Observation of multiple charge density wave phases in epitaxial monolayer 1T-VSe_(2) film

As a special order of electronic correlation induced by spatial modulation, the charge density wave(CDW) phenomena in condensed matters attract enormous research interests. Here, using scanning-tunneling microscopy in various temperatures, we discover a hidden incommensurate stripe-like CDW order besides the(■) CDW phase at low-temperature of 4 K in the epitaxial monolayer 1T-VSe_(2) film. Combining the variable-temperature angle-resolved photoemission spectroscopic(ARPES) measurements, we discover a two-step transition of an anisotropic CDW gap structure that consists of two parts △_(1) and△_(2). The gap part ?1 that closes around ~ 150 K is accompanied with the vanish of the(√7×√3) CDW phase. While another momentum-dependent gap part △_(2) can survive up to ~ 340 K, and is suggested to the result of the incommensurate CDW phase. This two-step transition with anisotropic gap opening and the resulted evolution in ARPES spectra are corroborated by our theoretical calculation based on a phenomenological form for the self-energy containing a two-gap structure △_(1) +△_(2), which suggests different forming mechanisms between the(√7×√3) and the incommensurate CDW phases. Our findings provide significant information and deep understandings on the CDW phases in monolayer 1T-VSe_(2) film as a two-dimensional(2D) material.

Charge Density Wave and Electron-Phonon Interaction in Epitaxial Monolayer NbSe_(2) Films

Understanding the interplay between superconductivity and charge-density wave(CDW)in NbSe_(2) is vital for both fundamental physics and future device applications.Here,combining scanning tunneling microscopy,angleresolved photoemission spectroscopy and Raman spectroscopy,we study the CDW phase in the monolayer NbSe_(2) films grown on various substrates of bilayer graphene(BLG),SrTiO_(3)(111),and Al_(2)O_(3)(0001).It is found that the two stable CDW states of monolayer NbSe_(2) can coexist on NbSe2/BLG surface at liquid-nitrogen temperature.For the NbSe_(2)/SrTiO_(3)(111)sample,the unidirectional CDW regions own the kinks at±41 meV and a wider gap at 4.2 K.It is revealed that the charge transfer from the substrates to the grown films will influence the configurations of the Fermi surface,and induce a 130 meV lift-up of the Fermi level with a shrink of the Fermi pockets in NbSe_(2)/SrTiO_(3)(111)compared with the NbSe2/BLG.Combining the temperature-dependent Raman experiments,we suggest that the electron-phonon coupling in monolayer NbSe2 dominates its CDW phase transition.

Electronic structures and Mott state of epitaxial TaS_(2) monolayers

Layered material TaS2hosts multiple structural phases and exotic correlated quantum states,including charge density wave(CDW),superconductivity,quantum spin liquid,and Mott insulating state.Here,we synthesized TaS_(2)monolayers in H and T phases using the molecular beam epitaxial(MBE)method and studied their electronic structures via angle-resolved photoemission spectroscopy(ARPES).We found that the H phase TaS_(2)(H-TaS_(2))monolayer is metallic,with an energy band crossing the Fermi level.In contrast,the T phase TaS_(2)(T-TaS_(2))monolayer shows an insulated energy gap at the Fermi level,while the normal calculated band structure implies it should be metallic without any band gap.However,by considering Hubbard interaction potential U,further density functional theory(DFT)calculation suggests that monolayer T-TaS_(2)could be a CDW Mott insulator,and the DFT+U calculation matches well with the ARPES result.More significantly,the temperature-dependent ARPES result indicates that the CDW Mott state in the T-TaS_(2)monolayer is more robust than its bulk counterpart and can persist at room temperature.Our results reveal that the dimensional effect can enhance the CDW Mott state and provide valuable insights for further exploring the exotic properties of monolayer TaS2.

通过调控范霍夫奇点在外延单层VSe_(2)中诱导出巡游铁磁性

处在费米能级处的范霍夫奇点(van Hove singularity)由于具有发散的态密度特征可以诱导出巡游铁磁性.本文在SrTiO_(3)(111)衬底上外延生长单层VSe_(2)薄膜,并通过调控范霍夫奇点诱导出巡游铁磁性.作者通过角分辨光电子能谱技术直接观测到了单层VSe_(2)中的范霍夫奇点能带结构.理论计算表明,当该范霍夫奇异点靠近费米能级时,可通过斯通纳(Stoner)不稳定性产生巡游铁磁性.实验上,作者利用SrTiO_(3)(111)在低温下极大增强的介电常数εr,通过界面电荷转移效应调控范霍夫奇点向费米能级处移动,并通过电输运测量观测到了3.3 K的巡游铁磁性转变.此外,通过改变薄膜层厚和更换衬底的方式,作者可以进一步调控界面电荷转移效果和范霍夫奇点的能量位置,并对巡游铁磁性进行调控.这项研究证明了范霍夫奇点可以成为巡游铁磁性的调控自由度,并拓展了二维磁体在未来电子信息技术的应用潜力.