Vacancy transport has a demonstrable impact on the microstructural evolution of polycrystalline metals, but existing models typically require knowledge of the stress state in order to describe lattice site generation and annihilation at interfaces. Using irreversible thermodynamics, the driving forces and equilibrium conditions dictating the response of incoherent interfaces are derived for a pure metal with vacancies under stress-free conditions. A phenomenological set of linear kinetic expressions that guarantees a decrease in the total energy upon diffusion is proposed. A near-equilibrium steady-state analytical solution for grain boundaries is obtained. In the stress-free limit, interface migration and transboundary diffusion are closely coupled, as are the production or annihilation of vacancies and the rigid-body dilation or contraction of the bulk grains. Solutions for various limiting kinetic regimes are also obtained, and the relevance of the kinetic parameters to pore nucleation at grain boundaries is discussed.

${\mathrm{Ca}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$ is a fascinating material that displays physical properties governed by spin-orbit interactions and structural distortions, showing a wide range of remarkable electronic phenomena. Here, we present a density-functional-based analysis of the interplay among degrees of freedom, such as magnetism, Coulomb repulsion (Hubbard $U$), and structural degrees of freedom, considering two exchange-correlation methods: local density approximation (LDA) and Perdew-Burke-Ernzerhof revised for solids (PBEsol). Our goal is twofold: first, to present a brief overview of the current state of the art on this compound underpinning to the last proposed theoretical models and experimental research, and second, to provide an alternative interpretation of the electronic properties compared with the previous theoretical models. Our findings show that ${\mathrm{Ca}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$ displays several electronic states (metal, semimetal, and narrow insulator) as a function of Hubbard $U$ while it exhibits structural transition depending on the functional. We disentangle the effect of the different degrees of freedom involved, clarifying the role of exchange correlation in the observed electronic and structural transitions.

We present an x-ray absorption spectroscopy study from the carbon $K$, sulfur $K$, and sulfur $L_{2,3}$ edges of crystalline oligothiophenes of varying length, i.e., bithiophene (2T), quaterthiophene (4T), and sexithiophene (6T), performed from first principles by means of all-electron density-functional theory and many-body perturbation theory. A comprehensive assignment of all relevant spectral features is performed based on the electronic structure and the character of the target conduction states. The inclusion of electron-hole effects leads to significant redistribution of oscillator strengths and to strongly bound excitons with binding energies ranging from 1.5 eV to 4.5 eV. When going from 2T to 6T, exciton binding energies decrease by up to 1 eV, which we attribute to the reduction of the average Coulomb attraction with increasing oligomer length. These high values are significantly larger than their counterparts in the optical excitations of these systems and indicative of their localization on the respective molecules. For the same reason, local-field effects which typically dominate the optical absorption of organic crystals turn out to play only a negligible role at all edges. We identify two sets of carbon atoms, i.e., with or without sulfur bonding, which exhibit distinct features at the C $K$ edge. The sulfur atoms, on the other hand, yield similar contributions in the S, $K$, and $L_{2,3}$ edge spectra. Our results show excellent agreement with available experimental data.

The optoelectronic properties of a semiconductor are determined by the combination of its elemental composition and the crystal structure. The vapor-liquid-solid nanowire growth mechanism offers the controlled epitaxy of the zinc-blende and wurtzite polytype for a number of semiconductors. Long, thin, and phase-pure wurtzite GaAs nanowires are desirable as epitaxial templates for the growth of hexagonal SiGe shells, but the growth of such nanowires remains a challenge. Here, we study the growth of wurtzite GaAs nanowires and find a diameter dependent critical length beyond which the crystal phase becomes mixed. The onset of uncontrolled polytypism is modeled with a small contribution of As diffusion during growth. Due to this increased supply of As during prolonged growth, Ga is depleted from the liquid catalyst, thereby decreasing the contact angle. We investigate possible pathways of As through diffusion on the facets and edges of the nanowire, and from the scaling of the critical length we deduce that edge diffusion has an important contribution. This study offers new insights for realizing long, phase-pure wurtzite GaAs nanowires with high aspect ratio.

Multifunctional materials with prominent properties such as electrical, ferroelectric, magnetic, optical, and magnetooptical are of keen interest to several practical implications. In the roadmap of designing such materials, in the present work, using density functional theory based first-principles calculations, we have investigated the properties of transition metals (TM)-substituted ${\mathrm{Na}}_{0.5}{\mathrm{Bi}}_{0.5}{\mathrm{TiO}}_3$ (NBT). Our calculations predict the emergence of half-metallic ferromagnetism in (TM)-substituted NBT. Nonzero magnetic moments of 1.5 and 1.9 ${\mu }_{\mathrm{B}}/\mathrm{f}.\mathrm{u}.$, respectively, are obtained for 25% Ni and Fe. Our data on optical properties for pure NBT are in excellent agreement with the available theory and experiments. For Ni/Fe-NBT, we observed a diverging nature of static dielectric constant, which could be attributed to the induced metallic character in the system. Our simulations on MOKE predict significant Kerr signals in both Ni and Fe-substituted NBT, which suggests transition metal substituted NBT as potential candidate for magnetooptic applications.

