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Materials Science (cond-mat.mtrl-sci)

Mon, 01 May 2023

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1.Subcycle control of valley-selective excitation via dynamical Franz-Keldysh effect in WSe$_2$ monolayer

Authors:Shunsuke Yamada, Kazuhiro Yabana, Tomohito Otobe

Abstract: This study performed first-principles calculations based on the time-dependent density functional theory to control the valley degree of freedom relating to the dynamical Franz-Keldysh effect (DFKE) in a monolayer of transition metal dichalcogenide. By mimicking the attosecond transient absorption spectroscopy, we performed numerical pump-probe experiments to observe DFKE around the $K$ or $K'$ valley in WSe$_2$ monolayer with a linearly-polarized pump field and a circularly-polarized probe pulse. We found that the circularly-polarized probe pulse with a given helicity can selectively observe the transient conductivity modulated by DFKE in each valley. The transient conductivity and excitation probability around each valley oscillate with the pump field frequency $\Omega$. The phases of the $\Omega$ oscillation for the $K$ and $K'$ valleys are opposite to each other. Furthermore, the pump-driven DFKE alters the absorption rate of WSe$_2$ monolayer and yields the valley-dependent $\Omega$ oscillation of the electron excitation induced by the pump plus probe field. With a simplified two-band model, we identified the $\Omega$ oscillation of the off-diagonal conductivity caused by the band asymmetry around the valleys as the physical mechanism responsible for the valley-selective DFKE.

2.Perpendicular magnetic anisotropy of an ultrathin Fe layer grown on NiO(001)

Authors:Soki Kobayashi, Hiroki Koizumi, Hideto Yanagihara, Jun Okabayashi, Takahiro Kondo, Takahide Kubota, Koki Takanashi, Yoshiaki Sonobe

Abstract: The magnetic anisotropy and magnetic interactions at the interface between Fe and NiO(001) were investigated. Depending on the growth conditions of the NiO(001) layers and the post-annealing temperature, the preferential magnetization direction of the ultrathin Fe layer grown on a NiO(001) layer changed from in-plane to a direction perpendicular to the film plane. The lattice constant of the NiO(001) layers parallel to the growth direction increased with O$_2$ flow rate, while that parallel to the in-plane were locked onto the MgO(001) substrate regardless of the growth conditions of the NiO layers. Moreover, perpendicular magnetization was observed only when the NiO layer was grown with O$_2$ flow rates higher than 2.0 sccm corresponding to oxygen-rich NiO. X-ray magnetic circular dichroism measurements revealed an enhancement in anisotropic orbital magnetic moments similar to the origin of perpendicular magnetic anisotropy at the Fe/MgO(001) interface. The interfacial magnetic anisotropy energies were 0.93 and 1.02 mJ/m$^2$ at room temperature and at 100 K, respectively, indicating less temperature dependence. In contrast, the coercivity $H_c$ exhibited a significant temperature dependence. Although no signature of exchange bias or unidirectional loop shift was observed, $H_c$ was strongly dependent on the NiO layer thickness, indicating that the exchange interaction at the interface between the ferromagnetic and antiferromagnetic layers was not negligible, despite the NiO(001) being a spin-compensated surface.

3.Crystalline Phase Effects on the Nonlinear Optical Response of MoS2 and WS2 Nanosheets

Authors:Michalis Stavrou, Nikolaos Chazapis, Eleni Nikoli, Raul Arenal, Nikos Tagmatarchis, Stelios Couris

Abstract: In the present work, some MoS2 and WS2 nanosheets were prepared and characterized. Depending on the preparation procedures, trigonal prismatic (2H) or octahedral (1T) coordination of the metal atoms were obtained, exhibiting metallic (1T) or semiconducting (2H) character. Both MoS2 and WS2 nanosheets were found exhibiting large nonlinear optical (NLO) response, strongly dependent on their metallic (1T) or semiconducting (2H) character. So, the semiconducting character 2H-MoS2 and 2H-WS2 exhibit positive nonlinear absorption and strong self-focusing, while their metallic character counterparts exhibit strong negative nonlinear absorption and important self-defocusing. In addition, the semiconducting MoS2 and WS2 were found exhibiting important and very broadband optical limiting action extended from 450 to 1750 nm. So, by selecting the crystalline phase of the nanosheets, i.e., their semiconduction/metallic character, their NLO response can be greatly modulated. The results of the present work demonstrate unambiguously that the control of the crystalline phase of MoS2 and WS2 provides an efficient strategy for 2D nanostructures with custom made NLO properties for specific optoelectronic and photonic applications.

