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Materials Science (cond-mat.mtrl-sci)

Wed, 12 Apr 2023

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1.Effect of Environmental Screening and Strain on Optoelectronic Properties of Two-Dimensional Quantum Defects

Authors:Shimin Zhang, Kejun Li, Chunhao Guo, Yuan Ping

Abstract: Point defects in hexagonal boron nitride (hBN) are promising candidates as single-photon emitters (SPEs) in nanophotonics and quantum information applications. The precise control of SPEs requires in-depth understanding of their optoelectronic properties. However, how the surrounding environment of host materials, including number of layers, substrates, and strain, influences SPEs has not been fully understood. In this work, we study the dielectric screening effect due to the number of layers and substrates, and the strain effect on the optical properties of carbon dimer and nitrogen vacancy defects in hBN from first-principles many-body perturbation theory. We report that the environmental screening causes lowering of the GW gap and exciton binding energy, leading to nearly constant optical excitation energy and exciton radiative lifetime. We explain the results with an analytical model starting from the BSE Hamiltonian with Wannier basis. We also show that optical properties of quantum defects are largely tunable by strain with highly anisotropic response, in good agreement with experimental measurements. Our work clarifies the effect of environmental screening and strain on optoelectronic properties of quantum defects in two-dimensional insulators, facilitating future applications of SPEs and spin qubits in low-dimensional systems.

2.Janus monolayer TaNF: a new ferrovalley material with large valley splitting and tunable magnetic properties

Authors:Guibo Zheng, Shuixian Qu, Wenzhe Zhou, Fangping Ouyang

Abstract: Materials with large intrinsic valley splitting and high Curie temperature are a huge advantage for studying valleytronics and practical applications. In this work, using first-principles calculations, a new Janus TaNF monolayer is predicted to exhibit excellent piezoelectric properties and intrinsic valley splitting, resulting from the spontaneous spin polarization, the spatial inversion symmetry breaking and strong spin-orbit coupling (SOC). TaNF is also a potential two-dimensional (2D) magnetic material due to its high Curie temperature and huge magnetic anisotropy energy. The effective control of the band gap of TaNF can be achieved by biaxial strain, which can transform TaNF monolayer from semiconductor to semi-metal. The magnitude of valley splitting at the CBM can be effectively tuned by biaxial strain due to the changes of orbital composition at the valleys. The magnetic anisotropy energy (MAE) can be manipulated by changing the energy and occupation (unoccupation) states of d orbital compositions through biaxial strain. In addition, Curie temperature reaches 373 K under only -3% biaxial strain, indicating that Janus TaNF monolayer can be used at high temperatures for spintronic and valleytronic devices.

3.An improved grand-potential phase-field model of solid-state sintering for many particles

Authors:Marco Seiz, Henrik Hierl, Britta Nestler

Abstract: Understanding the microstuctural evolution during the sintering process is of high relevance as it is a key part in many industrial manufacturing processes. Simulations are one avenue to achieve this understanding, especially field-resolved methods such as the phase-field method. Recent papers have shown several weaknesses in the most common phase-field model of sintering, which the present paper aims to ameliorate. The observed weaknesses are shortly recounted, followed by presenting model variations aiming to remove these deficiencies. The models are tested in the classical two-particle geometry, with the most promising model being run on large-scale three-dimensional packings to determine representative volume elements. A densification that is strongly dependent on the packing size is observed, which suggests that the model requires further improvement.

4.Exploring the Interfacial Coupling between Graphene and Antiferromagnetic Insulator MnPSe$_3$

Authors:Xin Yi, Qiao Chen, Kexin Wang, Yuanyang Yu, Yi Yan, Xin Jiang, Chengyu Yan, Shun Wang

Abstract: Interfacial coupling between graphene and other 2D materials can give rise to intriguing physical phenomena. In particular, several theoretical studies predict that the interplay between graphene and an antiferromagnetic insulator could lead to the emergence of quantum anomalous Hall phases. However, such phases have not been observed experimentally yet, and further experimental studies are needed to reveal the interaction between graphene and antiferromagnetic insulators. Here, we report the study in heterostructures composed of graphene and the antiferromagnetic insulator MnPSe$_3$. It is found that the MnPSe$_3$ has little impact on the quantum Hall phases apart from doping graphene via interfacial charge transfer. However, the magnetic order can contribute indirectly via process like Kondo effect, as evidenced by the observed minimum in the temperature-resistance curve between 20-40 K, far below the N\'eel temperature (70 K).

