In solid-state physics and materials science, comprehending the configuration space and structure-property relationships of covalent bond systems remains a fundamental challenge due to the high flexibility of local bonding characteristics. To systematically investigate the configuration space of materials featuring directional covalent bonds, we introduce an enumerated growth orientation method. This strategy extends beyond the nucleation process of isolated systems, as it can also be employed to identify hidden polymorphic materials with interesting properties. By applying this approach, we have uncovered a variety of polymorphic graphyne structures possessing desirable properties. Furthermore, our strategy can generate remarkable topological patterns that serve as fundamental components for covalent organic framework design. In summary, our approach offers a promising platform for materials design, which could catalyze the development of functional materials based on covalent bonds.

The anisotropic magnetoresistance (AMR) effect is one of the fundamental spin-dependent transport phenomena in ferromagnets and has been the subject of numerous experimental observations. However, the origin of AMR including the sign change of the magnetoresistance has not been fully clarified theoretically. In this paper, we observe a large negative AMR ratio in ${\mathrm{Fe}}_{0.75}{\mathrm{Co}}_{0.25}$ single-crystal thin films upon Ir addition and elucidate its origin by a theoretical model. ${\left({\mathrm{Fe}}_{0.75}{\mathrm{Co}}_{0.25}\right)}_{100-x}{\mathrm{Ir}}_x$ composition-spread thin films with $x$ up to 11% are fabricated on MgO(100) substrates by combinatorial sputtering technique. From x-ray diffraction results, the metastable $B2$-ordered phase of ${\mathrm{Fe}}_3\mathrm{Co}–\mathrm{Ir}$ is detected from $x=2.1\%$, which does not appear in the bulk equilibrium phase diagram. The $B2$ ordering is also confirmed by scanning transmission electron microscopy. The AMR ratio of the pure ${\mathrm{Fe}}_3\mathrm{Co}$ shows a small positive value of 0.3%. In contrast, once the Ir atoms are added, the AMR ratio becomes negative, and it exhibits the largest negative values of −4.7% at 10 K and −3.6% at 300 K for $x=11\%$. Using a theoretical model, the sign change of the AMR ratio from positive to negative upon $B2$ ordering is obtained for high-Ir concentration, which agrees with the experimental results. Thus, the metastable $B2$ ordering would be the key to observing the negative AMR. These findings provide deep insight into the origin of AMR in heavy-metal-doped ferromagnetic ordered alloys.

Antimony sulfide $\left({\mathrm{Sb}}_2{\mathrm{S}}_3\right)$ is an Earth-abundant and nontoxic material that is under investigation for solar energy conversion applications. However, it still suffers from poor power conversion efficiency and a large open circuit voltage loss that have usually been attributed to point or interfacial defects. More recently, there has been some discussion in the literature about the role of carrier trapping in the optical properties of ${\mathrm{Sb}}_2{\mathrm{S}}_3$, with some reporting self-trapped excitons as the microscopic origin for the performance loss, while others have found no evidence of carrier trapping with only large polarons existing in ${\mathrm{Sb}}_2{\mathrm{S}}_3$. By using first-principles methods, we demonstrate that ${\mathrm{Sb}}_2{\mathrm{S}}_3$ exhibits strong electron-phonon coupling, a prerequisite for carrier self-trapping in semiconductors, which results in a large renormalization of $200\mathrm{meV}$ of the absorption edge when temperature increases from 10 to $300\mathrm{K}$. When two electrons or holes are added to the system, corresponding to a carrier density of $1.6\times {10}^{20}{\mathrm{cm}}^{-3}$, we find wave function localization consistent with the presence of bipolarons accompanying a significant lattice distortion with the formation of Sb and S dimers. The formation energies of the electron and hole bipolarons are $-330$ and $-280$ meV per carrier, respectively. Our results reconcile some of the controversy in the literature regarding carrier trapping in ${\mathrm{Sb}}_2{\mathrm{S}}_3$ and demonstrate the existence of large electron-phonon coupling and carrier self-trapping that might place a fundamental limit on the open circuit voltage and, consequently, the maximum efficiency of the photovoltaic cells.

Semiconductor-superconductor hybrid nanowires are a leading material platform for the realization of Majorana zero modes. The semiconductors in previous studies are dominantly InAs or InSb. In this paper, we show the induced superconductivity in PbTe nanowires epitaxially coupled to a superconductor Pb. The Josephson junction devices based on this hybrid reveal a gate-tunable supercurrent in the open regime and a hard superconducting gap in the tunneling regime. By demonstrating the superconducting proximity effect, our result can enable Majorana searches and other applications, such as gate-tunable qubits in a new material platform.

