We present a study of electrical transport properties of $R{\mathrm{Bi}}_2$ $(R=\mathrm{La},\mathrm{Ce})$ under hydrostatic pressure up to $\sim 2.5$ GPa. These measurements are complemented by thermodynamic measurements of the specific heat on ${\mathrm{CeBi}}_2$ at different pressures up to 2.55 GPa. For ${\mathrm{CeBi}}_2$, we find a moderate increase of the antiferromagnetic transition, $T_{\text{N}}$, from 3.3 K to 4.4 K by pressures up to 2.55 GPa. Notably, resistance measurements for both ${\mathrm{CeBi}}_2$ and ${\mathrm{LaBi}}_2$ show signatures of superconductivity for pressures above $\sim 1.7$ GPa. However, the absence of superconducting features in specific-heat measurements for ${\mathrm{CeBi}}_2$ indicates that superconductivity in ${\mathrm{CeBi}}_2$ (and most likely ${\mathrm{LaBi}}_2$ as well) is not bulk and likely originates from traces of Bi flux, either on the surface of the platelike samples, or trapped inside the sample as laminar inclusions. Finally, we point out that extra caution should be exercised when claiming superconductivity based just on transport measurements, especially for Bi-containing compounds.

We have studied the anomalous Hall effect (AHE) in strained thin films of the frustrated antiferromagnet ${\mathrm{Mn}}_3\mathrm{NiN}$. The AHE does not follow the conventional relationships with magnetization or longitudinal conductivity and is enhanced relative to that expected from the magnetization in the antiferromagnetic state below $T_{\mathrm{N}}=260\mathrm{K}$. This enhancement is consistent with origins from the noncollinear antiferromagnetic structure, as the latter is closely related to that found in ${\mathrm{Mn}}_3\mathrm{Ir}$ and ${\mathrm{Mn}}_3\mathrm{Pt}$ where a large AHE is induced by the Berry curvature. As the Berry-phase-induced AHE should scale with spin-orbit coupling, yet larger AHE may be found in other members of the chemically flexible ${\mathrm{Mn}}_{3}A\mathrm{N}$ structure.

The impact of single-axis lattice expansion on the optical response of $\mathrm{BiFe}{\mathrm{O}}_3$ films is examined. Low-energy He implantation is used to tailor morphotropic phases of $\mathrm{BiFe}{\mathrm{O}}_3$ films and study changes in their optical spectra with continuously increasing lattice expansion. He ion implantation of epitaxial rhombohedral (R)- and tetragonal (T)-like $\mathrm{BiFe}{\mathrm{O}}_3$ films induces uniaxial out-of-plane strain that, on R-like films, eventually leads to a complete R-T phase transition. This approach allows us to provide insights into the optical response of $\mathrm{BiFe}{\mathrm{O}}_3$ films. Strain doping of T-like films leads to a significant redshift of the optical absorption spectra that is theoretically explained by a lowering of Fe $3d{t}_{2g}$ states. R-like films, on the other hand, show a less-pronounced sensitivity to uniaxial strain and a blueshift of about 250 meV at the strain-induced R-T transition. The results demonstrate that strain doping allows a deeper examination of the optical properties of epitaxial phases that are otherwise impossible to access by standard epitaxy.

A steplike density of states is the hallmark of a two-dimensional electron gas. Here, scanning tunneling spectroscopy is used to investigate this property in the differential conductance of an $\mathrm{I}{\mathrm{n}}_{0.53}\mathrm{G}{\mathrm{a}}_{0.47}\mathrm{As}$ quantum well grown on $p$-type doped InP(001). We show that the appearance of a steplike function in the conduction band strongly depends on both the hole concentration in the quantum well and the current injected into the quantized states. Based on four-point probe transport measurements and tunneling-induced light-emission spectroscopy, we discuss the physical mechanisms at the origin of the current and unveil the significant contribution of midgap surface states in the relaxation of the tunneling electrons. These surface states, via pinning of the Fermi level, also affect the potential landscape across the quantum well, as demonstrated by tight-binding calculations of the quantum well electronic structure.

