【公開日：2025.06.10】【最終更新日：2025.04.18】

課題データ / Project Data

課題番号 / Project Issue Number

24QS0009

利用課題名 / Title

Identifying the nature of low-energy excitation in bulk pyrochlore rhenate

利用した実施機関 / Support Institute

量子科学技術研究開発機構 / QST

機関外・機関内の利用 / External or Internal Use

外部利用/External Use

技術領域 / Technology Area

【横断技術領域 / Cross-Technology Area】（主 / Main）計測・分析/Advanced Characterization（副 / Sub）-

【重要技術領域 / Important Technology Area】（主 / Main）量子・電子制御により革新的な機能を発現するマテリアル/Materials using quantum and electronic control to perform innovative functions（副 / Sub）-

キーワード / Keywords

quantum material, superconductor, resonant inelastic x-ray scattering, orbital excitations,量子効果/ Quantum effect,放射光/ Synchrotron radiation,超常磁性/ Superparamagnetism

利用者と利用形態 / User and Support Type

利用者名（課題申請者）/ User Name (Project Applicant)

Jia Xun

所属名 / Affiliation

Institute of High Energy Physics, Chinese Academy of Sciences

共同利用者氏名 / Names of Collaborators in Other Institutes Than Hub and Spoke Institutes

Zhang Yujun,Huang Huan,Li Zelong,Zhang Jing

ARIM実施機関支援担当者 / Names of Collaborators in The Hub and Spoke Institutes

石井 賢司

利用形態 / Support Type

（主 / Main）共同研究/Joint Research（副 / Sub）-

利用した主な設備 / Equipment Used in This Project

QS-112：共鳴非弾性X線散乱装置

報告書データ / Report

概要（目的・用途・実施内容）/ Abstract (Aim, Use Applications and Contents)

The 5d pyrochlore oxides are a family of materials that exhibit several tunable quantum phases, including those with non-trivial electronic and magnetic collective phenomena. Among them, Cd₂Re₂O₇ (CRO) has recently drawn significant interest, including the identification of a hidden nematic phase. CRO is a correlated metal and undergoes a structural transition from centrosymmetric Fd-3m space group (phase I) to the I-4m2 (phase II) at Ts1 = 200 K [1]. Since phase II is noncentrosymmetric, the structural transformation is expected to be accompanied by transitioning into a topological state and potentially exotic superconductivity at low temperatures [2]. Recently, the low temperature phase was characterized as hosting multipolar nematic electronic order deviating from the symmetry of the lattice [3]. A theoretical analysis yielded that the phase below Ts1 may exhibit all-in-all-out (AIAO) magnetic order, which is yet to be experimentally confirmed [4]. Meanwhile, an NMR experimental work hints to rule out the existence of any long-range magnetic/charge order from 5 K to 100 K [5], which makes the debate become puzzled. More solid experimental exploration is further complicated by the availability of materials. We had a beamtime at BL11XU at 2023B period with No. 3591. The main message could be summarized from the RIXS map (fig. 1(a)) and q-dependent and temperature-dependent RIXS spectra in figs. 1(b) and 1(c). In fig. 1(a), three main features are displayed in RIXS map, with possible origins as intra-t_2g excitation, t_2g to E_g excitation and charge transfer excitation, respectively. The further detailed explanation needs to be done. Surprisingly, in figs. 1(b) and 1(c), we found temperature-dependent behavior of intra-t_2g excitation across the ground state – phase II transition, plus a low-energy excitation raising up at 8 K without clear origin. With these evidence at a very early stage, we sincerely apply for the 2024A beamtime to continue this research. This proposal seeks to further illuminate (a) the existence of low-energy feature in figs. 1(b) and 1(c) and if yes, chase its nature, as well as (b) the evolution of the electronic structure across the phase transition with more temperature points. We will use resonant inelastic X-ray spectroscopy (RIXS) to address both questions. Specifically, to address (a), we will explore the existence of possible magnetic excitations in CRO, especially along the [111] direction with magnetic peaks from the proposed AIAO magnetic peak. Moreover, we will track the evolution of orbital excitations that arise due to the strong spin-orbit coupling. The energy and width of the dd features will shed light into the orbital occupation. The proposed study is enabled by our recent success in obtaining magnetic excitations from pyrochlore iridates with AIAO ground states, including the observation of a persistent paramagnon surviving above the Neel temperature. Also, we have obtained high-quality large-size single crystals of CRO that are approximately 1 mm in each direction that is sufficiently large for the proposed measurements.

