Linear elasticity theory predicts a divergent strain field at the dislocation core, resulting from the continuum approximation breaking down at the atomic scale. We introduce a minimum model that includes elastic interactions and discrete lattice periodicity, and derive a set of equations that treat the core of an edge dislocation from a solely geometric perspective. We find an analytical formula for the displacement field of a straight dislocation of arbitrary mixed character, and we predict that the dislocation core widens as the screw character becomes more dominant. This finding is in qualitative and quantitative agreement with atomistic simulations of mixed dislocations in tungsten. The theory is based on a continuum form of the multistring Frenkel-Kontorova model, which is a nearest-neighbor model for atomic bonding that also takes into account the discreteness of the crystal lattice. Thus, we circumvent the need to use adjustable parameters in the treatment of a dislocation core.

We investigate the atomic energy mapping inferred by machine-learning potentials, in particular neural network potentials. We first show that the transferable atomic energy can be defined within the density functional theory, which means that the core of machine-learning potentials is to deduce a reference atomic-energy function from the given set of total energies. By utilizing invariant points in the feature space at which the atomic energy has a fixed reference value, we examine the atomic energy mapping of neural network potentials. Examples on Si consistently support that NNPs are capable of learning correct atomic energies. However, we also find that the neural network potential is vulnerable to ‘ad hoc’ mapping in which the total energy appears to be trained accurately while the atomic energy mapping is incorrect in spite of its capability. We show that the energy mapping can be improved by choosing the training set carefully and monitoring the atomic energy at the invariant points during the training procedure. The energy mapping in multicomponent systems is also discussed.

We study the effect of hydrostatic pressure on the electrical transport, magnetic, and structural properties of $\mathrm{MnB}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_4$ by measuring its resistivity, Hall effect, and x-ray diffraction under pressures up to 12.8 GPa supplemented by the first-principles calculations. At ambient pressure, $\mathrm{MnB}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_4$ shows a metallic conducting behavior with a cusplike anomaly at around $T_{\mathrm{N}}\approx 24\mathrm{K}$, where it undergoes a long-range antiferromagnetic (AF) transition. With increasing pressure, $T_{\mathrm{N}}$ determined from the resistivity anomaly first increases slightly with a maximum at around 2 GPa and then decreases until vanishing completely at about 7 GPa. Intriguingly, its resistivity is enhanced gradually by pressure and even evolves from metallic to semimetal or semiconductinglike behavior as $T_{\mathrm{N}}$ is suppressed. However, the density of the $n$-type charge carrier that remains dominant under pressure increases with pressure. In addition, the interlayer AF coupling seems to be strengthened under compression, since the critical field $H_{\mathrm{c}1}$ for the spin-flop transition to the canted AF state is found to increase with pressure. No structural transition was evidenced up to 12.8 GPa, but some lattice softening was observed at about 2 GPa, signaling the occurrence of an electronic transition or crossover from a localized to itinerant state. We have rationalized these experimental findings by considering the pressure-induced enhancement of antiferromagnetic/ferromagnetic competition and partial delocalization of $\mathrm{Mn}\text{−}3d$ electrons, which not only destroys long-range AF order but also promotes charge-carrier localization through enhanced spin fluctuations and/or the formation of a hybridization gap at high pressure.

Heavily doped semiconductors are by far the most studied class of materials for thermoelectric applications in the past several decades. They have Seebeck coefficient values which are 2–3 orders of magnitude higher than metals, making them attractive for thermoelectric applications. Conventional wisdom suggests that the optimal band gap of a semiconductor to achieve the largest power factor should be between 6 and 10 $k_{B}T$, yet the highest power factor materials known up to now do not have a band gap. In this paper, we show that semimetals with very small or no band gap, but high asymmetry between their conduction and valence bands, can also have large Seebeck coefficient values on the order of 200 $\mu \text{V/K}$, which is near the optimum value for thermoelectric applications. We have studied the band structure of a class of 18 semimetals using first principles calculations and calculated their Seebeck coefficient using the linearized Boltzmann equation within the constant relaxation time approximation. We conclude that despite the absence of the band gap, semimetals with band asymmetry can have good thermoelectric performance. We analyze the metrics often used to describe thermoelectric properties of materials and show that the ratio of electron and hole mass of density of states is a key parameter resulting in high Seebeck coefficient values in semimetals. This work is therefore suggesting some high thermoelectric performance materials could be of semimetallic nature.

