Temperature-dependent x-ray diffraction of the low-dimensional spin 1/2 quantum magnet TiOCl shows that the phase transition at T_{c2} = 90 K corresponds to a lowering of the lattice symmetry. Below T_{c1} = 66 K a twofold superstructure develops, that indicates the formation of spin-singlet pairs via direct exchange between neighboring Ti atoms, while the role of superexchange is found to be negligible. TiOCl thus is identified as a spin-Peierls system of pure 1D chains of atoms. The first-order character of the transition at T_{c1} is explained by the competition between the structurally deformed state below T_{c2} and the spin-Peierls state below T_{c1}.Comment: Phys. Rev. B (Rapid Communications) in pres

Molybdenum dioxide crystallizes in a monoclinic structure which deviates only slightly from the rutile structure and is characteristic of several early transition metal dioxides. We present results of all-electron electronic structure calculations based on density functional theory within the local density approximation and using the augmented spherical wave method. The electronic properties of MoO2 are dominated by strong hybridization of O 2p and crystal-field-split Mo 4d states with bands near the Fermi energy originating almost exclusively from Mo 4d t2g orbitals. In additional calculations for a hypothetical high-symmetry rutile structure these bands separate into quasi-one-dimensional d∥ states pointing along the rutile c-axis and the rather isotropically dispersing π* bands. On going to the monoclinic structure, the characteristic metal-metal dimerization causes strong splitting of the d∥ bands into bonding and antibonding branches which embrace the nearly inert π* bands at EF. As a consequence, large portions of the Fermi surface are removed. According to our calculations the monoclinic structure of MoO2 thus results from a Peierls-type instability of the d∥ bands in the presence of, but still rather unaffected by, an embedding background of π* states. Our work has strong implications for the current understanding of VO2 and the striking metal-insulator/structural transition displayed by this material.

A new approach is introduced to identify natural clusters of acoustic emission signals. The presented technique is based on an exhaustive screening taking into account all combinations of signal features extracted from the recorded acoustic emission signals. For each possible combination of signal features an investigation of the classification performance of the k-means algorithm is evaluated ranging from two to ten classes. The numerical degree of cluster separation of each partition is calculated utilizing the Davies-Bouldin and Tou indices, Rousseeuw's silhouette validation method and Hubert's Gamma statistics. The individual rating of each cluster validation technique is cumulated based on a voting scheme and is evaluated for the number of clusters with best performance. This is defined as the best partitioning for the given signal feature combination. As a second step the numerical ranking of all these partitions is evaluated for the globally optimal partition in a second voting scheme using the cluster validation methods results. This methodology can be used as an automated evaluation of the number of natural clusters and their partitions without previous knowledge about the cluster structure of acoustic emission signals. The suitability of the current approach was evaluated using artificial datasets with defined degree of separation. In addition the application of the approach to clustering of acoustic emission signals is demonstrated for signals obtained from failure during loading of carbon fiber reinforced plastic specimens.

We studied the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) like state established due to the proximity effect in superconducting Nb/Cu 41 Ni 59 bilayers. Using a special wedge-type deposition technique, series of 20-35 samples could be fabricated by magnetron sputtering during one run. The layer thickness of only a few nanometers, the composition of the alloy, and the quality of interfaces were controlled by Rutherford backscattering spectrometry, high resolution transmission electron microscopy, and Auger spectroscopy. The magnetic properties of the ferromagnetic alloy layer were characterized with superconducting quantum interference device (SQUID) magnetometry. These studies yield precise information about the thickness, and demonstrate the homogeneity of the alloy composition and magnetic properties along the sample series. The dependencies of the critical temperature on the Nb and Cu 41 Ni 59 layer thickness, T c (d S ) and T c (d F ), were investigated for constant thickness d F of the magnetic alloy layer and d S of the superconducting layer, respectively. All types of non-monotonic behaviors of T c versus d F predicted by the theory could be realized experimentally: from reentrant superconducting behavior with a broad extinction region to a slight suppression of superconductivity with a shallow minimum. Even a double extinction of superconductivity was observed, giving evidence for the multiple reentrant behavior predicted by theory. All critical temperature curves were fitted with suitable sets of parameters. Then, T c (d F ) diagrams of a hypothetical F/S/F spin-switch core structure were calculated using these parameters. Finally, superconducting spin-switch fabrication issues are discussed in detail in view of the achieved results.

We report on the first observation of a pronounced re-entrant superconductivity phenomenon in superconductor/ferromagnetic layered systems. The results were obtained using a superconductor/ferromagnetic-alloy bilayer of Nb/Cu1−xNix. The superconducting transition temperature Tc drops sharply with increasing thickness dCuNi of the ferromagnetic layer, until complete suppression of superconductivity is observed at dCuNi ≈4 nm. Increasing the Cu1−xNix layer thickness further, superconductivity reappears at dCuNi≈13 nm. Our experiments give evidence for the pairing function oscillations associated with a realization of the quasi-one dimensional Fulde-FerrellLarkin-Ovchinnikov (FFLO) like state in the ferromagnetic layer.The coexistence of superconductivity (S) and ferromagnetism (F) in a homogeneous material, described by Fulde-Ferrell and Larkin-Ovchinnikov (FFLO) [1,2], is restricted to an extremely narrow range of parameters [3]. So far no indisputable experimental evidence for the FFLO state exists.In general, superconductivity and ferromagnetism do not coexist, since superconductivity requires the conduction electrons to form Cooper pairs with antiparallel spins, whereas ferromagnetism forces the electrons to align their spins parallel. This antagonism can be overcome if superconducting and ferromagnetic regions are spatially separated, as for example, in artificially layered superconductor/ferromagnet (S/F) nanostructures (see, e.g. [4], for an early review). The two long-range ordered states influence each other via the penetration of electrons through their common interface. Superconductivity in such a proximity system can survive, even if the exchange splitting energy E ex ∼ k B θ Curie in the ferromagnetic layer is orders of magnitude larger than the superconducting order parameter ∆ ∼ k B T c , with T c the superconducting transition temperature. Cooper pairs entering from the superconducting into the ferromagnetic region experience conditions drastically different from those in a non-magnetic metal. This is due to the fact that spin-up and spin-down partners in a Cooper pair occupy different exchange-split spin-subbands of the conduction band in the ferromagnet. Thus, the spin-up and spin-down wave-vectors of electrons in a pair, which have opposite directions, cannot longer be of equal magnitude and the Cooper pair acquires a finite pairing momentum [5]. This results in a pairing function that does not simply decay as in a non-magnetic metal, but in addition oscillates on a characteristic length scale. This length scale is the magnetic coherence length ξ F , which will be specified below.Various unusual phenomena follow from the oscillation of the pairing wave function in ferromagnets (see, e.g. the recent reviews [6,7,8] and references therein). A prominent example is the oscillatory S/F proximity effect. It can be qualitatively described using the analogy with the interference of reflected light in a Fabry-Pérot interferometer at normal incidence. As the conditions change periodically between construc...