実験 / Experimental

The experiment was performed at the Re L₃-edge, providing an overall energy resolution of 80 meV. The experimental procedure consisted of the main steps as following: (a) X-ray absorption spectroscopy (XAS) measurements were conducted on both the sample and Re foil to determine the resonant photon energy. The energy resolution was verified through an energy-loss scan using Kapton tape. The incident energy was subsequently fixed at 10.5415 keV. (b) Resonant inelastic X-ray scattering (RIXS) measurements were performed in a horizontal scattering geometry with π-polarization. To minimize the influence of the horizontal photon footprint on the energy resolution, the incidence angle was maintained at or above 45°. After careful alignment of the crystal, data were collected around the (7 7 7) Bragg peak with 2θ ≈ 90°. (c) Momentum-dependent RIXS spectra were acquired at 8 K. During these measurements, a phase plate was employed to check the exact position of elastic line by taking a resolution function. (d) Temperature-dependent RIXS spectra were acquired between 8 K and 240 K at (7.2 7 7) and (7.6 7 7), respectively. During these measurements, a phase plate was employed to check the exact position of elastic line by taking a resolution function.

結果と考察 / Results and Discussion

In summary, the high-quality, high-resolution RIXS data confirm the existence of an unexpected low-energy excitation. Under our best knowledge, this is the first time of such observation. Detailed analysis of the RIXS spectra reveals two key characteristics of this excitation:(a) The excitation exhibits a nearly dispersionless behavior with an energy range of 50-60 meV at 8 K, as shown in Figure 2.(b). The excitation displays a pronounced softening behavior near Ts1 ≈ 200 K, as demonstrated in Figure 3. While Re in CRO adopts a 5d² electronic configuration, which precludes long-range magnetic ordering as supported by previous studies [5], the observed low-energy excitation may possess a complex magnetic origin and could be coupled to a structural phase transition. Further investigation is required to elucidate the origin of this excitation. This observation challenges the conventional theoretical framework of magnetic phase transitions.

図・表・数式 / Figures, Tables and Equations

Figure1: (a) RIXS map of single crystal Cd₂Re₂O₇ taken at room temperature. (b) RIXS spectra at (7.1 7.1 7.1) taken both at 8 K and 160 K; (c) RIXS spectra at (6.5 7.5 7.5) taken both at 8 K and 160 K. All these data were taken in 2023B beamtime.

Figure 2: (a) - (c) momentum-dependent RIXS spectra taken in 2024A beamtime along gamma-L, gamma-K and gamma-X, respectively. The black arrows guide the position of low-energy excitation.

Figure 3: (a) and (b) temperature-dependent RIXS spectra taken in 2024A beamtime on (7.2 7 7) and (7.6 7 7), respectively. The black arrows guide the position of low-energy excitation.

その他・特記事項（参考文献・謝辞等） / Remarks(References and Acknowledgements)

References:
[1] Jesse C. Petersen, et al., Nature Physics 2, 605-608 (2006).
[2] I. A. Sergienko, & S. H. Curnoe, J. Phys. Soc. Jpn 72, 1607-1610 (2003).
[3] J. W. Harter et al., Science 356, 295-299 (2017).
[4] S. Di Matteo and M. R. Norman, Phys. Rev. B 96, 115156 (2017).
[5] O. Vyaselev et al., Phys. Rev. Lett. 89, 017001(2002).
[6] Arun Paramekanti, et al., Phys. Rev. B 97, 235119 (2018).
[7] S. Calder et al., Nature Communications, 7, 11651 (2016).

成果発表・成果利用 / Publication and Patents

論文・プロシーディング（DOIのあるもの） / DOI (Publication and Proceedings)

口頭発表、ポスター発表および、その他の論文 / Oral Presentations etc.

特許 / Patents

特許出願件数 / Number of Patent Applications：0件
特許登録件数 / Number of Registered Patents：0件