We present a modeling and experimental investigation about the physics of nanofoam growth in pulsed laser deposition (PLD) experiments. Thanks to their unique features, ultralow density materials—known as nanofoams—are attracting a growing interest for many cutting-edge applications. PLD has emerged as one of the most promising and versatile techniques for the synthesis of nanofoam; however, the lack of a satisfactory comprehension of the nanofoam growth process hinders the unleashing of their full potential as innovative materials. In this work we propose a snowfall-like model that describes the nanofoam growth as the coalescence of micrometric-sized fractal aggregates of nanoparticles, where the aggregates are formed through an in-flight process whose timescale is determined by the shot-to-shot time interval. We exploit these insights to demonstrate an approach to control the nanofoam properties down to the nanoscale. These results, along with their interest from a fundamental point of view, open perspectives in the field of low density nanostructured materials.

Screened range-separated hybrid (SRSH) functionals are of potential interest as a computationally inexpensive yet accurate alternative approach for studying (opto)electronic properties in the solid state. At present, SRSH functionals are typically tuned to reproduce with high accuracy the properties of either bulk or low-dimensional structures, rendering such functionals not only material specific, but also structure specific. The transferability of tuned SRSH functionals between bulk and low-dimensional phases has not been examined systematically and is desirable for straightforward and consistent modeling of size effects in nanostructures. We present a simple yet effective approach for simultaneous tuning of the fraction of short-range exact exchange and the range-separation parameter, which delivers accurate and transferable SRSH functionals for two-dimensional and bulk phases of two prototypical layered materials, molybdenum disulfide (${\mathrm{MoS}}_2$) and hexagonal boron nitride (h-BN). The ground-state SRSH band structures, resulting from minimal fitting of the SRSH to a single quasiparticle energy, are found to be in excellent agreement with GW calculations over the entire Brillouin zone. Excited-state properties are predicted using time-dependent density functional theory calculations, based on the SRSH (TD-SRSH) functional. Calculated absorption spectra are found to be in excellent agreement with GW and Bethe-Salpeter equation (BSE) calculations for ${\mathrm{MoS}}_2$, but less so for h-BN; the failure of the TD-SRSH for the latter material is examined and a BSE approach based on the SRSH ground state is shown to restore accuracy.

The exchange bias effect is an essential component of magnetic memory and spintronic devices. Whereas recent research has shown that anisotropies perpendicular to the device plane provide superior stability against thermal noise, it has proven remarkably difficult to realize perpendicular exchange bias in thin-film structures. Here we demonstrate a strong perpendicular exchange bias effect in heterostructures of the quasi-two-dimensional canted antiferromagnet ${\mathrm{La}}_2{\mathrm{CuO}}_4$ and ferromagnetic $(\mathrm{La},\mathrm{Sr}){\mathrm{MnO}}_3$ synthesized by ozone-assisted molecular beam epitaxy. The magnitude of this effect can be controlled via the doping level of the cuprate layers. Canted antiferromagnetism of layered oxides is thus a new and potentially powerful source of uniaxial anisotropy in magnetic devices.

