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.

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.