We investigate the properties of hard antiphase boundaries in ${\mathrm{SrTiO}}_3$ using machine-learned force fields. In contrast to earlier findings based on standard ab initio methods, for all pressures up to $120\mathrm{kbar}$ the observed domain wall pattern maintains an almost perfect Néel character in quantitative agreement with Landau-Ginzburg-Devonshire theory, and the in-plane polarization $P_3$ shows no tendency to decay to zero. Together with the switching properties of $P_3$ under reversal of the Néel order parameter component, this provides hard evidence for the presence of rotopolar couplings. The present approach overcomes the severe limitations of ab initio simulations of wide domain walls and opens avenues toward concise atomistic predictions of domain-wall properties even at finite temperatures.

Heusler alloys, a class of easily prepared, highly ordered intermetallic compounds, were first reported in 1903. Since then, Heusler alloys have presented various physical phenomena in modern condensed-matter physics. Among Heusler alloys, Heusler-based fully compensated ferrimagnetic half-metals (FCF-HMs) are, particularly, relevant because they host fully spin polarization and have no net magnetic moment, making them have no stray field and less affected by external magnetic fields. Based on first-principles calculations and a tight-binding Hamiltonian model, we provide new insight into inverse-Heusler-based (IHB) FCF-HMs and reveal that they exhibit spin-polarized Weyl and quadratic nodal lines as well as spin-polarized drumheadlike surface states. This paper presents the electron-filling-based design rule and material candidates for IHB FCF-HMs and suggests that IHB FCF-HMs are promising candidates for follow-up investigations in the field of topological spintronics. Subsequent experimental confirmation of the topological states in IHB FCF-HMs is imminent.

We have performed first-principles calculations to investigate the electronic structure, configurations, formation, and binding energies of native and radiation induced point defects in pristine and Sc-doped wurtzite AlN. For the native defects, the nitrogen vacancy has the lowest formation energy in $p$-type material while the aluminum vacancy has the lowest formation energy in $n$-type material which is consistent with the previous studies. Several interstitial defect structures were modeled for Al, N, and Sc atoms. The effects of charge state on their relative stability were investigated. The binding energy of Sc with point defects was calculated and found to be dependent strongly on the defect type and charge state. The results obtained are discussed in light of the possible Sc effects on the radiation damage evolution in AlN. Thus the attraction of Sc atom to N vacancy and both Al and N interstitials reduces their mobility and increases Frenkel pair recombination distance.

Tetragonal CuMnAs was the first antiferromagnet where reorientation of the Néel vector was reported to occur by an inverse spin galvanic effect. A complicating factor in the formation of phase-pure tetragonal CuMnAs is the formation of an orthorhombic phase with nearly the same stoichiometry. Pure-phase tetragonal CuMnAs has been reported to require an excess of Cu to maintain a single phase in traditional solid state synthesis reactions. Here we show that subtle differences in diffraction patterns signal pervasive inhomogeneity and phase separation, even in Cu-rich ${\mathrm{Cu}}_{1.18}{\mathrm{Mn}}_{0.82}\mathrm{As}$. From calorimetry and magnetometry measurements, we identify two transitions corresponding to the Néel temperature ($T_N$) and an antiferromagnet to weak ferromagnet transition in ${\mathrm{Cu}}_{1.18}{\mathrm{Mn}}_{0.82}\mathrm{As}$ and ${\mathrm{CuMn}}_{0.964}{\mathrm{As}}_{1.036}$. These transitions have clear crystallographic signatures, directly observable in the lattice parameters upon in situ heating and cooling. The immiscibility and phase separation could arise from a spinoidal decomposition that occurs at high temperatures, and the presence of a ferromagnetic transition near room temperature warrants further investigation of its effect on the electrical switching behavior.

