Dilute stuffing in the pyrochlore iridate Eu2Ir2O7Eu_2Ir_2O_7

The pyrochlore Eu$_2$Ir$_2$O$_7$ has recently attracted significant attention as a candidate Weyl semimetal. The previous reports on this compound unanimously show a thermally induced metal to insulator (MI) transition, concomitant with antiferromagnetic (AFM) long-range ordering of the Ir-moments below T$_\textit{N} \sim$120 K. However, there are contradictory reports concerning the slope d$\rho/$dT of the resistivity plots ($\rho$) in the "metallic" state above the metal-insulator (MI) transition, and the value of $\rho$ in the insulating state, both of which show significant sample dependence. Here, we explore this issue by investigating six different Eu$_2$Ir$_2$O$_7$ samples with slightly varying Eu:Ir ratio. High-resolution synchrotron powder diffraction are done to probe minor variations in the cell parameters of the various Eu$_2$Ir$_2$O$_7$ samples investigated here. Specific heat (C$_p$) and magnetic susceptibility of all the samples showed long-range antiferromagnetic ordering upon cooling below T$_\textit{N} \sim$120 K. The transitions are, however, found to be smeared out for the off-stoichiometric samples. We show that the sign of d$\rho/$dT above the metal-insulator (MI) transition is highly sensitive to the unit cell length, which, in turn, depends on the level of Eu-stuffing at the Ir-site. Samples with composition close to the ideal stoichiometry (Eu : Ir $=$ 1) showed a change of sign of d$\rho/$dT from negative to positive upon cooling below a certain temperature T $^*$ $>$ T$_\textit{MI}$. With increasing Eu-stuffing T$^*$ decreased until a negative d$\rho/$dT persisted without any sign change down to T$_\textit{MI}$.Comment: 12 pages, 7 figure

Anomalous lattice contraction and emergent electronic phases in Bi-doped Eu2_2Ir2_2O7_7

We study the pyrochlore series (Eu$_{1-x}$Bi$_x$)$_2$Ir$_2$O$_7$ for $0 \leq x \leq 1$. We show that for small $x$, the lattice undergoes an anomalous contraction but the all-in/all-out and metal-to-insulator transitions remain robust, and the resistivity approaches a $1/T$ dependence at low-T, suggesting proximity to the Weyl semimetallic phase, as previously predicted theoretically. At the boundary between Eu$_2$Ir$_2$O$_7$ and Bi$_2$Ir$_2$O$_7$ a qualitatively different ground state emerges, which is characterized by its unusual metallic behavior and absence of magnetic ordering at least down to $0.02$ K.Comment: 5 Pages, 4 figure

Technology and Materials for Passive Manipulation of the Solar Spectrum in Greenhouses

Greenhouse horticulture grows increasingly important due to its ability to provide a controlled microclimate which is optimizable for highly efficient crop growth and resource use, although it may come at a significant energy and investment cost. One of the most crucial inputs in any greenhouse is sunlight, giving free energy and light for greenhouse crop growth. However, it is enormously variable, both geographically and seasonally. This review discusses materials and technologies usable in greenhouse cover and screen materials which can passively manipulate the incident sunlight to transmit a light spectrum that is ideal for crop growth, thereby improving the yield, and for greenhouse microclimate management, thereby reducing the energy usage of greenhouses. The current status of spectrum-manipulating technology in greenhouses, developments over the last few years, some potential innovations adaptable from diverse fields to greenhouse horticulture, and the associated challenges, are discussed

X-ray photoemission and absorption study of the pyrochlore iridates (Eu1−xBix)2Ir2O7,0⩽x⩽1(Eu_{1−x}Bi_x)_2Ir_2O_7, 0 ⩽ x ⩽ 1

The pyrochlore iridates (Eu$_{1-x}$Bi$_x$)$_2$Ir$_2$O$_7$ (0 $\leqslant$ x $\leqslant$ 1) undergo an anomalous negative lattice expansion for small Bi-doping ($x \leqslant 0.035$) (region I) and a normal lattice expansion for $x \geqslant0.1$ (region II); this is accompanied by a transition from an insulating (and magnetically ordered) to a metallic (and with no magnetic ordering) ground state. Here, we investigate (Eu$_{1-x}$Bix)$_2$Ir$_2$O$_7$ (0 $\leqslant$ x $\leqslant$ 1) using hard x-ray photoemission spectroscopy and x-ray absorption fine structure (XAFS) spectroscopy. By analyzing the Eu-L$_3$, Ir-L$_3$ and Bi-L$_2$ & L$_3$ edges x-ray absorption near edge structure spectra and Eu-3d core-level XPS spectra, we show that the metal cations retain their nominal valence, namely, Ir$^{4+}$, Bi$^{3+}$ and Eu$^{3+}$, respectively, throughout the series. The Ir-4f and Bi-4f core-level XPS spectra consist of screened and unscreened doublets. The unscreened component is dominant In the insulating range ($x\leqslant\,0.035)$, and in the metallic region ($x\geqslant\,0.1$), the screened component dominates the spectra. The Eu-3d core-level spectra remain invariant under Bi doping. The extended XAFS data show that the coordination around the Ir remains well preserved throughout the series. The evolution of the valence band spectra near the Fermi energy with increasing Bi doping indicates the presence of strong Ir(5d)–Bi(6p) hybridization which drives the metal-to-insulator transition

