, C 22 , softens importantly near T SP before its stiffening at lower temperature in the SP ordered state. A magnetic field of 16 Teslas widens and shifts the anomaly down to 15 K without affecting the ∆V L /V L data above T SP and only very weakly (within the uncertainties) in the low temperature limit where saturation is observed.…”

“…4, the transverse mode data in zero magnetic field can be reasonably fitted by the expression (6) using |∆ q | ∼ (1 − T /T SP ) β with a critical exponent β near 0.36 slightly smaller than for CuGeO 3 (0.42) [21] and (TMTTF) 2 PF 6 (0.49) [19]; a similar fit for the longitudinal mode in Fig. 3 is however not reliable because of a too much scatter in the data.…”

“…These quasi-1D materials, however, display a slightly dimerized structure leading to a Mott insulating gap below T ρ ≈ 230K [14], followed by a charge ordered superstructure (CO) at T CO ≈ 65K [15,16](T CO ≈ 84K in deuterated [17,18]) whose proximity with the SP transition has prevented the full characterization of its order parameter. Indeed, inhomogeneous dielectric [18] and elastic [19] behaviors were observed in the vicinity of the SP transition for the hydrogenated and deuterated PF 6 salts. Although the magnetic field dependence of the SP transition temperature agrees satisfactorily with the predicted one (D-U boundary), the field dependence of the order parameter appears to differ from the theoretical prediction [20], at variance with the inorganic CuGeO 3 material [21].…”

“…In other SP systems [19,21], softening and/or stiffening anomalies observed on elastic moduli below T SP were analyzed in a phenomenological framework of the Landau free energy model. This is achieved by introducing a coupling term between the SP distortion of wave vector q, which is proportional to the magnetic gap ∆ q , and the uniform elastic deformation e [30],…”