We study defect energy levels in hexagonal boron nitride $\left(h\text{−}\mathrm{BN}\right)$ with varying number of layers using a fragment many-body $GW$ formalism, taking as examples the paradigmatic carbon-dimer and ${\mathrm{C}}_{\mathrm{B}}{\mathrm{V}}_{\mathrm{N}}$ defects. We show that a single layer can be fragmented in polarizable finite-size areas reproducing faithfully the effect of the dielectric environment, dramatically facilitating the study at the many-body level of point defects in the dilute limit. The evolution of defect energy levels from the monolayer to a $n$-layer system due to increased screening, labeled polarization energies, follow a simple $(\mathrm{\Delta }P/n+P_{\infty })$ behavior. The coefficients $\mathrm{\Delta }P$ and $P_{\infty }$ are found to be close to universal, with opposite signs for holes and electrons, characterizing mainly the host and the position of the defect (surface or bulk), but hardly the defect type. Our results rationalize the evolution of defect energy levels with layer number, allowing to safely extrapolate results obtained for the monolayer to few layers, surface, or bulk $h\text{−}\mathrm{BN}$. The present many-body fragment approach further opens the door to studying disordered two-dimensional layers.

Since the discovery of graphene, numerous efforts have been made to seek new two-dimensional (2D) graphenelike materials with intriguing properties. Here,we report a stable graphenelike supersalt ${\mathrm{NaNO}}_3$ monolayer composed of superhalogen ${\mathrm{NO}}_3$. The 2D ${\mathrm{NaNO}}_3$ monolayer is found to possess an ultralow isotropic Young's modulus (2.983 N/m) and a low thermal conductivity (1.65 W/mK @300 K), which is much lower than that of 2D NaCl monolayer (3.72 W/mK @300 K). We attribute this behavior to the strong anharmonicity induced by the weak bonding between Na and ${\mathrm{NO}}_3$ cluster and the large atomic displacements of O in the ${\mathrm{NO}}_3$ cluster. A giant four-phonon scattering leads to a 60% reduction in lattice thermal conductivity at 300 K compared to that of three-phonon scattering. Furthermore, temperature-induced phonon hardening causes the lattice thermal conductivity to be nearly temperature independent in the studied temperature range (100–350 K), displaying significant difference from the atom-based 2D materials.

We have investigated the temperature $\left(T\right)$ dependence of the anisotropic magnetoresistance (AMR) effect of ${\mathrm{Co}}_2{\mathrm{FeGa}}_{0.5}{\mathrm{Ge}}_{0.5}$ (CFGG) epitaxial thin films having different compositions and atomic orders to examine the relation between AMR and the half-metallic electronic structure based on a developed theoretical model. The $T$ dependence of the resistance change (Δρ) of the AMR normalized at 10 K is minimal in the CFGG films having a standard composition and high atomic order. In contrast, the films having a large atomic disorder due to the Co-rich composition exhibit a large reduction of Δρ with $T$. Our theoretical model of AMR well explains this behavior; namely, the half-metallic ferromagnets having the Fermi level $\left(E_{\mathrm{F}}\right)$ around the center of the half-metallic gap are predicted to show a small $T$ dependence of Δρ because of no/few localized $d\downarrow$ states at $E_{\mathrm{F}}$. On the contrary, a large change of Δρ with $T$ is expected for the half-metallic materials having $E_{\mathrm{F}}$ close to gap edges or large in-gap states because of the contribution of thermally excited s-d scattering involving the $d\downarrow$ states. Moreover, we have also investigated the variation of Δρ with $T$ for the thin films of various half-metallic Co-based Heusler alloys and found the behavior that agrees with our theoretical prediction. The present study proves that the formation of a half-metallic gap and the position of $E_{\mathrm{F}}$ in a half-metallic material are readily predictable from the $T$ dependence of Δρ of the AMR effect, which can be a facile way for efficient screening of half-metallic materials.

