Cooperative dynamics in charge-ordered state of $\alpha$-(BEDT-TTF)${}_2$I${}_3$

Electric-field-dependent pulse measurements are reported in the charge-ordered state of $\alpha$-(BEDT-TTF)${}_2$I${}_3$. At low electric fields up to about 50 V/cm only negligible deviations from Ohmic behavior can be identified with no threshold field. At larger electric fields and up to about 100 V/cm a reproducible negative differential resistance is observed with a significant change in shape of the measured resistivity in time. These changes critically depend on whether constant voltage or constant current is applied to the single crystal. At high enough electric fields the resistance displays a dramatic drop down to metallic values and relaxes subsequently in a single-exponential manner to its low-field steady-state value. We argue that such an electric-field-induced negative differential resistance and switching to transient states are fingerprints of cooperative domain-wall dynamics inherent to two-dimensional bond-charge density waves with ferroelectric-like nature.

Figure 1

Schematic diagrams of the pulse measurement circuits employed for the constant-current mode (left and middle panels) and for the constant-voltage mode (right panel).

Figure 2

(a) Low- and (b, c, d) high-electric field behavior of $\alpha$-(BEDT-TTF)${}_2$I${}_3$, $\mathbf{E}\parallel a$, at a representative temperature 125 K. (a) Conductivity increase normalized to its Ohmic value vs electric field. (b) Current density vs electric field with corresponding resistivity vs time plots measured in the constant-current and (c) constant-voltage mode. The current density and electric field are evaluated as an average of 4–5 ms. (d) Conductivity vs current density measured in the constant-voltage mode. The full line is a fit to the power law $\sigma (J,T)={\sigma }_0\left(T\right)+A{J}^n$ with ${\sigma }_0=0.034$ ${\Omega }^{-1}$ cm${}^{-1}$ and $n=1.8\pm 0.2$. Constant-current mode data follow the same expression with ${\sigma }_0=0.04$ ${\Omega }^{-1}$ cm${}^{-1}$ and $n=2.1\pm 0.2$.

Figure 3

Effects of two extreme conditioning pulses with total source voltage $U$ on $\alpha$-(BEDT-TTF)${}_2$I${}_3$, $\mathbf{E}\parallel a$, at a representative temperature 125 K measured in constant-voltage mode. At sufficiently high electric fields within the conditioning pulse the sample becomes metallic and in time decays back to the high-resistance state (left panel). A low-field conditioning pulse keeps the resistance unchanged (right panel).

Figure 4

Comparison between measured resistance vs time (full lines) and an estimate (dashed lines) based on Joule heating and the dc curve, for three representative pulses with total source voltage $U$ applied to $\alpha$-(BEDT-TTF)${}_2$I${}_3$ along the $a$ axis at 125 K.