We investigate the translational and rotational relaxation dynamics of a crowded two-dimensional system of monodisperse Penrose kites, in which crystallization, quasicrystallization and nematic ordering are suppressed, from low to high area fractions along the metastable ergodic fluid branch. First, we demonstrate a decoupling between both the translational and the rotational diffusion coefficients and the relaxation time: the diffusivities are not inversely proportional to the relaxation time, neither in the low-density normal liquid regime nor in the high-density supercooled regime. Our simulations reveal that this inverse proportionality breaks in the normal liquid regime due to the Mermin-Wagner long-wavelength fluctuations and in the supercooled regime due to the dynamical heterogeneities. We then show that dynamical heterogeneities are mainly spatial for translational degrees of freedom and temporal for rotational ones, there is no correlation between the particles with largest translational and rotational displacements, and different dynamical length scales characterize the translational and the rotational motion. Hence, despite the translational and the rotational glass-transition densities coincide, according to a mode-coupling fit, translations and rotations appear to decorrelate via different dynamical processes.

Applications of commodity polymers are often hindered by their low thermal conductivity. In these systems, going from the standard polymers dictated by weak van der Waals interactions to biocompatible hydrogen-bonded smart polymers, the thermal transport coefficient $\kappa$ varies between 0.1 and 0.$4{\mathrm{Wm}}^{-1}{\mathrm{K}}^{-1}$. Combining all-atom molecular dynamics simulations with some experiments, we study thermal transport and its link to the elastic response of (standard and smart) commodity plastics. We find that there exists a maximum attainable stiffness (or sound-wave velocity), thus providing an upper bound of $\kappa$ for these solid polymers. The specific chemical structure and the glass transition temperature play no role in controlling $\kappa$, especially when the microscopic interactions are hydrogen-bonding based. Our results are consistent with the minimum thermal conductivity model and existing experiments. The effect of polymer stretching on $\kappa$ is also discussed.

Electronic properties and lattice dynamics of bulk $\varepsilon$-GaSe and one, two, and three tetralayers GaSe are investigated by means of density functional and density functional perturbation theory. The few-tetralayer systems are semiconductors with an indirect nature of the fundamental band gap and a Mexican-hat shape is observed at the top of the valence band. The phonon branch analysis reveals the dynamical stability for all systems considered together with the LO-TO splitting breakdown in two-dimensional systems. In-plane (E) and out-of-plane (A) zone-center lattice vibrations dominate the Raman and IR spectra.

The class of antiperovskite compounds $A_{3}B\mathrm{O}$ ($A$ = Ca, Sr, Ba; $B$ = Sn, Pb) has attracted interest as a candidate three-dimensional Dirac system with topological surface states protected by crystal symmetry. A key factor underlying the rich electronic structure of $A_{3}B\mathrm{O}$ is the unusual valence state of $B$, i.e., a formal oxidation state of $-4$. Practically, it is not obvious whether anionic $B$ can be stabilized in thin films, due to its unusual chemistry, as well as the polar surface of $A_{3}B\mathrm{O}$, which may render the growth-front surface unstable. We report x-ray photoelectron spectroscopy measurements of single-crystalline films of ${\mathrm{Sr}}_3\mathrm{SnO}$ and ${\mathrm{Sr}}_3\mathrm{PbO}$ grown by molecular beam epitaxy. We observe shifts in the core-level binding energies that originate from anionic Sn and Pb, consistent with density functional theory calculations. Near the surface, we observe additional signatures of neutral or cationic Sn and Pb, which may point to an electronic or atomic reconstruction with possible impact on putative topological surface states.

We have synthesized single crystals of dolerophanite ${\mathrm{Cu}}_2{\mathrm{OSO}}_4$ by means of chemical vapor transport and studied the direction-dependent magnetic susceptibility and dielectric properties of this low-dimensional $S=1/2$ system. Below the magnetic ordering temperature of $\sim 20$ K, we observe an unusually strong ferromagnetic component of roughly 1/4 ${\mu }_B$ per ${\mathrm{Cu}}^{2+}$ ion along the $b$ direction, parallel to the zigzag chains of copper-oxygen square plaquettes. It is also remarkable that the onset of magnetic ordering can be enhanced by about 20% if an external magnetic field is applied along the direction of these chains.