A huge magnetoresistance (MR) ratio exceeding 2000% at cryogenic temperature that was reported for half-metallic Heusler alloy based magnetic tunnel junctions showed large degradation at room temperature, which impedes practical application of Heusler alloy based MR devices. This motivates us to explore alternative Heusler alloys that show high spin polarization at finite temperatures. Here, we propose half-metallic Heusler alloys based on finite-temperature first-principles calculation via the disordered local moment method together with machine learning. We found several prospective materials at room temperature such as ${\mathrm{Co}}_2{\mathrm{MnGa}}_{0.2}{\mathrm{As}}_{0.8}$ and ${\mathrm{Co}}_2{\mathrm{FeAl}}_{0.4}{\mathrm{Sn}}_{0.6}$. We also investigated two combinatorial series, ${\mathrm{Co}}_2{\mathrm{MnGa}}_y{\mathrm{As}}_{1\text{−}y}$ and ${\mathrm{Co}}_2{\mathrm{FeAl}}_y{\mathrm{Sn}}_{1\text{−}y}$, to understand the effect of alloy mixing on temperature dependence and found that Fermi level tuning significantly improved the spin polarization and its temperature dependence, especially in ${\mathrm{Co}}_2{\mathrm{FeAl}}_y{\mathrm{Sn}}_{1\text{−}y}$.

Accurately describing the lattice dynamics and microscopic mechanism of thermal transport in materials with low-lying flat phonon modes remains an outstanding challenge due to the intrinsic strong anharmonicity. In this paper, we investigate the lattice dynamics and thermal transport in skutterudite ${\mathrm{YbFe}}_4{\mathrm{Sb}}_{12}$ using a state-of-the-art first-principles-based anharmonic phonon renormalization technique and a unified theory of lattice thermal transport. In contrast to the previous phenomenological models that introduce additional resonant scattering terms or hopping channels, we show that the unusual total lattice thermal conductivity in ${\mathrm{YbFe}}_4{\mathrm{Sb}}_{12}$ can be accurately predicted by considering anharmonic phonon renormalization and coherence contributions from the off-diagonal terms of heat flux operators. Both the cubic and quartic anharmonicities are essential for precisely predicting the significant shift in phonon energies. Specifically, the anharmonicity-induced phonon stiffening of the low-lying flat modes significantly enhances the thermal conductivity of particlelike phonons, e.g., by up to a factor of 1.6 at 300 K, by suppressing the cubic coupling strength and altering the scattering phase space, resulting in much-improved agreement with experiments. By further including the coherence contributions, the predicted total thermal conductivity increases by ∼22% throughout the entire temperature range, reproducing well the experimental values in both magnitude and temperature dependence. In this paper, we highlight the strong impact of higher-order anharmonicity on lattice dynamics and thermal transport in the filled skutterudite ${\mathrm{YbFe}}_4{\mathrm{Sb}}_{12}$. The insights gained in this paper will be helpful for manipulating the thermal properties of skutterudites and potentially other complex materials with strong anharmonicity, which can improve their performance in applications such as thermoelectrics, ferroelectrics, and photovoltaics.

Van der Waals heterostructures constitute a platform for investigating intriguing many-body quantum phenomena. In particular, transition-metal dichalcogenide (TMD) heterobilayers host long-lived interlayer excitons which exhibit permanent out-of-plane dipole moments. Here, we develop a microscopic theory for interlayer exciton-exciton interactions including both the dipolar nature of interlayer excitons as well as their fermionic substructure, which gives rise to an attractive fermionic exchange. We find that these interactions contribute to a drift force resulting in highly nonlinear exciton propagation at elevated densities in the ${\mathrm{MoSe}}_2-{\mathrm{WSe}}_2$ heterostructure. We show that the propagation can be tuned by changing the number of hBN spacers between the TMD layers or by adjusting the dielectric environment. In particular, although counterintuitive, we reveal that interlayer excitons in freestanding samples propagate slower than excitons in hBN-encapsulated TMDs—due to an enhancement of the net Coulomb drift with stronger environmental screening. Overall, our work contributes to a better microscopic understanding of the interlayer exciton transport in technologically promising atomically thin semiconductors.

Molecular dynamics simulations are used to study melts of asymmetric sphere-forming diblock copolymers with two significantly different values of the invariant degree of polymerization, $\overline{N}=$ 3820 and 960. In both systems, changes in parameters that correspond to decreasing temperature lead to the appearance of micelles at a critical micelle temperature (CMT) and crystallization at a lower order-disorder transition temperature (ODT). The CMT is identifiable in simulations by the appearance of large clusters with a strongly segregated core region, but has no equally clear signature in scattering experiments on systems of modest $\overline{N}$. The value of the product $\chi N$ at the CMT (where $\chi$ is the Flory-Huggins parameter and $N$ is degree of polymerization) is close to that predicted by SCFT for the ODT, while the value at the actual ODT is larger and increases with decreasing $\overline{N}$. Micelles exhibit significant and comparable dispersity in aggregation number in the crystalline and liquid phases near the ODT. Both the liquid and crystal phases exhibit transient dimers consisting of pairs of neighboring spherical micelles with cores connected by a bridge of core-block material.

