In this paper, we propose a convenient strategy to accelerate the evaluation of lattice thermal conductivity through combining phonon Boltzmann transport equation (PBTE) and on-the-fly machine learning potential (FMLP). The thermal conductivity of diamond silicon ($d\text{−}\mathrm{Si}$) is evaluated firstly by density functional theory (DFT), FMLP, and empirical potential with PBTE, respectively. The results demonstrate the proposed strategy integrates the prediction accuracy of DFT and computational speed of empirical potential, breaking the dilemma of traditional thermal conductivity assessment schemes. Based on this, the efficient strategy is applied to predict thermal conductivity of 102 low-energy metastable silicon crystals with energies between $d\text{−}\mathrm{Si}$ and experimentally ${\mathrm{Si}}_{24}$. Among them, the $Cmcm\text{−}{\mathrm{Si}}_{16}, P6/mmm\text{−}{\mathrm{Si}}_{36}$-2, $Pnma\text{−}{\mathrm{Si}}_{32}$-2 are predicted to host lowest lattice thermal conductivity in $xx$ (8.213 ${\mathrm{Wm}}^{-1}{\mathrm{K}}^{-1}$), $yy$ (10.917 ${\mathrm{Wm}}^{-1}{\mathrm{K}}^{-1}$), and $zz$ (11.807 ${\mathrm{Wm}}^{-1}{\mathrm{K}}^{-1}$) directions, respectively. Such low lattice thermal conductivity benefits from the combined effect of low phonon group velocity and intense phonon scattering caused by distorted $s{p}^3$ hybrid states in these metastable silicon crystals. The findings presented in this work provide new candidates and insights of silicon-based materials with ultra-low thermal conductivity, which will greatly expand the applications in thermoelectric and thermal insulation fields.

The phase-field crystal (PFC) model describes crystal lattices at diffusive timescales. Its amplitude expansion (APFC) can be applied to the investigation of relatively large systems under some approximations. However, crystal symmetries accessible within the APFC model are limited to basic ones, namely triangular and square in two dimensions, and body-centered cubic and face-centered cubic in three dimensions. In this work, we propose a general, amplitudes-based description of virtually any lattice symmetry. To fully exploit the advantages of this model, featuring slowly varying quantities in bulk and localized significant variations at dislocations and interfaces, we consider formulations suitable for real-space numerical methods supporting adaptive spatial discretization. We explore approaches originally proposed for the PFC model which allow for symmetries beyond basic ones through extended parametrizations. Moreover, we tackle the modeling of non-Bravais lattices by introducing an amplitude expansion for lattices with a basis and further generalizations. We study and discuss the stability of selected, prototypical lattice symmetries. As pivotal examples, we show that the proposed approach allows for a coarse-grained description of the kagome lattice, exotic square arrangements, and the diamond lattice, as bulk crystals and, importantly, hosting dislocations.

We employ machine-learning (ML) combined with first principles calculations to discover different rare-earth-free magnetic iron-cobalt silicide compounds. Deep machine-learning models are used to provide rapid screening of over 350 000 hypothetical structures to select a small fraction of promising structures and compositions for further studies by first-principles calculations. An adaptive genetic algorithm is used to search for lower energy structures based on the promising chemical compositions. Such a ML-guided approach dramatically accelerates the pace of materials discovery. We discover four new ternary Fe-Co-Si compounds, which exhibit desirable properties such as a large magnetic polarization $(J_s>1.0\mathrm{T})$, a significant easy-axis magnetic anisotropy $(K_1\ge 1.0\mathrm{MJ}/{\mathrm{m}}^3)$, and a high Curie temperature $(T_{\mathrm{C}}>840\mathrm{K})$. Moreover, the formation energies of these compounds are all within 70 meV/atom relative to the ternary convex hull, offering the possibility of synthesis.

