Low-temperature results of the statistical theory of noninteracting spin waves

Low-temperature predictions of a free-spin-wave theory that includes the correct spin-wave kinematics for theS=1/2, quantum Heisenberg ferromagnet, as described in a previous paper, are obtained for three dimensions using analytic and Monte Carlo techniques. The low-temperature propagator is found to be of boson form, and the low-temperature magnetization goes asT ^3/2, which agrees with the interacting theory at lowest order inT.     

Diaphragm opening effects on shock wave formation and acceleration in a rectangular cross section channel

Shock wave formation and acceleration in a high-aspect ratio cross section shock tube were studied experimentally and numerically. The relative importance of geometric effects and diaphragm opening time on shock formation are assessed. The diaphragm opening time was controlled through the use of slit-type (fast opening time) and petal-type (slow opening time) diaphragms. A novel method of fabricating the petal-type diaphragms, which results in a consistent burst pressure and symmetric opening without fragmentation, is presented. High-speed schlieren photography was used to visualize the unsteady propagation of the lead shock wave and trailing gas dynamic structures. Surface-mounted pressure sensors were used to capture the spatial and temporal development of the pressure field. Unsteady Reynolds-Averaged Navier–Stokes simulation predictions using the shear-stress-transport turbulence model are compared to the experimental data. Simulation results are used to explain the presence of high-frequency pressure oscillations observed experimentally in the driver section as well as the cause of the initial acceleration and subsequent rapid decay of shock velocity measured along the top and bottom channel surfaces. A one-dimensional theoretical model predicting the effect of the finite opening time of the diaphragm on the rate of driver depressurization and shock acceleration is proposed. The model removes the large amount of empiricism that accompanies existing models published in the literature. Model accuracy is assessed through comparisons with experiments and simulations. Limitations of and potential improvements in the model are discussed.     

Multiple photonic responses in films of organic-based magnetic semiconductor V(TCNE)x, x approximately 2

Concomitant photoinduced magnetic and electrical phenomena are reported for the organic-based magnetic semiconductor V(TCNE)x (x approximately 2; TCNE=tetracyanoethylene; magnetic ordering temperature Tc approximately 400 K). Upon optical excitation (457.9 nm), the system can be trapped in a thermally reversible photoexcited state, which exhibits reduced magnetic susceptibility and increased conductivity with a simultaneous change in IR absorption spectrum. The multiple photonic effects in V(TCNE)x are proposed to originate from structural changes induced by internal excitation in (TCNE)- anions, which lead to relaxation to a long-lived metastable state.     

Bonding, backbonding, and spin-polarized molecular orbitals: basis for magnetism and semiconducting transport in V[TCNE]x approximately 2

X-ray absorption spectroscopy (XAS) and magnetic circular dichroism (MCD) at the V L{2,3} and C and N K edges reveal bonding and backbonding interactions in films of the 400 K magnetic semiconductor V[TCNE]x approximately 2. In V spectra, d{xy}-like orbitals are modeled assuming V2+ in an octahedral ligand field, while d{z{2}} and d{x{2}-y{2}} orbitals involved in strong covalent sigma bonding cannot be modeled by atomic calculations. C and N MCD, and differences in XAS from neutral TCNE molecules, reveal spin-polarized molecular orbitals in V[TCNE]x approximately 2 associated with weaker pi bonding interactions that yield its novel properties.     

Hidden Higgs particles

In a class of extended Higgs structures containing a light or massless pseudoscalar it is quite likely the normal Wienberg-Salam Higgs particle decays preferentially into spin-zero bosons if its mass is below the t$\text{t}$ threshold. These spin-zero bosons may be invisible either because they do not decay or because they decay into neutrinos. Thus Higgs particles may be light enough to be produced at accessible energies but difficult to detect through their decay products.     

Reactions of furocoumarins. II. Synthetic aminomethyl psoralens via chloromethylation or benzylic bromination

Several psoralen derivatives have been synthesized in order to evaluate their efficacy as photochemotherapeutic (PUVA) agents, including a variety of 4′-substituted-4,5′,8-trimethypsoralen compounds ( 1d-j ). Improved synthesis of the very potent photosensitizers 8-methylpsoralen ( 6a ) and 4,8-dimethylpsoralen ( 6b ) are described and 6a has been shown to undergo formylation in the 4′-position. Free radical bromination of 6a and 6b with NBS affords primarily 8-bromomethyl derivatives ( 8a and d ), which are readily converted into the 8-aminomethyl derivatives ( 8c and f ) by the Gabriel method. If the 4′-position is blocked, electrophilic substitution apparently occurs primarily in the 5′-position of the psoralen system. At least, chloromethylation of 4′-methylpsoralen ( 9 ) affords mainly 5′-chloromethyl derivatives ( 10a and d ), which also lead to aminomethylpsoralens ( 10c and f ).     

A Cl nuclear quadrupole resonance study of NH4ICl4·H2O

The “missing” fourth ³⁵Cl NQR frequency postulated for NH₄ICl₄H₂O at room temperature has been observed. Four signals were also found for this compound at 195K, but the results at 77K were less clear-cut, with either five or (probably) six resonances detected. The occurrence of a phase transition between 77 and 195K seems likely. A structure is suggested for the ICl₄⁻ ion at room temperature from the relationship found previously between NQR frequency and bond length in tetrachloroiodates.