Magneto-optical Faraday and Kerr effect of orthoferrite thin films at high temperatures

Orthoferrites present, as bulk materials, reorientation transitions of their magnetic moment alignment at temperatures depending on the rare-earth (RE) ion. In particular, orthoferrites (REFeO₃) with RE = Sm, Dy, present this transition at T _SRT = 443 K and 36 K, respectively. The spectra of the complex Kerr and Faraday angle have been measured on orthoferrite thin films (RE = Sm, Dy, Y), which were prepared by pulsed laser deposition on amorphous quartz substrates. The obtained spectra exhibit contributions of both surfaces and interfaces. Propagation effects of the polarized light in the magneto-optical medium which is interpreted in terms of a simplified theoretical formalism, is also observed. For selected photon energies, temperature dependent Faraday rotation measurements, , on orthoferrite thin films (RE = Sm, Dy, Y) have been performed. A quite different thermal variation compared to the bulk magnetization has been observed. Curie temperatures are found to be close to the bulk values or slightly larger by 10 K to 20 K as in the case of DyFeO₃ and YFeO₃. For RE = Sm and Dy, increases with increasing temperature contrary to the saturation magnetization, passes through a maximum at about 460 K and vanishes with a T _C of 647±18 K, 695 K for RE = Sm and Dy respectively. Received 28 July 2000     

Resonant Raman scattering in Mn:ZnO dilute magnetic semiconductors

We report on micro-Raman measurements performed under various visible excitations on wurtzite Zn_1−xMn_xO thin films grown by MOCVD, for a wide range of Mn content (0<x<0.22). We observe a shift of the frequency of the E₂ phonon for increasing Mn content, indicating a substitution of the Mn atoms at the tetrahedral Zn sites of the host matrix. All spectra also reveal a dominant feature related to local vibrations of Mn atoms, as well as the A₁ (LO) phonon of the ZnMnO alloy. Both features are resonantly enhanced by the absorption of the incident laser light by internal transitions of the Mn²⁺ ions.     

Liquid chromatography–mass spectrometry (LC–MS): a powerful combination for selenium speciation in garlic (Allium sativum)

Liquid chromatography (LC) hyphenated with both elemental and molecular mass spectrometry has been used for Se speciation in Se-enriched garlic. Different species were separated by ion-pair liquid chromatography–inductively coupled plasma mass spectrometry (LC–ICP–MS) after hot-water extraction. They were identified by on-line reversed-phase liquid chromatography–electrospray ionization tandem mass spectrometry (RPLC–ESI–MS–MS). Se-methionine and Se-methylselenocysteine were determined by monitoring their product ions. Another compound, γ-glutamyl-Se-methylselenocysteine, shown to be the most abundant form of Se in the garlic, was determined without any additional sample pre-treatment after extraction and without the need for a synthesized standard. Product ions for this dipeptide were detected by LC–ESI–MS–MS for three isotopes of Se—⁷⁸ Se, ⁸⁰Se: and ⁸²Se. The method was extended to the species extracted during in-vitro gastrointestinal digestion. Because both Se-methylselenocysteine and γ-glutamyl-Se-methylselenocysteine have anticarcinogenic properties, their extractability and stability during human digestion are very important. Garlic was also treated with saliva, to enable detection and analysis of species extracted during mastication. Detailed information on the extractability of selenium species by both simulated gastric and intestinal fluid are given, and variation of the distribution of Se among the different species with time is discussed. Although the main species in garlic is the dipeptide γ-glutamyl-Se-methylselenocysteine, Se-methylselenocysteine is the main compound present in the extracts after treatment with gastrointestinal fluids. Two more, so far unknown compounds were observed in the chromatogram. The extracted species and their transformations were analysed by combining LC–ICP–MS and LC–ESI–MS–MS. In both the simulated gastric and intestinal digests, Se-methionine, Se-methylselenocysteine, and γ-glutamyl-Se-methylselenocysteine could be determined by LC–ESI–MS–MS by measuring their typical product ions.      

