Halide perovskites exhibit giant photostriction, that is, volume or shape changes upon illumination. However, the microscopic origin of this phenomenon remains unclear and there are experimental reports of both light-induced lattice expansion and contraction. In this paper we establish a general method, based on first-principles calculations and molecular orbital theory, which provides a microscopic picture of photostriction in insulators based on the orbital characters of their electronic bands near the Fermi level. For lead-halide perovskites, we find that different valence states have different bonding characters, leading to opposing strengthening or weakening of bonds depending on the photoexcitation energy. The overall trend is that light induces lattice contraction at low excitation energies, while giant lattice expansion occurs at high excitation energies, rationalizing experimental reports.
Multiferroic materials provide robust and efficient routes for the control of magnetism by electric fields, which has been diligently sought after for a long time. The two-dimensional (2D) vdW multiferroics is a more exciting endeavour. To date, the nonvolatile manipulation of magnetism through ferroelectric polarization still remains challenging in a 2D vdW heterostructure multiferroic. Here, we report a van der Waals (vdW) heterostructure multiferroic comprising atomically thin layered antiferromagnet (AFM) CrI3 and ferroelectric (FE) {\alpha}-In2Se3. We demonstrate anomalously layer-selective nonreciprocal and nonvolatile electric-field control of magnetization by the ferroelectric polarization. The nonreciprocal electric control originates from an intriguing antisymmetric enhancement of interlayer ferromagnetic coupling in the opposite ferroelectric polarization configurations of {\alpha}-In2Se3, which favor to selectively switch the spins in the second layer. Our work provides numerous possibilities for creating diverse heterostructure multiferroics at the limit of few atomic layers for multi-stage magnetic memories and brain inspired in-memory computing.
The intrinsic lattice thermal conductivity of MoS 2 is an important aspect in the design of MoS 2 ‐based nanoelectronic devices. We investigate the lattice dynamics properties of MoS 2 by first‐principle calculations. The intrinsic thermal conductivity of single‐layer MoS 2 is calculated using the Boltzmann transport equation for phonons. The obtained thermal conductivity agrees well with the measurements. The contributions of acoustic and optical phonons to the lattice thermal conductivity are evaluated. The size dependence of thermal conductivity is investigated as well. image
The recent synthesis of monolayer fullerene networks (Hou, L., et al. Nature 2022, 606, 507) provides new opportunities for photovoltaics and photocatalysis because of their versatile crystal structures for further tailoring of electronic, optical, and chemical function. To shed light on the structural aspects of the photocatalytic water splitting performance of fullerene nanomaterials, we compare the photocatalytic properties of individual polymeric fullerene chains and monolayer fullerene networks from first-principles calculations. We find that the photocatalytic efficiency can be further optimized by reducing the dimensionality from two-dimensional (2D) to one-dimensional (1D). The conduction band edge of the polymeric C60 chain provides an external potential for the hydrogen reduction reaction much higher than that of its monolayer counterparts over a wider range of pH values, and there are 2 times more surface active sites in the 1D chain than in the 2D networks from a thermodynamic perspective. These observations identify the 1D fullerene polymer as a more promising candidate as a photocatalyst for the hydrogen evolution reaction in comparison to monolayer fullerene networks.
We investigate the quantum Hall effect in a single Landau level in the presence of a square superlattice of δ-function potentials.The interplay between the superlattice spacing as and the magnetic length ℓB in clean system leads to three interesting characteristic regimes corresponding to as < ℓB, as ≫ ℓB and the intermediate one where as ∼ ℓB.In the intermediate regime, the continuous magnetic translation symmetry breaks down to discrete lattice symmetry.In contrast, we show that in the other two regimes, the same is hardly broken in the topological band despite the presence of the superlattice.In the presence of weak disorder (whitenoise) one typically expects a tiny fraction of extended states due to topological protection of the Landau level.Interestingly, we obtain a large fraction of extended states throughout the intermediate regime which maximizes at the special point as = √ 2πℓB.We argue the superlattice induced percolation phenomenon requires both the breaking of the time reversal symmetry and the continuous magnetic translational symmetry.It could have a direct implication on the integer plateau transitions in both continuous quantum Hall systems and the lattice based anomalous quantum Hall effect.
