A combination of scanning tunneling microscopy, low-energy electron diffraction,and low-energy electron microscopy (LEEM) has been used to identify the structural phases formed by 7,7,8,8-tetracyanoquinodimethane (TCNQ) on Ag(111). These comprise a two-dimensional gas phase, a low-density commensurate (LDC) phase, and a higher-density incommensurate (HDI) phase. LEEM also shows the presence of an additional “precursor-HDI” phase with a surface unit mesh area only ≈3% less than the HDI phase. Normal incidence x-ray standing-wave measurements of the HDI phase yield almost identical structural parameters to the LDC phase for which a full structure determination has been previously reported. The results show TCNQ does not adopt the inverted bowl distortion favored in earlier density functional theory calculations of TCNQ on coinage metal surfaces, but the N atoms are twisted out of the molecular plane, an effect found for the LDC phase to be due to incorporation of Ag adatoms. The possible role of Ag adatoms in the HDI phase, and in the transition from the precursor-HDI phase, is discussed.

The Kanzaki forces and their associated multipolar moments are standard ways of representing point defects in an atomistically informed way in the continuum. In this article, the Kanzaki force approach is extended to other crystalline defects. The article shows how the resulting Kanzaki force fields are to be computed for any general extended defect by first computing the relaxed defect's structure and then defining an affine mapping between the said defect structure and the original perfect lattice. This methodology can be employed to compute the Kanzaki force field of any mass-conserving defect, including dislocations, grain and twin boundaries, or cracks. Particular focus is then placed on straight edge dislocation in face-centered cubic (fcc) and body-centered cubic (bcc) pure metals, which are studied along different crystallographic directions. The particular characteristics of these force fields are discussed, drawing a distinction between the slip Kanzaki force field associated with the Volterra disregistry that characterizes the dislocation, and the core Kanzaki force field associated with the specific topology of the dislocation's core. The resulting force fields can be employed to create elastic models of the dislocation that, unlike other regularization procedures, offer a geometrically true representation of the core and the elastic fields in its environs, capturing all three-dimensional effects associated with the core.

The kamiokite ${\mathrm{Fe}}_2{\mathrm{Mo}}_3{\mathrm{O}}_8$ is regarded as a promising material exhibiting a giant magnetoelectric (ME) effect at the relatively high temperature $T$. Here, we explore this phenomenon on the basis of first-principles electronic structure calculations. For this purpose, we construct a realistic model describing the behavior of magnetic Fe $3d$ electrons and further map it onto the isotropic spin model. Our analysis suggests two possible scenarios for ${\mathrm{Fe}}_2{\mathrm{Mo}}_3{\mathrm{O}}_8$. The first one is based on the homogeneous charge distribution of the ${\mathrm{Fe}}^{2+}$ ions among tetrahedral $\left(t\right)$ and octahedral $\left(o\right)$ sites, which tends to lower the crystallographic $P{6}_{3}mc$ symmetry through the formation of an orbitally ordered state. Nevertheless, the effect of the orbital ordering on interatomic exchange interactions does not seem to be strong, so that the magnetic properties can be described reasonably well by averaged interactions obeying the $P{6}_{3}mc$ symmetry. The second scenario, which is supported by obtained parameters of on-site Coulomb repulsion and respects the $P{6}_{3}mc$ symmetry, implies the charge disproportionation involving the somewhat exotic $1+$ ionization state of the $t$-Fe sites (and $3+$ state of the $o$-Fe sites). Somewhat surprisingly, these scenarios are practically indistinguishable from the viewpoint of exchange interactions, which are nearly identical in these two cases. However, the spin-dependent properties of the electric polarization are expected to be different due to the strong difference in the polarity of the ${\mathrm{Fe}}^{2+}\text{−}{\mathrm{Fe}}^{2+}$ and ${\mathrm{Fe}}^{1+}\text{−}{\mathrm{Fe}}^{3+}$ bonds. Our analysis uncovers the basic aspects of the ME effect in ${\mathrm{Fe}}_2{\mathrm{Mo}}_3{\mathrm{O}}_8$. Nevertheless, the quantitative description should involve other ingredients, apparently related to the lattice and orbitals degrees of freedom.

