High-field properties of pure and doped MgB 2 and Fe-based superconductors
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oduction During the last 20 years, we have witnessed a stream of discoveries of new superconducting materials with critical temperatures Tc in the range of 10 < Tc < 60 K, filling the gap between conventional low-Tc superconductors (LTS) and high-Tc cuprates. In 2001, the discovery of MgB2 with Tc = 40 K was announced.1 It was quickly recognized that high Tc and other unusual properties of MgB2 result from the well-known multiband electron-phononmediated superconducting pairing but on different disconnected sheets of the Fermi surface (FS). In conventional metallic superconductors such as Nb or Pb, the superconducting gap Δ opens up below Tc in only a single band crossing the Fermi level. In contrast, the two-band MgB2 has two different superconducting gaps Δσ ≈ 7.2 meV and Δπ ≈ 2.3 meV on the σ and π sheets of the Fermi surface formed by the in-plane (σ) and out-of-plane (π) orbitals of Boron.2,3 In 2008, the discovery of superconductivity in LaFeAsO1–xFx at 26 K4 initiated investigations of the diverse family of ferropnictides. As a result, Tc has been increased up to 56 K by substituting La with other rare earths5 to create members of the family called RE-1111. Other Fe-based superconductor (FBS) families have also been found, most notably BaAs2(Fe1–xCox)2 and Ba1–xKxAs2Fe2 (Ba-122) with Tc ≤ 38 K;6 FeSe1–xTex (Fe-11) with Tc ≤ 15 K;7,8 and LiFeAs with Tc ≤ 18 K9. Like MgB2, FBS are also multiband superconductors, with several bands crossing the Fermi level and superconductivity occurring on several disconnected electron and hole sheets of the FS. Yet
MgB2 is a metallic superconductor with comparatively large in-plane coherence length at T = 0 K of ξ0 ≈ 5–10 nm, which quantifies the radius of the Cooper pairs. In contrast, the semimetallic FBS become superconducting only upon doping the parent antiferromagnetic compounds10 and have much shorter ξ0 ≈ ħvF/ 2 πkBTc = 1–2 nm because of their smaller Fermi velocities vF = 106–107 cm/s (the speed of the electrons on the FS).10 The significant difference in ξ0 in MgB2 and FBS suggests very different ways of improving Hc2 in these materials. Multiband superconductivity in MgB2 is different from that in FBS. MgB2 is a two-band superconductor with strong intraband and weak interband electron-phonon pairing, resulting in Cooper pairs in the s-orbital state (s-wave pairing).2,3 Superconductivity in FBS is dominated by strong interband coupling mediated by antiferromagnetic excitations. This coupling results in either d-wave pairing or in s-wave Cooper pairs, whose order parameter changes sign on different sheets of the FS (called s± pairing).11,12 As a result, MgB2 and FBS behave very differently in high magnetic fields. A magnetic field H suppresses superconductivity above the upper critical magnetic field Hc2. The temperature dependence of Hc2 is one of the fundamental properties of Type II superconductors and a key characteristic for high-field-magnet applications. For this reason, finding effective ways of increasing Hc2 is one of the challenges of materials science. In conventi
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