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1. Energy-valley-dependent charge transfer in few-layer transition metal dichalcogenide heterostructures

   https://doi.org/10.1103/PhysRevB.108.085302  

   Valencia-Acuna, Pavel; Amos, Stephanie; Peelaers, Hartwin; Zhao, Hui (August 2023, Physical Review B)

   The effect of the energy valley on interlayer charge transfer in transition metal dichalcogenide (TMD) heterostructures is studied by transient absorption spectroscopy and density functional theory. First-principles calculations confirm that the Λmin valley in the conduction band of few-layer WSe2 evolves from above its K valley in the monolayer (1L) to below it in 4L. Heterostructure samples of 𝑛⁢L−WSe2/1⁢L−MoS2, where 𝑛=1,2,3, and 4, are obtained by mechanical exfoliation and dry transfer. Photoluminescence spectroscopy reveals a thickness-dependent WSe2 band structure and efficient interlayer charge transfer. Transient absorption measurements show that the electron transfer time from the Λmin valley of 4L WSe2 to the K valley of MoS2 is on the order of 30 ps. This process is much slower than the K-K charge transfer in 1L/1L TMD heterostructures. The momentum-indirect interlayer excitons formed after charge transfer have lifetimes >1 ns. 

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2. Rapid and Rigorous Learning Simulation Methodology with High Accuracy for Electronic and Optical Superlattices

   Veresko, Martin; Liu, Yu; Hou, Daqing; Cheng, Ming-Cheng (July 2024, 24th Int'l Conf. Computational Science (ICCS2024), Malaga, Spain)

   A rigorous physics-informed learning methodology is proposed for predictions of wave solutions and band structures in electronic and optical superlattice structures. The methodology is enabled by proper orthogonal decomposition (POD) and Galerkin projection of the wave equation. The approach solves the wave eigenvalue problem in POD space constituted by a finite set of basis functions (or POD modes). The POD ensures that the generated modes are optimized and tailored to the parametric variations of the system. Galerkin projection however enforces physical principles in the methodology to further enhance the accuracy and efficiency of the developed model. It has been demonstrated that the POD-Galerkin methodology offers an approach with a reduction in degrees of freedom by 4 orders of magnitude, compared to direct numerical simulation (DNS). A computing speedup near 15,000 times over DNS can be achieved with high accuracy for either of the superlattice structures if only the band structure is calculated without the wave solution. If both wave function solution and band structure are needed, a 2-order reduction in computational time can be achieved with a relative least square error (LSE) near 1%. When the training is incomplete or the desired eigenstates are slightly beyond the training bounds, an accurate prediction with an LSE near 1%-2% still can be reached if more POD modes are included. This reveals its remarkable learning ability to reach correct solutions with the guidance of physical principles provided by Galerkin projection. 

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3. Density functional descriptions of interfacial electronic structure

   https://doi.org/10.1063/5.0156437  

   Liu, Zhen-Fei (September 2023, Chemical Physics Reviews)

   Heterogeneous interfaces are central to many energy-related applications in the nanoscale. From the first-principles electronic structure perspective, one of the outstanding problems is accurately and efficiently calculating how the frontier quasiparticle levels of one component are aligned in energy with those of another at the interface, i.e., the so-called interfacial band alignment or level alignment. The alignment or the energy offset of these frontier levels is phenomenologically associated with the charge-transfer barrier across the interface and therefore dictates the interfacial dynamics. Although many-body perturbation theory provides a formally rigorous framework for computing the interfacial quasiparticle electronic structure, it is often associated with a high computational cost and is limited by its perturbative nature. It is, therefore, of great interest to develop practical alternatives, preferably based on density functional theory (DFT), which is known for its balance between efficiency and accuracy. However, conventional developments of density functionals largely focus on total energies and thermodynamic properties, and the design of functionals aiming for interfacial electronic structure is only emerging recently. This Review is dedicated to a self-contained narrative of the interfacial electronic structure problem and the efforts of the DFT community in tackling it. Since interfaces are closely related to surfaces, we first discuss the key physics behind the surface and interface electronic structure, namely, the image potential and the gap renormalization. This is followed by a review of early examinations of the surface exchange-correlation hole and the exchange-correlation potential, which are central quantities in DFT. Finally, we survey two modern endeavors in functional development that focus on the interfacial electronic structure, namely, the dielectric-dependent hybrids and local hybrids. 

