Double exchange magnets: Spin-dynamics in the paramagnetic phase

The electronic structure of perovskite manganese oxides is investigated in terms of a Kondo lattice model with ferromagnetic Hund coupling and antiferromagnetic exchange between $t_{2g}$-spins using a finite temperature diagonalization technique. Results for the dynamic structure factor are consistent with recent neutron scattering experiments for the bilayer manganite La$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$ . The susceptibility shows Curie-Weiss behaviour and is used to derive a phase diagram. In the paramagnetic phase carriers are characterized as ferromagnetic polarons in an antiferromagnetic spin liquid.Comment: Revtex, 4 pages with 5 postscript figures include

Relation between crystal and magnetic structures of the layered manganites La2-2xSr1+2xMn2O7 (0.30 =< x =< 0.50)

Comprehensive neutron-powder diffraction and Rietveld analyses were carried out to clarify the relation between the crystal and magnetic structures of La2-2xSr1+2xMn2O7 (0.30 =< x =< 0.50). The Jahn-Teller (JT) distortion of Mn-O6 octahedra, i.e., the ratio of the averaged apical Mn-O bond length to the equatorial Mn-O bond length, is Delta_JT=1.042(5) at x=0.30, where the magnetic easy-axis at low temperature is parallel to the c axis. As the JT distortion becomes suppressed with increasing x, a planar ferromagnetic structure appears at x =< 0.32, which is followed by a canted antiferromagnetic (AFM) structure at x =< 0.39. The canting angle between neighboring planes continuously increases from 0 deg (planar ferromagnet: 0.32 =< x < 0.39) to 180 deg (A-type AFM: x=0.48 where Delta_JT=1.013(5)). Dominance of the A-type AF structure with decrease of JT distortion can be ascribed to the change in the eg orbital state from d3z^2-r^2 to dx^2-y^2

Global and regional effects of the photochemistry of CH_3O_2NO_2: evidence from ARCTAS

Using measurements from the NASA Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) experiment, we show that methyl peroxy nitrate (CH_3O_2NO_2) is present in concentrations of ~5–15 pptv in the springtime arctic upper troposphere. We investigate the regional and global effects of CH_3O_2NO_2 by including its chemistry in the GEOS-Chem 3-D global chemical transport model. We find that at temperatures below 240 K inclusion of CH_3O_2NO_2 chemistry results in decreases of up to ~20 % in NO_x, ~20 % in N_2O_5, ~5 % in HNO3, ~2 % in ozone, and increases in methyl hydrogen peroxide of up to ~14 %. Larger changes are observed in biomass burning plumes lofted to high altitude. Additionally, by sequestering NO_x at low temperatures, CH_3O_2NO_2 decreases the cycling of HO_2 to OH, resulting in a larger upper tropospheric HO_2 to OH ratio. These results may impact some estimates of lightning NO_x sources as well as help explain differences between models and measurements of upper tropospheric composition

Survey and Digital Documentation of Endangered Temple Wall Paintings in Shanxi Province, China

Shanxi Province is at the heart of China, and home to some of its richest architectural heritage. Covering an area of 156,000 square kilometres the Province is larger than England and Wales combined. Many earthen and timber buildings and temples contain wall paintings, witnessing Chinese folk religion, Buddhist and Daoist beliefs. Scattered over a large geographic area these remote village temples present a fairyland of Chinese traditional folklore. Many of these sites are endangered, and most are unrecorded. Funded by Arcadia, a charitable trust fund of Lisbet Rausing and Peter Baldwin, the Shanxi Digital Documentation of Endangered Temple Wall Painting Project (SDDP) is a four-year programme (2018–2021) aimed at recording these historic temples and wall paintings using high-resolution photographic and three-dimensional photogrammetric techniques, and also including selective capture of multi-spectral imagery. These records will form an open-access digital archive of temple paintings and associated architecture, structured by a Chinese-English bilingual database. The SDDP is a partnership between Zhejiang University, Shanxi Institute of Architecture Conservation and University College London (represented by two research centres within the Institute of Archaeology: the International Centre for Chinese Heritage and Archaeology and the Centre for Applied Archaeology). The programme of survey and research is guided by the Shanxi Provincial Bureau of Cultural Heritage (SBCH) and an international advisory board

