Contact resistance of semiconducting transition metal dichalcogenides has been shown to decrease in lateral heterojunctions formed with their metallic phases but its origins remain elusive. Here we combine first-principles and quantum transport calculations to rationalize the contact resistance of these structures in terms of phase, composition (${\mathrm{WTe}}_2, {\mathrm{MoTe}}_2, {\mathrm{WSe}}_2$, and ${\mathrm{MoSe}}_2$), and length of the channel. We find that charge injection in metallic 1T'-${\mathrm{WTe}}_2$/1T'-${\mathrm{MoTe}}_2$ junctions is nearly ideal as electrode Bloch states remain delocalized through the channel. Mixtures of 1T' selenides and tellurides depart from this scenario due to the momentum mismatch between states in the lead and channel. In semiconducting channels, the large Schottky barriers degrade the electrical contacts. Around band edges, contact resistance values are about an order of magnitude lower than those obtained experimentally suggesting that doping and phase-engineering could be employed to overcome this issue. We predict that transport regime in these junctions shifts from thermionic emission to tunneling for channels shorter than 3 nm at room temperature. We also discuss the presence of states at the metal/semiconductor interfaces. By underpinning mechanisms to control the contact resistance in heterogeneous two-dimensional materials, this work proves valuable towards the development of devices suitable for optoelectronics and phase-change materials applications.

The field of magnon spintronics is experiencing an increasing interest in the development of solutions for spin-wave-based data transport and processing technologies that are complementary or alternative to modern complementary metal-oxide semiconductor architectures. Nanometer-thin yttrium iron garnet (YIG) films have been the gold standard for insulator-based spintronics to date, but a potential process technology that can deliver perfect, homogeneous large-diameter films is still lacking. We report that liquid phase epitaxy (LPE) enables the deposition of nanometer-thin YIG films with low ferromagnetic resonance losses and consistently high magnetic quality down to a thickness of 20 nm. The obtained epitaxial films are characterized by an ideal stoichiometry and perfect film lattices, which show neither significant compositional strain nor geometric mosaicity, but sharp interfaces. Their magnetostatic and dynamic behavior is similar to that of single crystalline bulk YIG. We found that the in-plane Gilbert damping coefficient ${\alpha }_{\left|\right|}$ is independent of the film thickness and close to $1\times {10}^{-4}$, while the out-of-plane coefficient ${\alpha }_{\perp }$ increases with decreasing thickness. Together with an inhomogeneous peak-to-peak linewidth broadening of $\mathrm{\Delta }{H}_{0\left|\right|}=0.4\mathrm{G}$, these values are among the lowest ever reported for YIG films with a thickness smaller than 40 nm. These results suggest that nanometer-thin LPE films can be used to fabricate nano- and microscaled circuits with the required quality for magnonic devices. The LPE technique is easily scalable to YIG sample diameters of several inches.

We investigate phosphorus (P) donors in P-doped diamond epitaxial films by means of electron paramagnetic resonance spectroscopy (EPR). In a diamond thin film on IIa diamond substrate, we could observe an EPR signal of the NIMS1 center, which has been assigned to a neutral P donor with a $D_{2d}$ symmetry. In contrast, we could not observe this center in another free-standing diamond film with the same P concentration $(\left[\mathrm{P}\right]\sim 1\times {10}^{17}\mathrm{c}{\mathrm{m}}^{-3})$. This striking contrast can be reasonably accounted for in a model where the P donor in diamond has a nearly degenerate ground state due to $t_2$ and $e$, in contrast to P donors in group-IV semiconductors like Si and Ge, where $a_1$ singlet ground state is present. We observed a strong uniaxial strain only in the former diamond thin film, which was confirmed by Raman microscopy and preferential orientations of NIMS1 EPR centers. This strain splits the nearly degenerate ground state of the P donor, resulting in the observation of the NIMS1 center. We point out that the electronic system of the substitutional P donor in diamond is similar to that of interstitial Li donor in Si.