We use relativistic ab initio methods combined with model Hamiltonian approaches to analyze the normal-phase electronic and structural properties of the recently discovered WP superconductor. Remarkably, the outcomes of such study can be employed to set fundamental connections among WP and the CrAs and MnP superconductors belonging to the same space group. One of the key features of the resulting electronic structure is represented by the occurrence of multiple band crossings along specific high-symmetry lines of the Brillouin zone. In particular, we demonstrate that the eightfold band degeneracy obtained along the $\mathit{SR}$ path at $(k_x,k_y)=(\pi ,\pi )$ is due to inversion time-reversal invariance and a pair of nonsymmorphic symmetries. The presence of multiple degenerate Fermi points along the $\mathit{SR}$ direction constrains the topology of the Fermi surface, which manifests distinctive marks when considering its evolution upon band filling variation. If the Fermi level crosses the bands along the $\mathit{SR}$ line as it happens at the nominal filling of the MnP, these Fermi surfaces are open or closed Fermi pockets. Moving the relative position of the Fermi level away from the eightfold degenerate bands as for the WP and CrAs compounds, the electronic changeover exhibits a simultaneous modification of the Fermi-surface dimensionality and topology. Four two-dimensional (2D) Fermi-surface sheets are centered around the $\mathit{SR}$ line with a corrugated profile along the $k_z$ direction. Moreover, we show that the spin-orbit interaction determines a selective removal of the band degeneracy and, consequently, a splitting of the quasi-2D Fermi sheets, as it happens in WP. Finally, we comment on the connections between our results and recent experimental and theoretical proposals about the triplet superconductivity in this class of compounds.

The phase formations of high-entropy alloys (HEAs) are essential to their properties, but efficient prediction of them remains a challenge. In this work, we propose a support vector machine model that is capable of distinguishing stable body-centered cubic (BCC), face-centered cubic (FCC) HEA phases, and the remaining phases out of the 322 as-cast samples with a cross validation accuracy over 90% after training and test. With the model, we predicted 369 FCC and 267 BCC equiatomic HEAs from the composition space of 16 metallic elements, one order larger than the number of available experimental data. Furthermore, dozens of refractory HEAs with high ratios of melting temperature to density have been screened out. Eleven of them agree with recent experiments and the 20 quinary ones with highest melting temperatures are validated through first-principles calculations. The proposed model is complementary to the calculation of phase diagrams and ab initio methods and could serve as an effective guide for designing new HEAs.

This paper reports on the first ab initio molecular dynamics study of the ferroelectric sodium nitrite, shedding light on its ferroelctric-paraelectric phase transition. The remnant polarization $P_r$ was calculated using a Mulliken population analysis and maximally localized Wannier functions. Especially the Wannier based model is in outstanding agreement with experimental findings and previous Berry phase calculations. The simulations predict a ferroelectric Curie temperature $T_c$ between 425 and $450\mathrm{K}$ in excellent agreement with the experimental value of $437\mathrm{K}$. In addition, the anomalous lattice behavior (shrinking of the $c$ axis) during the phase transition is reproduced. Furthermore, the analysis of the phase transition revealed a combined displacive and order-disorder mechanism. The crystal field effect in the material could be quantified by investigating the molecular dipoles based on the maximally localized Wannier functions and the intermolecular charge transfer by analyzing the Mulliken charges. In agreement with earlier experimental and theoretical findings, the polarization reversal mechanism was found to be dominated by a $c$-axis rotation of the nitrite ions. The molecular insight into such a simple and prototypical material serves as a basis for a further development of more complex crystalline ferroelectrics, using a design principle inspired by ${\mathrm{NaNO}}_2$.

The formation and dynamics of a wide variety of binary two-dimensional ordered structures and superlattices are investigated through a phase field crystal model with sublattice ordering. Various types of binary ordered phases, the phase diagrams, and the grain growth dynamics and structural transformation processes, including the emergence of topological defects, are examined. The results are compared to the ordering and assembly of two-component colloidal systems. Two factors governing the binary phase ordering are identified, namely the coupling and competition between the length scales of two sublattices, and the selection of average particle densities of two components. The control and variation of these two factors lead to the prediction of various complex binary ordered patterns, with different types of sublattice ordering for integer versus noninteger ratios of sublattice length scales. These findings will enable further systematic studies of complex ordering and assembly processes of binary systems, particularly binary colloidal crystals.

