In the present work the longstanding issue of the structure and dynamics of smaller ions in oxides and its impact on the properties was investigated on 7% Li-doped $\mathrm{BaTi}{\mathrm{O}}_3$. The investigation combined several techniques, notably neutron powder diffraction (NPD), nuclear magnetic resonance (${}^7\mathrm{Li}$-NMR), electron paramagnetic resonance (EPR), electron microprobe, electric polarization (EP) measurement, and electronic structure calculations based on density-functional theory (DFT). Electron microprobe confirmed multiple phases, one containing incorporated Li in the $\mathrm{BaTi}{\mathrm{O}}_3$ host lattice and another glassy phase which breaks the host lattice due to excessive Li accumulation. While the average structure of Li in $\mathrm{BaTi}{\mathrm{O}}_3$ could not be determined by NPD, ${}^7\mathrm{Li}$-NMR revealed one broad “disordered” and multiple “ordered” peaks. Local structure models with different defect types involving ${\mathrm{Li}}^{+}$ were modeled and the corresponding chemical shifts $\left(\delta \right)$ were compared with experimental values. It is found that the closest defect model describing the ordered peaks, is with $\mathrm{T}{\mathrm{i}}^{4+}$ being replaced by four ${\mathrm{Li}}^{+}$ ions. The biexponential behavior of the spin-lattice relaxation of the ordered peaks each with a short and a long relaxation discloses the existence of paramagnetic ions. Finally, EPR revealed the existence of the paramagnetic ion $\mathrm{T}{\mathrm{i}}^{3+}$ as a charge-transfer defect. DFT calculations disclosed local antipolar displacements of Ti ions around both types of defect sites upon insertion of ${\mathrm{Li}}^{+}$. This is in accordance with the experimental observation of pinching effects of the EP in Li-doped $\mathrm{BaTi}{\mathrm{O}}_3$. These studies demonstrate the huge impact of the local structure of the doped smaller/lighter ions on the functional properties of oxides.

In order to answer the question of whether Schottky barriers on polar ZnO surfaces are different at Zn- and O-terminated surfaces, the interface formation of $n$-type ZnO and different high work function metals and metal oxides (Pt, ${\mathrm{PtO}}_x$, and ${\mathrm{RuO}}_2$) with Schottky barrier heights of up to 1.5 eV has been studied using photoelectron spectroscopy with in situ sample preparation. The experiments are designed to exclude the effects of substrate reduction and consequent Fermi level pinning by high concentrations of oxygen vacancies. Moreover, by including the Zn LMM Auger emission in the analysis, it is demonstrated that an accurate extraction of barrier heights needs to take into account that the screening of the photoelectron core hole can change in the course of contact formation. The polarization dependence of Schottky barriers, which is important for piezotronic applications, is in most cases dominated by the influence of defects. Reducing the influence of defects, up to $\sim 240$ meV higher Schottky barriers are revealed on oxygen-terminated surfaces. This is opposite to what has been reported in the literature but agrees with the dependence of barrier heights expected for an incomplete screening of the polarization of ZnO by the electrode as for ferroelectric materials.

In practical density-functional-theory-based simulations of defects in lead halide perovskites (LHPs), it is often assumed that spin-orbit coupling (SOC) and self-interaction corrections (SIC) mostly affect the electronic structure, whereas their effects on the structural properties are minor. Therefore, SOC and SIC are usually excluded during structural relaxation or molecular dynamics simulations. With such an assumption, previous studies predicted several unusual features of Br vacancies ($V_{\text{Br}}$) in $\mathrm{Cs}\mathrm{Pb}{\mathrm{Br}}_3$, including the formation of a Pb dimer, negative-$U$ character, and highly dynamic defect level. In this work, it is shown that SOC and SIC can play important roles in determining the equilibrium geometry and potential energy surface of $V_{\text{Br}}$ in $\mathrm{Cs}\mathrm{Pb}{\mathrm{Br}}_3$. Including SOC and SIC for structural relaxation results in a normal shallow level feature of $V_{\text{Br}}$, instead of the negative-$U$ character, and the Pb dimer becomes unstable in this case. Moreover, the highly dynamic defect level of $V_{\text{Br}}$ is absent when SOC is included for molecular dynamic trajectory generation. The contrasting results compared to previous studies are understood by the sensitive dependence of the occupation of the $V_{\text{Br}}$-associated states on the band edge positions of $\mathrm{Cs}\mathrm{Pb}{\mathrm{Br}}_3$, which is greatly affected by the strong SOC and SIC. These results thus highlight the importance of the correct prediction of the band edge positions for accurate modeling of defects in LHPs.

