We design and investigate the physical properties of new indium compounds $A\mathrm{In}{X}_3$ ($A$ = alkali metals, $X$ = F or Cl). We find nine new In-based materials in their ground state that are thermodynamically stable but are not reported in ICSD (Inorganic Crystal Structure Database). We also discuss several metastable structures. This new series of materials display multiple valences, charge and bond disproportionation, and dimerization. The most common valence of In is 3+. We also find two rare alternatives, one has ${\mathrm{In}}^{2+}$ with In-In dimerization and the other shows valence disproportionation to ${\mathrm{In}}^{1+}$ and ${\mathrm{In}}^{3+}$ with bond disproportionation. We study the possibility of superconductivity in these new In compounds and find that ${\mathrm{CsInF}}_3$ has a transition temperature about 24 K with sufficient hole doping and pressure.

The operation of Na-ion batteries is closely linked to the occurrence of phase transitions and Na ordering within the electroactive materials of their electrodes. We performed a first-principles study of ${\mathrm{Na}}_x{\mathrm{CoO}}_2$ ($0\le x\le 1$) to establish phase stability among O3, P3, O1, and staged hybrid O1-O3 host structures and to determine low-energy Na orderings within each host. We predict several staircases of ground-state orderings, many of which are found to remain ordered at room temperature. At high Na content, the ground states belong to a family of vacancy row orderings in O3 with distorted ${\mathrm{CoO}}_2$ layers. In P3, where the available Na sites form a honeycomb network, we discover three families of hierarchical ground-state orderings at intermediate compositions that essentially consist of domains of the $x=1/2$ ground state periodically separated by antiphase boundaries. Our results agree with the experimentally reported voltage profile and the observed changes in stacking sequence with Na concentration.

In this work, we demonstrate the possibility of controlling the orientation of the top-facets of GaN nanowires grown by plasma-assisted molecular beam epitaxy by means of an in situ thermal treatment at temperatures in the 800–910 ${}^{\circ }\text{C}$ range, at which the material decomposes with a non-negligible rate. Depending on whether the process is carried out under vacuum or active nitrogen exposure, the nanowires can develop either $\left\{1\overline{1}0\overline{2}\right\}$ or $\left\{1\overline{1}0\overline{8}\right\}$ semipolar top-facets. The shape transformation is reversible; namely, the original $\left(000\overline{1}\right)$ polar facets of as-grown nanowires can be recovered after further GaN growth. Using reflection high-energy electron diffraction, the reshaping process is monitored in vivo by tracking the formation and subsequent evolution of chevrons in the diffraction pattern. This analysis reveals that the reshaping takes place after a certain delay time that can last up to several tens of minutes. It also evidences that the shape transformation is not abrupt; instead there is a continuous evolution from high- to low-index semipolar facets. We observe energy barriers higher than 3 eV along the reshaping path toward the $\left\{1\overline{1}0\overline{2}\right\}$ top-facets. The formation of $\left\{1\overline{1}0\overline{8}\right\}$ top-facets is assigned to an intermediate energy minimum along the reshaping path where the system freezes in the presence of active nitrogen. A change in the Ga chemical potential at the nanowire tip is proposed as the trigger for the reshaping, a process that could be either thermodynamically or kinetically driven. The formation of semipolar facets after thermal decomposition is also observed for $\mathrm{GaN}\left(000\overline{1}\right)$ layers, demonstrating that the reshaping process is not related to the peculiar nanowire morphology. This unprecedented ability to form semipolar facets at the tip of N-polar GaN nanowires may pave the way for the fabrication of semipolar axial nanowire heterostructures along the $\left[000\overline{1}\right]$ direction.

Intrinsic, two-dimensional (2D) ferromagnetic semiconductors are an important class of materials for spintronics applications. ${\mathrm{Cr}}_2{X}_2{\mathrm{Te}}_6(X=\text{Si}\text{and Ge})$ semiconductors show 2D Ising-like ferromagnetism, which is preserved in few-layer devices. The maximum magnetic entropy change associated with the critical properties around the ferromagnetic transition for ${\mathrm{Cr}}_2{\mathrm{Si}}_2{\mathrm{Te}}_6-\mathrm{\Delta }{S}_M^{\text{max}}\sim 5.05$ J ${\mathrm{kg}}^{-1}{\mathrm{K}}^{-1}$ is much larger than $-\mathrm{\Delta }{S}_M^{\text{max}}\sim 2.64$ J ${\mathrm{kg}}^{-1}{\mathrm{K}}^{-1}$ for ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$ with an out-of-plane field change of 5 T. The rescaled $-\mathrm{\Delta }{S}_M(T,H)$ curves collapse onto a universal curve independent of temperature and field for both materials. This indicates similar critical behavior and 2D Ising magnetism, confirming the magnetocrystalline anisotropy that could preserve the long-range ferromagnetism in a few layers of ${\mathrm{Cr}}_2{X}_2{\mathrm{Te}}_6$.

