The electronic wave functions of an atom or molecule are affected by its interactions with its environment. These interactions dictate electronic and optical processes at interfaces, and is especially relevant in the case of thin film optoelectronic devices such as organic solar cells. In these devices, charge transport and interfaces between multiple layers occur along the thickness or vertical direction, and thus such electronic interactions between different molecules—same or different—are crucial in determining the device properties. Here, we introduce an in situ spectroscopic ellipsometry data analysis method called differential analysis in real time (DART) with the ability to directly probe electronic coupling due to intermolecular interactions along the thickness direction using vacuum-deposited organic semiconductor thin films as a model system. The analysis, which does not require any model fitting, reveals direct observations of electronic coupling between frontier orbitals under optical excitations leading to delocalization of the corresponding electronic wave functions with thickness or, equivalently, number of molecules away from the interface in C60 and MeO-TPD deposited on an insulating substrate $\left(\mathrm{Si}{\mathrm{O}}_2\right)$. Applying the same methodology for C60 deposited on phthalocyanine thin films, the analyses shows strong, anomalous features—in comparison to C60 deposited on $\mathrm{Si}{\mathrm{O}}_2$—of the electronic wave functions corresponding to specific excitation energies in C60 and phthalocyanines. Translation of such interactions in terms of dielectric constants reveals plasmonic type resonance absorptions resulting from oscillations of the excited state wave functions between the two materials across the interface. Finally, reproducibility, angstrom-level sensitivity, and simplicity of the method are highlighted showcasing its applicability for studying electronic coupling between any vapor-deposited material systems where real-time measurements during thin film growth are possible.

The iron-based superconductor ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ is emerging as a key material for high magnetic field applications owing to the recent developments in superconducting wires and bulk permanent magnets. Epitaxial thin films play important roles in investigating and artificially tuning physical properties; nevertheless, the synthesis of ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ epitaxial thin films remained challenging because of the high volatility of K. Herein, we report the successful growth of epitaxial ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ thin films by molecular-beam epitaxy with employing a combination of fluoride substrates $({\mathrm{CaF}}_2,$ ${\mathrm{SrF}}_2, \mathrm{and} {\mathrm{BaF}}_2)$ and a low growth temperature (350–420 °C). Our epitaxial thin film grown on ${\mathrm{CaF}}_2$ showed sharp superconducting transition at an onset critical temperature of 36 K, slightly lower than bulk crystals by ∼2 K due presumably to the strain effect arising from the lattice and thermal expansion mismatch. Critical current density $\left(J_{\mathrm{c}}\right)$ determined by the magnetization hysteresis loop is as high as $2.2\mathrm{MA}/\mathrm{c}{\mathrm{m}}^2$ at 4 K under self-field. In-field $J_{\mathrm{c}}$ characteristics of the film are superior to the bulk crystals. The realization of epitaxial thin films opens opportunities for tuning superconducting properties by epitaxial strain and revealing intrinsic grain boundary transport of ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$.

Hole doping in ${\mathrm{Nd}}_2{\mathrm{NiO}}_{4.00}$ can be achieved either by substituting the trivalent Nd atoms by bivalent alkaline-earth metals or by oxygen doping, yielding ${\mathrm{Nd}}_2{\mathrm{NiO}}_{4+\delta }$. While the alkaline-earth-metal atoms are statistically distributed on the rare-earth sites, the extra oxygen atoms in the interstitial lattice remain mobile down to ambient temperature and allow complex ordering scenarios depending on $\delta$ and $T$. Thereby the oxygen ordering, usually setting in far above room temperature, adds an additional degree of freedom on top of charge, spin, and orbital ordering, which appear at much lower temperatures. In this study, we investigated the interplay between oxygen and spin ordering for a low oxygen doping concentration, i.e., ${\mathrm{Nd}}_2{\mathrm{NiO}}_{4.10}$. Although the extra oxygen doping level remains rather modest with only 1 out of 20 possible interstitial tetrahedral lattice sites occupied, we observed by single-crystal neutron diffraction the presence of a complex three-dimensional (3D) modulated structure related to oxygen ordering already at ambient, the modulation vectors being $\pm 2/13$a*$\pm 3/13$b*, $\pm 3/13$b*$\pm 2/13$b*, and $\pm 1/5$a*$\pm 1/2$c*, and satellite reflections up to fourth order. Temperature-dependent neutron-diffraction studies indicate the coexistence of oxygen and magnetic ordering below $T_N\simeq 48$ K, the wave vector of the Ni sublattice being $\mathit{k}=\left(100\right)$. In addition, magnetic satellite reflections adapt exactly the same modulation vectors as found for the oxygen ordering, evidencing a unique coexistence of 3D modulated ordering for spin and oxygen ordering in ${\mathrm{Nd}}_2{\mathrm{NiO}}_{4.10}$. Temperature-dependent measurements of magnetic intensities suggest two magnetic phase transitions below 48 and 20 K, indicating two distinct onsets of magnetic ordering for the Ni and Nd sublattices, respectively.

