Carbon Concentration Dependence of the Superconducting Transition Temperature and Structure of MgCxNi3

The crystal structure of the superconductor MgCxNi3 is reported as a function of carbon concentration determined by powder neutron diffraction. The single-phase perovskite structure was found in only a narrow range of carbon content, 0.88 < x < 1.0. The superconducting transition temperature was found to decrease systematically with decreasing carbon concentration. The introduction of carbon vacancies has a significant effect on the positions of the Ni atoms. No evidence for long range magnetic ordering was seen by neutron diffraction for carbon stoichiometries within the perovskite phase stability range.Comment: 4 figure

Thermodynamic Comparison and the Ideal Glass Transition of A Monatomic Systems Modeled as an Antiferromagnetic Ising Model on Husimi and Cubic Recursive Lattices of the Same Coordination Number

Two kinds of recursive lattices with the same coordination number but different unit cells (2-D square and 3-D cube) are constructed and the antiferromagnetic Ising model is solved exactly on them to study the stable and metastable states. The Ising model with multi-particle interactions is designed to represent a monatomic system or an alloy. Two solutions of the model exhibit the crystallization of liquid, and the ideal glass transition of supercooled liquid respectively. Based on the solutions, the thermodynamics on both lattices was examined. In particular, the free energy, energy, and entropy of the ideal glass, supercooled liquid, crystal, and liquid state of the model on each lattice were calculated and compared with each other. Interactions between particles farther away than the nearest neighbor distance are taken into consideration. The two lattices show comparable properties on the transition temperatures and the thermodynamic behaviors, which proves that both of them are practical to describe the regular 3-D case, while the different effects of the unit types are still obvious.Comment: 27 pages, 13 figure

Structural and Magnetic Properties of Pyrochlore Solid Solutions (Y,Lu)2Ti2-x(Nb,Ta)xO7+/-y

The synthesis and characterization of the pyrochlore solid solutions, Y2Ti2-xNbxO7-y, Lu2Ti2-xNbxO7-y, Y2Ti2-xTaxO7-y and Lu2TiTaO7-y (-0.4<y<0.5), is described. Synthesis at 1600 C, and 10-5 Torr yields oxygen deficiency in all systems. All compounds are found to be paramagnetic and semiconducting, with the size of the local moments being less, in some cases substantially less, than the expected value for the number of nominally unpaired electrons present. Thermogravimetric analysis (TGA) shows that all compounds can be fully oxidized while retaining the pyrochlore structure, yielding oxygen rich pyrochlores as white powders. Powder neutron diffraction of Y2TiNbO7-based samples was done. Refinement of the data for oxygen deficient Y2TiNbO6.76 indicates the presence of a distribution of oxygen over the 8b and 48f sites. Refinement of the data for oxygen rich Y2TiNbO7.5 shows these sites to be completely filled, with an additional half filling of the 8a site. The magnetic and TGA data strongly suggest a preference for a Ti3+/(Nb,Ta)5+ combination, as opposed to Ti4+/(Nb,Ta)4+, in this pyrochlore family. In addition, the evidence clearly points to Ti3+ as the source of the localized moments, with no evidence for localized Nb4+ moments.Comment: Accepted to Journal of Solid State Chemistr

Pedestrian Solution of the Two-Dimensional Ising Model

The partition function of the two-dimensional Ising model with zero magnetic field on a square lattice with m x n sites wrapped on a torus is computed within the transfer matrix formalism in an explicit step-by-step approach inspired by Kaufman's work. However, working with two commuting representations of the complex rotation group SO(2n,C) helps us avoid a number of unnecessary complications. We find all eigenvalues of the transfer matrix and therefore the partition function in a straightforward way.Comment: 10 pages, 2 figures; eqs. (101) and (102) corrected, files for fig. 2 fixed, minor beautification

Interaction of ionic liquids with noble metal surfaces: Structure formation and stability of [OMIM][TFSA] and [EMIM][TFSA] on Au(111) and Ag(111)

