Generating unexpected spin echoes in dipolar solids with pi pulses

NMR spin echo measurements of 13C in C60, 89Y in Y2O3, and 29Si in silicon are shown to defy conventional expectations when more than one pi pulse is used. Multiple pi-pulse echo trains may either freeze out or accelerate the decay of the signal, depending on the pi-pulse phase. Average Hamiltonian theory, combined with exact quantum calculations, reveals an intrinsic cause for these coherent phenomena: the dipolar coupling has a many-body effect during any real, finite pulse.     

Spectroscopic evidence for the localization of Skyrmions near nu = 1 as T --> 0

Optically pumped nuclear magnetic resonance measurements of 71Ga spectra were carried out in an n-doped GaAs/Al(0.1)Ga0.9As multiple quantum well sample near the integer quantum Hall ground state nu = 1. As the temperature is lowered (down to T approximately 0.3 K), a "tilted plateau" emerges in the Knight shift data, which is a novel experimental signature of quasiparticle localization. The dependence of the spectra on both T and nu suggests that the localization is a collective process. The frozen limit spectra appear to rule out a 2D lattice of conventional Skyrmions.     

Electrodynamic analysis of helical radiators located on an elliptical surface

On the basis of integral representations of the electromagnetic field, mathematical models of the spiral radiators located on the surface of an ellipsoid are constructed. The internal electrodynamic problem is reduced to the set of linear algebraic equations written with respect to unknown values of current on radiating elements. In the band of normalized frequencies, the input resistance of antennas and the distribution of currents over the branches of spirals are calculated. Analysis of the frequency dependences of the input resistance makes it possible to reveal the phenomena of the parallel and serial macroresonance caused by the shape of the surface containing branches of spirals.     

Integral equation for a strip coil antenna located on a dielectric cylinder

The problem about the distribution of the surface current density in a narrow circular strip antenna as an infinitely thin perfectly conducting ribbon folded in a circle and positioned on the surface of a dielectric cylinder is reduced to a one-dimensional integral equation (IE). A method for solving the obtained IE is proposed. Complex distributions of the azimuthal component of the surface current density over the circular conductor are presented for different values of the dielectric permittivity of the cylinder.     

Water Interaction with Iron Oxides

We present a mechanistic study on the interaction of water with a well‐defined model Fe₃O₄(111) surface that was investigated by a combination of direct calorimetric measurements of adsorption energies, infrared vibrational spectroscopy, and calculations bases on density functional theory (DFT). We show that the adsorption energy of water (101 kJ mol⁻¹) is considerably higher than all previously reported values obtained by indirect desorption‐based methods. By employing ¹⁸O‐labeled water molecules and an Fe₃O₄ substrate, we proved that the generally accepted simple model of water dissociation to form two individual OH groups per water molecule is not correct. DFT calculations suggest formation of a dimer, which consists of one water molecule dissociated into two OH groups and another non‐dissociated water molecule creating a thermodynamically very stable dimer‐like complex.     

Effect of the cellular structure on thermal conductivity of rigid closed-cell foam polymers during long-term aging

The thermal conductivity of rigid closed-cell polyurethane foams during long-term aging has been studied. The similarity between the kinetics of changes in the physical and mechanical characteristics of PU foams on progressive aging is established, which is attributed to the effect of matrix destruction. It is found that rigid foams have cell walls of various strength, whose impact on the kinetics of changes in the physical characteristics of the foams during long-term aging is ascertained. The results of predicting the thermal conductivity of PU foams by the method of temperature-time analogy and establishing the limits of its application are discussed. The research presented is of interest both in determining the foam durability and in replacing freons by alternative, ecologically less harmful blowing agents.Translated from Mekhanika Kompozitnykh Materialov, Vol. 35, No. 2, pp. 187–198, March–April, 1999.     

Protein-based fluorescent bioassay for low-dose gamma radiation exposures

Analytical and Bioanalytical Chemistry - The study suggests an application of a coelenteramide-containing fluorescent protein (CLM-CFP) as a simplest bioassay for gamma radiation exposures....     

The Bridge Flow Regime in Microchannels

Doklady Physics - As a result of studies of the two-phase flow in slit microchannels with a height from 50 to 440 microns, a new flow regime, bridge flow, has been discovered and studied....     

Dynamic nuclear polarization in silicon microparticles

We report record high 29Si spin polarization obtained using dynamic nuclear polarization in microcrystalline silicon powder. Unpaired electrons in this silicon powder are due to dangling bonds in the amorphous region of this intrinsically heterogeneous sample. 29Si nuclei in the amorphous region become polarized by forced electron-nuclear spin flips driven by off-resonant microwave radiation while nuclei in the crystalline region are polarized by spin diffusion across crystalline boundaries. Hyperpolarized silicon microparticles have long T1 relaxation times and could be used as tracers for magnetic resonance imaging.