A $\mathrm{W}{\mathrm{Se}}_2$ monolayer of $1{\mathrm{T}}^{\prime }$ phase is a large band-gap quantum spin Hall insulator, supporting dissipationless charge and spin transports through the topologically protected edge states. In this paper, we explore the nanoribbon forms of $1{\mathrm{T}}^{\prime }$-phase $\mathrm{W}{\mathrm{Se}}_2$ by first-principles density functional calculations and the many-body perturbation GW and Bethe-Salpeter equation method. We found that the $1{\mathrm{T}}^{\prime }$ $\mathrm{W}{\mathrm{Se}}_2$ nanoribbon can show topological edge states with a ribbon width of $\sim 4\text{–}6\mathrm{nm}$. Those edge bands show crossing through the Fermi level an odd number of times, with one kind of spin-polarization connecting the valence band and conduction band continua. The topological features of the edge bands hold even under small and medium bending in the nanoribbon, while large bending induces large band splitting, resulting in a topological switch-off in the edge bands. The semiconducting $1{\mathrm{T}}^{\prime }$ $\mathrm{W}{\mathrm{Se}}_2$ nanoribbon shows a large tunability with bending in optical absorption spectra and exciton states. The lowest-energy exciton is changed from optically dark in the flat nanoribbon to bright in the bent nanoribbons. These properties in the $1{\mathrm{T}}^{\prime }$ $\mathrm{W}{\mathrm{Se}}_2$ nanoribbons suggest potential applications in controllable quantum electronics and exciton-based quantum information processes.

Endofullerene ${\mathrm{DySc}}_2{\mathrm{N}@\mathrm{C}}_{80}$ is a single-molecule magnet with a large magnetic anisotropy and high blocking temperature, which is promising for nanomagnetic applications. As the easy axis of magnetization coincides with the Dy-N bond direction, it is important to understand the structure of the ${\mathrm{DySc}}_2\mathrm{N}$ unit in the fullerene cage and to control the orientation of the molecules. Here we report on the experimental determination of Dy-N axis by x-ray absorption spectroscopy (XAS) with linear polarized light at the $\mathrm{Dy}\text{−}{\mathrm{M}}_{4,5}$ white lines. ${\mathrm{DySc}}_2{\mathrm{N}@\mathrm{C}}_{80}$ molecules were adsorbed on a Pt(111) surface and XAS was performed as a function of temperature in the range between 35 and 300 K. The ${\mathrm{M}}_5/{\mathrm{M}}_4$ branching ratio shows a clear and reversible variation with temperature which can be explained, on the basis of a thermodynamic model, by a change of average orientation of the molecules with temperature. The XAS spectra are well reproduced by ligand field multiplet calculations. It is shown that the angle between the magnetization (Dy-N) axis and the surface plane can be directly inferred from the XAS spectra with in-plane polarization by comparison with calculated spectra. It is found that the endohedral unit is randomly oriented at room temperature but tends towards orientation parallel to the surface at low temperature, indicating a weak but non-negligible interaction between the endohedral units and the metal surface.

In this paper $\mathrm{\Delta }$ learning is used to map orbital-free density functional theory (DFT) ionic forces to the corresponding Kohn-Sham (KS) DFT ionic forces. The development of the approximate force difference in terms of the ion positions is constructed and serves as a stand-in for the ground truth force difference. Descriptor vectors for ion configurations are constructed using all distances between ions in conjunction with an indexing based on a nearest neighbor ranking. It is demonstrated that such a scheme of descriptors can uniquely describe an ionic configuration up to a rotation and reflection when no ambiguity in the nearest neighbor ranking exists. How to handle the case when an ambiguity exists in the nearest neighbor ranking is discussed. As a proof of principle, the model is trained and tested on warm dense hydrogen at temperatures between 1 and 15 eV. Once tested, the model was used to perform molecular dynamic simulations of warm dense hydrogen. The resulting energies and pressures are within 1 and 2% of their respective target KS values.

