Negative permeability in a <Emphasis Type="Bold">Λ</Emphasis>-type three-level atomic vapor

A new approach is suggested to realize negative magnetic permeability that follows directly from quantum mechanics. It is shown that a Λ-type three-level atomic system with proper atomic parameters can give rise to striking magnetic responses, which could exhibit negative permeability in an optical frequency band. Both steady and transient behaviors of the magnetic permeability in the atomic vapor are studied. The present negative-permeability vapor could be mixed with a quantum coherent vapor whose electric permittivity is negative. Such a mixed vapor may give an isotropic left-handed vapor medium at the atomic-scale level. PACS 32.10.Dk; 42.50.Gy; 78.20.Ci     

Does confined water exhibit a fragile-to-strong transition?

The dynamics of supercooled confined water has recently been shown to have a pronounced, apparent fragile-to-strong transition (FST). Here we use broadband dielectric spectroscopy (10^-2–10⁹ Hz) to study the dynamics of water confined in silica matrices MCM-41 C10 and C18, with pore diameter of 21.4 and 36.1 ?, respectively. The local dynamics of water molecules and the dynamics of the hydroxyl groups on the inner wall of the pores are followed up to over 240 K. We argue that the reported FST for confined water is due to the vanishing of the cooperative α relaxation, which implies that it should not be interpreted as a true FST.     

Ab initio description of the first stages of laser-induced ultra-fast nonthermal melting of InSb

Using first principles, all-electron calculations and dynamical simulations we study the behavior of solid InSb immediately after intense femtosecond excitation. First, we determine the laser-excited potential energy surfaces with high accuracy for different electronic temperatures (corresponding to different laser fluences). Then, we demonstrate that, although most phonon modes become only slightly softened even for high electron temperatures, the transverse acoustic modes at the boundary of the Brillouin zone undergo dramatic changes and become unstable. This is the origin of nonthermal melting. Based on these results, the dynamics during the first hundreds of femtoseconds after laser excitation can be unambiguously elucidated. Our results are in agreement with recent experiments and support the predictions made by Stampfli and Bennemann for silicon.     

57 W, 27 fs pulses from a fiber laser system using nonlinear compression

We report on the generation of 27 fs pulses with an average output power of 57 W and a repetition rate of 78 MHz. The pulses are generated by combining a high average power fiber chirped pulse amplification (FCPA) system with a microstructured large-mode-area fiber for nonlinear compression. The FCPA system delivers 270 fs pulses in a linearly polarized beam with diffraction-limited quality. Nonlinear compression is achieved by launching the pulses into a short (few cm) piece of microstructured fiber and subsequent compression by a pair of chirped mirrors. PACS  42.55.Wd; 42.55.Xi, 42.65.Re     

Evidence for an isomeric 3/2- state in 53Co

The fragmentation of a 550MeV/u primary beam of ⁵⁸Ni on a ⁹Be target has been used to measure time- and energy-correlated γ decays following the implantation of event-by-event discriminated secondary fragments into a ⁹Be stopper plate. A new isomeric γ decay with T _1/2 = 14( ) ns and E _γ = 646.2(2) keV is observed and attributed to the decay of the yrast 3/2^- state in ⁵³ ₂₇Co₂₆ . This short-lived isomeric state has been populated by means of nuclear reactions during the stopping process of the secondary fragments. The experimental findings are discussed in the framework of large-scale spherical shell model calculations in conjunction with isospin symmetry-breaking residual interactions for the A = 53 , T _z = ±1/2 mirror nuclei ⁵³Co and ⁵³Fe .     

