77 Se and 87 Rb nuclear magnetic resonance (NMR) experiments on Rb 0.74 Fe 1.6 Se 2 reveal clearly distinct spectra originating from a majority antiferromagnetic (AF) and a minority metallic-superconducting (SC) phase. The very narrow NMR line of the SC phase evidences the absence of Fe vacancies and any trace of AF order. The Rb content of the SC phase is deduced from intensity measurements identifying Rb 0.3(1) Fe 2 Se 2 as the actual composition of the SC fraction. The resulting estimate of 0.15 electrons/Fe brings this class of superconductors 245 family closer to the other Fe-based superconductor families.DOI:

We have studied the low-energy spin-excitation spectrum of the single-crystalline Rb 2 Fe 4 Se 5 superconductor (T c = 32 K) by means of inelastic neutron scattering. In the superconducting state, we observe a magnetic resonant mode centered at an energy of hω res = 14 meV and at the (0.5 0.25 0.5) wave vector (unfolded Fe-sublattice notation), which differs from the ones characterizing magnetic resonant modes in other iron-based superconductors. Our finding suggests that the 245-iron-selenides are unconventional superconductors with a sign-changing order parameter, in which bulk superconductivity coexists with the 5 × 5 magnetic superstructure. The estimated ratios of hω res /k B T c ≈ 5.1 ± 0.4 and hω res /2∆ ≈ 0.7 ± 0.1, where ∆ is the superconducting gap, indicate moderate pairing strength in this compound, similar to that in optimally doped 1111-and 122-pnictides. PACS numbers: 74.70.Xa, 74.25.Ha, 78.70.Nx, 74.20.Rp Soon after the discovery of arsenic-free iron-selenide superconductors A 2 Fe 4 Se 5 (A = K, Rb, Cs), also known as 245-compounds [1], their unprecedented physical properties came to light, such as the coexistence of high-T c superconductivity with strong antiferromagnetism [2,3]. The pairing mechanism and the symmetry of the superconducting (SC) order parameter in this family of compounds still remain among the major open questions. In the majority of other Fe-based superconductors, it is widely accepted that the strong nesting between the holelike Fermi surface (FS) at the Brilliouin zone (BZ) center and electronlike FS at the BZ boundary leads to the sign-changing s-wave (s ± -wave) pairing symmetry [4]. This scenario has been supported by different experimental methods, such as angle-resolved photoemission spectroscopy (ARPES) [5], quasi-particle interference [6], and inelastic neutron scattering (INS) [7,8].On the other hand, recent theoretical calculations [9] and ARPES experiments [10, 11] on the 245-system revealed the absence of holelike FS at the BZ center in the electronic structure, implying that the nesting between the hole-and electronlike FS sheets is no longer present. Hence, several theoretical studies proposed alternative pairing instabilities, such as d-wave or another type of s ± -wave symmetry with sign-changing order parameter between bonding and antibonding states [12][13][14]. As a hallmark of sign-changing SC order parameter, several authors theoretically predicted a resonant mode in the magnetic excitation spectrum below the SC transition, yet its precise position in momentum space still remains controversial [12,13].A major complication in treating the 245-compounds theoretically arises from the presence of a crystallographic superstructure of Fe vacancies that has been consistently reported both from x-ray and neutron diffraction experiments [15]. This 5 × 5 superstructure is closely related to the static antiferromagnetic (AFM) order persisting up to the Néel temperature, T N ≈ 540 K [16]. Although most of the existing band structure calculations have so far negle...

Platelet-like single crystals of the Ca(Fe 1-x Co x ) 2 As 2 series having lateral dimensions up to 15 mm and thickness up to 0.5 mm were obtained from the high temperature solution growth technique using Sn flux. Upon Co doping, the c-axis of the tetragonal unit cell decreases, while the a-axis shows a less significant variation. Pristine CaFe 2 As 2 shows a combined spin-density-wave and structural transition near T = 166 K which gradually shifts to lower temperatures and splits with increasing Co-doping. Both transitions terminate abruptly at a critical Co-concentration of x c = 0.075. For x ≥ 0.05, superconductivity appears at low temperatures with a maximum transition temperature T C of around 20 K. The superconducting volume fraction increases with Co concentration up to x = 0.09 followed by a gradual decrease with further increase of the doping level. The electronic phase diagram of Ca(Fe 1-x Co x ) 2 As 2 (0 ≤ x ≤ 0.2) series is constructed from the magnetization and electric resistivity data. We show that the low-temperature superconducting properties of Co-doped CaFe 2 As 2 differ considerably from those of BaFe 2 As 2 reported previously. These differences seem to be related to the extreme pressure sensitivity of CaFe 2 As 2 relative to its Ba counterpart.

Resonant magnetic excitations are recognised as hallmarks of unconventional superconductivity in copper oxides, iron pnictides and heavy-fermion compounds. model calculations have related these modes to the microscopic properties of the pair wave function, but the mechanisms of their formation are still debated. Here we report the discovery of a similar resonant mode in the non-superconducting antiferromagnetic heavy-fermion metal CeB 6 . unlike conventional magnons, the mode is non-dispersive and is sharply peaked around a wave vector separate from those characterising the antiferromagnetic order. It is likely associated with a co-existing order parameter of the unusual antiferro-quadrupolar phase of CeB 6 , which has long remained hidden to neutron-scattering probes. The mode energy increases continuously below the onset temperature for antiferromagnetism, in parallel to the opening of a nearly isotropic spin gap throughout the Brillouin zone. These attributes are similar to those of the resonant modes in unconventional superconductors. This unexpected commonality between the two disparate ground states indicates the dominance of itinerant spin dynamics in the ordered lowtemperature phases of CeB 6 and throws new light on the interplay between antiferromagnetism, superconductivity and 'hidden' order parameters in correlated-electron materials.