Employing first-principles electronic structure calculations, we investigate the stability, electronic, and magnetic properties of ordered double transition metal-based two-dimensional maxene (MXene) compounds, ${\mathrm{Ti}}_{3-x}{\mathrm{Mo}}_x{\mathrm{C}}_2(x=0.5,1,1.5,2,2.5)$. Both unpassivated and compounds passivated with -OH, -O, or -F are considered. Based on this investigation, three narrow gap semiconducting compounds are identified, namely, ${\mathrm{Ti}}_2{\mathrm{MoC}}_2{\mathrm{F}}_2,{\mathrm{TiMo}}_2{\mathrm{C}}_2{\mathrm{F}}_2$, and ${\mathrm{TiMo}}_2{\mathrm{C}}_2{\left(\mathrm{OH}\right)}_2$ for subsequent study of thermoelectric properties within the formalism of Boltzmann transport. Our paper highlights the potential of ${\mathrm{Ti}}_2{\mathrm{MoC}}_2{\mathrm{F}}_2$ as a thermoelectric material with a ZT value higher than 1 within a large temperature range of 300–800 K upon $p$-type carrier doping. The ZT value of $p$-type ${\mathrm{Ti}}_2{\mathrm{MoC}}_2{\mathrm{F}}_2$ is found to reach a high value of 3.1 and an efficiency of $\sim 27\%$ at 800 K. Our exercise should motivate experimental study of yet-to-be synthesized double transition metal MXene compound, ${\mathrm{Ti}}_2{\mathrm{MoC}}_2$.

Accurate interatomic potentials are in high demand for large-scale atomistic simulations of materials that are prohibitively expensive by density functional theory (DFT) calculation. In this study, we apply machine learning potentials (MLPs) in a recently constructed repository to the prediction of the grain boundary energy in face-centered-cubic elemental metals, i.e., Ag, Al, Au, Cu, Pd, and Pt. The systematic application of machine learning potentials shows that they enable us to predict grain boundary structures and their energies accurately. The grain boundary energies predicted by the MLPs are in agreement with those calculated by DFT, although no grain boundary structures were included in training datasets of the present MLPs.

Two-dimensional 1H transition metal dichalcogenides (TMDs) provide a platform, analogous to group IV cubic semiconductor alloys (${\mathrm{Si}}_{1-x}\mathrm{Ge}$), that enables systematic investigations on the effects of alloying in 2D material systems. The existing literature on TMD alloys explores their electrical, magnetic, and optical properties, but lacks a comprehensive analysis of thermal transport in supported and nanostructured systems. Here we employ first-principles-driven phonon Boltzmann transport formalism and a 2D-3D thermal boundary conductance model to systematically study in-plane and cross-plane phonon transport of suspended and ${\mathrm{SiO}}_2$ supported single-layer TMD alloys. We find that the thermal conductivity of alloyed TMDs is dependent on system size up to tens of microns and that the combination of mass-difference and substrate scattering can significantly reduce thermal transport even in large systems ($>500$ nm). Beyond in-plane transport, we find that the thermal boundary conductance displays a qualitatively different trend and significantly weaker modulation with alloy composition as compared to the thermal conductivity. Our results help shed light on the in-plane and cross-plane thermal transport properties of 2D single-layer TMD alloys and further their applications in nanoelectronics, sensing, and energy devices.

Rare-earth monopnictides are predicted to be nontrivial semimetal candidates and show pressure-induced superconductivity. Here, we grow LuBi single crystal and study the magnetization, transport behaviors and electronic band structures to reveal its topological semimetal feature and superconductivity under pressure. At 0 GPa, the quantum oscillations indicate that there are several topologically nontrivial carrier pockets around the Fermi level, among which the hole ones are isotropic in shape, while the electron ones are anisotropic and responsible for the angular magnetoresistance. Upon compression, the superconductivity emerges in the titled compound, showing a similar pressure dependence as that observed in LaBi. Our calculation suggests that the electronic band structures are robust at low- and high pressure, respectively, and thus the topological features are always preserved. Besides, the nearly pressure-independent density of state in LuBi indicates that the conventional electron-phonon coupling appears to play a minor role in the superconductivity.