Manipulating spin-orbit coupling (SOC) is a key achievement for spin-orbitronic applications since SOC determines spin-diffusion lengths and spin-to-charge conversion efficiencies. While in most organic semiconductors SOC is inherently very weak due to being composed of primarily light elements, the SOC in conjugated polymer systems is also intimately tied to the polymer's structural conformation and thus may be manipulated. Here we report a modification of SOC in conjugated polymers by altering torsion angle between conjugated units. Spin-pumping experiments are performed on three poly(3-alkylthiophene) polymer films with decreasing conjugation lengths and concomitantly increasing torsion angle. The more twisted polymer exhibits a shorter spin-diffusion length and a giant spin-mixing conductance (up to ${10}^{21}{\mathrm{m}}^{-2}$ ), which is attributed to an increased SOC by structural conformation. This work offers a route for enhancing SOC and spin-injection efficiency in organic materials for spintronic applications.

This work presents a consistent formulation of the structural phase-field-crystal model of substitutional binary alloys that allows for the description of phases of unequal densities, a key feature in solidification. We further develop the dynamics of the model to be consistent with conserved Langevin dynamics in the true governing species densities. Additionally, this work expands on the ability to control pressure, so far only implemented in pure materials, to binary alloys by improving the control system that controls pressure from previous work. We study the equilibrium properties of the new model and demonstrate that control of pressure can drive various kinematic microscopic processes in materials such as grain boundary premelting, phase instability, and grain or interphase boundary motion.

Theoretical frameworks used to qualitatively and quantitatively describe nuclear dynamics in solids are often based on the harmonic approximation. However, this approximation is known to become inaccurate or to break down completely in many modern functional materials. Interestingly, there is no reliable measure to quantify anharmonicity so far. Thus, a systematic classification of materials in terms of anharmonicity and a benchmark of methodologies that may be appropriate for different strengths of anharmonicity is currently impossible. In this work, we derive and discuss a statistical measure that reliably classifies compounds across temperature regimes and material classes by their “degree of anharmonicity.” This enables us to distinguish “harmonic” materials, for which anharmonic effects constitute a small perturbation on top of the harmonic approximation, from strongly “anharmonic” materials, for which anharmonic effects become significant or even dominant and the treatment of anharmonicity in terms of perturbation theory is more than questionable. We show that the analysis of this measure in real and reciprocal space is able to shed light on the underlying microscopic mechanisms, even at conditions close to more complicated dynamical processes, e.g., phase transitions or defect formation. Eventually, we demonstrate that the developed approach is computationally efficient and enables rapid high-throughput searches by scanning over a set of several hundred binary solids. The results show that strong anharmonic effects beyond the perturbative limit are not only active in complex materials or close to phase transitions, but already at moderate temperatures in simple binary compounds.

Finding efficient ways of manipulating magnetic bits is one of the core goals in spintronic research. Electrically-generated spin-orbit torques (SOTs) are good candidates for this and the search for materials capable of generating highly-efficient SOTs has gained a lot of traction in recent years. While antiferromagnet/ferromagnet bilayer structures have been employed extensively for passive applications, e.g., by using exchange bias fields, their active properties are not yet widely employed. Here we show the presence of large interfacial SOTs in the bilayer of a ferromagnet and the two-dimensional layered antiferromagnetic insulator $\mathrm{Ni}\mathrm{P}{\mathrm{S}}_3$. We observe a large in-plane dampinglike interfacial torque, showing a torque conductivity of ${\sigma }_{\mathrm{DL}}\approx 1\times {10}^5\left(\frac{\hslash }{2e}\right)/\left(\mathrm{\Omega }\mathrm{m}\right)$ even at room temperature, comparable to the best devices reported in the literature for standard heavy-metal-based and topological insulators-based devices. Additionally, our devices also show an out-of-plane fieldlike torque arising from the $\mathrm{Ni}\mathrm{P}{\mathrm{S}}_3$/ferromagnet interface, further indicating the presence of an interfacial spin-orbit coupling in our structures. Temperature-dependent measurements reveal an increase of the SOTs with a decreasing temperature below the Néel temperature of $\mathrm{Ni}\mathrm{P}{\mathrm{S}}_3$ ($T_N\approx 170\mathrm{K}$), pointing to a possible effect of the magnetic ordering on our measured SOTs. Our findings show the potential of antiferromagnetic insulators and two-dimensional materials for future spintronic applications.

