Hans-Albrecht Krug von Nidda

Ordered Arrays of II/VI Diluted Magnetic Semiconductor Quantum Wires: Formation within Mesoporous MCM-41 Silica

We present a novel way of synthesising highly ordered arrays of hollow Cd(1-x)Mn(x)S quantum wires with lateral dimensions of 3-4 nm separated by 1-2 nm SiO2 barriers by forming Cd(1-x)Mn(x)S (0 < or = x < or = 1) semiconductors inside the pore system of mesoporous MCM-41 SiO2 host structures. X-ray diffraction and transmission electron microscopy (TEM) studies reveal the hexagonal symmetry of these arrays (space group p6m) and confirm the high degree of order. Physisorption measurements show the filling of the pores of the MCM-41 SiO2. The X-ray absorption near-edge structure (XANES), extended X-ray absorption fine structure (EXAFS), electron paramagentic resonance (EPR), and Raman studies confirm the good crystalline quality of the incorporated (Cd,Mn)S guest. The effects of reducing the lateral dimensions on the magnetic and electronic properties of the diluted magnetic semiconductor were studied by photoluminescence (PL) and PL excitation spectroscopy and by SQUID and EPR measurements in the temperature range 2-400 K. Due to the quantum confinement of the excitons in the wires, an increase of about 200 meV in the direct band gap was observed. In addition, the p-d hybridisation-related bowing of the band gap as a function of Mn concentration in the wires is much stronger than in the bulk. This effect is related to the increase in the band gap due to quantum confinement, which shifts the p-like valence band edge closer to the 3d-related states of Mn in the valence band. Thus, the p-d hybridisation and the strength of the band gap bowing are increased. Compared to bulk (II,Mn)VI compounds, antiferromagnetic coupling between the magnetic moments of the Mn2+ ions is weaker. For the samples with high Mn concentrations (x > 0.8) this leads to a suppression of the phase transition of the Mn system from paramagnetic to antiferromagnetic. This effect can be explained by the fact that the lateral dimensions of the wires are smaller than the magnetic length scale of the antiferromagnetic ordering.

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.

Ultra-robust high-field magnetization plateau and supersolidity in bond-frustrated MnCr2S4

High magnetic field studies in MnCr2S4 reveal an extended magnetization plateau and the realization of supersolid phase. Frustrated magnets provide a promising avenue for realizing exotic quantum states of matter, such as spin liquids and spin ice or complex spin molecules. Under an external magnetic field, frustrated magnets can exhibit fractional magnetization plateaus related to definite spin patterns stabilized by field-induced lattice distortions. Magnetization and ultrasound experiments in MnCr2S4 up to 60 T reveal two fascinating features: (i) an extremely robust magnetization plateau with an unusual spin structure and (ii) two intermediate phases, indicating possible realizations of supersolid phases. The magnetization plateau characterizes fully polarized chromium moments, without any contributions from manganese spins. At 40 T, the middle of the plateau, a regime evolves, where sound waves propagate almost without dissipation. The external magnetic field exactly compensates the Cr–Mn exchange field and decouples Mn and Cr sublattices. In analogy to predictions of quantum lattice-gas models, the changes of the spin order of the manganese ions at the phase boundaries of the magnetization plateau are interpreted as transitions to supersolid phases.

Traces of Z2-Vortices in CuCrO2, AgCrO2, and PdCrO2

The spin-spin relaxation of the two-dimensional triangular lattice antiferromagnets CuCrO 2 , AgCrO 2 , and PdCrO 2 is investigated by electron spin resonance spectroscopy. The data proof excellent agreement with very recent predictions for the Z 2 -vortex ordering scenario in triangular Heisenberg antiferromagnets derived by Okubo and Kawamura.

[Co5Tp*4(Me2bta)6]: A Highly Symmetrical Pentanuclear Kuratowski Complex Featuring Tris(pyrazolyl)borate and Benzotriazolate Ligands.

