Correlation effects in the iron pnictides

One of the central questions about the iron pnictides concerns the extent to which their electrons are strongly correlated. Here we address this issue through the phenomenology of the charge transport and dynamics, single-electron excitation spectrum, and magnetic ordering and dynamics. We outline the evidence that the parent compounds, while metallic, have electron interactions that are sufficiently strong to produce incipient Mott physics. In other words, in terms of the strength of electron correlations compared to the kinetic energy, the iron pnictides are closer to intermediately-coupled systems lying at the boundary between itinerancy and localization, such as V2O3 or Se-doped NiS2, rather than to simple antiferromagnetic metals like Cr. This level of electronic correlations produces a new small parameter for controlled theoretical analyses, namely the fraction of the single-electron spectral weight that lies in the coherent part of the excitation spectrum. Using this expansion parameter, we construct the effective low-energy Hamiltonian and discuss its implications for the magnetic order and magnetic quantum criticality. Finally, this approach sharpens the notion of magnetic frustration for such a metallic system, and brings about a multiband matrix t-J1-J2 model for the carrier-doped iron pnictides.Comment: 14 pages, 2 figures, discussions on several points expanded, published in the Focus Issue on Iron-Based Superconductor

Stabilizing the spiral order with spin-orbit coupling in an anisotropic triangular antiferromagnet

We study the effects of spin-orbit coupling (SOC) on the large-U Hubbard model on anisotropic triangular lattice at half-filling using the Schwinger-boson method. We find that the SOC will in general lead to a zero temperature condensation of the Schwinger bosons with a single condensation momentum. As a consequence, the spin-spin correlation vanishes along the z-axis but develops in the $x$-$y$ plane, with the ordering wave vector being dramatically dependent on the SOC. Moreover, the phase boundary of the magnetic ordered state extends to the region of large spatial anisotropy with increased condensation density, demonstrating that the spiral order is always stabilized by the SOC.Comment: 4 pages, 4figure

Band Narrowing and Mott Localization in Iron Oxychalcogenides La2O2Fe2O(Se,S)2

Bad metal properties have motivated a description of the parent iron pnictides as correlated metals on the verge of Mott localization. What has been unclear is whether interactions can push these and related compounds to the Mott insulating side of the phase diagram. Here we consider the iron oxychalcogenides La2O2Fe2O(Se,S)2, which contain an Fe square lattice with an expanded unit cell. We show theoretically that they contain enhanced correlation effects through band narrowing compared to LaOFeAs, and we provide experimental evidence that they are Mott insulators with moderate charge gaps. We also discuss the magnetic properties in terms of a Heisenberg model with frustrating J1-J2-J2' exchange interactions on a "doubled" checkerboard lattice.Comment: 4 pages, 5 eps figures. Version to appear in Phys. Rev. Let

Plant-level real-time monitoring data reveal substantial abatement potential of air pollution and CO₂ in China's cement sector

China is the world’s greatest cement producer, generating significant air pollution and CO2 emissions. To combat these impacts, China introduced stricter air pollution standards for the cement industry in 2015, yet no plant-level analysis exists to determine their effectiveness. To analyze the impacts of emission regulations, we coupled 2014–2018 smokestack-level real-time observations with plant-specific information and constructed an hourly based dataset of air pollutants (particulate matter [PM], sulfur dioxide [SO2], nitrogen oxide [NOX]) and CO2 emissions. Our analysis shows that regulations introduced in 2015 led to PM, SO2, and NOX reductions of 50.3%, 43.6%, and 34.2%, respectively, but CO2 increased by 5%. Interestingly, 9.4% of the plants analyzed reached China’s 2020 ultralow air pollution standards in 2018. Further analysis shows that if small and old plants are phased out and all remaining plants implement advanced equipment and improve fuels and energy efficiency, PM, SO2, NOX, and CO2 could be further reduced by 68.8%, 66.1%, 82.2%, and 62.0% by 2060. Our results reveal the co-benefits of clean air and climate policies for cement production

Two novel hierarchical homogeneous nanoarchitectures of TiO2 nanorods branched and P25-coated TiO2 nanotube arrays and their photocurrent performances

We report here for the first time the synthesis of two novel hierarchical homogeneous nanoarchitectures of TiO2 nanorods branched TiO2 nanotube arrays (BTs) and P25-coated TiO2 nanotube arrays (PCTs) using two-step method including electrochemical anodization and hydrothermal modification process. Then the photocurrent densities versus applied potentials of BTs, PCTs, and pure TiO2 nanotube arrays (TNTAs) were investigated as well. Interestingly, at -0.11 V and under the same illumination condition, the photocurrent densities of BTs and PCTs show more than 1.5 and 1 times higher than that of pure TNTAs, respectively, which can be mainly attributed to significant improvement of the light-absorbing and charge-harvesting efficiency resulting from both larger and rougher surface areas of BTs and PCTs. Furthermore, these dramatic improvements suggest that BTs and PCTs will achieve better photoelectric conversion efficiency and become the promising candidates for applications in DSSCs, sensors, and photocatalysis

Impurity Quantum Phase Transition in a Current-Carrying d-Wave Superconductor

We study an Anderson impurity embedded in a d-wave superconductor carrying a supercurrent. The low-energy impurity behavior is investigated by using the numerical renormalization group method developed for arbitrary electronic bath spectra. The results explicitly show that the local impurity state is completely screened upon the non-zero current intensity. The impurity quantum criticality is in accordance with the well-known Kosterlitz-Thouless transition.Comment: 6 pages, 8 figures, revised version with a derivation of the host Hamiltonian by a local gauge transformation and a finite size effect analysis added as two appendices, to appear in Phys. Rev.