Fe-Au core-shell nanoparticles embedded in an amorphous alumina matrix are synthesized at high temperature using magnetron sputtering. The nanoparticles display a single-crystal Fe core covered by a crystalline Au shell epitaxially grown on the different core facets. The morphology of the grown nanoparticles is analyzed by transmission electron microscopy, while their structural details are resolved at the atomic scale using probe-corrected high-angle annular dark-field scanning transmission microscopy. Two different epitaxial relationships at the Au/Fe interface are observed at low Au coverage, whereas only one type of interface orientation remains when the shell thickness exceeds 3 monolayers (MLs). This leads to a drastic Fe core transformation from a Wulff shaped crystal towards a nanocube. This core transformation drives a surface reconstruction resulting in a combination of open and close-packed strain free facets, offering different opportunities for molecule binding. In addition, our experiments show that the magnetic properties of the Fe core are preserved by the Au shell and that the 10 nm size of the grown nanoparticles favors a superparamagnetic behavior, suitable for biomedical applications.

The acoustic phonons in the organic-inorganic lead halide perovskites have been reported to have anomalously short lifetimes over a large part of the Brillouin zone. The resulting shortened mean free paths of the phonons have been implicated as the origin of the low thermal conductivity. We apply neutron spectroscopy to show that the same acoustic phonon energy linewidth broadening (corresponding to shortened lifetimes) occurs in the fully inorganic ${\mathrm{CsPbBr}}_3$ by comparing the results on the organic-inorganic ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbCl}}_3$. We investigate the critical dynamics near the three zone boundaries of the cubic $Pm\overline{3}m$ Brillouin zone of ${\mathrm{CsPbBr}}_3$ and find energy and momentum broadened dynamics at momentum points where the Cs-site ($A$-site) motions contribute to the cross section. Neutron diffraction is used to confirm that both the Cs and Br sites have unusually large thermal displacements with an anisotropy that mirrors the low temperature structural distortions. The presence of an organic molecule is not necessary to disrupt the low-energy acoustic phonons at momentum transfers located away from the zone center in the lead halide perovskites and such damping may be driven by the large displacements or possibly disorder on the $A$ site.

The continuous improvement of the sublimation process of SiC allows using epitaxial graphene nowadays for quantum metrology. While it is known that the interface between graphene and the SiC surface is crucial for graphene's transport properties, almost no information about the composition of the SiC substrate after the sublimation process is available. In this study we present high resolution ${\mathrm{c}}_{\text{s}}$-corrected scanning transmission electron microscopy (STEM) experiments on $6H$-SiC(0001) samples after growth of graphene. A Si deficiency within the first three SiC bilayers was found by atomically resolved energy dispersive x-ray spectroscopy (EDX). The Si concentration within the first bilayer can be reduced up to 50%. In addition, as probed by electron energy loss spectroscopy, the hybridization state of C within the first five bilayers revealed a ${\mathrm{sp}}^2$ contribution, which we refer to as the precipitation of small carbon clusters. Our analysis clearly shows that the electronic interface of epitaxial graphene on $6H$-SiC(0001) is not atomically sharp.

The study of insulator-to-metal transitions is of interest from the viewpoint of fundamental understanding of the underlying physics, and materials at the brink of such transitions possess useful functionality. Driving this transition through compositional tuning can help engineer useful material properties. Here we study the role of disorder in the form of cation off-centering on the compositionally-controlled insulator-to-metal transition in the solid solution oxide pyrochlore (${\mathrm{Pr}}_{1-x}{\mathrm{Bi}}_x{)}_2{\mathrm{Ru}}_2{\mathrm{O}}_7$. Prior work has established site disorder by the ${\mathrm{Bi}}^{3+}$ cations shifting incoherently away from their ideal crystallographic site in the Bi end-member pyrochlore as a consequence of stereochemical activity of the lone pair of electrons. However, less is known about the consequences of such off-centering in solid solutions and its role in determining the electronic ground state. Here we demonstrate through total scattering studies that even a small substitution of Bi on the pyrochlore A site leads to site disorder that enhances the average effective size of the A-site cation. This indirectly increases Ru-O-Ru covalency, which appears to play a crucial role in the crossover from insulating to metallic behavior in the solid solution. Further, density functional electronic structure calculations suggest the combination of primary and secondary (due to size) electronic effects of the lone pair-driven incoherent cation displacements drive the solid solution into a metallic state.

