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The Janus VSBrI monolayer, identified by first-principles calculations, emerges as a promising semiconducting material with desirable ferromagnetic and ferroelectric properties. It's in-plane magnetic anisotropic energy is up to 460 $\mu$eV/V , and in-plane and out-of-plane piezoelectric strain coefficients are larger than many other known two-dimensional materials. The energy variations among different magnetic states show a strong correlate with polarization. Interestingly, the stability of the ferroelectric phase can be further enhanced by the application of biaxial tensile strain. These intriguing properties make the Janus VSBrI monolayer highly desirable for practical applications in piezoelectronic devices, and a promising candidate for multifunctional spintronic devices., Comment: 14 pages, 4 figures

Machine learning models are susceptible to being misled by biases in training data that emphasize incidental correlations over the intended learning task. In this study, we demonstrate the impact of data bias on the performance of a machine learning model designed to predict the synthesizability likelihood of crystal compounds. The model performs a binary classification on labeled crystal samples. Despite using the same architecture for the machine learning model, we showcase how the model's learning and prediction behavior differs once trained on distinct data. We use two data sets for illustration: a mixed-source data set that integrates experimental and computational crystal samples and a single-source data set consisting of data exclusively from one computational database. We present simple procedures to detect data bias and to evaluate its effect on the model's performance and generalization. This study reveals how inconsistent, unbalanced data can propagate bias, undermining real-world applicability even for advanced machine learning techniques., Comment: 8 pages, 4 figures, 3 appendices

The exploitation of piezoelectric ferromagnetism (PFM) in two-dimensional (2D) materials with large out-of-plane piezoelectric response is motivated not only by technological applications but also scientific interest. In this study, the CrONM monolayer family (N=F, Cl; M=Br, Cl) was investigated using first-principles calculations, revealing that the Janus CrOFBr monolayer exhibits intrinsic ferromagnetic semiconductor behavior along with a significant out-of-plane piezoelectric effect. The calculated out-of-plane piezoelectric strain coefficients d$_{31}$ and d$_{32}$ are up to 1.21 and 0.63 pm/V, respectively. These values are greater than those of the majority of 2D materials. Furthermore, our findings demonstrate that applying tensile strain can enhance the out-of-plane piezoelectric response, leading to a respective 27% and 67% augmentation in the piezoelectric strain coefficients d$_{31}$ and d$_{32}$ compared to the unstrained configurations. This discovery holds great potential for propelling the field of nanoelectronics forward and facilitating the development of multifunctional semiconductor spintronic applications. Finally, by comparing d$_{31}$ and d$_{32}$ of the CrONM monolayer family (N=F, Cl; M=Br, Cl), we find that the magnitudes of d$_{31}$ and d$_{32}$ are correlated with the electronegativity difference between the M and N atoms. These findings provide valuable insights for the design of 2D piezoelectric materials with enhanced vertical piezoelectric responses., Comment: 21 pages, 6 figures

Metastable materials are abundant in nature and technology, showcasing remarkable properties that inspire innovative materials design. However, traditional crystal structure prediction methods, which rely solely on energetic factors to determine a structure's fitness, are not suitable for predicting the vast number of potentially synthesizable phases that represent a local minimum corresponding to a state in thermodynamic equilibrium. Here, we present a new approach for the prediction of metastable phases with specific structural features, and interface this method with the XtalOpt evolutionary algorithm. Our method relies on structural features that include the local crystalline order (e.g., the coordination number or chemical environment), and symmetry (e.g., Bravais lattice and space group) to filter the parent pool of an evolutionary crystal structure search. The effectiveness of this approach is benchmarked on three known metastable systems: XeN$_8$, with a two-dimensional polymeric nitrogen sublattice, brookite TiO$_2$, and a high pressure BaH$_4$ phase that was recently characterized. Additionally, a newly predicted metastable melaminate salt, $P$-1 WC$_{3}$N$_{6}$, was found to possess an energy that is lower than two phases proposed in a recent computational study. The method presented here could help in identifying the structures of compounds that have already been synthesized, and developing new synthesis targets with desired properties.