Coulomb interactions in two-dimensional semimetals like graphene are not screened in the usual way, and sizable long-range interaction has been found. With the aim of achieving new materials with nonconventional screening, as well as, in order to investigate various effects such as reduced dimensionality and electronic band dispersion, we calculate the strength of the effective Coulomb interaction $U$ between $p_z$ electrons in recently developed hexagonal ${\mathrm{C}}_3\mathrm{N}$ materials from the first principles using the constrained random-phase approximation. We find that the calculated parameters in the monolayer of the ${\mathrm{C}}_3\mathrm{N}$ are larger than the ones in graphene and remain sizable even in metallic bilayers. Nonlocal Coulomb interactions $U\left(r\right)$ in nonmetallic ${\mathrm{C}}_3\mathrm{N}$ nanoribbons are almost 1 eV smaller than the ones for ${\mathrm{C}}_3\mathrm{N}$ monolayer. Our results show that similar to graphene, screening in hexagonal ${\mathrm{C}}_3\mathrm{N}$ is also nonconventional. This controversial screening of Coulomb interaction stems from the electronic structure and massless Dirac dispersion below Fermi energy, which has to be preserved for symmetry reasons in hexagonal ${\mathrm{C}}_3\mathrm{N}$ layers.

Two-dimensional layered materials, particularly ternary chalcogenides, are promising for technological applications, as they exhibit tunable electronic, optical, and magnetic properties. Here we present detailed first-principles calculations of electronic, optical, and thermal transport properties of ${\text{Fe}X}_2{Y}_4$ $(X=\mathrm{Ga}, \mathrm{In}; Y=\mathrm{S}, \mathrm{Se}, \mathrm{Te})$ ternary layered chalcogenides, relevant for sustainable energy applications. We show that the monolayers, similar to bulk, are dynamically and thermally stable, as demonstrated by phonon dispersion and molecular dynamics simulations. The exfoliation energy of monolayers is comparable to graphene and transition-metal dichalcogenides, suggesting possible synthesis of monolayers by mechanical exfoliation. In the hexagonal crystal structure, these materials are nonmagnetic semiconductors with varied band-gap values, that are interesting for photocatalysis, photovoltaics, and thermoelectric applications. ${\mathrm{FeGa}}_2{\mathrm{S}}_4$ and ${\mathrm{FeIn}}_2{\mathrm{S}}_4$ monolayers exhibit suitable band gaps and band-edge positions for photocatalytic water splitting and ${\mathrm{CO}}_2$ reduction. ${\mathrm{FeGa}}_2{\mathrm{Se}}_4$ and ${\mathrm{FeIn}}_2{\mathrm{Se}}_4$ monolayers are promising photocatalysts for hydrogen evolution reaction. The calculated optical absorption spectra indicate that ${\mathrm{FeIn}}_2{\mathrm{Te}}_4$, ${\mathrm{FeIn}}_2{\mathrm{Se}}_4$, and ${\mathrm{FeGa}}_2{\mathrm{Se}}_4$ are promising photovoltaic absorbers with high spectroscopic limited maximum efficiencies of $\sim$ 30%, 27%, and 24.5%, respectively, comparable to existing high-performance thin-film absorber materials, such as CdTe $(\sim 31.5\%)$ and ${\mathrm{CuInSe}}_2$ $(\sim 28\%)$. The narrow band gaps and relatively low room-temperature lattice thermal conductivity of ${\mathrm{FeGa}}_2{\mathrm{Te}}_4$ (11.00 W/mK) and ${\mathrm{FeIn}}_2{\mathrm{Te}}_4$ (8.84 W/mK) monolayers suggest potential applications in thermoelectrics. We thereby propose ${\text{Fe}X}_2{Y}_4$ ternary layered chalcogenides as promising materials for sustainable energy applications due to their suitable electronic, optical, and thermal properties.

