Hopping conduction in polydiacetylene single crystals

The charge transport in polydiacetylene quasi-1D single crystals (PDA-PTS) has been studied as a function of temperature, electric and magnetic fields. In the Ohmic regime the temperature dependence of the resistivity, rho(T), is characteristic of hopping conduction with a crossover at T<50 K from activated rho(T)=rho(0) exp [(E-A/k(B)T)], with E(A)similar to13-19 meV to variable-range hopping transport rho(T)=rho(0) exp [(T-0/T)(p)], with psimilar to0.65-0.70. At modest electric fields the resistivity depends as rho(E,T)=rho(0,T)exp (-eEL/k(B)T), where the characteristic hopping length changes as Lsimilar toT(-m) with msimilar to0.5 at T>50 K and msimilar to0.75 at T<50 K. At high electric fields the low temperature current becomes temperature independent and follows: I(E)=I-0 exp [-(E-0/E)(0.5)], which corresponds to the regime of activation-free phonon-emission-assisted hopping conduction. Magnetoresistance at T<4.2 K is negative and depends on magnetic fields H as In[rho(H,T)/rho(0,T)]similar toH(0.5). The results demonstrate that at low temperature the charge transport is mainly supported due to quasi-1D variable-range hopping with the influence of Coulomb interactions. At higher temperatures the thermally activated nearest-neighbor transport by small polarons is dominant.