【公開日：2025.06.10】【最終更新日：2025.04.16】
課題データ / Project Data
課題番号 / Project Issue Number
24QS0007
利用課題名 / Title
Exploring temperature evolution of electronic excitations in 1T-TaS2
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 Excluding Supporters in the Hub and Spoke Institutes
Zhang Yujun,Shi Xiongfei,Su Zhibin,Li Zelong
ARIM実施機関支援担当者 / Names of Supporters 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
報告書データ / Report
概要（目的・用途・実施内容）/ Abstract (Aim, Use Applications and Contents)
1T-TaS2 is among a large group of transition metal dichalcogenides (TMDCs) with charge orders, which are actively studied due to their proximity to superconductivity and potential applications in quasi-two-dimensional electronics. 1T-TaS2 features a commensurate charge order of Q_CCDW = (3/13, 1/13, 1/3) below 225 K, 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 [1]. For both the CCDW and NCDW phase, the Q-vector corresponds to the ‘star of David’-like charge reconstruction in the 2D plane. Despite decades of research, the nature of the insulating state in the CCDW phase 1T-TaS2 is actively debated. On the one hand, the 13 Ta atoms per CCDW supercell have a nominal 5d1 electronic configuration [1, 2, 3], prompting the widely accepted argument that the Coulomb repulsion plays a vital role and that the onset of insulator behavior is of Mott type. On the other hand, the band dispersion in the CCDW phase observed using the angle-resolved photoemission spectroscopy (ARPES) can be captured reasonably well through band folding without significant electron correlation [4]. Very recently, combining Resonant inelastic X-ray scattering (RIXS) at Ta resonance and calculations, we suggest that the nature of 1T-TaS2 is not a textbook Mott insulator but one close to a band insulator since the electronic excitations we observed can be described qualitatively and semi-quantitatively using extended electron wavefunctions [5]. Furthermore, rare research focus on the temperature evolution of electronic excitations in 1T-TaS2. In real space, NCCDW phase could be treated as the discrete zones of ‘Star of David’ and hosts a similar Q vector of charge order as that in CCDW phase. In sharp contrast, there’s huge change in both real structure and electronic structure across the ICCDW – NCCDW phase transition, see fig. (a), which may cause a huge change in electronic excitations’ dispersion. This proposal will broaden our current knowledge of electronic correlation in 1T-TaS2 and other layered materials. Also, technically, this will be the very a few RIXS measurement at high temperature.
実験 / Experimental
The experiment was performed at the Ta L3-edge with a fixed incident photon energy of 9.882 keV, 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 TaS2 and Ta 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 9.882 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 (1 0 6) Bragg peak with 2θ ≈ 90°. (c) Momentum-dependent RIXS spectra were acquired at 300 K (NCCDW phase) and 400 K (ICDW 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 RIXS spectra exhibit similar features across the CCDW [5], NCCDW and ICDW phases (this beamtime) at equivalent momentum transfer, as demonstrated in Figure 1; (b) Momentum-dependent RIXS measurements reveal systematic variations in the fitted energies of dd excitations, as presented in Figure 2; (c) No evidence of a Mott gap (predicted at ~0.5 eV) was observed in either the NCCDW or ICDW phases.Combined with previous findings reported in [5], these preliminary results taken in this beamtime suggest that the ground state of 1T-TaS2 deviates significantly from a Mott insulating state. Furthermore, the phase transition between NCCDW and ICDW phases shows no characteristic signatures of Mott insulation. Further analysis of the data will provide more detailed insights into these observations.
図・表・数式 / Figures, Tables and Equations
Fig. 1: (a) and (b) display the momentum-dependent RIXS spectra acquired at 300 K (NCCDW phase) and 400 K (ICDW phase) during this experiment. The gray dashed lines indicate the overall fitting curves of the RIXS spectra, while the gray shaded regions represent characteristic spectral features.(c) and (d) present representative RIXS spectra measured at (0.2, 0, 6) and (0.5, 0, 6) for the NCCDW (300 K) and ICDW (400 K) phases, respectively.
Fig. 2: the fitted dispersions of the two dd excitations both at 300 K (NCCDW phase) and at 400 K (ICDW phase).
その他・特記事項（参考文献・謝辞等） / Remarks(References and Acknowledgements)
References:
[1] B. Sipos, et al., Nat. Mat. 7, 960-965 (2008).
[2] P. Fazekas, and E. Tosatti, Physica B+C 99, 183-187 (1980).
[3] L. Perfetti, T. A. Gloor, F. Mila, H. Berger, and M. Grioni, Phys. Rev. B 71, 153101 (2005).
[4] Y. D. Wang, et al., Nat. Commun. 11, 4215 (2020).
[5] 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