In this work, rhodamine B-doped GPTS/TEOS-derived organic/silica monolithic xerogels were prepared by the predoping sol-gel process. Thermal lens method was applied to determine thermo-optical properties of the samples. Since the samples present a high optical absorption coefficient, the high absorption theoretical TL model was introduced. Thermal diffusivities, optical absorption coefficients, and fluorescence quantum yield values were determined for samples with different rhodamine B (RhB) concentrations, from ${10}^{-5}$ to ${10}^{-2}$ M. The dye-doped organic/silica xerogels present a high fluorescence quantum yield (up to 90%), maintaining the value over 50% until a concentration of $\sim 1.6$ mM. To the best of our knowledge, RhB-doped transparent monoliths with a high fluorescence quantum yield with the highest RhB concentration reported in a solid hybrid matrix were obtained.

The role of rare-earth ($R$) ions in magnetodielectric (MD) coupling is always intriguing and markedly different for different systems. Although many reports are available concerning this aspect in frustrated $3d$-transition metal oxides, no such reports exist on higher $d$ ($4d/5d$)-orbital based systems due to the rare availability of highly insulating $4d/5d$ systems. Here, we systematically investigated the magnetic, dielectric, ferroelectric, and magnetodielectric behavior of the 6H-perovskites ${\mathrm{Ba}}_{3}R{\mathrm{Ru}}_2{\mathrm{O}}_9$ for different $R$-ions, namely $R=\mathrm{Sm},\mathrm{Tb}$, and Ho, which magnetically order at 12, 9.5, and 10.2 K, respectively. For $R=\mathrm{Tb}$ and Ho, the temperature- and magnetic-field-dependent complex dielectric constant traces the magnetic features, which manifests MD coupling in this system. A weak magnetic-field ($H$) induced transition is observed for $\sim 30\mathrm{kOe}$, which is clearly captured in $H$-dependent dielectric measurements. No MD coupling is observed for ${\mathrm{Ba}}_3{\mathrm{SmRu}}_2{\mathrm{O}}_9$. The MD coupling is enhanced by a factor of 3 and 20 times for $R=\mathrm{Tb}$ and Ho, respectively, when compared to that of the Nd-counterpart. These results evidence the gradual enhancement of MD coupling with the introduction of heavier $R$-ions in this series, which is attributed to their larger moment values. Our investigation establishes dominating $4d\left(\mathrm{Ru}\right)-4f\left(R\right)$ magnetic correlation in this series for the heavier $R$-members.

A thermodynamic model for metalorganic vapor-phase epitaxy (MOVPE) of the N-polar $\left(000\overline{1}\right)$ binary group-III nitrides (AlN, GaN, and InN) in the step-flow growth mode is proposed based on the Burton, Cabrera, and Frank (BCF) theory. The coverages of the group-III adatoms are thermodynamically evaluated under competitive adsorption with hydrogen, which is used as a carrier gas or dissociated from the ${\mathrm{NH}}_3$ source gas during MOVPE. The chemical potentials of the group-III and H adatoms on N-polar group-III nitride surfaces are modeled using the respective bond energies with the surface N atoms of the nitride and the vibrational frequencies of the adatoms. The coverages of the coadsorbed group-III and H adatoms are calculated using the Langmuir adsorption isotherm with these chemical potentials. The configuration entropy of the group-III adatoms bridges the gap between the thermodynamic model and the BCF theory. The coverage of the group-III adatoms plays a role like partial pressure of the group-III gas in the thermodynamics. The equilibrium coverage of the group-III adatoms and the equilibrium pressure of the ${\mathrm{NH}}_3$ gas are evaluated from the conditions of Gibbs energy balance between the sources (group-III adatom and ${\mathrm{NH}}_3$ gas molecule) and products (group-III nitride and $3/2{\mathrm{H}}_2$ gas molecules) and of speed balance between group-III and N incorporation into step kinks. Fair agreements with the experimentally optimized growth conditions for MOVPE of N-polar GaN and InN are obtained by this method. Among the examined binary group-III nitrides, AlN growth is hardly affected by ${\mathrm{H}}_2$ gas pressure, GaN growth is controlled well by ${\mathrm{H}}_2$ gas pressure, and InN growth is strongly inhibited by ${\mathrm{H}}_2$ gas. A criterion for selecting the ${\mathrm{NH}}_3$/group-III flow ratio for maximum products/cost and minimum waste of the materials is demonstrated using the growth model and the estimated growth parameters. The offcut angle dependence of the growth rate on the vicinal substrates is also investigated.