${\mathrm{Al}}_{0.35}{\mathrm{In}}_{0.65}\mathrm{As}$ is a direct semiconductor whose multivalley conduction-band structure has been proposed for use in so-called valley photovoltaics. In such hot-carrier solar cells, energetic (hot) photocarriers are stored in satellite valleys away from the Γ point, allowing them to be extracted prior to thermalization and to thereby increase power-conversion efficiency. While prior theoretical work has highlighted the potential of ${\mathrm{Al}}_{0.35}{\mathrm{In}}_{0.65}\mathrm{As}$—a widely used barrier material in electronic and optoelectronic devices, for use in valley photovoltaics—surprisingly little is known about its electrical properties, especially how these are impacted by the application of high fields. In this work, we therefore undertake a detailed characterization of the electrical properties of Te-doped ($n$-type) ${\mathrm{Al}}_{0.35}{\mathrm{In}}_{0.65}\mathrm{As}$, over wide ranges of temperature (3–400 K) and electric field (<50 kV/cm). Using pulsed measurements to suppress the influence of Joule heating, we reveal the presence of clear negative-differential conductance in the current-voltage characteristics of the films, suggestive of the intervalley transfer of hot electrons. This conclusion is supported by the results of ensemble Monte Carlo simulations of the hot-carrier action, which confirm the connection of the observed negative-differential conductance to hot-electron transfer from the conduction-band (Γ) minimum, to the side valleys at the L point. The quantitative features of the experimentally determined velocity-field curves are found to be in good agreement with the results of these calculations, providing further confidence in the role of the implied intervalley transfer mechanism. Overall, these results confirm the excellent potential of ${\mathrm{Al}}_{0.35}{\mathrm{In}}_{0.65}\mathrm{As}$ for use as the absorber material in hot-carrier solar cell technology.

Herein we report on a subset of the elastic constants, $c_{ij}$, and hardness values of ${\mathrm{V}}_2\mathrm{AlC}$ and $\mathrm{C}{\mathrm{r}}_2\mathrm{AlC}$ single crystals by means of microindentation/nanoindentation techniques. Density functional theory (DFT) is also used to calculate the elastic constants. The $c_{33}$ and $c_{11}$ values determined using a Berkovich tip and those calculated by DFT are all found to fall in the relatively narrow range of 320–350 GPa. These results confirm once again that many of MAX phases are relatively elastically isotropic, especially when compared to many other known layered solids such as graphite and mica. Similarly, the hardness values, obtained using Vickers, Berkovich, and a 5 µm spherical tip on the two orthogonal $\mathrm{C}{\mathrm{r}}_2\mathrm{AlC}$ surfaces, are quite comparable and average 9.0 ± 1 GPa. In all cases, the hardness values are at most 20% higher when the basal planes are loaded along [0001] than when they are loaded edge-on. The $\mathrm{C}{\mathrm{r}}_2\mathrm{AlC}$ surfaces record an average microyielding stress of 2.7 ± 0.3 GPa, while the less defective ${\mathrm{V}}_2\mathrm{AlC}$ crystals linearly sustain stresses of the order of 20 GPa, after which, in approximately 60% of the cases, pop-ins, some of which are substantial, are recorded. Postindentation scanning electron microscope micrographs clearly evidence the plastic anisotropy of these crystals. Large pileups near the indent edges and delamination cracks after loading along the [0001] and $\left[10\overline{1}0\right]$ directions, respectively, are consistent with deformation by ripplocations and ripplocation boundaries, as are the fully and spontaneously recoverable hysteresis stress-strain loops.

The deterministic production of single photons from two-dimensional materials promises to usher in a new generation of photonic quantum devices. In this work, we outline criteria by which single-photon emission can be realized in two-dimensional materials: spatial isolation, spectral filtering, and low excitation of quantum emitters. We explore how these criteria can be fulfilled in atomically thin transition metal dichalcogenides, where excitonic physics dictates the observed photoemission. In particular, we model the effect of defects and localized strain, in accordance with the most common experimental realizations, on the photon statistics of emitted light. Moreover, we demonstrate that an optical cavity has a negative impact on the photon statistics, suppressing the single-photon character of the emission by diminishing the effect of spectral filtering on the emitted light. Our work provides a theoretical framework revealing criteria necessary to facilitate single-photon emission in two-dimensional materials and thus can guide future experimental studies in this field.

