NCAFM2023 Programme Booklet

Thursday 0940 - 1000

Nikhil Seeja Sivakumar 1 , Nikhil Seeja Sivakumar 1 , Marion van Midden Mavric 2 , Nora Huijgen 1 , Sebastian Scherb 1 , Umut Kamber 3 , Daniel Wegner 1 , Alexander A. Khajetoorians 1 , Nadine Hauptmann 1. Scanning Probe Microscopy (SPM) department, Institute for Molecules and Materials (IMM), Radboud Universiteit, Nijmegen, Nederland 2. Condensed Matter Physics department, Jožef Stefan Institute, Ljubljana 3. Department of Physics, Princeton University, USA Email: nikhil.seejasivakumar@ru.nl CAN WE DISENTANGLE THE CHARGE MODULATION FROM A GEOMETRIC CORRUGATION IN THE CHARGE DENSITY WAVE SYSTEM 2H-NbSe 2 ? Single or few layers of van der Waals materials often exhibit novel types of correlated electron ground states below a critical temperature, e.g. charge density waves (CDW). Materials exhibiting CDWs have been extensively studied to understand the dominant mechanism underlying the correlated ground state, i.e. if the formation of the CDW is driven by electron-electron or electron-phonon interaction. For the example of a one dimensional atomic chain, a CDW is well understood within the Peierls model, where the CDW is due to an electronic instability accompanied by a structural re-arrangement of the atoms due to electron-phonon coupling. In higher dimensions however, this model alone does often not suffice to explain the occurrence of a CDW [1]. The CDW observed in bulk 2H-NbSe 2 has been studied for decades and the mechanism has been highly debated. However, there is now a consensus regarding the key role of electron-phonon coupling in stabilization of the CDW [2]. Recent works highlight the role of lateral displacements of Nb atoms as an important ingredient for CDW formation as well as an axial movement of Se atoms which will result in a surface buckling [3,4]. Since 1988, Scanning Tunnelling Microscopy (STM) has been routinely used for surface characterization of 2H-NbSe 2 at the atomic scale [5]. However, the measured tunnelling current is a convolution of the local density of state and the geometric corrugation. Thus, a surface buckling cannot be easily disentangled from a modulation of the local density of states. Therefore, we utilize combined low-temperature STM and non-contact Atomic Force Microscopy (STM/nc-AFM) based on the qPlus design to study the CDW at the surface of 2H-NbSe 2 . We compare the distance dependent contrast of the atomic lattice and the CDW super lattice in both current and frequency shift images. We analyze the role of electrostatic and chemical forces to the observed contrast via force spectroscopy at selected regions on the CDW unit cell.

Fig. Simultaneously imaged (a) current and (b) frequency shift.

References [1] X. Zhu et.al. Adv. in Phys. 2 , 622 (2017). [2] F. Flicker et. al. Nat Commun 6 , 7034 (2015). [3] D. E. Moncton et. al. PRB 16 , 801 (1977). [4] D. Lin et.al. Nano Letters 22 ,9365-9389 (2022). [5] B. Giambattista et.al. PRB 37 ,5 (1988).

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