Determination of the neuropeptides arginine vasotocin and isotocin in brains of three-spined sticklebacks (Gasterosteus aculeatus) by off-line solid phase extraction-liquid chromatography-electrospray tandem mass spectrometry

A method based on solid phase extraction (SPE) followed by liquid chromatography-electrospray ionisation tandem mass spectrometry for the determination of the nonapeptides arginine vasotocin (AVT) and isotocin (IT) in brains of three-spined sticklebacks (Gasterosteus aculeatus) is described. Separation and detection were optimized using synthetic standards. Limits of detection (LOD) for standard solutions were 160 pg mL(-1) for AVT and 250 pg mL(-1) for IT. The SPE procedure hardly affected the LODs for standard solutions. Mainly because of ion suppression, LODs for AVT and IT in brains were approximately 5 and 25 pg mg(-1), respectively. The concentrations determined in the brain of several fishes ranged from 10 to 500 pg mg(-1) for AVT and from 400 to 4000 pg mg(-1) for IT.     

Comparison of charge models for fixed-charge force fields: small-molecule hydration free energies in explicit solvent

In molecular simulations with fixed-charge force fields, the choice of partial atomic charges influences numerous computed physical properties, including binding free energies. Many molecular mechanics force fields specify how nonbonded parameters should be determined, but various choices are often available for how these charges are to be determined for arbitrary small molecules. Here, we compute hydration free energies for a set of 44 small, neutral molecules in two different explicit water models (TIP3P and TIP4P-Ew) to examine the influence of charge model on agreement with experiment. Using the AMBER GAFF force field for nonbonded parameters, we test several different methods for obtaining partial atomic charges, including two fast methods exploiting semiempirical quantum calculations and methods deriving charges from the electrostatic potentials computed with several different levels of ab initio quantum calculations with and without a continuum reaction field treatment of solvent. We find that the best charge sets give a root-mean-square error from experiment of roughly 1 kcal/mol. Surprisingly, agreement with experimental hydration free energies does not increase substantially with increasing level of quantum theory, even when the quantum calculations are performed with a reaction field treatment to better model the aqueous phase. We also find that the semiempirical AM1-BCC method for computing charges works almost as well as any of the more computationally expensive ab initio methods and that the root-mean-square error reported here is similar to that for implicit solvent models reported in the literature. Further, we find that the discrepancy with experimental hydration free energies grows substantially with the polarity of the compound, as does its variation across theory levels.     

Polarization effects in photoisomerization of azo dyes in polymeric films

The reversible cis-trans photoisomerization of disperse red 1 (DR1) in PMMA thin films has been demonstrated previously by using the Attenuated Total Reflection (ATR) method. In this communication photoisomerization of DR1 is shown to be strongly polarization sensitive. This new property of dye doped polymeric films could lead to practical applications in integrated optics. A simple molecular interpretation is given. Illumination of a DR1 sample by polarization fringes produces a grating of molecular orientation able to diffract a probe beam.     

Minimal cocycles with the scaling property and substitutions

‘Fractal’ functions are formulated as a minimal cocycle on a topological dynamics which admits nontrivial scaling transformations. In this paper, it is proved that if in addition it admits a continuous family of scaling transformations, then itscapacity is not ino(N ²). We define minimal cocycles with nontrivial scaling transformations coming from substitutions on a finite alphabet which are proved to have capacityO(N), so that they admit only a discrete family of scaling transformations. We also construct one which has capacityO(N ²) and admit a continuous family of scaling transformations. Supported by C.N.R.S.(U.R.A 225); Partially supported by the Japan Society for the Promotion of Science.     

Inclusive neutral kaon production in 70 GeV/c K+p interactions

The inclusive production of neutral kaons in 70 GeV/c K⁺p interactions is studied with the CERN BEBC bubble chamber. The (semi-)inclusive cross sections are interpreted in terms of the various strangeness channels leading to neutral kaon production. The invariant inclusive cross section for kaon production is studied as a function of p_t² and the Feynman variable x. The latter distributions are considered both “raw” and corrected for the presence of K⁰'s resulting from K¹ decay. They are compared with the predictions expected from the Regge-Mueller formalism, the recombination model and fragmentation models.     

Dipolar interaction in the transverse Ising model of H-bonded ferroelectrics

Numerical solutions are presented for the set of approximated kinetic equations previously obtained to describe the dynamical behaviour of the pseudospin correlation functions of the transverse Ising model in H-bonded ferroelectrics. Stability of the soft mode and spectral characteristics of the correlation functions as T → ∞ are discussed in terms of an interaction made of both nearest-neighbour and dipole-dipole contributions. This infinite-temperature calculation shows how shift (towards zero frequency) and width of the soft mode vary with the interdipolar strength of the interaction. The asymmetry of the imaginary part of susceptibility _X″zz(ω) exhibited by KDP in its paraelectric phase is also shown to be due to the dipolar character of the interaction.