Semilocal density functional approximations are widely used. None of them can capture the long-range van der Waals (vdW) attraction between separated subsystems, but they differ remarkably in the extent to which they capture intermediate-range vdW effects responsible for equilibrium bonds between neighboring small closed-shell subsystems. The local density approximation (LDA) often overestimates this effect, while the Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA) underestimates it. The strongly constrained and appropriately normed (SCAN) meta-GGA often estimates it well. All of these semilocal functionals require an additive nonlocal correction such as the revised Vydrov-Van Voorhis 2010 (rVV10) to capture the long-range part. This work reports adsorption energies and the corresponding geometry of aromatic thiophene (${\mathrm{C}}_4{\mathrm{H}}_4\mathrm{S}$) bound to transition-metal surfaces. The adsorption process requires a genuine interplay of covalent and weak binding and requires a simultaneously accurate description of surface and adsorption energies with the correct prediction of the adsorption site. All these quantities must come from well-balanced short- and long-range correlation effects for a universally applicable method for weak interactions with chemical accuracy. Our methods indicate that the correct interplay is not present in any combination of recent meta-GGA's and rVV10. The simple short-range damping of the vdW correction scheme that is practically successful in the combination of GGA's and vdW approximations is less transferable in SCAN+rVV10 or in the revised version, revSCAN+rVV10. In addition, we present accurate random-phase-approximation-quality adsorption energies from a model based on the one of Zaremba and Kohn.

We present the results of a photon energy and polarization dependent angle-resolved photoemission spectroscopy (ARPES) study on high quality, epitaxial ${\mathrm{SrNbO}}_3$ thin films prepared in situ by pulsed laser deposition (PLD). We show that the Fermi surface is composed of three bands mainly due to ${\mathrm{t}}_{2g}$ orbitals of Nb $4d$, in analogy with the $3d$-based perovskite systems. The bulk band dispersion for the conduction and valence states obtained by density functional theory (DFT) is generally consistent with the ARPES data. The small discrepancy in the bandwidth close to the Fermi level seems to result from the interplay of correlation effects and the presence of vacancies. The ARPES results are complemented by soft x-ray photoemission spectroscopy measurements in order to provide indications on the chemical states and the stoichiometry of the material.

Understanding the microscopic kinetics of quasicrystal formation via nucleation and growth is crucial. Here, we report unusual pathways to nucleation and growth of dodecagonal quasicrystals via a phase field crystal model incorporating a two-length-scale potential. Under certain thermodynamic parameters, both quasicrystal growths via heterogeneous and homogeneous nucleation may be associated with a multistep behavior and the transient appearance of triangular and intermediate phases, different from classical nucleation pathways. The metastable intermediate phase spontaneously occurs to bridge the triangular phase and quasicrystal nuclei of different orientations to reduce the total free energy of the system. Decomposition of an undercooled fluid phase into quasicrystal phase shows a multistep pathway wherein the triangular phase and the intermediate phase may occur faster than the quasicrystal phase, when the growth rate of one length-scale ordering is significantly different from the other and the subsequent competing and coupling of both length scales are involved. The calculated structure factor, radial distribution function, and the aperiodic tiling structure of the intermediate phase explain why it appears during the quasicrystal formation.

Alkali-based semiconducting photocathodes, due to their high quantum efficiency (QE) in the visible light spectrum, are promising candidates to replace traditional metal photocathodes for high-brightness beam applications such as x-ray free electron lasers (XFELs). However, they suffer from rapid degradation which significantly limits their operational lifetimes. Coating them with two-dimensional (2D) materials has been proposed as a possible avenue to prevent the degradation. Ideally, the 2D coating layer should not increase the work function of semiconducting photocathodes, thus maintaining the high QE of semiconducting photocathodes in visible light. Herein, we report a computational screening of over 4000 2D materials in the Computational 2D Materials Database (C2DB). The assessment of their potential to be good coating layers is based on their effects to the surface electronic properties. We discover several candidate materials that are even capable of decreasing the work function of semiconducting photocathodes. Some of the experimentally synthesized 2D materials, such as hydrogenated graphene (graphane) and several hydroxylated transition metal carbides/nitrides (MXenes), are particularly appealing for this application.

