In this work, we investigate the effects of the ${\mathrm{V}}_2{\mathrm{O}}_3$ structural phase transition on the magnetic properties of an amorphous magnetic thin film of CoFeB in contact with it. ${\mathrm{V}}_2{\mathrm{O}}_3$ thin films are deposited epitaxially on sapphire substrates, reaching bulklike properties after few nm of growth. By means of temperature dependent Kerr effect characterizations, we prove that crossing the ${\mathrm{V}}_2{\mathrm{O}}_3$ structural phase transition induces reproducible and reversible changes to CoFeB magnetic properties, especially to its coercive field. By decreasing the oxide layer thickness, its effects on the magnetic layer decreases, while reducing the magnetic layer thickness maximizes it, with a maximum of 330% coercive field variation found between the two ${\mathrm{V}}_2{\mathrm{O}}_3$ structural phases. By simply tuning the temperature, this systematic study shows that the engineering of ${\mathrm{V}}_2{\mathrm{O}}_3$ structural transition induces large interfacial strain and thus strong magnetic property variations to an amorphous thin film, opening wide possibilities in implementing strain-driven control of the magnetic behavior without strict requirements on epitaxial coherence at the interface.

We calculate the effects of conduction band electrons (CBEs), introduced by doping or photoexcitation, on the ferroelectricity and phonon dynamics of ${\mathrm{BaTiO}}_3$ (BTO). We show that CBEs destabilize ferroelectricity, which would lower the Curie temperature and coercive field, and might help to improve the speed or efficiency with which polarization domains can be switched in ferroelectric devices. We show that CBEs lower the frequencies of the $A_1$ soft and ferroelectric modes in BTO's ferroelectric phases, and raise the soft-mode frequency in its paraelectric phase. We also show that femtosecond laser pulses could be used to selectively excite a coherent $A_1$ ferroelectric-mode phonon. This would allow this much-studied excitation to be monitored by pump-probe spectroscopy as it decays into other modes. We show that many of the properties of doped and undoped BTO have simple and intuitive explanations, within an ionic picture of BTO's bonding, if it is assumed that ferroelectricity is not driven by long-range interactions, but by the attraction between Ti and O neighbors. Most of the effects of CBEs are consequences of them reducing ions' charges and increasing their polarizabilities, thereby weakening the Ti-O attraction. As the CBE density increases, so does the density of delocalized interstitial electrons, which would increase conductivity. We argue that a polar metallic phase exists if the threshold for metallic conductivity can be reached before the CBEs make polar distortions energetically unfavorable by critically weakening the Ti-O attraction.

$\mathrm{BaTi}{\mathrm{O}}_3$ (BTO) is an emergent material in the field of silicon-integrated photonics, as its thin films have been demonstrated to have a very large electro-optic (Pockels) coefficient that can be used for optical modulators. However, BTO grown directly on $\mathrm{SrTi}{\mathrm{O}}_3$ (STO)-buffered Si (STO is required for epitaxial growth) initially grows with a polarization direction not suitable for the geometries currently used in photonic devices. Here, we grow BTO on a $\mathrm{BaSn}{\mathrm{O}}_3$-buffered STO substrate to form orthorhombic $mm2$ BTO, which has in-plane polarization orientation needed for photonic devices. Extensive crystalline characterization is done to confirm the high quality of the films, along with electro-optic measurements. Theoretical simulations coupled with the experimental results provide a foundational understanding of the properties of strain-stabilized orthorhombic BTO. Large electro-optic coefficients of 121 pm/V are observed in films as thin as 40 nm.

The hunt for an ideal topological insulator, where the Dirac point is situated in a desirable energetic position and the bulk remains insulating, has motivated experiments on band structure engineering in these materials. To achieve this, ${\mathrm{Sb}}_2{\mathrm{Te}}_3$ and ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ are commonly combined in ternary compounds or, less frequently, in heterostructures. Here we report on the growth of ${\mathrm{Sb}}_2{\mathrm{Te}}_3/{\mathrm{Bi}}_2{\mathrm{Te}}_3$ heterostructures by means of molecular-beam epitaxy. Using angle-resolved photoelectron spectroscopy, we are able to differentiate between the shift of the chemical potential and the changes in the electronic structure, causing the lift-off of the Dirac point away from the bulk valence band when varying the ${\mathrm{Sb}}_2{\mathrm{Te}}_3$ adlayer thickness. Our paper demonstrates that the important modulation of the surface states takes place for the very first ${\mathrm{Sb}}_2{\mathrm{Te}}_3$ layers, while thicker adlayers only cause a gradual change of the bulk states and a rigid shift of the chemical potential. Furthermore, we observe the occurrence of diffusion between the ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ and ${\mathrm{Sb}}_2{\mathrm{Te}}_3$ layers and conclude that a growth at room temperature, followed by annealing, maintains an acceptable crystalline quality while substantially reducing the interdiffusion.