For nanostructured interfaces between open-shell molecules and metal surfaces that involve charge transfer upon adsorption, the investigation of molecular magnetic properties is an interesting yet difficult task, because in principle different magnetic configurations with distinct properties can be found. Here, we study the magnetic properties of CuPc-Ag and CoPc-Ag interfaces, which constitute interesting test cases because charge is transferred to the initially open-shell Pc molecules upon adsorption. Using hybrid density functional theory, we examine the stability of the various magnetic configurations occurring at these nanoscale interfaces, as well as for the corresponding gas-phase anions, and compare our findings to those of previous experimental studies. For CuPc-Ag, we identify a high-spin triplet configuration as the most likely configuration at the interface, whereas for CoPc-Ag a quenching of the total magnetic moment is found. Interestingly, such quenching is consistent with two distinctly different interfacial electronic configurations. These important differences in the magnetic properties of CuPc and CoPc on Ag are rationalized by variations in the interaction of their central metal atoms with the substrate. Our work facilitates a deeper understanding of the magnetic configuration and interlinked electronic-structure properties of molecule-metal interfaces. Furthermore, it highlights the necessity of an appropriate choice of methodology in tandem with a detailed evaluation of the different emerging magnetic properties.

In this work, the findings of an in situ fluorescence spectroscopy and microscopy investigation of the growth of $\alpha$-sexithiophene ($\alpha \text{−}6\mathrm{T}$) thin films on muscovite mica are presented. The sensitive dependence of the fluorescence emission on the molecular arrangement is exploited to reveal the details of the $\alpha \text{−}6\mathrm{T}$ thin film growth from the initial stage. A Stranski-Krastanov growth mode with a two-monolayer-thick wetting layer is observed. The molecules within the wetting layer lie flat on the mica surface and their molecular arrangement evolves as a function of the surface coverage. After the completion of the wetting layer, needlelike three-dimensional islands consisting of flat-lying molecules and flat islands containing nearly-upright-standing molecules grow competitively. Exposing the $\alpha \text{−}6\mathrm{T}$ thin films to air leads to dewetting and this process is found to be partially reversible, i.e., a large fraction of the wetting layer recovers gradually after the sample is transferred back into vacuum. This dewetting (rewetting) process is attributed to the surface modification due to the adsorption (desorption) of water molecules.

Single crystals of the layered, van der Waals compound $\mathrm{VB}{\mathrm{r}}_3$ were synthesized by a vapor transport technique, and characterized by single-crystal x-ray diffraction, temperature- and field-dependent magnetization, and temperature-dependent specific-heat measurements. $\mathrm{VB}{\mathrm{r}}_3$ has a honeycomb-based $\mathrm{Bi}{\mathrm{I}}_3$-type structure with space group $R\text{−}3$ (no. 148) at 100 K, analogous to the structure of other honeycomb trihalides. Structural stacking faults were observed as evidenced by the streaking of reflections along $c*$ in the diffraction data. $\mathrm{VB}{\mathrm{r}}_3$ goes through a structural phase transition at 90.4 K and a subsequent antiferromagnetic phase transition at 26.5 K, below which the magnetic moments are primarily aligned along the $c$ axis.

We calculate the lattice thermal conductivity ($\kappa$) for cubic (zinc-blende) and hexagonal (wurtzite) phases for eight semiconductors using ab initio calculations and solving the phonon Boltzmann transport equation, explaining the different behavior of the ratio ${\kappa }_{\mathrm{hex}}/{\kappa }_{\mathrm{cub}}$ between the two phases. We show that this behavior depends on the relative importance of two antagonistic factors: anharmonicity, which we find to be always higher in the cubic phase, and the accessible phase space, which is higher for the less symmetric hexagonal phase. Based on that, we develop a method that predicts the most conducting phase—cubic or hexagonal—where other more heuristic approaches fail. We also present results for nanowires made of the same materials, showing the possibility to tune ${\kappa }_{\mathrm{hex}}/{\kappa }_{\mathrm{cub}}$ over a wide range by modifying their diameter, thus making them attractive materials for complex phononic and thermoelectric applications and systems.

