In weakly interacting organic semiconductors, static disorder and dynamic disorder often have an important impact on transport properties. Describing charge transport in these systems requires an approach that correctly takes structural and electronic fluctuations into account. Here, we present a multiscale method based on a combination of molecular-dynamics simulations, electronic-structure calculations, and a transport theory that uses time-dependent nonequilibrium Green's functions. We apply the methodology to investigate charge transport in ${\mathrm{C}}_{60}$-containing self-assembled monolayers, which are used in organic field-effect transistors.

Magnetic refrigeration based on the magnetocaloric effect at room temperature is one of the most attractive alternatives to the current gas compression/expansion method routinely employed. Nevertheless, in giant magnetocaloric materials, optimal refrigeration is restricted to the narrow temperature window of the phase transition ($T_c$). In this work, we present the possibility of varying this transition temperature in the same giant magnetocaloric material by ion irradiation. We demonstrate that the transition temperature of iron rhodium thin films can be tuned by the bombardment of ions of ${\mathrm{Ne}}^{5+}$ with varying fluences up to ${10}^{14}$ ions ${\mathrm{cm}}^{-2}$, leading to optimal refrigeration over a large 270–380 K temperature window. The $T_c$ modification is found to be due to the ion-induced disorder and to the density of new pointlike defects. The variation of the phase transition temperature with the number of incident ions opens new perspectives in the conception of devices using giant magnetocaloric materials.

In the nanoregime, chemical species can reorganize in ways not predicted by their equilibrium bulk behavior. Here, we engineer Ni-Cr nanoalloys at the magnetic end of their compositional range (i.e., 0–15 at. % Cr), and we investigate the effect of Cr incorporation on their structural stability and resultant magnetic ordering. To ensure their stoichiometric compositions, the nanoalloys are grown by cluster beam deposition, a method that allows one-step, chemical-free fabrication of bimetallic nanoparticles. While full Cr segregation toward nanoparticle surfaces is thermodynamically expected for low Cr concentrations, metastability occurs as the Cr dopant level increases in the form of residual Cr in the core region, yielding desirable magnetic properties in a compensatory manner. Using nudged elastic band calculations, residual Cr in the core is explained based on modifications in the local environment of individual Cr atoms. The resultant competition between ferromagnetic and antiferromagnetic ordering gives rise to a wide assortment of interesting phenomena, such as a cluster-glass ground state at very low temperatures and an increase in Curie temperature values. We emphasize the importance of obtaining the commonly elusive magnetic nanophase diagram for $M$-Cr $(M=\mathrm{Fe}$, Co, and Ni) nanoalloys, and we propose an efficient single-parameter method of tuning the Curie temperature for various technological applications.

We develop a minimal continuum dislocation dynamics model for slip in bcc metals that accounts explicitly for non-Schmid behavior of screw dislocations in these materials. The dislocation substructure is represented by a continuum distribution of $1/2\left[111\right]$ screw dislocations moving on three possible $\left\{110\right\}$ planes according to the flow rule that describes the response of isolated screw dislocations to external loads. The cross-slip of dislocations is assessed using the master equation which takes into account different energy barriers for dislocations moving on the three slip planes. To demonstrate the performance of the model, we study the buildup of plastic strain at 77 K in both tension and compression for a number of loading directions covering the entire area of the standard stereographic triangle. The non-Schmid behavior of screw dislocations is shown to persist to the continuum level, whereby interactions between dislocations merely affect the rate of plastic flow.

We examine analytically equilibrium conformations of elastic chains of paramagnetic beads in the presence of a precessing magnetic field. Conformations of these filaments are determined by minimizing their total energy, given in the harmonic approximation by the sum of the bending energy, quadratic in its curvature, and the magnetic dipolar interaction energy, quadratic in the projection of the vector tangent to the filament onto the precession axis. In particular, we analyze two families of open filaments with their ends aligned along the precession axis and described by segments of planar curves and helices. These configurations are characterized in terms of two parameters encoding their features such as their length, separation between their ends, as well as their bending and magnetic moduli, the latter being proportional to the magnitude and precession angle of the magnetic field. Based on energetic arguments, we present the set of parameter values for which each of these families of curves is probable to occur.