The quaternary arsenide compounds $X\mathrm{Cu}Y\mathrm{As}$ ($X=\text{Zr,}\text{Hf}$; $Y=\text{Si,}\text{Ge}$) belong to the vast family of the 1111-type quaternary compounds, which possess outstanding physical properties ranging from $p$-type transparent semiconductors to high-temperature Fe-based superconductors. In this paper, we study the electronic structure topology, the spin Hall effect (SHE), and the spin Nernst effect (SNE) in these compounds based on density functional theory calculations. First we find that the four considered compounds are Dirac semimetals with the nonsymmorphic symmetry-protected Dirac line nodes along the Brillouin zone boundary $A\text{−}M$ and $X\text{−}R$ and low density of states near the Fermi level ($E_F$). Second, the intrinsic SHE and SNE in some of these considered compounds are found to be large. In particular, the calculated spin Hall conductivity (SHC) of HfCuGeAs is as large as $-514(\hslash /e)(\mathrm{S}/\mathrm{cm})$. The spin Nernst conductivity (SNC) of HfCuGeAs at room temperature is also large, being $-0.73$ ($\hslash /e$) (A/m K). Moreover, both the magnitude and sign of the SHC and SNC in these compounds can be manipulated by varying either the applied electric field direction or the spin current direction. The SHE and SNE in these compounds can also be enhanced by tuning the Fermi level via chemical doping or electric gating. Finally, a detailed analysis of the band-decomposed and $k$-resolved spin Berry curvatures reveals that these large SHC and SNC as well as their notable tunabilities originate largely from the presence of a large number of spin-orbit coupling-gapped Dirac points near the Fermi level as well as the gapless Dirac line nodes, which give rise to large spin Berry curvatures. Our findings thus suggest that the four $X\mathrm{Cu}Y\mathrm{As}$ compounds not only provide a valuable platform for exploring the interplay between SHE, SNE, and band topology, but they also have promising applications in spintronics and spin caloritronics.

We present an investigation on the controlled growth of epitaxial iron telluride films of multiple phases by molecular beam epitaxy. By optimizing the substrate temperature, we fabricate different phases of α-FeTe, β-FeTe, and $\mathrm{Fe}{\mathrm{Te}}_2$, respectively, whose crystalline morphologies are determined by means of in situ scanning tunneling microscopy/spectroscopy and ex situ scanning transmission electron microscopy. While both α- and β-FeTe films are metallic, we uncover a ∼185- and 65-mV semiconducting band gap for the (100) and (011) facets of $\mathrm{Fe}{\mathrm{Te}}_2$ film, respectively, with the former one being compatible with the first principles calculations. Moreover, for $\mathrm{Fe}{\mathrm{Te}}_2$, we observe reduced gaps with enhanced conductance in the vicinity of edge boundaries, which are dependent on the geometries of step orientation and possibly in correlation to the magnetic anisotropy with structural distortion. Our study provides insight into the controllable synthesis of Fe-Te compounds with variation of stoichiometries and surface terminations, which may generalize to other epitaxial Fe chalcogenide films.

High refractive index dielectric nanoparticles have provided a new platform for exotic light manipulation through the interference of multipole modes. The Kerker effect is one example of a Huygens source design. Rather than exploiting interference between the electric dipole and magnetic dipole, as in many conventional Huygens source designs, we explore Kerker-type suppressed backward scattering mediated by the dominant electric dipole, toroidal dipole, and magnetic quadrupole. These modes are provided by a designed and fabricated CMOS compatible ultrathin silicon nanodisk metasurface with a suppressed magnetic dipole contribution and verified through multipole decomposition. The nontrivial substrate effect is considered using a semianalytical transfer matrix model. The model successfully predicts the observed reflection dip. By applying a general criterion for constructive and destructive interference, it is shown that while constructive interference occurs between the electric and toroidal dipole contributions, the experimentally observed suppressed backward Kerker-type scattering arises from the destructive interference between backward scattered contributions due to the electric dipole and the magnetic quadrupole. Our study paves the way towards new types of Huygens sources or metasurface design, such as for peculiar transverse Kerker scattering.