Heterogeneous stacks of two-dimensional transition-metal dichalcogenides can be arranged so as to have a type-II band alignment, where the valence band maximum and the conduction band minimum are located on different layers. These structures can host long-living interlayer excitons with inhibited charge recombination and enhanced charge-carrier separation. Interlayer excitons appear as photoluminescence peaks below the band gap, but not in absorption experiments, indicating that they may form after and not during absorption. In order to quantify the interlayer component of the absorption spectra of such heterostructures, we perform first-principles calculations of the layer-decomposed dielectric function of the ${\mathrm{HfS}}_2/{\mathrm{PtS}}_2$ heterobilayer. This has a type-II band alignment and a relatively small interlayer distance, which should facilitate the formation of interlayer excitons. We find that the interlayer component is always only a small fraction of the total dielectric function, owing to the large spatial separation between the electron and the hole. However, the interlayer contribution is greatly enhanced upon reducing the interlayer distance. Compression of the layers produces a split-off band at the top of the valence bands. This remains localized on ${\mathrm{PtS}}_2$ so that the heterostructure preserves the type-II character. At the same time the type-II bandgap is reduced, moving the interlayer absorption peak to a lower energy and to a position well separated from the rest of the absorption spectrum.

Due to their combination of mechanical stiffness and flexibility, two-dimensional (2D) materials have received significant interest as potential electromechanical materials. Flexoelectricity is an electromechanical coupling between strain gradient and polarization. Unlike piezoelectricity, which exists only in noncentrosymmetric materials, flexoelectricity theoretically exists in all dielectric materials. However, most work on the electromechanical energy conversion potential of 2D materials has focused on their piezoelectric and not flexoelectric behavior and properties. In the present paper, we demonstrate that the intrinsic structural asymmetry present in monolayer Janus transition-metal dichalcogenides (TMDCs) enables significant flexoelectric properties. We report these flexoelectric properties using a recently developed charge-dipole model that couples with classical molecular dynamics simulations. By employing a prescribed bending deformation, we directly calculate the flexoelectric constants while eliminating the piezoelectric contribution to the polarization. We find that the flexoelectric response of a Janus TMDC is positively correlated to its initial degree of asymmetry, which contributes to stronger $\sigma \text{−}\sigma$ interactions as the initial degree of asymmetry rises. In addition, the high transfer of charge across atoms in Janus TMDCs leads to larger electric fields due to $\pi \text{−}\sigma$ coupling. These enhanced $\sigma \text{−}\sigma$ and $\pi \text{−}\sigma$ interactions are found to cause the flexoelectric coefficients of the Janus TMDCs to be several times higher than traditional TMDCs, such as ${\mathrm{MoS}}_2$, whose flexoelectric constant is already ten times larger than graphene.

The growth of Tb-induced nanostructures on vicinal Si(111) substrates with an offcut toward the $\left[11\overline{2}\right]$ direction, so-called Si(hhk) substrates, was studied. The nanostructures were analyzed using scanning tunneling microscopy and spectroscopy as well as core-level photoemission spectroscopy. For low Tb coverages, terraces covered by a Tb-induced submonolayer structure are observed. At higher coverages, metallic Tb disilicide monolayer structures are dominant. Their appearance strongly depends on the offcut direction. Well-defined nanowires with straight edges are only present for Si(hhk) substrates with $h<k$. Furthermore, only these substrates may host very narrow metallic nanolines. In general, narrower nanostructures are observed for larger offcut angles enabling the formation of Tb disilicide nanowires with average widths as low as $2.3\mathrm{nm}$ for Si(335) substrates.

Ni-Mn-In magnetic shape-memory Heusler alloys exhibit generally a large thermal hysteresis at their first-order martensitic phase transition which hinders a technological application in magnetic refrigeration. By optimizing the Cu content in ${\mathrm{Ni}}_2{\mathrm{Cu}}_x{\mathrm{Mn}}_{1.4-x}{\mathrm{In}}_{0.6}$, we obtained a thermal hysteresis of the martensitic phase transition in ${\mathrm{Ni}}_2{\mathrm{Cu}}_{0.2}{\mathrm{Mn}}_{1.2}{\mathrm{In}}_{0.6}$ of only 6 K. We can explain this very small hysteresis by an almost perfect habit plane at the interface of martensite and austenite phases. Application of hydrostatic pressure does not reduce the hysteresis further, but shifts the martensitic transition close to room temperature. The isothermal entropy change does not depend on warming or cooling protocols and is pressure independent. Experiments in pulsed-magnetic fields on ${\mathrm{Ni}}_2{\mathrm{Cu}}_{0.2}{\mathrm{Mn}}_{1.2}{\mathrm{In}}_{0.6}$ find a reversible magnetocaloric effect with a maximum adiabatic temperature change of $-13$ K.