Among the iron-based superconductors, “122”-type compounds have been widely studied due to the availability of high-quality single crystals. However, due to lack of a natural cleavage plane, the cleaved surface is usually polarized, which allows doping change or structure modification on the surface. In this paper, we propose a simple method to successfully transform the 122-type parent compound $\mathrm{Ba}{\mathrm{Fe}}_2{\mathrm{As}}_2$ to a superconductor via room-temperature relaxation or uniaxial strain. Using scanning tunneling microscopy/spectroscopy, we demonstrate that this superconductivity results directly from the disordered surface morphologies. Based on the transport results and the disappearance of superconductivity with every new cleave, we conclude that the superconductivity only manifests at the most superficial surface. Our finding opens up an unexpected way to achieve superconductivity in undoped $\mathrm{Ba}{\mathrm{Fe}}_2{\mathrm{As}}_2$, other than chemical doping, which may offer an alternative channel to understand the iron-based superconductivity and its connection with disorders.

Oxygen vacancies are ubiquitous in oxides and strongly influence the material's electronic structure and catalytic and transport properties. Here we focus on a seemingly simple defective oxide, MgO, and on the electronic properties of oxygen vacancies, which remain controversial in spite of numerous studies. We present an ab initio investigation of the photoexcitation and photoionization process of these defects, using a newly developed embedding Bethe-Salpeter equation approach, implemented in the WEST code. We find absorption and emission energies in good agreement with experiments. Our results provide a detailed, microscopic understanding of the absorption and emission processes of the neutral and positively charged oxygen vacancy, reconciling different views present in the chemistry and condensed-matter physics communities.

Structural prediction for the discovery of novel materials is a long sought-after goal of computational physics and materials sciences. The success is rather limited for methods such as the simulated annealing method (SA) that require expensive density functional theory (DFT) calculations and follow unintelligent search paths. Here a machine-learning based crystal combinatorial optimization program (CCOP) with a fitting-search style is proposed to drastically improve the efficiency of structural search in SA. CCOP uses a graph neural network energy prediction model to reduce the DFT cost and a deep reinforcement learning algorithm to direct the search path. Tests on six multicomponent alloys show the energy prediction model is capable of extracting the bonding characteristics of the complex alloys to achieve interpretability. It also achieves high accuracy with a tiny training set (an increment of 30 samples per iteration) by active learning in less than five iterations. Comparison with a few conventional methods shows that CCOP finds the lowest-energy structures with the smallest number of search steps. CCOP cuts the computing cost of SA by two orders of magnitude, while providing better search results than SA. CCOP is promising for serving as a broadly applicable tool for the efficient crystal structure predictions.

Blue phosphorene (blue-P) is a promising candidate for developing next-generation nanoelectronics devices due to its unique properties. Therefore, the controllable growth of a two-dimensional (2D) wafer-scale blue-P monolayer on substrates is a fundamental issue. However, the direct growth of blue-P on substrates currently remains a daunting challenge. In this paper, by using first-principles calculations, we propose a feasible route for fabricating blue-P via epitaxial growth. The growth mechanism of one-dimensional (1D) phosphorene chains to finite-sized blue-P clusters on Ag(111) is elucidated, and the reason why blue-P clusters cannot grow into a large-area monolayer on the Ag(111) surface is revealed. Moreover, the growth of blue-P on vicinal Ag(111) is explored and the energetic benefit of 1D zigzag blue-P nanoribbon growth along the $\mathrm{Ag}\langle 110\rangle$ step edge is confirmed. More importantly, we propose that the unidirectionally orientated blue-P nanoribbons can merge into large-area blue-P nanosheets through a step-guide growth mode. The feasibility of this strategy is validated by using molecular dynamics simulations, and a series of candidate substrates is selected. This study not only provides a promising substrate for epitaxial growth of blue-P but also sheds light on the preparation of more 2D materials.

Borophene has several configurations and shows many unique properties, such as the Dirac point, the negative Poisson's ratio, inherent metallicity, but it is oxidized easily. Recently, ordered chemical modulation of ${\beta }_{12}$ borophene grown on the Ag(111) substrate by hydrogenation was experimentally reported, and three configurations of ${\beta }_{12}$ borophane were proposed by scanning tunneling microscopy and spectroscopy [Science 371, 1143 (2021)]. In this paper, using first-principles calculations, we study seven configurations of hydrogen atoms adsorbed on freestanding ${\beta }_{12}$ borophene not only including the three adsorption structures in above literature, but also other four possible configurations with one or two hydrogen atoms adsorbed on it per unit cell. Among the seven configurations, we predict two stable ${\beta }_{12}$ borophanes. They possess the dynamic stability and different anisotropic metallicity from pristine ${\beta }_{12}$ borophene and show superconductivity. Based on the Eliashberg equation, the calculated electron-phonon coupling strength are about 0.82 and 0.52, and their superconducting transition temperature $T_{\mathrm{c}}$ are 20.51 and 5.90 K, respectively. Thus, these borophanes provide new platforms for two-dimensional superconductivity and antioxidation of borophene.

