We report the synthesis and characterization of a rare-earth dichalcogenide ${\mathrm{EuTe}}_2$. An antiferromagnetic transition was found at $T_{\mathrm{N}}=11\mathrm{K}$. The antiferromagnetic order can be tuned by an applied magnetic field to access a first-order spin-flop transition and a spin-flip transition. These transitions are associated with a large negative magnetoresistance with a change of magnitude of resistivity over five orders. Magnetic susceptibility, heat capacity, and Hall coefficient measurements reveal that the moments of ${\mathrm{Eu}}^{2+}$ align along the $c$ axis and holes are the majority carriers. Furthermore, density functional theory calculations demonstrate that the carriers near the Fermi surface mainly originate from the Te $5p$ orbitals and the magnetism is dominated by localized electrons from the Eu $4f$ orbitals. Our results suggest that ${\mathrm{EuTe}}_2$ is an $A$-type antiferromagnetic material with large negative magnetoresistance.

In a combined experimental and theoretical study, we investigate the influence of the material source arrangement in a molecular beam epitaxy system on the growth of nanowire (NW) core-shell structures. In particular, we study the shell growth of GaN around GaN template NWs under the boundary condition that Ga and N do not impinge on a given sidewall facet at the same time. Our experiments with different V/III ratios and substrate temperatures show that obtaining shells with homogeneous thickness along the whole NW length is not straightforward. Analyzing in detail the shell morphology with and without substrate rotation, we find that the different azimuthal angles of the sources have a major impact on the Ga adatom kinetics and the final shell morphology. In general, growth is possible only under directly impinging N, and Ga adatoms diffuse between the sidewalls and the top facet as well as the substrate, but not between adjacent sidewall facets. On the basis of these experimental results, we develop a diffusion model which takes into account different NW facets and the substrate. The model allows to describe well the experimental shell profiles and predicts that homogeneous shell growth can be achieved if the Ga and N sources are arranged next to each other or for very high rotation speeds. Moreover, the modeling reveals that the growth on a given side facet can be categorized within one rotation in four different phases: the Ga wetting phase, the metal-rich growth phase, the N-rich growth phase, and the dissociation phase. The striking difference to growth processes on planar samples is that, in our case, diffusion takes place between different regions, i.e. the sidewall vs the top facet and substrate, out of which on one N impinges not continuously, resulting in complex gradients in chemical potential that are modulated in time by substrate rotation. The comprehensiveness of our model provides a deep understanding of diffusion processes and the resulting adatom concentration, and could be applied to other three-dimensional structures and material systems.

The structural and magnetic properties of the face-centered-cubic double perovskite $\mathrm{B}{\mathrm{a}}_2\mathrm{MnW}{\mathrm{O}}_6$ were investigated using neutron powder diffraction, DC magnetometry, muon spin relaxation, and inelastic neutron scattering. $\mathrm{B}{\mathrm{a}}_2\mathrm{MnW}{\mathrm{O}}_6$ undergoes type II long-range antiferromagnetic ordering at a Néel temperature of 8(1) K with a frustration index, $f\approx 8$. Inelastic neutron scattering was used to identify the magnetic coupling constants $J_1$ and $J_2$, which were found to equal −0.080 and −0.076 meV, respectively. This indicates that both of the magnetic coupling constants are antiferromagnetic with similar magnitudes, which is in contrast to other known $3d$ metal double perovskites of the form $\mathrm{B}{\mathrm{a}}_{2}M\mathrm{W}{\mathrm{O}}_6$. Above the Néel temperature, muon spin-relaxation measurements and inelastic neutron-scattering techniques identify a short-range correlated magnetic state that is similar to that observed in the archetypical face-centered-cubic lattice antiferromagnet MnO.

