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1. Proximity-enhanced valley Zeeman splitting at the WS 2 /graphene interface

   https://doi.org/10.1088/2053-1583/acd5df  

   Faria Junior, Paulo E. ; Naimer, Thomas ; McCreary, Kathleen M. ; Jonker, Berend T. ; Finley, Jonathan J. ; Crooker, Scott A. ; Fabian, Jaroslav ; Stier, Andreas V. ( May 2023 , 2D Materials)

   The valley Zeeman physics of excitons in monolayer transition metal dichalcogenides provides valuable insight into the spin and orbital degrees of freedom inherent to these materials. Being atomically-thin materials, these degrees of freedom can be influenced by the presence of adjacent layers, due to proximity interactions that arise from wave function overlap across the 2D interface. Here, we report 60 T magnetoreflection spectroscopy of the A- and B- excitons in monolayer WS₂, systematically encapsulated in monolayer graphene. While the observed variations of the valley Zeeman effect for the A- exciton are qualitatively in accord with expectations from the bandgap reduction and modification of the exciton binding energy due to the graphene-induced dielectric screening, the valley Zeeman effect for the B- exciton behaves markedly different. We investigate prototypical WS₂/graphene stacks employing first-principles calculations and find that the lower conduction band of WS₂at the$K/K^{\mathrm{\prime }}$valleys (the${\mathrm{C}\mathrm{B}}^{-}$band) is strongly influenced by the graphene layer on the orbital level. Specifically, our detailed microscopic analysis reveals that the conduction band at theQpoint of WS₂mediates the coupling between${\mathrm{C}\mathrm{B}}^{-}$and graphene due to resonant energy conditions and strong coupling to the Dirac cone. This leads to variations in the valley Zeeman physics of the B- exciton, consistent with the experimental observations. Our results therefore expand the consequences of proximity effects in multilayer semiconductor stacks, showing that wave function hybridization can be a multi-step energetically resonant process, with different bands mediating the interlayer interactions. Such effects can be further exploited to resonantly engineer the spin-valley degrees of freedom in van der Waals and moiré heterostructures.

    

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2. Sputtered L 1 0 ‐FePd and its Synthetic Antiferromagnet on Si/SiO 2 Wafers for Scalable Spintronics

   https://doi.org/10.1002/adfm.202214201  

   Lyu, Deyuan ; Shoup, Jenae E. ; Huang, Dingbin ; García‐Barriocanal, Javier ; Jia, Qi ; Echtenkamp, William ; Rojas, Geoffrey A. ; Yu, Guichuan ; Zink, Brandon R. ; Wang, Xiaojia ; et al ( February 2023 , Advanced Functional Materials)

   As a promising alternative to the mainstream CoFeB/MgO system with interfacial perpendicular magnetic anisotropy (PMA),L1₀‐FePd and its synthetic antiferromagnet (SAF) structure with large crystalline PMA can support spintronic devices with sufficient thermal stability at sub‐5 nm sizes. However, the compatibility requirement of preparingL1₀‐FePd thin films on Si/SiO₂wafers is still unmet. In this paper, high‐qualityL1₀‐FePd and its SAF on Si/SiO₂wafers are prepared by coating the amorphous SiO₂surface with an MgO(001) seed layer. The preparedL1₀‐FePd single layer and SAF stack are highly (001)‐textured, showing strong PMA, low damping, and sizeable interlayer exchange coupling, respectively. Systematic characterizations, including advanced X‐ray diffraction measurement and atomic resolution‐scanning transmission electron microscopy, are conducted to explain the outstanding performance ofL1₀‐FePd layers. A fully‐epitaxial growth that starts from MgO seed layer, induces the (001) texture ofL1₀‐FePd, and extends through the SAF spacer is observed. This study makes the vision of scalable spintronics more practical.

