The phase field method has emerged as the tool of choice to simulate complex pattern formation processes in various domains of materials sciences. For the phase field model to faithfully reproduce the dynamics of a prescribed free-boundary problem with transport equations in the bulk and boundary conditions at the interfaces, the so-called thin-interface limit should be performed. For a phase transformation driven by diffusion, the kinetic cross-coupling between the phase field and the diffusion field has recently been introduced, allowing a control on interface boundary conditions in the general case where the diffusivity in the growing phase $D_S$ neither vanishes (one-sided model) nor equals the one of the disappearing phase $D_L$ (symmetric model). Here, we investigate the capabilities of this nondiagonal phase field model in the case of two-dimensional dendritic growth. We benchmark our model with Green's function calculations (sharp-interface model) for the symmetric and one-sided cases, and our results for arbitrary $D_S/D_L$ allow us to propose a generalization of the theory by Barbieri and Langer [Phys. Rev. A 39, 5314 (1989)] for finite anisotropy of interface energy. We also perform simulations that evidence the necessity of introducing the kinetic cross-coupling and of eliminating surface diffusion. Our work opens up the way for quantitative phase field simulations of phase transformations with diffusion in the growing phases playing an important role in the pattern and velocity selections.

The soft magnetic properties in iron-based nanocrystalline alloys have generally been achieved through the addition of nonmagnetic additives, but this has the unwanted effect of reducing the saturation magnetization. In a recent advance, it was shown that magnetically soft nanostructured alloys could be prepared from a simple, binary Fe-B amorphous precursor by ultrarapid annealing (URA). These materials had a saturation magnetization comparable to that of silicon steel, while a refined grain structure resulted in considerably lower core losses through the exchange averaging effect of the magnetic anisotropy. As yet, the formation mechanism of the soft magnetic nanostructure in this simple binary system has not been identified. We investigated the crystallization process of amorphous ${\mathrm{Fe}}_{86}{\mathrm{B}}_{14}$ using in situ transport measurements. The relationship between the resistivity and the volume fraction of bcc-Fe crystallites was calibrated by ${}^{57}\mathrm{Fe}$ Mössbauer spectroscopy. In this paper, we show that the isothermal crystallization behavior of these Fe-B amorphous alloys is well described by the Johnson-Mehl-Avrami-Kolmogorov kinetic model and that the the Avrami exponent ($n$) shows a tendency to increase with increasing annealing temperature ($T_{\mathrm{a}}$); the exponent $n$ is approximately 1.5 at $T_{\mathrm{a}}<700\mathrm{K}$ while $n>2$ at $T_{\mathrm{a}}=729\mathrm{K}$. This suggests that below 700 K the nucleation rate is governed solely by a limited number of quenched-in nuclei whereas at higher temperatures, homogeneous nucleation starts to dominate. Since the onset of crystallization is increased dramatically by rapid heating, the formation of the refined nanostructure induced by URA can be well understood by an increase in the number density of nuclei due to homogeneous nucleation. The increased homogeneous nucleation at higher annealing temperatures can be linked to enhanced atomic transport due to viscous flow.

We investigate the ferroelectric domain architecture and its operando response to an external electric field in ${\mathrm{BaTiO}}_3$-based electro-optic heterostructures integrated on silicon. By noninvasive optical second-harmonic generation, we identify the preexistence of in-plane ($a$-) domains dispersed within a predominantly out-of-plane- ($c$-) oriented matrix. Monitoring the poling behavior of the respective domain populations, we show that the spontaneous polarization of these $a$-domains lack a predominant orientation in the pristine state, yet can be selectively aligned with an in-plane electric field, leaving the $c$-domain population intact. Hence, domain reorientation of a ferroelastic $c$-to-$a$ type was directly excluded. Such independent electrical control of ferroelectric $a$-domains in a $c$-oriented ${\mathrm{BaTiO}}_3$ film on silicon is a valuable platform for engineering multidirectional electro-optic functionality in integrated photonic devices.