The large Landé $g$ factor, high spin-orbit coupling, and low effective mass of the two-dimensional electron gas in InSb quantum wells combined with proximal superconductivity may realize a scalable platform for topological quantum computation. Aluminum thin films directly deposited on top of InSb planar structures result in the formation of a reactive AlInSb layer at the interface. This interlayer progressively consumes the whole Al film, resulting in a disordered AlInSb layer after a few months at room temperature. We report on a heterostructure design that results in a significant increase of the durability of these hybrid Al-InSb heterostructures with the preservation of a pure Al film and sharp superconductor-semiconductor interface for more than one year. Two monolayers of epitaxial InAs at the superconductor-semiconductor interface prevent interfacial reactivity as evidenced by x-ray reflectivity and energy dispersive spectroscopy measurements. Structural characterizations of the Al films by transmission electron microscopy reveal the presence of tens of nanometers wide grains predominantly oriented with Al(110) parallel to InSb(001).

Molecular beam epitaxy offers an exciting avenue for investigating the behavior of the topological semimetal $\mathrm{C}{\mathrm{d}}_3\mathrm{A}{\mathrm{s}}_2$, by providing routes for doping, alloying, strain engineering, and heterostructure formation. To date, however, minimal exploration has been devoted to the impact of defects that are incorporated into epilayers due to constraints imposed by the substrate and narrow growth window. Here, we use a combination of lattice-matched $\mathrm{Z}{\mathrm{n}}_x\mathrm{C}{\mathrm{d}}_{1-x}\mathrm{Te}$ buffer layers, miscut substrates, and broadband illumination to study how dislocations, twins, and point defects influence the electron mobility of $\mathrm{C}{\mathrm{d}}_3\mathrm{A}{\mathrm{s}}_2$. A combination of defect suppression approaches produces $\mathrm{C}{\mathrm{d}}_3\mathrm{A}{\mathrm{s}}_2$ epilayers with electron mobilities upwards of $15000\mathrm{c}{\mathrm{m}}^2/\mathrm{V}\mathrm{s}$ at room temperature.

We have explored the effect of magnetic rare-earth dopants substitutionally incorporated on the Ba sites of ${\mathrm{BaSnO}}_3$ in terms of electronic transport, magnetism, and optical properties. We show that for ${\mathrm{Ba}}_{0.92}{R}_{0.08}{\mathrm{SnO}}_3$ thin films (where $R=\text{La,}\text{Pr,}\text{Nd,}\text{Gd}$), there is a linear increase of mobility with carrier concentration across all doping schemes. La-doped films have the highest mobilities, followed by Pr- and Nd-doped films. Gd-doped samples have the largest ionic size mismatch with the Ba site and correspondingly the lowest carrier concentrations and electron mobilities. However, crystallinity does not appear to be a strong predictor of transport phenomena; our results suggest that point defects more than grain boundaries are key ingredients in tuning the conduction of ${\mathrm{BaSnO}}_3$ films grown by pulsed laser deposition. Pronounced, nonhysteretic x-ray magnetic dichroism signals are observed for Pr-, Nd-, and Gd-doped samples, indicating paramagnetism. Finally, we probe the optical constants for each of the ${\mathrm{BaSnO}}_3$ doping schemes and note that there is little change in the transmittance across all samples. Together these results shed light on conduction mechanisms in ${\mathrm{BaSnO}}_3$ doped with rare-earth cations.

By performing extensive molecular dynamics simulations, we investigate the deformation behavior in $\mathrm{A}{\mathrm{l}}_{90}\mathrm{S}{\mathrm{m}}_{10}$ and $\mathrm{C}{\mathrm{u}}_{64.5}\mathrm{Z}{\mathrm{r}}_{35.5}$ alloys after elongated isothermal annealing in the vicinity of the glass-transition temperature ($T_g$). Different microstructural response to the annealing process was observed: $\mathrm{A}{\mathrm{l}}_{90}\mathrm{S}{\mathrm{m}}_{10}$ maintains the glassy structure with improved energetic stability, enhanced short-range order (SRO), and a more pronounced spatial network that extends beyond the first atomic shell, while $\mathrm{C}{\mathrm{u}}_{64.5}\mathrm{Z}{\mathrm{r}}_{35.5}$ forms nanocrystalline Laves $\mathrm{C}{\mathrm{u}}_2\mathrm{Zr}$ phases. Shear banding occurs in both annealed systems under shear loading. For $\mathrm{A}{\mathrm{l}}_{90}\mathrm{S}{\mathrm{m}}_{10}$, the spatial network formed by the local clusters characterizing the SRO of the system is significantly weakened but remains appreciable in the shear band. In contrast, the crystalline ordering in the $\mathrm{C}{\mathrm{u}}_{64.5}\mathrm{Z}{\mathrm{r}}_{35.5}$ is completely destroyed during shear banding. Consequently, while displaying higher yield strength, the annealed $\mathrm{C}{\mathrm{u}}_{64.5}\mathrm{Z}{\mathrm{r}}_{35.5}$ sample appears to be less ductile. By carefully examining the effect of microstructures on the structural ordering in the shear band and the consequent mechanical response, our work contributes to a better understanding of the deformation mechanism of amorphous alloys as compared with that in crystalline materials.

