Exchange interactions and magnetic phases of transition metal oxides: benchmarking advanced ab initio methods

The magnetic properties of the transition metal monoxides MnO and NiO are investigated at equilibrium and under pressure via several advanced first-principles methods coupled with Heisenberg Hamiltonian MonteCarlo. The comparative first-principles analysis involves two promising beyond-local density functionals approaches, namely the hybrid density functional theory and the recently developed variational pseudo-self-interaction correction method, implemented with both plane-wave and atomic-orbital basis sets. The advanced functionals deliver a very satisfying rendition, curing the main drawbacks of the local functionals and improving over many other previous theoretical predictions. Furthermore, and most importantly, they convincingly demonstrate a degree of internal consistency, despite differences emerging due to methodological details (e.g. plane waves vs. atomic orbitals

Ammonium recovery from agro-industrial digestate using bioelectrochemical systems

Abstract Growing food and biomass production at the global scale has determined a corresponding increase in the demand for and use of nutrients. In this study, the possibility of recovering nitrogen from agro-industrial digestate using bioelectrochemical systems was investigated: two microbial electrolysis cells (MECs) were fed with synthetic and real digestate (2.5 gNH4+-N L−1). Carbon felt and granular graphite were used as anodes in MEC-1 and MEC-2, respectively. As to synthetic wastewater, the optimal nitrogen load (NL) for MEC-1 and -2 was 1.25 and 0.75 gNH4+-N d−1, respectively. MEC-1 showed better performance in terms of NH4+-N removal efficiency (39 ± 2.5%) and recovery rate (up to 70 gNH4+-N m−2d−1), compared to MEC-2 (33 ± 4.7% and up to 30 gN m−2d−1, respectively). At the optimal hydraulic retention time, lower NH4+-N removal efficiencies and recovery rates were observed when real digestate was fed to MEC-1 (29 ± 6.6% and 60 ± 13 gNH4+-N m−2d−1, respectively) and MEC-2 (21 ± 7.9% and 10 ± 3.6 gNH4+-N m−2d−1, respectively), likely due to the higher complexity of the influent. The average energy requirements were 3.6–3.7 kWh kgNremoved−1, comparable with values previously reported in the literature and lower than conventional ammonia recovery processes. Results are promising and may reduce the need for costly and polluting processes for nitrogen synthesis

Fermi-surface pockets in magnetic underdoped cuprates from first principles

Using an innovative first-principles band theory enabling the exploration of Mott-insulating magnetic cuprates, we study the Fermi surface of underdoped Y$_{1-x}$Ca$_x$Ba$_2$Cu$_3$O$_6$ in a selection of magnetically ordered and polaronic states. All phases exhibit qualitatively similar, hole-like nodal-point small pockets. Their properties (area, masses, mass sign) only partially match those extracted from recent quantum-oscillation experiments. Ab initio calculations, therefore, do not straightforwardly support a magnetic origin of quantum oscillations.Comment: 4 pages, 4 figure

Diazotroph Activity in Surface Narragansett Bay Sediments in Summer is Stimulated by Hypoxia and Organic Matter Delivery

Bacteria that carry out many processes of the nitrogen cycle inhabit estuarine sediments. Denitrification is known to be a dominant process causing estuarine sediments to behave as net nitrogen sinks. However, measurements of nitrogen fluxes in the sediments of Narragansett Bay, Rhode Island, USA, have at times revealed high rates of net nitrogen (N2) fixation. Whereas changes in primary production, in magnitude and phenology, within Narragansett Bay have been identified as possible causes for these changes in nitrogen cycling within the benthos, a factor that has not been examined thus far is seasonal hypoxia. Since anaerobic diazotrophs figure so prominently within the sediments of Narragansett Bay, we hypothesized that dissolved oxygen concentrations in the bottom waters affect their activity. In order to explore this relationship, we measured the activity of diazotrophs in the surface sediments of 3 study areas during the summers of 2013 and 2014 using the acetylene reduction assay. We explored the effects of several water quality parameters on nitrogenase activity including, among others, dissolved oxygen and chlorophyll concentrations. Our measurements of nitrogenase activity were generally low, ranging between 2 and 5 nmol ethylene g-1 d-1 but spiked to 16 nmol ethylene g-1 d-1 at an area experiencing severe hypoxia in July 2013. Our data suggest that diazotrophy in estuarine sediments is enhanced when the benthos experiences very low dissolved oxygen in conjunction with recent influxes of autochthonous organic matter. Experiments with sediment core incubations conducted in the laboratory support our hypothesis that low dissolved oxygen and organic matter additions promote N2 fixation

A variational pseudo-self-interaction correction approach: ab-initio description of correlated oxides and molecules

We present a fully variational generalization of the pseudo self-interaction correction (VPSIC) approach previously presented in two implementations based on plane-waves and atomic orbital basis set, known as PSIC and ASIC, respectively. The new method is essentially equivalent to the previous version for what concern the electronic properties, but it can be exploited to calculate total-energy derived properties as well, such as forces and structural optimization. We apply the method to a variety of test cases including both non-magnetic and magnetic correlated oxides and molecules, showing a generally good accuracy in the description of both structural and electronic properties.Comment: 23 pages, 9 tables, 16 figure

Evidence for the weakly coupled electron mechanism in an Anderson-Blount polar metal

Over 50 years ago, Anderson and Blount proposed that ferroelectric-like structural phase transitions may occur in metals, despite the expected screening of the Coulomb interactions that often drive polar transitions. Recently, theoretical treatments have suggested that such transitions require the itinerant electrons be decoupled from the soft transverse optical phonons responsible for polar order. However, this decoupled electron mechanism (DEM) has yet to be experimentally observed. Here we utilize ultrafast spectroscopy to uncover evidence of the DEM in LiOsO_3, the first known band metal to undergo a thermally driven polar phase transition (T_c ≈ 140 K). We demonstrate that intra-band photo-carriers relax by selectively coupling to only a subset of the phonon spectrum, leaving as much as 60% of the lattice heat capacity decoupled. This decoupled heat capacity is shown to be consistent with a previously undetected and partially displacive TO polar mode, indicating the DEM in LiOsO_3

Evidence for an extended critical fluctuation region above the polar ordering transition in LiOsO₃

Metallic Li Os O 3 undergoes a continuous ferroelectric-like structural phase transition below T c = 140 K to realize a polar metal. To understand the microscopic interactions that drive this transition, we study its critical behavior above T c via electromechanical coupling—distortions of the lattice induced by short-range dipole-dipole correlations arising from Li off-center displacements. By mapping the full angular distribution of second harmonic electric-quadrupole radiation from Li Os O 3 and performing a simplified hyper-polarizable bond model analysis, we uncover subtle symmetry-preserving lattice distortions over a broad temperature range extending from T c up to around 230 K, characterized by nonuniform changes in the short and long Li-O bond lengths. Such an extended region of critical fluctuations may explain anomalous features reported in specific heat and Raman scattering data and suggests the presence of competing interactions that are not accounted for in existing theoretical treatments. More broadly, our results showcase how electromechanical effects serve as a probe of critical behavior near inversion symmetry-breaking transitions in metals