Orbital magnetism and spin-orbit effects in the electronic structure of BaIrO3.

The electronic structure and magnetism of Ir 5d5 states in nonmetallic, weakly ferromagnetic BaIrO3 are probed with x-ray absorption techniques. Contrary to expectation, the Ir 5d orbital moment is found to be ~1.5 times larger than the spin moment. This unusual, atomiclike nature of the 5d moment is driven by a strong spin-orbit interaction in heavy Ir ions, as confirmed by the nonstatistical large branching ratio at Ir L(2,3) absorption edges. As a consequence, orbital interactions cannot be neglected when addressing the nature of magnetic ordering in BaIrO3. The local moment behavior persists even as the metallic-paramagnetic phase boundary is approached with Sr doping or applied pressure.

Orientational distributions and nematic order of rodlike magnetic nanoparticles in dispersions.

Using small-angle neutron scattering (SANS), we have investigated the orientational order of iron nanoparticles dispersed in cyclohexanone. The particles have rodlike shape and size distributions with an average length of 200 nm and an average diameter of 25 nm. SANS shows an anisotropy, which is a measure of orientational order, in magnetic dispersions with a volume fraction of 3.2% and 3.9% iron particles in shear flow and/or magnetic field. The scattering anisotropy can be fitted by a model assuming an Onsager distribution of the orientation of the particles in shear flow. The orientational distribution of particles oriented by a magnetic field can be described by a different model assuming the Maier-Saupe orientational distribution for uniaxial ferromagnetic particles. The orientational distribution parameter m for the Maier-Saupe distribution or alpha for the Onsager distribution and the orientational order parameter S have been determined at shear rates gamma[over ] of to 0-4000 s(-1) and in magnetic fields of 0-18 mT. The S values indicate that the particles start to orient either in a shear flow of 100 s(-1) or in a magnetic field of 6 mT. Applying only shear results in an orientational order, with the dispersion returning to the disordered state when the shear rate is decreased to zero. In sharp contrast, application of magnetic fields greater than 6 mT results in orientational order in the field-increasing cycle, and two-thirds of the orientational order remains when the field is decreased to zero. This shows that the order in a magnetic field is different from the order in a shear flow, the action of magnetizing the particles along a certain direction is irreversible, and the orientational order parameter exhibits hysteresis.

Robust singlet dimers with fragile ordering in two-dimensional honeycomb lattice of Li2RuO3.

When an electronic system has strong correlations and a large spin-orbit interaction, it often exhibits a plethora of mutually competing quantum phases. How a particular quantum ground state is selected out of several possibilities is a very interesting question. However, equally fascinating is how such a quantum entangled state breaks up due to perturbation. This important question has relevance in very diverse fields of science from strongly correlated electron physics to quantum information. Here we report that a quantum entangled dimerized state or valence bond crystal (VBC) phase of Li2RuO3 shows nontrivial doping dependence as we perturb the Ru honeycomb lattice by replacing Ru with Li. Through extensive experimental studies, we demonstrate that the VBC phase melts into a valence bond liquid phase of the RVB (resonating valence bond) type. This system offers an interesting playground where one can test and refine our current understanding of the quantum competing phases in a single compound.

Synthesis and antiviral activity of pyranosylphosphonic acid nucleotide analogues.

Pyranosyl nucleotide analogues have been designed so that the intramolecular base to phosphorus distance closely approximates that of natural nucleotides. This was achieved by attaching the phosphorus directly at the anomeric position and the base at the 4-position of the carbohydrate. A series of compounds incorporating natural bases and having this novel structure were made via a short synthesis starting from commercially available glycals. Addition of triisopropyl phosphite to the glycals furnished alpha- and beta-2-enopyranosylphosphonates which were then substituted with the heterocycle using Mitsunobu chemistry. Deprotection afforded the 2',3'-unsaturated isonucleotide analogue. In some cases the deprotection sequence induced double-bond migration leading to the 1',2'-unsaturated derivative. NMR spectroscopic structural analysis established an axial preference for the base and an equatorial preference for the beta-phosphorus which results in intramolecular base to phosphorus distances within 1 A of that of natural nucleotides. All of the deprotected compounds were screened for inhibition of HCMV, HSV-2, and HIV replication. Several compounds inhibited HCMV and HSV-2, the most potent of which was the unsaturated cytosine analogue 18 (HCMV IC50 = 10 microM, HSV-2 IC50 = 85 microM). None of the compounds were cytotoxic at the highest dose (1 mM) tested. None of the compounds were inhibitory to HIV.

Improved performance accordion heteronuclear multiple-bond correlation spectroscopy-IMPEACH-MBC.

