MAGnetism InduCed VALLEY polarization in large scale 2D materials – MAGICVALLEY

This project targets the fabrication of high quality MX2 and magnetic 2D layers on large scale (in the range of cm²) using techniques that can be easily scaled up. Magnetism in 2D layers will be either intrinsic (in MnSe2 and VSe2) or by doping MX2 layers with magnetic impurities (Mn, V and Fe). In this respect, our objective is to reach higher Curie temperatures than in CrI3 (45 K) and Cr2Ge2Te6 (40 K), possibly up to room temperature.
A great part of the efforts will be devoted to the growth of materials by MBE and CVD on suitable substrates (graphene, mica and oxides) and to their atomic, electronic and magnetic characterizations at various scales. Growth techniques (MBE, CVD) as well as usual lab characterization tools (AFM, reflectivity, photoluminescence and Raman spectroscopy, x-ray diffraction, XPS, SQUID, MOKE, FMR and magnetotransport) are all available in the consortium. The strength of this project is the use of advanced characterization techniques to get deeper insight into the properties of 2D layers and their vdW heterostructures. They include scanning tunneling microscopy and spectroscopy (STM/STS) at low temperature to access the local atomic and electronic structure; spin and angle resolved photoemission spectroscopy (spin-ARPES) to access the spin resolved band structure over large areas (10-100 µm); (scanning) transmission electron microscopy (TEM/STEM) to access the structure and chemical composition at the atomic scale and Kelvin probe force microscopy (KPFM) to access interface potentials and band alignment in vdW heterostructures. This experimental effort will be backed by a strong theoretical support based on ab-initio calculations focusing on the electronic and magnetic properties of 2D magnetic layers and on proximity effects.

- Development of the 2D ferromagnetic diselenide compound: V1-xPtxSe2
- Successful vanadium doping of WSe2 by MBE: the V atoms substitute partly the W atoms. STM studies were performed on pristine and magnetically doped WSe2 layers grown on graphene. V atoms, identified as substitutional dopants from STEM studies, were found inside the WSe2 layers. STS data show that they correspond to negatively charged entities, which are thus probably non-magnetic. The negative charge of V dopants results from the capture of an electron from the graphene substrate by an empty V induced state located just above the WSe2 valence band, as confirmed by ab-initio calculations.
- Experimental evidence of the valley Nernst effect in WSe2 using the spin pumping-ferromagnetic resonance technique
- Optimization of the growth of MoSe2 and WSe2 on SiO2 (by solid phase epitaxy) and mica (for very low metal flux and hig hgrowth temperature)
- Study of the charge transfer between WSe2 and graphene by Kelvin probe microscopy and ab initio calculations
- Measure of the splitting between dark and bright excitons in MoS2 monolayers
- Experimental evidence of the inter-layer excitonic states in MoS2 bilayers and demonstration of the associated giant Stark effect
- Measure of the excitonic dynamics in van der Waals heterostructures TMD/Graphene
- Demonstration of the control of the radiative lifetime of excitons by the Purcell effect.

Highlight:
We succeeded in incorporating vanadium atoms in substitution of W atoms in WSe2 by molecular beam epitaxy. The STM/STS work on MBE grown samples has led to the unambiguous identification/characterization of individual V dopants. This is a noticeable result since, although point defects were commonly reported in TMD samples, their correct identification remained largely uncertain.

Perspectives:
- Development and optimization of the growth by MBE of MoSe2 and WSe2 on hBN flakes provided by the LPCNO Partner who will study the optical properties (reflectivity, luminescence) of this TMD monolayers.
- Development of the growth of ferromagnetic Te-based 2D materials: Fe3GeTe2 and Fe5GeTe2
- Growth of vdW hétérostructures 2D ferro (VPtSe2, FexGeTey)/MoSe2, WSe2
- STM/STS and STEM/EELS study of the VPtSe2 compound
- Effect of the dielectric environment on the excitonic spin dynamics.

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DOI:10.1103/PhysRevLett.125.036802

In the monolayer limit, two dimensional (2D) transition metal dichalcogenides (2H-MX2, with M=Mo, W and X=S, Se) are semiconductors with a sizeable (1-2 eV) and direct electronic bandgap as well as (degenerate) valleys at the K+/K- corners of the Brillouin zone. Beyond their use as classical semiconductors, this peculiar electronic structure opens new and exciting possibilities for information processing that exploit the quantum degree of freedom known as the valley index. This emergent field of research is known as « valleytronics ». It has been established that K+/K- valleys can be selectively addressed by using circularly polarized light and that the valley Hall effect allows resolving the valley polarization of charge carriers. However, for practical use of these materials in valleytronics, a means of reaching a permanent (and tunable) lifting of the valley degeneracy at K+ and K- has to be developed. The aim of the MAGICVALLEY project is to develop a new and promising method to reach this objective, which consists in breaking the time-reversal symmetry in MX2 through magnetic exchange coupling with a 2D (para)magnetic material. First, efforts will be dedicated to the growth and to the extensive characterization of magnetic 2D layers which can be either a ferromagnetic 2D layer (VSe2, MnSe2) or MSe2 (M=Mo, W) layers doped with magnetic impurities. Then, magnetic 2D layer/MX2 van der Waals heterostructures will be developed to explore the valley polarization by magnetic exchange proximity. The materials will be grown over large areas (1 cm2) by molecular beam epitaxy and chemical vapor deposition. This approach will provide the ultraclean interfaces needed for an efficient exchange coupling. Moreover, using van de Waals bonded layers will largely suppress interface reactivity. The project involves a comprehensive set of characterization techniques to firmly establish the atomic, magnetic and electronic structures of the constituent materials. Finally, the splitting of the valleys, a key step for valleytronics, will be investigated by both optical spectroscopy and electrical measurements. The experimental work will be supported by an important theoretical effort. All the partners of the project have a strong expertise in the study of 2D materials in their own field.