Hall coefficient and H in underdoped LaFeAsO_0.95F_0.05

¹  MPA-NHMFL, Los Alamos National Laboratory - Los Alamos, NM 87545, USA
²  Materials and Structures Laboratory, Tokyo Institute of Technology - Mail-Box: R3-7, 4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8503, Japan
³  ERATO-SORST, Japan Science and Technology Agency in Frontier Research Center, Tokyo Institute of Technology - Mail-Box: S2-13, 4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8503, Japan
⁴  Frontier Research Center, Tokyo Institute of Technology - Mail-Box: S2-13, 4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8503, Japan

Corresponding author: ykohama@lanl.gov

Received: 24 June 2008
Accepted: 23 September 2008

Abstract

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 Δ E = 630 K. The upper critical field, H, estimated from resistivity measurements, exceeds 75 T with zero-field T_c = 26.3 K, suggesting an unconventional nature for superconductivity.