In this work, we present our discovery and characterization of a new kagome prototype structure, ${\mathrm{KV}}_3{\mathrm{Sb}}_5$. We also present the discovery of the isostructural compounds ${\mathrm{RbV}}_3{\mathrm{Sb}}_5$ and ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$. All materials exhibit a structurally perfect two-dimensional kagome net of vanadium. Density-functional theory calculations indicate that the materials are metallic, with the Fermi level in close proximity to several Dirac points. Powder and single-crystal syntheses are presented, with postsynthetic treatments shown to deintercalate potassium from single crystals of ${\mathrm{KV}}_3{\mathrm{Sb}}_5$. Considering the proximity to Dirac points, deintercalation provides a convenient means to tune the Fermi level. Magnetization measurements indicate that ${\mathrm{KV}}_3{\mathrm{Sb}}_5$ exhibits behavior consistent with a the Curie-Weiss model at high temperatures, although the effective moment is low $(0.22{\mu }_{\text{B}}$ per vanadium ion). An anomaly is observed in both magnetization and heat capacity measurements at 80 K, below which the moment is largely quenched. Elastic neutron scattering measurements find no obvious evidence of long-range or short-range magnetic ordering below 80 K. The possibility of an orbital-ordering event is considered. Single-crystal resistivity measurements show the effect of deintercalation on the electron transport and allow estimation of the Kadowaki-Woods ratio in ${\mathrm{KV}}_3{\mathrm{Sb}}_5$. We find that $A/{\gamma }^2\sim 61\mu \mathrm{Ohm}$ cm ${\mathrm{mol}}_{\text{FU}}^2{\mathrm{K}}^2{\mathrm{J}}^{-2}$, suggesting that correlated electron transport may be possible. ${\mathrm{KV}}_3{\mathrm{Sb}}_5$ and its cogeners ${\mathrm{RbV}}_3{\mathrm{Sb}}_5$ and ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ represent a new family of kagome metals, and our results demonstrate that they deserve further study as potential model systems.

The electronic, magnetic, transport, and magneto-optical properties of the $D{0}_3$ and amorphous $\mathrm{F}{\mathrm{e}}_3\mathrm{Si}$ are investigated by using the first-principles calculations. A strong correlation between local magnetization and atomic arrangement is established. The amorphization significantly alters the spin polarization and the magneto-optical Kerr rotation spectrum but has a little influence on anomalous Hall conductivity. Analyses in band structures and interband matrix elements provide clear insights for the understanding of these results.

The $\lambda$ phase of nitrogen was reported in 2016 and is one of more than a dozen high-pressure solid nitrogen forms that have been discovered. However, its crystal structure could not be solved unambiguously from powder diffraction alone; rather the reported structure was determined by combining experimental monoclinic lattice parameters with atomic positions from an earlier, computationally predicted structure that had similar unit cell dimensions. Here, we revisit this structure using density functional theory and higher-level fragment-based second-order Møller-Plesset perturbation theory (MP2) and coupled cluster singles, doubles, and perturbative triples [CCSD(T)]. Crystal structure prediction is performed to demonstrate that the reported $P{2}_1/c$ structure is indeed the likeliest candidate for the $\lambda$ phase. Furthermore, we provide further evidence for the structural assignment by demonstrating reasonable agreement between its predicted and experimental structural parameters and Raman spectra. Finally, the thermodynamic stability of the $\lambda$ phase relative to other phases has been uncertain, but the calculations do suggest that it may be the thermodynamically most stable phase for at least part of the pressure range over which it has been observed.

Converting solar energy into chemical energy by splitting water is a promising method to generate a sustainable and renewable solution without detrimental environmental impact. The two-dimensional semiconductors serve as potential catalysts in this regard, and here we combine Janus transition-metal dichalcogenides $(\mathrm{Mo}XY,X/Y=\mathrm{S},\mathrm{Se},\mathrm{Te})$ and graphitic carbon nitride in a van der Waals heterostructure. Within the first-principles calculations, we investigate the electronic, optical and excitonic properties that determine the photocatalytic activity. Due to the internal electric field, the photogenerated electrons and holes are separated in the $\mathrm{Mo}XY$ layers, and also generates high overpotentials for the redox reactions. The high optical absorptions span throughout the entire visible and near ultraviolet regime in these heterostructure nanocomposites. Further, the lower exciton binding, calculated within the two-dimensional hydrogenic model, indicates efficient charge separation. The maximum solar-to-hydrogen efficiency using the entire solar spectrum reaches up to 16.5% in these heterostructures. The present results indicate these heterostructures to be excellent candidate materials for photocatalytic applications and also demonstrate the enormous possibilities in two-dimensional heterostructures that should attract considerable theoretical and experimental attention in future.