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Register a new userPressure-induced unusual metallic state in EuNiO$_3$
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The perovskite antiferromagnetic ($T_{rm N}$ $sim$ 220 K) insulator EuNiO$_3$ undergoes at ambient pressure a metal-to-insulator transition at $T_{rm MI}$ = 460 K which is associated with a simultaneous orthorhombic-to-monoclinic distortion, leading to charge disproportionation. We have investigated the change of the structural and magnetic properties of EuNiO$_3$ with pressure (up to $sim$ 20 GPa) across its quantum critical point (QCP) using low-temperature synchrotron angle-resolved x-ray diffraction and $^{151}$Eu nuclear forward scattering of synchrotron radiation, respectively. With increasing pressure we find that after a small increase of $T_{rm N}$ ($p$ $leq$ 2 GPa) and the induced magnetic hyperfine field $B_{rm hf}$ at the $^{151}$Eu nucleus ($p$ $leq$ 9.7 GPa), both $T_{rm N}$ and $B_{rm hf}$ are strongly reduced and finally disappear at $p_{rm c}$ $cong$ 10.5 GPa, indicating a magnetic QCP at $p_{rm c}$. The analysis of the structural parameters up to 10.5 GPa reveals no change of the lattice symmetry within the experimental resolution. Since the pressure-induced insulator-to-metal transition occurs at $p_{rm IM}$ $cong$ 6 GPa, this result implies the existence of an antiferromagnetic metallic state between 6 and 10.5 GPa. We further show from the analysis of the reported high pressure electrical resistance data on EuNiO$_3$ at low-temperatures that in the vicinity of the QCP the system behaves as non-Fermi-liquid, with the resistance changing as $T^{rm n}$, with n=1.6, whereas it becomes a normal Fermi-liquid, n = 2, for pressures above $sim$15 GPa. On the basis of the obtained data a magnetic phase diagram in the ($p$, $T$) space is suggested.
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A complex interplay of different energy scales involving Coulomb repulsion, spin-orbit coupling and Hunds coupling energy in two-dimensional (2D) van der Waals (vdW) material produces novel emerging physical state. For instance, ferromagnetism in vdW charge transfer insulator CrGeTe$_3$, that provides a promising platform to simultaneously manipulate the magnetic and electrical properties for potential device implementation using few layers thick materials. Here, we show a continuous tuning of magnetic and electrical properties of CrGeTe$_3$ single crystal using pressure. With application of pressure, CrGeTe$_3$ transforms from a FM insulator with Curie temperature, $T_{rm{C}} sim $ 66 K at ambient condition to a correlated 2D Fermi metal with $T_{rm{C}}$ exceeding $sim$ 250 K. Notably, absence of an accompanying structural distortion across the insulator-metal transition (IMT) suggests that the pressure induced modification of electronic ground states are driven by electronic correlation furnishing a rare example of bandwidth-controlled IMT in a vdW material.
The charge transfer antiferromagnetic (T$_{N}$ =220 K) insulator EuNiO$_{3}$ undergoes, at ambient pressure, a temperature-induced metal insulator MI transition at T$_{MI}$=463 K. We have investigated the effect of pressure (up to p~20 GPa) on the electronic, magnetic and structural properties of EuNiO$_{3}$ using electrical resistance measurements, ${151}^$Eu nuclear resonance scattering of synchrotron radiation and x-ray diffraction, respectively. With increasing pressure we find at p$_{c}$ =5.8 GPa a transition from the insulating state to a metallic state, while the orthorhombic structure remains unchanged up to 20 GPa. The results are explained in terms of a gradual increase of the electronic bandwidth with increasing pressure, which results in a closing of the charge transfer gap. It is further shown that the pressure-induced metallic state exhibits magnetic order with a lowervalue of T$_{N}$ (T$_{N}$ ~120 K at 9.4 GPa) which disappears between 9.4 and 14.4 GPa.
LiOsO$_3$ has been recently identified as the first unambiguous ferroelectric metal, experimentally realizing a prediction from 1965 by Anderson and Blount. In this work, we investigate the metallic state in LiOsO$_3$ by means of infrared spectroscopy supplemented by Density Functional Theory and Dynamical Mean Field Theory calculations. Our measurements and theoretical calculations clearly show that LiOsO$_3$ is a very bad metal with a small quasiparticle weight, close to a Mott-Hubbard localization transition. The agreement between experiments and theory allows us to ascribe all the relevant features in the optical conductivity to strong electron-electron correlations within the $t_{2g}$ manifold of the osmium atoms.
We study the electronic structure, magnetic state, and phase stability of paramagnetic BiNiO$_3$ near a pressure-induced Mott insulator-to-metal transition (MIT) by employing a combination of density functional and dynamical mean-field theory. We obtain that BiNiO$_3$ exhibits an anomalous negative-charge-transfer insulating state, characterized by charge disproportionation of the Bi $6s$ states, with Ni$^{2+}$ ions. Upon a compression of the lattice volume by $sim$4.8%, BiNiO$_3$ is found to make a Mott MIT, accompanied by the change of crystal structure from triclinic $Pbar{1}$ to orthorhombic $Pbnm$. The pressure-induced MIT is associated with the melting of charge disproportionation of the Bi ions, caused by a charge transfer between the Bi $6s$ and O $2p$ states. The Ni sites remain to be Ni$^{2+}$ across the MIT, which is incompatible with the valence-skipping Ni$^{2+}$/Ni$^{3+}$ model. Our results suggest that the pressure-induced change of the crystal structure drives the MIT in BiNiO$_3$.
Hyperhoneycomb iridate $beta$-Li$_2$IrO$_3$ is a three-dimensional analogue of two-dimensional honeycomb iridates, such as $alpha$-Li$_2$IrO$_3$, which recently appeared as another playground for the physics of Kitaev-type spin liquid. $beta$-Li$_2$IrO$_3$ shows a non-collinear spiral ordering of spin-orbital-entangled $J_{rm eff}$ = 1/2 moments at low temperature, which is known to be suppressed under a pressure of $sim$2 GPa. With further increase of pressure, a structural transition is observed at $P_{rm S}$ $sim$ 4 GPa at room temperature. Using the neutron powder diffraction technique, the crystal structure in the high-pressure phase of $beta$-Li$_2$IrO$_3$ above $P_{rm S}$ was refined, which indicates the formation of Ir$_2$ dimers on the zig-zag chains, with the Ir-Ir distance even shorter than that of metallic Ir. We argue that the strong dimerization stabilizes the bonding molecular orbital state comprising the two local $d_{zx}$-orbitals on the Ir-O$_2$-Ir bond plane, which conflicts with the equal superposition of $d_{xy}$-, $d_{yz}$- and $d_{zx}$- orbitals in the $J_{rm eff}$ = 1/2 wave function produced by strong spin-orbit coupling. The results of resonant inelastic x-ray scattering (RIXS) measurements and the electronic structure calculations are fully consistent with the collapse of the $J_{rm eff}$ = 1/2 state. A subtle competition of various electronic phases is universal in honeycomb-based Kitaev materials.
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