Regarded as a promising candidate for absorber material in photovoltaic applications, $\mathrm{Ba}{\mathrm{Si}}_2$ confronts the challenge of high-quality material synthesis via low-cost processes. Here, we fabricated $\mathrm{Ba}{\mathrm{Si}}_2$ thin films through the industrially applicable sputtering technique with the face-to-face annealing (FTFA) approach. The employment of the FTFA approach leads to an improvement of the sputtered $\mathrm{Ba}{\mathrm{Si}}_2$ from perspectives of surface homogeneity and crystal quality. Various covers are applied in the FTFA, including $\mathrm{Ba}{\mathrm{Si}}_2$, glass, and Si, which causes alterations in the film's electrical and optical properties. These impacts of the FTFA method on sputtered $\mathrm{Ba}{\mathrm{Si}}_2$ films stem from two aspects, i.e., heat redistributions caused by the variation of thermal networks, and interfacial interactions within the confined space between the cover and the film. The FTFA approach provides a facile strategy for minimizing the impacts of $\mathrm{Ba}{\mathrm{Si}}_2$ surface oxidation during high-temperature processes. These results and findings can push forward the material development of $\mathrm{Ba}{\mathrm{Si}}_2$ and its photovoltaic applications.

Shear-coupled grain boundary (GB) migration has been evidenced as an effective plastic mechanism in absence of the usual intragranular dislocation activity. GB migration occurs through the nucleation and further motion of disconnections. For perfect GBs, the activation barrier for migration is dominated by the disconnection nucleation. In this study, we examine the effects of a dipole of sessile disconnections on the shear-coupled GB migration using molecular dynamics simulations on a $\mathrm{\Sigma }41\left[001\right]\left(540\right)$ symmetric tilt GB in aluminum. The first effect is observed on disconnection nucleation: we show that the disconnection dipole can operate as a source of mobile disconnections. The corresponding yield stress is weakly affected, but the GB migration energy barrier can be reduced by $35\%$. The second effect is seen on the disconnection mobility: the sessile disconnection dipole impedes the disconnection motion by repulsing or attracting it. We conclude that the influence of such dipole on the GB migration favors the nucleation of mobile disconnections on one hand but slows down their motion on the other hand.

Using topological band theory analysis we show that the nonsymmorphic symmetry operations in hexagonal lattices enforce Weyl points at the screw-invariant high-symmetry lines of the band structure. The corepresentation theory and connectivity group theory show that Weyl points are generated by band crossings in accordionike and hourglasslike dispersion relations. These Weyl points are stable against weak perturbations and are protected by the screw rotation symmetry. Based on first-principles calculations we found a complete agreement between the topological predicted energy dispersion relations and real hexagonal materials. Topological charge (chirality) and Berry curvature calculations show the simultaneous formation of Weyl points and nodal lines in $4d$ transition-metal trifluorides such as ${\mathrm{AgF}}_3$ and ${\mathrm{AuF}}_3$. Furthermore, a large intrinsic spin Hall conductivity was found due to the combined strong spin-orbit coupling and multiple Weyl-point crossings in the electronic structure. These materials could be used for the spin/charge conversion in more energy-efficient spintronic devices.

Using high-resolution angle-resolved photoemission spectroscopy, we systematically investigate the electronic structure of β-InSe, a van der Waals semiconductor with a direct band gap. Our measurements show a good agreement with ab initio calculations, which helps reveal the important impact of spin-orbit coupling on the electronic structure of β-InSe. Using surface potassium doping, we tune the chemical potential of the system and observe the unoccupied conduction band. The direct band gap is determined to be about 1.3 eV. Interestingly, we observe a global band shift when the sample is illuminated by a continuous-wave laser at 632.8 nm, which can be understood by an efficient surface photovoltaic effect. The surface photovoltaic can be tuned by in situ surface potassium doping. Our results not only provide important insights into the semiconducting properties of InSe, but also suggest a feasible method to study and engineer the surface photovoltaic effect in InSe-based devices.

Epitaxial perovskite oxide interfaces with different symmetry of the epitaxial layers have attracted considerable attention due to the emergence of novel behaviors and phenomena. In this paper, we show by aberration-corrected transmission electron microscopy (TEM) that orthorhombic ${\mathrm{LaInO}}_3$ films grow in form of three different types of domains on the cubic ${\mathrm{BaSnO}}_3$ pseudosubstrate. Quantitative evaluation of our TEM data shows that $c_{\mathrm{pc}}$-oriented and $a_{\mathrm{pc}}/b_{\mathrm{pc}}$-oriented domains are present with similar probability. While continuum elasticity theory suggests that $c_{\mathrm{pc}}$-oriented domains should exhibit a significantly higher strain energy density than $a_{\mathrm{pc}}/b_{\mathrm{pc}}$-oriented domains, density-functional calculations confirm that $c_{\mathrm{pc}}$- and $a_{\mathrm{pc}}$-oriented domains on ${\mathrm{BaSnO}}_3$ have similar energies.