La-doped $\mathrm{BaSn}{\mathrm{O}}_3$ films were grown on $\mathrm{DySc}{\mathrm{O}}_3$ substrates by molecular beam epitaxy using La, Ba, and $\mathrm{Sn}{\mathrm{O}}_2$ sources with and without additional oxidant, respectively. Lattice parameter measurements as a function of growth conditions show a reduced lattice parameter that is likely due to substitution of $\mathrm{S}{\mathrm{n}}^{2+}$ on the Ba site. The propensity for the antisite defect is discussed as being due to the combination of oxygen-poor, Sn-rich conditions and the dual valence of Sn. Although electron mobilities are highest for films with reduced lattice parameters, antisite defects will pose an upper limit to thin film mobility. Less Sn-rich conditions lead to the formation of another defect that causes a lattice expansion. The combined effects of these defects on the lattice parameter can compensate each other and cause the appearance of a stoichiometric lattice parameter for nonstoichiometric films with poor electrical behavior.

The development of nanoplasmonic devices is hampered by the lack of metallic systems that are at the same time chemically stable, mechanically robust, easy to grow, and have tailorable optical properties. Gathering together all these characteristics in a single material is very unusual: in particular the controlled modification of the plasmonic properties of metallic bulk and films still represents an unsolved task. Here, we present a first principles study of the interplay between composition and plasmonic properties in substoichiometric ${\mathrm{TiN}}_x$ crystals, which combine superior structural properties and tunable optical response. We investigate the stability of defective ${\mathrm{TiN}}_x$ over a large range of vacancy concentrations, and we describe the microscopic character and the plasmonic properties of these systems. Our results indicate a clear trend in the modification of the plasmonic activity of ${\mathrm{TiN}}_x$ as a function of nitrogen concentration, and show that for this class of materials the fine manipulation of the internal stoichiometry is a unique tool to tailor the plasmonic properties.

Dynamic networks formed by physically crosslinked, entangled polymers have emerged as self-healing, stretchable, and functional materials. Entangled associative gels with remarkable toughness and extensibility have been produced by several distinct chemical approaches, suggesting that these enhanced mechanical properties result from molecular-scale topology. Previously, artificially engineered associative proteins were designed to provide an ideal model system to investigate the role of entanglement on gel mechanics via well-defined entangled or unentangled physical gels. Herein, uniaxial strain-induced structural changes in these model gels were observed using in situ small-angle x-ray scattering (SAXS) and in situ polarized optical microscopy (POM) up to 2000% engineering strain. Anisotropic optical responses to uniaxial strain at the nano-, micro-, and macroscales suggest that stress dissipation mechanisms enable high extensibility and toughness. Nano- and microscopic anisotropy observed by SAXS indicate stretching and alignment of flexible polymer strands along the straining axis, and nonmonotonic macroscopic anisotropy observed by POM suggests relaxation within the hydrogel due to rearrangement of associative network junctions. Unentangled hydrogels exhibit low toughness and a strain-rate-dependent transition from ductile to brittle tensile behavior, which is typical for associative polymer solutions. These findings indicate that topological entanglements and the freedom of individual chains to align at the nanoscale due to junction relaxation are both critical to achieving high toughness and elongation in entangled physical gels.

Light-matter interactions are key in providing fundamental information about materials. Even in the linear-response regime, the spectroscopic response of a material encodes in it many properties of the ground state as well as of its excitations. This knowledge has been critical in our understanding of novel quantum materials, and the further improvement and extensions of linear spectroscopy will continue to be key in the exploration of novel states of matter. We report the development of broadband circular polarization spectroscopy in the terahertz range of the electromagnetic spectrum. We take advantage of a recent design of a broadband quarter wave plate, based on the Fresnel rhomb concept, and use it in conjunction with a polarization modulation technique to provide direct information of the response of a material to circularly polarized THz radiation. As an example of this technique we study the cyclotron resonance of a 2D electron gas from a AlGaAs-GaAs quantum well. We demonstrate the unique advantages that this technique will bring in the study of novel quantum materials.