Self-assembly of defects in materials can create novel physical properties with potential applications in various technological fields. Here, we studied the physical mechanism of self-assembly of helium gas bubbles in molybdenum under ion implantation and unified the formation window of gas bubble/void superlattice in terms of irradiation temperatures and helium-atomic parts per million/displacements per atom damage levels. The ion fluence and temperature-dependent formation of gas bubble superlattice in molybdenum was examined via both transmission electron microscopy and synchrotron-based small-angle x-ray scattering. The formation of gas bubble superlattice is linked with specific implantation conditions, including ion fluence and implantation temperature. The bubble lattice constant increases with increasing the implantation temperature from 150 to 450 ˚C. Once the gas bubble superlattice forms, increasing fluence has no effect on the bubble lattice constant. Both experiments and atomic kinetic Monte Carlo modeling indicate a three-stage formation process of gas bubble superlattice, from random bubbles to planar ordering and then to three-dimensional superlattices, suggesting that one-dimensional diffusion of self-interstitial atoms can cause the formation of gas bubble superlattice. Our study advances the understanding of defect self-assembly in materials in nonequilibrium states and provides an approach of managing the defect formation and transforming them from a liability into an asset in a controllable way.

$\mathrm{C}{\mathrm{o}}_3\mathrm{S}{\mathrm{n}}_2{\mathrm{S}}_2$ has been attracting attention as a model quasi-2D kagome ferromagnet as well as a ferromagnetic Weyl semimetal. Here, we report the evolution of the magnetic domain within $\mathrm{C}{\mathrm{o}}_3\mathrm{S}{\mathrm{n}}_2{\mathrm{S}}_2$ as a function of temperature and external field as studied by in situ Lorentz microscopy. The domain structure typical for anisotropy-controlled material was observed, and domain wall (DW) motion was found to depend strongly on temperature. It was inactive due to strong pinning in a low-temperature range yet activated as a result of thermal depinning with increasing temperature. The activation temperature decreased as the external field increased. In the temperature range just below the Curie temperature, spontaneous domain reorganization associated with DW creep occurred even in the absence of an external field. The low-field anomalous magnetic phase just below the Curie temperature and split between zero-field-cooled and field-cooled magnetization-temperature curves found in our previous study [M. A. Kassem et al., Phys. Rev. B 96, 014429 (2017)] are explained in terms of temperature-dependent DW motion including the DW creep.

Ion intercalation was recently used to explore two-dimensional (2D) transition metal dichalcogenides (TMDs) with precise tuning of ion concentration in a field-effect-transistor configuration. However, how to systematically change the properties of 2D TMDs, e.g., superconductivity and charge density waves, by ion intercalation has not been explored. We report in this paper results of electrical transport measurements on 2D crystals of $2\text{H}\text{−}{\mathrm{TaSe}}_2$ intercalated with Li ions that is tuned continuously by ionic gating. Shubnikov–de Haas magnetoconductance oscillation and Hall coefficient measurements on crystals of $2\text{H}\text{−}{\mathrm{TaSe}}_2$ revealed an ion intercalation induced multi- to single-band change in the Fermi surface (FS) topology, deep in the charge density wave phase, resulting in a reduction of the number of independent channels for electronic conduction. A remarkable crossover from weak antilocalization to weak localization tuned by gate voltage or temperature was found and attributed to the ion intercalation induced variations in the spin-orbital coupling and electron-phonon interaction. These observations provide new insight into the enhancement of superconductivity and the suppression of charge density waves in 2D $2\text{H}\text{−}{\mathrm{TaSe}}_2$ induced by ion intercalation and demonstrate furthermore the great potential of ion intercalation for engineering electronic properties of 2D TMDs.