Principles of structure formation and adsorbate–adsorbate interactions in ionic liquid adlayers on metal surfaces were investigated in a comparative STM study on Ag(111) and Au(111) surfaces.</p

ρ\rho Polarization and `Model Independent' Extraction of ∣Vub∣/∣Vcd∣|V_{ub}|/|V_{cd}| from D→ρℓνD\to\rho\ell\nu and B→ρℓνB\to\rho\ell\nu

We briefly discuss the predictions of the heavy quark effective theory for the semileptonic decays of a heavy pseudoscalar to a light one, or to a light vector meson. We point out that measurement of combinations of differential helicity decay rates at Cleo-c and the $B$ factories can provide a model independent means of extracting the ratio $|V_{ub}|/|V_{cd}|$. We briefly discuss the corrections to this prediction.Comment: 8 pages, LaTeX, 1 figur

Structural Disorder, Octahedral Coordination, and 2-Dimensional Ferromagnetism in Anhydrous Alums

The crystal structures of the triangular lattice, layered anhydrous alums KCr(SO4)2, RbCr(SO4)2 and KAl(SO4)2 are characterized by X-ray and neutron powder diffraction at temperatures between 1.4 and 773 K. The compounds all crystallize in the space group P-3, with octahedral coordination of the trivalent cations. In all cases, small amounts of disorder in the stacking of the triangular layers of corner sharing MO6 octahedra and SO4 tetrahedra is seen, with the MO6-SO4 network rotated in opposite directions between layers. The electron diffraction study of KCr(SO4)2 supports this model, which on average can be taken to imply trigonal prismatic coordination for the M3+ ions; as was previously reported for the prototype anhydrous alum KAl(SO4)2. The temperature dependent magnetic susceptibilities for ACr(SO4)2 (A = K,Rb,Cs) indicate the presence of predominantly ferromagnetic interactions. Low temperature powder neutron diffraction reveals that the magnetic ordering is ferromagnetic in-plane, with antiferromagnetic ordering between planes below 3 K.Comment: Accepted to the Journal of Solid State Chemistr

Modeling and simulation of bulk gallium nitride power semiconductor devices

Bulk gallium nitride (GaN) power semiconductor devices are gaining significant interest in recent years, creating the need for technology computer aided design (TCAD) simulation to accurately model and optimize these devices. This paper comprehensively reviews and compares different GaN physical models and model parameters in the literature, and discusses the appropriate selection of these models and parameters for TCAD simulation. 2-D drift-diffusion semi-classical simulation is carried out for 2.6 kV and 3.7 kV bulk GaN vertical PN diodes. The simulated forward current-voltage and reverse breakdown characteristics are in good agreement with the measurement data even over a wide temperature range

OmOm Diagnostic for Dilaton Dark Energy

$Om$ diagnostic can differentiate between different models of dark energy without the accurate current value of matter density. We apply this geometric diagnostic to dilaton dark energy(DDE) model and differentiate DDE model from LCDM. We also investigate the influence of coupled parameter $\alpha$ on the evolutive behavior of $Om$ with respect to redshift $z$. According to the numerical result of $Om$, we get the current value of equation of state $\omega_{\sigma0}$=-0.952 which fits the WMAP5+BAO+SN very well.Comment: 6 pages and 6 figures

Thermal Recombination: Beyond the Valence Quark Approximation

Quark counting rules derived from recombination models agree well with data on hadron production at intermediate transverse momenta in relativistic heavy-ion collisions. They convey a simple picture of hadrons consisting only of valence quarks. We discuss the inclusion of higher Fock states that add sea quarks and gluons to the hadron structure. We show that, when recombination occurs from a thermal medium, hadron spectra remain unaffected by the inclusion of higher Fock states. However, the quark number scaling for elliptic flow is somewhat affected. We discuss the implications for our understanding of data from the Relativistic Heavy Ion Collider.Comment: 5 pages, 5 figure