Owing to its unique thermodynamical description, phase separation has largely been modeled in the Cahn-Hilliard framework. In the present work, as a computationally efficient alternative, a multicomponent, multiphase-field model operating in Allen-Cahn framework is presented and subsequently employed to simulate spinodal decomposition. Stability analysis shows that the formulation can cover the effect of phase separation while simplifying the extension to multiple phases and incorporation of additional driving forces. Computational efficiency of the proposed approach is compared with the conventional technique by modeling intercalation in a representative one-dimensional domain. Moreover, intercalation within a multigrain system involving multiparticle interaction is studied. Our results suggest initiation of phase transformation at higher order junctions as well as a grain-by-grain intercalation behavior in a two-phase cathode material such as the well-studied ${\text{LiFePO}}_4$.

Principles of design to create dynamically stable transition metal-, lanthanide-, and actinide-based low-dimensional borides are presented. A charge transfer analysis of donor metal atoms to electron deficient honeycombed B lattices allows to predict complex covalent heterostructures hosting Dirac states. The applicable guidelines are supported with the analysis of phonon spectra computed with first-principles calculations to demonstrate the physical stability of nanometer-thick heterostructures. Similar or dissimilar layered borides can be stacked on top of each other in a layer-by-layer fashion creating compounds that can be fundamentally different from the individual layers, opening a rich playground to explore novel physical properties and new materials. Functionalities such as multiple Dirac states, highly dispersive electronic bands, and decoupled acoustic-optical phonon branches are studied. The combination of appealing electronic properties and physical realization makes of predicted layered borides promising materials to integrate a new generation of two-dimensional materials.

We report the room temperature elastic constants of boron phosphide (BP) and boron arsenide (BAs) single crystals derived from Brillouin frequencies measured by picosecond interferometry. The synthesis of BP and BAs with thermal conductivity as high as 540 and $1000\mathrm{W}{\mathrm{m}}^{\text{–}1}{\mathrm{K}}^{\text{–}1}$, respectively, has made them promising materials for thermal management. Accurate measurements of elastic constants are needed to assess the accuracy of computational modeling of the lattice dynamics. The crystals are cut and polished in different orientations to access waves traveling along different directions. The surface normal orientations of the crystals are determined using electron backscattering diffraction. We studied the Brillouin frequencies of quasilongitudinal waves in five different orientations of BP and BAs crystals. Quasishear waves were observed in two orientations of BP and one orientation of BAs. The propagation directions and acoustic velocities are used to construct Christoffel equations which are then solved for the elastic constants. We report $C_{11}$, $C_{12}$, and $C_{44}$ values of 354 ± 5 GPa, 83 ± 15 GPa, and 190 ± 8 GPa for BP and 291 ± 5 GPa, 76 ± 13 GPa, and 173 ± 6 GPa for BAs. The measured elastic constants for BAs differ by less than 5% and 17% from calculated elastic constants obtained through local density approximation and Perdew-Burke-Ernzerhof density functional calculations, respectively. In most cases, the measured elastic constants are larger than the calculated elastic constants.

The generation of nonequilibrium electron spin polarization, spin transport, and spin detection are fundamental in many quantum devices. We demonstrate that a lattice of magnetic nanodots enhances the electron spin polarization in monolayer graphene via carrier exchange. We probed the spin polarization through a resistively detected variant of electron spin resonance and observed resonance amplification mediated by the presence of the nanodots. Each nanodot locally injects a surplus of spin-polarized carriers into the graphene, and the ensemble of all spin hot spots generates a nonequilibrium electron spin polarization in the graphene layer at macroscopic lengths. This occurs whenever the interdot distance is comparable or smaller than the spin diffusion length.