Metal monochalcogenides $\left(MX\right)$ have recently been rediscovered as two-dimensional materials with electronic properties highly dependent on the number of layers. Although some intriguing properties appear in the few-layer regime, the carrier mobility of $MX$ compounds increases with the number of layers, motivating the interest in multilayered heterostructures or bulk materials. By means of angle-resolved photoemission spectroscopy (ARPES) measurements and density functional theory calculations, we compare the electronic band structure of bulk $\epsilon \text{−}\mathrm{GaSe}$ and $\epsilon$-InSe semiconductors. We focus our attention on the top valence band of the two compounds along main symmetry directions, discussing the effect of spin-orbit coupling and contributions from post-transition-metal (Ga or In) and Se atoms. Our results show that the top valence band at $\mathrm{\Gamma }$ point is dominated by Se $p_z$ states, while the main effect of Ga or In appears more deeply in binding energy, at the Brillouin zone corners, and in the conduction band. These findings explain also the experimental observation of a hole effective mass rather insensitive to the post-transition metal. Finally, by means of spin-resolved ARPES and surface band structure calculations we describe Rashba-Bychkov spin splitting of surface states in $\epsilon \text{−}\mathrm{InSe}$.

Structural and phonon properties, formation and cohesive energies, and electronic structures of ${\mathrm{II}}_A$-IV-${\mathrm{N}}_2$ (where ${\mathrm{II}}_{\mathrm{A}}=\mathrm{Be}$ or Mg; $\mathrm{IV}=\mathrm{Si}$, Ge, or Sn) monolayers with a graphenelike planar structure are systematically studied. Stability and property evolution with the variation of constituent group ${\mathrm{II}}_A$ and group IV elements are revealed. Dynamical and elastic stability of ${\mathrm{II}}_A$-IV-${\mathrm{N}}_2$ monolayers is justified by phonon and elasticity calculations, respectively. Their wide band gaps ranging from 3.32 to 5.61 eV are predicted by the $GW$ method on top of density functional calculations. The fat-band analysis and charge-density calculation of selected eigenstates unveil the effect of chemical environment and elemental substitution on electronic states near the band-gap region. The quasi-free-electron state and the parabolic dispersion of the lowest conduction band are advantageous for electron transport in electronic applications. A close examination of the band alignment and structural similarity between monolayers of ${\mathrm{II}}_A$-IV-${\mathrm{N}}_2$ compounds and III nitrides indicates that it is possible to form type-I or type-II heterostructures and alloy systems composed of monolayers of ${\mathrm{II}}_A$-IV-${\mathrm{N}}_2$ compounds and III nitrides. The findings in this work will promote research aiming at the synthesis, characterization, and application of novel two-dimensional materials, alloys, and heterostructures.

Despite intense efforts on all known quasi-two-dimensional superconductors, the origin and exact boundary of the electronic orderings, particularly charge density waves and superconductivity, are still attractive problems with several open questions. Here, in order to reveal how the superconducting gap evolves, we report on high quality complementary measurements of magneto-optical imaging, specific heat, magnetic susceptibility, resistivity measurements, Andreev spectroscopy, and London penetration depth ${\lambda }_{ab}\left(T\right)$ measurements supplemented with theoretical calculations for $2\mathrm{H}\text{−}\mathrm{Nb}{\mathrm{Se}}_2$ and $2\mathrm{H}\text{−}\mathrm{Nb}{\mathrm{S}}_2$ single crystals. The temperature dependence of ${\lambda }_{ab}\left(T\right)$ calculated from the lower critical field and Andreev spectroscopy can be well described by using a two-band model with $s$-wave-like gaps. The effect of pressure on the superconducting gap of both systems illustrates that both bands are practically affected. Upon compression, the Fermi surfaces do not change significantly, and the nesting remains almost unaffected compared to that at ambient condition. However, a strong bending in the upper critical fields ($H_{\mathrm{c}2}$) curves is obtained under pressure and support the presence of a strong Pauli paramagnetic effect. In $\mathrm{Nb}{\mathrm{Se}}_2$, using a two-band model with $s$-wave-like gaps, the temperature dependence $H_{\mathrm{c}2}\left(T\right)$ can be properly described. In contrast to that, the behavior of $H_{\mathrm{c}2}$ for $\mathrm{Nb}{\mathrm{S}}_2$ is ruled by the spin paramagnetic effect. The estimated values of the penetration depth at $T=0\mathrm{K}$ confirm that $\mathrm{Nb}{\mathrm{Se}}_2$ and $\mathrm{Nb}{\mathrm{S}}_2$ superconductors depart from a Uemura-style relationship between $T_{\mathrm{c}}$ with ${\mathit{\lambda }}_{ab}^{-2}\left(T\right)$, the in-plane superconducting penetration depth.

