Using photoconduction techniques, we demonstrate that the low-temperature Ohmic conduction of o-TaS3 is not provided by band motion or hopping of single-particle excitations-electrons and holes excited over the Peierls gap. Instead, the low-temperature Ohmic conduction is mostly provided by collective excitations having an activation energy much less than the Peierls gap value and shunting the contribution of electrons and holes responsible for photoconduction.

Light illumination of thin crystals of CDW conductor TaS3 is found to result in dramatic changes of both linear (G) and nonlinear conduction. The increase of G is accompanied by suppression of the collective conduction, growth of the threshold field ET , and appearance of the switching and hysteretic behavior in the nonlinear conduction. The effects in the nonlinear conduction are associated with increase of CDW elasticity due to illumination that leads in particular to appearance of a relation ET ∝ G 1/3 expected for the one-dimensional pinning. Quasi-one-dimensional (quasi-1D) conductors with charge-density-waves (CDW) [1] are one of the most interesting physical systems with collective electron transport. The interaction between electrons condensed into the CDW dominates in elastic properties of the electron crystal-CDW. The elastic properties of the CDW affect such characteristics of quasi-1D conductors as the value of the threshold field for CDW sliding, E T , phase-correlation length, dielectric constant, et al. In its turn the elastic properties are controlled by quasiparticles (electrons and holes) thermally excited over the Peierls gap in the energy spectrum and screening the electric fields caused by CDW deformations. Thus a variation of the quasiparticle concentration (or the total carrier concentration) may be a tool controlling the properties of CDW conductors. An attempt to vary the total carrier concentration has been undertaken in the field effect experiment [2]. In particular it was found that 1%-change of the total concentration of the current carriers by the transverse electric field leads to 40%-change of the threshold field value. Another well-known way to modify the carrier concentration is excitation of nonequilibrium current carriers by light. For example, illumination of a semiconductor may result in increase of the carrier concentration by orders of magnitude. Such a change can be easily detected as a variation of the conduction (photo-conduction). Photoconduction is one of the most fruitful methods to study the details of the energy structure, current carrier recombination time, and other semiconductor parameters. The similarity of numerous physical properties of the CDW conductors and semiconductors arising from the existence of the gap in the electron state density is well known [3]. Several attempts of experimental search for photocon-duction of CDW materials [4, 5, 6, 7] reveal contradictory results. In Ref. [4, 5] no noticeable photoconduction in TaS 3 was observed. Instead, the bolometric response was found and employed for detailed study of the energy structure in TaS 3. In addition an enhancement of the bolometric response was reported in nonlinear regime Ref. [4]. In Ref. [6] photoinduced CDW conduction was observed in blue bronze K 0.3 MoO 3. The red boundary of the effect was found to correspond to the Peierls gap value. The phenomenon was associated with initiation of the CDW depinning by optically excited single electrons. No light-induced variation of the linear conductio...

Light illumination of thin crystals of CDW conductor TaS3 is found to result in dramatic changes of both linear (G) and nonlinear conduction. The increase of G is accompanied by suppression of the collective conduction, growth of the threshold field ET , and appearance of the switching and hysteretic behavior in the nonlinear conduction. The effects in the nonlinear conduction are associated with increase of CDW elasticity due to illumination that leads in particular to appearance of a relation ET ∝ G 1/3 expected for the one-dimensional pinning. Published in: Pis'ma v ZhETF, 79, 680 (2004) [JETP Letters, 79, 550 (2004)] Quasi-one-dimensional (quasi-1D) conductors with charge-density-waves (CDW) [1] are one of the most interesting physical systems with collective electron transport. The interaction between electrons condensed into the CDW dominates in elastic properties of the electron crystal -CDW. The elastic properties of the CDW affect such characteristics of quasi-1D conductors as the value of the threshold field for CDW sliding, E T , phasecorrelation length, dielectric constant, et al. In its turn the elastic properties are controlled by quasiparticles (electrons and holes) thermally excited over the Peierls gap in the energy spectrum and screening the electric fields caused by CDW deformations. Thus a variation of the quasiparticle concentration (or the total carrier concentration) may be a tool controlling the properties of CDW conductors. An attempt to vary the total carrier concentration has been undertaken in the field effect experiment [2]. In particular it was found that 1%-change of the total concentration of the current carriers by the transverse electric field leads to 40%-change of the threshold field value. Another well-known way to modify the carrier concentration is excitation of nonequilibrium current carriers by light. For example, illumination of a semiconductor may result in increase of the carrier concentration by orders of magnitude. Such a change can be easily detected as a variation of the conduction (photoconduction). Photoconduction is one of the most fruitful methods to study the details of the energy structure, current carrier recombination time, and other semiconductor parameters. The similarity of numerous physical properties of the CDW conductors and semiconductors arising from the existence of the gap in the electron state density is well known [3].Several attempts of experimental search for photoconduction of CDW materials [4,5,6,7] reveal contradictory results. In Ref. [4,5] no noticeable photoconduction in TaS 3 was observed. Instead, the bolometric response was found and employed for detailed study of the energy structure in TaS 3 . In addition an enhancement of the bolometric response was reported in nonlinear regime Ref. [4]. In Ref.[6] photoinduced CDW conduction was observed in blue bronze K 0.3 MoO 3 . The red boundary of the effect was found to correspond to the Peierls gap value. The phenomenon was associated with initiation of the CDW depinning by optically e...

Photoconduction study of quasi-1D conductors allows to distinguish between the single-particle and collective {\it linear} conduction, investigate the effect of screening on collective transport and obtain interesting new details of the electronic energy structure of pure and doped CDW conductors. Here we present results of photoconduction study in quasi-1D conductors o-TaS$_3$, K$_{0.3}$MoO$_3$, and NbS$_3$(I).Comment: Invited talk at ECRYS 2011, 6 pages, 13 figures, to be published in Physica B: Condensed Matte

Photoconduction in the monoclinic phase of quasi-one-dimensional conductor TaS 3 has been observed at T < 70 K. It was studied jointly with low-temperature ohmic and non-linear dark conduction. The strong sample quality dependence of both photoconduction and dark conduction at this temperature region has been observed. Together with a similarity of the main features of the photoconduction characteristic of both monoclinic (m-TaS 3 ) and orthorhombic (o-TaS 3 ) samples the following new peculiarities of photoconduction in m-TaS 3 were found: 1) the dependence of the activation energy of photoconduction on temperature, T , 2) the change of the recombination mechanism from the linear type to the collisional one at low T with a sample quality growth, 3) the existence of a fine structure of the electric-field dependence of photoconduction. Spectral study gives the Peierls energy gap value 2∆ * = 0.18 eV.