Unusual Nonlinear Conductivity in ChargeOrdered α(BEDTTTF)_{2}I_{3}
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Thirdorder Electrical Conductivity of the ChargeOrdered Organic Salt Alpha(BEDTTTF)_{2}I_{3}
J. Phys. Soc. Jpn. 91, 023703 (2022).
Unusual nonlinear conduction is detected in organic molecular salt α(BEDTTTF)_{2}I_{3}. The observed thirdorder nonlinear conductivity implies a quadrupole instability hidden in this chargeordered organic salt.
When voltage V is applied to the sample, electrical current I proportional to the voltage flows as V = R × I or I = G × V, where R and G (= 1/R) denote the resistance and conductance, respectively. This is the wellknown Ohm’s law, also called as the linear conduction because of a linear relationship between current I and voltage V.
In contrast, several materials show nonlinear conduction, in which current I is not proportional to voltage V. The representatives are the nonlinear semiconductor devices such as diodes and transistors, as widely used in modern electronics. Recently, nonlinear conduction has also been observed in various correlated insulators, and it becomes a challenging issue to clarify the underlying mechanism of the nonlinearity.
A peculiar class of the organicmolecularbased compounds hosts an emergent electron crystal called “charge order,” in which the electrons are arranged periodically owing to the Coulomb repulsion. It provides a fascinating playground to study the nonlinear conduction because such a charge crystal may be highly sensitive to the external electric field as the normal crystal is affected by the external mechanical force. Indeed, organic salt α(ET)_{2}I_{3}, in which the charge order emerges at T_{CO} = 136 K, exhibits interesting nonlinear transport such as a collective sliding motion of charge order above the threshold field.
In the present study, the authors aim to examine the lowfield nonlinear response of chargeordered salt α(ET)_{2}I_{3} by a harmonic conductivity experiment. Generally, the nonlinear conduction can be expressed as a Taylor series of current I as I = G_{1}V + G_{2}V^{2} + G_{3}V^{3} …, where G_{i} denotes the ithorder conductance. The first term on the righthand side is the Ohmic term. In this experiment, a bridge circuit technique was utilized to accurately detect thirdorder conductance G_{3}.
The authors note that the thirdorder conductance of α(ET)_{2}I_{3} is critically enhanced near the charge order transition temperature. As the origin, a close relation between the thirdorder conductance and the thirdorder electric susceptibility has been discussed. Thermodynamically, the thirdorder susceptibility may relate to a quadrupole moment; thus, the present results imply a quadrupole instability hidden in the chargeordered organic salt.
Although the crystal structure of molecularbased compounds appears very complicating at the first glance, it is greatly simplified by regarding the complicated molecule as a pointlike charge (electric monopole). The present results indicate that the asymmetric charge distribution in the molecule unit with a higherorder quadrupole moment is essential to thoroughly understand various intriguing properties of molecular solids.
(Written by R. Okazaki on behalf of all authors)
Thirdorder Electrical Conductivity of the ChargeOrdered Organic Salt Alpha(BEDTTTF)_{2}I_{3}
J. Phys. Soc. Jpn. 91, 023703 (2022).
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