We comprehensively investigated the magnetic, ferroelectric, and ME properties of ${\mathrm{Ba}}_{2-x}{\mathrm{Sr}}_x{\mathrm{Co}}_2{\left({\mathrm{Fe}}_{1-y}{\mathrm{Al}}_y\right)}_{12}{\mathrm{O}}_{22}$ single crystals in broad doping ranges of Sr $(1.0\le x\le 1.8)$ and Al $(0.00\le y\le 0.08)$. Most of the investigated compounds exhibit an intriguing coexistence of two apparently competing magnetic phases: a transverse conical (TC) and alternating longitudinal conical (ALC) spin structure. The magnetic properties show that the ${\mathrm{Ba}}_{0.2}{\mathrm{Sr}}_{1.8}{\mathrm{Co}}_2{\left({\mathrm{Fe}}_{0.96}{\mathrm{Al}}_{0.04}\right)}_{12}{\mathrm{O}}_{22}$ crystal has the highest ordering temperature and largest volume fraction of the ALC phase at zero H; further, after the application of an in-plane H, it exhibits a maximized volume fraction of the metastable TC phase, resulting in the highest ME susceptibility and electric polarization at all temperatures below 300 K. Our findings demonstrate that securing the thermal stability of the ALC phase is a crucial prerequisite to achieve optimized ME coupling in ${\mathrm{Co}}_2\mathrm{Y}$-type hexaferrites, pointing to a general strategy applicable to other hexaferrites as well.

In this work, we investigate the radiation-induced segregation (RIS) resulting from the coupling between the atomic and point defect (PD) fluxes toward the structural defects of the microstructure. This flux coupling depends on the migration mechanisms of PDs and atoms, including thermal diffusion mechanisms and forced atomic relocations (FAR) occurring in displacement cascades. We derive an analytic model of the PD and solute RIS profiles accounting for PD production and mutual recombination, the FAR mechanism, and the overall sink strength of the microstructure controlling the elimination of PDs at structural defects. From this model, we present a parametric investigation of diffusion and RIS properties in dilute $\mathrm{Fe}\text{−}B$ $(B=\mathrm{P}, \mathrm{Mn}, \mathrm{Cr}, \mathrm{Si}, \mathrm{Ni}, \mathrm{and} \mathrm{Cu})$ binary alloys, in the form of quantitative temperature/radiation flux/sink strength maps. As in previous works, we distinguish three kinetic domains for the diffusion and RIS properties: the recombination domain, the sink domain, and the thermal domain. Both our analytical approach and numerical applications demonstrate that the diffusion and RIS behaviors of PDs and solute atoms largely differ from one kinetic domain to another. Moreover, at high radiation flux, low temperature, and large sink strength, FARs tend to destroy the solute RIS profiles and therefore reduce the overall amount of RIS by forcing the mixing of solute and host atoms, especially close to PD sinks. Finally, we provide quantitative criteria to emulate in-reactor RIS behaviors by ion irradiation.

Bipolar magnetic semiconductors (BMSs) are a class of compounds possessing different valence band maximum (VBM) and conduction band minimum (CBM) in terms of electron spins, which are related to but distinct from half-metals. They allow one to switch one spin current to another by applying external gate voltage or chemical doping, which can meet the requirement of bipolar manipulation in spintronic devices. Designing and searching BMSs are still a challenge. Here we predict that monolayer ${\mathrm{CrPS}}_4$ and ${\mathrm{CrPSe}}_4$ are bipolar ferromagnetic semiconductors with Curie temperature 58 K and 82 K, respectively. We predicted that they can be exfoliated from their A-type antiferromagnetic bulk form. Electric-field gating could drive ${\mathrm{CrPS}}_4$ and ${\mathrm{CrPSe}}_4$ into half-metals with reversible spin-polarization directions, where the bipolar doping is relatively easier to access for ${\mathrm{CrPSe}}_4$ in experiment than ${\mathrm{CrPS}}_4$ for its suitable electron affinity and ionic potential. Furthermore, we highlight the role played by the different magnitude between spin exchange splitting and crystal field splitting in the formation of BMSs. The results presented here may provide new clues in designing or searching BMSs.

Despite the fundamental importance of mass transport, there is still a lack of knowledge regarding the mechanisms in many materials. In particular, how the magnetic spin structure interacts with migrating defects is unclear. Here, using density functional theory to examine cation interstitial transport in antiferromagnetic corundum-structured oxides, we reveal an interplay between the migration of the interstitial and the magnetic structure of the oxide. The magnetic spin configuration impacts the migration energy of the migrating defect but, critically, the defect modifies the spin configuration. Thus, the two aspects change in concert to modify one another. This has profound implications for mass transport in magnetic materials.