The sensitivity of materials properties, particularly those of perovskite oxides, to epitaxial strain has been exploited to great advantage to create materials with new or enhanced properties. Although it has certainly been recognized that mismatch in the thermal expansion coefficients of the bulk and substrate material will contribute to the misfit strain, the significance of this contribution for ferroelectric perovskite thin films has not been systematically explored. We use first-principles density functional theory and the example of ferroelectric ${\mathrm{PbTiO}}_3$ thin films on various substrates to show that ignoring the thermal expansion of the substrate (that is, assuming that the in-plane lattice parameter of the film remains roughly constant as a function of temperature) results in ferroelectric transition temperatures and structural trends that are completely qualitatively different from calculations in which thermal expansion mismatch is properly taken into account. Our work suggests that the concept of a misfit strain defined as a single number is particularly ill defined for ${\mathrm{PbTiO}}_3$ and invites further study of the interplay between thermal expansion mismatch and structural and functional properties in other thin-film materials.

The reorientational dynamics of ammonium borohydride (${\mathrm{NH}}_4{\mathrm{BH}}_4$) was studied using quasielastic neutron scattering in the temperature interval from 10 to 240 K, which covers both the dynamically ordered and disordered polymorphs of ${\mathrm{NH}}_4{\mathrm{BH}}_4$. In the low-temperature $(<50\mathrm{K})$ ordered polymorph of ${\mathrm{NH}}_4{\mathrm{BH}}_4$, analysis of the quasielastic neutron scattering data reveals that no reorientational dynamics is present within the probed timescale region of 0.1 to 100 ps. In the high-temperature $(>50\mathrm{K})$ disordered polymorph, the analysis establishes the onset of ${\mathrm{NH}}_4^{+}$ and ${\mathrm{BH}}_4^{-}$ dynamics at around 50 and 125 K, respectively. The relaxation time at 150 K for ${\mathrm{NH}}_4^{+}$ is approximately 1 ps, while around 100 ps for ${\mathrm{BH}}_4^{-}$. The ${\mathrm{NH}}_4^{+}$ dynamics at temperatures below 125 K is associated with preferential tetrahedral tumbling motions, where each of the hydrogen atoms in the ${\mathrm{NH}}_4^{+}$ tetrahedron can visit any of the four hydrogen sites, however, reorientations around a specific axis are more frequently occurring ($C_2$ or $C_3$). At higher temperatures, the analysis does not exclude a possible evolution of the ${\mathrm{NH}}_4^{+}$ dynamics from tetrahedral tumbling to either cubic tumbling, where the hydrogen atoms can visit any of the eight positions corresponding to the corners of a cube, or isotropic rotational diffusion, where the hydrogen atoms can visit any location on the surface of a sphere. The ${\mathrm{BH}}_4^{-}$ dynamics can be described as cubic tumbling. The difference in reorientational dynamics between the two ions is related to the difference of the local environment where the dynamically much slower ${\mathrm{BH}}_4^{-}$ anion imposes a noncubic environment on the ${\mathrm{NH}}_4^{+}$ cation.