Vertically oriented layered $\mathrm{I}{\mathrm{n}}_2\mathrm{S}{\mathrm{e}}_3$ nanoplates have been grown by chemical vapor deposition. It is demonstrated that the growth is subject to the species of substrates because the morphologies of nanoplates grown on mica and $\mathrm{Si}{\mathrm{O}}_2/\mathrm{Si}$ substrates are quite different. We obtained vertical $\mathrm{I}{\mathrm{n}}_2\mathrm{S}{\mathrm{e}}_3$ nanosheets on $\mathrm{Si}{\mathrm{O}}_2/\mathrm{Si}$ and sapphire substrates with surface dangling bonds, while we obtained horizontal $\mathrm{I}{\mathrm{n}}_2\mathrm{S}{\mathrm{e}}_3$ nanosheets on mica without surface dangling bonds. A continuum model based on surface diffusion and a moving boundary is developed to describe the intermediate states of the steps and the edge. The dangling-bond-assisted heterogeneous nucleation mechanism is proposed to describe the growth of vertical $\mathrm{I}{\mathrm{n}}_2\mathrm{S}{\mathrm{e}}_3$ nanoplates: difficult diffusion of $\mathrm{I}{\mathrm{n}}_2\mathrm{S}{\mathrm{e}}_3$ adatoms on dangling-bonded substrate surface due to large interface energy between $\mathrm{I}{\mathrm{n}}_2\mathrm{S}{\mathrm{e}}_3$ adatoms and $\mathrm{Si}{\mathrm{O}}_2/\mathrm{Si}$ (and sapphire) substrates results in $\mathrm{I}{\mathrm{n}}_2\mathrm{S}{\mathrm{e}}_3$ seeds growing perpendicularly to the substrate; then vertical nanoplates inherit the growth direction of $\mathrm{I}{\mathrm{n}}_2\mathrm{S}{\mathrm{e}}_3$ seeds. This work paves the way for the growth of monodisperse vertical two-dimensional nanoplates on $\mathrm{Si}{\mathrm{O}}_2/\mathrm{Si}$ substrates, and it expands their utilizations from conventional lateral structured devices to nonplanar vertical electronic and optoelectronic devices.

We present a simple geometrical “fluidic” approximation to the nonadiabatic part of the Kohn-Sham potential, $v_{\mathrm{KS}}$, of time-dependent density-functional theory (DFT). This part of $v_{\mathrm{KS}}$ is often crucial, but most practical functionals utilize an adiabatic approach based on ground-state DFT, limiting their accuracy in many situations. For a variety of model systems, we calculate the exact time-dependent electron density and find that the fluidic approximation corrects a large part of the error arising from the “exact adiabatic” approach, even when the system is evolving far from adiabatically.

We report on the electronic structure of doped $\mathrm{LaFe}{\mathrm{O}}_3$ at the crossover from an insulating-to-metallic phase transition. Comprehensive x-ray spectroscopic methodologies are used to understand core and valence electronic structure as well as crystal structure distortions associated with the electronic transition. Despite the antiferromagnetic (AFM) ordering at room temperature, we show direct evidence of itinerant carriers at the Fermi level revealed by resonant photoemission spectroscopy (RPES) at the Mo $L_3$ edge. RPES data taken at the Fe $L_3$ edge show spectral weight near the valence band edge and significant hybridization with O $2p$ states required for AFM ordering. Resonant inelastic x-ray scattering spectra taken across Fe $L_{2,3}$ edges show electron correlation effects ($U$) driven by Coulomb interactions of $d$ electrons as well as broad charge-transfer excitations for $x\ge 0.2$ where the compound crosses over to a metallic state. Site substitution of Fe by Mo ions in the Fe-${\mathrm{O}}_6$ octahedra enhances the separation of the two Fe-O bonds and Fe-O-Fe bonding angles relative to the orthorhombic $\mathrm{LaFe}{\mathrm{O}}_3$, but no considerable distortions are present to the overall structure. Mo ions appear to be homogeneously doped, with average valency of both metal sites monotonically decreasing with increasing Mo concentration. This insulator-to-metal phase transition with AFM stability is primarily understood through intermediate interaction strengths between correlation ($U$) and bandwidth ($W$) at the Fe site, where an estimation of this ratio is given. These results highlight the important role of extrinsic carriers in stabilizing a unique phase transition that can guide future efforts in antiferromagnetic-metal spintronics.

Magnetic skyrmions in chiral-lattice ferromagnets are currently attracting enormous research interest because of their potential applications in spintronic devices. However, they emerge in bulk specimens only in a narrow window of temperature and magnetic field. This limited stability regime is recognized as an obstacle to technical applications. Recent experiments demonstrated that the thermodynamic stability of magnetic skyrmions is enhanced or suppressed by the application of a uniaxial strain depending on its axial direction in bulk chiral-lattice ferromagnets MnSi [Y. Nii et al., Nat. Commun. 6, 8539 (2015); A. Chacon et al., Phys. Rev. Lett. 115, 267202 (2015)] and ${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$ [S. Seki et al., Phys. Rev. B 96, 220404(R) (2017)]. Motivated by these experimental discoveries, we theoretically investigated the effects of anisotropic Dzyaloshinskii–Moriya interactions on the stability of magnetic skyrmions caused by this uniaxial strain. We find that magnetic skyrmions are significantly stabilized (destabilized) in the presence of anisotropic DM interactions when an external magnetic field lies perpendicular (parallel) to the anisotropy axis, along which the DM coupling is strengthened. Our results account completely for the experimentally observed strain-induced stabilization and destabilization of magnetic skyrmions and provide a firm ground for possible strain engineering of skyrmion-based electronic devices.