We present electronic structure calculations on layered nickelate oxyfluorides derived from the Ruddlesden-Popper aristotype structure in search of unidentified materials that may host nickelate superconductivity. By performing anion exchange of oxygen with fluorine, we create two heteroanionic ${\mathrm{La}}_2{\mathrm{NiO}}_3\mathrm{F}$ polymorphs and stabilize ${\mathrm{Ni}}^{1+}$ in four-coordinate and five-coordinate square planar and square pyramidal geometries, respectively. We further predict chemical reactions with high thermodynamic driving forces to guide their synthesis. These oxyfluorides are weakly correlated antiferromagnetic insulators, and their nonmagnetic phases exhibit quasi-two-dimensional Fermi surfaces dominated by Ni $d_{x^2-y^2}$ states, which strikingly resemble undoped cuprate superconductors. We discuss how the oxyfluoride anion chemistry promotes Ni-O covalency and single-band character that is more similar to the cuprates than homoanionic infinite-layer nickelates. We use our understanding to propose doping strategies and layered ${\mathrm{LaSrNiO}}_2{\mathrm{F}}_2$ and ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_4{\mathrm{F}}_3$ nickelate oxyfluorides with tunable electronic and magnetic structures for experimentation.

The vibrational properties of $2H\text{−}{\mathrm{TaSe}}_{2-x}{\mathrm{S}}_x$ $(0\le x\le 2)$ single crystals were probed using Raman spectroscopy and density functional theory calculations. The end members revealed two out of four symmetry-predicted Raman active modes, together with the pronounced two-phonon structure, attributable to the enhanced electron-phonon coupling. Additional peaks become observable due to crystallographic disorder for the doped samples. The evolution of the $E_{2g}^2$ mode Fano parameter reveals that the disorder has a weak impact on electron-phonon coupling, which is also supported by the persistence of two-phonon structure in doped samples. As such, this research provides thorough insights into the lattice properties, the effects of crystallographic disorder on Raman spectra, and the interplay of this disorder with the electron-phonon coupling in $2H\text{−}{\mathrm{TaSe}}_{2-x}{\mathrm{S}}_x$ compounds.

It is well established that physical aging of amorphous solids is governed by a marked change in dynamical properties as the material becomes older. Conversely, structural properties such as the radial distribution function exhibit only a very weak age dependence, usually deemed negligible with respect to the numerical noise. Here we demonstrate that the extremely weak age-dependent changes in structure are, in fact, sufficient to reliably assess the age of a glass with the support of machine learning. We employ a supervised learning method to predict the age of a glass based on the system's instantaneous radial distribution function. Specifically, we train a multilayer perceptron for a model glass former quenched to different temperatures and find that this neural network can accurately classify the age of our system across at least 4 orders of magnitude in time. Our analysis also reveals which structural features encode the most useful information. Overall, this work shows that through the aid of machine learning, a simple structure-dynamics link can, indeed, be established for physically aged glasses.

Thermoelectric materials can serve for conversion between thermal and electrical energy. In the search for thermoelectric materials, layered SnSe and ${\mathrm{SnSe}}_2$ are promising candidates. We have successfully synthesized SnSe and ${\mathrm{SnSe}}_2$ single crystals by the modified Bridgman method and studied their thermoelectric properties: thermopower $\left(S\right)$, thermal conductivity $\left(\kappa \right)$, and electrical conductivity $(\sigma )$ in the temperature range between 2 and 400 K, which are absent in the literature. The kink observed in the thermopower corresponds to the metallic-nonmetallic crossover temperature for both SnSe and ${\mathrm{SnSe}}_2$, reflecting their inherent electronic nature. Compared with ${\mathrm{SnSe}}_2$, >100 K, we find that SnSe exhibits higher electrical conductivity, higher thermopower, and lower thermal conductivity, thus resulting in the higher figure of merit. Hall effect measurements reveal that the Hall mobility in SnSe is an order higher than that in ${\mathrm{SnSe}}_2$, advancing its thermoelectric performance. These experimental results are supported by first-principles calculations, which indicate that the inequivalent Sn-Se bonding lengths help improve the figure of merit of SnSe.

High-temperature superconductivity is reported in a series of compositionally complex cuprates with varying degrees of size and spin point-defect disorder. Three compositions of Y-site alloyed ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-x}$, i.e., (5Y)BCO, were prepared via solid-state methods using different sets of rare earth ions on the Y site. Synchrotron x-ray diffraction and energy-dispersive x-ray spectroscopy confirm these samples have high phase purity and homogeneous mixing of the Y-site elements. For samples near optimal doping, electrical resistivity and magnetometry measurements reveal the superconducting transition temperature, $T_C$, is greater than 91 K for all 5Y alloying compositions. The lack of $T_C$ suppression observed in these materials, of order $1\%$ relative to pure YBCO ($T_C=93$ K), contrasts recent results on other multicomponent cuprates based on the La-214 system where superconductivity was not observed. Therefore, a key result from this work is the demonstration that high-temperature superconductivity can exist in complex ceramic compositions. The difference in superconducting behavior between these compositionally complex cuprate families is discussed in terms of impurity potentials and sample form (powder vs film). This work sets the stage for future studies to leverage the larger composition and disorder phase space of compositionally complex cuprates to isolate different types of disorder and their effect on the various electronic phases exhibited by high-$T_C$ superconductors.

