Strong localization in a suspended monolayer graphene by intervalley scattering

A gate induced insulating behavior at zero magnetic field is observed in a high mobility suspended monolayer graphene near the charge neutrality point. The graphene device initially cleaned by a current annealing technique was undergone a thermo-pressure cycle to allow short range impurities to be adsorbed directly on the ultra clean graphene surface. The adsorption process generated a strong temperature and electric field dependent behavior on the conductance of the graphene device. The conductance around the neutrality point is observed to be reduced from around $e^2\mathrm{/}h$ at 30 K to $\sim 0.01e^2\mathrm{/}h$ at 20 mK. A direct transition from insulator to quantum Hall conductor within $\approx 0.4T$ accompanied by broken-symmetry-induced $\nu =0,\pm 1$ plateaux confirms the presence of intervalley scatterers.