Noise spectroscopy across electric field-induced transition in an organic charge-transfer complex

Alkali-metal-TCNQ salt, K-TCNQ, is a quasi-one-dimensional organic charge-transfer complex with segregated columns of alkali cations and TCNQ anion. The compound undergoes structural transition at Tc ~ 395 K as a result of strong electron-lattice interaction inherent in quasi-one-dimensional systems. Below Tc, an electric-induced transition from insulating to metallic phase is observed and contains two threshold behaviors in current-voltage characteristics, akin to charge density wave (CDW) systems. In CDW systems, the lower threshold is attributed to Coulomb blockade for soliton nucleation and the upper one is the depinning field. The upper threshold field follows an exponential temperature-dependence like in CDW systems implying collective charge dynamics near the transition. Low-frequency conductance noise spectroscopy is employed to understand charge carrier dynamics across both the thresholds. The power spectral density (PSD) of the measured current fluctuation increases by almost three orders around the lower threshold implying possible nucleation of phase solitons and on approaching the upper threshold field possible indications of CDW sliding are seen in the behavior of PSD. The existence and dynamics of CDW in these organic charge-transfer systems will be presented.