arXiv subjects

Congwei Xie

Publications and source records attributed to Congwei Xie.

10 recordsLinked to original sources

arXiv2026

A new framework for atom-resolved decomposition of second-harmonic generation in nonlinear-optical crystals

In this work, we develop a new framework for computing atom-resolved contributions to optical properties based on atoms-in-molecules (AIM) schemes. The formalism is independent of the specific AIM method and is made rigorous by partitioning momentum matrix elements into atomic contributions while exactly satisfying the relevant sum rules. We apply it to second-harmonic generation (SHG) in six representative UV and deep-UV nonlinear-optical crystals, namely $\beta$-\ce{BaB2O4} (BBO), \ce{LiB3O5} (LBO), \ce{CsB3O5} (CBO), \ce{CsLiB6O10} (CLBO), \ce{KBe2BO3F2} (KBBF), and \ce{LiCs2PO4} (LCPO). The atom-triplet decomposition reveals a clear hierarchy for the largest SHG component of each crystal. In general, two-center terms provide the leading contribution, one-center terms remain comparatively small, and fully three-center terms supply an important secondary contribution. A motif-triplet decomposition further indicates behavior dominated by the anionic framework in KBBF and LBO. In BBO, CBO, and CLBO, contributions from the anionic framework and the cation sublattice act cooperatively, although the cation contribution is crystal dependent. Moreover, cooperative contributions from the phosphate framework and the Cs sublattice are also observed in LCPO, where the O-Cs contribution is particularly significant. These results may provide a new perspective for understanding the microscopic origin of SHG in nonlinear-optical materials.

physics.chem-ph↗

arXiv2026

Impact of scissors-correction schemes on first-principles calculations of second-harmonic generation in ultraviolet nonlinear-optical crystals

In this work, we assess two widely used scissors-correction schemes for first-principles calculations of second-harmonic generation in representative borate and phosphate ultraviolet nonlinear-optical (UV-NLO) crystals, namely scheme-L [Phys.\ Rev.\ Lett.\ \textbf{63}, 1719 (1989)] and scheme-N [Phys.\ Rev.\ B \textbf{72}, 045223 (2005)]. To enable controlled and numerically robust comparisons, we derive a unified static-limit formulation that avoids spurious divergences and is applicable to both schemes, thereby extending earlier static-limit treatments that were effectively restricted to scheme-L. Benchmark calculations show that both schemes largely preserve the spectral line shape while mainly rescaling the overall response. Scheme-N systematically yields 15\%--25\% larger SHG magnitudes than scheme-L, although for some tensor components and experimental datasets scheme-L shows closer agreement with experiment. We further show that Kleinman symmetry is satisfied in the static limit at the level of the formal theory, whereas apparent violations in practical calculations arise mainly from the numerical approximation used to evaluate generalized derivatives.

physics.chem-ph↗

arXiv2025

Self-Optimizing Machine Learning Potential Assisted Automated Workflow for Highly Efficient Complex Systems Material Design

Machine learning interatomic potentials have revolutionized complex materials design by enabling rapid exploration of material configurational spaces via crystal structure prediction with ab initio accuracy. However, critical challenges persist in ensuring robust generalization to unknown structures and minimizing the requirement for substantial expert knowledge and time-consuming manual interventions. Here, we propose an automated crystal structure prediction framework built upon the attention-coupled neural networks potential to address these limitations. The generalizability of the potential is achieved by sampling regions across the local minima of the potential energy surface, where the self-evolving pipeline autonomously refines the potential iteratively while minimizing human intervention. The workflow is validated on Mg-Ca-H ternary and Be-P-N-O quaternary systems by exploring nearly 10 million configurations, demonstrating substantial speedup compared to first-principles calculations. These results underscore the effectiveness of our approach in accelerating the exploration and discovery of complex multi-component functional materials.

cond-mat.mtrl-sci↗

arXiv2025

Graph Neural Network Prediction of Nonlinear Optical Properties

Nonlinear optical (NLO) materials for generating lasers via second harmonic generation (SHG) are highly sought in today's technology. However, discovering novel materials with considerable SHG is challenging due to the time-consuming and costly nature of both experimental methods and first-principles calculations. In this study, we present a deep learning approach using the Atomistic Line Graph Neural Network (ALIGNN) to predict NLO properties. Sourcing data from the Novel Opto-Electronic Materials Discovery (NOEMD) database and using the Kurtz-Perry (KP) coefficient as the key target, we developed a robust model capable of accurately estimating nonlinear optical responses. Our results demonstrate that the model achieves 82.5% accuracy at a tolerated absolute error up to 1 pm/V and relative error not exceeding 0.5. This work highlights the potential of deep learning in accelerating the discovery and design of advanced optical materials with desired properties.

cond-mat.mtrl-sci↗

arXiv2025

Two-Dimensional Graphene-like BeO Sheet: A Promising Deep-Ultraviolet Nonlinear Optical Materials System with Strong and Highly Tunable Second Harmonic Generation

Two-dimensional (2D) materials with large band gaps and strong and tunable second-harmonic generation (SHG) coefficients play an important role in the miniaturization of deep-ultraviolet (DUV) nonlinear optical (NLO) devices. Despite the existence of numerous experimentally synthesized 2D materials, none of them have been reported to meet DUV NLO requirements. Herein, to the first time, an experimentally available graphene-like BeO monolayer only formed by NLO-active [BeO3] unit is suggested as a promising 2D DUV NLO material due to its ultrawide band gap (6.86 eV) and a strong SHG effect (\{chi}_"22" ^((2))(2D) = 6.81 {\AA}\times pm/V) based on the first-principles calculations. By applying stacking, strain, and twist engineering methods, several 2D BeO sheets have been predicted, and the flexible structural characteristics endow them with tunable NLO properties. Remarkably, the extremely stress-sensitive out-of-plane \{chi}_"15" ^((2))(2D) and \{chi}_"33" ^((2))(2D) (exceptional 30% change) and the robust in-plane \{chi}_"22" ^((2))(2D) against large strains can be achieved together in AC-, AAC-, AAE, and ACE-stacking BeO sheets under in-plane biaxial strain, exhibiting emergent phenomena uniquely not yet seen in other known 2D NLO materials. Our present results reveal that 2D BeO systems should be a new option for 2D DUV NLO materials.