Twelve layered hybrid organic-inorganic perovskites combining oligoacene derivatives [phenylmethylammonium (PMA), naphthylmethylammonium (NMA), anthrylmethylammonium (AMA), and tetrylmethylammonium (TMA)] and lead halides (${\mathrm{Cl}}^{-}, {\mathrm{Br}}^{-}$, and ${\mathrm{I}}^{-}$ anions) are investigated by first-principles density functional theory (DFT), showing broad, rational tunability of band gap, quantum well type, and spin-dependent energy band properties. Six compounds are known from previous syntheses and are used to devise a computational search space for likely low-energy structures. Among the six known compounds, a refined structure is identified for (${\mathrm{NMA})}_2{\mathrm{PbCl}}_4$ and a new, lower-energy structure is suggested for (${\mathrm{AMA})}_2{\mathrm{PbCl}}_4$. The DFT based search methodology is next applied to predict the likely structures for the six unknown compounds. Computationally predicted energy levels for all 12 compounds from spin-orbit coupled hybrid DFT reveal tunable type I quantum well alignments, with frontier orbitals located on the inorganic component, on the organic component, or on both, as controlled by cation and anion selection. Several structures spontaneously break local and global inversion symmetry, causing strong spin splitting of the conduction bands [up to 0.12 eV for (${\mathrm{PMA})}_2{\mathrm{PbCl}}_4$] and showing potential for spin-dependent transport properties and future spintronic and tunable chiroptical applications.

Two-dimensional (2D) Mo-based MXenes $\left({\mathrm{Mo}}_{n+1}{C}_n{T}_x\right)$ are recognized to have significant potential as hydrogen evolution reaction (HER) activity electrocatalysts. However, appropriate descriptors are absent to predict the H-adsorption Gibbs energy $\left(\mathrm{\Delta }{G}_{\mathrm{H}}\right)$ due to the unique delocalized electronic properties of the Mo atom. In this paper, we used first-principles calculations and machine learning to study the HER activity of ${\mathrm{Mo}}_2\mathrm{C}{\mathrm{O}}_2$ with single transition metal-doped (${\mathrm{Mo}}_2\mathrm{C}{\mathrm{O}}_2$-STM), and elucidate the mechanisms by which single transition metals (STMs) regulate the hydrogen evolution reaction. Our results revealed that $\mathrm{\Delta }{G}_{\mathrm{H}}$ has a “W” shape as a function of the doped atom changing in one period. The electronic structure analysis indicates that the electronic delocalized Mo has a longer range affecting not only the nearest atoms, but the second-nearest neighbor (STM-Mo) bonding effect controls the periodic distribution of $\mathrm{\Delta }{G}_{\mathrm{H}}$. Using machine-learning method, we quantized the STM regulation mechanism using five key structural and electronic descriptors, and predicted the $\mathrm{\Delta }{G}_{\mathrm{H}}$ of ${\mathrm{Mo}}_2\mathrm{C}{\mathrm{O}}_2$-STM, which were also extended to ${\mathrm{W}}_2\mathrm{C}{\mathrm{O}}_2$-STM successfully. Our findings highlight the importance of considering second-nearest-neighbor bonding effects in similar delocalized materials systems research.

Atomically thin layers exfoliated from magnetic van der Waals layered materials are currently of high interest in solid state physics. VOCl is a quasi-two-dimensional layered antiferromagnet which was recently synthesized in monolayer form. Previous theoretical studies have assumed the high-temperature orthorhombic lattice symmetry also in the low-temperature range, where the bulk system is known to be monoclinic due to a strong magnetoelastic coupling. We demonstrate from ab initio calculations that this monoclinic distortion is prevalent also in monolayers, which is in line with recent experimental indications of monoclinic symmetry. Our calculations also show that competing ferromagnetic and antiferromagnetic interactions cause a frustrated twofold magnetic superstructure where higher-order magnetic interactions play a key role.

We report on the structural and superconducting properties of the intermetallic compound BaGaGe. We find that this material undergoes a structural second-order phase transition from a distorted ${\mathrm{AlB}}_2$-type structure [1H, $a=4.3254\left(2\right)$ Å, $c=5.1078\left(3\right)$ Å, $P6/mmm$] into a ${\mathrm{CaIn}}_2$-type structure [2H, $a=4.3087\left(3\right)$ Å, $c=10.2117\left(6\right)$ Å, $P{6}_3/mmc$] at a transition temperature of $T_{\mathrm{S}}=253\mathrm{K}$. We find that the structural phase transition corresponds to a coherent buckling of the honeycomb layers, which we can interpret as a disorder-to-order transition of the atoms located within this layer. We show that the 2H-BaGaGe phase becomes superconducting at a critical temperature of $T_{\mathrm{c}}=2.1\mathrm{K}$. The bulk nature of the superconductivity in 2H-BaGaGe is confirmed by means of specific-heat measurements, where we determine a value of $\mathrm{\Delta }C/\gamma T_{\mathrm{c}}=1.59$, which is close to the expected BCS value in the weak-coupling limit.

