La(Fe,Si)13-based magnetic refrigerants obtained by novel processing routes

The structure and magnetic properties of LaFe_13−xSi_x and Co-substituted LaFe_11.8−xCo_xSi_1.2 alloys prepared by melt spinning, as well as of LaFe_11.57Si_1.43H_x hydrides prepared by reactive milling are investigated. The hysteresis in the temperature- and field-induced phase transitions is significantly reduced as compared with conventional bulk alloys, which makes these materials very attractive for magnetic refrigerant applications. The unusual combination of features characteristic of first- and second-order phase transitions in the La(Fe,Si)₁₃-based compounds is discussed on the basis of density-functional electronic structure calculations.     

Rotating metrics admitting non-perfect fluids

In this paper an application of Newman-Janis algorithm in spherically symmetric metrics with the functions M(u,r) and e(u,r) has been discussed. After the transformation of the metric via this algorithm, these two functions M(u,r) and e(u,r) will be transformed to depend on the three variables u,r,. With these functions of three variables, all the Newman–Penrose (NP) spin coefficients, the Ricci as well as the Weyl scalars have been calculated from the Cartans structure equations. Using these NP quantities, we first give examples of rotating solutions of Einsteins field equations like Kerr–Newman, rotating Vaidya solution and rotating Vaidya–Bonnor solution. It is found that the technique developed by Wang and Wu can be used to give further examples of embedded rotating solutions, that the rotating Kerr–Newman solution can be combined smoothly with the rotating Vaidya solution to derive the Kerr–Newman–Vaidya solution, and similarly, Kerr–Newman–Vaidya–Bonnor solution of the field equations. It has also shown that the embedded universes like Kerr–Newman de Sitter, rotating Vaidya–Bonnor–de Sitter, Kerr–Newman–Vaidya–de Sitter can be derived from the general solutions with Wang–Wu function. All rotating embedded solutions derived here can be written in Kerr–Schild forms, showing the extension of Xanthopouloss theorem. It is also found that all the rotating solutions admit non-perfect fluids.     

On a Poisson–Lie Analogue of the Classical Dynamical Yang–Baxter Equation for Self-dual Lie Algebras

We derive a generalization of the classical dynamical Yang–Baxter equation (CDYBE) on a self-dual Lie algebra G by replacing the cotangent bundle T*G in a geometric interpretation of this equation by its Poisson–Lie (PL) analogue associated with a factorizable constant r-matrix on G. The resulting PL-CDYBE, with variables in the Lie group G equipped with the Semenov-Tian-Shansky Poisson bracket based on the constant r-matrix, coincides with an equation that appeared in an earlier study of PL symmetries in the WZNW model. In addition to its new group theoretic interpretation, we present a self-contained analysis of those solutions of the PL-CDYBE that were found in the WZNW context and characterize them by means of a uniqueness result under a certain analyticity assumption.     

Intramolecular Diels–Alder reaction in enyne–allenes: a computational investigation and comparison with the Myers–Saito and Schmittel reactions

The reaction mechanisms as well as substituted effect and solvent effect of the enyne–allenes are investigated by Density Functional Theory (DFT) method and compared with the Myers–Saito and Schmittel reactions. The Myers–Saito reaction of non‐substituted enyne–allenes is kinetically and thermodynamically favored as compared to the Schmittel reaction; while the concerted [4 + 2] cycloaddition is only 1.32 kcal/mol higher than the C₂? C₇ cyclization and more exothermic (Δ_RE = ?69.38 kcal/mol). For R₁ = CH₃ and t‐Bu, the increasing barrier of the C₂? C₇ cyclization is higher than that for the C₂? C₆ cyclization because of the steric effect, so the increased barrier of the [4 + 2] cycloaddition is affected by such substituted electron‐releasing group. Moreover, the strong steric effect of R₁ = t‐Bu would shift the C₂? C₇ cyclization to the [4 + 2] cycloaddition. On the other hand, for R₁ = Ph, NH₂, O^?, NO₂, and CN substituents, the barrier of the C₂? C₆ cyclization would be more diminished than the C₂? C₇ cyclization due to strong mesomeric effect; the reaction path of C₂? C₇ cyclization would also shift to the [4 + 2] cycloaddition. The solvation does not lead to significant changes in the potential‐energy surface of the reaction except for the more polar surrounding solvent such as dimethyl sulfoxide (DMSO), or water. Copyright © 2009 John Wiley & Sons, Ltd.     