We present the current-induced switching of the internal magnetization direction in a magnetic topological insulator/topological insulator heterostructure in the quantum anomalous Hall regime. The switching process is based on the bias current dependence of the coercive field, which is attributed to either Joule heating or the effect of the spin transfer provided by the unpolarized bias current. Increasing the bias current leads to a decrease in the magnetic order in the sample. When the applied current is subsequently reduced, the magnetic moments align with an externally applied magnetic field, resulting in repolarization in the opposite direction. This includes a reversal of the spin polarization and hence a reversal of the chiral edge mode. Possible applications in spintronic devices are discussed.

Innovative materials, particularly multiferroics, have the potential to transform technology. Full optimization of their usage will require an in-depth understanding of the underlying electronic and magnetic interactions. One such material is $\epsilon \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$. However, because this hard ferrimagnet with strong magnetoelectric coupling is composed of only ${\mathrm{Fe}}^{3+}$, distinguishing the role of the various sites is incredibly challenging. To overcome this challenge, ${\mathrm{Cr}}^{3+}$ ions were doped into the ${\mathrm{DO}}_{h1}$ sites of $\epsilon \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$ nanoparticles, effectively creating electron deficient defects. The $\epsilon$-(${\mathrm{Fe}}_{1-x}{\mathrm{Cr}}_x{)}_2{\mathrm{O}}_3$ ($x=0.01$ to 0.12) nanoparticles have reduced ${\mu }_0{H}_C\left(T\right)$ and $M_S\left(T\right)$ as the Cr concentration increases. The impact of Cr doping on the local electronic and magnetic structure is characterized. At 10 K, subtle changes are measured, with the ${\mathrm{T}}_d$ site electrons becoming increasingly localized as the concentration of electron-deficient ${\mathrm{DO}}_{h1}$ sites increases. Far more dramatic changes occur at 300 K, when the ${\mathrm{T}}_d$ site of the $\epsilon$-(${\mathrm{Fe}}_{1-x}{\mathrm{Cr}}_x{)}_2{\mathrm{O}}_3$ splits into two distinct local environments with ${\mathrm{Fe}}^{2+}$ and ${\mathrm{Fe}}^{4+}$ character. The nature of this splitting suggests that dynamic electron interactions play a significant role in $\epsilon \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$'s magnetic anisotropy and magnetoelectric properties.

The rectified bulk photovoltaic effect (BPVE) in noncentrosymmetric semiconductors, also called shift current, is considered promising for optoelectronic devices, terahertz emission, and possibly solar energy harvesting. A clear understanding of the shift current mechanism and search for materials with large shift current is, therefore, of immense interest. $ABC$ semiconductors $\mathrm{LiZn}X$ $(X=\mathrm{N}, \mathrm{P}, \mathrm{As}, \text{and} \mathrm{Sb})$ can be stabilized in cubic as well as hexagonal morphologies lacking inversion symmetry—an ideal platform to investigate the significant contributing factors to shift current, such as the role of structure and chemical species. Using density-functional calculations properly accounting for the electronic bandgaps, the shift current conductivities in $\mathrm{LiZn}X$ $(X=\mathrm{P}, \mathrm{As}, \mathrm{Sb})$ are found to be approximately an order of magnitude larger than the well-known counterparts and peak close to the maximum solar radiation intensity. Notably, hexagonal LiZnSb shows a peak shift current conductivity of about $-75\mu \mathrm{A}/{\mathrm{V}}^2$ and Glass coefficient of about $-20\times {10}^{-8} \mathrm{cm}/\mathrm{V}$, comparable to the highest predicted values in literature. Our comparative analysis reveals a quantitative relationship between the shift current response and the electronic polarization. These findings not only posit Li-Zn-based $ABC$ semiconductors as viable material candidates for potential applications but also elucidates key aspects of the structure-BPVE relationship.

Strong and conductive carbon nanotube films are ideal candidates for lightning-strike protection. Understanding their failure mechanisms by considering the anisotropic and single-fiber nature is essential to improve the performance. Our experimental studies show that the single-layer, nanometer-thick films fail under electrification by crack nucleation and propagation, reminiscent of brittle and ductile fracture of materials under mechanical loads. Sharp and diffuse patterns of fracture are identified in aligned and nonwoven films, respectively, signaling the strong effect of material anisotropy that is absent in common engineering materials. The fracture is driven by local Joule heating concentrated at the crack fronts instead of force-induced breakage, which is validated by experimental characterization and simulation results at both continuum and atomistic levels.