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Because of the importance of ferromagnetism at room temperature, we search for new materials that can exhibit a non-vanishing magnetic moment at room temperature and at the same time can be used in spintronics. The experimental results indicate that d0 ferromagnetism without any magnetic impurities takes place in PbS films made of close-packed lead sulfide nanoparticles of 30 nm. To explain the existence of the d0 ferromagnetism, we propose a model where various PbS bulk and surface configurations of Pb-vacancies are analyzed. The bulk configurations have a zero magnetic moment while the two surface configurations with Pb vacancies with the same non-vanishing magnetic moments and lowest ground state energies contribute to the total magnetization. Based on the experimental value of the saturation magnetization, 0.2 emu g−1, we have found that the calculated Pb vacancy concentration should be about 3.5%, which is close to typical experimental values. Besides being very important for applications, there is one feature of PbS d0 ferromagnetism that makes this material special for fundamental research: PbS ferromagnetism can exhibit topologically driven spatial magnetic moment distributions (e.g., magnetic skyrmions) due to large spin–orbit coupling.

The aggregation of ammonium borate salts in hydrocarbon was investigated by 1H, 19F, and 11B pulsed field gradient (PFG) NMR. The molecular self-diffusion coefficients of the borate salts, [C(C6H5)3][B(C6F5)4], [N(CH3)2(n-C18H37)2][B(C6F5)4], [N(n-C18H37)4][B(C6F5)4], a neutral borane compound, and a siloxane model compound were measured in toluene, cyclohexane, hexane, and a solvent of mixed alkanes. Diffusion coefficients were determined from the echo attenuation of the stimulated echo pulsed field gradient NMR signal as described by Stejskal and Tanner. In all of the samples studied, the echo decay was observed to be a pure exponential decay, corresponding to a single diffusion coefficient within the resolution of the NMR experiment. The hydrodynamic radius of an equivalent diffusing sphere was calculated from the experimental diffusion coefficients using the Stokes−Einstein relation. We found that the neutral borane and siloxane model compounds are monomeric (nonaggregated) in both the aliphatic and a...

Both the oxygen diffusion rate and the oxygen solubility vary with depth into the interior of biological membranes. The product of these two gradients generates a single gradient, a permeability gradient, which is a smooth continuous function of the distance from the center of the membrane. Using electron paramagnetic resonance and the spin-probe method, the relaxation gradient of oxygen, which is directly proportional to the permeability gradient, is the quantity that can be directly measured in membranes under physiological conditions. The gradient obtained provides a calibrated ruler for determining the membrane depth of residues either from loop regions of membrane-binding proteins or from the membrane-exposed residues of transmembrane proteins. We have determined the relaxation gradient of oxygen in zwitterionic and anionic phospholipid membranes by attaching a single nitroxide probe to a transmembrane alpha-helical polypeptide at specific residues. The peptide ruler was used to determine the depth of penetration of the calcium-binding loops of the C2 domain of cytosolic phospholipase A(2). The positions of selected residues of this membrane-binding protein that penetrate into the membrane, determined using this ruler, compared favorably with previous determinations using more complex methods. The relaxation gradient constrains the possible values of the membrane-dependent oxygen concentration and the oxygen diffusion gradients. The average oxygen diffusion coefficient is estimated to be at least 2-fold smaller in the membrane than that in water.

The optical properties of organic chromophores, in the class of nonlinear optical molecules, have improved dramatically. These chromophores have large hyperpolarizability and large permanent dipole moments, in excess of 10 D. When these molecules are mixed with a host polymer at high chromophore density and processed using an aligning electric poling field, the resulting material does not have the electro-optic (EO) response promised by the individual molecules. We have used equilibrium statistical mechanics, and Monte Carlo (MC) methods in a lattice-based model, to investigate how the molecular properties translate into the EO response of the material. We compare our MC results with a previously developed analytic theory and find good agreement when certain assumptions about the analytic theory are made. By use of the MC methods to compute order parameters for chromophores at high density on a lattice in the presence of an external uniform poling field, we find that greater order and hence greater EO per...