The lanthanide pyrochlore oxides $L{n}_2{B}_2{\mathrm{O}}_7$ are one of the most intensely studied classes of materials within condensed matter physics, firmly centered as one of the pillars of frustrated magnetism. The extensive chemical diversity of the pyrochlores, coupled with their innate geometric frustration, enables realization of a wide array of exotic and complex magnetic ground states. Thus, the discovery of new pyrochlore compositions has been a persistent theme that continues to drive the field in exciting directions. The recent focus on the mixed $B$-site pyrochlores offers a unique route towards tuning both local coordination chemistry and sterics, while maintaining a nominally pristine magnetic sublattice. Here, we present a broad overview of the pyrochlore stability field, integrating recent synthetic efforts in mixed $B$-site systems with the historically established $L{n}_2{B}_2{\mathrm{O}}_7$ families. In parallel, we present the discovery and synthesis of the entire $L{n}_2{\mathrm{InSbO}}_7$ family ($Ln$ = La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) located near the boundary of the pyrochlore stability field using a rapid, hybrid mechanicochemical and microwave-assisted synthesis technique. Magnetic characterization on the entire class of compounds draws striking parallels to the stannate analogs, suggesting that these compounds may host a breadth of exotic magnetic ground states.

The quasi-two-dimensional van der Waals magnet ${\mathrm{Fe}}_{5-\delta }{\mathrm{GeTe}}_2$ has emerged as a promising platform for electronic and spintronic functionalities at room temperature, owing to its large ferromagnetic ordering temperature $T_{\text{C}}\approx$ 315 K. Interestingly, by cobalt (Co) substitution of iron in F5GT, $\text{i.e.,}{\left({\text{Fe}}_{1-x}{\text{Co}}_x\right)}_{5-\delta }{\text{GeTe}}_2$ (Co-F5GT), not only can its magnetic transition temperature be further enhanced, but the magnetic and structural ground states can also be tuned. Specifically, an antiferromagnetic (AFM) order is induced beyond the Co doping level $x\ge 0.4$. Here, we investigate the magnetic properties of a Co-F5GT single crystal at $x=0.45\left(1\right)$, by utilizing the element-specific, resonant magnetic x-ray scattering technique. Our study reveals an A-type, Ising-like AFM ground state, with a transition temperature $T_{\text{N}}\approx$ 340 K. In addition, our work unveils an important contribution from Co magnetic moments to the magnetic order. The application of the in-plane magnetic fields gradually polarizes the spin moments along the field direction, but without inducing incommensurate spin texture(s).

Hybrid metal-polymer micropillar arrays are elaborated by solvent-assisted embossing techniques and subsequent gold deposition. The polymer is poly(methyl methacrylate) (PMMA) grafted with Disperse Red 1 (DR1) azobenzene derivatives. This photochromic material exhibits spectacular photomechanical properties. Under off-normal $p$-polarized illumination in the absorption band of the DR1, the metal-polymer hybrid micropillars bend in a direction determined by the light polarization. The deformation remains stable when the light excitation is turned off. The optically driven modification of the pillar shape allows us to tune the optical properties of the pillar arrays. Very large intensity changes are measured in the diffraction spectra at zero order (specular reflection), first order, and second order, with almost on and off switching of the diffraction efficiency. The photoinduced bending of the micropillars can be fully reversed by switching the linear light polarization to the orthogonal $s$-polarization state. This allows us to restore the initial optical properties of the pillar array. The polarization-controlled reversibility of the pillar deformation suggests a complex photoinduced deformation mechanism involving stress and stress release assisted by the change in the viscoelastic properties of the polymer under illumination in the absorption band of the DR1 chromophore.

The formation and degradation of the solid electrolyte interphase (SEI) and its underlying transport properties play an essential role in the overall performance of lithium-ion batteries. This paper presents classical molecular dynamics studies on polycrystalline inorganic lithium fluoride (LiF) layers to model and predict the SEI transport properties. The ionic conductivity is obtained from the lithium-ion diffusivity in polycrystalline structures of LiF using the Nernst-Einstein relation. The predicted molecular dynamics data are used in a continuum scale phase-field model to evaluate the plating kinetics under fast charging conditions. The analysis emphasizes that the SEI ionic conductivity properties impact the plating dynamics, where SEI's low ion conductivity value is prone to large plating and subsequent capacity degradation. The combination of atomic and continuum scale studies shown herein lays a foundation to tune in SEI transport properties to decrease the amount of lithium plating and improve the performance of fast-charging batteries.