We report the magnetic and electronic transport properties of the inversion and time-reversal symmetry breaking Weyl semimetal NdAlGe. This material is analogous to NdAlSi, whose helical magnetism presents a rare example of a Weyl-mediated collective phenomenon, but with a larger spin-orbit coupling. Our neutron diffraction experiments revealed that NdAlGe, similar to NdAlSi, supports an incommensurate spin density wave ($T_{\text{inc}}=6.8$ K) whose spins are predominantly pointing along the out-of-plane direction and have a small helical spin canting of $3^{\circ }$. The spin density wave has a long wavelength of $\approx 35$ nm and transitions to a commensurate ferrimagnetic state below $T_{\text{com}}=5.1\mathrm{K}$. Using small-angle neutron scattering, we showed that the zero-field cooled ferrimagnetic domains form stripes in real space with characteristic length scales of 18 and 72 nm parallel and perpendicular to the [110] direction, respectively. Interestingly, for the transport properties, NdAlSi does not exhibit an anomalous Hall effect (AHE) that is commonly observed in magnetic Weyl semimetals. In contrast to NdAlSi, we identify two different AHE regimes in NdAlGe that are, respectively, governed by intrinsic Berry curvature and extrinsic disorders/spin fluctuations. Our paper suggests that Weyl-mediated magnetism prevails in this group of noncentrosymmetric magnetic Weyl semimetals $\mathrm{NdAl}X$, but transport properties including AHE are affected by material-specific extrinsic effects such as disorders, despite the presence of prominent Berry curvature.

The structure and growth behavior of Bi(110) ultrathin films on Si(111)-$7\times 7$ substrate are studied using real-time x-ray crystal truncation rod scattering measurements. The film grows in units of bilayers up to four atomic layers and shows a quasi–layer-by-layer growth above four layers in which even-layer-height domains are preferentially formed. It is revealed that the even-layer-height domains fully consist of black-phosphorus-like paired layers while the odd-layer-height domains have an additional bulk-Bi(110)-like layer beneath the top bilayer. This suggests that a structure change occurs at the subsurface when an additional single layer grows on the even or odd domains. Monte Carlo simulations indicate that a smaller free energy of the even structure than the odd structure leads to the preferential formation and that a small interface energy at the even/odd domain boundary allows the frequent transitions during the growth.

Lithium niobate and lithium tantalate crystals are technologically important metal oxides with exceptional combinations of ferroelectric, piezoelectric, acoustic, optical, and electrical properties. The self-diffusion of both, the ionic constituents and the underlying point defects, is especially important for the overall electrical conductivity. To get insight into their dynamics, we investigate in this work Li self-diffusion in congruent ${\mathrm{LiNbO}}_3$ and ${\mathrm{LiTaO}}_3$ single crystals from different suppliers up to a temperature of $800{}^{\circ }\mathrm{C}$, using isotope-enriched ${}^6\mathrm{LiNbO}_3$ and ${}^6\mathrm{LiTaO}_3$ tracer layers in combination with secondary ion mass spectrometry depth-profile analysis. The diffusivities of the two isostructural materials are identical within error limits and can be described by the Arrhenius law with an activation energy of 1.35 eV in the range from ${150}^{\circ }$ to $800{}^{\circ }\mathrm{C}$. Furthermore, the electrical conductivity is determined between $400{}^{\circ }\mathrm{C}$ and $600{}^{\circ }\mathrm{C}$ and can described by an activation energy of about 1.34 eV. This is in excellent agreement with the energy barrier for the diffusion of a single Li vacancy as determined by nudged elastic band calculations based on density-functional theory. The Li-ion conductivities calculated from the diffusivities in ${\mathrm{LiNbO}}_3$ and ${\mathrm{LiTaO}}_3$ are identical within the error margins with the overall conductivities obtained from impedance spectroscopy measurements. This indicates that the migration of ${\mathrm{Li}}^{+}$ is able to explain the overall electrical conductivity below $600{}^{\circ }\mathrm{C}$ down to $180{}^{\circ }\mathrm{C}$.