Nanocrystals based on metal-halide perovskites offer a promising material platform for highly efficient lighting. Using transient optical spectroscopy, we study excitation recombination dynamics in manganese-doped CsPb(Cl,Br)3 perovskite nanocrystals. We find an increase in the intrinsic excitonic radiative recombination rate upon doping, which is typically a challenging material property to tailor. Supported by ab initio calculations, we can attribute the enhanced emission rates to increased exciton localization through lattice periodicity breaking from Mn dopants, which increases exciton effective masses and overlap of electron and hole wavefunctions and thus the oscillator strength. Our report of a fundamental strategy for improving luminescence efficiencies in perovskite nanocrystals will be valuable for maximizing efficiencies in light-emitting applications.
Controlling heat transport through material design is one important step toward thermal management in 2D materials. To control heat transport, a comprehensive understanding of how structure influences heat transport is required. It has been argued that a buckled structure is able to suppress heat transport by increasing the flexural phonon scattering. Using a first principles approach, we calculate the lattice thermal conductivity of 2D mono-elemental materials with a buckled structure. Somewhat counterintuitively, we find that although 2D group-V materials have a larger mass and higher buckling height than their group-IV counterparts, the calculated κ of blue phosphorene (106.6 W mK-1) is nearly four times higher than that of silicene (28.3 W mK-1), while arsenene (37.8 W mK-1) is more than fifteen times higher than germanene (2.4 W mK-1). We report for the first time that a buckled structure has three conflicting effects: (i) increasing the Debye temperature by increasing the overlap of the pz orbitals, (ii) suppressing the acoustic-optical scattering by forming an acoustic-optical gap, and (iii) increasing the flexural phonon scattering. The former two, corresponding to the harmonic phonon part, tend to enhance κ, while the last one, corresponding to the anharmonic part, suppresses it. This relationship between the buckled structure and phonon behaviour provides insight into how to control heat transport in 2D materials.
oneAPI is based on Data Parallel C++ (DPC++) and incorporates SYCL from the Khronos Group to support a cross-architecture programming environment. It delivers the freedom to choose suitable hardware for specific applications. In this paper, we developed a cross-architecture real-time medical ultrasound imaging application using oneAPI based on an open-source project SUPRA [1]. The application takes raw ultrasound data from ultrasound equipment as input and processes it to obtain B-mode images. The ultrasound processing pipeline contains four modules: Beamforming, Envelope Detection, Log-compression, and Scan-conversion. The proposed ultrasound medical beamforming algorithms are implemented with Intel's oneAPI, which enables the algorithms to target multiple hardware architectures: GPU, FPGA, and CPU. In this paper, we show how to migrate and optimize these medical ultrasound algorithms on the GPU and FPGA in a unified programming language supported by DPC++. We also compare the GPU and FPGA performance of the algorithms. The results show that the ultrasound application achieved 140.0, 176.1 and 168.4 FPS using Intel Iris Xe integrated graphics, DG1 GPU and Arria 10 FPGA, respectively. Finally, we also evaluate the computation results correctness of our implementations with the original SUPRA application.
To date, research on defect-induced magnetism in SiC has been conducted to clarify the relationship between the ferromagnetic signal and the carrier concentration. It has been experimentally proven that there is an interaction between the d0 magnetic moment and the hole carrier in p-type 4H-SiC. However, for n-type SiC, the existing theoretical predictions are insufficient to explain the variation in magnetization with the doping concentration. To solve this problem, we prepared 4H-SiC epitaxial layers with different nitrogen doping concentrations and introduced defects by ion implantation. By measuring and analyzing the magnetic properties, we found that the ferromagnetic composition depends on both the implantation dose and the doping concentration. By performing first-principles calculations, we studied the magnetic moments and interactions of defects with different charge states, which is related to defect-induced ferromagnetism. These defects include not only the paramagnetic centers reported in previous studies, such as silicon vacancies and divacancies, but also the NCVSi complex defect of recent interest, which are indicated by positron annihilation experiments. Combining experimental observations with theoretical calculations, we explained the relationship between magnetic properties and the nitrogen doping concentration in the epitaxial samples. Our research will help us to better understand the physical mechanism of defect-induced magnetism in doped semiconductors and provide a potential platform for the control of defect-induced magnetism by carrier density modulation and the fabrication of SiC spintronic devices without transition metals.
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