engleski

Fingerprints of hopping conductivity in disordered Charge density wave systems

Electric conductivity of charge density wave (CDW) systems exhibits rich variety of behavior ; thermal activation across the gap at low fields below transition temperature TP, collective contribution (nonlinear conductivity channel) above the threshold field (ET) and variable range hopping (VRH) at low temperatures and in granulated thin films in the whole temperature range . Particularly the origin of the hopping conductivity is still unclear. We have investigated the influence of disorder on conductivity phenomena in CDW systems TaS3 and blue bronze (BB: K0.3MoO3) in a wide range of temperatures and electric fields using both the DC and pulse measurements at low and high fields respectively. Disorder has been introduced in several ways: (1) synthesis of TaS3 samples doped with Nb, (2) irradiation of nominally pure TaS3 samples and (3) deposition of granular thin BB films by pulsed laser deposition (PLD). Our results show that the nonlinear conductivity can be described by VRH already below 50 K in contrast to the linear channe, where it appears only below 20 K. Moreover, the point defects in TaS3 introduced by doping and irradiation have no effect on VRH. Together with the dielectric data, it suggests the microscopic picture of soliton hopping in CDW at low temperatures. In granular thin BB films, on the other hand, the influence of grain boundaries is overwhelming even above TP, which is in contrast to the previous experiments on thin BB films. The results can be understood by applying recent theoretical results for Beloborodov's Efros- Shlovskii VRH in granular materials.

charge density waves; hopping; TaS3; blue bronze; transport; dielectric spectroscopy; solitons

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