Zr-doped indium oxide ($\mathrm{I}{\mathrm{n}}_2{\mathrm{O}}_3:\mathrm{Zr}$) has been shown to satisfy the requirements of low resistance, wide band gap, and high infrared transmittance for application as a front contact in broadband solar cells. However, the reduction of indium usage in front of transparent electrodes is still an unsatisfied requirement. With the goal of reducing the amount of indium while leveraging its properties, in this work, $\mathrm{I}{\mathrm{n}}_2{\mathrm{O}}_3:\mathrm{Zr}$ films with reduced thickness compared to those standardly used in solar cells are studied. 100 to 15-nm-thick films were sputtered at room temperature and annealed in distinct atmospheres to study the links between thickness, microstructure, and optoelectronic properties. As-deposited films exhibit an amorphous microstructure embedding bixbyite $\mathrm{I}{\mathrm{n}}_2{\mathrm{O}}_3$ nanocrystals. Annealing in neutral (${\mathrm{N}}_2$) or reducing atmosphere (${\mathrm{H}}_2$) allows a slight growth of these crystallites but the layers remain mostly amorphous. Whereas annealing in air results in polycrystalline films with an average grain lateral size ranging from 350 to 500 nm. The large crystalline grains formed during air annealing lead to increased electron mobility for all thickness: up to $100\mathrm{c}{\mathrm{m}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ for 100-nm-thick films and up to $50\mathrm{c}{\mathrm{m}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ for 15-nm-thick films, which is remarkable for such thin polycrystalline films. Conversely, ${\mathrm{H}}_2$ annealing ensures high free-carrier densities ($>1\times {10}^{20}\mathrm{c}{\mathrm{m}}^{-3}$) but not high mobilities, still achieving conductivities between 1000 and $2000\mathrm{S}\mathrm{c}{\mathrm{m}}^{-1}$, with the films less than 50-nm-thick keeping high broadband transmittance. The possibility of thinning down $\mathrm{I}{\mathrm{n}}_2{\mathrm{O}}_3:\mathrm{Zr}$ to a few tens of nanometers while keeping both high lateral conductivity and good transparency makes this material a promising candidate to reduce the amount of indium in optoelectronic applications, such as flexible touch screens and solar cells.

Two-dimensional (2D) materials are strongly affected by the dielectric environment, including substrates, making it an important factor in designing materials for quantum and electronic technologies. Yet, first-principles evaluations of charged defect energetics in 2D materials typically do not include substrates due to the high computational cost. We present a general continuum model approach to incorporate substrate effects directly in density-functional theory calculations of charged defects in the 2D material alone. We show that this technique accurately predicts charge defect energies compared to much more expensive explicit substrate calculations, but with the computational expediency of calculating defects in freestanding 2D materials. Using this technique, we rapidly predict the substantial modification of charge transition levels of two defects in ${\mathrm{MoS}}_2$ and ten defects promising for quantum technologies in hBN, due to ${\mathrm{SiO}}_2$ and diamond substrates. This establishes a foundation for high-throughput computational screening of new quantum defects in 2D materials that critically accounts for substrate effects.

Magnetism is an important property of materials that plays a key role in many different applications. In the present paper, we use a combination of computational tools: a machine-learning technique for screening of stable candidates, an evolutionary algorithm for crystal structure determination, and first-principles calculations for characterization of electronic and magnetic properties to make predictions on magnetic double perovskites, which are yet to be synthesized. Out of 412 scanned candidates of $A_2{BB}^{\prime }{O}_6$ composition with $3d$ and $4d$ or $5d$ transition metals at B and $B^{\prime }$ sites, we found 33 compounds to form stable double-perovskite structures, 25 of which were further considered for characterization of their structure and properties. Our exercise predicted 21 double perovskites of varying magnetic and electronic properties, ranging from ferromagnetic half metals to ferri- and antiferromagnetic insulators to ferromagnetic metals and a rare example of antiferromagnetic metals. Our computational study is expected to help in discovering new magnetic double perovskites.