Metal intercalation is an effective method to modify the physical and chemical properties of low-dimensional systems such as epitaxial graphene. Here, we show that the nucleation and growth of metal nanostructures on epitaxial graphene on SiC(0001) can be dramatically changed by metal intercalation. Using scanning tunneling microscopy experiments, we demonstrate that dysprosium (Dy) metal islands are selectively nucleated on the area with Dy intercalation under both graphene and carbon buffer layers, while the adjacent area with only buffer layer intercalated remains relatively bare. Using first-principles calculations based on density functional theory, we show that adsorption of Dy adatom on the preferred nucleation area is energetically more favorable than on other areas. Moreover, changes in the electronic structure and the interlayer spacing upon Dy intercalation obtained from our calculations are also consistent with experimental observations. Our results indicate that metal intercalation is a promising way to manipulate the interaction between graphene and deposited adatoms.

Using the Ioffe-Regel-Mott criterion for strong localization crossover in disordered doped two-dimensional (2D) electron systems, we theoretically study the relationships between the three key experimentally determined localization quantities: critical density $n_{\mathrm{c}}$, critical resistance ${\rho }_{\mathrm{c}}$, and sample quality defined by the effective impurity density (as experimentally diagnosed by the sample mobility ${\mu }_{\mathrm{m}}$ at densities much higher than critical densities). Our results unify experimental results for 2D metal-insulator transitions (MITs) in Si systems over a 50-year period (1970–2020), showing that $n_{\mathrm{c}}$ $\left({\rho }_{\mathrm{c}}\right)$ decreases (increases) with increasing sample quality, explaining why the early experiments in the 1970s, using low-quality samples $[{\mu }_{\mathrm{m}}\sim {10}^3$ ${\mathrm{cm}}^2/\left(\mathrm{V}\mathrm{s}\right)]$, reported strong localization crossover at $n_{\mathrm{c}}\sim {10}^{12}$ ${\mathrm{cm}}^{-2}$ with ${\rho }_{\mathrm{c}}\sim {10}^3$ $\mathrm{\Omega }$ whereas recent experiments (after 1995), using high-quality samples $[{\mu }_{\mathrm{m}}>{10}^4$ ${\mathrm{cm}}^2/\left(\mathrm{V}\mathrm{s}\right)]$, report $n_{\mathrm{c}}\sim {10}^{11}$ ${\mathrm{cm}}^{-2}$ with ${\rho }_{\mathrm{c}}>{10}^4$ $\mathrm{\Omega }$. Our theory establishes the 2D MIT to be primarily a screened Coulomb disorder-driven strong localization crossover phenomenon, which happens at different sample-dependent critical density and critical resistance values, thus unifying Si 2D MIT phenomena over a 50-year period.

Deformation mechanisms in sputter-deposited crystalline Al/amorphous Si nanocomposites with nanolaminate or nanofibrous morphology are characterized by nanoindentation, micropillar compression testing and transmission electron microscopy (TEM). The nanofibrous composite having crystalline Al nanofibers with ∼40–50 nm in length and 15–20 nm in diameter embedded in amorphous Si exhibits strain hardening to a maximum flow stress of 2.9 GPa and no shear band (SB) formation in compression up to plastic strain exceeding 24%. On the other hand, nanolaminate composite that is composed of 80 nm crystalline Al layers and 20 nm amorphous Si layers exhibits catastrophic SBs starting at plastic strains in the range of 5–10%. Cross-sectional TEM of the deformed samples reveals a high density of stacking faults and twin boundaries in Al nanofibers and no microshear bands in the nanofibrous composite, suggesting plastic deformation in amorphous Si phase and crystalline Al nanofibers. Molecular dynamics simulations revealed that the plastic deformation in amorphous Si phase in the cosputtered films could be favored by the decrease in flow strength of amorphous Si with increasing Al solute concentration trapped in Si.