Virtues and Limitations of Markovian Master Equations with a Time-Dependent Generator

A Markovian master equation with time-dependent generator is constructed that respects basic constraints of quantum mechanics, in particular the von Neumann conditions. For the case of a two-level system, Bloch equations with time-dependent parameters are obtained. Necessary conditions on the latter are formulated. By employing a time-local counterpart of the Nakajima–Zwanzig equation, we establish a relation with unitary dynamics. We also discuss the relation with the weak-coupling limit. On the basis of a uniqueness theorem, a standard form for the generator of time-local master equations is proposed. The Jaynes–Cummings model with atomic damping is solved. The solution explicitly demonstrates that reduced dynamics can be described by time-local master equations only on a finite time interval. This limitation is caused by divergencies in the generator. A limit of maximum entropy is presented that corroborates the foregoing statements. A second limiting case demonstrates that divergencies may even occur for small perturbations of the weak-coupling regime.     

Lowering the Hartree–Fock Minimizer by Electron–Positron Pair Correlation

We prove by a simple computation that a suitable coupling to the positronic sector lowers the energy of the purely electronic minimizer of the electron–positron Hartree–Fock functional.     

Boundary G/G Theory and Topological Poisson–Lie Sigma Model

We study a boundary version of the gauged WZW model with a Poisson–Lie group G as the target. The Poisson–Lie structure of G is used to define the Wess–Zumino term of the action on surfaces with boundary. We clarify the relation of the model to the topological Poisson sigma model with the dual Poisson–Lie group G ^* as the target and show that the phase space of the theory on a strip is essentially the Heisenberg double of G introduced by Semenov–Tian–Shansky.     

HOW REYE'S CONFIGURATION HELPS IN PROVING THE BELL–KOCHEN–SPECKER THEOREM: A CURIOUS GEOMETRICAL TALE

It is shown that the 24 quantum states or rays used by Peres (J. Phys. A 24, 174-8 (1991)) to give a proof of the Bell–Kochen–Specker (BKS) theorem have a close connection with Reye's configuration, a system of twelve points and sixteen lines known to projective geometers for over a century. The interest of this observation stems from the fact that it provides a ready explanation for many of the regularities exhibited by the Peres rays and also permits a systematic construction of all possible non-coloring proofs of the BKS theorem based on these rays. An elementary exposition of the connection between the Peres rays and Reye's configuration is given, following which its applications to the BKS theorem are discussed.     

Spectroscopic evidence of co-insertion of carbon atoms into Li intercalated layered compounds

Co-insertion of solvent into layered compounds intercalated with lithium has been evidenced by Raman scattering measurements. The penetration of solvated species into the host is observed in samples prepared by an electrochemical technique as well as by direct chemical reaction in n-butyl lithium. The co-insertion is evidenced by the appearance of peaks in the spectral range 1200–1800 cm^–1 which are attributed to vibrations due to carbon atoms.     

My Life with Fisher

This is based on the after-dinner talk given at the 70th Birthday Conference for Michael Fisher at Rutgers in December, 2001. It is longer than the talk, incorporating additional text from the after-dinner talk I gave at Fisher's 60th Birthday Conference at the National Academy in Washington D.C. in 1991.     

Affine Defects and Gravitation

We argue that the structure general relativity (GR) as a theory of affine defects is deeper than the standard interpretation as a metric theory of gravitation. Einstein–Cartan theory (EC), with its inhomogeneous affine symmetry, should be the standard-bearer for GR-like theories. A discrete affine interpretation of EC (and gauge theory) yields topological definitions of momentum and spin (and Yang–Mills current), and their conservation laws become discrete topological identities. Considerations from quantum theory provide evidence that discrete affine defects are the physical foundation for gravitation.     