The structure of the material responsible for the room temperature and near ambient pressure superconductivity reported in an N-doped lutetium hydride [Nature, 615, 244 (2023)] has not been conclusively determined. Herein, density functional theory calculations are performed in an attempt to uncover what it might be. Guided by a range of strategies including crystal structure prediction and modifications of existing structure types, we present an array of Lu-N-H phases that are dynamically stable at experimentally relevant pressures. Although none of the structures found are thermodynamically stable, and none are expected to remain superconducting above 17 K at 10 kbar, a number of metallic compounds with fcc Lu lattices -- as suggested by the experimental X-ray diffraction measurements of the majority phase -- are identified. The system whose calculated equation of states matches best with that measured for the majority phase is fluorite-type LuH2, whose 10 kbar superconducting critical temperature was estimated to be 0.09 K using the Allen-Dynes modified McMillan equation., Comment: 11 pages, 8 figures

Ab initio evolutionary structure searches coupled with quasiharmonic calculations predict that the insulating Na hP4 phase transitions to a novel P63/m phase between 200 GPa at 150 K, and 350 GPa at 1900 K. P63/m Na is a topological semimetal with a Dirac nodal surface that is protected by a non-symmorphic symmetry, S2z . It is characterized by localized non-nuclear charge within 1D honeycomb channels and 0D cages rendering it an electride. These results highlight the complexity of warm dense sodiums electronic structure and free energy landscape that emerges at conditions where ionic cores overlap.

Previous researches only reported very small interfacial thermal resistances at room temperature due to limitations in sample combinations and methods. Taking cognizance of the importance of mismatched phonon structures, we report values up to $2*10^{-4}W^{-1}m^{2}K$, thousand times larger than highest values reported to date. This enables substantial tuning of the thermal conductivity in composites, and does not constrain other characteristics. Our findings inspire new design strategies, for heat control in integrated circuits and thermoelectric composites, that harness thermal transport at interfaces.

MASnI$_3$, an organometallic halide, has great potential in the field of lead-free perovskite solar cells. Ultraviolet photons have been shown to generate deep trapping electronic defects in mesoporous TiO$_2$-based perovskite, affecting its performance and stability. In this study, the structure, electronic properties, and optical properties of the cubic, tetragonal, and hexagonal phases of MASnI$_3$ were studied using first-principles calculations. The results indicate that the hexagonal phase of MASnI$_3$ possesses a larger indirect band gap and larger carrier effective mass along the \emph{c}-axis compared with the cubic and tetragonal phases. These findings were attributed to the enhanced electronic coupling and localization in the hexagonal phase. Moreover, the hexagonal phase exhibited high absorption of ultraviolet photons and high transmission of visible photons, particularly along the \emph{c}-axis. These characteristics demonstrate the potential of hexagonal MASnI$_3$ for application in multijunction perovskite tandem solar cells or as coatings in mesoporous TiO$_2$-based perovskite solar cells to enhance ultraviolet stability and photon utilization., Comment: 7 pages, 4 figures

Topological nodal line semimetals, hosting one-dimensional Fermi lines with symmetry protection, has become a hot topic in topological quantum matter. Due to the breaking of time reversal symmetry in magnetic system, nodal lines require protection by additional symmetries. Here, we report the discovery of antiferromagnetic type-I and type-II nodal lines coexist in the monolayer CrAs$_{2}$ based on a systematic first-principles calculation. Remarkably, the type-I nodal line in CrAs$_{2}$ form a concentric loop centered around the $\Gamma$ point is filling-enforced by nonsymmorphic analogue symmetry and robust against spin-orbital coupling. The type-II nodal lines, a kind of open nodal lines appear around the Fermi level, are protected by the mirror symmetry in the absence of spin-orbital coupling. The antiferromagnetic monolayer CrAs$_{2}$ proposed here may provide a platform for the correlation between magnetism and exotic topological phases., Comment: 15 pages, 5 figures

A large bulk band gap is critical for the applications of quantum spin hall (QSH) insulators in spintronics at room temperature. Based on first-principles calculations, we predict that the methyl-functionalized III-Bi monolayers, namely III-Bi-(CH3)2 (III=Ga, In, Tl) thin films, own QSH states with band gap as large as 0.260, 0.304 and 0.843 eV, respectively, making them suitable for room-temperature applications. The topological characteristics are confirmed by s-px,y band inversion, topological invariant Z2, and the topologically protected edge states. Noticeably, for GaBi/InBi-(CH3)2 films, the s-px,y band inversion occurred in the progress of spin-orbital coupling (SOC), while for TlBi(CH3)2 film, the s-px,y band inversion happened in the progress of chemical bonding. Significantly, the QSH states in III-Bi-(CH3)2 films are robust against the mechanical strains and various methyl coverages, making these films particularly flexible to substrate choice for device applications. Besides, the h-BN substrate is an ideal substrate for III-Bi-(CH3)2 films to realize large gap nontrivial topological states.. These findings demonstrate that the methyl-functionalized III-Bi films may be good QSH effect platforms for topological electronic devices design and fabrication in spintronics.