157 research outputs found
Exponential localization of Wannier functions in insulators
No full textThe exponential localization of Wannier functions in two or three dimensions is proven for all insulators that display time-reversal symmetry, settling a long-standing conjecture. Our proof relies on the equivalence between the existence of analytic quasi-Bloch functions and the nullity of the Chern numbers (or of the Hall current) for the system under consideration. The same equivalence implies that Chern insulators cannot display exponentially localized Wannier functions. An explicit condition for the reality of the Wannier functions is identified
X-ray Optical Activity: Applications of Sum Rules
No full textEdge-selective sum rules are proposed for a variety of X-ray dichroisms related to natural or non- reciprocal optical activity. Four spherical operators are identified that mix orbitals of different parities in what is assumed to be the ground state. The orbital anapole moment W(1) is primarily responsible for the magne- tochiral dichroism; the time-even rank-2 tensor N(2) = [L, W](2) for natural circular dichroism; the time-odd rank-2 tensor W(2) = [L, n](2) for nonreciprocal magnetic linear dichroisms. At higher orders, the time-odd rank- 3 tensor G(3) = [L, L, W](3) can also contribute to all nonreciprocal dichroisms. The physical content of these operators is analyzed. For every magnetoelectric group, one can predict which dichroic effect can be measured with either a single crystal or a powdered sample. Experimental spectra are produced to illustrate the value of the sum rules and the practical conditions of their application. Regarding nonreciprocal activity, one should be
cautious about discussing magnetic symmetry because the deep core hole can couple the true ground state with low-lying excited states
Magnetic X-ray dichroism in the soft X-ray energy range: application to the study of magnetism in intermetallic and modulated (multilayers, superlattices) systems
No full text
4f orbital and spin magnetism in cerium intermetallic compounds studied by magnetic circular x-ray dichroism
No full text
Green function hierarchy for open shells.
No full textIn many systems containing open shell ions, the wave function of the ions cannot be described by a single Slater determinant. This invalidates standard many-body theory and requires the use of nonequilibrium quantum field theory with initial correlations. Quantum group techniques are used here to deal with this combinatorially complex theory. The hierarchy of interacting Green functions for general initial correlations is derived and the case of a system with two-particle correlations is treated in detail
Soil carbon sequestration and greenhouse gas fluxes in the eastern Corn Belt
No full textThe relative impact of different crop production systems on global warming is not completely documented. This study was conducted to enhance the current understanding of carbon sequestration and greenhouse gas sources/production/fluxes in the Corn Belt as affected by N source and crop rotation. Soil surface fluxes of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) were measured by static-chamber, soil organic matter (SOM) fractions to a soil depth of 15 cm were determined by physical (particulate organic matter, POM) and chemical (non-hydrolyzable C, NHC) fractionations, as well as dry matter (DM), and N accumulation and/or partitioning were assessed by harvest index (HI) during two consecutive growing seasons. Agricultural systems evaluated were continuous corn with either spring or fall liquid swine manure (CCSM and CCFM, respectively), continuous corn and corn phase of corn-soybean rotation with urea-ammonium nitrate (CCUAN and CSUAN, respectively), soybean phase of corn-soybean rotation (SC) and restored prairie grass (PG) grown in Drummer and Raub soil series. In addition, soil N2O production as influenced by redox potential as well as source-partitioning (nitrification vs. denitrification) by 15N-labeling were assessed in aerobic and anaerobic incubations, respectively, evaluating CCUAN and CCFM. Among corn treatments, CO2 and N2O contributed 86 and 14%, respectively, to global warming potential. Seasonal CO2 emissions did not differ among treatments, but N2O emissions were sharply impacted by treatment (Grouping of treatments: CCSM 8.2a, CCUAN 6.4ab, CSUAN 4.9bc, CCFM 3.3cd, SC 1.0de, PG 0.2e kg N2O – N ha -1) as they were driven by pulse emissions following N fertilizer inputs coinciding with major rainfall events. Both greater N2O production with increasing soil water content in manured soils and the source-partitioning results suggested that denitrification was the predominant pathway. Redox potential range for N2O production occurred at moderate reducing conditions (420 to 575 mV) and was not affected by N source. Partitioning of DM and N into corn grain was slightly enhanced (3-4%) by UAN and corn-soybean rotation, respectively. As an N management system, CCFM appears to mitigate global warming because it enhanced soil C, POM-C and NHC concentrations as well as mitigated N2O emissions at the soil surface. However, CCFM had the lowest productivity (9 Mg DM grain ha-1 yr -1). Timing of N application had a large impact on global warming mitigation in agricultural soils
- …
