Magnetic Properties of the Superconducting State of Iron-Based

 Superconductors

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Abstract

We show that features of the dynamical spin susceptibility can unambiguously distinguish between different pairing symmetries of the superconducting state in iron-based superconductors. A magnetic resonance is a unique feature for the extended \(s_{x^2y^2}\)-wave \(\cos k_x\cos k_y\) pairing symmetry. It is present in the pure superconducting (SC) state, but weakens in the mixed SC and magnetically ordered state. We calculate the the RPA correction to the NMR spin relaxation rate \(1/T_1\) and the Knight shift in the above states and show a good agreement with experimental results. Moreover, we argue that the energy dispersion of the magnetic resonance along c axis observed in neutron scattering experiments is also an indirect evidence supporting the \(s_{x^2y^2}\) pairing symmetry.

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Superconductivity at 43 K in an iron-based layered compound LaO(1-x)F(x)FeAs.

Hiroki Takahashi, Kazumi Igawa, Kazunobu Arii … (2008)

The iron- and nickel-based layered compounds LaOFeP (refs 1, 2) and LaONiP (ref. 3) have recently been reported to exhibit low-temperature superconducting phases with transition temperatures T(c) of 3 and 5 K, respectively. Furthermore, a large increase in the midpoint T(c) of up to approximately 26 K has been realized in the isocrystalline compound LaOFeAs on doping of fluoride ions at the O2- sites (LaO(1-x)F(x)FeAs). Experimental observations and theoretical studies suggest that these transitions are related to a magnetic instability, as is the case for most superconductors based on transition metals. In the copper-based high-temperature superconductors, as well as in LaOFeAs, an increase in T(c) is often observed as a result of carrier doping in the two-dimensional electronic structure through ion substitution in the surrounding insulating layers, suggesting that the application of external pressure should further increase T(c) by enhancing charge transfer between the insulating and conducting layers. The effects of pressure on these iron oxypnictide superconductors may be more prominent than those in the copper-based systems, because the As ion has a greater electronic polarizability, owing to the covalency of the Fe-As chemical bond, and, thus, is more compressible than the divalent O2- ion. Here we report that increasing the pressure causes a steep increase in the onset T(c) of F-doped LaOFeAs, to a maximum of approximately 43 K at approximately 4 GPa. With the exception of the copper-based high-T(c) superconductors, this is the highest T(c) reported to date. The present result, together with the great freedom available in selecting the constituents of isocrystalline materials with the general formula LnOTMPn (Ln, Y or rare-earth metal; TM, transition metal; Pn, group-V, 'pnicogen', element), indicates that the layered iron oxypnictides are promising as a new material platform for further exploration of high-temperature superconductivity.

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Superconductivity at 43 K in Samarium-arsenide Oxides \(SmFeAsO_{1-x}F_x\)

D. Fang, H. Chen, R H Liu … (2008)

Since the discovery of high-transition temperature (\(T_c\)) superconductivity in layered copper oxides, extensive efforts have been devoted to explore the higher \(T_c\) superconductivity. However, the \(T_c\) higher than 40 K can be obtained only in the copper oxide superconductors so far. The highest reported value of \(T_c\) for non-copper-oxide bulk superconductivity is 39 K in \(MgB_2\).\cite{jun} The \(T_c\) of about 40 K is close to or above the theoretical value predicted from BCS theory.\cite{mcmillan} Therefore, it is very significant to search for non-copper oxide superconductor with the transition temperature higher than 40 K to understand the mechanism of high-\(T_c\) superconductivity. Here we report the discovery of bulk superconductivity in samarium-arsenide oxides \(SmFeAsO_{1-x}F_x\) with ZrCuAiAs type structure. Resistivity and magnetization measurements show strong evidences for transition temperature as high as 43 K. \(SmFeAsO_{1-x}F_x\) is the first non-copper oxide superconductor with \(T_c\) higher than 40 K. The \(T_c\) higher than 40 K may be a strong argument to consider \(SmFeAsO_{1-x}F_x\) as an unconventional superconductor.

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Nearly Isotropic superconductivity in (Ba,K)Fe2As2

J. Luo, S. A. Baily, F Balakirev … (2008)

Superconductivity was recently observed in the iron-arsenic-based compounds with a superconducting transition temperature (Tc) as high as 56K [1-7], naturally raising comparisons with the high Tc copper oxides. The copper oxides have layered crystal structures with quasi-two-dimensional electronic properties, which led to speculations that reduced dimensionality (that is, extreme anisotropy) is a necessary prerequisite for superconductivity at temperatures above 40 K [8,9]. Early work on the iron-arsenic compounds seemed to support this view [7,10]. Here we report measurements of the electrical resistivity in single crystals of (Ba,K)Fe2As2 in a magnetic field up to 60 T. We find that the superconducting properties are in fact quite isotropic, being rather independent of the direction of the applied magnetic fields at low temperature. Such behaviour is strikingly different from all previously-known layered superconductors [9,11], and indicates that reduced dimensionality in these compounds is not a prerequisite for high-temperature superconductivity. We suggest that this situation arises because of the underlying electronic structure of the iron-arsenide compounds, which appears to be much more three dimensional than that of the copper oxides. Extrapolations of low-field single-crystal data incorrectly suggest a high anisotropy and a greatly exaggerated zero-temperature upper critical field.