The successful preparation of platinum metal pernitrides ($\mathrm{Pm}{\mathrm{N}}_2$) in high-temperature and high-pressure experiments has aroused great scientific interest, since it has long been thought that these systems could not be prepared, and also because of their intriguing mechanical properties. Although it is now widely recognized that $\mathrm{Pm}{\mathrm{N}}_2$ phases can be stabilized under high pressure, the physical origin explaining their stability remains unknown. By means of high-throughput first-principles schemes, we reveal that the choice of specific space group of these pernitrides at high pressure can be quantified by the anchoring effect of distorted $\mathrm{Pm}{\mathrm{N}}_6$ octahedra. The competition between baddeleyite and marcasite is attributed uniquely to Pm dimerization, resulting in a profound enhancement of Pm-Pm bonding and N-N $\pi$ antibonding, while Pm-N bonding plays a secondary role. The observed mechanical strength and atomic deformation mechanism of $\mathrm{Pm}{\mathrm{N}}_2$ suggest that they are ultraincompressible yet soft. This is attributed to the breaking of elongated Pm-N bonds in the $\mathrm{Pm}{\mathrm{N}}_6$ octahedra, which is accompanied by a continuous semiconductor-semimetal-metal transition for the semiconducting $\mathrm{Pm}{\mathrm{N}}_2$ during straining. These findings shed light on the physical origin of high-pressure stabilization and highlight the importance of exploring deformation mechanisms in designing novel strong solids.

While computational methods for the study of point defects in materials have received significant attention, methods for the investigation of grain boundaries require further development. In this study, we applied a genetic algorithm to find the atomic structure of grain boundaries in semiconductor materials with given Miller indices and investigated their electronic structure. Our study of the $\mathrm{\Sigma }3$ (112) grain boundary in CdTe shows that the stability of grain boundaries can be greatly enhanced by breaking mirror symmetry, locally or globally. The resulting grain boundary can be electrically less harmful because the origin of the defect levels are removed from the middle of the band gap and the grain boundaries can serve as a channel for electron extraction.

We present a theoretical study of the modification of the near-field radiative heat transfer due to surface phonon-surface plasmon polariton coupling in bilayer systems made of a doped semiconductor and a polar dielectric. By tuning the surface-plasmon mode of the former material in resonance with the surface phonon modes of the latter one, the near-field spectral radiative heat flux can be suppressed in a frequency band gap. We show how mode hybridization is responsible for the opening of this spectral gap. By changing the carrier concentration we detune the surface plasmon-polariton from the surface phonon polariton. In this case, the band gap does not open. Furthermore, we show that the surface plasmon polariton of the top-most layer allows optimizing the total heat transfer between the bilayer structure as a function of the separation between them.

Disconnection nucleation governs the incipient plasticity and strength of grain boundaries (GBs) that are subject to shear coupling. Here we report a link between the structure and critical shear stress for disconnection nucleation in a GB via atomic slip that is predictive based on the dichromatic pattern. A transition in shear strength and its dependence on tension/compression in a series of [0 0 1] symmetric tilt GBs in Cu at 0 K found from atomistic simulations perfectly match the predictions from the atomic slip analysis.