    

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3. Quadrupolar excitons and hybridized interlayer Mott insulator in a trilayer moiré superlattice

   https://doi.org/10.1038/s41467-023-40288-9  

   Lian, Zhen ; Chen, Dongxue ; Ma, Lei ; Meng, Yuze ; Su, Ying ; Yan, Li ; Huang, Xiong ; Wu, Qiran ; Chen, Xinyue ; Blei, Mark ; et al ( August 2023 , Nature Communications)

   Transition metal dichalcogenide (TMDC) moiré superlattices, owing to the moiré flatbands and strong correlation, can host periodic electron crystals and fascinating correlated physics. The TMDC heterojunctions in the type-II alignment also enable long-lived interlayer excitons that are promising for correlated bosonic states, while the interaction is dictated by the asymmetry of the heterojunction. Here we demonstrate a new excitonic state, quadrupolar exciton, in a symmetric WSe₂-WS₂-WSe₂trilayer moiré superlattice. The quadrupolar excitons exhibit a quadratic dependence on the electric field, distinctively different from the linear Stark shift of the dipolar excitons in heterobilayers. This quadrupolar exciton stems from the hybridization of WSe₂valence moiré flatbands. The same mechanism also gives rise to an interlayer Mott insulator state, in which the two WSe₂layers share one hole laterally confined in one moiré unit cell. In contrast, the hole occupation probability in each layer can be continuously tuned via an out-of-plane electric field, reaching 100% in the top or bottom WSe₂under a large electric field, accompanying the transition from quadrupolar excitons to dipolar excitons. Our work demonstrates a trilayer moiré system as a new exciting playground for realizing novel correlated states and engineering quantum phase transitions.

    

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4. Digital Tuning of the Transition Temperature of Epitaxial VO 2 Thin Films on MgF 2 Substrates by Strain Engineering

   https://doi.org/10.1002/admi.202001790  

   Howard, Sebastian A. ; Evlyukhin, Egor ; Páez Fajardo, Galo ; Paik, Hanjong ; Schlom, Darrell G. ; Piper, Louis F. J. ( March 2021 , Advanced Materials Interfaces)

   Straining the vanadium dimers along the rutilec‐axis can be used to tune the metal‐to‐insulator transition (MIT) of VO₂but has thus far been limited to TiO₂substrates. In this work VO₂/MgF₂epitaxial films are grown via molecular beam epitaxy (MBE) to strain engineer the transition temperature (T_MIT). First, growth parameters are optimized by varying the synthesis temperature of the MgF₂(001) substrate (T_S) using a combination of X‐ray diffraction techniques, temperature dependent transport, and soft X‐ray photoelectron spectroscopy. It is determined thatT_Svalues greater than 350 °C induce Mg and F interdiffusion and ultimately the relaxation of the VO₂layer. Using the optimized growth temperature, VO₂/MgF₂(101) and (110) films are then synthesized. The three film orientations display MITs with transition temperatures in the range of 15–60 °C through precise strain engineering.

    

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5. Tuning moiré excitons and correlated electronic states through layer degree of freedom

   https://doi.org/10.1038/s41467-022-32493-9  

   Chen, Dongxue ; Lian, Zhen ; Huang, Xiong ; Su, Ying ; Rashetnia, Mina ; Yan, Li ; Blei, Mark ; Taniguchi, Takashi ; Watanabe, Kenji ; Tongay, Sefaattin ; et al ( August 2022 , Nature Communications)

   Moiré coupling in transition metal dichalcogenides (TMDCs) superlattices introduces flat minibands that enable strong electronic correlation and fascinating correlated states, and it also modifies the strong Coulomb-interaction-driven excitons and gives rise to moiré excitons. Here, we introduce the layer degree of freedom to the WSe₂/WS₂moiré superlattice by changing WSe₂from monolayer to bilayer and trilayer. We observe systematic changes of optical spectra of the moiré excitons, which directly confirm the highly interfacial nature of moiré coupling at the WSe₂/WS₂interface. In addition, the energy resonances of moiré excitons are strongly modified, with their separation significantly increased in multilayer WSe₂/monolayer WS₂moiré superlattice. The additional WSe₂layers also modulate the strong electronic correlation strength, evidenced by the reduced Mott transition temperature with added WSe₂layer(s). The layer dependence of both moiré excitons and correlated electronic states can be well described by our theoretical model. Our study presents a new method to tune the strong electronic correlation and moiré exciton bands in the TMDCs moiré superlattices, ushering in an exciting platform to engineer quantum phenomena stemming from strong correlation and Coulomb interaction.

    

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