Evidence of Collective Charge Transport in the Ohmic Regime of $o\mathrm{\text{−}}{\mathrm{TaS}}_3$ in the Charge-Density-Wave State by a Photoconduction Study

Using photoconduction techniques, we demonstrate that the low-temperature Ohmic conduction of $o\mathrm{\text{−}}{\mathrm{TaS}}_3$ 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.

Figure 1

Photoresponse of the $o\mathrm{\text{−}}{\mathrm{TaS}}_3$ sample to a meander light pulse sequence (light is on at $t=0$ and off at $t=1\mathrm{s}$) at different light intensities. $T=40\mathrm{K}$, $V=200\mathrm{mV}$. Sample parameters: $L=0.4\mathrm{mm}$, $R_{300\mathrm{K}}=7\mathrm{k}\Omega$. The inset shows the shape of the pulse $\delta I(t)/\delta I(1\mathrm{s})=[I(t)-I(0)]/[I(1\mathrm{s})-I(0)]$ at $W=10$, 1, 0.1, and $0.01\mathrm{mW}/{\mathrm{cm}}^2$.

Figure 2

Photoresponse of $o\mathrm{\text{−}}{\mathrm{TaS}}_3$ sample to a meander light pulse sequence (light is on at $t=0$ and off at $t=1\mathrm{s}$) at different light intensities. Sample parameters: $L=0.4\mathrm{mm}$, $R_{300\mathrm{K}}=7\mathrm{k}\Omega$, $T=20\mathrm{K}$, $V=2\mathrm{V}$.

Figure 3

An amplitude of photoresponse of a $o\mathrm{\text{−}}{\mathrm{TaS}}_3$ sample to a meander light pulse sequence (light pulse duration 1 s, a period 2 s) vs light intensity at different temperatures. The dashed line shows the slope of $\delta I\propto W^{1/2}$ expected for the quadratic recombination mechanism. Sample parameters: $L=0.4\mathrm{mm}$, $R_{300\mathrm{K}}=7\mathrm{k}\Omega$.

Figure 4

Temperature variation of the linear conductance of $o\mathrm{\text{−}}{\mathrm{TaS}}_3$ in the dark (upper curve). Set of curves shows temperature dependencies of ac photoconduction measured at light intensities (from top to bottom) 10, 4, 1.8, 1, 0.5, 0.28, 0.077, 0.038, 0.02, 0.0082, 0.005, 0.0024, 0.0014, 0.00 062, 0.00 031, and $0.00019\mathrm{mW}/{\mathrm{cm}}^2$. The lines corresponds to the activation law with the activation energies $E_a=1250\mathrm{K}$ and $E_a^{*}=-1250\mathrm{K}$. Inset shows the same set of $\delta G$ curves normalized by the light intensities. The solid line results from collapse of the activation dependences with $E_a=1250\mathrm{K}$ shown by lines in Fig. 4. Sample parameters: $L=0.7\mathrm{mm}$, $R_{300\mathrm{K}}=95\mathrm{k}\Omega$, $f=4.5\mathrm{Hz}$. See text for details.