A spontaneous ferromagnetic moment can be induced in ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ thin films below a temperature $T\approx 16$ K by the introduction of Mn dopants. We demonstrate that films grown via molecular beam epitaxy with the stoichiometry ${\mathrm{Mn}}_{0.14}{\mathrm{Bi}}_{1.86}{\mathrm{Te}}_3$ maintain the crystal structure of pure ${\mathrm{Bi}}_2{\mathrm{Te}}_3$. The van der Waals nature of interlayer forces in the ${\mathrm{Mn}}_{0.14}{\mathrm{Bi}}_{1.86}{\mathrm{Te}}_3$ crystal causes lattice mismatch with the underlayer to have a limited effect on the resulting crystal structure, as we demonstrate by thin film growth on tetragonal ${\mathrm{MgF}}_2$ (110) and ${\mathrm{NiF}}_2$ (110). Electronic transport and magnetic moment measurements show that the ferromagnetic moment of the ${\mathrm{Mn}}_{0.14}{\mathrm{Bi}}_{1.86}{\mathrm{Te}}_3$ thin films is enhanced as the Fermi level moves from the bulk conduction band and toward the bulk band gap, suggesting that electronic surface states play an important role in mediating the ferromagnetic order. Ferromagnetic ${\mathrm{Mn}}_{0.14}{\mathrm{Bi}}_{1.86}{\mathrm{Te}}_3$/antiferromagnetic ${\mathrm{NiF}}_2$ bilayers show evidence that the ferromagnetic moment of the ${\mathrm{Mn}}_{0.14}{\mathrm{Bi}}_{1.86}{\mathrm{Te}}_3$ film is suppressed, suggesting the existence of an interface effect between the two magnetic layers.

${\mathrm{Mn}}_2\mathrm{GaC}$ is a MAX phase belonging to a family of naturally nanolaminated materials with formula $M_{n+1}A{X}_n$ ($n=1,2,3$), where $M$ is a transition metal, $A$ is an A-group element, and $X$ is carbon or nitrogen. It has a complex magnetic phase diagram, and there are many open questions regarding its magnetic properties. Here we study epitaxial films of ${\mathrm{Mn}}_2\mathrm{GaC}$ with two different crystal orientations on MgO(1 1 1) substrates: a (0 0 0 l) dominated orientation and a mixed (0 0 0 l) and (1 0 $\overline{1}$ 3) orientation. Magnetic measurements between 3 and 320 K are presented for in-plane and out-of-plane magnetic fields on both types of film, which show that ${\mathrm{Mn}}_2\mathrm{GaC}$ has a magnetocrystalline anisotropy with (0 0 0 l) as easy planes. This provides clear experimental evidence of the anisotropic properties associated with the nanolaminated structure of a MAX phase. In addition, a close look at the magnetic response at low temperature shows that the noncollinear magnetic state is unchanged below 50 K, contrary to previous results, with a magnetic moment of 0.38 ${\mu }_B$ per Mn atom at a temperature of 3 K and applied field of 5 T.

Bilayer Strontium Ruthenate ${\mathrm{Sr}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$ is a strongly correlated electronic system that shows diverse electronic and structural phases. Upon doping with Mn, an orbital selective Mott phase is observed before the material transitions to a Mott insulating state. Additionally, Mn doping leads to the emergence of an antiferromagnetic state with ${\mathit{q}}_{\mathit{AFM}}=(\pi /2,\pi /2)$ ordering wave vector. Quasiparticle interference (QPI) experiments find a sharp but highly dispersive peak at the AFM wave vector. Another set of QPI peaks is observed at ${\mathit{q}}^{*}=(\pi ,0),$ possibly due to a charge order effect. In this work we utilize a tight binding model relevant to Mn doped ${\mathrm{Sr}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$, and show that the origin of observed orbital selective Mott phase is inherently dependent upon the presence of a strong onsite exchange interaction and oxygen octahedral rotation suppression induced by the Mn doping. We further find that the experimentally observed QPI spectra, including the peaks at ${\mathit{q}}_{\mathit{AFM}}$, and ${\mathit{q}}^{*}$ wave vectors can be concomitantly explained within this formalism.