The thermal conductivity (Λ) of wide-band-gap semiconductors GaN and SiC is critical for their application in power devices and optoelectronics. Here, we report time-domain thermoreflectance measurements of Λ in GaN, ${}^{71}\mathrm{GaN}$, and SiC between 150 and 850 K. The samples include bulk $c$- and $m$-plane wurtzite GaN grown by hydride vapor phase epitaxy (HVPE) and ammonothermal methods; homoepitaxial natural isotope abundant GaN and isotopically enriched ${}^{71}\mathrm{GaN}$ layers with thickness of 6–12 μm grown on $c$-, $m$-, and $a$-plane GaN substrates grown by HVPE; and bulk crystals of 4H and 6H SiC. In low dislocation density ($<{10}^7\mathrm{c}{\mathrm{m}}^{-2}$) bulk and homoepitaxial GaN, Λ is insensitive to crystal orientation and doping concentration (for doping $<{10}^{19}\mathrm{c}{\mathrm{m}}^{-3}$); $\mathrm{\Lambda }\approx 200\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ at 300 K and $\approx 50\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ at 850 K. In ${}^{71}\mathrm{GaN}$ epilayers at 300 K, Λ is $\approx 15\%$ higher than in GaN with natural isotope abundance. The measured temperature dependence of Λ in GaN is stronger than predicted by first-principles based solutions of the Boltzmann transport equation that include anharmonicity up to third order. This discrepancy between theory and experiment suggests possible significant contributions to the thermal resistivity from higher-order phonon scattering that involve interactions between more than three phonons. The measured Λ of 4H and 6H SiC is anisotropic, in good agreement with first-principles calculations, and larger than GaN by a factor of $\approx 1.5$ in the temperature range $300<T<850\mathrm{K}$. This paper provides benchmark knowledge about the thermal conductivity in wide-band-gap semiconductors of GaN, ${}^{71}\mathrm{GaN}$, and SiC over a wide temperature range for their applications in power electronics and optoelectronics.

Precision control of interfacial structures and electronic properties is the key to the realization of functional heterostructures. Here, utilizing the scanning tunneling microscope (STM) both as a manipulation and characterization tool, we demonstrate the fabrication of a heterostructure in a single atomically precise graphene nanoribbon (GNR) and report its electronic properties. The heterostructure is made of a seven-carbon-wide armchair GNR and a lower band gap intermediate ribbon synthesized bottom-up from a molecular precursor on an Au substrate. The short GNR segments are directly written in the ribbon with a STM tip to form atomic precision intraribbon heterostructures. Based on STM studies combined with density functional theory calculations, we show that the heterostructure has a type-I band alignment, with manifestations of quantum confinement and orbital hybridization. Our finding demonstrates a feasible strategy to create a double-barrier quantum dot structure with atomic precision for functionalities, such as negative differential resistance devices in GNR-based nanoelectronics.

Spin chirality in metallic materials with noncoplanar magnetic order can give rise to a Berry phase induced topological Hall effect. Here, we report the observation of a large topological Hall effect in high-quality films of ${\mathrm{Mn}}_{1.5}\mathrm{PtSn}$ that were grown by means of magnetron sputtering on MgO(001). The topological Hall resistivity is present up to ${\mu }_{0}H\approx 4\mathrm{T}$ below the spin reorientation transition temperature, $T_s=185$ K. We find that the maximum topological Hall resistivity is of comparable magnitude as the anomalous Hall resistivity. Owing to the size, the topological Hall effect is directly evident prior to the customarily performed subtraction of magnetometry data. Further, we underline the robustness of the topological Hall effect in ${\mathrm{Mn}}_{2-x}\mathrm{PtSn}$ by extracting the effect for multiple stoichiometries $\left(x=0.5,0.25,0.1\right)$ and film thicknesses $(t=104,52,35$ nm) with maximum topological Hall resistivities between 0.76 and $1.55\mu \mathrm{\Omega }\mathrm{cm}$ at 150 K.

Relationships between concentration of unintentionally doped carbon in GaN and its metalorganic vapor phase epitaxy conditions were investigated theoretically. A kinetic-thermodynamic model which considers kinetic behavior of adsorbed atoms on vicinal surface was proposed. Thermodynamic properties of gas species and adsorption energies obtained by first-principles calculation were used in the model. The predicted carbon concentration range, ${10}^{15}\sim {10}^{17}\mathrm{c}{\mathrm{m}}^{-3}$, agreed with that of experimental results quantitatively. The calculation results also reproduced experimental tendency: Carbon concentration decreases with increase of ${\mathrm{NH}}_3$ partial pressure and total pressure and/or decrease of trimethylgallium partial pressure.