Role of spin-phonon and electron-phonon interactions in the phonon renormalization of (Eu1−x_{1−x}Bix_x)2_2 Ir2_2O7_7 across the metal-insulator phase transition: Temperature-dependent Raman and x-ray studies

We report temperature-dependent Raman scattering and x-ray diffraction studies of pyrochlore iridates (Eu$_{1−x}$Bi$_x$)$_2$ Ir$_2$O$_7$, for x=0, 0.02, 0.035, 0.05, and 0.1. The temperature variation in Raman experiments spans from 4 to 300 K, covering the metal-insulator phase transition accompanied by paramagnetic–to–all-in/all-out (AIAO) spin ordering (T$_N$). These systems also show a Weyl semimetal (WSM) phase at low temperatures (<∼50K). The Ir-O-Ir bond bending mode A$_{1g}$ (510 cm$^{−1}$) shows anomalous softening (for x=0.0, 0.02, 0.035, and 0.05) in the magnetically ordered AIAO state, arising primarily from the spin-phonon interaction due to the phonon modulation of the Dzyaloshinskii-Moriya spin-exchange interaction. The two stretching modes T$^{1}_{2g}$ (307 cm$^{−1}$) and T$^{2}_{2g}$ (382 cm$^{−1}$) harden significantly in the magnetic insulating phase. The T$_{2g}$ phonons (for x=0.0, 0.02, 0.035, and 0.05) also show anomalous temperature dependence of their mode frequencies above T$_N$ due to strong electron-phonon coupling. The signatures of the WSM state are observed clearly in phonon renormalization <50K (in x=0.02) due to strong electron-phonon interaction. Our experimental results establish strong magneto-elastic coupling below T$_N$ and significant electron-phonon interactions in the metallic phase above T$_N$ as well as in the low-temperature WSM state

Ultrafast Demagnetization Control in Magnetophotonic Surface Crystals

Magnetic memory combining plasmonics and magnetism is poised to dramatically increase the bit density and energy efficiency of light-assisted ultrafast magnetic storage, thanks to nanoplasmon-driven enhancement and confinement of light. Here we devise a new path for that, simultaneously enabling light driven bit downscaling, reduction of the required energy for magnetic memory writing, and a subtle control over the degree of demagnetization in a magnetophotonic surface crystal. It features a regular array of truncated-nanocone-shaped Au-TbCo antennas showing both localized plasmon and surface lattice resonance modes. The ultrafast magnetization dynamics of the nanoantennas show a 3-fold resonant enhancement of the demagnetization efficiency. The degree of demagnetization is further tuned by activating surface lattice modes. This reveals a platform where ultrafast demagnetization is localized at the nanoscale and its extent can be controlled at will, rendering it multistate and potentially opening up so-far-unforeseen nanomagnetic neuromorphic-like systems operating at femtosecond time scales controlled by light

Ultrafast demagnetization in a ferrimagnet under electromagnetic field funneling

The quest to improve the density, speed and energy efficiency of magnetic memory storage has led to the exploration of new ways of optically manipulating magnetism at the ultrafast time scale, in particular in ferrimagnetic alloys. While all-optical magnetization switching is well-established on the femtosecond timescale, lateral nanoscale confinement and thus the potential significant reduction of the size of the magnetic element remains an outstanding challenge. Here we employ resonant electromagnetic energy funneling through plasmon nanoantennas to influence the demagnetization dynamics of a ferrimagnetic TbCo alloy thin film. We demonstrate how Ag nanoring-shaped antennas under resonant optical femtosecond pumping reduce the overall demagnetization in the underlying films up to three times compared to non-resonant illumination. We attribute such a substantial reduction to the nanoscale confinement of the demagnetization process. This is qualitatively supported by the electromagnetic simulations that strongly evidence the resonant optical energy-funneling to the nanoscale from the nanoantennas into the ferrimagnetic film. This observation is an important step for reaching deterministic ultrafast all-optical magnetization switching at the nanoscale in such systems, opening a route to develop nanoscale ultrafast magneto-optics

Magnetic and all-optical switching properties of amorphous TbxCo100-x alloys

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