J. Deisenhofer

Observation of a griffiths phase in paramagnetic La1-xSrxMnO3.

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.

NMR study in the iron-selenide Rb0.74Fe1.6Se2: determination of the superconducting phase as iron vacancy-free Rb0.3Fe2Se2.

77Se and 87Rb nuclear magnetic resonance (NMR) experiments on Rb0.74Fe1.6Se2 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))Fe2Se2 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.

Superconductivity at Tc = 44 K in LixFe2Se2(NH3)y

Following a recent proposal by Burrard-Lucas et al. [arXiv:1203.5046] we intercalated FeSe with Li in liquid ammonia. We report on the synthesis of new LixFe2Se2(NH3)y phases as well as on their magnetic and superconducting properties. We suggest that the superconducting properties of these new hybride materials appear not to be influenced by the presence of electronically-innocent Li(NH2) molecules. Indeed, high onset temperatures of 44 K and shielding fractions of almost 80% were only obtained in samples containing exclusively Lix(NH3)y moieties acting simultaneously as electron donors and spacer units. The c-axis lattice parameter of the new intercalated phases is strongly enhanced when compared to the alkali-metal intercalated iron selenides A1−xFe2−ySe2 with A = K, Rb, Cs, Tl with Tc = 32 K.

Magnetic resonant mode in the low-energy spin-excitation spectrum of superconducting Rb2Fe4Se5 single crystals.

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 ℏω(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 ℏω(res)/k(B)T(c)≈5.1±0.4 and ℏω(res)/2Δ≈0.7±0.1, where Δ is the superconducting gap, indicate moderate pairing strength in this compound, similar to that in optimally doped 1111 and 122 pnictides.

Competition of magnetism and superconductivity in underdoped (Ba1- xKx)Fe2As2

Polycrystalline samples of underdoped (Ba1- xKx)Fe2As2 (x≤0.4) were synthesized and studied by x-ray powder diffraction, magnetic susceptibility, specific heat and 57Fe-Mosbauer spectroscopy. The structural phase transition from tetragonal to orthorhombic lattice symmetry shifts towards lower temperatures, becomes less pronounced at x=0.1–0.2 and is no longer present at x=0.3. Bulk superconductivity is observed in all samples except (Ba0.9K0.1)Fe2As2 by resistivity and magnetic susceptibility measurements. Specific heat data show a broad spin-density-wave (SDW) phase transition in (Ba0.9K0.1)Fe2As2, which is hardly discernible in (Ba0.8K0.2)Fe2As2. No SDW anomaly is found in the specific heat of optimally doped (Ba0.6K0.4)Fe2As2, where C changes by 0.1 J K− 1 at Tc=37.3 K. 57Fe-Mosbauer spectra show full magnetic hyperfine field splitting, indicative of antiferromagnetic (AF) ordering at 4.2 K in samples with x=0–0.2, but zero magnetic hyperfine field in samples with x=0.3. The spectra of (Ba0.9K0.1)Fe2As2 and (Ba0.8K0.2)Fe2As2 in the phase transition regions are temperature-dependent superpositions of magnetic and non-magnetic components, caused by inhomogeneous potassium distribution. Our results suggest the co-existence of AF ordering and superconductivity without mesoscopic phase separation in the underdoped region and show unambiguously homogeneous superconducting phases close to optimal doping. This is in contrast to recently reported results about single-crystal (Ba1- xKx)Fe2As2.

Nanoscale Layering of Antiferromagnetic and Superconducting Phases in Rb 2 Fe 4 Se 5 Single Crystals

We studied phase separation in the single-crystalline antiferromagnetic superconductor Rb(2)Fe(4)Se(5) (RFS) using a combination of scattering-type scanning near-field optical microscopy and low-energy muon spin rotation (LE-μSR). We demonstrate that the antiferromagnetic and superconducting phases segregate into nanometer-thick layers perpendicular to the iron-selenide planes, while the characteristic in-plane size of the metallic domains reaches 10  μm. By means of LE-μSR we further show that in a 40-nm thick surface layer the ordered antiferromagnetic moment is drastically reduced, while the volume fraction of the paramagnetic phase is significantly enhanced over its bulk value. Self-organization into a quasiregular heterostructure indicates an intimate connection between the modulated superconducting and antiferromagnetic phases.

Melanin in the extracellular matrix of germlings of Botrytis cinerea

Previous work on the composition of the extracellular matrix of germlings of the plant pathogenic fungus Botrytis cinerea demonstrated the presence of carbohydrate, protein, and simple lipids; which, together, comprised 50-60% of the dry weight. Here we show that most of the remaining mass of the extracellular matrix consists of a chemically inert dark pigment with the electron paramagnetic resonance characteristics of a melanin. Scanning electron micrographs of the purified pigment, and transmission electron micrographs of thin sections made using the pigment indicate that it has a filamentous structure. We conclude that melanin is an important component of the extracellular matrix of germlings of B. cinerea. This is the first report of a melanin present in the extracellular matrix of a plant pathogenic fungus.

Crystal field, Dzyaloshinsky-Moriya interaction, and orbital order in La 0.95 Sr 0.05 MnO 3 probed by ESR

We present a comprehensive analysis of the Dzyaloshinsky-Moriya interaction and crystal-field parameters using the angular dependence of the paramagnetic-resonance shift and linewidth in single crystals of

Optical spectroscopy in CoO: Phononic, electric, and magnetic excitation spectrum within the charge-transfer gap

{\mathrm{La}}_{0.95}{\mathrm{Sr}}_{0.05}{\mathrm{MnO}}_{3}

Unconventional magnetostructural transition in CoCr2O4 at high magnetic fields

within the orthorhombic Jahn-Teller distorted phase. The Dzyaloshinsky-Moriya interaction