Human group IIA phospholipase A(2) (hGIIA) is secreted from a number of cells during inflammation and is known to interact strongly with anionic membranes and to exhibit potent Gram-positive bactericidal activity. This protein contains 23 cationic residues, which are scattered over its entire surface, resulting in a high pI of 9.39. To understand the molecular basis for the selective binding of hGIIA to anionic membranes, 14 single-site, spin-labeled hGIIA proteins were analyzed in the presence and absence of vesicles of anionic phospholipid by time domain and continuous wave electron paramagnetic resonance (EPR) spin relaxant techniques. Surprisingly, for hGIIA bound to anionic vesicles, all of the spin labels were highly protected from water-soluble spin relaxants. Together with light scattering studies, these EPR results suggest the formation of a supramolecular aggregate involving clusters of hGIIA molecules bridging together multiple vesicles. This anomalous mode of binding of hGIIA to anionic phospholipid explains previous data in which charge reversal mutation of a few cationic residues on multiple faces of hGIIA leads to a comparable and modest reduction in affinity of the protein for anionic vesicles. In the presence of mixed micelles composed of 10% anionic phospholipids in Triton X-100 a monodisperse protein-lipid complex is formed. Under these conditions, the EPR methods were used to map the surface of hGIIA that constitutes the interfacial binding site (IBS). The IBS of hGIIA consists of the highly hydrophobic surface that surrounds the opening to the active site slot.

Apparent rate constants, at acidic pH and neutral pH for the reaction of a family of ester-containing 5-carboxyl-5-methyl-1-pyrroline N-oxides with superoxide (O2*-) were estimated, using ferricytochrome c as a competitive inhibitor. It was of interest to note that the rate constants were similar among the different nitrones and not that significantly different from that found for 5-(diethoxyphosphoryl)-5-dimethyl-1-pyrroline N-oxide. At acidic pH, the rate constant for spin trapping O2*- was 3-fold greater than that at physiological pH. Subsequent experiments determined the half-life of aminoxyls, derived from the reaction of these nitrones with O2*-. The EPR spectra were modeled by using a global analysis method. The results clearly demonstrated that EPR spectra of all the aminoxyls were inconsistent with a model that included a single gamma-hydrogen splitting. A better interpretation modeled them as two diastereomers with identical nitrogen splittings and slightly different beta-hydrogen splittings. Detailed line width analyses slightly favored an equal line width-unequal population ratio for the two diastereomers.

Utilizing guidance from quantum and statistical mechanics, the electro-optic coefficients of organic materials have been increased to values greater than 100 pm/V at telecommunication wavelengths (e.g., to 130 pm/V at 1.3 microns). Electro-optic materials now afford significant advantages in terms of bandwidth and electro-optic activity over inorganic materials such as lithium niobate. Moreover, organic materials have also been found to be quite processable permitting the fabrication, by reactive ion etching and photolithographic techniques, of 3-D active waveguide structures and integration with both VLSI semiconductor electronics and silica fiber optics. Stripline, cascaded prism, and microresonator structures have been fabricated, as have low-optical-loss coupling structures. A number of prototype devices demonstrating superior performance have been produced; however, the long-term, in-field performance of such devices still remains to be evaluated. Nevertheless, significant advances have been made in improving the thermal and photochemical stability of organic materials and in defining the mechanisms that define these stabilities (by testing under accelerated conditions). The role of nanoscale architecture in systematically improving stability of organic electro-optic materials, as well as contributing to enhanced electro-optic activity and reduced optical loss, has been clarified.

Employing guidance from quantum and statistical mechanics, the electro-optic activity of organic materials has been increased to values greater than 100 pm/V at telecommunication wavelengths (e.g., 130 pm/V at 1.3 microns). Electro-optic materials now afford significant advantages in terms of bandwidth and electro-optic activity over competitive inorganic materials such as lithium niobate. Organic materials have also been found to be quite processable permitting the fabrication by reactive ion etching and photolithographic techniques of 3-D active waveguide structures and integration with both VLSI semiconductor electronics and silica fiber optics. Both stripline and microresonator structures have been fabricated, as have low-optical-loss coupling structures. A number of prototype devices demonstrating superior performance have been demonstrated; however, the long-term, in-field performance of such devices still remains to be evaluated. This article focuses on statistical mechanical theoretical methods that have aided the design of improved materials.