Entropy-stabilized oxides possess a large configurational entropy that allows for the unique ability to include typically immiscible concentrations of species in different configurations. Particularly in oxides, where the physical behavior is strongly correlated to stereochemistry and electronic structure, entropic stabilization creates a unique platform to tailor the interplay of extreme structural and chemical disorder to realize unprecedented functionalities. Here, we control stereochemically driven structural disorder in single crystalline, rocksalt, (MgCoNiCuZn)O-type entropy-stabilized oxides through the incorporation of ${\mathrm{Cu}}^{2+}$ cations. We harness the disorder to tune the degree of glassiness in the antiferromagnetic structure. Structural distortions driven by the Jahn-Teller effect lead to a difference in valence on the Co cation sites, which extends to dilution and disorder of the magnetic lattice. A spin glass model reveals that the fractional spin ordering of the magnetic lattice can be tuned by ∼65%. These findings demonstrate entropy-stabilization as a tool for control of functional phenomena.

We have studied the magnetic properties of multilayers composed of ferromagnetic metal Co and heavy metals with strong spin orbit coupling (Pt and Ir). Multilayers with symmetric (ABA stacking) and asymmetric (ABC stacking) structures are grown to study the effect of broken structural inversion symmetry. We compare the perpendicular magnetic anisotropy (PMA) energy of symmetric Pt/Co/Pt, Ir/Co/Ir multilayers and asymmetric Pt/Co/Ir, Ir/Co/Pt multilayers. First, the interface contribution to the PMA is studied using the Co layer thickness dependence of the effective PMA energy. Comparison of the interfacial PMA between the Ir/Co/Pt, Pt/Co/Ir asymmetric structures and Pt/Co/Pt, Ir/Co/Ir symmetric structures indicate that the broken structural inversion symmetry induced PMA is small compared to the overall interfacial PMA. Second, we find the magnetic anisotropy field is significantly increased in multilayers when the ferromagnetic layers are antiferromagnetically coupled via interlayer exchange coupling (IEC). Macrospin model calculations can qualitatively account for the relation between the anisotropy field and the IEC. Among the structures studied, the IEC is the largest for the asymmetric Ir/Co/Pt multilayers: the exchange coupling field exceeds 3 T and consequently, the anisotropy field approaches 10 T. Third, comparing the asymmetric Ir/Co/Pt and Pt/Co/Ir structures, we find the IEC and, to some extent, the interface PMA are stronger for the former than the latter. X-ray magnetic circular dichroism (XMCD) studies suggest that the proximity-induced magnetization in Pt is larger for the Ir/Co/Pt multilayers than the inverted structure (Pt/Co/Ir), which may partly account for the difference in the magnetic properties. These results show the intricate relation between PMA, IEC, and the proximity-induced magnetization that can be exploited to design artificial structures with unique magnetic characteristics.

This article reports on the interplay between structural properties of the interface region and emerging proton conductivity in thin films of insulating $\mathrm{BaZr}{\mathrm{O}}_3$ perovskite deposited by pulsed laser ablation onto $\mathrm{NdGa}{\mathrm{O}}_3$ wide-band-gap insulators. High-resolution transmission electron microscopy and surface x-ray diffraction reveal the presence of a large number of misfit dislocations at the interface, allowing the full relaxation of the epitaxial strain. An analysis of the x-ray diffraction patterns reveals strain relaxation that occurs over a thickness of about 3 nm, equally divided between the film and the substrate. Electrical impedance spectroscopy measurements show a sizeable proton conductance that can mainly be attributed to the defective interface. Assuming that proton conduction occurs in the interface layer, values of interface conductivity of about 0.5 S/cm at 650 °C are estimated with an activation energy of about 0.86 eV. Experimental findings are explained assuming that the defective interface layer can accommodate a very large concentration of carriers whose mobility is somewhat hindered by the high concentration of structural defects at the interface.

We investigate the variation of the band gap across the Ruddlesden-Popper (RP) series $\left({\mathrm{A}}_{n+1}{\mathrm{B}}_n{\mathrm{X}}_{3n+1}\right)$ in model chalcogenide, oxide, and halide materials to understand the factors influencing the band gap evolution with $n$. In contrast to the oxides and halides, we find the band gaps of the chalcogenides evolve differently with the thickness of the perovskite blocks in these natural superlattices. We show that octahedral rotations (i.e., deviation of the B-X-B bond angles from ${180}^{\circ })$ and quantum confinement effects compete to decide the band gap evolution of RP phases. The insights gained here will allow us to rationally design layered perovskite phases for electronics and optoelectronics.

