Thermal conductivities of $\mathrm{V}{\mathrm{N}}_x/\mathrm{MgO}$(001) ($0.76\le x\le 1.00$) epitaxial layers, grown by reactive magnetron sputter deposition, are measured in the temperature range $300<T<1000\mathrm{K}$ using time-domain thermoreflectance (TDTR). Data for the total thermal conductivity are compared to the electronic contribution to the thermal conductivity calculated from the measured electrical conductivity, the Wiedemann-Franz law, and an estimate of the temperature dependence of the Lorenz number $L$($T$). The total thermal conductivity is dominated by electron contribution and varies between $13\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ at $x=0.76$ and $20\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ at $x=1.00$ for $T=300\mathrm{K}$ and between 25 and $35\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ for $T=1000\mathrm{K}$. The lattice thermal conductivity vs $x$ ranges from 5 to $7\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ at 300 K and decreases by 20% at 500 K. The low magnitude and weak temperature dependence of the lattice thermal conductivity are attributed to strong electron-phonon coupling in VN.

Advances in understanding heat transport in solids were recently reported by both experiment and theory. However an efficient and predictive quantum simulation framework to investigate thermal properties of solids, with the same complexity as classical simulations, has not yet been developed. Here we present a method to compute the thermal conductivity of solids by performing ab initio molecular dynamics at close to equilibrium conditions, which only requires calculations of first-principles trajectories and atomic forces, thus avoiding direct computation of heat currents and energy densities. In addition the method requires much shorter sequential simulation times than ordinary molecular dynamics techniques, making it applicable within density functional theory. We discuss results for a representative oxide, MgO, at different temperatures and for ordered and nanostructured morphologies, showing the performance of the method in different conditions.

The modification of polymer dynamics in the presence of strongly interacting nanoparticles has been shown to significantly change the macroscopic properties above the glass transition temperature of polymer nanocomposites (PNCs). However, much less attention has been paid to changes in the dynamics of glassy PNCs. Analysis of neutron and light scattering data presented herein reveals a surprising enhancement of local dynamics, e.g., fast picosecond and secondary relaxations, in glassy PNCs accompanied with a strengthening of mechanical modulus. We ascribe this counter-intuitive behavior to the complex interplay between chain packing and stretching within the interfacial layer formed at the polymer-nanoparticle interface.

An approach to greatly enhance the piezoelectric properties ($\sim 400$ pC/N) of the tetragonal $\mathrm{BaTi}{\mathrm{O}}_3$ polycrystal using a small number of $A$-site acceptor-donor substitutions [D. Xu et al., Acta Mater. 79, 84 (2014)] has been proposed. In this study, $\mathrm{Pb}\left(\mathrm{ZrTi}\right){\mathrm{O}}_3$ (PZT) based polycrystals with various crystal symmetries (tetragonal, rhombohedral, and so on) were chosen to investigate the piezoelectricity enhancement mechanism. X-ray diffraction results show that doping generates an intrinsic uniaxial compressive stress along the ${\left[001\right]}_{\mathrm{pc}}$ direction in the $AB{\mathrm{O}}_3$ lattices. Piezoelectric maps in the parameter space of temperature and Ti concentration in the PZT and doped system show a more significant enhancement effect of $\mathrm{L}{\mathrm{i}}^{+}\text{−}\mathrm{A}{\mathrm{l}}^{3+}$ codoping in tetragonal PZT than in the rhombohedral phase. Phenomenological thermodynamic analysis indicates that the compressive stress results in more serious flattening of the free-energy profile in tetragonal PZT, compared with that in the rhombohedral phase. The chemical stress obtained by this acceptor-donor codoping can be utilized to optimize the piezoelectric performance on the tetragonal-phase site of the morphotropic phase boundary in the PZT system. The present study provides a promising route to the large piezoelectric effect induced by chemical-stress-driven flattening of the free-energy profile.

We introduce a simple but efficient electronic fitness function (EFF) that describes the electronic aspect of the thermoelectric performance. This EFF finds materials that overcome the inverse relationship between $\sigma$ and $S$ based on the complexity of the electronic structures regardless of specific origin (e.g., isosurface corrugation, valley degeneracy, heavy-light bands mixture, valley anisotropy or reduced dimensionality). This function is well suited for application in high throughput screening. We applied this function to 75 different thermoelectric and potential thermoelectric materials including full- and half-Heuslers, binary semiconductors, and Zintl phases. We find an efficient screening using this transport function. The EFF identifies known high-performance $p$- and $n\text{−}\mathrm{type}$ Zintl phases and half-Heuslers. In addition, we find some previously unstudied phases with superior EFF.

