Robust D-wave pairing correlations in a hole-doped spin-fermion model

Pairing correlations are studied numerically in a hole-doped spin-fermion model. Simulations performed on up to 12 x 12 clusters provide indications of D-wave superconductivity away from half-filling comparable to those of the 2D t-J model. The pairing correlations are the strongest in the direction perpendicular to the dynamic stripes that appear in the ground state at some densities. An optimal doping, where correlations are maximized, was observed at approximately 25% doping with an estimated T(c) approximately 100-200 K, in qualitative agreement with high-T(c) cuprates' phenomenology, while pairing correlations are suppressed by static stripe inhomogeneities.     

Double-exchange model study of multiferroic RMnO<Subscript>3</Subscript> perovskites

In this proceeding, recent theoretical investigations by the authors on the multiferroic RMnO₃ perovskites are briefly reviewed at first. Using the double-exchange model, the realistic spiral spin order in undoped manganites such as TbMnO₃ and DyMnO₃ is well reproduced by incorporating a weak next-nearest neighbor superexchange (~10% of nearest neighbor superexchange) and moderate Jahn-Teller distortion. The phase transitions from the A-type antiferromagnet (as in LaMnO₃), to the spiral phase (as in TbMnO₃), and finally to the E-type antiferromagnet (as in HoMnO₃), with decreasing size of the R ions, were also explained. Moreover, new results of phase diagram of the three-dimensional lattice are also included. The ferromagnetic tendency recently discovered in the LaMnO₃ and TbMnO₃ thin films is explained by considering the substrate stress. Finally, the relationship between our double-exchange model and a previously used J₁-J₂-J₃ model is further discussed from the perspective of spin wave excitations.     

Activity concentrations of238U and226Ra in agricultural samples

Distributions of²³⁸U and²²⁶Ra in agricultural samples and cultivated soils have been studied over ten years. The crops are rice, spinach and Chinese cabbage. Two investigated areas have been selected (35° 18 N, 113° 35 E). The agricultural samples and soils were collected annually from May 1982 through October 1991. The activity concentrations of²²⁶Ra in agricultural samples are greater than those of²³⁸U. The transfer factors of²³⁸U,²²⁶Ra are from 0.06·10^–3 to 1.2·10^–3. The²²⁶Ra/²³⁸U ratios for three agricultural samples have their characteristic values.     

Background levels of238U and226Ra in atmospheric aerosols

The²³⁸U and²²⁶Ra contents of small-volume aerosols are determined by a chemical analysis technique. Mean activity concentrations of²³⁸U and²²⁶Ra in aerosols over approximately ten years are 0.29·10^–5 and 0.93·10^–5 Bq/m³, respectively. The yearly variation of²³⁸U and²²⁶Ra in aerosols is small. The concentrations of²²⁶Ra are always larger than those of²³⁸U in the same sampling time. The correlation of²³⁸U and²²⁶Ra cannot be recogonized (r=0.18). The concentrations of summer samples are greater than those of winter samples for²³⁸U. One of the causes of seasonal difference may be due to the fact that the components of aerosols are different according to soil size, soil components, weathering states, etc.     

Giant cluster coexistence in doped manganites and other compounds

Computational studies of models for manganese oxides show the generation of large coexisting metallic and insulating clusters with equal electronic density, in agreement with the recently discovered micrometer-sized inhomogeneities in manganites. The clusters are induced by disorder on exchange and hopping amplitudes near first-order transitions of the nondisordered strongly coupled system. The random-field Ising model illustrates the qualitative aspects of our results. Percolative characteristics are natural in this context. The conclusions are general and apply to a variety of compounds.     

Investigation of stability of (Ge,Sn)-(S,or Se)42 cluster vibrational spectra

The insistent two mode type behavior of the A₁ and high frequency F₂ bands in the tin contained glasses is reproduced by our vibrational calculations based on the outrigger-raft with some modifications. High pressure Raman spectra suggest an instability of the tin tetrahedral bonding. Comparative study of glassy and crystalline Ge(Se, or S)₂ by high pressure Raman scattering gives important informations about the strain field and the topological order in the glasses.     

Effects of crosslinker density on the polymer network structure in poly‐N,N‐dimethylacrylamide hydrogels

To investigate the effects of crosslinker density on the properties of hydrogels, compression tests, scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and Raman measurements were performed on poly‐N,N‐dimethylacrylamide hydrogels. The results of the compression tests showed that the Young's modulus increases as the crosslinker density increases. To understand the mechanism of the change in the mechanical properties, the structures of the polymer networks and water and the molecular vibrations were analyzed using SEM, DSC, and Raman methods. From the SEM images, it was found that the porosity estimated from the mesh size and cell density increases with increasing crosslinker density. In addition, the DSC and Raman results show that the thickness of the bound water increases as the porosity increases, although the density of the polymer chains in the porous wall remains nearly constant. The increase in the number density of polymer chains can be one of the mechanisms contributing to the increase in the mechanical strength of the hydrogels at lower crosslinker density below 5 mol %, as proposed by previous studies. At higher crosslinker density, however, the number density of polymer chains does not increase with increasing crosslinker density. The present results suggest that the bound water plays an important role in strengthening the hydrogel. The water structure may be one of the dominant factors governing the chemical and physical properties of hydrogels. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013 , 51, 1017–1027     

Microscopic study of a spin-orbit-induced Mott insulator in Ir oxides

Motivated by recent experiments of a novel 5d Mott insulator in Sr2IrO4, we have studied the two-dimensional three-orbital Hubbard model with a spin-orbit coupling λ. The variational Monte Carlo method is used to obtain the ground state phase diagram with varying an on-site Coulomb interaction U as well as λ. It is found that the transition from a paramagnetic metal to an antiferromagnetic insulator occurs at a finite U=U(MI), which is greatly reduced by a large λ, characteristic of 5d electrons, and leads to the "spin-orbit-induced" Mott insulator. It is also found that the Hund's coupling induces the anisotropic spin exchange and stabilizes the in-plane antiferromagnetic order. We have further studied the one-particle excitations by using the variational cluster approximation and revealed the internal electronic structure of this novel Mott insulator. These findings are in agreement with experimental observations on Sr2IrO4.     

New family of three-dimensional topological insulators with antiperovskite structure

All known topological insulators are crystallographically related to either the noncentrosymmetric zinc-blende HgTe-type family or to the hexagonal centrosymmetric Bi2Se3 one. Through first-principles calculations, here we show evidence that under a proper uniaxial strain cubic ternary centrosymmetric antiperovskite compounds (M3N)Bi (M=Ca, Sr, and Ba) are three-dimensional topological insulators. This proposed family of materials is chemically inert and the lattice structure is well matched to important semiconductors, which provides a rich platform to easily integrate with electronic devices.     

Possible path to a new class of ferromagnetic and half-metallic ferromagnetic materials

We introduce a path to a possibly new class of magnetic materials whose properties are determined entirely by the presence of a low concentration of specific point defects. Using model Hamiltonian and ab initio band structure methods we demonstrate that even large band gap nonmagnetic materials as simple as CaO with a small concentration of Ca vacancies can exhibit extraordinary properties. We show that such defects will initially bind the introduced charge carriers at neighboring sites and depending on the internal symmetry of the clusters so formed, will exhibit "local" magnetic moments which for concentrations as low as 3% transform this nonmagnetic insulator into a half-metallic ferromagnet.