Rare-earth (RE) oxyhydride thin films show a color-neutral, reversible photochromic effect at ambient conditions. The origin of the photochromism is the topic of current investigations. Here, we investigated the lattice defects, electronic structure, and crystal structure of photochromic ${\mathrm{YH}}_x{\mathrm{O}}_y$ and $\mathrm{Gd}{\mathrm{H}}_x{\mathrm{O}}_y$ thin films deposited by magnetron sputtering using positron annihilation techniques and x-ray diffraction, in comparison with Y, ${\mathrm{YH}}_{\sim 1.9}$, ${\mathrm{Y}}_2{\mathrm{O}}_3$, Gd, $\mathrm{Gd}{\mathrm{H}}_{\sim 1.8}$, and ${\mathrm{Gd}}_2{\mathrm{O}}_3$ films. Positron annihilation lifetime spectroscopy (PALS) reveals the presence of cation monovacancies in the as-deposited Y and ${\mathrm{YH}}_{\sim 1.9}$ films at concentrations of $\sim {10}^{-5}$ per cation. In addition, vacancy clusters and nanopores are found in the as-prepared ${\mathrm{YH}}_x{\mathrm{O}}_y$ and ${\mathrm{Y}}_2{\mathrm{O}}_3$ films. Doppler broadening positron annihilation spectroscopy (DB-PAS) of the Y- and Gd-based films reflects the transition from a metallic to an insulating nature of the RE metal, metal hydride, semiconducting oxyhydride and insulating oxide films. In-situ illumination DB-PAS shows the irreversible formation predominantly of di-vacancies, as PALS showed that cation mono-vacancies are already abundantly present in the as-prepared films. The formation of di-vacancies supports conjectures that ${\mathrm{H}}^{\text{–}}$ (and/or ${\mathrm{O}}^{2\text{–}}$) ions become mobile upon illumination, as these will leave anion vacancies behind, some of which may subsequently cluster with cation vacancies present. In addition, in RE oxyhydride films, partially reversible shifts in the Doppler parameters are observed that correlate with the photochromic effect and point to the formation of metallic domains in the semiconducting films. Two processes are discussed that may explain the formation of these metallic domains and the changes in optical properties associated with the photochromic effect. The first process considers the reversible formation of metallic nanodomains with reduced O : H composition by transport of light-induced mobile hydrogen and local oxygen displacements. The second process considers metallic nanodomains resulting from the trapping of photoexcited electrons in an $e_g$ orbital at the yttrium ions surrounding positively charged hydrogen vacancies that are formed by light-induced removal of hydrogen atoms from octahedral sites. When a sufficiently large concentration, on the order of ∼10%, is reached in a certain domain of the film, band formation of the $e_g$ electrons may occur, leading to an Anderson-Mott insulator-metal transition like the case of yttrium trihydride in these domains.

Relaxor ferroelectrics are an eminent group of functional materials, characterized by complex micro- and nanoscale structures, accounting for their enhanced piezoelectric properties. In the $\left(1-x\right){\mathrm{Na}}_{1/2}{\mathrm{Bi}}_{1/2}{\mathrm{TiO}}_3\text{−}x{\mathrm{BaTiO}}_3$ (NBT-BT) solid solution, the evolution of nanoscale domains and their hierarchical association with ferroelectric domains is investigated using conventional and scanning transmission electron microscopy on compositions with 6, 9, and 12 mol % BT. Short-range fluctuations in the local polar displacement (polar nanoregions, PNRs) account for a heterogeneous nanostructure at the morphotropic phase boundary (6 mol % BT). Platelike nanodomains of tetragonal $P4bm$ symmetry coexist with a minor volume fraction of rhombohedral $R3c$ nanodomains. Their overall population decreases with increasing BT content. However, ferroelectric $P4mm$ domains in the composition with 12 mol % BT still exhibit nanoscale regions, which deviate from the average polarization. Small volume fractions of both $P4bm$ and $R3c$ nanodomains remain embedded within the ferroelectric domains. This hierarchical domain configuration underpins the complex structural characteristics of NBT-based relaxor ferroelectrics.

Understanding ferroelectricity is of both fundamental and technological importance to further stimulate the development of new materials designs and manipulations. In this respect, supertetragonal (ST) phases with high $c/a$ ratio of $\sim 1.3$ have recently resulted in outstanding ferroelectric polarization values, which urges for a microscopic origin of these novel structural phases. Here, we perform an in-depth first-principle study on the well-known ferroelectric barium titanate ${\mathrm{BaTiO}}_3$ under a hydrostatic negative pressure, showing an isosymmetric phase transition to such a ST phase. The chemical origin and driving mechanisms of this phase transition are identified as a drastic change of the covalently $\pi$-bonded electrons. These findings provide guidance in the search for new ST phases, with great opportunities for novel multiferroic materials, and can be generalized in the understanding of other isosymmetric phase transitions.

Picosecond acoustics combined with a diamond anvil cell is used to study liquid indium and to determine with high accuracy both the sound velocity and the melting curve over an extended pressure and temperature range. The sound velocities, determined by phonon surface imaging, complement previous inelastic x-ray scattering determinations and are in good agreement with estimations according to a thermodynamic model. Based on exact thermodynamic relations, the equation of state of the liquid phase is obtained using the isothermal bulk modulus $B_{T,0}$ and its first pressure derivative $B_T^{\prime }$. These quantities are derived from the precise experimental determination of the variation of the sound velocity as a function of pressure. Melting is determined via the detection of abrupt changes in the elastic properties between solid and liquid phases and through the monitoring of the solid-liquid coexistence. The melting curve constrained up to 6 GPa and 673 K is shown to be well described by the Simon-Glatzel equation in the full $(p,T)$ range explored.