The spin-state transition in ${\mathrm{LaCoO}}_3$ has eluded description for decades despite concerted theoretical and experimental effort. In this study, we approach this problem using fully charge self-consistent density functional theory + embedded dynamical mean field theory (DFT+DMFT). We show from first principles that ${\mathrm{LaCoO}}_3$ cannot be described by a single, pure spin state at any temperature. Instead, we observe a gradual change in the population of higher-spin multiplets with increasing temperature, with the high-spin multiplets being excited at the onset of the spin-state transition followed by the intermediate-spin multiplets being excited at the metal-insulator-transition temperature. We explicitly elucidate the critical role of lattice expansion and oxygen octahedral rotations in the spin-state transition. We also reproduce, from first principles, that the spin-state transition and the metal-insulator transition in ${\mathrm{LaCoO}}_3$ occur at different temperature scales. In addition, our results shed light on the importance of electronic entropy in driving the spin-state transition, which has so far been ignored in all first-principles studies of this material.

Microstructure features in correlation with the incommensurate modulation and oxygen interstitials in $\mathrm{B}{\mathrm{i}}_2\left(\mathrm{S}{\mathrm{r}}_{2-x}\mathrm{L}{\mathrm{a}}_x\right)\mathrm{Cu}{\mathrm{O}}_{6+\delta }$ superconducting materials were studied by Cs-corrected scanning transmission electron microscopy. Atomic displacements following the modulation wave were well characterized by a sinusoidal wave for each atomic layer, which highlighted clear changes resulting from increases in the La concentration. Careful investigations of the alterations in the local atomic structure revealed that remarkable microstructural features, i.e., notable soliton lines, which arise from the prominent interplay between incommensurate modulation and the basic lattice, appear at the $\mathrm{Cu}{\mathrm{O}}_2$ sheets yielding visible structural anomalies for $x$ ranging from 0.40 to 0.85. The interstitial oxygen atoms between the SrO-BiO layers became clearly visible for $X\ge 0.73$ and showed well-defined ordered states in the $x=1.10$ sample. These structural features, in particular the strong structural effects of the soliton lines on the $\mathrm{Cu}{\mathrm{O}}_2$ sheets, could evidently affect the physical properties of layered La-Bi2201 systems.

Based on the combination of first-principles calculations, Boltzmann transport equation, and electron-phonon interaction (EPI), we investigate the thermal and electronic transport properties of crystalline cinnabar $\left(\alpha \text{−}\mathrm{HgS}\right)$. The calculated lattice thermal conductivity ${\kappa }_L$ is remarkably low, e.g., $0.60{\mathrm{Wm}}^{-1}{\mathrm{K}}^{-1}$ at $300\mathrm{K}$, which is about $30\%$ of the value for the typical thermoelectric material PbTe. Via taking fully into account the k dependence of the electron relaxation time computed from the EPI matrix, the accurate numerical results of thermopower $S$, electrical conductivity $\sigma$, and electronic thermal conductivity ${\kappa }_E$ are obtained. The calculated power factor $S^2\sigma$ is relatively high while the value of ${\kappa }_E$ is negligible, which, together with the fairly low ${\kappa }_L$, leads to a good thermoelectric performance in the $n$-type doped $\alpha \text{−}\mathrm{HgS}$, with the figure of merit $zT$ even exceeding 1.4. Our analyses reveal that (i) the large weighted phase space and the quite low phonon group velocity result in the low ${\kappa }_L$, (ii) the presence of flat band around the Fermi level combined with the large band gap causes the high $S$, and (iii) the small electron linewidths of the conduction band lead to a large relaxation time and thus a relatively high $\sigma$. These results support that $\alpha \text{−}\mathrm{HgS}$ is a potential candidate for thermoelectric applications.

