Photoconduction and photocontrolled collective effects in the Peierls conductor TaS3

Abstract

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...

Charge-density waves physics revealed by photoconduction

“…This effect was observed in the blue bronze, K 0.3 MoO 3 , where illumination-induced growth of the threshold field was found [6]. The effect was then reproduced and studied in details in o-TaS 3 [2,4]. It was shown that the effect results from the illuminationinduced change of the screening which affects in its turn the elastic modulus of the CDW.…”

“…As the LEDemitted light power is controlled by the current flowing through the LED, the light intensity control is also simple. This method was widely used for investigation of the temperature and time evolution of the photoconduction current in q-1D conductors [2,3,4,5].…”

“…Slow CDW motion (CDW creep) at E E T suppresses phococonduction [2]. Negative photoconduction (δG < 0) is observed at E > E T [2,4,6] due to illumination-induced growth of E T .…”

“…In its turn, the relaxation time can be obtained from direct measurements of temporary response of photocurrent after switching the illumination on or off. The photoresponse kinetics was measured in blue bronzes by using the laser excitation [6] and in o-TaS 3 with LED excitation [2]. In both cases the millisecond time scale of the characteristic time, τ, was observed.…”

“…τ depends on the voltage applied and decreases with the voltage when it exceeds the threshold one for onset of nonlinear conduction [6] 2. τ follows the activation law at T T P /3 [2] 3. τ depends on the light intensity in a way which corresponds to the collisional recombination mechanism [2], intensity-dependent relaxation times are observed in the low-temperature region at T T P /4 [2, 3, 4] 4. long-time logarithmic-type relaxation appears in the low-temperature region at T T P /4 [4] Relatively long relaxation time observed in photoconduction measurements allows to consider the current carriers (electrons and holes) as well-defined physical objects justifying thereby the usage of the semiconductor model [12] for description of many properties of the Peielrs conductors. Note, however, that the initial and final energy states of optically excited electrons and holes may be different due to intrinsically high polarizability of crystal lattices of studied q-1D materials [13].…”