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

Monopole-limited nucleation of magnetism in Eu2_{2}Ir2_{2}O7_{7}

We present an in-depth analysis of muon-spin spectroscopy measurements of Eu$_{2}$Ir$_{2}$O$_{7}$ under the effect of the Eu$_{1-x}$Bi$_{x}$ isovalent and diamagnetic substitution as well as of external pressure. Our results evidence an anomalously slow increase of the magnetic volume fraction upon decreasing temperature only for stoichiometric Eu$_{2}$Ir$_{2}$O$_{7}$, pointing towards highly unconventional properties of the magnetic phase developing therein. We argue that magnetism in Eu$_{2}$Ir$_{2}$O$_{7}$ develops based on the nucleation of magnetic droplets at $T_{N}$, whose successive growth is limited by the need of a continuous generation of magnetic hedgehog monopoles.Comment: 9 pages, 4 figure

Research Data Supporting: Ultrafast melting and recovery of collective order in the excitonic insulator Ta2NiSe5

This data includes that which is shown in the main text figures 1-4 of the associated publication. We have used pump-probe measurements to investigate the ordered phase in excitonic insulator candidate, Ta2NiSe5. The measurements are performed with a single pump, and two pump configuration (also sometimes called pump-push-probe). While all the data shown is taken in reflection, we did similar measurements also in a transmission configuration

Ultrafast melting and recovery of collective order in the excitonic insulator Ta2NiSe5

AbstractThe layered chalcogenide Ta2NiSe5 has been proposed to host an excitonic condensate in its ground state, a phase that could offer a unique platform to study and manipulate many-body states at room temperature. However, identifying the dominant microscopic contribution to the observed spontaneous symmetry breaking remains challenging, perpetuating the debate over the ground state properties. Here, using broadband ultrafast spectroscopy we investigate the out-of-equilibrium dynamics of Ta2NiSe5 and demonstrate that the transient reflectivity in the near-infrared range is connected to the system’s low-energy physics. We track the status of the ordered phase using this optical signature, establishing that high-fluence photoexcitations can suppress this order. From the sub-50 fs quenching timescale and the behaviour of the photoinduced coherent phonon modes, we conclude that electronic correlations provide a decisive contribution to the excitonic order formation. Our results pave the way towards the ultrafast control of an exciton condensate at room temperature.We are also grateful to the Engineering and Physical Science Research Council (EPSRC) and the Winton Programme for the Physics of Sustainability for funding. We acknowledge the financial support from the Department of Science and Technology (DST), India [Grant No. SR/WOS-A/PM-33/2018 (G)] and IISER Pune for providing the facilities for crystal growth and characterization. We acknowledge funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (Grant Agreement 758826). We thank the Department of Science and Technology, India for support under Nanoemission and Year of Science Professorship

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