We introduce a phase-field crystal model that creates an array of complex three- and two-dimensional crystal structures via a numerically tractable three-point correlation function. The three-point correlation function is designed in order to energetically favor the principal interplanar angles of a target crystal structure. This is achieved via an analysis performed by examining the crystal's structure factor. This approach successfully yields energetically stable simple cubic, diamond cubic, simple hexagonal, graphene layers, and ${\mathrm{CaF}}_2$ crystals. To illustrate the ability of the method to yield a particularly complex and technologically important crystal structure, we show how this three-point correlation function method can be used to generate perovskite crystals.

Thin-film synthesis methods that have developed over the past decades have unlocked emergent interface properties ranging from conductivity to ferroelectricity. However, our attempts to exercise precise control over interfaces are constrained by a limited understanding of growth pathways and kinetics. Here we demonstrate that shuttered molecular beam epitaxy induces rearrangements of atomic planes at a polar/nonpolar junction of ${\mathrm{LaFeO}}_3$ (LFO)/$n\text{−}{\mathrm{SrTiO}}_3$ (STO) depending on the substrate termination. Surface characterization confirms that substrates with two different $({\mathrm{TiO}}_2$ and SrO) terminations were prepared prior to LFO deposition; however, local electron-energy-loss spectroscopy measurements of the final heterojunctions show a predominantly LaO/${\mathrm{TiO}}_2$ interfacial junction in both cases. Ab initio simulations suggest that the interfaces can be stabilized by trapping extra oxygen (in LaO/${\mathrm{TiO}}_2)$ and forming oxygen vacancies (in ${\mathrm{FeO}}_2$/SrO), which points to different growth kinetics in each case and may explain the apparent disappearance of the ${\mathrm{FeO}}_2$/SrO interface. We conclude that judicious control of deposition time scales can be used to modify growth pathways, opening new avenues to control the structure and properties of interfacial systems.

In a series of studies related to laser-induced damage of optical materials and deposition of plastics, we discovered a broadly emitting photoluminescence with fast lifetimes that we termed quasicontinuum photoluminescence (QC-PL). Here, we suggest that a high density of optically active defects leads to QC-PL, where interactions between defects affect the temporal and spectral characteristics of both excitation and emission. We develop a model that predicts the temporal characteristics of QC-PL, based on energy transfer interactions between high densities of defects. Our model does not explain all spectral broadening and redshifts found in QC-PL, since we do not model spectral changes in defects due to proximity to other defects. However, we do provide an example of a well-defined system that exhibits the QC-PL characteristics of a distribution in shortened lifetimes and broadened, redshifted energy levels: an organic chromophore (fluorescein) that has been dried rapidly on a fused silica surface. Recently, we showed that regions of fused silica exposed to up to 1 billion high-fluence laser shots at 351 rm nm at subdamage fluences exhibit significant transmission losses at the surface. Here, we find that these laser-exposed regions also exhibit QC-PL. Increases in the density of induced defects on these laser-exposed surfaces, as measured by the local transmission loss, lead to decreases in the observed lifetime and redshifts in the spectrum of the QC-PL, consistent with our explanation for QC-PL. We have found QC-PL in an increasing variety of situations and materials, and we believe it is a phenomenon commonly found on surfaces and nanostructured materials.

Machine learning interatomic potentials (MLIPs) based on a large data set obtained by density functional theory calculation have been developed recently. This study gives both conceptual and practical bases for the high accuracy of MLIPs, although MLIPs have been considered to be simply an accurate black-box description of atomic energy. We also construct the most accurate MLIP of elemental Ti ever reported using a linearized MLIP framework and many angular-dependent descriptors, which also corresponds to a generalization of the modified embedded atom method potential.