Angle-resolved photoemission spectroscopy (ARPES) is probably the most important experimental technique for the investigation of the electronic structure of solids. With respect to the interpretation of measured complex photoemission spectra, extensive theoretical efforts are often involved, as one needs a calculation of the band structure, a mapping of the band structure to the observed complex photoemission spectrum, and the inclusion of many-body effects. Typical methods that include the mentioned necessary steps for the interpretation are usually theoretically costly (such as time-dependent density functional theory). In contrast to purely numerical methods, analytical physical theories grant an easier approach in the implementation and, in general, allow for a more direct insight into the involved physical processes. Here, we show that combining a Keldysh Green's function formulation of the photoelectric current, together with a tight-binding parametrization of band structures, gives a powerful tool set for the simulation of ARPES spectra. Our approach is fast, capable of including many-body effects, and easy to be implemented and, therefore, from the viewpoint of an experimentalist, a valuable tool for the interpretation of photoemission spectra.

Topological superconductors showing Majorana fermions or bound states at the edge or surface are promising components for potential quantum computation and thus attract much attention in physical sciences. Recently, topological Weyl fermion states have been discovered in nonsuperconducting TaP crystals, which may offer a great platform to investigate topological nontrivial superconductivity if superconductivity can be induced wherein. Here we report the discovery of nonmagnetic and ferromagnetic tips induced unconventional interface superconductivity around the nanoscale point contact region on TaP crystals. The tip-induced superconductivity (TISC) at the contact is enhanced with the ferromagnetic Ni tip, comparing with nonmagnetic Au and PtIr tips. The point contact spectroscopy shows evidences of anisotropic superconductivity with the mixed spin-triplet pairing, indicating unconventional superconducting nature. Moreover, the Shubnikov–de Haas oscillations in magnetoresistance are detected from the tip-point-contact measurement, which demonstrate that the nonzero Berry phase or topological nontrivial property of TaP maintains under the tip point contact. The theoretical analysis shows that surface superconductivity plays a major role in this TISC and there exists a large spin-triplet mixing in the gap function. Therefore, our work suggests an intriguing Weyl surface superconducting state on TaP.

We combine polarization-resolved low-frequency Raman microscopy and dispersion-corrected density-functional calculations (DFT-D3) to study polymorph-dependent lattice vibrations in coronene, a model molecular system for nanographenes and disklike organic semiconductors that exhibits two crystalline structures with distinct electronic and optical properties. Changes in low-energy Raman-active lattice phonons are followed across the $\gamma$- to $\beta$-phase transition at 150 K. Raman frequencies calculated using DFT-D3 agree to within 4 ${\mathrm{cm}}^{-1}$, and on the basis of polarization dependence of peak positions and intensities we achieve a clear mode assignment. Studies of the associated atomic motions show how the pure librational and rotational modes of $\gamma$-coronene change into mixed roto-librations in the $\beta$-phase, explaining the remarkable differences in Raman spectra of the two phases.

Theoretical formulations capable of modeling chemical interactions over 3–4 orders of magnitude of bond strength, from covalent to van der Waals (vdW) forces, are one of the primary goals in materials physics, and chemistry. Development of vdW corrections for density-functional theory has thus been a major research field for two decades. While many of these corrections are semiempirical, more theoretically rigorous ab initio functionals have been developed. The ab initio functional vdW-DF2, when coupled with the reoptimized B86 exchange function (vdW-DF2-rB86), has typically performed as well, if not better than most semiempirical formulations. Here we present a system, Co intercalation of graphene on Ir(111), for which a semiempirical correction predicts local corrugation maxima in locations at which the vdW-DF2-rB86 functional predicts global minima. Sub-angstrom precision quantitative structural measurements show better agreement with the semiempirical correction. We posit that it is balancing the weak vdW interaction with the stronger, even covalent, interactions that proves a challenge for the vdW-DF2-rB86 functional.

