Correlated electrons in the flat band in the charge density wave state of 4Hb−TaSexS2−x

Correlated electrons in the flat band in the charge density wave state of 4Hb−TaSexS2−x

Yanyan Geng#, Jianfeng Guo#, Fanyu Meng#, Manyu Wang, Shuo Mi, Li Huang, Rui Xu, Fei Pang, Kai Liu, Shancai Wang, Hong-Jun Gao, Weichang Zhou, Wei Ji*, Hechang Lei*, and Zhihai Cheng*

Many intriguing quantum states of matter, such as unconventional superconductivity, magnetic phases, and fractional quantum Hall physics, emerge from the spatially correlated localized electrons in the flat bands of solid materials. By using scanning tunneling microscopy and spectroscopy (STM/STS), we report on the real-space investigation of correlated electrons in the flat band of superlattice 4⁢𝐻𝑏⁡−TaS⁢e𝑥⁢S2−𝑥. In contrast with the pristine 4⁢𝐻𝑏⁡−Ta⁢S2, the selenium (Se) substitutions significantly affect the interfacial transfer of correlated electrons between the charge density wave (CDW) states of 1⁢𝑇- and 1⁢𝐻⁡−Ta⁢S2 layers and contribute the real-space fractional electron-filling configurations with the distributed electron-filled and void Star of David (SoD) clusters of the 1⁢𝑇 layer. The site-specific STS spectra directly reveal their respective prominent spectra weight above 𝐸F and symmetric Mott-like spectra. In addition, the spatial distributions of these electron-filled SoDs in the 1⁢𝑇 layer of 4⁢𝐻𝑏⁡−TaS⁢e0.7⁢S1.3 demonstrate different local short-range order, clearly indicating the complex neighboring interactions among the localized electrons in the flat band of the 1⁢𝑇 layer. Our results not only provide in-depth insight into correlated electrons in the flat CDW band but also provide a simple platform to manipulate the electron-correlation-related quantum states.

Strain-engineered rippling at the bilayer-MoS2 interface identified by advanced atomic force microscopy

Strain-engineered rippling at the bilayer-MoS2 interface identified by advanced atomic force microscopy

Haoyu Dong, Songyang Li, Shuo Mi, Jianfeng Guo, Zhaxi Suonan, Hanxiang Wu, Yanyan Geng, Manyu Wang, Huiwen Xu, Li Guan,Fei Pang, Wei Ji, Rui Xu, Zhihai Cheng

Hafnia-based ferroelectric materials, like Hf0.5Zr0.5O2 (HZO), have received tremendous attention owing to their potentials for building ultra-thin ferroelectric devices. The orthorhombic(O)-phase of HZO is ferroelectric but metastable in its bulk form under ambient conditions, which poses a considerable challenge to maintaining the operation performance of HZO-based ferroelectric devices. Here, we theoretically addressed this issue that provides parameter spaces for stabilizing the O-phase of HZO thin-films under various conditions. Three mechanisms were found to be capable of lowering the relative energy of the O-phase, namely, more significant surface-bulk portion of (111) surfaces, compressive caxis strain, and positive electric fields. Considering these mechanisms, we plotted two ternary phase diagrams for HZO thin-films where the strain was applied along the in-plane uniaxial and biaxial, respectively. These diagrams indicate the O-phase could be stabilized by solely shrinking the film-thickness below 12.26 nm, ascribed to its lower surface energies. All these results shed considerable light on designing more robust and higher-performance ferroelectric devices.

Interlayer coupling modulated tunable magnetic states in superlattice MnBi2⁢Te4(Bi2⁢Te3)𝑛 topological insulators

Interlayer coupling modulated tunable magnetic states in superlattice MnBi2⁢Te4(Bi2⁢Te3)𝑛 topological insulators

Jianfeng Guo, Huan Wang, Haoyan Zhang, Shuo Mi, Songyang Li, Haoyu Dong, Shiyu Zhu, Jiawei Hu, Xueyun Wang, Yanjun Li, Yasuhiro Sugawara, Rui Xu, Fei Pang, Wei Ji, Tianlong Xia, and Zhihai Cheng

The intrinsic superlattice magnetic topological insulators of MnBi2⁢Te4(Bi2⁢Te3)𝑛 (𝑛=0,1,2…) provides a promising material platform for the realization of diverse exotic topological quantum states, such as quantum anomalous Hall effect and axion-insulator state. All these quantum states are sensitively dependent on the complex interplay and intertwinement of their band topology, magnetism, and defective structural details. Here, we report a comprehensive real-space investigation on the magnetic ordering states of MnBi2⁢Te4(Bi2⁢Te3)𝑛 using cryogenic magnetic force microscopy. The MnBi2⁢Te4(Bi2⁢Te3)𝑛 crystals exhibit a distinctive magnetic evolution from A-type antiferromagnetic to ferromagnetic states via the increased Bi2⁢Te3 intercalation layers. The magnetic field- and temperature-dependent phase evolution behaviors of MnBi6⁢Te10 and MnBi8⁢Te13 are comparatively investigated to obtain the complete 𝐻−𝑇 phase diagrams. The combination impact of the intrinsic and defect-mediated interlayer coupling on their magnetic states were further discussed. Our results pave a possible way to realize more exotic quantum states via the tunable magnetic configurations in the artificial-stacking MnBi2⁢Te4(Bi2⁢Te3)𝑛 multilayers.

Hysteretic electronic phase transitions in correlated charge density wave state of 1T-TaS2

Hysteretic electronic phase transitions in correlated charge density wave state of 1T-TaS2

Physical Review B 107, 195401 (2023)

Yanyan Geng, Le Lei, Haoyu Dong, Jianfeng Guo, Shuo Mi, Yan Li, Li Huang, Fei Pang, Rui Xu, Weichang Zhou, Zheng Liu, Wei Ji, and Zhihai Cheng

The layered transition metal dichalcogenide 1T−TaS2 has evoked great interest owing to its particularly rich electronic phase diagram including different charge density wave (CDW) phases. However, few studies have focused on its hysteretic electronic phase transitions based on the in-depth discussion of the delicate interplay among temperature-dependent electronic interactions. Here, we report a sequence of spatial electronic phase transitions in the hysteresis temperature range (160–230 K) of 1T−TaS2 via variable-temperature scanning tunneling microscopy. Several emergent electronic states are investigated at multiscale during the commensurate CDW–triclinic CDW (CCDW-TCDW) phase transitions: a spotty-CDW state above ∼160K, a network-CDW (NCDW) state above ∼180K during the warmup process, a belt-TCDW state below ∼230K, a NCDW state below ∼200K, and finally a mosaic-CDW state below ∼160K during cooldown from the TCDW phase. These emergent electronic states are associated with the delicate temperature-dependent competition and/or cooperation of stacking-dependent interlayer interactions, intralayer electron-electron correlations, and electron-phonon (e−ph) coupling of 1T−TaS2. Our results not only provide insight to understand the hysteretic electronic phase transitions in the correlated CDW state, but also pave a way to realize more exotic quantum states by accurately and effectively tuning various interior interactions in correlated materials.

DOI:10.1103/PhysRevB.107.195401