Detailed knowledge of the phase diagram and the nature of the competing magnetic and superconducting phases is imperative for a deeper understanding of the physics of iron-based superconductivity. Magnetism in the iron-based superconductors is usually a stripe-type spin-density-wave, which breaks the tetragonal symmetry of the lattice, and is known to compete strongly with superconductivity. Recently, it was found that in some systems an additional spin-density-wave transition occurs, which restores this tetragonal symmetry, however, its interaction with superconductivity remains unclear. Here, using thermodynamic measurements on Ba1−xKxFe2As2 single crystals, we show that the spin-density-wave phase of tetragonal symmetry competes much stronger with superconductivity than the stripe-type spin-density-wave phase, which results in a novel re-entrance of the latter at or slightly below the superconducting transition.

The coupling between superconductivity and othorhombic distortion is studied in vapor-grown FeSe single crystals using high-resolution thermal-expansion measurements. In contrast to the Ba122-based (Ba122) superconductors, we find that superconductivity does not reduce the orthorhombicity below Tc. Instead we find that superconductivity couples strongly to the in-plane area, which explains the large hydrostatic pressure effects. We discuss our results in light of the spinnematic scenario and argue that FeSe has many features quite different from the typical Fe-based superconductors.

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Using resistivity, heat-capacity, thermal-expansion, and susceptibility measurements we study the normal-state behavior of KFe2As2. We find that both the Sommerfeld coefficient (γ ≈ 103 mJ mol −1 K −2 ) and the Pauli susceptibility (χ ≈ 4×10 −4 ) are strongly enhanced, which confirm the existence of heavy quasiparticles inferred from previous de Haas-van Alphen and ARPES experiments. We discuss this large enhancement using a Gutzwiller slave-boson mean-field calculation, which reveals the proximity of KFe2As2 to an orbital-selective Mott transition. The temperature dependence of the magnetic susceptibility and the thermal expansion provide strong experimental evidence for the existence of a coherence-incoherence crossover, similar to what is found in heavy fermion and ruthenate compounds, due to Hund's coupling between orbitals.PACS numbers: 74.70. Xa, 74.25.Bt, 65.40.Ba, 75.20.En, 71.38.Cn Soon after the discovery of high-temperature superconductivity in iron pnictide compounds, 1,2 their unique electronic structure, displaying electron and hole sheets, was revealed. In the Ba 1−x K x Fe 2 As 2 series, superconductivity emerges in the vicinity of an antiferromagnetic spin density wave (SDW) instability (x ≈ 0.3) and is maximal at x ≈ 0.4 with T c ≈ 38 K. At this optimal concentration, the superconducting order parameter is fully gapped with either s ++ or s +− symmetry. 3,4 In the latter case, it is believed that pairing is due to repulsive interband interactions enhanced by the magnetic fluctuations which develop around the nesting vector that connects the two different sheets. 3 However, superconductivity is not confined to this hypothetical quantum critical region and persists to x = 1 (with a strongly depressed T c ≈ 3 K), 2 where only hole pockets are present. 5-8 Moreover, the Sommerfeld coefficient for KFe 2 As 2 , γ ≈ 100 mJ mol −1 K −2 , is paradoxally about twice larger than observed at the optimal concentration 9,10 and recent laser ARPES measurements reveal that some of the energy gaps have nodes. 11 Clearly, more experimental investigations are necessary to elucidate the situation in the overdoped region of Ba 1−x K x Fe 2 As 2 and to understand the origin of the strong mass enhancement observed in quantum oscillation and ARPES experiments in KFe 2 As 2 . LDA+DMFT calculations stress that the mass enhancement in iron pnictides is not related to the proximity to a quantum critical point, but because the electrons, being rather localized at high temperature, start to form coherent quasiparticle bands with the underlying Fermi surface. 12,13 In this scenario, Hund's rule coupling is responsible for the large mass enhancement and a coherenceincoherence crossover is expected to occur for increasing temperature.In this Letter, we combine resistivity, specific-heat, thermal-expansion and susceptibility measurements to study in detail the normal state of KFe 2 As 2 . We clearly show that both the Sommerfeld coefficient (γ ≈ 102 mJ mol −1 K −2 ) and the Pauli susceptibility (χ ≈ 4×10 −4 ) are strongly en...

