[Paper Review] Pressure-induced unusual metallic state in EuNiO$_3$

Hisao Kobayashi, Shugo Ikeda+8 more|arXiv (Cornell University)|May 18, 2015

Magnetic and transport properties of perovskites and related materials1 references3 citations

TL;DR

This study investigates pressure-induced quantum phase transitions in the antiferromagnetic insulator EuNiO₃ using synchrotron-based x-ray diffraction and ¹⁵¹Eu nuclear forward scattering. It identifies a pressure-driven insulator-to-metal transition at ~6 GPa and a magnetic quantum critical point at ~10.5 GPa, where an unusual antiferromagnetic metallic state emerges, exhibiting non-Fermi-liquid behavior (resistivity ∝ T¹.⁶) near the critical point before transitioning to a Fermi-liquid state above 15 GPa.

ABSTRACT

The perovskite antiferromagnetic ($T_{ m N}$ $\sim$ 220 K) insulator EuNiO$_3$ undergoes at ambient pressure a metal-to-insulator transition at $T_{ m MI}$ = 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 $T_{ m N}$ ($p$ $\leq$ 2 GPa) and the induced magnetic hyperfine field $B_{ m hf}$ at the $^{151}$Eu nucleus ($p$ $\leq$ 9.7 GPa), both $T_{ m N}$ and $B_{ m hf}$ are strongly reduced and finally disappear at $p_{ m c}$ $\cong$ 10.5 GPa, indicating a magnetic QCP at $p_{ m c}$. 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 $p_{ m IM}$ $\cong$ 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 $T^{ m n}$, 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.

Motivation & Objective

• To investigate the evolution of structural, magnetic, and electronic properties of EuNiO₃ under high pressure.
• To identify the presence and nature of a quantum critical point (QCP) in the antiferromagnetic insulator EuNiO₃.
• To determine whether charge disproportionation persists in the metallic phase under pressure.
• To analyze the electronic behavior near the QCP, distinguishing between non-Fermi-liquid and Fermi-liquid regimes.
• To construct a comprehensive (pressure, temperature) phase diagram for EuNiO₃ based on experimental data.

Proposed method

• Low-temperature synchrotron angle-resolved x-ray diffraction was used to probe structural changes and lattice parameters up to ~20 GPa.
• ¹⁵¹Eu nuclear forward scattering (NFS) measured the magnetic hyperfine field at Eu nuclei, serving as a local probe of Ni sublattice magnetism.
• The pressure dependence of the Néel temperature (T_N) and hyperfine field (B_hf) was extracted to identify the magnetic quantum critical point.
• Analysis of reported low-temperature electrical resistance data was performed using power-law fitting R ∝ T^n to classify non-Fermi-liquid (n ≈ 1.6) vs. Fermi-liquid (n = 2) behavior.
• Structural parameters were analyzed to detect symmetry changes or anomalies at the insulator-to-metal (IM) and quantum critical transitions.
• A (p, T) phase diagram was constructed based on combined x-ray, NFS, and resistivity data.

Experimental results

Research questions

• RQ1 Does the antiferromagnetic insulating ground state of EuNiO₃ persist under high pressure, and at what pressure does it collapse?
• RQ2 Is there evidence for a quantum critical point in EuNiO₃, and what is its pressure location?
• RQ3 What is the nature of the electronic state between the insulator-to-metal transition at ~6 GPa and the magnetic QCP at ~10.5 GPa?
• RQ4 How does the electronic behavior (resistivity vs. temperature) evolve across the quantum critical point, and what does this imply about the nature of the ground state?
• RQ5 Does charge disproportionation survive in the metallic phase under high pressure, as suggested by structural and electronic data?

Key findings

• The insulator-to-metal transition in EuNiO₃ occurs at p_IM ≈ 6 GPa, with no detectable structural anomaly in lattice parameters within experimental resolution.
• The Néel temperature T_N and magnetic hyperfine field B_hf at ¹⁵¹Eu nuclei decrease monotonically with pressure and vanish at p_c ≈ 10.5 GPa, indicating a magnetic quantum critical point.
• An antiferromagnetic metallic state exists between p_IM ≈ 6 GPa and p_c ≈ 10.5 GPa, as confirmed by the persistence of magnetic order in a metallic phase.
• Near the QCP (10.5 GPa ≤ p ≤ 14.8 GPa), the electrical resistivity follows a non-Fermi-liquid power law R ∝ T^n with n = 1.6.
• For pressures above ~15 GPa, the resistivity follows a Fermi-liquid behavior with n = 2, indicating a recovery of conventional quasiparticle behavior.
• The analysis suggests that charge disproportionation likely persists in the metallic phase, as no significant symmetry change or structural anomaly was detected at the IM transition.