A modification of the recently reported ACCORD-HMBC long-range heteronuclear shift correlation experiment is described. The new experiment, IMPEACH-MBC (improved performance accordion heteronuclear multiple-bond correlation), introduces a new pulse sequence element, a constant time variable delay. The incorporation of the constant time variable delay into the IMPEACH-MBC sequence suppresses (1)H-(1)H coupling modulation inherent to the utilization of the accordion principle to sample a broad range of potential long-range heteronuclear couplings. (1)H-(1)H coupling modulation, which introduces an F(1) modulation or a "skew" of responses in the second frequency domain of the ACCORD-HMBC experiment, is suppressed in the IMPEACH-MBC experiment. Results of identically optimized IMPEACH-MBC and ACCORD-HMBC experiments performed on a sample of strychnine are compared.

Shear- and magnetic-field-induced ordering in magnetic nanoparticle dispersion from small-angle neutron scattering.

Small-angle neutron scattering experiments have been performed to investigate orientational ordering of a dispersion of rod-shaped ferromagnetic nanoparticles under the influence of shear flow and static magnetic field. In this experiment, the flow and flow gradient directions are perpendicular to the direction of the applied magnetic field. The scattering intensity is isotropic in zero-shear-rate or zero-applied-field conditions, indicating that the particles are randomly oriented. Anisotropic scattering is observed both in a shear flow and in a static magnetic field, showing that both flow and field induce orientational order in the dispersion. The anisotropy increases with the increase of field and with the increase of shear rate. Three states of order have been observed with the application of both shear flow and magnetic field. At low shear rates, the particles are aligned in the field direction. When increasing shear rate is applied, the particles revert to random orientations at a characteristic shear rate that depends on the strength of the applied magnetic field. Above the characteristic shear rate, the particles align along the flow direction. The experimental results agree qualitatively with the predictions of a mean field model.

Ferrimagnetism in EuFe4Sb12 due to the interplay of f-electron moments and a nearly ferromagnetic host.

We combine x-ray magnetic circular dichroism spectroscopy at Fe L2,3 edges, at Eu M4,5 edges, x-ray absorption spectroscopy (XAS) investigation of Eu valence, and local spin density calculations, to show that the filled skutterudite Eu0.95Fe4Sb12 is a ferrimagnet in which the Fe 3d moment and the Eu2+ 4f moment are magnetically ordered with dominant antiferromagnetic coupling. From Eu L3 edge XAS, we find that about 13% of the Eu have a formal valence of 3+. We ascribe the origin of ferrimagnetism at a relatively high transition temperature TC of 85 K in Eu0.95Fe4Sb12 to f-electron interaction with the nearly ferromagnetic [Fe4Sb12]2.2- host lattice.

Pharmacokinetics and metabolism of selected prodrugs of PMEA in rats.

The oral bioavailability of PMEA [9-[2-(phosphonomethoxy)ethyl]adenine; adefovir) has been determined in rats from three bisester prodrugs of PMEA: bis-(pivaloyloxymethyl) PMEA (bis-POM PMEA), bis-(phenyl) PMEA, and bis-(o-ethoxyphenyl) PMEA. The prodrugs were each administered to 9 male rats as solutions in PEG 400 at a dose of 10 mg-equivalent of PMEA per kg. Plasma samples were obtained over the course of 12 hr and concentrations of PMEA were determined by fluorescence derivatization and analysis by HPLC. Concentrations of PMEA observed in plasma following oral administration of PMEA prodrugs were compared with levels observed for intravenous PMEA. The observed oral bioavailabilities of PMEA from bis-POM PMEA, bis-(phenyl) PMEA, and bis-(o-ethoxyphenyl) PMEA were 38.2%, 2.46%, and 40.1%, respectively. PMEA was the only metabolite formed after oral administration of bis-POM PMEA. Three metabolites were detected after oral administration of either bis-(phenyl) PMEA or bis-(o-ethoxyphenyl) PMEA to rats: PMEA, the corresponding monoester, and 2-adenylacetic acid. The major metabolite of bis-(phenyl) PMEA was 2-adenylacetic acid following oral administration. 2-Adenylacetic acid appears to have been formed from the intact prodrugs by a P450 mediated oxidation of the ethyl side chain.

Non-Fermi-liquid spin dynamics in CeCoGe3-xSi(x) for x = 1.2 and 1.5.

Muon spin relaxation has been measured in CeCoGe3-xSi(x) at the magnetic/nonmagnetic boundary compositions of x = 1.2 and x = 1.5. Both the alloys are found to exhibit an ordered region and a disordered region. At x = 1.2, short-range magnetic ordering is observed below 0.86 K in the ordered region. The disordered region is paramagnetic and the muon spin-lattice relaxation rate lambda2 in this region displays non-Fermi-liquid (NFL) spin dynamics, i.e., the power law lambda2 proportional to T0.72 which shows the formation of Griffiths phase. lambda2 in the x = 1.5 alloy displays logarithmic (NFL) scaling below 1 K, in agreement with the theory of a T = 0 K magnetic transition.