Two-band superconductivity in LaFeAsO0.89F0.11 at very high magnetic fields

Abstract

[7,8] These discoveries have generated much interest in the mechanisms and manifestations of unconventional superconductivity in the family of doped quaternary layered oxypnictides LOMPn (L = La, Pr, Ce, Sm; M = Mn, Fe, Co, Ni; Pn=P, As) [9,10], because many features of these materials clearly set them apart from other superconductors. First, ab-initio calculations indicate that superconductivity originates from the d-orbitals of what would normally be expected to be pairbreaking magnetic Fe ions, suggesting that new non-phonon pairing mechanisms are responsible for the high T c [11,12]. Second, F-doped LaFeAsO is a semimetal, which exhibits strong ferromagnetic and antiferromagnetic fluctuations and a possible spin density wave instability around 150K in the parent undoped LaFeAsO [5,[13][14][15][16]. And third, superconductivity emerges on several disconnected pieces of the Fermi surface [11,12,17,18], thus exhibiting the multi-gap pairing, which has recently attracted so much attention in MgB 2 [19].Given the importance of magnetic correlations in the doped oxypnictides, transport measurements at very high magnetic fields are vital to probe the mechanisms of superconductivity. Indeed, first measurements of the upper critical field B c2 at low fields B < 7T have yielded a slope B c2 / (T c ) = dB c2 /dT º 2T/K near T c , for both La and Sm based oxypnictides [2][3][4][5][6]. From the conventional one-band Werthamer-Helfand-Hohenberg (WHH) theory [20] such slopes already imply rather high values B c2 (0) = 0.69T c B c2/ º 36T for LaFeAsO 0.89 F 0.1 , B c2 (0) º 59.3T for SmFeAsO 0.89 F 0.1 , and B c2 (0) º 72 T for PrFeAsO 0.89 F 0.1 , all well above B c2 of Nb 3 Sn. However, studies of the high-field superconductivity in MgB 2 alloys have shown that the upward curvature of B c2 (T) resulting from the multiband effects can significantly increase B c2 (0) as compared to the WHH one-band extrapolation (see, e.g., the review [21] and references therein). To address these important issues, we have performed high-field dc transport measurements on LaFeAsO 0.89 F 0.1 samples up to 45T. We show that B c2 (T) indeed exhibits two-gap behavior similar to that in MgB 2 , and B c2 (0) values exceed the WHH extrapolation by the factor ~ 2. Moreover, the observed B c2 (0) also exceeds the BCS paramagnetic limitPolycrystalline LaFeAsO 0.89 F 0.11 samples were made by solid state synthesis [4]. A sample ~ 3 x 1 x 0.5 mm was used for our four probe transport measurements in the 45T Hybrid magnet at the NHMFL, supplemented by low field measurements in a 9T superconducting magnet. Our low-field data agreed well with the earlier data taken at ORNL on the same sample [4], indicating its good temporal and atmospheric stability. The 45T Hybrid magnet was swept only from 11.5T to 45T due to the constant 11.5T background of the outsert magnet while lower fields were swept from 0T to 9T in a PPMS with resistivity measured in AC mode using a 5mA excitation current, whereas the high field resistance R(B) was measured by a Keithley nanovo...

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