The $\left({\mathrm{Li}}_{1-x}{\mathrm{Fe}}_x\mathrm{OH}\right)\mathrm{FeSe}$ superconductor has been suspected of exhibiting long-range magnetic ordering due to Fe substitution in the LiOH layer. However, no direct observation such as magnetic reflection from neutron diffraction has been reported. Here, we use a chemical design strategy to manipulate the doping level of transition metals in the LiOH layer to tune the magnetic properties of the $\left({\mathrm{Li}}_{1-x-y}{\mathrm{Fe}}_x{\mathrm{Mn}}_y\mathrm{OD}\right)\mathrm{FeSe}$ system. We find Mn doping exclusively replaces Li in the hydroxide layer resulting in enhanced magnetization in the $\left({\mathrm{Li}}_{0.876}{\mathrm{Fe}}_{0.062}{\mathrm{Mn}}_{0.062}\mathrm{OD}\right)\mathrm{FeSe}$ superconductor without significantly altering the superconducting behavior as resolved by magnetic susceptibility and electrical/thermal transport measurements. As a result, long-range magnetic ordering was observed below 12 K with neutron diffraction measurements. This work has implications for the design of magnetic superconductors for the fundamental understanding of superconductivity and magnetism in the iron chalcogenide system as well as exploitation as functional materials for next-generation devices.

The surface structures resulting from scratching monolayer ${\mathrm{MoS}}_2$ on ${\mathrm{SiO}}_2$ and multilayer ${\mathrm{MoS}}_2$ using diamond nanotips with normal forces up to $3\mu \mathrm{N}$ and scan velocities up to $10\mu \mathrm{m}/\mathrm{s}$ in ambient conditions are compared. In both cases the damage process initiates with normal forces of about $2\mu \mathrm{N}$. As shown by postmortem AFM images the monolayer is peeled off along the zigzag direction of ${\mathrm{MoS}}_2$ and folded in the form of flat triangular flakes. Wrinkling and multiple folding can be associated to debris accumulation originated from the ${\mathrm{SiO}}_2$ substrate, which is also scratched by the tip. On the multilayer ${\mathrm{MoS}}_2$ thicker chips with linear size of about 100 nm and height of few tens of nm are rolled out. The chipping is associated to a distinct stick-slip motion of the AFM tip with repetition period logarithmically increasing with the velocity but definitely below the chip length, suggesting that the process occurs through a series of exfoliation events. Simple energetic considerations allow to explain why peeled monolayers tend to fold over the untreated surface while thicker layers tend to bend and form chips.

Weyl semimetals such as the TaAs family (TaAs, TaP, NbAs, NbP) host quasiparticle excitations resembling the long-sought-after Weyl fermions at special band-crossing points in the band structure denoted as Weyl nodes. They are predicted to exhibit a negative longitudinal magnetoresistance (LMR) due to the chiral anomaly if the Fermi energy is sufficiently close to the Weyl points. However, current jetting effects, i.e., current inhomogeneities caused by a strong, field-induced conductivity anisotropy in semimetals, have a similar experimental signature and therefore have hindered a determination of the intrinsic LMR in the TaAs family so far. This work investigates the longitudinal magnetoresistance of all four members of this family along the crystallographic $a$ and $c$ directions. Our samples are of similar quality as those previously studied in the literature and have a similar chemical potential, as indicated by matching quantum-oscillation frequencies. Care was taken to ensure homogeneous currents in all measurements. As opposed to previous studies where this was not done, we find a positive LMR that saturates in fields above 4 T in TaP, NbP, and NbAs for $B\left|\right|c$. Using Fermi-surface geometries from band-structure calculations that had been confirmed by experiment, we show that this is the behavior expected from a classical purely orbital effect, independent of the distance of the Weyl node to the Fermi energy. The TaAs family of compounds is the first to show such a simple LMR without apparent influences of scattering anisotropy. In configurations where the orbital effect is small, i.e., for $B\left|\right|a$ in NbAs and NbP, we find a nonmonotonous LMR, including regions of negative LMR. We discuss a weak antilocalization scenario as an alternative interpretation to the chiral anomaly for these results, since it can fully account for the overall field dependence.

