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, ρ(T), is characteristic of hopping conduction with a crossover at T<50 K from activated ρ(T) =ρ₀ exp [(E_A/k_BT)], with E_A∼13-19 meV to variable-range hopping transport ρ(T)=ρ₀ exp [(T₀/T)^p], with p∼0.65-0.70. At modest electric fields the resistivity depends as ρ(E,T)=ρ(0,T)exp (-eEL/k_BT), where the characteristic hopping length changes as L∼T^-m with m∼0.5 at T>50 K and m∼0.75 at T<50 K. At high electric fields the low temperature current becomes temperature independent and follows: I(E) =I₀ exp [-(E _/0E)^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[ρ(H,T)/ρ(0,T)]∼H^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.