cond-mat.mes-hall↗

arXiv2022

PNO: A Promising Deep-UV Nonlinear Optical Material with Extremely High Second Harmonic Generation Effect

In this work, the polar tetrahedron [PN$_2$O$_2$] was revealed as a new deep-ultraviolet (deep-UV) nonlinear optically active unit. Accordingly, a thermodynamically stable compound (PNO) consisting of the polar [PN$_2$O$_2$] units was predicted and suggested as a promising candidate of deep-UV nonlinear optical (NLO) material. Compared with other deep-UV materials known to date, PNO possesses the strongest second harmonic generation (SHG) coefficient (about 6 times that of KH$_2$PO$_4$ (KDP)). Moreover, its three-dimensional connectivity endows it with good mechanical and thermal properties. Therefore, PNO should be a new option for non-$π$-conjugated deep-UV NLO materials.

physics.optics↗

arXiv2018

Novel high-pressure calcium carbonates

Calcium and magnesium carbonates are believed to be the host compounds for most of the oxidized carbon in the Earth's mantle. Here, using evolutionary crystal structure prediction method USPEX, we systematically explore the MgO-CO2 and CaO-CO2 systems at pressures ranging from 0 to 160 GPa to search for thermodynamically stable magnesium and calcium carbonates. While MgCO3 is the only stable magnesium carbonate, three calcium carbonates are stable under pressure: well-known CaCO3, and newly predicted Ca3CO5 and CaC2O5. Ca3CO5 polymorphs are found to contain isolated orthocarbonate (CO4)4- tetrahedra, and are stable at relatively low pressures (>11 GPa), whereas CaC2O5 is stable above 33 GPa and its polymorphs feature polymeric motifs made of CO4 tetrahedra. Detailed analysis of chemical stability of CaCO3, Ca3CO5 and CaC2O5 in the environment typical of the Earth's lower mantle reveals that none of these compounds can exist in the Earth's lower mantle. We conclude that MgCO3 is the main host of oxidized carbon throughout the lower mantle.

cond-mat.mtrl-sci↗

arXiv2015

Effect of carbon vacancies on structural and mechanical properties of stable zirconium carbides: A first principles study

By using evolutionary algorithm USPEX, we have predicted a number of stable zirconium carbides. In addition to the well-known rocksalt-type stoichiometric ZrC (Fm-3m), present prediction also identifies five stable substoichiometric zirconium carbides adopting rocksalt-type structures with ordered carbon vacancies, Zr8C7 (P-1), Zr6C5 (C2/m), Zr5C4 (P-1), Zr3C2 (C2/m), and Zr2C (Fd-3m). The effects of carbon vacancies on structural and mechanical properties are investigated. We highlight that the distribution of carbon vacancies has significant influence on volume, Pugh's ratio, and hardness. We further propose that hardness can be enhanced by replacing carbon vacancies with suitable elements, in particular nitrogen and oxygen.

cond-mat.mtrl-sci↗

arXiv2013

Prediction of stable hafnium carbides: their stoichiometries, mechanical properties, and electronic structure

Hafnium carbides are studied by a systematic search for possible stable stoichiometric compounds in the Hf-C system at ambient pressure using variable-composition ab initio evolutionary algorithm implemented in the USPEX code. In addition to well-known HfC, we predicted two additional compounds Hf3C2 and Hf6C5. The structure of Hf6C5 with space group C2/m contains 11 atoms in the primitive cell and this prediction revives the earlier proposal by A. I. Gusev. The stable structure of Hf3C2 also has space group C2/m, and is more energetically favorable than the Immm, P-3m1, P2 and C2221 structures put forward by A. I. Gusev. Dynamical and mechanical stability of the newly predicted structures have been verified by calculations of their phonons and elastic constants. The bulk and shear moduli of Hf3C2 are 195.8 GPa and 143.1 GPa, respectively, while for Hf6C5 they are 227.9 GPa and 187.2 GPa, respectively. Their mechanical properties are inferior to those of HfC due to the presence of structural vacancies. Chemical bonding, band structure, and Bader charge are presented and discussed.

cond-mat.mtrl-sci↗

arXiv2013

Evolutionary search for new high-k dielectric materials: methodology and applications to hafnia-based oxides

High-k dielectric materials are important as gate oxides in microelectronics and as potential dielectrics for capacitors. In order to enable computational discovery of novel high-k dielectric materials, we propose a fitness model (energy storage density) that includes the dielectric constant, bandgap, and intrinsic breakdown field. This model, used as fitness function in conjunction with first-principles calculations and global optimization evolutionary algorithm USPEX, efficiently leads to practically important results. We found a number of high-fitness structures of SiO2 and HfO2, some of which correspond to known phases and some are new. The results allow us to propose characteristics (genes) common to high-fitness structures - these are the coordination polyhedra and their degree of distortion. Our variable-composition searches in the HfO2-SiO2 system uncovered several high-fitness states. This hybrid algorithm opens up a new avenue of discovering novel high-k dielectrics with both fixed and variable compositions, and will speed up the process of materials discovery.

cond-mat.mtrl-sci↗