The pentanuclear Co(II) complex [Co5Tp*4(Me2bta)6] containing N-donor ligands (5,6-dimethyl benzotriazolate; Me2bta6) and N-donor capping ligands (tris(3,5-dimethyl-1-pyrazolyl)borate; Tp*) was prepared by a simple and efficient ligand exchange reaction from [Co5Cl4(Me2bta)6] and tetra-n-butyl ammonium tris(3,5-dimethyl-1-pyrazolyl)borate. Compared to the precursor complex [Co5Cl4(Me2bta)6], which contains one Co(II) ion in octahedral and four Co(II) ions in tetrahedral coordination geometry, the title compound features all five Co(II) ions in an octahedral coordination environment while keeping a high complex symmetry. This results in modified properties including improved solubility and distinct magnetic behavior as compared to the precursor complex. The molecular structure and phase purity of the compound was verified by XRPD, UV-vis, ESI-MS, IR, and NMR measurements. Thermal stability of the compound was determined via TGA. The magnetic properties of here reported novel complex [Co5Tp*4(Me2bta)6] as well as its precursor [Co5Cl4(Me2bta)6] were examined in detail via ESR and SQUID measurements, which indicated weak anti-ferromagnetic exchange interactions between high-spin Co(II) centers at T < 20 and 50 K, respectively.

Magnetoelectrics: Is CdCr2S4 a multiferroic relaxor? (reply)

Catalan and Scott propose an alternative interpretation for our findings for CdCr2S4 in terms of Maxwell–Wagner effects. They also quote related isomorphs, such as HgCr2S4 (ref. 3), which has been discussed elsewhere. As we have shown, Maxwell–Wagner relaxations can indeed strongly affect the dielectric properties of transition-metal oxides and semiconductors. We do not find Catalan and Scotts arguments about CdCr2S4 convincing, however, and think that our experiments provide ample evidence for multiferroic relaxor behaviour.

Anisotropic Exchange in Spin Chains

Quasi-one-dimensional (1D) spin systems are highly unconventional materials, which exhibit a wide variety of phenomena, including spin-Peierls transition and charge ordering. In this paper, we show that electron spin resonance (ESR) is a very powerful tool to study spin relaxation mechanisms in these systems. We review the microscopic theory of superexchange and include a discussion of some recent experimental and theoretical developments concerning ESR in 1D solids. Furthermore, we evaluate the anisotropy and the temperature dependence of the ESR linewidth in three 1D systems (LiCuVO4, CuGeO3, α′-NaV2O5). Thus, we can determine the type and the magnitude of the anisotropic exchange interactions between spins and investigate the effects of fluctuations of charge and lattice degrees of freedom in the vicinity of phase transitions in these systems.

Is CdCr2S4 a multiferroic relaxor? (reply): Magnetoelectrics

Catalan and Scott propose an alternative interpretation for our findings for CdCr2S4 in terms of Maxwell–Wagner effects. They also quote related isomorphs, such as HgCr2S4 (ref. 3), which has been discussed elsewhere. As we have shown, Maxwell–Wagner relaxations can indeed strongly affect the dielectric properties of transition-metal oxides and semiconductors. We do not find Catalan and Scotts arguments about CdCr2S4 convincing, however, and think that our experiments provide ample evidence for multiferroic relaxor behaviour.

Magnetoelectrics: Is CdCr 2 S 4 a multiferroic relaxor? (reply)

Catalan and Scott propose an alternative interpretation for our findings for CdCr2S4 in terms of Maxwell–Wagner effects. They also quote related isomorphs, such as HgCr2S4 (ref. 3), which has been discussed elsewhere. As we have shown, Maxwell–Wagner relaxations can indeed strongly affect the dielectric properties of transition-metal oxides and semiconductors. We do not find Catalan and Scotts arguments about CdCr2S4 convincing, however, and think that our experiments provide ample evidence for multiferroic relaxor behaviour.