We report the synthesis of a new layered iridium oxyfluoride, $\mathrm{S}{\mathrm{r}}_2\mathrm{Ir}{\left(\mathrm{O},\mathrm{F}\right)}_{6-\delta }$, using a topochemical fluorination method. The $c$ axis is elongated compared to that in the mother compound $\mathrm{S}{\mathrm{r}}_2\mathrm{Ir}{\mathrm{O}}_4$, because the fluorine layer is inserted into the rock-salt layer while tetragonal symmetry is preserved. Resistivity measurements for the compound show nonmetallic behavior similar to that of $\mathrm{S}{\mathrm{r}}_2\mathrm{Ir}{\mathrm{O}}_4$. In magnetization measurements, suppression of antiferromagnetic order and paramagnetic behavior was observed after topochemical fluorination. Meanwhile, muon spin relaxation (μSR) experiments suggest the development of antiferromagnetic spin correlation between Ir spins. Furthermore, fluorine content was estimated at 1.5 the chemical composition of the fluorinated product being $\mathrm{S}{\mathrm{r}}_2\mathrm{Ir}{\mathrm{O}}_x{\mathrm{F}}_{1.5}$ with $x=3.25$, assuming $\mathrm{I}{\mathrm{r}}^{4+}$. Suppression of magnetic order in $\mathrm{S}{\mathrm{r}}_2\mathrm{Ir}{\left(\mathrm{O},\mathrm{F}\right)}_{6-\delta }$ is attributed to antiferromagnetic instability between $\mathrm{Ir}{\mathrm{O}}_2$ layers by elongation of the $c$ axis due to fluorine intercalation.

A phase-field crystal model that spontaneously undergoes displacive phase transitions is introduced by explicitly studying a two-component two-dimensional square crystal reminiscent of a perovskite. When the intercomponent free energy is a simple polynomial, the crystal undergoes displacive transitions in the $\langle 10\rangle$ and $\langle 11\rangle$ directions. When the interaction is a correlation function, displacements in any direction can occur. This displacive phase-field crystal (DPFC) model maps to Landau-Ginzburg-Devonshire (LGD) theories for ferroelectrics, and the DPFC and LGD models are compared in terms of phase transitions and domain walls. Dynamical simulations of quadrijunctions were also performed and found stable spiraling quadrijunctions and unstable nonspiraling quadrijunctions. Last, domain coarsening across a small-angle grain boundary demonstrates multiple forms of non-mean-curvature-driven growth.

The formation of nanopores under electron irradiation is an ideal process to quantify chemical bonds in two-dimensional materials. Nowadays, high-resolution transmission electron microscopy (HRTEM) allows investigating such nucleation and growth phenomena with incomparable spatial and temporal resolution. Moreover, theoretical calculations are usually exploited to confirm characteristic features of these atomic-scale observations. Nevertheless, the full understanding of the ejection mechanisms of atoms requires a detailed investigation of the interplay between the very dynamic edge structure of expanding nanopores and the displacement energy of edge atoms $\left(E_D\right)$. Here, the dynamics of triangular nanopores in hexagonal boron nitride (h-BN) under various electron dose rates was followed by aberration-corrected HRTEM with high temporal resolution to provide new in situ insights into their growth processes. We reveal that the ejection of atomic pairs is an elemental mechanism that considerably speeds up the expansion of nanopores. Atomic-scale calculations were exploited to quantify the structure-dependent $E_D$ of all the ejected edge atoms. They revealed strong variations of this threshold energy during the growth processes. This quantitative study reconciles theoretical and experimental measurements of the ejection rate of atoms in h-BN under electron irradiation, which is essential for nanopore engineering in this atomically thin membrane.