Two-dimensional materials open up tremendous opportunities for nanoelectronics and optoelectronics. Using first-principles density functional methods, we predict a family of two-dimensional boron-pnictogen materials. Our results show that these materials have excellent energetic, dynamical, thermal, mechanical, and chemical stabilities. The intrinsic structural anisotropy found in these materials leads to highly direction-dependent mechanical, electronic, and optical properties. They possess highly anisotropic Young's modulus and Poisson's ratio. The tensile strength under uniaxial and biaxial deformations is found to be very high for these materials. Electronically, they are all semiconductors with narrow band gaps. The band gap energies can be tuned by alloying, strain engineering, and chemical functionalization. They exhibit anisotropic and high carrier mobility. All these electronic properties make them promising candidates for nanoelectronic device applications. Using state-of-the-art $GW$- Bethe-Salpeter equation approach, taking the electron-hole effect into account, the prominent optical absorption structure with strong anisotropy in the visible light region endow the boron-pnictogen materials with great potential in optoelectronics.

We report on a temperature-driven reversible change of the in-plane magnetic anisotropy of ${\mathrm{V}}_2{\mathrm{O}}_3/\mathrm{Ni}$ bilayers. This is caused by the rhombohedral to monoclinic structural phase transition of ${\mathrm{V}}_2{\mathrm{O}}_3$ at $T_C=160$ K. The in-plane magnetic anisotropy is uniaxial above $T_C$, but as the bilayer is cooled through the structural phase transition, a secondary magnetic easy axis emerges. Ferromagnetic resonance measurements show that this change in magnetic anisotropy is reversible with temperature. We identify two structural properties of the ${\mathrm{V}}_2{\mathrm{O}}_3/\mathrm{Ni}$ bilayers affecting the in-plane magnetic anisotropy: (1) a growth-induced uniaxial magnetic anisotropy associated with steplike terraces in the bilayer microstructure and (2) a low-temperature strain-induced biaxial anisotropy associated with the ${\mathrm{V}}_2{\mathrm{O}}_3$ structural phase transition. Magnetoresistance measurements corroborate the change in magnetic anisotropy across the structural transition and suggest that the negative magnetostriction of Ni leads to the emergence of a strain-induced easy axis. This shows that a temperature-dependent structural transition in ${\mathrm{V}}_2{\mathrm{O}}_3$ may be used to tune the magnetic anisotropy in an adjacent ferromagnetic thin film.

Semiconductors with a wide band gap ($>3.0$ eV), high dielectric constant ($>10$), good thermal dissipation, and capable of $n$- and $p$-type doping are highly desirable for high-energy power electronic devices. Recent studies indicate that ${\mathrm{ZnGa}}_2{\mathrm{O}}_4$ may be suitable for these applications, standing out as an alternative to ${\mathrm{Ga}}_2{\mathrm{O}}_3$. The simple face-centered-cubic spinel structure of ${\mathrm{ZnGa}}_2{\mathrm{O}}_4$ results in isotropic electronic and optical properties, in contrast to the large anisotropic properties of the $\beta$-monoclinic ${\mathrm{Ga}}_2{\mathrm{O}}_3$. In addition, ${\mathrm{ZnGa}}_2{\mathrm{O}}_4$ has shown, on average, better thermal dissipation and potential for $n$- and $p$-type conductivity. Here we use density functional theory and hybrid functional calculations to investigate the electronic, optical, and point defect properties of ${\mathrm{ZnGa}}_2{\mathrm{O}}_4$, focusing on the possibility for $p$- and $n$-type conductivity. We find that the cation antisite ${\mathrm{Ga}}_{\mathrm{Zn}}$ is the lowest-energy donor defect that can lead to unintentional $n$-type conductivity. The stability of self-trapped holes (small hole polarons) and the high formation energy of acceptor defects make it difficult to achieve $p$-type conductivity. However, with an excess of Zn, forming ${\mathrm{Zn}}_{(1+2x)}{\mathrm{Ga}}_{2(1-x)}{\mathrm{O}}_4$ alloys, this compound can display an intermediate valence band, facilitating $p$-type conductivity. Due to the localized nature of this intermediate valence band, $p$-type conductivity by polaron hopping is expected, explaining the low mobility and low hole density observed in recent experiments.