The design of novel two-dimensional (2D) materials with unique atomistic configurations and exotic properties are highly desirable for material science. Here, we report the prediction of 2D ${\mathrm{Cu}}_2\mathrm{C}$ layers featuring unique carbon motifs with Dirac nodal lines through evolutionary algorithm searches in conjunction with first-principles calculations. The global minimum $\alpha \text{−}{\mathrm{Cu}}_2\mathrm{C}$ is an exciting new structure featuring one-dimensional (1D) zigzag carbon chains sandwiched by two hexagonal-close-packed copper monolayers, conferring to our predicted ground-state 2D $\alpha \text{−}{\mathrm{Cu}}_2\mathrm{C}$ an inverse coordination structure. This polyacetylene-like motif $\left(\mathrm{poly}\text{−}{\mathrm{C}}_2\right)$ is also encountered in $\gamma \text{−}{\mathrm{Cu}}_2\mathrm{C}$. Remarkably, the electronic band structure of $\alpha$ and $\gamma \text{−}{\mathrm{Cu}}_2\mathrm{C}$ phases containing polyacetylene-like chains display a 1D Dirac nodal line, which is protected by the glide plane symmetry. Fermi velocities $\left(v_f\right)$ as high as $2.45\times {10}^5$ and $3.85\times {10}^5$ m/s are calculated for $\alpha$ and $\gamma \text{−}{\mathrm{Cu}}_2\mathrm{C}$ phases, respectively. This work is an effective effort to design and stabilize the 2D copper carbide layers with exotic structures and nodal lines.

In this paper, a two-dimensional phase field crystal model of graphene and hexagonal boron nitride (hBN) is extended to include out-of-plane deformations in stacked multilayer systems. As proof of principle, the model is shown analytically to reduce to standard models of flexible sheets in the small deformation limit. Applications to strained sheets, dislocation dipoles, and grain boundaries are used to validate the behavior of a single flexible graphene layer. For multilayer systems, parameters are obtained to match existing theoretical density functional theory calculations for graphene/graphene, hBN/hBN, and graphene/hBN bilayers. More precisely, it is shown that the parameters can be chosen to closely match the stacking energies and layer spacing calculated by Zhou et al. [Phys. Rev. B 92, 155438 (2015)]. Further validation of the model is presented in a study of rotated graphene bilayers and stacking boundaries. The flexibility of the model is illustrated by simulations that highlight the impact of complex microstructures in one layer on the other layer in a graphene/graphene bilayer.

To understand the magnetic property of layered van der Waals materials $\mathrm{CrO}X$ $(X=\text{Cl,Br})$, we performed detailed first-principles calculations for both the bulk and monolayer. We found that the charge-only density functional theory combined with the explicit on-site interaction terms (so-called $\mathrm{cDFT}+U$) well reproduces the experimental magnetic ground state of bulk $\mathrm{CrO}X$, which is not the case for the use of spin-dependent density functional theory (so-called $\mathrm{sDFT}+U$). Unlike some of the previous studies, our results show that $\mathrm{CrO}X$ monolayers are antiferromagnetic as in the bulk. It is also consistent with our magnetic force linear response calculation of exchange couplings, $J_{\mathrm{ex}}$. The result of orbital-decomposed $J_{\mathrm{ex}}$ calculations shows that the Cr $t_{2g}\text{−}{t}_{2g}$ component contributes mainly to the antiferromagnetic order in both the bulk and monolayer. Our result and analysis show that taking the correct Hund's physics into account is of key importance in constructing the magnetic phase diagram and describing the electronic structure.

Lead-free perovskites $\mathrm{CsSn}{\mathrm{Br}}_3$ were predicted to be promising for photovoltaic applications, yet the measured performance is rather disappointing. To understand the mechanism behind the large discrepancy and to further improve the functionality, we study the stability, transition energy levels, and diffusion of different intrinsic defects in pure $\alpha -\mathrm{CsSn}{\mathrm{Br}}_3$ using density-functional theory calculations. Our results reveal the unique characteristics of the intrinsic defects and their effects on the optoelectronic properties of perovskite $\mathrm{CsSn}{\mathrm{Br}}_3$. Among the low formation energy defects, the vacancy defects $V_{\mathrm{Sn}}$ and $V_{\mathrm{Cs}}$ can form shallow energy levels to provide $p$-type conductivity, while the antisite defects ${\mathrm{Br}}_{\mathrm{Sn}}$ and $\mathrm{Br}{}_{\mathrm{Cs}}$ can act as detrimental recombination centers to deteriorate the performance of devices. Moreover, the mobile defects $V_{\mathrm{Sn}}^{2-}$ and $V_{\mathrm{Cs}}^{-}$ also degrade device performance by the field-screening effect. The effect of the chemical potential modulation of $\mathrm{Sn}{\mathrm{F}}_2$ addition into $\mathrm{CsSn}{\mathrm{Br}}_3$ is also discussed. These results provide in-depth insights into defect behaviors in the Sn-based perovskites and shed light on related optoelectronic applications.