(Co)cyclic (Co)homology of Bialgebroids: An Approach via (Co)monads

For a (co)monad T _l on a category , an object X in , and a functor , there is a (co)simplex in . The aim of this paper is to find criteria for para-(co)cyclicity of Z ^*. Our construction is built on a distributive law of T _l with a second (co)monad T _r on , a natural transformation , and a morphism in . The (symmetrical) relations i and w need to satisfy are categorical versions of Kaygun’s axioms of a transposition map. Motivation comes from the observation that a (co)ring T over an algebra R determines a distributive law of two (co)monads and on the category of R-bimodules. The functor Π can be chosen such that is the cyclic R-module tensor product. A natural transformation is given by the flip map and a morphism is constructed whenever T is a (co)module algebra or coring of an R-bialgebroid. The notion of a stable anti-Yetter-Drinfel’d module over certain bialgebroids, the so-called  ×  _R -Hopf algebras, is introduced. In the particular example when T is a module coring of a  ×  _R -Hopf algebra and X is a stable anti-Yetter-Drinfel’d -module, the para-cyclic object Z _* is shown to project to a cyclic structure on . For a -Galois extension , a stable anti-Yetter-Drinfel’d -module T _S is constructed, such that the cyclic objects and are isomorphic. This extends a theorem by Jara and Ştefan for Hopf Galois extensions. As an application, we compute Hochschild and cyclic homologies of a groupoid with coefficients in a stable anti-Yetter-Drinfel’d module, by tracing it back to the group case. In particular, we obtain explicit expressions for (coinciding relative and ordinary) Hochschild and cyclic homologies of a groupoid. The latter extends results of Burghelea on cyclic homology of groups.     

RF-plasma assisted pulsed laser deposition of nitrogen-doped SrTiO3 thin films

Perovskite-type nitrogen substituted SrTiO₃ thin films were deposited with a one-step process by RF-plasma assisted pulsed laser deposition from a SrTiO₃ target using a N₂ plasma, while deposition with a NH₃ plasma yields films with almost no incorporated nitrogen. The deposited films exhibit a cubic perovskite-type crystal structure and reveal oriented growth on MgO(100) substrates. The unit cell parameters of the studied N-doped SrTiO₃ films range within 3.905<a<3.918 Å, which is slightly larger than for SrTiO₃ (a=3.905 Å). The nitrogen content in the deposited films varies from 0.2 to 0.7 atom%. The amount of incorporated nitrogen in the films decreases with increasing RF-power, while the N₂ flow rate does not have any pronounced influence on the N content. Nitrogen incorporation results in an increased optical absorption at 400–600 nm, which is associated with N(2p) energy states that have a higher energy level than the valence band in strontium titanate. The optical band gap energies in the studied N-doped SrTiO₃ films are at 3.2–3.3 eV, which is very similar to that of pure strontium titanate (～3.2 eV). Films deposited with NH₃ for the RF-plasma exhibit a lower degree of crystallinity and reveal almost no nitrogen incorporation into the crystal lattice.     

Lifetime measurements and transition probability calculations in singly ionized tungsten (W<Subscript>II</Subscript>)

New measurements of radiative lifetimes for 9 levels in singly ionized tungsten (W_II) have been performed with the time-resolved laser-induced-fluorescence technique. Transition probabilities have been obtained from a combination of experimental lifetimes and theoretical branching fractions. The reliability of the present results is assessed through the good agreement observed between the calculated lifetimes and the experimental values from this work and from previous publications. These new results fill in a gap in the available data for this atomic species particularly important for fusion reactors.     

Observation and theoretical description of periodic geometric rearrangement in electronically excited nonstoichiometric sodium-fluoride clusters

We present two-color fs pump-probe spectra of Na2F which were recorded by employing excitation wavelengths around 1208 nm (pump) and ionization wavelengths around 405 nm (probe). The observed oscillatory structure of the signal with a period of 185 fs shows an excellent agreement with our simulated spectra. The employed ab initio Wigner distribution approach provides clear evidence that this observation is caused by photoinduced metal bond breaking followed by a butterfly-type periodic geometric rearrangement.     

Self-diffusion rates in Al from combined first-principles and model-potential calculations

Monovacancy diffusion alone dominates over diffusion due to divacancies and interstitials in Al for all temperatures up to the melting point. Deviations from a single Arrhenius dependence are due to anharmonicity. The conclusion is based on a combination of theoretical methods, from density functional theory to thermodynamic integration, without fitting to experimental data. The calculated diffusion rate agrees with experimental data over 11 orders of magnitude.