4.Molybdenum diselenide-manganese porphyrin bifunctional electrocatalyst for hydrogen evolution reaction and selective hydrogen peroxide production

Authors:Antonia Kagkoura, Christina Stangel, Raul Arenal, Nikos Tagmatarchis

Abstract: Electrochemical reactions for hydrogen and hydrogen peroxide production are essential for energy conversion to diminish energy crisis, but still lack efficient electrocatalysts. Development of non\-noble metal bifunctional electrocatalysts for hydrogen evolution and 2e oxygen reduction reaction to ease reaction kinetics is a challenging task. Integration of single components by employing easy strategies provides a key\-step towards the realization of highly active electrocatalysts. In this vein, MoSe2 owns catalytic active sites and high specific surface area but suffers from insufficient conductivity and high catalytic performance that noble\-metals provide. Herein, MoSe2 was used as a platform for the incorporation of manganese metallated porphyrin. The developed hybrid, namely MoSe2\-MnP, by the initial metal\-ligand coordination and the subsequent grafting with MnP was fully characterized and electrochemically assessed. The bifunctional electrocatalyst lowered the overpotential toward hydrogen evolution, improved reaction kinetics and charge transfer processes and was extremely stable after 10000 ongoing cycles. Simultaneously, rotating ring disk electrode analysis showed that oxygen reduction proceeds through the 2e pathway for the selective production of hydrogen peroxide with a high yield of 97 percent. The new facile modification route can be applied in diverse transition metal dichalcogenides and will help the development of new advanced functional materials.

5.Discovery and construction of surface kagome electronic states induced by p-d electronic hybridization

Authors:Li Huang, Xianghua Kong, Qi Zheng, Yuqing Xing, Hui Chen, Yan Li, Zhixin Hu, Shiyu Zhu, Jingsi Qiao, Yu-Yang Zhang, Haixia Cheng, Zhihai Cheng, Xianggang Qiu, Enke Liu, Hechang Lei, Xiao Lin, Ziqiang Wang, Haitao Yang, Wei Ji, Hong-Jun Gao

Abstract: Kagome-lattice materials possess attractive properties for quantum computing applications, but their synthesis remains challenging. Herein, we show surface kagome electronic states (SKESs) on a Sn-terminated triangular Co3Sn2S2 surface, which are imprinted by vertical p-d electronic hybridization between the surface Sn (subsurface S) atoms and the buried Co kagome lattice network in the Co3Sn layer under the surface. Owing to the subsequent lateral hybridization of the Sn and S atoms in a corner-sharing manner, the kagome symmetry and topological electronic properties of the Co3Sn layer is proximate to the Sn surface. The SKESs and both hybridizations were verified via qPlus non-contact atomic force microscopy (nc-AFM) and density functional theory calculations. The construction of SKESs with tunable properties can be achieved by the atomic substitution of surface Sn (subsurface S) with other group III-V elements (Se or Te), which was demonstrated theoretically. This work exhibits the powerful capacity of nc-AFM in characterizing localized topological states and reveals the strategy for synthesis of large-area transition-metal-based kagome lattice materials using conventional surface deposition techniques.

6.Accelerating microstructure modelling via machine learning: a new method combining Autoencoder and ConvLSTM

Authors:Owais Ahmad, Naveen Kumar, Rajdip Mukherjee, Somnath Bhowmick

Abstract: Phase-field modeling is an elegant and versatile computation tool to predict microstructure evolution in materials in the mesoscale regime. However, these simulations require rigorous numerical solutions of differential equations, which are accurate but computationally expensive. To overcome this difficulty, we combine two popular machine learning techniques, autoencoder and convolutional long short-term memory (ConvLSTM), to accelerate the study of microstructural evolution without compromising the resolution of the microstructural representation. After training with phase-field generated microstructures of ten known compositions, the model can accurately predict the microstructure for the future nth frames based on previous m frames for an unknown composition. Replacing n phase-field steps with machine-learned microstructures can significantly accelerate the in silico study of microstructure evolution.