Using parameter-free density functional approximation and maximally localized Wannier functions analysis, we demonstrate how the ${\mathrm{Ni}}^{3+}$-based heterointerface can present a complete orbital polarization of the Ni-$e_g$ orbitals without introducing additional electronic correlation, which is very inspiring for superconductivity research in two-dimensional confined nickelate layers. The polarized internal electric field from the insulating $\mathrm{Dy}\mathrm{Sc}{\mathrm{O}}_3$ layer, spontaneous oxygen octahedral distortion, and in-plane tensile strain can fine-tune means for the generation of single occupied Ni-$d_{x^2-y^2}$ band structures and cuperate-like Fermi surfaces. Furthermore, by including electronic correlation through on-site Hubbard $U$, we also find a stronger antiferromagnetic correlation than that in the recent synthetic infinite-layer $\mathrm{Nd}\mathrm{Ni}{\mathrm{O}}_2$ with hole-doped ${\mathrm{Ni}}^{1+}$ superconductivity. The low-energy electronic and spin excitations in the nickelate layer resemble those of high-temperature cuprate superconductors. Our study sheds light on promising directions for the preparation of nickelate superconductors.

Formation of stable dispersions of CdSe/CdS core-shell quantum rods (CSQRs) in an inverse nematic gel (N-gel) is reported. The CSQRs distribute uniformly in the N-gel matrix and spontaneously form an ordered arrangement of bright clusters. Estimates show that ∼90% of CSQRs are homogeneously dispersed and ∼10% form clusters in the N-gel phase. Optical properties exhibited by individual CdSe/CdS CSQRs are retained in the dispersions, including a moderate polarization dependence. Extensive fluorescence confocal microscopy studies show evidence for suppressed fluorescence blinking of the clusters in the N-gel phase. The spatial constancy of such clusters is another attractive feature which makes them suitable as stable nanoemitters. The results are interpreted in terms of the unusual type of distribution of the helical gelator fibers in the N-gel phase which promotes self-assembly of the CSQRs and interactions within the clusters.

Using scanning tunneling microscopy and spectroscopy, we visualized the native defects in antiferromagnetic topological insulator ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$. Two native defects, ${\mathrm{Mn}}_{\mathrm{Bi}}$ and ${\mathrm{Bi}}_{\mathrm{Te}}$ antisites, can be well resolved in the topographic images. ${\mathrm{Mn}}_{\mathrm{Bi}}$ tend to suppress the density of states at the conduction band edge. Spectroscopy imaging reveals a localized peaklike local density of state at $\sim 80$ meV below the Fermi energy. A careful inspection of topographic and spectroscopic images, combined with density functional theory calculation, suggests this results from ${\mathrm{Bi}}_{\mathrm{Mn}}$ antisites at Mn sites. The random distribution of ${\mathrm{Mn}}_{\mathrm{Bi}}$ and ${\mathrm{Bi}}_{\mathrm{Mn}}$ antisites results in spatial fluctuation of local density of states near the Fermi level in ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$.

Microstrip line ferromagnetic resonance (MS-FMR) and Brillouin light scattering (BLS) in the Damon-Eshbach geometry were used to investigate the perpendicular magnetic anisotropy (PMA), the damping and the interfacial Dzyaloshinskii-Moriya (iDMI) interaction in ferromagnetic $\left(\mathrm{FM}\right)/\mathrm{Ta}{\mathrm{O}}_x\text{−}\mathrm{based}$ systems as a function of the ferromagnetic ($\mathrm{FM}=\mathrm{Co}$ or ${\mathrm{Co}}_8{\mathrm{Fe}}_{72}{\mathrm{B}}_{20}$) and the $\mathrm{Ta}{\mathrm{O}}_x$ thicknesses (oxidation level). The analysis of the experimental FMR and BLS data has shown that the effective magnetization, the Gilbert damping parameter α and the iDMI are inversely proportional to the CoFeB and the Co films thickness. The BLS investigation of the iDMI variation versus the $\mathrm{Ta}{\mathrm{O}}_x$ thickness and oxidation level reveals a contribution of $\mathrm{FM}/\mathrm{Ta}{\mathrm{O}}_x$ mediated by the presence of a Rashba field at this interface. Finally, we evidenced a correlation between iDMI and PMA by varying the Cu spacer thickness in the $\mathrm{Pt}/\mathrm{Cu}/\mathrm{Co}/\mathrm{Ta}{\mathrm{O}}_x$ system and we showed that both PMA and iDMI are localized at the first atomic monolayers of the Pt/Co interface. The observed non-linear dependence of PMA versus iDMI constant is attributed to similar interface orbital hybridizations involved in both quantities.