The nematic susceptibility, χφ, of hole-doped Ba(1-x)K(x)Fe2As2 and electron-doped Ba(Fe(1-x)Co(x))2As2 iron-based superconductors is obtained from measurements of the elastic shear modulus using a three-point bending setup in a capacitance dilatometer. Nematic fluctuations, although weakened by doping, extend over the whole superconducting dome in both systems, suggesting their close tie to superconductivity. Evidence for quantum critical behavior of χφ is, surprisingly, only found for Ba(Fe(1-x)Co(x))2As2 and not for Ba(1-x)K(x)Fe2As2--the system with the higher maximal Tc value.

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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-

The low-temperature electronic phase diagram of Ba1−xNaxFe2As2, obtained using highresolution thermal-expansion and specific-heat measurements, is shown to be considerably more complex than previously reported, containing nine different phases. Besides the magnetic C2 and reentrant C4 phases, we find evidence for an additional, presumably magnetic, phase below the usual SDW transition, as well as a possible incommensurate magnetic phase. All these phases coexist and compete with superconductivity, which is particularly strongly suppressed by the C4-magnetic phase due to a strong reduction of the electronic entropy available for pairing in this phase.High-temperature superconductivity in Fe-based systems usually emerges when a stripe-type antiferromagnetic spin-density-wave (SDW) is suppressed by either doping or pressure [1][2][3]. The SDW transition is accompanied, or sometimes even slightly preceeded, by a structural phase transition from a high-temperature tetragonal (C 4 ) to a low-temperature orthorhombic (C 2 ) state, which has sparked the lively debate about electronic nematicity and the respective role of spin and orbital physics in these materials [4][5][6][7][8]. In the holedoped compounds, Ba 1−x Na x Fe 2 As 2 , Ba 1−x K x Fe 2 As 2 , and Sr 1−x Na x Fe 2 As 2 , recent studies have shown that the C 4 symmetry is restored in a small pocket within the magnetic C 2 phase region [9][10][11][12]. Mössbauer studies on Sr 0.63 Na 0.37 Fe 2 As 2 find that only half of the Fe sites carry a magnetic moment in this phase [12], which is consistent with the double-Q magnetic structure predicted within the itinerant spin-nematic scenario [6,9,12,13]. Moreover, neutron studies have shown that the spins flip from in-plane in the C 2 phase to out of plane in the C 4 reentrant phase [14], indicating that spin-orbit interactions cannot be neglected. In the Ba 1−x K x Fe 2 As 2 system, the reentrant C 4 phase reverts back to the C 2 phase near the onset of superconductivity, due to a stronger competition of the C 4 phase with superconductivity [10]. The presence of this phase in the hole-doped systems presents strong evidence that the physics of these Fe-based systems can be treated in an itinerant picture, and recent theoretical studies based upon the spin-nematic scenario can reproduce phase diagrams very similar to the experimental ones [15], as well as the spin-reorientation in the C 4 phase if spin-orbit interactions are included [16].Here, we reinvestigate in greater detail the low-temperature electronic phase diagram of Ba 1−x Na x Fe 2 As 2 using high-resolution thermalexpansion and specific-heat measurements and show that it is considerably more complex than previously * liran.wang@kit.edu † present address: The Ames Laboratory, U.S. Department of Energy, Iowa State University, Ames, Iowa 50011, USA ‡ christoph.meingast@kit.edu reported, containing nine different phases. Besides the usual C 2 and reentrant C 4 magnetic phases, we find evidence for an additional, presumably magnetic, C 2 phase, in which the orth...