Occurrence of Superconductivity and Magnetism in Nominally Undoped LaOFeAs |
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Authors: | Anurag Gupta Hannu Huhtinen Chandra Shekhar Kim Schlesier Pankaj Srivastava Amit Srivastava O. N. Srivastava Reino Laiho A. V. Narlikar |
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Affiliation: | 1.Division of Superconductivity and Cryogenics,National Physical Laboratory (CSIR),New Delhi,India;2.Wihuri Physical Laboratory, Department of Physics and Astronomy,University of Turku,Turku,Finland;3.Centre of Advanced Studies for Physics of Materials, Department of Physics,Banaras Hindu University,Varanasi,India;4.UGC-DAE Consortium for Scientific Research,Indore,India;5.Graduate School of Materials Research,Turku,Finland |
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Abstract: | Superconductivity in a LaOFeAs system is known to get introduced by F-doping (LaO1−x F x FeAs) even under ambient conditions and oxygen deficiency (LaO1−x FeAs) under high pressure conditions. Hitherto unreported, superconductivity in F-free undoped LaOFeAs samples is observed and confirmed for the first time by various characterization tools–resistive ρ(T,B), magnetic M(T,B) and modulated microwave absorption (MMA) measurements. The ρ(T) at B=0 shows a clear superconducting transition with an onset at T con∼17 K and a tail-like behavior when R goes to zero at T c0∼8 K. In the presence of B, the superconducting transition shifts to lower T with a rate ∼−5.5 and −1.65 T/K, depending on whether the ρ(T) has dropped to 90% or 5% of its normal state value, respectively. M(T) in zero field cooling at B=10 mT shows diamagnetic downturn below at ∼12 K. At T<T c0 the change in MMA with B shows a low field (B∼1 mT) peak, which vanishes at T>T c0, indicating the presence of weak link superconducting networks in the sample. The sample shows a complex electrical and magnetic behavior in the normal state. For instance, ρ(T) reveals a weak SDW-like anomaly at T∼132 K along with a resistivity minimum at T min ∼78 K. M(T) also shows the presence of a magnetic anomaly at T∼130 K. Both below and above T con, presence of an additional ferromagnetic component is observed in the isothermal M(B) loop measurements. The superconducting and normal state features of our sample are compared with other undoped and doped LaOFeAs systems reported in the literature. |
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