Porous materials are widely used for applications in gas storage and separation. The diffusive properties of a variety of gases in porous media can be modeled using molecular dynamics simulations that can be computationally demanding depending on the pore geometry, complexity, and amount of gas adsorbed. We explore a dimensionality reduction approach for estimating the self-diffusion coefficient of gases in simple pores using Langevin dynamics, such that the three-dimensional (3D) atomistic interactions that determine the diffusion properties of realistic systems can be reduced to an effective one-dimensional (1D) diffusion problem along the pore axis. We demonstrate the approach by modeling the transport of nitrogen molecules in single-walled carbon nanotubes of different radii, showing that 1D Langevin models can be parametrized with a few single-particle 3D atomistic simulations. The reduced 1D model predicts accurate diffusion coefficients over a broad range of temperatures and gas densities. Our work paves the way for studying the diffusion process of more general porous materials such as zeolites or metal-organics frameworks with effective models of reduced complexity.

We report optimized crystal growth conditions for the quaternary compound ${\mathrm{CuCrP}}_2{\mathrm{S}}_6$ by chemical vapor transport. Compositional and structural characterization of the obtained crystals were carried out by means of energy-dispersive x-ray spectroscopy and powder x-ray diffraction. ${\mathrm{CuCrP}}_2{\mathrm{S}}_6$ is structurally closely related to the $M_2{\mathrm{P}}_2{\mathrm{S}}_6$ family ($M$: transition metal), which contains several compounds that are under investigation as 2D magnets. As-grown crystals exhibit a platelike, layered morphology as well as a hexagonal habitus. We present successful exfoliation of such as-grown crystals down to thicknesses of 2.6 nm corresponding to four layers. ${\mathrm{CuCrP}}_2{\mathrm{S}}_6$ crystallizes in the monoclinic space group $C2/c$. Magnetization measurements reveal an antiferromagnetic ground state with $T_{\text{N}}\approx 30$ K and a positive Curie-Weiss temperature in agreement with dominant ferromagnetic intralayer coupling. Specific heat measurements confirm this magnetic phase transition and the magnetic order is suppressed in an external magnetic field of about 6 T (8 T) applied parallel (perpendicular) to the $ab$ plane. At higher temperatures between 140–200 K, additional broad anomalies associated with structural changes accompanying antiferroelectric ordering are detected in our specific heat studies.

The recent synthesis of two-dimensional (2D) layered crystals of ${\mathrm{MoSi}}_2{\mathrm{N}}_4$ and ${\mathrm{WSi}}_2{\mathrm{N}}_4$ has received significant attention due to their novel properties and potential applications. The inclusion of silicon during chemical vapor deposition growth enhances the ambient stability of transition metal nitrides and enables large-scale growth in monolayer limit. The transition metal and nitrogen elements can potentially be replaced with other metal and pnictogen atoms, which can constitute a new and large 2D family. In this respect, based on first-principles calculations, we design $M{\mathrm{Si}}_2{Z}_4$ ($M$: Pd and Pt, $Z$: N and P) monolayers and investigate their structural, vibrational, mechanical, electronic, and optical properties. The dynamical, thermal, and mechanical stabilities of the proposed systems are confirmed through phonon band dispersion calculations, ab initio molecular dynamics simulations, and elastic tensor analyses, respectively. The computed Raman and infrared spectra of the nanosheets reveal that the frequency and the intensity of the most prominent peaks are specified by the type of pnictogen atom in the structures. The mechanical response of the considered systems in the elastic regime is investigated in terms of in-plane stiffness ($Y_{\text{2D}}$) and the Poisson's ratio ($\nu$). The obtained values of $Y_{\text{2D}}$ indicate that $M{\mathrm{Si}}_2{\mathrm{N}}_4$ monolayers are stiffer than $M{\mathrm{Si}}_2{\mathrm{P}}_4$ nanosheets and the $\nu$ values are found to be within the same range and close to the ductile-brittle transition limit. The electronic structure investigation shows that while $M{\mathrm{Si}}_2{\mathrm{N}}_4$ monolayers are quasidirect wide-band-gap semiconductors, the $M{\mathrm{Si}}_2{\mathrm{P}}_4$ monolayers are semiconductors with an indirect narrow band gap. Additionally, these monolayers have isotropic optical spectra and, depending on the type of pnictogen atom, significant optical absorption peaks within the infrared, visible, and ultraviolet regions can be attained. Additionally, it is found that $M{\mathrm{Si}}_2{\mathrm{N}}_4$ monolayers possess multiple bound excitons with strong exciton binding when electron-hole interactions are taken into account. Our results not only expand the class of 2D $M{\mathrm{Si}}_2{Z}_4$ crystals but also offer valuable insights on the physical properties of the designed systems and suggest them as promising materials in diverse optoelectronic applications.