Pressure, a fundamental thermodynamic variable, enables phase transitions to exotic phases with unique physical properties, such as superconductivity. In this work we perform a complete study of crystal structures and relevant electronic properties of La–S crystalline systems in a pressure range of 0–200 GPa. A structural search based on first-principles swarm-intelligence identifies two hitherto unknown pressure-stabilized stoichiometries, namely, ${\mathrm{LaS}}_3$ and ${\mathrm{LaS}}_5$, in addition to the previously reported compounds. We find that the S-S bonding patterns in La–S compounds evolve in the following sequence with increasing S content and pressure: Atomic S, ${\mathrm{S}}_2$ dimers, one-dimensional linear S chains, and two-dimensional S ladders. Further electron-phonon calculations show that both ${\mathrm{LaS}}_3$ and ${\mathrm{LaS}}_5$ are superconductors with critical temperatures of 13.6 K at 100 GPa and 11 K at 120 GPa, respectively. The softened acoustic phonon branches are responsible for their superconductivity. Our current work is expected to guide future experimental studies investigating superconductivity and structural features of La–S system and more, in general, of other rare-earth chalcogenides.

We investigate the superconducting properties of ${\mathrm{Sc}}_5{\mathrm{Co}}_4{\mathrm{Si}}_{10}$ using low-temperature resistivity, magnetization, heat capacity, and muon-spin rotation and relaxation ($\mu \mathrm{SR}$) measurements. We find that ${\mathrm{Sc}}_5{\mathrm{Co}}_4{\mathrm{Si}}_{10}$ exhibits type-II superconductivity with a superconducting transition temperature $T_{\mathrm{C}}=3.5\left(1\right)\mathrm{K}$. The temperature dependence of the superfluid density obtained from transverse-field $\mu \mathrm{SR}$ spectra is best modeled using an isotropic Bardeen-Cooper-Schrieffer type $s$-wave gap symmetry with $2\mathrm{\Delta }/k_{\mathrm{B}}{T}_{\mathrm{C}}=2.84\left(2\right)$. However, the zero-field muon-spin relaxation rate reveals the appearance of a spontaneous magnetic field below $T_{\mathrm{C}}$, indicating that time-reversal symmetry (TRS) is broken in the superconducting state. Although this behavior is commonly associated with nonunitary or mixed singlet-triplet pairing, our group-theoretical analysis of the Ginzburg-Landau free energy alongside density functional theory calculations indicates that unconventional mechanisms are pretty unlikely. Therefore, we have hypothesized that TRS breaking may occur via a conventional electron-phonon process.

A class of septuple-atomic-layer two-dimensional (2D) materials, $\mathrm{Mo}{\mathrm{Si}}_2{\mathrm{N}}_4$ and $\mathrm{W}{\mathrm{Si}}_2{\mathrm{N}}_4$, was fabricated recently, and the three constituent atoms may be replaced by other elements in the Periodic Table, thus forming a large family. In the current work, using density functional theory calculations, we systematically investigate 18 $M{\mathrm{Si}}_2{\mathrm{N}}_4$ (where $M$ denotes groups IV-B, V-B, and VI-B transition metals) monolayers in both H- and T-phases for their stabilities and their electronic, magnetic, and spintronic properties, and we highlight the spintronic properties and their van der Waals heterostructures. The electronic and magnetic properties of these 2D monolayers are correlated with the $d$-levels splitting (from crystal field and exchange field) and electron filling. Spin-dependent band alignment of $M{\mathrm{Si}}_2{\mathrm{N}}_4$ heterostructures shows that most of them are type II heterostructures for both spin channels. In addition, the valence-band edge of seven $M{\mathrm{Si}}_2{\mathrm{N}}_4$ monolayers resides at the Brillouin zone center ($\mathrm{\Gamma }$-point), which may form momentum-matched heterostructures by stacking with other 2D semiconductors. The ferromagnetic semiconductors (H-$\mathrm{V}{\mathrm{Si}}_2{\mathrm{N}}_4$, H-$\mathrm{Nb}{\mathrm{Si}}_2{\mathrm{N}}_4$, and H-$\mathrm{Ta}{\mathrm{Si}}_2{\mathrm{N}}_4$) and the heterostructures composed of them (e.g., H-$\mathrm{Mo}{\mathrm{Si}}_2{\mathrm{N}}_4$/H-$\mathrm{V}{\mathrm{Si}}_2{\mathrm{N}}_4$ or H-$\mathrm{W}{\mathrm{Si}}_2{\mathrm{N}}_4$/H-$\mathrm{V}{\mathrm{Si}}_2{\mathrm{N}}_4$) can be a half-semiconductor or half-metal. These results, in addition to the literature reporting high Curie temperatures, indicate that $M{\mathrm{Si}}_2{\mathrm{N}}_4$ and their heterostructures are promising for room-temperature spintronics and optoelectronics.