Magnetite (${\mathrm{Fe}}_3{\mathrm{O}}_4$) thin-films are among the most stimulating systems for electronic applications, in particular given that their electric and magnetic properties can be controlled by substrate strain. Here we investigate the electronic structure of a 38 nm ${\mathrm{Fe}}_3{\mathrm{O}}_4/{\mathrm{SrTiO}}_3\left(001\right)$ thin-film by a unique set of x-ray magnetic linear dichroism (XMLD) measurements. We show that it is only possible to uncover the orbital character of the Fe sites in ${\mathrm{Fe}}_3{\mathrm{O}}_4$ by a systematic analysis of the XMLD angular distribution. The local symmetry of the ${\mathrm{Fe}}^{2+}$ B site in the thin-film is found to be trigonally distorted. Our results highlight that the combination of state-of-the-art XMLD measurements and theoretical simulations is indispensable for investigating the electronic structure of oxide thin-films and heterostructures.

Slow nonradiative recombination is a key factor in achieving high open-circuit voltages or high luminescence yields in any optoelectronic material. Whether a defect is contributing substantially to nonradiative recombination is often estimated by defect statistics based on the model by Shockley, Read, and Hall. However, defect statistics are agnostic to the origin of the capture coefficients and therefore conclude that essentially every defect between the two quasi-Fermi levels is equally likely to be a recombination-active defect. Here, we combine Shockley-Read-Hall statistics with microscopic models for defect-assisted recombination to study how the microscopic properties of a material affect how recombination active a defect is depending on its energy level. We then use material parameters representative of typical photovoltaic absorber materials ($\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3\mathrm{Pb}{\mathrm{I}}_3$, Si, and GaAs) to illustrate the relevance, but also the limitations of our model.

Knowledge of the nitrogen-vacancy (NV) center formation kinetics in diamond is critical to engineering sensors and quantum information devices based on this defect. Here we utilize the longitudinal tracking of single NV centers to elucidate NV defect kinetics during high-temperature annealing from 800–1100 ${}^{\circ }\mathrm{C}$ in high-purity chemical-vapor-deposition diamond. We observe three phenomena which can coexist: NV formation, NV quenching, and NV orientation changes. Of relevance to NV-based applications, a 6- to 24-fold enhancement in the NV density, in the absence of sample irradiation, is observed by annealing at 980 ${}^{\circ }\mathrm{C}$, and NV orientation changes are observed at 1050 ${}^{\circ }\mathrm{C}$. With respect to the fundamental understanding of defect kinetics in ultrapure diamond, our results indicate a significant vacancy source can be activated for NV creation between 950–980 ${}^{\circ }\mathrm{C}$ and suggest that native hydrogen from ${\mathrm{NVH}}_y$ complexes plays a dominant role in NV quenching, supported by recent ab initio calculations. Finally, the direct observation of orientation changes allows us to estimate an NV diffusion barrier of 4.7 $\pm$ 0.9 eV.

By means of density functional theory, we perform a focused study of both body-centered-cubic (bcc) and face-centered-cubic (fcc) Fe-Ni random solid solutions, represented by special quasirandom structures. The whole concentration range and various magnetic configurations are considered. Excellent agreement on the concentration dependence of magnetization is found between our results and experimental data, except in the Invar region. Some locally antiferromagnetic fcc structures are proposed to approach experimental values of magnetization. Vibrational entropies of ordered and disordered systems are calculated for various concentrations, showing an overall good agreement with available experimental data. The vibrational entropy systematically contributes to stabilize disordered rather than ordered structures and is not negligible compared to the configurational entropy. Free energy of mixing is estimated by including the vibrational and ideal configurational entropies. From them, low- and intermediate-temperature Fe-Ni phase diagrams are constructed, showing a better agreement with experimental data than the one from a recent thermodynamic assessment for some phase boundaries below 700 K. The determined order-disorder transition temperatures for the $\mathrm{L}{1}_0$ and $\mathrm{L}{1}_2$ phases are in good agreement with the experimental values, suggesting an important contribution of vibrational entropy.