Ir-buffered ${\mathrm{Fe}}_{0.7}{\mathrm{V}}_{0.3}$ (FeV) thin films of variable thickness ($1.4\mathrm{nm}\le t_{\mathrm{FeV}}\le 10\mathrm{nm}$) were sputtered on thermally oxidized Si substrate and capped with Cu or MgO films. Vibrating sample magnetometery measurements allowed deducing their magnetization at saturation and the magnetic dead layer thickness, found to be capping layer independent. Microstrip line ferromagnetic resonance measurements under in-plane and perpendicular to the film plane applied magnetic fields are employed to investigate the perpendicular magnetic anisotropy (PMA) and damping. PMA is found to result from a uniaxial interfacial contribution which is slightly capping layer dependent. In contrast, the second-order PMA constant, which could result from the inhomogeneous distribution and the spatial fluctuations at the nanoscale of the uniaxial PMA at interface, is significantly stronger for Ir/FeV/Cu films. The damping dependence on the inverse of FeV effective thickness revealed a low Gilbert damping constant for FeV $({\alpha }_{\mathrm{FeV}}=1.7\times {10}^{-3})$ and allowed concluding that damping enhancement is mainly induced by Ir/FeV. The zero frequency linewidth for in-plane configuration is attributed to two magnon scattering contribution, found to be higher for Cu capped FeV. The investigation of the FeV thickness dependence of interfacial Dzyaloshinskii-Moriya interaction (iDMI) of both systems via Brillouin light scattering allowed to conclude to the zero iDMI constant of FeV/Cu. It also led to deduce iDMI constants of Ir/FeV and FeV/MgO, estimated to be $D_{\mathrm{s}}^{\mathrm{Ir}/\mathrm{FeV}}=(0.13\pm 0.02)\mathrm{pJ}/\mathrm{m}$ et $D_s^{\mathrm{FeV}/\mathrm{MgO}}=(-0.07\pm 0.02)\mathrm{pJ}/\mathrm{m}$, respectively.

Metal-halide perovskite (MHP) thin films for next-generation solar cells are typically fabricated by wet-chemical synthesis in which surface properties such as the orientation of surface facets and their termination cannot be controlled. MHP device efficiencies depend critically on those surface properties. We demonstrate the epitaxial growth of several nanometer thick purely (001)-orientated films of ${\mathrm{CsPbBr}}_3$ and ${\mathrm{CsSnBr}}_3$ MHPs on Au(001) by molecular beam epitaxy. The epitaxial films are in a cubic phase aligned with the substrate. Their surfaces are singly terminated and can be modified for the first time. While the CsBr and ${\mathrm{PbBr}}_2$ surface terminations differ in terms of their atomic structure and defect content, the films are intrinsic semiconductors irrespective of the termination. The work function of the ${\mathrm{PbBr}}_2$-terminated surface is increased by $0.7\mathrm{eV}$, which has drastic implications for the level alignment at MHP interfaces.