Solution of Duffin–Kemmer–Petiau Equation for the Step Potential

The Duffin–Kemmer–Petiau (DKP) equation for spin 0 and 1 in the pressence of the step potential is solved. The problem is reduced to the solution of an equation of Feshbach–Villars type.The reflection and transmission coefficients are correctly deduced. The Klein paradox persists and is discussed using the charge interpretation.     

Model of laser-induced chemical etching of silicon in chlorine atmosphere

A mathematical model for the chemical etching of silicon in a chlorine atmosphere induced by laser irradiation is described. The model takes into account: dissociation of molecules having absorbed radiant energy into chlorine atoms and their diffusion onto the substrate surface, generation of photocarriers in the silicon substrate, kinetics of chlorine atom chemisorption on the silicon surface, chemical reaction of chemisorbed chlorine atoms with silicon atoms, and desorption of reaction products. The obtained results are compared with experimental data.     

Simultaneous formation of contacts and diffusion barriers for VLSI by rapid thermal silicidation of TiW

The formation of (Ti_xW_1–x)Si₂/(Ti_xW_1–x)N, by rapid thermal processing of Ti_xW_1–x on Si in an N₂ ambient is investigated. An activation energy of 1.7 eV is obtained for silicide formation. A distinct snow-ploughing of As atoms is observed during silicide formation whereas the interfacial B concentration decreases with increasing silicide formation temperature. The diffusion barrier properties of the (Ti_xW_1–x)Si₂/(Ti_xWi_1–x)N stack in contact with Al is investigated upon post-metal annealing. No interaction between the layers is found for temperatures as high as 475°C after 60 min. The improved thermal stability of the (Ti_xW_1–x)N layer in contact with Al is attributed to nitrogen blocking of the grain boundaries.     

On the Specific Heat of Fermi–Pasta–Ulam Systems and Their Glassy Behavior

The specific heat of Fermi–Pasta–Ulam systems has until now been estimated through the energy fluctuations of a suitable subsystem, and opposite answers were apparently provided concerning its possible vanishing for vanishing temperatures. In the present paper a more realistic numerical implementation of the specific heat measurement is discussed, which mimics the interaction of the FPU system with a calorimeter. It is found that there exists a freezing critical temperature below which the relaxation times to equilibrium between FPU system and calorimeter become relevant, so that the system presents aging and hysteresis features very similar to those familiar in glasses and spin glasses. In particular, in the framework of such a point of view involving finite long times, the specific heat appears to vanish for vanishing temperatures.     

Self-organization of cavities under irradiation

In the present paper two examples of self-organization in solids under irradiation are considered on the basis of original mechanisms, namely, the ordering of voids in void lattices under high temperature irradiation and the alignment of gas bubbles in bubble lattices under low-temperature gas atom implantation. The ordering of cavities (i.e. voids or gas bubbles) is proposed to arise due to a dissipative interaction between cavities induced by the interstitial dislocation loop absorption and punching, respectively, which represent anisotropic mechanisms of atomic transport.     

Symmetrizer and Antisymmetrizer of the Birman–Wenzl–Murakami Algebras

Explicit formulas for the symmetrizer and the antisymmetrizer of the Birman–Wenzl–Murakami algebras BWM(r,q) _n are given.     

Intensity-induced nonlinear effects in UV window materials

Intensity-induced nonlinear effects in optical window materials have been investigated at 308 nm. The absolute two-photon absorption coefficients for fused silica, CaF₂, BaF₂, Al₂O₃ and ADP crystals have been measured by using a single 120 ps, transform-limited pulse from the second harmonic of a distributed feedback (DFB) dye laser. The nonlinear refractive index coefficient has been obtained from measurements of far-field intensity distributions.