The proximity of superconductivity and antiferromagnetism in the phase diagram of iron arsenides [1, 2], the apparently weak electron-phonon coupling [3] and the "resonance peak" in the superconducting spin excitation spectrum [4, 5, 6, 7] have fostered the hypothesis of magnetically mediated Cooper pairing. However, since most theories of superconductivity are based on a pairing boson of sufficient spectral weight in the normal state, detailed knowledge of the spin excitation spectrum above the superconducting transition temperature T c is required to assess the viability of this hypothesis [8,9]. Using inelastic neutron scattering we have studied the spin excitations in optimally doped BaFe 1.85 Co 0.15 As 2 (T c = 25 K) over a wide range of temperatures and energies. We present the results in absolute units and find that the normal state spectrum carries a weight comparable to underdoped cuprates [10,11]. In contrast to cuprates, however, the spectrum agrees well with predictions of the theory of nearly antiferromagnetic metals [12], without complications arising from a pseudogap [13, 14, 15] or competing incommensurate spin-modulated phases [16]. We also show that the temperature evolution of the resonance energy follows the superconducting energy gap ∆, as expected from conventional Fermi-liquid approaches [17, 18]. Our observations point to a surprisingly simple theoretical description of the spin dynamics in the iron arsenides and provide a solid foundation for models of magnetically mediated superconductivity.

The specific heat of high-purity Ba(0.68)K(0.32)Fe2As2 single crystals with the highest reported superconducting Tc=38.5 K was studied. The electronic specific heat Cp below Tc shows two gap features, with Δ1≈11 meV and Δ2≈3.5 meV obtained from an α-model analysis. The reduced gap value, 2Δ(max)/kBTc≈6.6, the magnitude of the specific-heat jump, ΔCp(Tc)/Tc, and its slope below Tc exhibit a strong-coupling character. We also show that an Eliashberg model with two hole and two electron bands gives the correct values of Tc, the superconducting gaps, and the free-energy difference.

We study the symmetry of spin excitation spectra in 122-ferropnictide superconductors by comparing the results of first-principles calculations with inelastic neutron scattering (INS) measurements on BaFe 1.85 Co 0.15 As 2 and BaFe 1.91 Ni 0.09 As 2 samples that exhibit neither static magnetic phases nor structural phase transitions. In both the normal and superconducting (SC) states, the spectrum lacks the threedimensional (3D) 4 2 /m screw symmetry around the ( 1 2 1 2 L) axis that is implied by the I4/mmm space group. This is manifest both in the in-plane anisotropy of the normal-and SC-state spin dynamics and in the out-ofplane dispersion of the spin-resonance mode. We show that this effect originates from the higher symmetry of the magnetic Fe-sublattice with respect to the crystal itself, hence the INS signal inherits the symmetry of the unfolded Brillouin zone (BZ) of the Fe-sublattice. The in-plane anisotropy is temperature-independent and can be qualitatively reproduced in normal-state density-functional-theory calculations without invoking a symmetry-broken ("nematic") ground state that was previously proposed as an explanation for this effect. Below the SC transition, the energy of the magnetic resonant mode ω res , as well as its intensity and the SC spin gap inherit the normal-state intensity modulation along the out-of-plane direction L with a period twice larger than expected from the body-centered-tetragonal BZ symmetry. The amplitude of this modulation decreases at higher doping, providing an analogy to the splitting between even and odd resonant modes in bilayer cuprates. Combining our and previous data, we show that at odd L a universal linear relationship ħ hω res ≈ 4.3 k B T c holds for all the studied Fe-based superconductors, independent of their carrier type. Its validity down to the lowest doping levels is consistent with weaker electron correlations in ferropnictides as compared to the underdoped cuprates.

A comparative study of the upper critical field H c2 and second magnetization peak H sp was performed using high-quality single crystals of hole-doped Ba 0.68 K 0.32 Fe 2 As 2 and electron-doped BaFe 1.85 Co 0.15 As 2 and BaFe 1.91 Ni 0.09 As 2 . The H c2 was extracted from both resistivity and magnetization measurements using varying magnetic fields on H ʈ c and H Ќ c orientations. The anisotropic ratio, ␥ = H c2Ќc / H c2 ʈc , was observed to decrease to ϳ2.5 for the hole-doped and ϳ3.0 for both electron-doped samples as the magnetic fields were increased up to 9 T. It demonstrates that the anisotropic properties only show slight change by doping aliovalent ions either in or out-of the basal plane of FeAs. For the hole-doped Ba 0.68 K 0.32 Fe 2 As 2 the H c2 and H sp shift toward the higher temperature and higher field regime in the temperature-normalized ͑T / T c ͒ vortex phase diagram, suggesting a stronger vortex pinning by the comparison with the electron-doped BaFe 1.85 Co 0.15 As 2 and BaFe 1.91 Ni 0.09 As 2 . In contrast to the As-deficiency or inhomogeneous doping distribution of K, Co, and Ni, the dense pinning centers in Ba 0.68 K 0.32 Fe 2 As 2 may be attributed to the disordered structural domains and suggested to be responsible for the intrinsic disorder and anisotropy of iron arsenides.

