Dissipative quantum Hall effect in polycrystalline CVD graphene

We report on a study of the quantum Hall effect in large area Hall bars made of polycrystalline graphene grown by chemical vapor deposition and then transferred on SiO₂/Si substrate. The longitudinal conductivity σ_xx measured near Landau Level filling factors ±2, ±6 evidences a strong backscattering of carriers even at T=0.3 K and B=19 T. It results that the Hall resistance is not quantized at the metrological level. σ_xx increases (decreases) as a function of the temperature (magnetic induction) following unexpected power laws that are not compatible with usual backscattering mechanisms like variable range hopping or inter-Landau level activation. With the support of structural characterizations and numerical simulations, we discuss the role of line defects (wrinkles and grain boundaries) crossing the Hall bar which can short-circuit the counter-propagating edge states.