Structural and electronic transitions in gadolinium iron borate GdFe<Superscript>3</Superscript>(BO<Superscript>3</Superscript>)<Superscript>4</Superscript> at high pressures

The optical properties and structure of gadolinium iron borate GdFe₃(BO₃)₄ crystals are studied at high pressures produced in diamond-anvil cells. X-ray diffraction data obtained at a pressure of 25.6 GPa reveal a firstorder phase transition retaining the trigonal symmetry and increasing the unit cell volume by 8%. The equation of state is obtained and the compressibility of the crystal is estimated before and after the phase transition. The optical spectra reveal two electronic transitions at pressures ～26 GPa and ～43 GPa. Upon the first transition, the optical gap decreases jumpwise from 3.1 to ～2.25 eV. Upon the second transition at P=43 GPa, the optical gap deceases down to ～0.7 eV, demonstrating a dielectric-semiconductor transition. By using the theoretical model developed for a FeBO₃ crystal and taking into account some structural analogs of these materials, the anomalies of the high-pressure optical spectra are explained.     

Mechanism of Crystal Phosphor Luminescence

An experimental method has been developed for determining the mechanism of luminescence of crystals by the changes arising in the intensity of a burst of their luminescence under the action of a constant electric field. An example of determining the mechanism of luminescence of AgBr crystals and AgBr_0.95 and AgBr_0.60Cl_0.40 solid solutions by the method proposed is presented. We were the first to establish that the low-temperature photoluminescence of AgBr_0.95I_0.05 in the band with _max = 540 nm proceeds by the Schone–Klasens recombination mechanism. It has also been established that the longwave luminescence of AgBr, AgBr_0.95I_0.05, and AgBr_0.60Cl_0.40 in the band with _max = 630 nm arises as a result of the recombination of electrons localized on a luminescence center with free holes.     

The band structure of <Emphasis Type="Italic">n</Emphasis>-type cuprate superconductors with the <Emphasis Type="Italic">T</Emphasis>′(<Emphasis Type="Italic">T</Emphasis>) structure taking into account strong electron correlation

The spectral density, dispersion relations, and the position of the Fermi level for n-doped compositions based on NCO and LCO were calculated within the framework of the generalized tight binding method. As distinguished from LCO, the dielectric gap in NCO is nonlinear in character. We observe a virtual level both at the bottom of the conduction band and at the top of the valence band in both compounds. However, its position corresponds to the extreme bottom of the conduction band in LCO and is 0.1–0.2 eV above the bottom in NCO. This explains why we observe Fermi level pinning in n-LCO as the concentration of the doping component grows and reproduce its absence in NCCO at low doping values. We also found both compositions to be unstable in a narrow concentration range with respect to a nonuniform charge density distribution. The relation between the phase diagram for NCCO and the calculated electronic structure is discussed.     

Halide-induced supramolecular ligand rearrangement

A novel reaction involving the halide-induced rearrangement of ligands within supramolecular Rh(I) complexes containing hemilabile ligands is presented. Three analogous bis- and trishemilabile ligands have been synthesized to construct bi- and trimetallic Rh(I) macrocyclic complexes. An intentionally added halide source results in the formal rotation of only one hemilabile ligand along the axis that is perpendicular to the plane defined by the aryl backbone of the hemilabile ligands. X-ray structures, as determined by X-ray crystallography, of key intermediates and products are presented.     

Regge oscillations in integral cross sections for proton impact on atomic hydrogen

The integral cross sections for elastic scattering and spin exchange for proton impact on atomic hydrogen show several oscillations in the energy range 0.01-1.0 eV that cannot be associated with resonances or the glory effect. A complex angular momentum analysis using computed Regge trajectories shows that each peak of the oscillatory structure is predominantly associated with at most three trajectories. In this way, the peaks are related to the L=0 bound states of H+ 2. The complex angular momentum theory for integral cross sections that we introduce shows that such oscillations are a general feature of potential scattering.     

On a quantum plateau of magnetization in metal-organic quasi-one-dimensional ferrimagnets

The possibility of realizing a quantum plateau of magnetization in [Mn(hfac)₂BNO_R] metal-organic compounds is investigated theoretically. A model of a one-dimensional ferrimagnetic chain (5/2, 1) is used for calculating the magnetization as a function of an external field by the method of discrete path integral representation (DPIR). Within this model, the coexistence of classical and quantum plateaus of magnetization is revealed. It is shown that the critical field H_c1 that destroys the classical plateau (ground-state magnetization) is determined by the optical gap in zero field, which is estimated by the matrix-product method and a numerical method of exact diagonalization (recursion method).     

Temperature and concentration dependences of the electronic etructure of copper oxides in the generalized tight binding method

The electronic structure of p-type doped HTSC cuprates is calculated by explicitly taking into account strong electron correlations. The smooth evolution of the electronic structure from undoped antiferro-magnetic to optimally and heavily doped paramagnetic compositions is traced. For a low doping level, in-gap impurity-type states are obtained, at which the Fermi level is pinned in the low-doping region. These states are separated by a pseudogap from the valence band. The Fermi surfaces calculated for the paramagnetic phase for various concentrations of holes are in good agreement with the results of ARPES experiments and indicate a gradual change in the Fermi surface from the hole type to the electron type.     

Frustrated antiferromagnetism in the Sr<Subscript>2</Subscript>YRuO<Subscript>6</Subscript> double perovskite

The spins of Ru⁵⁺ ions in Sr₂YRuO₆ form a face-centered cubic lattice with antiferromagnetic nearest neighbor interaction J≈25 meV. The antiferromagnetic structure of the first type experimentally observed below the Néel temperature T _N =26 K corresponds to four frustrated spins of 12 nearest neighbors. In the Heisenberg model in the spin-wave approximation, the frustrations already cause instability of the antiferromagnetic state at T=0 K. This state is stabilized by weak anisotropy D or exchange interaction I with the next-nearest neighbors. Low D/J～I/J～10^?3 values correspond to the experimental T _N and sublattice magnetic moment values.