We report on the discovery of a novel triangular phase regime in the system La1-xSrxMnO3 by means of electron spin resonance and magnetic susceptibility measurements. This phase is characterized by the coexistence of ferromagnetic entities within the globally paramagnetic phase far above the magnetic ordering temperature. The nature of this phase can be understood in terms of Griffiths singularities arising due to the presence of correlated quenched disorder in the orthorhombic phase.

57 Fe-Mössbauer studies of superconducting Rb 0.8 Fe 1.6 Se 2.0 with T C = 32.4 K were performed on singlecrystalline and polycrystalline samples in the temperature range 4.2-295 K. They reveal the presence of 88% magnetic and 12% nonmagnetic Fe 2+ species with the same polarization dependence of their hyperfine spectra. The magnetic species are attributed to the 16i sites of the √ 5 × √ 5 × 1 superstructure and the nonmagnetic Fe species to a nanosized phase observed in recent structural studies of superconducting K x Fe 2−y Se 2 systems rather than to the vacant 4d sites in the √ 5 × √ 5 × 1 superstructure. The 57 Fe spectrum of a single-crystalline sample in an external field of 50 kOe applied parallel to the crystallographic c axis confirms the antiferromagnetic order between the fourfold ferromagnetic Fe(16i) supermoments and the absence of a magnetic moment at the Fe sites in the minority phase. A discussion of all spectral information and comparison with superconducting FeSe provides convincing evidence that the nanoscale phase separation is monitored by Mössbauer spectroscopy in Rb 0.8 Fe 1.6 Se 2.0 .

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...

Spin-polarized scanning tunneling microscopy (SP-STM) has been used extensively to study magnetic properties of nanostructures. Using SP-STM to visualize magnetic order in strongly correlated materials on an atomic scale is highly desirable, but challenging. We achieved this goal in iron tellurium (Fe(1+ y)Te), the nonsuperconducting parent compound of the iron chalcogenides, by using a STM tip with a magnetic cluster at its apex. Our images of the magnetic structure reveal that the magnetic order in the monoclinic phase is a unidirectional stripe order; in the orthorhombic phase at higher excess iron concentration (y > 0.12), a transition to a phase with coexisting magnetic orders in both directions is observed. It may be possible to generalize the technique to other high-temperature superconductor families, such as the cuprates.