5.Non-Speckle-based DVC for Measuring Large Deformations in Homogeneous Solids using Laboratory X-ray CT

Authors:Zifan Wang, Akshay Joshi, Angkur Jyoti Dipanka Shaikeea, Vikram Susdhir Deshpande

Abstract: X-ray computed tomography (XCT) has become a reliable metrology tool for measuring internal flaws and other microstructural features in engineering materials. However, tracking of material points to measure three-dimensional (3D) deformations has hitherto relied on either artificially adding tracer particles (speckles) or exploiting inherent microstructural features such as inclusions. This has greatly limited the spatial resolution and magnitude of the deformation measurements. Here we report a novel Flux Enhanced Tomography for Correlation (FETC) technique that leverages the inherent inhomogeneities within nominally homogeneous engineering polymers to track 3D material point displacements without recourse to artificial speckles or microstructural features such as inclusions. The FETC is then combined with a Eulerian/Lagrangian transformation in a multi-step Digital Volume Correlation (DVC) methodology to measure all nine components of the deformation gradient within the volume of complex specimens undergoing extreme deformations. FETC is a powerful technique that greatly expands the capabilities of laboratory-based XCT to provide amongst other things the inputs required for data-driven constitutive modelling approaches.

6.Electrical Characteristics of in situ Mg-doped beta-Ga2O3 Current-Blocking Layer for Vertical Devices

Authors:Sudipto Saha, Lingyu Meng, A F M Anhar Uddin Bhuiyan, Ankit Sharma, Chinmoy Nath Saha, Hongping Zhao, Uttam Singisetti

Abstract: The lack of p-type doping has impeded the development of vertical gallium oxide (Ga2O3) devices. Current blocking layers (CBL) using implanted deep acceptors has been used to demonstrate vertical devices. This paper presents the first demonstration of in situ Mg-doped beta-Ga2O3 CBLs grown using metalorganic chemical vapor deposition. Device structures were designed with in-situ Mg doped layers with varied targeted Mg doping concentrations, which were calibrated by quantitative secondary ion mass spectroscopy (SIMS). The effectiveness of the CBL is characterized using temperature dependent current-voltage measurements using n-Mg-doped-n structures, providing crucial insight into the underlying mechanisms. To further validate the experimental results, a TCAD simulation is performed and the electrically active effective doping is found to be dependent on the Mg-doping density, offering a new perspective on the optimization of CBL performance. Breakdown measurements show a 3.4 MV/cm field strength. This study represents a significant step forward in the development of Ga2O3-based devices and paves the way for future advancements in this exciting field.

7.Machine-Learning Recognition of Dzyaloshinskii-Moriya Interaction from Magnetometry

Authors:Bradley J. Fugetta, Zhijie Chen, Dhritiman Bhattacharay, Kun Yue, Kai Liu, Amy Y. Liu, Gen Yin

Abstract: The Dzyaloshinskii-Moriya interaction (DMI), which is the antisymmetric part of the exchange interaction between neighboring local spins, winds the spin manifold and can stabilize non-trivial topological spin textures. Since topology is a robust information carrier, characterization techniques that can extract the DMI magnitude are important for the discovery and optimization of spintronic materials. Existing experimental techniques for quantitative determination of DMI, such as high-resolution magnetic imaging of spin textures and measurement of magnon or transport properties, are time consuming and require specialized instrumentation. Here we show that a convolutional neural network can extract the DMI magnitude from minor hysteresis loops, or magnetic `fingerprints', of a material. These hysteresis loops are readily available by conventional magnetometry measurements. This provides a convenient tool to investigate topological spin textures for next-generation information processing.