Charge disproportionation transitions in complex oxides most commonly link high-temperature phases containing identical cations with the same oxidation state and crystallographic site, to low-temperature phases in which charge transfer between these ions results in unequal oxidation states. Here we propose, based on density functional calculations, a related concept that we term heteronuclear charge disproportionation, in which charge transfer occurs between different elements on different crystallographic sites. We show for the case of ${\mathrm{EuMnO}}_3$ that such a transition from the experimentally observed ${\mathrm{Eu}}^{3+}{\mathrm{Mn}}^{3+}{\mathrm{O}}_3$ phase to the, so far, unknown ${\mathrm{Eu}}^{2+}{\mathrm{Mn}}^{4+}{\mathrm{O}}_3$ phase can be triggered by pressure and epitaxial strain. We then identify measurable signatures to aid in an experimental exploration of the complex pressure- and biaxial strain-dependent phase stability of ${\mathrm{EuMnO}}_3$ that we hope to motivate with our predictions. We suggest other candidate crystal chemistries for heteronuclear charge disproportionation, in which novel physics could emerge from the coexistence of instabilities.

The intrinsic spin Hall conductivity of a two-dimensional gas confined to ${\mathrm{SrTiO}}_3$, such as occurs at an ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ interface, is calculated from the Kubo formula. The effect of strain in the [001] and the [111] directions is incorporated into a full tight-binding Hamiltonian for a quantum well with a [001] growth direction. We show that the spin-charge conversion ratio can be significantly altered through the strain and gate voltage by tuning the chemical potential. The strain direction is also a significant factor in the spin Hall response, as this direction affects the alignment of the conduction bands.

Understanding, designing, and processing functional metal sulfides are significant challenges in part because of the high temperatures and pressures and the number of secondary phases encountered in these complex systems. In particular, the underlying thermochemical properties are not well understood that would allow prediction of equilibrium conditions and driving forces. In addition, obtaining accurate values for the energetics of metal sulfide systems is far from complete, suggesting application of density functional theory (DFT) calculations. Here, the results of an examination of 69 phases by DFT using 12 exchange-correlation (X-C) functionals indicate that (i) the key source of error in predicting the Gibbs energy of a phase is the enthalpy calculated at 0 K rather than entropy at finite temperatures from phonon calculations and (ii) an improved prediction of the thermodynamic properties at 0 K relies on the selected nonlocal X-C functional such as the hybrid potential. Regarding metal sulfides, we conclude from the present DFT results that (1) the secondary phase ${\mathrm{Cu}}_2{\mathrm{ZnSn}}_3{\mathrm{S}}_8$, associated with the desired photovoltaic material ${\mathrm{Cu}}_2{\mathrm{ZnSnS}}_4$, is not stable at 0 K, but it becomes slightly stable with increasing temperature (i.e., >800 K), primarily due to the vibrational entropy, which makes it difficult to be detected in a typical thin-film growth process; (2) the hybrid X-C functional improves the predicted energetics for most of the layered transition-metal disulfides such as ${\mathrm{TiS}}_2$, ${\mathrm{MoS}}_2$, and ${\mathrm{WS}}_2$, but not for the nonlayered ${\mathrm{RuS}}_2$, ${\mathrm{OsS}}_2$, and ${\mathrm{IrS}}_2$ as well as the layered ${\mathrm{PdS}}_2$; and (3) the formation of the solid-state electrolyte ${\mathrm{Na}}_3{\mathrm{PS}}_4$ is thermodynamically favored. We further conclude that accurate energetics as a function of temperature for the materials of interest is feasible to be achieved beyond the semilocal DFT calculations with the key being enthalpy predicted at 0 K.

Investigations into the phase behavior of Brownian systems of nonspherical colloids have received considerable attention because these systems can exhibit very rich self-assembled structures, some of which have potential for applications. In this work, we explore the phase behavior of corner-rounded hexagons in two dimensions, both through experiments and Monte Carlo (MC) simulations. Our experiments, using lithographically shape-designed hexagons, which have corner-rounded vertices and nearly hard in-plane interactions, reveal three different solid phases for increasing particle area fraction ${\varphi }_{\mathrm{A}}$: hexagonal rotator crystal (RX), hexagonal crystal (HX), and frustrated hexagonal crystal (FHX). In the RX phase, hexagons form a hexagonal lattice, but they are randomly oriented and their rotations are ergodic. In the HX phase, hexagons orient uniformly on average, but their rotations are still ergodic via slower hopping motion. In the FHX phase, all hexagons are uniformly oriented, but their rotations are highly bound and nonergodic. MC simulation results on matching rounded hexagons confirm this experimentally observed sequence of phases. Using simulations, we increase the corner roundness and show that the molecular-orientational order decreases at fixed ${\varphi }_{\mathrm{A}}$. Our results provide insights into controlling the large-scale self-assembly of spatially and orientationally ordered two-dimensional arrays of colloids through particle shape design and crowding conditions.