Canonical matrices of bilinear and sesquilinear forms

Canonical matrices are given for (a) bilinear forms over an algebraically closed or real closed field; (b) sesquilinear forms over an algebraically closed field and over real quaternions with any nonidentity involution; and (c) sesquilinear forms over a field F of characteristic different from 2 with involution (possibly, the identity) up to classification of Hermitian forms over finite extensions of F. A method for reducing the problem of classifying systems of forms and linear mappings to the problem of classifying systems of linear mappings is used to construct the canonical matrices. This method has its origins in representation theory and was devised in [V.V. Sergeichuk, Math. USSR-Izv. 31 (1988) 481-501].Comment: 44 pages; misprints corrected; accepted for publication in Linear Algebra and its Applications (2007

Memory Effect and Triplet Pairing Generation in the Superconducting Exchange Biased Co/CoOx/Cu41Ni59/Nb/Cu41Ni59 Layered Heterostructure

We fabricated a nanolayered hybrid superconductor-ferromagnet spin-valve structure, the resistive state of which depends on the preceding magnetic field polarity. The effect is based on a strong exchange bias (about -2 kOe) on a diluted ferromagnetic copper-nickel alloy and generation of a long range odd in frequency triplet pairing component. The difference of high and low resistance states at zero magnetic field is 90% of the normal state resistance for a transport current of 250 {\mu}A and still around 42% for 10 {\mu}A. Both logic states of the structure do not require biasing fields or currents in the idle mode.Comment: 9 pages, 4 figures, Accepted to Applied Physics Letter

Reentrant Superconductivity and Superconducting Critical Temperature Oscillations in F/S/F trilayers of Cu41Ni59/Nb/Cu41Ni59 Grown on Cobalt Oxide

Ferromagnet/Superconductor/Ferromagnet (F/S/F) trilayers constitute the core of a superconducting spin valve. The switching effect of the spin valve is based on interference phenomena occurring due to the proximity effect at the S/F interfaces. A remarkable effect is only expected if the core structure exhibits strong critical temperature oscillations, or most favorable, reentrant superconductivity, when the thickness of the ferromagnetic layer is increased. The core structure has to be grown on an antiferromagnetic oxide layer (or such layer to be placed on top) to pin by exchange bias the magnetization-orientation of one of the ferromagnetic layers. In the present paper we demonstrate that this is possible, keeping the superconducting behavior of the core structure undisturbed.Comment: 22 pages, 12 figures, 1 tabl

Chain Formation by Spin Pentamers in eta-Na9V14O35

The nature of the gapped ground state in the quasi-one-dimensional compound eta-Na9V14O35 cannot easily be understood, if one takes into account the odd number of spins on each structural element. Combining the results of specific heat, susceptibility and electron spin resonance measurements we show that eta-Na9V14O35 exhibits a novel ground state where multi-spin objects build up a linear chain. These objects - pentamers - consist of five antiferromagnetically arranged spins with effective spin 1/2. Their spatial extent results in an exchange constant along the chain direction comparable to the one in the high-temperature state.Comment: 6 pages, 5 figure

A new criteria for zero quantum discord

We propose a new criterion to judge zero quantum discord for arbitrary bipartite states. A bipartite quantum state has zero quantum discord if and only if all blocks of its density matrix are normal matrices and commute with each other. Given a bipartite state with zero quantum discord, how to find out the set of local projectors, which do not disturb the whole state after being imposed on one subsystem, is also presented. A class of two-qubit X-state is used to test the criterion, and an experimental scheme is proposed to realize it. Consequently, we prove that the positive operator-valued measurement can not extinguish the quantum correlation of a bipartite state with nonzero quantum discord.Comment: 10 pages, 1 figur

Selecting a single orientation for millimeter sized graphene sheets

We have used Low Energy Electron Microscopy (LEEM) and Photo Emission Electron Microscopy (PEEM) to study and improve the quality of graphene films grown on Ir(111) using chemical vapor deposition (CVD). CVD at elevated temperature already yields graphene sheets that are uniform and of monatomic thickness. Besides domains that are aligned with respect to the substrate, other rotational variants grow. Cyclic growth exploiting the faster growth and etch rates of the rotational variants, yields films that are 99 % composed of aligned domains. Precovering the substrate with a high density of graphene nuclei prior to CVD yields pure films of aligned domains extending over millimeters. Such films can be used to prepare cluster-graphene hybrid materials for catalysis or nanomagnetism and can potentially be combined with lift-off techniques to yield high-quality, graphene based electronic devices

Surface Morphology and Strain Relief in Surfactant Mediated Growth of Germanium on Silicon (111)

The growth of Ge on Si is strongly modified by adsorbates called surfactants. The relevance of the stress on surface morphology and the growth mode of Ge on Si(111) is presented in a detailed in situ study by high resolution low energy electron diffraction (LEED) during the deposition. The change from islanding to layer-by-layer growth mode is seen in the oscillatory intensity behaviour of the 00-spot. As a strain relief mechanism, the Ge-film forms a microscopic rough surface of small triangular and defect-free pyramids in the pseudomorphic growth regime up to 8 monolayers. As soon as the pyramids are completed and start to coalesce, strain relieving defects are created at their base, finally arranging to the dislocation network. Without the driving force for the micro-roughness, the stress, the surface flattens again showing a much larger terrace length. The formation process of the dislocation network results in a spot splitting in LEED, since the periodic dislocations at the interface give rise to elastic deformation of the surface. Surprisingly the Ge-film is relaxed to 70% immediately after 8 monolayers of coverage, which is attributed to the micro rough surface morphology, providing innumerous nucleation sites for dislocation