Nanoscale phonon transport is a key process that governs thermal conduction in a wide range of materials and devices. Creating controlled phonon populations by resonant excitation at terahertz (THz) frequencies can drastically change the characteristics of nanoscale thermal transport and allow a direct real-space characterization of phonon mean-free paths. Using metamaterial-enhanced terahertz excitation, we tailored a phononic excitation by selectively populating low-frequency phonons within a nanoscale volume in a ferroelectric $\mathrm{BaTi}{\mathrm{O}}_3$ thin film. Real-space time-resolved x-ray diffraction microscopy following THz excitation reveals ballistic phonon transport over a distance of hundreds of nm, two orders of magnitude longer than the averaged phonon mean-free path in $\mathrm{BaTi}{\mathrm{O}}_3$. On longer length scales, diffusive phonon transport dominates the recovery of the transient strain response, largely due to heat conduction into the substrate. The measured real-space phonon transport can be directly compared with the phonon mean-free path as predicted by molecular dynamics modeling. This time-resolved real-space visualization of THz-matter interactions opens up opportunities to engineer and image nanoscale transient structural states with new functionalities.

We theoretically demonstrate how competition between band inversion and spin-orbit coupling (SOC) results in nontrivial evolution of band topology, taking antiperovskite ${\mathrm{Ba}}_{\text{3}}\mathrm{SnO}$ as a prototype material. A key observation is that when the band inversion dominates over SOC, there appear “twin” Dirac cones in the band structure. Due to the twin Dirac cones, the band shows highly peculiar structure in which the upper cone of one of the twin continuously transforms to the lower cone of the other. Interestingly, the relative size of the band inversion and SOC is controlled in this series of antiperovskite $A_{3}E\mathrm{O}$ by substitution of $A$ (Ca, Sr, Ba) and/or $E$ (Sn, Pb) atoms. Analysis of an effective model shows that the emergence of twin Dirac cones is general, which makes our argument a promising starting point for finding a singular band structure induced by the competing band inversion and SOC.

The laser ablation of materials induced by an ultrashort femtosecond pulse is a complex phenomenon, which depends on both the material properties and the properties of the laser pulse. A combination of molecular dynamics (MD) and momentum scaling model (MSM) methods is applied to a large atomic system to study the process of ultrafast laser-material interactions, behavior of matter in a highly nonequilibrium state, material disintegration, and formation of nanoclusters (NCs). Laser pulses with several fluences in the range from $500\mathrm{J}/{\mathrm{m}}^2$ to $5000\mathrm{J}/{\mathrm{m}}^2$ interacting with a large system of aluminum atoms are simulated. The response of Al to laser energy deposition is investigated within the finite-size laser spot. It is found that the shape of the plasma plume is dynamically changing during expansion. At several tens of picoseconds it can be characterized as a long hollow ellipsoid surrounded by Al atoms and NCs. The time evolution of NCs in the plume is investigated. The collisions between single Al atoms and generated NCs and fragmentation of large NCs determine the fractions of different-size NCs in the plume. The MD-MSM simulations show that laser fluence greatly affects the size distribution of NCs, their polar angles, and magnitude and direction vectors of NC velocities. These results and predictions are consistent, in many aspects, with the experimental data and previous MD simulations.

In this study, we examine the mechanisms leading to ${}^{29}\mathrm{Si}$ incorporation into highly enriched ${}^{28}\mathrm{Si}$ films deposited by hyperthermal ion beams at elevated temperatures in the dilute presence of natural abundance silane $\left({\mathrm{SiH}}_4\right)$ gas. Enriched ${}^{28}\mathrm{Si}$ is a critical material in the development of quantum information devices because ${}^{28}\mathrm{Si}$ is free of nuclear spins that cause decoherence in a quantum system. We deposit epitaxial thin films of ${}^{28}\mathrm{Si}$ enriched in situ beyond 99.999 98% ${}^{28}\mathrm{Si}$ onto Si(100) using an ion-beam deposition system and seek to develop the ability to systematically vary the enrichment and measure the impact on quantum coherence. We use secondary ion mass spectrometry to measure the residual ${}^{29}\mathrm{Si}$ isotope fraction in enriched samples deposited from $\approx 250{}^{\circ }\mathrm{C}$ up to 800 °C. The ${}^{29}\mathrm{Si}$ isotope fraction is found to increase from $<1\times {10}^{-6}$ at the lower temperatures, up to $>4\times {10}^{-6}$ at around 800 °C. From these data, we estimate the temperature dependence of the incorporation fraction $s$ of ${\mathrm{SiH}}_4$, which increases sharply from about $2.9\times {10}^{-4}$ at 500 °C to $2.3\times {10}^{-2}$ at 800 °C. We determine an activation energy of 1.00(8) eV associated with the abrupt increase in incorporation and conclude that below 500 °C, a temperature-independent mechanism such as activation from ion collisions with adsorbed ${\mathrm{SiH}}_4$ molecules is the primary incorporation mechanism. Direct incorporation from the adsorbed state is found to be minimal.