We report neutron diffraction studies of FeS single crystals obtained from ${\mathrm{Rb}}_x{\mathrm{Fe}}_{2-y}{\mathrm{S}}_2$ single crystals via a hydrothermal method. While no $\sqrt{5}\times \sqrt{5}$ iron vacancy order or block antiferromagnetic order typical of ${\mathrm{Rb}}_x{\mathrm{Fe}}_{2-y}{\mathrm{S}}_2$ is found in our samples, we observe $C$-type short-range antiferromagnetic order with moments pointed along the $c$ axis hosted by a different phase of FeS with an expanded interlayer spacing. The Néel temperature for this magnetic order is determined to be $170\pm 4$ K. Our finding of a variant FeS structure hosting this $C$-type antiferromagnetic order demonstrates that the known FeS phase synthesized in this method is in the vicinity of a magnetically ordered ground state, providing insights into understanding a variety of phenomena observed in FeS and the related ${\mathrm{FeSe}}_{1-x}{\mathrm{S}}_x$ iron chalcogenide system.

Heterostructures of transition metal oxides perovskites represent an ideal platform to explore exotic phenomena involving the complex interplay between the spin, charge, orbital and lattice degrees of freedom available in these compounds. At the interface between such materials, this interplay can lead to phenomena that are present in none of the original constituents such as the formation of the interfacial two-dimensional electron system (2DES) discovered at the $\mathrm{LA}{\mathrm{O}}_3/\mathrm{ST}{\mathrm{O}}_3$ (LAO/STO) interface. In samples prepared by growing a LAO layer onto a STO substrate, the 2DES is confined in a band bending potential well, whose width is set by the interface charge density and the STO dielectric properties, and determines the electronic band structure. Growing LAO (2 nm) /STO $(x$ nm)/LAO (2 nm) heterostructures on STO substrates allows us to control the extension of the confining potential of the top 2DES via the thickness of the STO layer. In such samples, we explore the electronic structure trend under an increase of the confining potential with using soft x-ray angle-resolved photoemission spectroscopy combined with ab initio calculations. The results indicate that varying the thickness of the STO film modifies the quantization of the $3d{t}_{2\mathrm{g}}$ bands and, interestingly, redistributes the charge between the $d_{xy}$ and $d_{xz}/d_{yz}$ bands.

In semiconducting solar cell absorbers, high absorption coefficient (α) near the band-edge is critical to maximize the photocurrent generation and collection. Nevertheless, despite the importance of the band-edge absorption characteristics, the quantitative analysis of the band-edge optical transitions has not been performed. In this study, we have implemented systematic density-functional theory calculation, focusing on the band-edge oscillator strength ($f$) of eight practical solar cell absorbers (GaAs, InP, CdTe, $\mathrm{CuInS}{\mathrm{e}}_2$, $\mathrm{CuGaS}{\mathrm{e}}_2$, $\mathrm{C}{\mathrm{u}}_2\mathrm{ZnSnS}{\mathrm{e}}_4$, $\mathrm{C}{\mathrm{u}}_2\mathrm{ZnSn}{\mathrm{S}}_4$, and $\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3\mathrm{Pb}{\mathrm{I}}_3$) with zincblende, chalcopyrite, kesterite, and hybrid perovskite structures. We find that all the crystals exhibit very high $f$ in the band-gap $\left(E_{\mathrm{g}}\right)$ region, compared with $f$ in the higher-energy region, revealing the fact that α in the $E_{\mathrm{g}}$ region is enhanced significantly by the high $f$. We have further established that the magnitude of $f$ can be interpreted from the charge distributions involved in the optical transition. In contrast to general understanding that the band-edge light absorption is derived from the joint density of state, our result reveals the critical role of $f$ in the determination of absolute α values near $E_{\mathrm{g}}$ in quite general solar cell materials.

