Emergent Ferromagnetism with Fermi-Liquid Behavior in Proton Intercalated ${\mathrm{CaRuO}}_{3}$

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Emergent Ferromagnetism with Fermi-Liquid Behavior in Proton Intercalated ${CaRuO}_{3}$

Shengchun Shen, Zhuolu Li, Zijun Tian, Weidong Luo, Satoshi Okamoto, and Pu Yu

Phys. Rev. X 11, 021018 – Published 21 April 2021

Abstract

The evolution between Fermi-liquid and non-Fermi-liquid states in correlated electron systems has been a central subject in condensed matter physics because of the coupled intriguing magnetic and electronic states. An effective pathway to explore the nature of non-Fermi-liquid behavior is to approach its phase boundary. Here we report a crossover from non-Fermi-liquid to Fermi-liquid state in metallic ${CaRuO}_{3}$ through ionic liquid gating induced protonation with electric field. This electronic transition subsequently triggers a reversible magnetic transition with the emergence of an exotic ferromagnetic state from this paramagnetic compound. Our theoretical analysis reveals that hydrogen incorporation plays a critical role in both the electronic and magnetic phase transitions via structural distortion and electron doping. These observations not only help understand the correlated magnetic and electronic transitions in perovskite ruthenate systems, but also provide novel pathways to design electronic phases in correlated materials.

Received 9 September 2020
Revised 9 March 2021
Accepted 22 March 2021

DOI:https://doi.org/10.1103/PhysRevX.11.021018

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Magnetic phase transitions

Thin films Transition metal oxides

Density functional theory Dynamical mean field theory Hall bar Resistivity measurements X-ray diffraction

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Shengchun Shen¹, Zhuolu Li¹, Zijun Tian², Weidong Luo ^2,3,*, Satoshi Okamoto ^4,†, and Pu Yu ^1,5,6,‡

¹State Key Laboratory of Low Dimensional Quantum Physics and Department of Physics, Tsinghua University, Beijing 100084, China
²Key Laboratory of Artificial Structures and Quantum Control, School of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai 200240, China
³Institute of Natural Sciences, Shanghai Jiao Tong University, Shanghai 200240, China
⁴Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
⁵Frontier Science Center for Quantum Information, Beijing 100084, China
⁶RIKEN Center for Emergent Matter Science (CEMS), Wako 351-198, Japan

^*wdluo@sjtu.edu.cn
^†okapon@ornl.gov
^‡yupu@mail.tsinghua.edu.cn

Popular Summary

Strange metals are indeed strange—many of their observable properties, such as electrical resistivity, magnetic susceptibility, and specific heat, exhibit unusual temperature dependence. Such states—also known as non-Fermi-liquid (NFL) states—appear in the vicinity of exotic states like high-temperature superconductivity and heavy fermions. Understanding the onset or loss of NFL behavior could therefore shed light on the nature of such unusual phases. Here, we observe an NFL-to-FL transition, which appears prior to a paramagnetic-to-ferromagnetic phase transition, in calcium ruthenate.

In our experiments, we induce a destabilization of the NFL state by injecting protons into a thin film of calcium ruthenate, a process known as protonation. These protons, introduced via electrolysis of an adjacent layer of water, alter the atomic lattice structure as well as electron concentration of the sample, which, in turn, transforms it from an NFL state to a “normal” Fermi-liquid state. In situ measurements of the electric and magnetic properties of the sample are carried out during the protonation. We find that protonation induces an NFL-to-FL transition in a controllable, reversible manner. We also discover that the Fermi-liquid state is a precursor of an emergent ferromagnetic state from the paramagnetic state of the initial compound.

This intriguing finding raises further questions, such as how the magnetic ordering relates to the NFL and FL phases as well as what happens in that brief window after the transition to the FL state but before the onset of ferromagnetism. Our work also highlights how voltage-controlled protonation can provide an excellent tuning knob for triggering electronic and magnetic phase transitions.