Recently, a transition of deformation mechanism from localized dislocation slip to delocalized plasticity via an anomalous tensile detwinning mechanism has been discovered in bitwinned metallic nanowires (NWs) with a single twin boundary (TB) running parallel to the NW length. However, experiments showed that the anomalous tensile detwinning in most of bitwinned NWs does not propagate through the whole NW, which limits the NWs failure strain when compared to the twinning-induced superplasticity in single-crystalline NWs. An elusive but fundamentally important question is that what factors might affect the propagation of tensile detwinning in such bitwinned NWs. In addition, can this tensile detwinning mechanism be applied to other types of twinned NWs? Here, based on in situ transmission electron microscopy testing and molecular dynamics simulations, a competition between shear localization and tensile detwinning is identified. By dividing the tensile detwinning mechanism into two steps and investigating each step separately, it is found that the quality of a single-crystalline embryo formed during step one determines the succeeding detwinning propagation (step two) and the final plastic strain. Furthermore, this anomalous tensile detwinning mechanism is extended to other metallic NWs with multiple TBs running parallel to the length direction, such as asymmetric pentatwinned NWs and NWs with multiple parallel TBs. This work highlights the important role of detwinning in large plasticity in metallic NWs with different twin structures.

We present a model for the thermally activated propagation of cracks in elastic matrices. The propagation is considered as a subcritical phenomenon, the kinetics of which is described by an Arrhenius law. In this law, we take the thermal evolution of the crack front into account, assuming that a portion of the released mechanical energy is transformed into heat in a zone surrounding the tip. We show that such a model leads to a two-phase crack propagation: the first phase at low velocity in which the temperature elevation is of little effect and the propagation is mainly governed by the mechanical load and by the toughness of the medium, and the second phase in which the crack is thermally weakened and propagates at greater velocity. We illustrate, with numerical simulations of mode I cracks propagating in thin disordered media, how such a dual behavior can explain the usual stick-slip in brittle fracturing. In addition, we predict the existence of a limiting ambient temperature above which the weakened phase ceases to exist and we propose this critical phenomenon as a novel explanation for the brittle-ductile transition of solids.

Understanding the crystal-field splitting and orbital polarization in noncentrosymmetric systems such as ferroelectric materials is fundamentally important. In this study, taking ${\mathrm{BaTiO}}_3$ as a representative material, we investigate titanium crystal-field splitting and orbital polarization in noncentrosymmetric ${\mathrm{TiO}}_6$ octahedra with resonant x-ray linear dichroism at the Ti $L_{2,3}$ edge. The high-quality ${\mathrm{BaTiO}}_3$ thin films were deposited on ${\mathrm{DyScO}}_3$ (110) single crystal substrates in a layer-by-layer way by pulsed laser deposition. The reflection high-energy electron diffraction and element specific x-ray absorption spectroscopy were performed to characterize the structural and electronic properties of the films. In sharp contrast to conventional crystal-field splitting and orbital configuration ($d_{xz}/d_{yz}<d_{xy}<d_{3z^2-r^2}<d_{x^2-y^2}$ or $d_{xy}<d_{xz}/d_{yz}<d_{x^2-y^2}<d_{3z^2-r^2}$) expected from compressive or tensile epitaxial strain, respectively, it is revealed that $d_{xz}$, $d_{yz}$, and $d_{xy}$ orbitals are nearly degenerate, whereas $d_{3z^2-r^2}$ and $d_{x^2-y^2}$ orbitals are split with an energy gap $\sim 100$ meV in the epitaxial ${\mathrm{BaTiO}}_3$ films. We find that the unexpected degenerate orbitals $d_{xz}/d_{yz}/d_{xy}$ result from the competition between the orbital splitting induced by epitaxial strain and that induced by polar distortions of ${\mathrm{BaTiO}}_3$ films. Our results provide a route to manipulate orbital degree of freedom by switching electric polarization in ferroelectric materials.

