Doping evolution of superconducting gaps and electronic densities of states in Ba(Fe _1−x Co _x ) ₂ As ₂ iron pnictides

Abstract

Abstract. -An extensive calorimetric study of the normal-and superconducting-state properties of Ba(Fe1−xCox)2As2 is presented for 0 < x < 0.2. The normal-state Sommerfeld coefficient increases (decreases) with Co doping for x < 0.06 (x > 0.06), which illustrates the strong competition between magnetism and superconductivity to monopolize the Fermi surface in the underdoped region and the filling of the hole bands for overdoped Ba(Fe1−xCox)2As2. All superconducting samples exhibit a residual electronic density of states of unknown origin in the zero-temperature limit, which is minimal at optimal doping but increases to the normal-state value in the strongly under-and over-doped regions. The remaining specific heat in the superconducting state is well described using a two-band model with isotropic s-wave superconducting gaps.Introduction. -Despite the fact that the theoretical background has been available since the late 50's with the pioneering papers of Suhl et al.[1] and Moskalenko [2], multiband superconductivity (MBSC) emerged as an unanimously accepted phenomenon only after the discovery of the MgB 2 superconductor in 2001 [3]. Rapidly, calorimetric signatures, like the excess specific heat observed at low temperature (with respect to the single-band BCS curve) [4], the initial rapid rise of the mixed-state heat capacity with magnetic field [5], and the anomalous positive curvature of the upper critical field [6] provided the most convincing evidence of its existence and are now textbook hallmarks of the existence of two gaps. Later, significant interband contributions to the Eliashberg function, reminiscent of MBSC, were experimentally detected using tunneling experiments [49]. Since then, the occurrence of MBSC has been discussed for many different compounds including heavy fermions [7], cobaltates [8], chalcogenides [9], A15 compounds [50], and the recently discovered iron-pnictide family. The aforementioned characteristic signatures of MBSC are less pronounced in iron pnictide superconductors since the interband coupling, pro-

Signature of multigap nodeless superconductivity in fluorine-doped NdFeAsO

“…It is clearly seen in Fig. 4(a) that the larger gap, a tendency for strong-coupling effects, increases stronger than linear with T c for T c ≥ 30 K. The larger gap converges to the BCS value, which has previously been reported also for Ba(Fe 1−x Co x ) 2 As 2 and Mg 1−x Al x B 2 [44,45]. By analogy, the difference in gap amplitude could be comes from the coupling strength and the interband scattering between the 2D σ bands (which exhibit the larger gap) and the 3D π bands are weak because of their difference in character.…”

Muon spin rotation study of magnetism and superconductivity in Ba(Fe1−xCox)2Bernhard1, Wang2, Nuccio3 et al. 2012Phys. Rev. BSelf Cite5422871View full textAdd to dashboardCite

“…32 Specific heat measurements on crystals from the same or similar growth batches are reported in Ref. 26. These measurements suggest a bulk superconducting state of the samples in the Co doping range of 0.035 x 0.13 with a maximum transition temperature T c, max = 24.5 K at x = 0.065.…”

Iron-based superconductors: Magnetism, superconductivity, and electronic structure (Review Article)

“…In 111 systems, superconductivity emerges already at zero doping instead of magnetic order (in LiFeAs) or together with it (in NaFeAs). There are several important experimentally established tendencies, which are followed by many representatives of iron-based family with highest c T [146]: large difference in superconducting gap magnitude on different FS pockets [39,43,150,151], / c T Δ value, that is similar to cuprates and much higher than expected from BCS [43,59,150], correlation of c T with anion height [152]. The complexity of the electronic structure of FeSC was originally an obstacle on the way to its understanding [39,40], but at closer look, such a variety of electronic states turned out to be extremely useful for uncovering the correlation between orbital character and pairing strength [146] and, more general, between electronic structure and superconductivity [17].…”