No abstract

We report that the ͑Ba, K͒Fe 2 As 2 crystal with T c = 32 K shows a pinning potential, U 0 , as high as 10 4 K, with U 0 showing very little field dependence. The ͑Ba, K͒Fe 2 As 2 single crystals become isotropic at low temperatures and high magnetic fields, resulting in a very rigid vortex lattice, even in fields very close to H c2 . The isotropic rigid vortices observed in the two-dimensional ͑2D͒ ͑Ba, K͒Fe 2 As 2 distinguish this compound from 2D high-T c cuprate superconductors with 2D vortices. The vortex avalanches were also observed at low temperatures in the ͑Ba, K͒Fe 2 As 2 crystal. It is proposed that it is the K substitution that induces both almost isotropic superconductivity and the very strong intrinsic pinning in the ͑Ba, K͒Fe 2 As 2 crystal.A high critical current density, J c , upper critical field, B c2 , and irreversibility field, B irr , a high superconducting transition temperature, T c , strong magnetic-flux pinning, good grain connectivity, and isotropic superconductivity are the major physical requirements for superconducting materials used in practical applications operating at low and, in particular, high magnetic fields. The conventional low-T c superconductors, where H c2 is also small, can only carry large J c at very low temperatures. The cuprate high-T c superconductors suffer from poor grain connectivity and easy melting of the vortex lattice, leading to small J c in high magnetic fields at relatively high temperatures. For MgB 2 superconductor with T c of 39 K, B irr is far below H c2 , and J c drops quickly with both field and temperature, preventing its use above 20 K. The newly discovered Fe-based superconductors 1-7 show T c as high as 55 K and B c2 above 200 T, in combination with a small anisotropy for REFeAsO 1−x F x ͑RE-1111 phase, with RE a rare-earth element͒ 8 and an almost isotropic superconductivity for ͑Ba, K͒Fe 2 As 2 ͑122 phase͒. 9 These properties make the Fe-based superconductors extremely promising candidates for high magnetic field applications at relatively high temperatures. The current carrying ability of these superconductors at high fields and temperatures is largely determined by the flux-pinning strength and the behavior of the vortex matter. Therefore, the determination of their intrinsic vortex pinning strength is a central issue from both an applied and a fundamental perspective. Both 1111 and 122 phase compounds have typical two-dimensional ͑2D͒ crystal structures. In RE-1111 phase, where RE is a rare-earth element, the FeAs superconducting layers are separated by insulating LaO layers 10 while in Ba͑K͒-122 phase, the FeAs layer is sandwiched between conductive Ba layers. 5 It is expected that the 122 phase containing two FeAs layers would have small anisotropy and thus higher intrinsic pinning compared to the single layer 1111 phase. Co-doped BaFe 2 As 2 single crystal shows an anisotropy of 1-3 and upper criticalfield values of B c2 ͑B ʈ ab͒ = 20 T and B c2 ͑B ʈ c͒ = 10 T at 20 K, with dB c2 / dT Ϸ 5 T/ K. 11 For single crystals of the optimally do...

A quantum critical point is a point in a system's phase diagram at which an order is completely suppressed at absolute zero temperature (T). The presence of a quantum critical point manifests itself in the finite-T physical properties, and often gives rise to new states of matter. Superconductivity in the cuprates and in heavy fermion materials is believed by many to be mediated by fluctuations associated with a quantum critical point. In the recently discovered iron-pnictide superconductors, we report transport and NMR measurements on BaFe 2 À x Ni x As 2 (0rxr0.17). We find two critical points at x c1 ¼ 0.10 and x c2 ¼ 0.14. The electrical resistivity follows r ¼ r 0 þ AT n , with n ¼ 1 around x c1 and another minimal n ¼ 1.1 at x c2 . By NMR measurements, we identity x c1 to be a magnetic quantum critical point and suggest that x c2 is a new type of quantum critical point associated with a nematic structural phase transition. Our results suggest that the superconductivity in carrier-doped pnictides is closely linked to the quantum criticality.

No abstract