Understanding the structure of ferromagnetic/molecular interfaces obtained by the deposition of metal on a molecular layer is a key parameter to master for the development of molecular spintronic devices. Here we studied by means of grazing incidence x-ray diffraction and x-ray reflectivity a model $\mathrm{Co}/{\mathrm{C}}_{60}/\mathrm{Co}\left(0001\right)$ system. Prior to Co deposition, the grown ${\mathrm{C}}_{60}$ layer is crystallized in a face centered cubic lattice with the presence of two twins. The Co overlayer, presenting a polycrystalline hexagonal close packed structure with stacking faults, induces an amorphization of the topmost ${\mathrm{C}}_{60}$ layers. Most importantly, we observe a deep diffusion of Co atoms in the octahedral sites of the crystallized ${\mathrm{C}}_{60}$ film which could strongly affect the spin transport properties of the molecular layer and thus the magnetoresistance of the final device.

We investigated the influence of oxygen overstoichiometry on apical oxygen disorder and magnetic correlations in $\mathrm{N}{\mathrm{d}}_2\mathrm{Ni}{\mathrm{O}}_{4+\delta }$ ($\delta \sim 0.11$) in the temperature range of 2–300 K by means of synchrotron x-ray powder diffraction, neutron single-crystal and powder diffraction studies, combined with macroscopic magnetic measurements. In the investigated temperature range, the compound crystallizes in a tetragonal commensurate structure with a $P{4}_2/ncm$ space group with excess oxygen atoms occupying the $4b$ (¾ ¼ ¼) interstitial sites, coordinated by four apical oxygen atoms. Large and anisotropic thermal displacement parameters are found for equatorial and apical oxygen atoms, which are strongly reduced on an absolute scale compared to the $\mathrm{N}{\mathrm{d}}_2\mathrm{Ni}{\mathrm{O}}_{4.23}$ phase. Maximum entropy analysis of the neutron single-crystal diffraction data uncovered anharmonic contributions to the displacement parameters of the apical oxygen atoms, toward the nearest vacant $4b$ interstitial site, related to the phonon-assisted oxygen diffusion mechanism. Macroscopic magnetization measurements and neutron powder diffraction studies reveal long-range antiferromagnetic ordering of the Ni sublattice at $T_{\mathrm{N}}\sim 53\mathrm{K}$ with a weak ferromagnetic component along the $c$ axis, while the long-range magnetic ordering of the Nd sublattice occurs below 10 K. Temperature-dependent neutron diffraction patterns show the appearance of a commensurate magnetic order at $T_{\mathrm{N}}$ with the propagation vector $\mathit{k}=\left(100\right)$ and the emergence of an additional incommensurate phase below 30 K, while both phases coexist at 2 K. The commensurate magnetic structure is best described by the $P{4}_2/n{c}^{\prime }{m}^{\prime }$ Shubnikov space group. Refined magnetic moments of the Ni and Nd sites at 2 K are 1.144(76) and $1.632\left(52\right){\mu }_B$, respectively. A possible origin of the incommensurate phase is discussed and a tentative magnetic phase diagram is proposed.

ZrSiS has been identified as a topological material made from nontoxic and earth-abundant elements. Together with its extremely large and uniquely angle-dependent magnetoresistance, this makes it an interesting material for applications. We study the origin of the so-called butterfly magnetoresistance by performing magnetotransport measurements on four different devices made from exfoliated crystalline flakes. We identify near-perfect electron-hole compensation, tuned by the Zeeman effect, as the source of the butterfly magnetoresistance. Furthermore, the observed Shubnikov-de Haas oscillations are carefully analyzed using the Lifshitz-Kosevich equation to determine their Berry phase and thus their topological properties. Although the link between the butterfly magnetoresistance and the Berry phase remains uncertain, the topological nature of ZrSiS is confirmed.