【公開日：2025.06.10】【最終更新日：2025.04.15】

課題データ / Project Data

課題番号 / Project Issue Number

24QS0108

利用課題名 / Title

The nature of electronic structures in 1T-Ta(S, Se)₂ resolved by RIXS

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, charge density wave, resonant inelastic x-ray scattering, orbital excitations,量子効果/ Quantum effect,放射光/ Synchrotron radiation,超伝導/ Superconductivity

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

Li Zelong

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)

In a low-dimensional electron system, the interplay between electron correlation, superconductivity (SC), and charge order or charge density wave (CDW) is a central issue. The 1T phase Ta-based dichalcogenides are exemplary systems in this regard. The electronic phase diagram of 1T-TaS_2−xSe_x, shown in Figure 1(a), as a function of temperature and doping level, exhibits complexity. Below 225 K, 1T-TaS₂ displays a commensurate charge order of Q_CCDW = (3/13, 1/13, 1/3), a nearly commensurate charge order at Q_NCDW = (0.245, 0.068, 1/3), and an incommensurate charge order of Q_ICDW = (0.283, 0, 1/3) above 355 K. In contrast, 1T-TaSSe exhibits a ground state intertwined between superconductivity and a nearly commensurate charge density wave (NCDW) state, transitioning to a normal state above 400 K. Furthermore, with increasing doping levels, the superconducting phase diminishes, and 1T-TaSe₂ manifests a commensurate charge order phase as ground state and a normal state above 500 K.   Previously, a physical model addressing the electron correlation in such complex systems was proposed, as depicted in Figures 1(b) – 1(d). The Mott-band-Mott transition occurs at varying doping levels, depending on the hybridization strength of Ta 5d orbitals and S(Se) p orbitals. However, our recent Resonant Inelastic X-ray Scattering (RIXS) study challenges the traditional Mott model for 1T-TaS₂ [2]. The 1T-TaS_2−xSe_x system serves as an ideal platform to investigate this model further.   This proposed research aims to enhance our understanding of electronic correlations in such systems and other layered materials. Technically, it represents one of the few RIXS measurements conducted at high temperatures (above 400 K in this study).

実験 / Experimental

The experiment was performed around the Ta L₃-edge, providing an overall energy resolution of 130 meV. The experimental procedure consisted of three main steps: (a) X-ray absorption spectroscopy (XAS) measurements were conducted on both TaSe₂ and Ta foil to determine the resonant photon energy. The energy resolution was verified through an energy-loss scan using Kapton tape. (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°. Incident-energy dependent RIXS spectra were preformed at room temperature, which is shown in figure 2. Several distinct features are displayed in figure 2. Since the low-energy features are the primary goal of this beamtime, the incident energy was subsequently fixed at 9.88 keV where these two excitations show maximum intensity. (c) After careful alignment of the crystal, data were collected around the (1 0 6) Bragg peak with 2θ ≈ 90°. (d) Momentum-dependent RIXS spectra were acquired at 8K, 300 K (CCDW phase) and above 400 K ( metallic phase). During these measurements, a phase plate was employed to obtain the resolution function for energy calibration using the elastic line.

結果と考察 / Results and Discussion

In summary, high-quality RIXS data were successfully obtained, yielding significant results as follows:(a) The momentum-dependent RIXS maps exhibit remarkable similarity to those previously reported for 1T-TaS₂, demonstrating characteristics distinctly different from those expected for a Mott insulator/metal system. (b) Temperature-dependent scans were systematically performed around a CCDW peak above 400 K to precisely determine the critical temperature (Tc). Above Tc, the acquired RIXS spectra revealed substantially different features compared to the low-temperature data. Detailed analysis confirmed that these spectral changes resulted from sample degradation during high-temperature measurements. Further analysis, supported by theoretical calculations, will be conducted to elucidate the electronic properties of TaX₂ (X = S, Se).

図・表・数式 / Figures, Tables and Equations

Figure1: (a) Schematic electronic phase diagram of 1T-TaS_2−xSe_x as a function of temperature and the doping level x. (b) – (d) Schematic orbital characters derived from the present ARPES experiment as a function of x, where UHB, LHB and E_Mott refer to the upper Hubbard band, the lower Hubbard band and Mott-Hubbard gap, respectively. All these results and the corresponding figures are adapted from reference [1].

Figure 2: (a) Incident energy-dependent RIXS spectra on 1T-TaSe₂ at room temperature. (b) 2D RIXS map on 1T-TaSe₂ at room temperature. The white line represents summed RIXS intensity from the low-energy dd excitations with energies of ~ 1 eV and 2 eV, respectively.

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

References:
[1] R. Ang et al., Phys. Rev. B 88, 115145 (2013).
[2] X. Jia et al., Phys. Rev. B 108, 205105 (2023).

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