报告人：Andrew Wee 教授（新加坡国立大学）

报告题目：二维磁性材料和二维量子光源

时间：2023年1月12日（周四）下午15：00

地点：#腾讯会议：366 720 815

Abstract:

The first part of this talk will focus on our work on 2D TMD van der Waals magnets, which if they exist would be ideal atomically thin building blocks for 2D spintronics [1]. Theories have predicted intrinsic magnetism in 2D VX2, such as vanadium diselenide and vanadium ditelluride. Bonilla et al. reported strong room-temperature ferromagnetism in 1T-VSe2 monolayers on van der Waals substrates [2]. We show however, that 2D 1T-VSe2 is not intrinsically ferromagnetic, but displays evidence of spin frustration [3]. Nevertheless, a magnetic transition in 2D VSe2 is induced at the contamination-free interface between Co and VSe2 via interface hybridization [4]. Promotion of ferromagnetism in 2D VSe2 is accompanied by antiferromagnetic coupling to Co and a reduction in the spin moment of Co. We demonstrate that the reconstructed VSe2 monolayer with Se-deficient line defects displays room-temperature ferromagnetism under X-ray magnetic circular dichroism and magnetic force microscopy, consistent with the DFT calculations [5]. This work possibly resolves the controversy on whether monolayer VSe2 is intrinsically ferromagnetic, and highlights the importance of controlling surface defects in 2D TMDs.

The second part of this talk introduces our recent work on the novel 2D material NbOCl2. Unlike TMDs, due to its vanishing interlayer electronic coupling and monolayer-like excitonic behaviour in the bulk form, it exhibits scalable second-harmonic generation intensity. The strong second-order nonlinearity enables correlated parametric photon pair generation, through a spontaneous parametric down-conversion (SPDC) process, in flakes as thin as about 46 nm. To our knowledge,

this is the first SPDC source unambiguously demonstrated in 2D materials, and the thinnest SPDC source ever reported.

References:

[1] W. Zhang, P. K. J. Wong, R. Zhu, A. T. S. Wee, InfoMat (2019) 1, 1–17.

[2] M. Bonilla et al., Nat. Nano. (2018) 13, 289-293.

[3] P. K. J. Wong et al., Adv. Mat. (2019) 31, 1901185.

[4] W. Zhang et al., ACS Nano (2019) 13, 8997-9004.

[5] R. Chua et al., Adv. Mater. (2020) 32, 2000693.

[6] Q. B. Guo et al., Nature 613 (2023) 53.

CV:

Andrew Wee is a Class of ’62 Professor of Physics at the National University of Singapore (NUS), He was previously Dean of Science and Vice President (University and Global Relations). His research interests are in surface and nanoscale science, scanning tunneling microscopy (STM) and synchrotron radiation studies of the molecule-substrate interface, graphene and related 2D materials. He was a Commonwealth Fellow as well as a Rhodes Scholar at the University of Oxford, where he received his received his DPhil (1990). He holds a Bachelor of Arts (Honours) in Physics (1994) as well as a Masters degree from the University of Cambridge. He is an Associate Editor of the journal ACS Nano, and serves or has served on several other journal editorial boards.

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