MnSi is a prototypical chiral cubic magnet, in which swirling magnetic objects referred to as magnetic skyrmions appear. It is expected that the thin film of MnSi may expand the range of the magnetic skyrmion lattice phase in the temperature–magnetic field diagram. On the other hand, the MnSi film on an achiral Si (111) substrate may suffer from chiral twin formation. We succeeded in visualizing the domain structure of the chirality and axis orientation of a MnSi thin film by using a combination of transmission electron microscopy and x-ray reflectivity measurement. Only two types of domains were found on a Si (111) substrate. The information is essential for the challenge of homochiral film fabrication.

At ambient pressure the spinel $\mathrm{Li}{\mathrm{V}}_2{\mathrm{O}}_4$ is a metal with a heavy fermion ground state. However, under applied pressure a transition to an insulating, nonmagnetic state occurs. Powder x-ray diffraction has been used to study structural changes associated with this transition, and reveals that the ambient-pressure $Fd\overline{3}m$ spinel structure distorts to a monoclinic cell of $C2/m,C2$, or $Cm$ symmetry above 11 GPa at low temperatures. The changes of structure and properties provide evidence for orbital molecule formation in the cubic phase at high pressures and low temperatures that leads to long-range orbital molecule order in the monoclinic phase.

The thermal conductivities (TCs) of the vast majority of amorphous polymers are in a very narrow range, 0.1–0.5 W ${\mathrm{m}}^{-1}$ ${\mathrm{K}}^{-1}$, although single polymer chains possess TCs of orders of magnitude higher. The chemical structure of polymer chains plays an important role in determining the TC of bulk polymers. We propose a thermal resistance network (TRN) model for the TC in amorphous polymers taking into account the chemical structure of molecular chains. Our model elucidates the physical origin of the low TC universally observed in amorphous polymers with various chemical structures. The empirical formulas of the pressure and temperature dependence of TC can be successfully reproduced not only in solid polymers but also in polymer melts. We further quantitatively explain the anisotropic TC in oriented polymers.

The wide measured range of thermal conductivities $\left(k\right)$ for monolayer ${\mathrm{MoS}}_2$ and the corresponding incongruent calculated values in the literature all suggest that extrinsic defect thermal resistance is significant and varied in synthesized samples of this material. Here we present defect-mediated thermal transport calculations of ${\mathrm{MoS}}_2$ using interatomic forces derived from density functional theory combined with Green's function methods to describe phonon-point-defect interactions and a Peierls-Boltzmann formalism for transport. Conductivity calculations for bulk and monolayer ${\mathrm{MoS}}_2$ using different density functional formalisms are compared. Nonperturbative first-principles methods are used to describe defect-mediated spectral functions, scattering rates, and phonon $k$, particularly from sulfur vacancies $\left(V_{\mathrm{S}}\right)$, and in the context of the plethora of measured and calculated literature values. We find that $k$ of monolayer ${\mathrm{MoS}}_2$ is sensitive to phonon-$V_{\mathrm{S}}$ scattering in the range of experimentally observed densities, and that first-principles $k$ calculations using these densities can explain the range of measured values found in the literature. Furthermore, measured $k$ values for bulk ${\mathrm{MoS}}_2$ are more consistent because $V_{\mathrm{S}}$ defects are not as prevalent.

We report the observation of two distinct large-scale defects in ${\left(\mathrm{Bi},\mathrm{Sb}\right)}_2\mathrm{T}{\mathrm{e}}_3$ topological insulator (TI) thin films grown by molecular beam epitaxy. Small-angle rotations are detected between quintuple layers of the TI film, extending throughout a grain and beyond, and nm-sized Te formations are discovered that extend along grain boundaries. Density functional theory calculations suggest that the rotational defects can affect the local band structure of the film while preserving spin-momentum locking in the Dirac bands, and that the Te nanostructures at grain boundaries can result in wider-band-gap regions between the grains.