Here, we present an ab initio investigation of the pressure-induced behavior of ${\mathrm{MgGeO}}_3$ and ${\mathrm{NaMgF}}_3$ perovskite (PV), traditional low-pressure analogs (LPAs) of ${\mathrm{MgSiO}}_3$ PV. The latter is an exceedingly important system in planetary sciences displaying novel phases and phase reactions under pressure that are impractically high and still challenging to experiments. Specifically, we investigate the possibility of ${\mathrm{MgGeO}}_3$ and ${\mathrm{NaMgF}}_3$ to display at lower pressures the novel phases, $I\overline{4}2d$-type ${\mathrm{A}}_2\mathrm{B}{X}_4$, $P{2}_1/c$-type ${\mathrm{AB}}_2{X}_5$, and dissociation/recombination transitions displayed by ${\mathrm{MgSiO}}_3$ above $\sim 500\mathrm{GPa}$, alone or in the presence of its binary compounds, MgO and ${\mathrm{SiO}}_2$. We find that, although neither ${\mathrm{MgGeO}}_3$ nor ${\mathrm{NaMgF}}_3$ are perfect LPAs of ${\mathrm{MgSiO}}_3$, they are useful for several reasons: (i) both display the post-PV transition observed in ${\mathrm{MgSiO}}_3$ PV; (ii) starting at $\sim 20\mathrm{GPa}$, the Na-Mg-F system produces a novel $P{2}_1/c$-type ${\mathrm{NaMg}}_2{\mathrm{F}}_5$ phase by dissociation of ${\mathrm{NaMgF}}_3$ or by its compression with ${\mathrm{MgF}}_2$. Instead, the Mg-Ge-O system produces a $I\overline{4}2d$-type ${\mathrm{Mg}}_2{\mathrm{GeO}}_4$ by dissociation of ${\mathrm{MgGeO}}_3$ or by its compression with MgO starting at $\sim 200\mathrm{GPa}$. Such pressures are routinely accessible in laser-heated diamond-anvil cells today; (iii) like ${\mathrm{MgSiO}}_3$, both systems ultimately dissociate into the binary AX and $\mathrm{B}{X}_2$ compounds, confirming this trend in ternary systems first predicted in ${\mathrm{MgSiO}}_3$. We also predict potential metastable phase transitions into a ${\mathrm{Gd}}_2{\mathrm{S}}_3$-type structure in ${\mathrm{MgGeO}}_3$ and into a ${\mathrm{U}}_2{\mathrm{S}}_3$-type structure in ${\mathrm{NaMgF}}_3$. Metastable polymorphic transitions may occur more easily than dissociation/recombination reactions under insufficiently heated compression.

The importance of organic molecular cation orientation and interaction with surrounding inorganic framework in the optoelectronic hybrid halide perovskites is a topic of considerable interest. To that end, we study the effect of organic molecule rotations on the properties of such hybrid semiconductors using the swarm intelligence-based structure prediction method combined with ab initio density functional calculations. Adopting the cubic phases of ${\mathrm{APbI}}_3$ [$\mathrm{A}={\mathrm{CH}}_3{\mathrm{NH}}_3$(MA), ${\mathrm{CHNH}}_2{\mathrm{NH}}_2$(FA) and ${\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{NH}}_3$(EA)], we determine the energetically stable/metastable configurations of organic molecules inside the quasicubic perovskite cages. Different cation orientations result in up to $\sim 250\mathrm{meV}/\mathrm{formula}$ changes in the material free energy, reflecting the complex energy landscape of these hybrid materials containing dynamically rotating organic components. Notably, the [012] orientation of MA is found to give the most stable structure of ${\mathrm{MAPbI}}_3$, while the conventional [001], [011], or [111] directions remain slightly higher in energy. Molecular orientations clearly influence the fundamental electronic bandgap and also modify the magnitude of Rashba-type energy band splitting resulting in indirect gap behavior. However, the dielectric screening remains large for all orientations thus creating weakly bound excitons. Based on the analysis of many organic molecule configurations obtained through the structure search technique, our results provide insightful understanding on the effects of organic molecule rotation on optoelectronic properties and carrier dynamics of hybrid halide perovskites.