This computational paper examines the effect of local atomic environments on the electron charge density in $\delta \text{−}\mathrm{U}{\mathrm{O}}_3,\gamma \text{−}\mathrm{U}{\mathrm{O}}_3$, and $\mathrm{L}{\mathrm{a}}_6\mathrm{U}{\mathrm{O}}_{12}$. In particular, this paper reveals differences in the uranium local atomic environments in these model oxide compounds. To examine the differences in a quantitative way, atoms-in-molecule (AIM) and Bader analysis methods were used to interrogate the electron charge density. The electron charge-density distribution in each compound was obtained using density functional theory. The AIM-Bader analyses provided estimates for the so-called Bader charges on individual lattice atoms, as well as the locations of the bond critical points (BCPs) between bonding atoms and the charge densities at the BCPs. Calculation results revealed a quantitative inverse correlation between the charge density at the BCP and the U-O bond length. In addition, this inverse correlation was found to be surprisingly similar to a well-established crystal chemical relationship between bond strength and bond length.

Drying of particle suspensions is an ubiquitous phenomenon with many natural and practical applications. In particular, in unidirectional drying, the evaporation of the solvent induces flows which accumulate particles at the liquid/air interface. The progressive buildup of a dense region of particles can be used, in particular, in the processing of advanced materials and architectures while the development of heterogeneities and defects in such systems is critical to their function. A lot of attention has thus been paid to correlating the flow and particle dynamics to the ordering of particles. However, dynamic observation at the particle scale and its correlation with local particle ordering are still missing. Here we show by measuring the particle velocities with high frame rate laser scanning confocal microscopy that the ordering of weakly Brownian particles during unidirectional drying in a Hele-Shaw cell opened on one side depends on the velocity of particles that impinge at the pinned liquid/solid interface. Under the ambient and experimental conditions presented in the following, the particle velocities accumulate in two branches. A higher degree of ordering is found for the branch of faster particle velocity which we explain by an increase in the pressure drop which drags the particles into a denser packing as the flow velocity increases. This counterintuitive behavior is the opposite to what is found with Brownian particles, which can reorganize by Brownian motion into denser packing during drying, as long as the flow velocity is not too high. These results show that different kinetic conditions can be used to obtain dense, defect-free regions of particles after drying. In particular, it suggests that rapid, directional drying could be used to control the crystallinity of particle deposits.

We create a two-dimensional electron system (2DES) at the interface between EuO, a ferromagnetic insulator, and ${\mathrm{SrTiO}}_3$, a transparent nonmagnetic insulator considered the bedrock of oxide-based electronics. This is achieved by a controlled in situ redox reaction between pure metallic Eu deposited at room temperature on the surface of ${\mathrm{SrTiO}}_3$—an innovative bottom-up approach that can be easily generalized to other functional oxides and scaled to applications. Additionally, we find that the resulting EuO capping layer can be tuned from paramagnetic to ferromagnetic, depending on the layer thickness. These results demonstrate that the simple, novel technique of creating 2DESs in oxides by deposition of elementary reducing agents [T. C. Rödel et al., Adv. Mater. 28, 1976 (2016)] can be extended to simultaneously produce an active, e.g., magnetic, capping layer enabling the realization and control of additional functionalities in such oxide-based 2DESs.

We show the existence of Fano resonances in the context of near-field radiative heat transfer, which enables the strong suppression and enhancement of the spectral heat flux at specific wavelengths. We make use of the plasmon-phonon coupling in a symmetric nanocavity composed of a polaritonic material coated with a metallic layer. Each side of the cavity is kept at different temperatures. The hybridization of the plasmonic and phononic modes sustained by the multilayer structure is determined by the matching of their polarization. This leads to the opening of a thermal band gap, where heat transfer in the coupled system is inhibited for all wave vectors and for wavelengths at which the individual constituents are thermally transmissive.

