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TDLI RESEARCH ACHIEVEMENTS
近日，上海交通大学李政道研究所青年学者严智明与英国圣安德鲁斯大学Peter Wahl教授团队和Phil D. C. King 教授团队、意大利CNR-SPIN的Silvia Picozzi教授团队、德国马克斯·普朗克固体化学物理研究所Andrew Mackenzie教授团队在Nature Communications上共同发表题为 “Avoided metallicity in a hole-doped Mott insulator on a triangular lattice” 的研究论文，揭示了被掺杂的莫特绝缘表面可通过电荷歧化（charge disproportionation）形成一种与块体中的莫特绝缘态截然不同的绝缘基态。
掺杂莫特绝缘体可导致高温超导和电荷、自旋以及轨道序等涌现态的形成。然而，能够很好理解其进化的物理学却知之甚少。在该论文中，严智明等人通过扫描隧道显微（STM）、角分辨光电子能谱学（ARPES）和密度泛函理论（DFT）研究了反铁磁金属材料铜铁矿（delafossite）PdCrO2的莫特绝缘CrO2表面，其固有的极性突变为表面掺杂提供了一条干净的途径（图一a）。
图一：a，PdCrO2两个极性表面（Pd 和 CrO2）发生的表面掺杂效应示意图；b-d，从两个极性表面获得的STM（b）和ARPES（c-d）数据。
通过实验，严智明等人观测到PdCrO2的Pd和CrO2两个极性面存在的证据（图一、b-d），并在CrO2表面观测到一个邻近费米能级（EF）、大小约500meV的绝缘能隙（图二），这与预期有所不同。
图二、a-e，PdCrO2两个极性面的ARPES实验数据；f-g，在 CrO2面出现大小约500meV的能隙的ARPES（f）和STM/S（g）实验证据；h， CrO2面在低能量、低电流时的STM表面形貌图；插图，对应高能量时的表面形貌图。i，对应（h）的傅里叶变换。
去进一步探索该能隙的起源，严智明等人在低偏压、低隧穿电流条件下对CrO2表面进行STM表征，并发现在邻近EF时，该面出现一个短程有序态（见图二、h-i）。基于实验发现，论文合作者Srdjan Stavrić博士采用了周期性为√7×√7的超级晶胞（晶胞中 4个Cr原子自旋向上，3个Cr原子自旋向下）来建模并对CrO2的表面电子结构进行计算。结果显示，通过电荷歧化（charge disproportionation），该短程有序态的形成会使表面出现两种不同类型的Cr原子（CrA和CrB），两者有着不同的Bader电荷、自旋磁矩和垂直位移（图三）。
图三、a，块状PdCrO2的投射态密度（PDOS）。计算时采用了周期性为√7×√7的超级晶胞，其中 4个Cr原子自旋向上，3个Cr原子自旋向下；b，计算结果显示，经过松弛，部分Cr原子的垂直位移高达~0.1Å，同时表面存在两种不同类型的Cr原子；c，CrO2表面的PDOS。EF处的态来自亚表面Pd层，而Cr态的间隙约为0.2 eV；插图，较窄能量范围内CrA和CrB的PDOS。
另外，在微弱外力的干扰下，该短程有序态会随着时间演化，从而展示其像玻璃一样的特性（图四）。
图四、a-d，时间相关的STM形貌图数据展示出CrO2面的短程有序态在微弱外力的干扰下随着时间的演化；e-g，对应的差分图像；h，显示基态演化时间特征的方差-时间图
上海交通大学李政道研究所严智明为论文的共同第一作者（STM实验）兼共同通讯作者，其他共同第一作者包括圣安德鲁斯大学博士后Gesa-R. Siemann（ARPES实验）和CNR-SPIN博士后Srdjan Stavrić（理论计算），其他共同通讯作者包括英国圣安德鲁斯大学Peter Wahl教授和Phil D. C. King教授、和意大利CNR-SPIN Silvia Picozzi 教授。
本工作得到上海市科委和上海交通大学的资助，在此深表感谢。
https://www.nature.com/articles/s41467-024-52007-z
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Yim et al. reveals a doped Mott-insulating surface can, through charge-disproportionation, form an insulating ground state that is distinct from the hidden Mott-insulating state in the bulk
TDLI RESEARCH ACHIEVEMENTS
Recently, T. D. Lee (Tenure-track) Fellow Chi Ming Yim of Tsung Dao Lee Institute, Shanghai Jiao Tong University, together with research groups of Prof. Peter Wahl and Prof. Phil D. C. King of University of St Andrews (United Kingdom), Prof. Silvia Picozzi of CNR-SPIN (Italy) and Prof. Andrew Mackenzie of MPI-CPfS (Germany) published a research article titled “Avoided metallicity in a hole-doped Mott insulator on a triangular lattice” in Nature Communications. The article points out that a doped Mott-insulating surface can stabilize itself through charge disproportionation into an insulating ground state that is distinct from its hidden Mott insulating state in the bulk.