An intermittent pattern is observed in the modeling of interfacial cyclic-loading crack growth at high-angle grain boundaries in ternary Fe-Ni-Cr alloys. Different from conventional wisdom of stress-intensity factor, the abrupt crack advances are found driven by extreme value statistics—namely, the aggregation of atoms with most compressive residual stresses. In addition, inherently non-affine atomic stress fluctuations are discovered, and the fluctuations peak at intermediate level of chemical heterogeneity, causing the fastest crack growth. Implications of such nonmonotonic mechanism in regard to the origin of intermediate-temperature embrittlement phenomena are also discussed.

Lead halide perovskites are widely applied in photovoltaic devices and solar cells. However, the economic potential is restricted by the toxicity of lead. Two-dimensional (2D) layered organic-inorganic hybrid perovskite (OIHP) copper-based perovskites with good stability and hypotoxicity greatly fulfill environmentally favorable standards. Herein, we systematically report a synthesis and band-gap modulation of 2D layered OIHP $PE{A}_2\mathrm{Cu}{\mathrm{Cl}}_4\left(PE{A}^{+}={\mathrm{C}}_6{\mathrm{H}}_5{\mathrm{C}}_2{\mathrm{H}}_4\mathrm{N}{\mathrm{H}}_3\right)$ nanocrystals (NCs). The direct band gap of $PE{A}_2\mathrm{Cu}{\mathrm{Cl}}_4$ NCs is effectively decreased from 2.93 to 1.77 eV by using high-pressure technique, accompanied by the piezochromism of products from yellow to dark red and then to opaque black, which enhances the photovoltaic application to a greater extent. Combining experiments and theoretical analysis indicates that direct band-gap narrowing is caused by the shorted average Cl–Cu–Cl bond length, resulting in increased $\mathrm{Cu}\text{−}3d$ and $\mathrm{Cl}\text{−}3p$ interaction, which raises the valence-band maximum toward the conduction-band minimum. The study contributes to a better understanding of 2D layered OIHP $PE{A}_2\mathrm{Cu}{\mathrm{Cl}}_4$ NCs and enables pressure processing as a reliable technique for improving materials by design in applications.

Ongoing advances in superconductors continue to revolutionize technology thanks to the increasingly versatile and robust availability of lossless supercurrents. In particular, high supercurrent density can lead to more efficient and compact power transmission lines, high-field magnets, as well as high-performance nanoscale radiation detectors and superconducting spintronics. Here, we report the discovery of an unprecedentedly high superconducting critical current density ($17\mathrm{MA}/\mathrm{c}{\mathrm{m}}^2$ at 0 T and $7\mathrm{MA}/\mathrm{c}{\mathrm{m}}^2$ at 8 T) in $1{\mathrm{T}}^{\prime }\text{−}\mathrm{W}{\mathrm{S}}_2$, exceeding those of all reported two-dimensional superconductors to date. $1{\mathrm{T}}^{\prime }\text{−}\mathrm{W}{\mathrm{S}}_2$ features a strongly anisotropic (both in- and out-of-plane) superconducting state that violates the Pauli paramagnetic limit signaling the presence of unconventional superconductivity. Spectroscopic imaging of the vortices further substantiates the anisotropic nature of the superconducting state. More intriguingly, the normal state of $1{\mathrm{T}}^{\prime }\text{−}\mathrm{W}{\mathrm{S}}_2$ carries topological properties. The band structure obtained via angle-resolved photoemission spectroscopy and first-principles calculations points to a ${\mathrm{Z}}_2$ topological invariant. The concomitance of topology and superconductivity in $1{\mathrm{T}}^{\prime }\text{−}\mathrm{W}{\mathrm{S}}_2$ establishes it as a topological superconductor candidate, which is promising for the development of quantum computing technology.

Molecular strains can greatly alter chemical reactions in covalent systems. Experimental and computational tools designed to characterize this have focused on simple elongation forces. Yet, mechanical loading in condensed matter results in complex, many-body deformations. Hence, we use four-body external potentials designed to reproduce these strains with reactive molecular dynamics. Mimicking these deformations results in significant lowering of activation barriers and different reaction pathways in the energetic material 1,3, 5-trinitro-2,4,6-triaminobenzene (TATB) and a lower-energy reaction pathway for isomerization in spiropyran.