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4. ΔK =0 M1 Excitation Strength of the Well-Deformed Nucleus 164Dy from K Mixing

   T. Beck, V. Werner (August 2020, Physical review letters)

   The size of a ΔK ¼ 0 M1 excitation strength has been determined for the first time in a predominantly axially deformed even-even nucleus. It has been obtained from the observation of a rare K-mixing situation between two close-lying Jπ ¼ 1þ states of the nucleus 164Dy with components characterized by intrinsic projection quantum numbers K ¼ 0 and K ¼ 1. Nuclear resonance fluorescence induced by quasimonochromatic linearly polarized γ-ray beams provided evidence for K mixing of the 1þ states at 3159.1(3) and 3173.6(3) keV in excitation energy from their γ-decay branching ratios into the ground-state band. The ΔK ¼ 0 transition strength of BðM1; 0 þ 1 → 1 þ K¼0 Þ ¼ 0.008ð1Þμ2 N was inferred from a mixing analysis of their M1 transition rates into the ground-state band. It is in agreement with predictions from the quasiparticle phonon nuclear model. This determination represents first experimental information on the M1 excitation strength of a nuclear quantum state with a negative R-symmetry quantum number. 

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5. High quality wurtzite BAlN with high B content by metalorganic chemical vapor deposition

   Li, Xiao-Hang; Wang, Shuo; Ponce, Fernando A; Detechprohm, Theeradetch; Ougazzaden, Abdallah; Dupuis, Russell D. (June 2015, 57th Electronic Materials Conference)

   BAlN films were grown by flow-rate modulation epitaxy on AlN. Figure 1 shows x-ray diffraction (XRD) peaks of 3-µm AlN/(0001) sapphire template layer and 45-nm BAlN layer at 2θ angles of 36.146o and 36.481o, corresponding to c-lattice constants of 4.966 and 4.922Å, respectively. The BAlN XRD peak is very clear and distinct given the small thickness, indicating good wurtzite crystallinity. It is not possible to directly calculate the B content from XRD alone because of uncertainty of the lattice parameters and strain. However, based on the angular separation of the XRD peaks and c-lattice constant difference, the B content is estimated to be ~7% [ ], which is considerably higher than those of high-quality wurtzite BAlN layers reported before [ , , ]. To obtain the accurate B content, Rutherford backscattering spectrometry (RBS) measurements are being made. Figures 2(a)-(b) show a high-resolution cross-sectional transmission electron microscopy (TEM) image with a magnification of 150 kx taken at a-zone axis ([11-20] projection) and diffraction pattern after fast-Fourier transform (FFT). A sharp interface between the AlN and BAlN layers is observed. In addition, the BAlN film exhibits a highly ordered lattice throughout the entire 45nm thickness without the polycrystalline columnar structures found in previous reports [1, ]. The FFT image confirms a wurtzite structure oriented along c-axis. Figure 3 shows a 5×5 µm2 atomic force microscopy (AFM) image of BAlN layer surface. The root-mean-square (RMS) surface roughness is ~1.7nm. Surface macro-steps were found on the surface due to longer diffusion length of group-III atoms than the expected step terrace width. This indicates there is potential to lower the growth temperature to create smoother surfaces while maintaining crystallinity which has been observed for AlN [ ]. In summary, a high-quality wurtzite BAlN layer with relatively high B content ~7% was demonstrated by MOCVD. Refractive index will be measured to facilitate design of distributed Bragg reflector (DBR) for deep UV vertical-cavity surface-emitting laser (VCSEL). 

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