We have deposited In-doped $\mathrm{S}{\mathrm{b}}_x\mathrm{T}{\mathrm{e}}_y$ ($x:y=2:3$, 1:1, 3:1, 4:1) phase-change films, and studied their physics properties like crystallization temperature, activation energy, and optical band gap. On the basis of these parameters, a balance between thermal stability and crystallization speed can be found when In content is 20 at. %. We thus further studied their crystallization kinetics using the flash differential scanning calorimetry and the generalized Mauro-Yue-Ellison-Gupta-Allan viscosity model to investigate the potential fragile-to-strong crossover (FSC) in $\mathrm{I}{\mathrm{n}}_{20}{\left(\mathrm{S}{\mathrm{b}}_2\mathrm{T}{\mathrm{e}}_3\right)}_{80}, \mathrm{I}{\mathrm{n}}_{20}{\left(\mathrm{SbTe}\right)}_{80}, \mathrm{I}{\mathrm{n}}_{20}{\left(\mathrm{S}{\mathrm{b}}_3\mathrm{Te}\right)}_{80}$, and $\mathrm{I}{\mathrm{n}}_{20}{\left(\mathrm{S}{\mathrm{b}}_4\mathrm{Te}\right)}_{80}$ supercooled liquids. It was found that, $\mathrm{I}{\mathrm{n}}_{20}{\left(\mathrm{S}{\mathrm{b}}_3\mathrm{Te}\right)}_{80}$ has a large crossover magnitude ($f$) of 2.4 with a distinct FSC behavior, but its maximum crystal growth rate ($U_{\max }$) of $0.047\mathrm{m}{\mathrm{s}}^{-1}$ is too low to satisfy the high-speed phase-change application. The crystal growth rate of $\mathrm{I}{\mathrm{n}}_3\mathrm{SbT}{\mathrm{e}}_2$ ($\mathrm{I}{\mathrm{n}}_{51}\mathrm{S}{\mathrm{b}}_{17}\mathrm{T}{\mathrm{e}}_{32}$) was found to be $0.08\mathrm{m}{\mathrm{s}}^{-1}$ without distinct FSC. In contrast, $\mathrm{I}{\mathrm{n}}_{20}{\left(\mathrm{S}{\mathrm{b}}_4\mathrm{Te}\right)}_{80}$ was demonstrated to have a larger $U_{\max }$ of $0.425\mathrm{m}{\mathrm{s}}^{-1}$ and a distinct FSC behavior with a $f$ value of 2.6, which is larger than that of typical phase-change supercooled liquid $\mathrm{A}{\mathrm{g}}_{5.5}\mathrm{I}{\mathrm{n}}_{6.5}\mathrm{S}{\mathrm{b}}_{59}\mathrm{T}{\mathrm{e}}_{29}$. The results strongly support that, obvious FSC is unique only in some phase-change supercooled liquids, but not a universal dynamic feature.

Mechanical behavior of polycrystalline materials is intimately connected to migration of grain boundaries, which in turn is dramatically impacted by the presence of defects. In this paper, we present atomistic simulations to elucidate the elementary mechanism that dictates the role of vacancies in enhancing grain boundary migration via shear-coupled normal motion. The minimum energy pathway and the associated energy barriers are calculated using the nudged elastic band method. Fully three-dimensional atomistic simulations provide excellent verification of the three-dimensional disconnection model and furnish quantitative evidence that vacancies facilitate grain boundary migration by weakening the line tension of a disconnection loop. It is also revealed that vacancies serve as energetically favorable sites for the nucleation of grain boundary disconnections, thereby inducing shear-coupled grain boundary migration.

We report x-ray photon correlation spectroscopy (XPCS) experiments with in situ rheometry performed on a soft glass composed of a concentrated suspension of charged silica nanoparticles subjected to step strains that induce yielding and flow. The XPCS measurements characterize the particle-scale and mesoscale motions within the glass that underlie the highly protracted decay of the macroscopic stress following the step strains. These dynamics are anisotropic, with slow, convective particle motion along the direction of the preceding shear that persists for surprisingly large times and that is accompanied by intermittent motion in the perpendicular (vorticity) direction. A close correspondence between the convective dynamics and stress relaxation is demonstrated by power-law scaling between the characteristic velocity of the collective particle motion and the rate of stress decay.

In this work we study the effect of different synthetic conditions on thermal transport properties of poly(3,4-ethylenedioxythiophene) (PEDOT) by focusing in particular on the role of proton scavengers. To this aim, different PEDOT samples were generated in silico using a novel computational algorithm based on a combination of first-principles density functional theory and classical molecular dynamics simulations. The corresponding thermal conductivities were then estimated using the approach to equilibrium molecular dynamics methodology. The results show that the initial synthetic conditions strongly affect the corresponding thermal conductivities, which display variations up to a factor of $\sim 2$ depending on the proton scavenger. By decomposing the thermal conductivity tensor along the direction of maximum chain alignment and the corresponding perpendicular directions, we attribute the thermal conductivity differences to the variations in the average polymer chain length ${\lambda }_{\mathrm{ave}}$. A dependence of the thermal conductivity with the polydispersity index was finally observed, suggesting a possible role of intercrystallite chains in enhancing thermal transport properties. By means of the Green-Kubo modal analysis, we eventually characterize the vibrational modes involved in PEDOT thermal transport and investigate how they are related to the thermal conductivity values of the samples.