Doping Mott insulators can lead to the formation of a variety of emergent states such as high temperature superconductivity as well as charge, spin and orbital orders etc. However, the underlying physics for understanding their evolution is rare. In this Article, Yim et al. employed low temperature scanning tunnelling microscopy (STM), angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT), to study the Mott insulating CrO2 surface layer of delafossite PdCrO2, whose instinct polar catastrophe provides a clean route for surface doping (see Fig. 1a).
Fig 1. a, Schematics of surface doping effect occurring on the two different surface terminations of PdCrO2. b-d, STM (b) and ARPES (c-d) data obtained from the two surface terminations.
Through experiment, Yim et al. have found the evidence for the existence of the two different surface terminations of PdCrO2 (Fig. 1, b-d), and unexpectedly, detected on the CrO2 terminated surface an insulating gap with size of 500 mV centred around the Fermi level (Fig. 2).
Fig. 2. a-e, ARPES data obtained from the two surface terminations of PdCrO2; f-g, ARPES (f) and STM/S (g) data revealing an energy gap of 500 meV on the CrO2 terminated surface; h, STM topographic image of the CrO2 terminated surface recorded at low bias voltage and extremely low tunnelling current. Inset of (h), that recorded at higher sample bias; i, Fourier transformation of (h).
To further explore the origin of the energy gap, Yim et al. performed STM measurement on the CrO2 terminated surface at extremely low bias and tunnelling current condition, through which discovered that there exists a short-range ordered state (see Figure 2, h-i). Based on the experimental observation, Dr Srdjan Stavrić (one of the collaborators) performed DFT calculations using a model with a √7×√7 spatial periodicity and comprising 4 Cr atoms with their spins pointing upward and 3 with spins pointing downward, to calculate the electronic structure of the CrO2 surface. The calculation results show that through charge disproportionation, the formation of the short-range ordered state leads to two different types of Cr atoms (CrA, CrB), both having different Bader charges, magnetic moments, and vertical displacements from the surface layer.
Fig. 3. a, Projected density of states (PDOS) of bulk PdCrO2，calculated using a supercell having √7×√7 spatial periodicity and comprising 4 Cr atoms with spins pointing up and 3 with spins pointing down. b, Results show that following surface relaxation, the vertical displacement of some Cr atoms can exceed ~0.1Å, meanwhile there exists two different types of Cr atoms on the CrO2 surface. c, Surface electronic structure of the CrO2 terminated surface; inset of (c), PDOS of CrA and CrB in narrower energy range near EF.
Furthermore, under weak perturbation (e.g., tens of meVs tip pulses), the short-range ordered state shows a temporal evolution, hence exhibiting a glassy dynamical behaviour (Fig. 4).
Fig. 4. a-d, time-dependent STM images showing temporal evolution of the short-range ordered state following a small perturbation (a 70 mV, 2 s tip pulse in this case); e-g，corresponding difference images；h，Variance versus time plot showing the glassy dynamics of the short-range ordered state.
Chi Ming Yim of Tsung Dao Lee Institute, Shanghai Jiao Tong University is the co-first author (STM experiment) and co-corresponding author of the paper. Other co-first authors include Dr. Gesa -R. Siemann of University of St Andrews (ARPES) and Dr. Srdjan Stavrić of CNR-SPIN (theoretical calculations). Other co-corresponding authors include Prof. Peter Wahl and Prof. Phil D. C. King of University of St Andrews, and Prof. Silvia Picozzi of CNR-SPIN, Italy.
https://www.nature.com/articles/s41467-024-52007-z
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Author | Chi Ming Yim
Editor | Wenzhuo Meng
