Comparative study of ordered and disordered Y1-xSrxCoO3-d

We have succeeded in preparing A-site ordered- and disordered-Y1/4Sr3/4CoO3-d with various oxygen deficiencies delta, and have made comparative study of their structural and physical properties. In the A-site ordered structure, oxygen vacancies order, and d = 0.34 sample shows a weak ferromagnetic transition beyond 300 K. On the other hand, in the A-site disordered structure, no oxygen vacancy ordering is observed, and d = 0.16 sample shows a ferromagnetic metallic transition around 150 K. A-site disordering destroys the orderings of oxygen-vacancies and orbitals, leading to the strong modification of the electronic phases.Comment: 3 pages, 3 figures, proceeding of 52nd Mangetism and Magnetic Materials Conference (MMM 2007), published in Journal of Applied Physic

Refinement Type Inference via Horn Constraint Optimization

We propose a novel method for inferring refinement types of higher-order functional programs. The main advantage of the proposed method is that it can infer maximally preferred (i.e., Pareto optimal) refinement types with respect to a user-specified preference order. The flexible optimization of refinement types enabled by the proposed method paves the way for interesting applications, such as inferring most-general characterization of inputs for which a given program satisfies (or violates) a given safety (or termination) property. Our method reduces such a type optimization problem to a Horn constraint optimization problem by using a new refinement type system that can flexibly reason about non-determinism in programs. Our method then solves the constraint optimization problem by repeatedly improving a current solution until convergence via template-based invariant generation. We have implemented a prototype inference system based on our method, and obtained promising results in preliminary experiments.Comment: 19 page

Multiferroic properties of an \aa kermanite Sr2_2CoSi2_2O7_7 single crystal in high magnetic fields

The magnetic and dielectric properties of \aa kermanite Sr$_2$CoSi$_2$O$_7$ single crystals in high magnetic fields were investigated. We have observed finite induced electric polarization along the c axis in high fields, wherein all Co spins were forcibly aligned to the magnetic field direction. Existence of the induced polarization in the spin-polarized state accompanied with the finite slope in the magnetization curve suggests the possible role of the orbital angular momenta in the excited states as its microscopic origin. The emergence of the field-induced polarization without particular magnetic order can be regarded as the magnetoelectric effects of the second order from the symmetry point of view. A low magnetic field-driven electric polarization flip induced by a rotating field, even at room temperature, has been successfully demonstrated.Comment: 12 pages, 4 figure

Neutrophil Oxygen Radical Production by Dialysis Membranes

The ability of different dialysis membranes to activate polymorphonuclear neutrophil oxygen radical production was investigated with chemiluminescence. All the six membranes, namely cuprophan, cellulose acetate, polycarbonate, polysulphone, polyacrilonitrile and polymethylmethacrylate were able to interact with neutrophils and stimulate their oxygen radical production, the highest responses being seen with polyacrilonitrile, polymethylmethacrylate and polycarbonate. To analyse the role of complement in this interaction, fresh plasma, heat inactivated and zymosan-activated plasma were added: with fresh plasma oxygen radical production was stimulated on cuprophan, cellulose acetate and polysulphone, not modified on polycarbonate, and decreased on polyacrilonitrile and polymethylmethacrylate. With heat-inactivated plasma, the responses were decreased or abrogated on all the membranes except polycarbonate and polymethylmethacrylate, whereas with zymosanactivated plasma similar responses to fresh plasma were observed. In addition, when plasma was used to precoat the membrane, cuprophan, cellulose acetate and polysulphone disclosed an enhanced neutrophil oxidative burst, while precoated polyacrilonitrile and polymethylmethacrylate were less stimulatory than uncoated membranes. In contrast the precoating of polycarbonate did not modify oxygen radical production. These data suggest that neutrophil activation occurs by direct membrane neutrophil interaction. Plasmatic factors modulate this interaction but complement seems involved on cellulosic and polysulphone membranes only. Therefore, it appears that oxygen radicals produced from contact of neutrophils with the dialysis membrane might play an initial and/or additional role in the events occurring at the initiation of haemodialysi

Magnetic-field-induced switching between ferroelectric phases in orthorhombic-distortion-controlled RRMnO3_{3}