The electronic structure and half-metallic gap of ${\mathrm{Co}}_2\mathrm{MnSi}$ in the presence of crystallographic defects, partial Fe substitution for Mn, and thermal spin fluctuations are studied using the coherent potential approximation and the disordered local moment method. In the presence of 5% Co or Mn vacancies the Fermi level shifts down to the minority-spin valence-band maximum. In contrast to NiMnSb, both types of Mn antisite defects in ${\mathrm{Co}}_2\mathrm{MnSi}$ are strongly exchange coupled to the host magnetization, and thermal spin fluctuations do not strongly affect the half-metallic gap. Partial substitution of Mn by Fe results in considerable changes in the Bloch spectral function near the Fermi level, which strongly deviates from the rigid-band picture. In particular, a light band with the Fe character crosses the Fermi level at about 50% concentration. At room temperature, Fe substitution of up to 30% slightly increases the spin polarization at the Fermi level.

Amorphous solids display a ductile to brittle transition as the kinetic stability of the quiescent glass is increased, which leads to a material failure controlled by the sudden emergence of a macroscopic shear band in quasistatic protocols. We numerically study how finite deformation rates influence ductile and brittle yielding behaviors using model glasses in two and three spatial dimensions. We find that a finite shear rate systematically enhances the stress overshoot of poorly annealed systems, without necessarily producing shear bands. For well-annealed systems, the nonequilibrium discontinuous yielding transition is smeared out by finite shear rates and is accompanied by the emergence of multiple shear bands, as reported in metallic glass experiments. We show that the typical size of the bands and the distance between them increases algebraically with the inverse shear rate. We provide a dynamic scaling argument for the corresponding lengthscale, based on the competition between the deformation rate and the propagation time of the shear bands.

Ferromagnets crystallizing in structures described by chiral cubic space groups, including compounds with the B20 or $\beta$-Mn structures, are known to host long-period chiral spin textures such as skyrmion lattices. These spin textures are stabilized by a competition between ferromagnetic exchange and antisymmetric Dzyaloshinskii-Moriya (DM) exchange, which is enhanced by the spin-orbit coupling associated with high-atomic-number elements. For real-world application, it is desirable to find materials that can host compact skyrmion lattices at readily accessible temperatures. Here, we report on the crystal chemistry and magnetic phase diagrams of a family of compounds with the filled $\beta$-Mn structure, ${\mathrm{FePd}}_{1-x}{\mathrm{Pt}}_x{\mathrm{Mo}}_3\mathrm{N}$, with $T_C$ ranging from 175 to 240 K. DC and AC magnetization measurements reveal magnetic phase diagrams consistent with the formation of a skyrmion pocket just below $T_C$. The magnitudes of ferromagnetic and DM exchanges are determined from the phase diagrams, demonstrating that the introduction of increasing amounts of Pt can be used to increase spin-orbit coupling in order to control the expected skyrmion lattice parameter between 140 and 65 nm while simultaneously increasing $T_C$.

We study geometries and electronic properties of unstrained and strained zigzag $\mathrm{Mo}X{}_2$ ($X$ = O, S, and Se) nanotubes and $\mathrm{Mo}XY$ ($X,Y$ = O, S, and Se) nanotubes in the framework of density functional theory. Using the local density approximation, we obtain fully relaxed geometries of (10,0) and (14,0) nanotubes for all $\mathrm{Mo}X{}_2$, and (7,0) and (28,0) nanotubes for selected $\mathrm{Mo}X{}_2$. From the detailed analysis of the optimized geometries it is revealed that, although in the planar case these transition metal dichalcogenides (TMDs) normally take so-called H structures, zigzag nanotubes turn into geometries between H-like and T-like nanotubes having different height for inner and outer chalcogen site. Electronic property investigations reveal that TMD nanotubes without oxygen atoms are found to have narrower gaps than TMD nanotubes with oxygen atoms. These narrow direct gaps of TMD nanotubes should be attractive properties for infrared optoelectronic devices. On the other hand, energy-gap values of the TMD nanotubes with oxygen atoms are found to depend rather strongly on the strain. Therefore, they should be potential future device materials for mechanical sensors. We also find a global strong correlation between the fundamental-gap values and some combined structural parameters in strained $\mathrm{Mo}X{}_2$ ($X$ = S, Se) nanotubes and planar $\mathrm{Mo}X{}_2$ ($X$ = S, Se), which will be useful to predict gap values of other strained and unstrained ($n,0$) nanotubes.