Magnetic and transport properties of transition-metal implanted ZnO single crystals

ZnO single crystals were implanted with Mn, Co and Ni with fluences between 1 × 10¹⁶ cm^-2 and 1 × 10¹⁷ cm^-2 and energy of 200 keV. Results indicate that aggregation of transition metal ions in the as implanted state occurs only in the case of Ni. After an annealing stage to recover the ZnO structure aggregation occurs for the higher fluences of all implanted species. For lower concentrations paramagnetic behaviour with magnetic moments close to those of individual ions is observed. No polarised impurity band is formed as a result of the presence of transition metal ions and all samples show electrical conduction by carriers in extended states of ZnO. Significant values of magnetoresistance are measured at low temperatures, where electrical transport is described by hopping mechanisms between localized states. The sign of the magnetoresistance is dependent of the doping ion and is correlated with the observed aggregation.     

Anisotropic hydroxypropylcellulose films as alignment layers of a bistable ferroelectric device

Hydroxypropylcellulose films are used as the alignment substrate in a liquid crystal bistable electro-optical device. These alignment films were characterized by atomic force microscopy. The electro-optical behaviour of this device, which operates in the Clark-Lagerwall mode, is identical to that observed for typical SSFLC cells. In particular the bistability of the cell was clearly observed, the threshold voltage was measured and an estimation of the anchoring energy of the LC mixture used, in this type of system, was made.     

Investigation of the iron site localization in doped ZnO

Iron doped (5%) ZnO powders were prepared by hydrothermal and combustion methods. Subsequent heating treatments on these samples were performed under air and also under a reductive atmosphere. Powder XRD, Mössbauer spectroscopy and magnetization studies were used to characterize the products and investigate the localization of iron ions in the ZnO lattice. It was shown that a mixture with a ZnFe₂O₄ type phase was obtained when the sample was prepared by the hydrothermal method, while Fe³⁺ occupy tetrahedral sites of the ZnO lattice in the case of the sample obtained by the combustion method. The magnetic measurements show that no ferromagnetism was detected in the iron doped ZnO compound, neither in the as prepared state nor after reduction.     

Semifree symplectic circle actions on -orbifolds

A theorem of Tolman and Weitsman states that all symplectic semifree circle actions with isolated fixed points on compact symplectic manifolds must be Hamiltonian and have the same equivariant cohomology and Chern classes of equipped with the standard diagonal circle action. In this paper, we show that the situation is much different when we consider compact symplectic orbifolds. Focusing on -orbifolds with isolated cone singularities, we show that such actions, besides being Hamiltonian, can now be obtained from either or a weighted projective space, or a quotient of one of these spaces by a finite cyclic group, by a sequence of special weighted blow-ups at fixed points. In particular, they can have any number of fixed points.