To advance fundamental understanding and ultimate application of transition-metal dichalcogenide (TMD) monolayers, it is essential to develop capabilities for the synthesis of high-quality single-layer samples. Molecular beam epitaxy (MBE), a leading technique for the fabrication of the highest-quality epitaxial films of conventional semiconductors has, however, typically yielded only small grain sizes and suboptimal morphologies when applied to the van der Waals growth of monolayer TMDs. Here, we present a systematic study on the influence of adatom mobility, growth rate, and metal:chalcogen flux on the growth of ${\mathrm{NbSe}}_2$, ${\mathrm{VSe}}_2$, and ${\mathrm{TiSe}}_2$ using MBE. Through this, we identify the key drivers and influence of the adatom kinetics that control the epitaxial growth of TMDs, realizing four distinct morphologies of the as-grown compounds. We use this to determine optimized growth conditions for the fabrication of high-quality monolayers, ultimately realizing the largest grain sizes of monolayer TMDs that have been achieved to date via MBE growth.

According to experimental observations, the temperature dependence of self-diffusion coefficient in most body-centered cubic metals (bcc) exhibits non-Arrhenius behavior. The origin of this behavior is likely related to anharmonic vibrational effects at elevated temperatures. However, it is still debated whether anharmonicity affects more the formation or migration of monovacancies, which are known to govern the self-diffusion. In this extensive atomistic simulation study we investigated thermodynamic properties of monovacancies in bcc molybdenum, here taken as a representative model system, from zero temperature to the melting point. We combined first-principles calculations and classical simulations based on three widely used interatomic potentials for Mo. In our analysis we employ static and dynamic atomistic calculations as well as statistical sampling techniques and thermodynamic integration to achieve thorough information about temperature variations of vacancy formation and migration free energies and diffusivities. In addition, we carry out large-scale molecular dynamics simulations that enable direct observation of high-temperature self-diffusion at the atomic scale. By scrutinizing the results obtained by different models and methods, we conclude that the peculiar self-diffusion behavior is likely caused by strong temperature dependence of the vacancy formation energy.

Lattice defects play a key role in determining the properties of crystalline materials. Probing the three-dimensional (3D) lattice strains that govern their interactions remains a challenge. Bragg coherent diffraction imaging (BCDI) allows strain to be measured with nanoscale 3D resolution. However, it is currently limited to materials that form microcrystals. Here we introduce a technique that allows the manufacture of BCDI samples from bulk materials. Using tungsten as an example, we show that focused ion-beam machining can be used to extract, from macroscopic crystals, micron-sized BCDI samples containing specific preselected defects. To interpret the experimental data, we develop a displacement-gradient-based analysis for multireflection BCDI. This allows accurate recovery of the full lattice strain tensor from samples containing multiple dislocations. These capabilities open the door to BCDI as a microscopy tool for studying complex real-world materials.

We investigated Li-ion distribution profiles at the interfaces between $\gamma \text{−}{\mathrm{Li}}_3{\mathrm{PO}}_4$ and metal electrodes using first-principles and one-dimensional continuum-model calculations. Within the allowed range of the chemical potential of Li in $\gamma \text{−}{\mathrm{Li}}_3{\mathrm{PO}}_4$, $-6.11\mathrm{eV}\le {\mu }_{\mathrm{Li}}\le -2.59\mathrm{eV}$, which is estimated from the chemical potential diagram of Li, P, O-related chemical compounds, we predict upward band bending near Au(111) and Ni(111) interfaces. We found interstitial Li-ion accumulation for the larger ${\mu }_{\mathrm{Li}}$ values and its depth is ca. 3 Å for ${\mu }_{\mathrm{Li}}=-2.59\mathrm{eV}$. For the Li(100) interface with ${\mu }_{\mathrm{Li}}=-2.59\mathrm{eV}$, we predict slight upward band bending and the formation of interstitial Li-ions. However, the downward band bending occurs for ${\mu }_{\mathrm{Li}}=-6.11\mathrm{eV}$, where the accumulated charge carrier corresponds to the Li-ion vacancies. Finally, we suggest that the interstitial Li-ions accumulated within a few Å from the Au(111) interface and the Li-Au alloying play a central role in the switching of the novel memory device.