We report superconductivity at 1.4 K in the ternary ${\mathrm{SrPt}}_{10}{\mathrm{P}}_4$ with a complex new structure. ${\mathrm{SrPt}}_{10}{\mathrm{P}}_4$ crystallizes in a monoclinic space-group C2/c (#15) with lattice parameters $a=22.9151\left(9\right)\text{Å}, b=13.1664$(5)$\text{Å}, c=13.4131$(5)$\text{Å}$, and $\beta =90.0270\left(5\right){}^{\circ }$. Bulk superconductivity in the samples has been demonstrated through resistivity, ac susceptibility, and heat capacity measurements. High pressure measurements have shown that the superconducting ${\mathrm{T}}_C$ is systematically suppressed upon application of pressure, with a ${\mathrm{dT}}_C/\mathrm{dP}$ coefficient of $-0.016$ K/GPa.

We report a systematic investigation on the magnetization relaxation properties of iron-based half-metallic Heusler alloy ${\mathrm{Fe}}_2{\mathrm{Cr}}_{1-x}\mathrm{Co}{}_x\mathrm{Si}$ (FCCS) thin films using broadband angular-resolved ferromagnetic resonance. Band structure engineering through Co doping ($x$) demonstrated by first-principles calculations is shown to tune the intrinsic magnetic damping over an order of magnitude, namely $1\times {10}^{-2}–8\times {10}^{-4}$. Notably, the intrinsic damping constants for samples with high Co concentration are among the lowest reported for Heusler alloys and even comparable to magnetic insulator yttrium iron garnet. Furthermore, a significant reduction of both isotropic and anisotropic contributions of extrinsic damping of the FCCS alloys was found in the FCCS films with $x=0.5–0.75$, which is of particular importance for applications. These results demonstrate a practical recipe to tailor functional magnetization for Heusler alloy-based spintronics at room temperature.

Modulating the polarization of a beam of quantum particles is a powerful method to tailor the macroscopic properties of the ensuing energy flux as it directly influences the way in which its quantum constituents interact with other particles, waves, or continuum media. To date, practical polarizers, being well developed for electric and electromagnetic energy, have not yet been proposed for heat fluxes carried by phonons. Here, we report on atomistic phonon transport calculations demonstrating that ferroelectric domain walls can operate as phonon polarizers when a heat flux pierces them. Our simulations for the representative ferroelectric perovskite ${\mathrm{PbTiO}}_3$ show that the structural inhomogeneity associated with the domain walls strongly suppresses transverse phonons, while longitudinally polarized modes can travel through multiple walls in series, largely ignoring their presence.

In this study, thermal unpinning of edge dislocations from voids in $\alpha$-Fe is investigated by means of molecular dynamics simulations. The activation energy as a function of shear stress and temperature is systematically determined. Simulations with a constant applied stress are compared with dynamic simulations with a constant strain rate. We found that a constant applied stress results in a temperature-dependent activation energy. The temperature dependence is attributed to the elastic softening of iron. If the stress is normalized with the softening of the specific shear modulus, the activation energy is shown to be temperature-independent. From the dynamic simulations, the activation energy as a function of critical shear stress was determined using previously developed methods. The results from the dynamic simulations are in good agreement with the constant stress simulations, after the normalization. This indicates that the computationally more efficient dynamic method can be used to obtain the activation energy as a function of stress and temperature. The obtained relation between stress, temperature, and activation energy can be used to introduce a stochastic unpinning event in larger-scale simulation methods, such as discrete dislocation dynamics.

The 6000 series Al alloys, which include a few percent of Mg and Si, are important in automotive and aviation industries because of their low weight, as compared to steels, and the fact their strength can be greatly improved through engineered precipitation. To enable atomistic-level simulations of both the processing and performance of this important alloy system, a neural network (NN) potential for the ternary Al-Mg-Si has been created. Training of the NN uses an extensive database of properties computed using first-principles density functional theory, including complex precipitate phases in this alloy. The NN potential accurately reproduces most of the pure Al properties relevant to the mechanical behavior as well as heat of solution, solute-solute, and solute-vacancy interaction energies, and formation energies of small solute clusters and precipitates that are required for modeling the early stage of precipitation and mechanical strengthening. This success not only enables future detailed studies of Al-Mg-Si but also highlights the ability of NN methods to generate useful potentials in complex alloy systems.