Similar effects of metal and boron vacancies on phonon scattering and lattice thermal conductivity $\left({\kappa }_l\right)$ of ${\mathrm{ZrB}}_2$ and ${\mathrm{HfB}}_2$ are reported. These defects challenge the conventional understanding that associates larger impacts to bigger defects. We find the underlying reason to be a strong local perturbation caused by boron vacancy that substantially changes the interatomic force constants. In contrast, a long ranged but weaker perturbation is seen in the case of metal vacancy. We show that these behaviors originate from a mixed metallic and covalent bonding nature in the metal diborides. The thermal transport calculations are performed in a complete ab initio framework based on Boltzmann transport equation and density functional theory. Phonon-vacancy scattering is calculated using ab initio Green's function approach. Effects of natural isotopes and grain boundaries on ${\kappa }_l$ are also systematically investigated, however we find an influential role of vacancies to explain large variations seen in the experiments. We further report a two-order of magnitude difference between the amorphous and pure-crystal limits. Our results outline significant material design aspects for these multifunctional high-temperature ceramics.

Ferroelectrics have polar surfaces that can undergo large structural and stoichiometric modifications to be neutral. These changes can have major implications on the surface stability and physicochemical properties. We have studied the growth and structure of ferroelectric GeTe thin films on Si(111) by a combination of scanning tunneling microscopy, low-energy electron microscopy, and low-energy electron diffraction. We show that GeTe growth occurs with a single epitaxy and proceeds via a step-flow mode hindered by the advance of electrically neutral step edges exhibiting triangular notches. We demonstrate the presence of ferroelectric nanodomains with in-plane components of polarization and a complex restructuring of their polar surface. $2\times 2$ surface reconstruction, missing row reconstruction and extended 2D modulations of the surface structure are demonstrated on these nanodomains. We show that these structures stabilize the surface termination of the low-symmetry polar nanodomains.

We report atomistic simulation results which indicate that the location of shear banding in a metallic glass (MG) can be ascertained with reasonably high accuracy solely from the undeformed static structure. Correlation is observed between the location of the initiation of shear bands in a simulated MG and the initial distribution of the density of fertile sites (DFS) for stress-driven shear transformations identified a priori based on a deep learning model devised in our recent work [Fan and Ma, Nat. Commun. 12, 1506 (2021)]. In addition, we demonstrated that one can judge whether a glass is brittle or ductile solely based upon its initial DFS distribution. These validate that shear bands in MG arise from nonlinear instabilities and that the as-quenched glass structure contains inhomogeneities that influence these instabilities. In this paper, we also reveal an important subtlety regarding the nondeterministic nature of athermal quasistatic shear simulations.

We derive a simple analytical line integral expression for the relaxation volume tensor of an arbitrary interconnected dislocation network. This quantity determines the magnitude of dislocation contribution to the dimensional changes and volumetric swelling of a material, and highlights the fundamental dual role of dislocations as sources of internal strain as well as carriers of plastic deformation. To illustrate applications of the method, we compute the relaxation volume of a stacking fault tetrahedron, a defect commonly occurring in fcc metals; the volume of an unusual tetrahedral configuration formed by the $(a/2)\langle 111\rangle$ and $a\langle 001\rangle$ dislocations in a bcc metal; and estimate the relative contribution of extended dislocations to the volume relaxation of heavily irradiated tungsten.

We study the linear response to strain in a mean-field elastoplastic model for athermal amorphous solids, incorporating the power-law mechanical noise spectrum arising from plastic events. In the “jammed” regime of the model, where the plastic activity exhibits a nontrivial slow relaxation referred to as aging, we find that the stress relaxes incompletely to an age-dependent plateau, on a timescale which grows with material age. We determine the scaling behavior of this aging linear response analytically, finding that key scaling exponents are universal and independent of the noise exponent $\mu$. For $\mu >1$, we find simple aging, where the stress relaxation timescale scales linearly with the age $t_{\mathrm{w}}$ of the material. At $\mu =1$, which corresponds to interactions mediated by the physical elastic propagator, we find instead a $t_{\mathrm{w}}^{1/2}$ scaling arising from the stretched exponential decay of the plastic activity. We compare these predictions with measurements of the linear response in computer simulations of a model jammed system of repulsive soft athermal particles, during its slow dissipative relaxation towards mechanical equilibrium, and find good agreement with the theory.