Hall coefficient and H_c2 in underdoped LaFeAsO _0.95F_0.05

The electrical resistivity and Hall coefficient of LaFeAsO _0.95F_0.05 polycrystalline samples were measured in pulsed magnetic fields up to μ₀H=60 T from room temperature to 1.5 K. The resistance of the normal state shows a negative temperature coefficient (dρ/ dT<0) below 70 K for this composition, indicating insulating ground state in underdoped LaFeAsO system in contrast to heavily doped compound. The charge carrier density obtained from Hall effect can be described as constant plus a thermally activated term with an energy gap ΔE=630 K. The upper critical field, H_c2, estimated from resistivity measurements, exceeds 75 T with zero-field T_c=26.3 K, suggesting an unconventional nature for superconductivity.