Anomalous dielectric response in tetrathiafulvalene-p-chloranil as observed in temperature- and pressure-induced neutral-to-ionic phase transition

Infrared Investigations of the Neutral-Ionic Phase Transition in TTF-CA and Its Dynamics

“…Upon cooling, the resistivity follows an Arrhenius development ρ dc (T) = ρ 0 exp {∆/T}, at which ∆ is the temperature-independent activation energy, or energy gap in a semiconductor picture, respectively. In the neutral phase, the activation energy is 0.12 eV (corresponding to 1276 K), which is close to the literature values lying between 0.095 eV and 0.065 eV [9,11,12]. In the inset (e) of Figure 2, the derivative of the logarithmic resistivity is presented, in order to illustrate the variations of the slope upon cooling.…”

“…Furthermore, we also varied the laser intensity Q p by more than a factor of three and λ therm between 0.12 W/Kcm (value for TTF-TCNQ [102] at 40 K) and 0.001 W/Kcm (for α-(BEDT-TTF) 2 I 3 [103] in the charge ordered phase) in Equation (9). However, we cannot observe any significant changes in the temperature drop which can be correlated with our measured data.…”

“…By the generation of the one-dimensional, neutral, non-dimerized region, domain walls are formed between the neutral and ionic parts (NIDW). The excitation energy of the NIDW is about 0.1 eV [9,11] and corresponds to the activation energy of 0.12 eV and 0.065 eV in the ionic phase, as we will determine below.…”

“…However, the simulations confirm the influence of these NIDW on the transport properties and on the pressure-induced phase transition. The NIDW are also responsible for the feature observed in the dielectric function above the phase transition [9], the electric transport [25] and the nonlinear conductivity [12]. Originally, this was predicted theoretically [22,23] and then confirmed quantitatively by Soos and Painelli [24], by taking into account the interaction of the lattice and the molecular vibrations with the electronic system.…”

“…[2], occurring after the loss of the twofold screw axis which is caused by the break of symmetry and the structural phase transition; (b) Temperature evolution of the real part 1 of the dielectric function, recorded at a frequency of 100 Hz (light blue) and 100 kHz (dark blue). 1 diverges for all frequencies at the critical temperature T NI and marks a first-order para-to ferroelectric transition [9]. The feature, appearing between T = 100 and 200 K for 100 Hz, is attributed to the excitation of neutral-ionic domain walls; (c) At the phase transition, the specific heat capacity C p of TTF-CA diverges similar to 1 , indicating a first-order phase transition [10]; (d) Arrhenius plot of the resistivity ρ dc (green) of TTF-CA along the stacking direction.…”