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Vol. 11, Iss. 2 — April - June 2021

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Figure 1
Structural transformation of ${CaRuO}_{3}$ through ionic liquid gating induced protonation. (a) Schematic illustration of proton intercalated induced phase transformation in ${CaRuO}_{3}$ through ionic liquid gating (ILG). The upper panel shows a paramagnetic non-Fermi-liquid (PM NFL) metallic state in ${CaRuO}_{3}$ to ferromagnetic Fermi-liquid behavior (FM FL) in $H_{x} {CaRuO}_{3}$ ( $x = 0.5$ ). The optimized crystalline structures were obtained from DFT calculations. The lower panel shows a schematic for the protonation process during the ILG. (b) In situ XRD $θ − 2 θ$ scans around (001) peak of ${CaRuO}_{3}$ as a function of gating voltage ( $V_{G}$ ). The peak position shifts gradually toward lower angle (from 23.07° to 22.17°) as the $V_{G}$ increases, corresponding to a distinct lattice expansion along out of plane for up to 4.1%. (c) Measured residual hydrogen concentration through ex situ SIMS on both pristine ${CaRuO}_{3}$ and proton intercalated $H_{x} {CaRuO}_{3}$ (gated at 3.5 V) samples. A referenced ${Al}^{3 +}$ signature was also shown to highlight the interface position between film and substrate.
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Figure 2
Emergent ferromagnetism in proton intercalated ${CaRuO}_{3}$ thin films. (a) In situ temperature dependence of resistivity at different $V_{G}$ . (b) Magnetic field dependent Hall resistivity at 2 K for pristine and gated ( $V_{G} = 2.5 V$ ) states, respectively. (c) Temperature-dependent anomalous Hall resistivity $ρ_{X Y}^{AHE}$ at different $V_{G}$ . The inset illustrates the method to obtain the $ρ_{X Y}^{AHE}$ from a representative Hall loop ( $V_{G} = 2.0 V$ and $T = 2 K$ ). (d) Summary of $ρ_{X Y}^{AHE}$ and carrier density ( $n$ , extracted from linear part of Hall signal at high-magnetic-field region) as a function of $V_{G}$ at 2 K.
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Figure 3
Evolution from non-Fermi-liquid to Fermi-liquid behavior accompanied by magnetic transition. Normalized longitude resistivity $ρ_{x x /} ρ_{2 K}$ as a function of (a) $T^{3 / 2}$ and (b) $T^{2}$ at low-temperature region with different $V_{G}$ . Insets show the deviation from $T^{2}$ ( $T^{3 / 2}$ ) for pristine (gated at $V_{G} > 1.0 V$ ) states for comparison. The dashed lines are guides to the eyes. (c) Obtained fitting exponent $α$ as a function of $V_{G}$ for $ρ (T) \sim T^{α}$ , forming two regions labeled by NFL and FL. The $V_{G}$ dependence of $ρ_{X Y}^{AHE}$ is also shown. (d) Reversible evolution of fitting exponent $α$ and $ρ_{X Y}^{AHE}$ , as $V_{G}$ cycled between 0 and 3.5 V.
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Figure 4
Theoretically calculated phase diagram of proton intercalated $H_{x} {CaRuO}_{3}$ . (a) Spin polarized density of states (DOS) of $H_{0.5} {CaRuO}_{3}$ obtained through $DFT + DMFT$ calculations. The vertical dashed line marks the Fermi level which is taken to be the origin of the horizontal axis. (b) Imaginary part of the self-energy as a function of Matsubara frequency $ω_{n}$ for ${CaRuO}_{3}$ (filled symbols) and $H_{0.5} {CaRuO}_{3}$ (open symbols). A paramagnetic state is considered for both cases. The color codes used denote different orbitals in the same way as (a), and the dashed lines indicate ${(ω_{n})}^{1 / 2}$ behavior. (c) Calculated Ru magnetic moment $M$ as a function of Ru $d$ electron number ( $N_{e}$ ) for pristine ${CaRuO}_{3}$ (blue square), $H_{0.25} {CaRuO}_{3}$ (light blue circle), $H_{0.5} {CaRuO}_{3}$ (green triangle), and ${HCaRuO}_{3}$ (red diamond), respectively. The inset shows a summarized intensity map of magnetization ( $M$ ) as a function of both $N_{e}$ and $x$ , in which the color bar denotes $M$ . The yellow line shows guideline for the boundary between ferromagnetism (FM) and paramagnetism (PM), while the dashed black line represents the actual protonation effect with simultaneous structural expansion and electron doping (as $N_{e} = 4 + x$ ).
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Physical Review X

Emergent Ferromagnetism with Fermi-Liquid Behavior in Proton Intercalated CaRuO3

Shengchun Shen, Zhuolu Li, Zijun Tian, Weidong Luo, Satoshi Okamoto, and Pu Yu

Phys. Rev. X 11, 021018 – Published 21 April 2021

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Emergent Ferromagnetism with Fermi-Liquid Behavior in Proton Intercalated ${CaRuO}_{3}$