Pyrite $\mathrm{Fe}{\mathrm{S}}_2$ has long been recognized as a high potential photovoltaic material, due to its exceptionally high optical absorption, low toxicity, and the abundance and low cost of its constituents. Despite the suitable band gap (0.95 eV), minority carrier diffusion length (100–1000 nm), and short-circuit current density, power conversion efficiencies in $\mathrm{Fe}{\mathrm{S}}_2$-based solar cells have never exceeded 3% however, primarily due to low open circuit voltages $\left(V_{\mathrm{oc}}\sim 0.1\mathrm{V}\right)$. Surface phenomena have been implicated as the root cause of this low $V_{\mathrm{oc}}$, recent experiments on $n$-type crystals providing evidence for surface conduction, including $p$-type surface inversion. Here we report a detailed study of electronic transport in a large set (∼120 samples) of thoroughly characterized vapor-transport-grown $n\text{−}\mathrm{Fe}{\mathrm{S}}_2$ single crystals, with both as-prepared and mechanically polished surfaces. Abundant evidence for surface conduction is obtained from the temperature dependence of the resistance and its anisotropy, the thickness dependence of the resistivity, the sensitivity to surface preparation, and the nature of an observed surface insulator-metal transition. While the bulk transport is relatively reproducible, as-grown crystals display striking diversity in surface behavior, which is suppressed by polishing. Via detailed analyses, we demonstrate that the $\mathrm{Fe}{\mathrm{S}}_2$ surface conduction is truly two dimensional, that it can influence in-plane transport even at room temperature, and that a p-type surface inversion layer can be unambiguously established, with no possibility of artifacts from hopping conduction. A nonlinear Hall effect is also observed, allowing us to constrain a two-channel conduction model we show capable of describing all field- and temperature-dependent transport data. Combined with simple arguments, these results place limits on the thickness of the surface conduction layer, which lie below ∼3 nm. Finally, in some crystals, for unknown reasons, the as-grown surface is definitively $n$ type. These results highlight that while surface conduction is clearly important in pyrite $\mathrm{Fe}{\mathrm{S}}_2$, and is gradually yielding to understanding, additional work is clearly warranted to further understand and control it.

Damping in magnetization dynamics characterizes the dissipation of magnetic energy and is essential for improving the performance of spintronics-based devices. While the damping of ferromagnets has been well studied and can be artificially controlled in practice, the damping parameters of antiferromagnetic materials are nevertheless little known for their physical mechanisms or numerical values. Here we calculate the damping parameters in antiferromagnetic dynamics using the generalized scattering theory of magnetization dissipation combined with the first-principles transport computation. For the PtMn, IrMn, PdMn, and FeMn metallic antiferromagnets, the damping coefficient associated with the motion of magnetization $\left({\alpha }_m\right)$ is 1–3 orders of magnitude larger than the other damping coefficient associated with the variation of the Néel order $\left({\alpha }_n\right)$, in sharp contrast to the assumptions made in the literature.

Boron-containing nitride alloys such as BAlN are being explored as novel members of the nitride family of materials for electronic and optoelectronic applications. Using hybrid density functional calculations we determine structural properties, band gaps, and band-gap bowing of random wurtzite BAlN alloys. The fundamental band gap of BN is indirect while AlN is a direct-band-gap semiconductor. This leads to a crossover in the band gap from direct to indirect at $28\%$ boron. We find that the direct band gap experiences extreme bowing, leading to a fundamental gap that changes very little up to 17% boron incorporation.

The structural and electronic properties of naphthalene adsorbed on graphene are studied from first principles using the van der Waals density functional method. It is shown that naphthalene molecules are stabilized by forming a superstructure with the periodicity of $(2\sqrt{3}\times 2\sqrt{3})$ and a tilted molecular adsorption geometry on graphene, in good agreement with the scanning tunneling microscopy (STM) experiments on highly oriented pyrolytic graphite. Our results predict that image potential states (IPSs) are induced by intermolecular interaction on the naphthalene overlayer, hybridizing with the IPSs derived from graphene. The resultant hybrid IPSs are characterized by anisotropic effective mass reflecting the molecular structure of naphthalene. By means of STM simulations, we reveal that one of the hybrid IPSs manifests itself as an oval protrusion distinguishable from naphthalene molecular orbitals, which identifies the origin of an experimental STM image previously attributed to the lowest unoccupied molecular orbital of naphthalene.