Two centrosymmetric gallium nitride phases, one with a body-centered-tetragonal structure (bct-GaN) and the other base-centered orthorhombic ($Z$-GaN), are predicted. The inversion symmetry inherently inhibits spontaneous polarization, and more importantly, piezoelectric polarization regardless of heteroepitaxial growth direction and even in the presence of shear strain, offering alternative solutions to the polarization-induced electrostatic field problem encountered by group-III nitride-based light-emitting diodes (LEDs), beyond those promised by nonpolar and semipolar wurtzite (w-GaN) and zinc-blende GaN. Density functional theory calculations, validated by agreement with experiment for w-GaN, reveal equilibrium structures, phonon dispersions, and band structures of bulk bct-GaN and $Z$-GaN. To ensure their stability at room temperature, we first carry out an analysis of dynamical and thermal stability using phonon calculations and molecular dynamics simulations, which indicate that these centrosymmetric structures are dynamically stable and remain stable at high temperature. Moreover, the relative stability of GaN polymorphs is highly dependent on sample thickness. We predict that bct-GaN and $Z$-GaN are energetically more favorable than w-GaN for freestanding ultrathin films up to 47 and 70 layers, respectively, thus suggesting the possibility of the kinetic growth of these two phases. The $\mathit{GW}$ self-energy corrected, direct band gaps of bct-GaN and $Z$-GaN are close to the w-GaN value, promising LED applications in the same spectral ranges.

Ferroelectric nanostructures have recently attracted considerable research interests for the discovery of novel topological dipole states. However, currently the preparation of high-quality nanodot arrays remains challenging, precluding the clear characterization of the dipole states in the nanodots. Combining pulsed laser deposition method with circular anodic aluminum oxide template, we reported an unexpected growth of high crystalline quality $\mathrm{PbZ}{\mathrm{r}}_{0.52}\mathrm{T}{\mathrm{i}}_{0.48}{\mathrm{O}}_3$ nanodot array in quadrilateral shape with size being ∼80 nm. Piezoresponse force microscopy measurement on various regions of the sample shows that the nanodots form rather diverse in-plane domain patterns, with the appearance of abundant topological domain defects including flux-closure vortices, center-divergent vortices, center-convergent vortices, and antivortices. A deterministic control of the domain structure of the nanodots via applying an electric field is further achieved. The nanodots were found to favor a center-divergent vortex pattern under negative tip bias, although the domain patterns of the nanodots at the as-grown state are different. Phase field simulations were performed to reproduce the experimental observation, and our results indicate that the screening condition of the nanodots is far from the ideal ones assumed in previous theoretical modellings. The nanodots are potentially useful in developing multi-state and reconfigurable electronic devices.

Thermal properties (thermal diffusivity, heat capacity, and thermal conductivity) have been retrieved for the layered compounds $\mathrm{C}{\mathrm{u}}_{1-x}\mathrm{A}{\mathrm{g}}_x\mathrm{In}{\mathrm{P}}_2{\left(\mathrm{S},\mathrm{Se}\right)}_6, \mathrm{AgBi}{\mathrm{P}}_2{\left(\mathrm{S},\mathrm{Se}\right)}_6$, and $\mathrm{CuBi}{\mathrm{P}}_2\mathrm{S}{\mathrm{e}}_6$ in the temperature range 30–350 K, using single crystals. A complete explanation of the thermal properties as ions are substituted has been developed, showing the role played by disorder, electronic levels hybridization, ion coordination, or size on the second-order Jahn-Teller effect. The evolution of the thermal diffusivity and the thermal conductivity with temperature has shown, on the one hand, strong thermal anisotropies and, on the other hand, that heat is effectively transferred by phonons, with some of the compounds (especially those with Bi) presenting very low values due to the enhancement of phonon scattering events, expressing a strong anharmonic behavior, which is justified in detail, case by case, on the basis of interactions among optical and acoustic phonon branches as well as the presence of electron lone pairs. This broad study opens up the possibility of designing new materials for applications where a low thermal conductivity is essential.