Determination of the Upper Critical Field of a Single Crystal LiFeAs: The Magnetic Torque Study up to 35 Tesla

Abstract

We report on the upper critical field B c2 of a superconducting LiFeAs single crystal with T c $ 16 K, determined from magnetic torque measurements in dc-magnetic fields up to 35 T and at temperatures down to 0.3 K. B c2 at 0.3 K is obtained to be 26.4 and 15.5 T for the applied field B a k ab and B a k c, respectively. The anisotropy parameter À ¼ B ab c2 =B c c2 is $3 at T c and decreases to 1.7 as T ! 0, showing rather isotropic superconductivity. While B c2 is orbitally-limited for B a k c, the spin-paramagnetic effect is evident in the temperature dependence of B c2 for B a k ab. KEYWORDS: iron-based superconductor, LiFeAs, upper critical field, anisotropySince the discovery of superconductivity in LaFeAs(O,F) with T c ¼ 26 K, 1) a variety of related compounds containing FeAs-layers has been found to exhibit superconductivity.2)The parent compounds RFeAsO (R = rare earth, ''1111'' system) and AFe 2 As 2 (A = alkaline earth or Eu, ''122'' system) with the ZrCuSiAs-and ThCr 2 Si 2 -type structures, respectively, undergo antiferromagnetic and structural transitions. The transitions can be suppressed by several kinds of doping effects 1,[3][4][5][6] or application of pressure, [7][8][9] and T c reaches $56 K in some compounds. [10][11][12] The magnetic long range order usually competes with superconductivity, but the fluctuation likely plays a crucial role in the pairing mechanism of the Fe-based high-T c systems. It is also of interest that, around the optimal condition where T c shows its maximum, deviation from conventional Fermi-liquid behavior has been observed such as $ T , 13,14) anomalous Hall angle, 13) an enhancement of effective masses, 15) etc. These superconducting and normal-state features bear resemblance with those widely reported for strongly-correlated electron systems including cuprates and heavy fermion compounds.The title compound LiFeAs, categorized as the ''111'' system with the CeFeSi-type structure, has distinctive characteristics: (i) the stoichiometric superconductivity with T c as high as $17 K, 16) (ii) no experimental evidence for the magnetic/structural transitions, 17,18) and (iii) single crystals with high quality (residual resistivity ratio up to 50). 19,20) Therefore, LiFeAs provides a unique opportunity to probe the intrinsic properties of Fe-based high-T c superconductivity.Up to now, there are few reports on systematic measurements of the upper critical field B c2 of Fe-based superconductors to address the issue of the pair-breaking mechanism. This is mainly due to the fact that high-T c superconductors including Fe-based systems generally have extremely high B c2 . In many cases, accordingly, the lowtemperature behavior is extrapolated from the high temperature data around T c , which may lead to misleading conclusions. A precise determination of B c2 over the whole temperature range could provide important clues to the pairbreaking mechanism of high-T c superconductivity.Here, we present the first report on the whole temperature dependence of B c2 and its angular variatio...

Influence of s± symmetry on unconventional superconductivity in pnictides above the Pauli limit – two-band model study

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Anomalous Fermi surface in FeSe seen by Shubnikov–de Haas oscillation measurements

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Pauli-limiting effects in the upper critical fields of a clean LiFeAs single crystal

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