The production and persistence of ΣRONO2 in the Mexico City plume

Alkyl and multifunctional nitrates (RONO2, ΣANs) have been observed to be a significant fraction of NOy in a number of different chemical regimes. Their formation is an important free radical chain termination step ending production of ozone and possibly affecting formation of secondary organic aerosol. ΣANs also represent a potentially large, unmeasured contribution to OH reactivity and are a major pathway for the removal of nitrogen oxides from the atmosphere. Numerous studies have investigated the role of nitrate formation from biogenic compounds and in the remote atmosphere. Less attention has been paid to the role ΣANs may play in the complex mixtures of hydrocarbons typical of urban settings. Measurements of total alkyl and multifunctional nitrates, NO2, total peroxy nitrates (ΣPNs), HNO3 and a representative suite of hydrocarbons were obtained from the NASA DC-8 aircraft during spring of 2006 in and around Mexico City and the Gulf of Mexico. ΣANs were observed to be 10–20% of NOy in the Mexico City plume and to increase in importance with increased photochemical age. We describe three conclusions: (1) Correlations of ΣANs with odd-oxygen (Ox) indicate a stronger role for ΣANs in the photochemistry of Mexico City than is expected based on currently accepted photochemical mechanisms, (2) ΣAN formation suppresses peak ozone production rates by as much as 40% in the near-field of Mexico City and (3) ΣANs play a significant role in the export of NOy from Mexico City to the Gulf Region

The Skyrme model predictions for the 27J=3/2{\bf 27}_{J=3/2} mass spectrum and the 273/2{\bf 27}_{3/2}-10ˉ\bar{\bf 10} mass splittings

The ${\bf 27}_{J=3/2}$-plet mass spectrum and the ${\bf 27}_{3/2}$-$\bar{\bf 10}$ mass splittings are computed in the framework of the minimal SU(3)$_f$ extended Skyrme model. As functions of the Skyrme charge $e$ and the SU(3)$_f$ symmetry breaking parameters the predictions are presented in tabular form. The predicted mass splitting ${\bf 27}_{3/2}$-$\bar{\bf 10}$ is the smallest among all SU(3)$_f$ baryonic multiplets.Comment: 4 pages, 2 tables, version to appear in Phys. Rev.

Optical Studies of a Layered Manganite La_{1.2}Sr_{1.8}Mn_2O_7 : Polaron Correlation Effect

Optical conductivity spectra of a cleaved ab-plane of a La_{1.2}Sr_{1.8}Mn_2O_7 single crystal exhibit a small polaron absorption band in the mid-infrared region at overall temperatures. With decreasing temperature (T) to Curie temperature (T_C), the center frequency of the small polaron band moves to a higher frequency, resulting in a gap-like feature, and that it collapses to a lower frequency below T_C. Interestingly, with decreasing T, the stretching phonon mode hardens above T_C and softens below T_C. These concurring changes of lattice and electronic structure indicate that short range polaron correlation exist above T_C but disappear with a magnetic ordering.Comment: 4 pages including 5 figures. submitted to Phys. Rev.

Doping dependence of the exchange energies in bilayer manganites: Role of orbital degrees of freedom

Recently, an intriguing doping dependence of the exchange energies in the bilayer manganites $La_{2-2x}Sr_{1+2x}Mn_2O_7$ has been observed in the neutron scattering experiments. The intra-layer exchange only weakly changed with doping while the inter-layer one drastically decreased. Here we propose a theory which accounts for these experimental findings. We argue, that the observed striking doping dependence of the exchange energies can be attributed to the evaluation of the orbital level splitting with doping. The latter is handled by the interplay between Jahn-Teller effect (supporting an axial orbital) and the orbital anisotropy of the electronic band in the bilayer structure (promoting an in-plane orbital), which is monitored by the Coulomb repulsion. The presented theory, while being a mean-field type, describes well the experimental data and also gives the estimates of the several interesting energy scales involved in the problem.Comment: Added references, corrected typos. To appear in Phys. Rev.