We have investigated the dielectric and magnetic properties of Eu$_{0.595}$Y$_{0.405}$MnO$_{3}$ $without$ the presence of the 4$f$ magnetic moments of the rare earth ions, and have found two ferroelectric phases with polarization along the $a$ and $c$ axes in a zero magnetic field. A magnetic field induced switching from one to the other ferroelectric phase took plase in which the direction of ferroelectric polarization changed from the a axis to the c axis by the application of magnetic fields parallel to the a axis. In contrast to the case of TbMnO$_{3}$, in which the 4$f$ moments of Tb$^{3+}$ ions play an important role in such a ferroelectric phase switching, the magnetic-field-induced switching between ferroelectric phases in Eu$_{0.595}$Y$_{0.405}$MnO$_{3}$ does not originate from the magnetic transition of the rare-earth 4$f$ moments, but from that of the Mn 3$d$ spins.Comment: 8 pages, 3 figures, RevTeX4, Proceedings of MMM 2005, to appear in J. Appl. Phy

Scanning tunneling microscopy and spectroscopy of the electronic local density of states of graphite surfaces near monoatomic step edges

We measured the electronic local density of states (LDOS) of graphite surfaces near monoatomic step edges, which consist of either the zigzag or armchair edge, with the scanning tunneling microscopy (STM) and spectroscopy (STS) techniques. The STM data reveal that the $(\sqrt{3} \times \sqrt{3}) R 30^{\circ}$ and honeycomb superstructures coexist over a length scale of 3-4 nm from both the edges. By comparing with density-functional derived nonorthogonal tight-binding calculations, we show that the coexistence is due to a slight admixing of the two types of edges at the graphite surfaces. In the STS measurements, a clear peak in the LDOS at negative bias voltages from -100 to -20 mV was observed near the zigzag edges, while such a peak was not observed near the armchair edges. We concluded that this peak corresponds to the graphite "edge state" theoretically predicted by Fujita \textit{et al.} [J. Phys. Soc. Jpn. {\bf 65}, 1920 (1996)] with a tight-binding model for graphene ribbons. The existence of the edge state only at the zigzag type edge was also confirmed by our first-principles calculations with different edge terminations.Comment: 20 pages, 11 figure

Transport and structural study of pressure-induced magnetic states in Nd0.55Sr0.45MnO3 and Nd0.5Sr0.5MnO3

Pressure effects on the electron transport and structure of Nd1-xSrxMnO3 (x = 0.45, 0.5) were investigated in the range from ambient to ~6 GPa. In Nd0.55Sr0.45MnO3, the low-temperature ferromagnetic metallic state is suppressed and a low temperature insulating state is induced by pressure. In Nd0.5Sr0.5MnO3, the CE-type antiferromagnetic charge-ordering state is suppressed by pressure. Under pressure, both samples have a similar electron transport behavior although their ambient ground states are much different. It is surmised that pressure induces an A-type antiferromagnetic state at low temperature in both compounds

Electronic structure theory of the hidden order material URu2_2Si2_2

We report a comprehensive electronic structure investigation of the paramagnetic (PM), the large moment antiferromagnetic (LMAF), and the hidden order (HO) phases of URu$_2$Si$_2$. We have performed relativistic full-potential calculations on the basis of the density functional theory (DFT), employing different exchange-correlation functionals to treat electron correlations within the open $5f$-shell of uranium. Specifically, we investigate---through a comparison between calculated and low-temperature experimental properties---whether the $5f$ electrons are localized or delocalized in URu$_2$Si$_2$. We also performed dynamical mean field theory calculations (LDA+DMFT) to investigate the temperature evolution of the quasi-particle states at 100~K and above, unveiling a progressive opening of a quasi-particle gap at the chemical potential when temperature is reduced. A detailed comparison of calculated properties with known experimental data demonstrates that the LSDA and GGA approaches, in which the uranium $5f$ electrons are treated as itinerant, provide an excellent explanation of the available low-temperature experimental data of the PM and LMAF phases. We show furthermore that due to a materials-specific Fermi surface instability a large, but partial, Fermi surface gapping of up to 750 K occurs upon antiferromagnetic symmetry breaking. The occurrence of the HO phase is explained through dynamical symmetry breaking induced by a mode of long-lived antiferromagnetic spin-fluctuations. This dynamical symmetry breaking model explains why the Fermi surface gapping in the HO phase is similar but smaller than that in the LMAF phase and it also explains why the HO and LMAF phases have the same Fermi surfaces yet different order parameters. Suitable derived order parameters for the HO are proposed to be the Fermi surface gap or the dynamic spin-spin correlation function.Comment: 23 pages, 20 figure