Although intermetallics with a $B2$-type crystal structure are typically brittle, a class of $B2$ intermetallics that demonstrates unusually high ductility has been reported. A set of recently developed $B2$-like quaternary precious metal-rare earth alloys also includes compositions with significant ductility. To predict ductility in these systems, we have adapted a computational energy-based metric based on slip systems and relative stability of planar defects, developed to predict ductility in $B2$ binary systems, for use with quaternary $B2$-like alloys. The computational metric successfully predicts the experimentally-determined ductility or brittleness of 15 $B2$-like quaternary precious metal-rare earth and refractory alloys.

Compensated ferrimagnets are promising materials for fast spintronic applications based on domain-wall motion as they combine the favorable properties of ferromagnets and antiferromagnets. They inherit from antiferromagnets immunity to external fields, fast spin dynamics, and rapid domain-wall motion. From ferromagnets they inherit straightforward ways to read out the magnetic state, especially in compensated half metals, where electrons flow in only one spin channel. Here, we investigate domain structure in compensated half-metallic ${\mathrm{Mn}}_2{\mathrm{Ru}}_{0.5}\mathrm{Ga}$ films and assess their potential in domain-wall motion-based spin-electronic devices. Our focus is on understanding and reducing domain-wall pinning in unpatterned epitaxial thin films. Two modes of magnetic reversal, driven by nucleation or domain-wall motion, are identified for different thin film deposition temperatures ($T_{\mathrm{dep}}$). The magnetic aftereffect is analyzed to extract activation volumes ($V^{*}$), activation energies ($E_{\mathrm{A}}$), and their variation ($\mathrm{\Delta }{E}_{\mathrm{A}}$). The latter is decisive for the magnetic reversal regime, where domain-wall motion dominated reversal (weak pinning) is found for $\mathrm{\Delta }{E}_{\mathrm{A}}<0.2\mathrm{eV}$ and nucleation dominated reversal (strong pinning) for $\mathrm{\Delta }{E}_{\mathrm{A}}>0.5\mathrm{eV}$. A minimum $\mathrm{\Delta }{E}_{\mathrm{A}}=28\mathrm{meV}$ is found for $T_{\mathrm{dep}}=290{}^{\circ }\mathrm{C}$. Prominent pinning sites are visualized by analyzing virgin domain patterns after thermal demagnetization. In the sample investigated they have spacings of order 300 nm, which gives an upper limit of the track width of spin-torque domain-wall motion-based devices.

Herein we present our results on the investigation of the influence of external magnetic fields on the anisotropic collective dynamics of core/shell colloidal cubes having a hematite core and silica shell. Owing to the hematite cores, these micrometer-sized particles possess permanent dipole moments, which are at an angle with respect to the long diagonal of the cubes. As a result, they self-assemble into chains, which subsequently sediment to form higher-order structures. Using multispeckle ultrasmall-angle x-ray photon correlation spectroscopy, the anisotropic dynamics within these structures at the nearest-neighbor length scale was probed. The relaxation of the intermediate scattering function follows a compressed exponential behavior along all the different directions with respect to the external field—parallel, perpendicular, and at an angle $\approx {45}^{\circ }$—indicating hyperdiffusive behavior. We believe that the inhomogeneous distribution of stress points originating from the interplay of external field-induced (both gravitational and magnetic) alignment of the chains are responsible for the anomalous dynamics. The effective diffusion coefficients along and at $\approx {45}^{\circ }$ angle exhibit mild de Gennes narrowing, which is not very common for hyperdiffusive dynamics. We rationalize our observations by considering a superposition of diffusive and stress-induced ballistic processes and argue that depending on the azimuthal direction the relative contribution from these two processes changes.