Frustration of the interlayer coupling by mobile holes in La2-xSrxCuO4 (x<0.02)

We have studied the interlayer coupling in the antiferromagnetic (AF) phase of Sr and Zn doped La2CuO4 by analyzing the spin flip transition in the magnetization curves. We find that the interlayer coupling strongly depends on the mobility of the hole charge carriers. Samples with the same hole content as well as the same Neel temperature but a different hole mobility, which we adjusted by Zn co-doping, can have a very different interlayer coupling. Our results suggest that only mobile holes can cause a strong frustration of the interlayer coupling.Comment: 4 pages, 4 figure

Blindness incidence in Germany - A population-based study from Württemberg-Hohenzollern

Few data on the incidence of blindness in Germany are available. We analysed causes of legal blindness for the region Württemberg-Hohenzollern (population 5.5 million) in order to help fill in this gap. Material and Methods: Population-based investigation on the incidence of legal blindness (visual acuity <1/50) based on materials from the social servies. Age-dependent blindness incidences were modelled via logistic regression models. Results: 647 blind persons were newly registered in 1994 (blindness incidence 11.6/100,000). The blindness incidence is moderate in infants (4.5/100,000) and decreases further during childhood. At the age of 20 years, the incidence again rises to the former level and remains relatively constant. After the age of 60 years, the incidence increases sharply: 5-year odds ratios are 1.76 (CI: 1.68-1.85) in women and 1.72 (CI: 1.60-1.84) in men. The blindness incidence is higher in women, 15.6/100,000, compared to 12.2/100,000 in men. The major causes of blindness are: macular degeneration, 3.92/100,000; diabetic retinopathy, 2.01/100,000; glaucoma, 1.6/100,000; high myopia, 0.77/100,000; optic atrophy, 0.68/100,000; central nervous system-triggered blindness; 0.56/100,000, and tapetoretinal degenerations, 0.52/100,000. Discussion: Due to monetary incentives for the blind persons, social service files offer accurate and complete data. Besides macular degeneration, glaucoma and dia betic retinopathy are major causes of blindness. Thus, this study suggests further blindness prevention activities for diabetic retinopathy and glaucoma

Evolution of two-gap behavior of the superconductor FeSe_1-x

The superfluid density, \rho_s, of the iron chalcogenide superconductor, FeSe_1-x, was studied as a function of pressure by means of muon-spin rotation. The zero-temperature value of \rho_s increases with increasing transition temperature T_c (increasing pressure) following the tendency observed for various Fe-based and cuprate superconductors. The analysis of \rho_s(T) within the two-gap scheme reveals that the effect on both, T_c and \rho_s(0), is entirely determined by the band(s) where the large superconducting gap develops, while the band(s) with the small gap become practically unaffected.Comment: 5 pages, 3 figure

Magnetic order and spin dynamics across a ferromagnetic quantum critical point: μ\muSR investigations of YbNi4_4(P1−x_{1-x}Asx_x)2_2

In the quasi-1D heavy-fermion system YbNi$_4$(P$_{1-x}$As$_x$)$_2$ the presence of a ferromagnetic (FM) quantum critical point (QCP) at $x_c$ $\approx 0.1$ with unconventional quantum critical exponents in the thermodynamic properties has been recently reported. Here, we present muon-spin relaxation ($\mu$SR) experiments on polycrystals of this series to study the magnetic order and the low energy 4$f$-electronic spin dynamics across the FM QCP. The zero field $\mu$SR measurements on pure YbNi$_4$(P$_{2}$ proved static long range magnetic order and suggested a strongly reduced ordered Yb moment of about 0.04$\mu_B$. With increasing As substitution the ordered moment is reduced by half at $x = 0.04$ and to less than 0.005 $\mu_B$ at $x=0.08$. The dynamic behavior in the $\mu$SR response show that magnetism remains homogeneous upon As substitution, without evidence for disorder effect. In the paramagnetic state across the FM QCP the dynamic muon-spin relaxation rate follows 1/$T_{1}T\propto T^{-n}$ with $1.01 \pm 0.04 \leq n \leq 1.13 \pm 0.06$. The critical fluctuations are very slow and are even becoming slower when approaching the QCP.Comment: 6 pages, 4 figure

Weak phase separation and the pseudogap in the electron-doped cuprates

We study the quantum transition from an antiferromagnet to a superconductor in a model for electron- and hole-doped cuprates by means of a variational cluster perturbation theory approach. In both cases, our results suggest a tendency towards phase separation between a mixed antiferromagnetic-superconducting phase at low doping and a pure superconducting phase at larger doping. However, in the electron-doped case the energy scale for phase separation is an order of magnitude smaller than for hole doping. We argue that this can explain the different pseudogap and superconducting transition scales in hole- and electron-doped materials.Comment: Final version, accepted for publication in Europhysics Letter

Pressure Induced Static Magnetic Order in Superconducting FeSe_1-x

We report on a detailed investigation of the electronic phase diagram of FeSe_1-x under pressures up to 1.4GPa by means of AC magnetization and muon-spin rotation. At a pressure \simeq0.8GPa the non-magnetic and superconducting FeSe_1-x enters a region where long range static magnetic order is realized above T_c and bulk superconductivity coexists and competes on short length scales with the magnetic order below T_c. For even higher pressures an enhancement of both the magnetic and the superconducting transition temperatures as well as of the corresponding order parameters is observed. These exceptional properties make FeSe1-x to be one of the most interesting superconducting systems investigated extensively at present.Comment: 5 pages, 3 figure

High temperature ferromagnetism of Li-doped vanadium oxide nanotubes

The nature of a puzzling high temperature ferromagnetism of doped mixed-valent vanadium oxide nanotubes reported earlier by Krusin-Elbaum et al., Nature 431 (2004) 672, has been addressed by static magnetization, muon spin relaxation, nuclear magnetic and electron spin resonance spectroscopy techniques. A precise control of the charge doping was achieved by electrochemical Li intercalation. We find that it provides excess electrons, thereby increasing the number of interacting magnetic vanadium sites, and, at a certain doping level, yields a ferromagnetic-like response persisting up to room temperature. Thus we confirm the surprising previous results on the samples prepared by a completely different intercalation method. Moreover our spectroscopic data provide first ample evidence for the bulk nature of the effect. In particular, they enable a conclusion that the Li nucleates superparamagnetic nanosize spin clusters around the intercalation site which are responsible for the unusual high temperature ferromagnetism of vanadium oxide nanotubes.Comment: with some amendments published in Europhysics Letters (EPL) 88 (2009) 57002; http://epljournal.edpsciences.or

Microscopic Evidence of Spin State Order and Spin State Phase Separation in Layered Cobaltites RBaCo2O5.5 with R=Y, Tb, Dy, and Ho

We report muon spin relaxation measurements on the magnetic structures of RBaCo_2O_5.5 with R=Y, Tb, Dy, and Ho. Three different phases, one ferrimagnetic and two antiferromagnetic, are identified below 300 K. They consist of different ordered spin state arrangements of high-, intermediate-, and low-spin Co^3+ of CoO_6 octahedra. Phase separation into well separated regions with different spin state order is observed in the antiferromagnetic phases. The unusual strongly anisotropic magnetoresistance and its onset at the FM-AFM phase boundary is explained.Comment: 4 pages, accepted for publication in Phys. Rev. Let