The mechanical behavior of $\langle 111\rangle$ ultrathin gold nanowires (NWs) and their dependence on the correlated parameters of size (i.e., diameter $\sim 1–1.6$ nm), morphology (rectangular and hexagonal prism), and ultrahigh density twin boundaries was investigated using density functional theory calculations. The parameters of size, morphology, and the presence of twins all significantly influence the mechanical behavior of Au NWs, and their effects are interdependent. Importantly, an ultrahigh density of twins in NWs enhances their strength in both morphologies, and across all sizes studied. Our calculations reveal a remarkable range of Young's modulus (60–158 GPa) and ultimate tensile strengths (3.7–14.0 GPa), suggesting that the scatter observed in experimental research involving ultrathin gold nanowires is a feature of intrinsic complexity. In particular, a “wrinkling” of the atomic planes along the wire axis due to a combination of anisotropic surface stresses and wire core response, is demonstrated to be largely suppressed by the presence of high-density twins. The high tensile strengths of the studied nanowires with twins is attributed to a combination of the suppression of this anisotropic phenomena and slip plane disruption. This work demonstrates that precise control of these parameters is required during synthesis to achieve target mechanical behaviour in industrial device applications. Moreover, the work demonstrates that this system has the potential to be tuned for access to a vast range of materials design space.

We report an optimized chemical vapor transport method that enables the growth of FeP single crystals up to 500 mg in mass and $80{\mathrm{mm}}^3$ in volume. The high quality of the crystals obtained by this method was confirmed by means of energy-dispersive x-ray spectroscopy, high-resolution transmission electron microscopy, low-temperature single-crystal x-ray diffraction, and neutron diffraction experiments. We investigated the transport and magnetic properties of the single crystals, and we calculated the electronic structure of FeP. We show both theoretically and experimentally that the ground state of FeP is metallic. The examination of the magnetic data reveals antiferromagnetic order below $T_{\mathrm{N}}=119\mathrm{K}$ while transport remains metallic in both the paramagnetic and the antiferromagnetic phase. The analysis of the neutron diffraction data shows an incommensurate magnetic structure with the propagation vector $\mathbf{Q}=\left(0,0,\pm \delta \right)$, where δ is close to 0.2. For a full understanding of the magnetic state, further experiments are needed. The successful growth of large high-quality single crystals paves the way for further investigations of itinerant magnets with incommensurate spin structures using a wide range of experimental tools.

The presence of facets and line junctions connecting facets on grain boundaries (GBs) has a strong impact on the properties of structural, functional, and optoelectronic materials: They govern the mobility of interfaces, the segregation of impurities, as well the electronic properties. In the present paper, we employ density-functional theory and modified embedded atom method calculations to systematically investigate the energetics and thermodynamic stability of these defects. As a prototype system, we consider $\mathrm{\Sigma }3$ tilt GBs in Si. By analyzing the energetics of different faceted GBs, we derive a diagram that describes and predicts the reconstruction of these extended defects as a function of facet length and boundary inclination angle. The phase diagram sheds light upon the fundamental mechanisms causing GB faceting phenomena. It demonstrates that the properties of faceting are not determined solely by anisotropic GB energies but by a complex interplay between geometry and microstructure, boundary energies as well as long-range strain interactions.

Predicting the degree of localization and calculating the trapping energies of polarons in insulators by density functional theory (DFT) is challenging. Hybrid functionals are often reparametrized to obtain accurate results and the a priori selection of these parameters is still an open question. Here we test the accuracy of several range-separated hybrid functionals, all reparametrized to produce an accurate band gap, by calculating the charge transition levels (CTLs) of experimentally well-studied hole polaron defect centers in MgO. We show that the functional with screened long-range exact exchange is moderately but consistently more accurate than functionals which do not include long-range exact exchange. We provide evidence that the source of the improved accuracy is the eigenvalue associated with the valence band maximum of the bulk material. We discuss the extent to which this accuracy relates to Koopmans' compliance of the defect energy level.

Hyperbolic materials are of particular interest for the next generation of photonic and optoelectronic devices. Since artificial metamaterials are intrinsically limited by the size of their nanostructured components, there has been a hunt for natural hyperbolic materials in the last few years. In a first-principles work based on density-functional theory and many-body perturbation theory, we investigate the fundamental dielectric response of $\mathrm{Mo}{\mathrm{Te}}_2$ in monolayer, bilayer, and bulk form and find that it is a natural type-II hyperbolic material with low losses between 3 and 6 eV. Going from the monolayer to the bulk, the energy window of hyperbolic dispersion is blueshifted by a few tenths of an eV. We show that excitonic effects and optical anisotropy play a major role in the hyperbolic behavior of $\mathrm{Mo}{\mathrm{Te}}_2$. Our results confirm the potential of layered materials as hyperbolic media for optoelectronics, photonics, and nanoimaging applications.