The superconducting properties of high-$T_c$ materials are functions of the concentration of charge carriers, which is controlled by the concentration of defects including heterovalent cations, interstitial oxygen ions, and oxygen vacancies. Here we combine low-temperature thermal treatment of ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ epitaxial thin films and confocal Raman spectroscopy to control and investigate oxygen vacancies. We demonstrate that the apical site is the most favorable position to accommodate oxygen vacancies under low-temperature annealing conditions. Additionally we show that in high-quality films of overdoped ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$, oxygen vacancies strongly deform the oxygen environment around the copper ions. This observation is consistent with previous defect-chemical studies, and calls for further investigation of the defect-induced properties in the overdoped regime of the hole-doped lanthanum cuprates.

Crack nucleation, in which a crack is propagated via the concentration of stress at its tip, is a ubiquitous phenomenon. Here, we show via simulations and theory that in systems such as fiber networks that are below the point of mechanical stability, continuous nonlinear alignments lead to a steady state in which new load-bearing fiber chains emerge to replace those lost to fracture, preventing stress concentration and leading to the accumulation of distributed damage over a divergent length scale. In contrast to linear models that display diverging length scales at a critical point, this phenomenon occurs over a large parameter range, and is expected to be observed in biopolymer networks and porous artificial materials. This mixture of fiber alignment and fracture leads to massively greater energy dissipation and to fracture avalanche statistics distinct from those present in linear models.

Oxide heterostructures offer a pathway to control emergent phases in complex oxides, but their creation often leads to boundaries that have a polar discontinuity. In order to fabricate atomic-scale arrangements of dissimilar materials, we need a clear understanding of the pathways by which materials resolve polarity issues. By examining the real-time lattice structure in situ during growth for the case of polar ${\mathrm{LaNiO}}_3$ synthesized on nonpolar ${\mathrm{SrTiO}}_3$ (001), we demonstrate how films in ultrathin limit form as ${\mathrm{LaNiO}}_{2.5}$ and then evolve into ${\mathrm{LaNiO}}_3$ as the thickness increases. Theory explains how the polar energetics drives the formation of oxygen vacancies and the stability of these phases with thickness and structure.

The properties of carbon-vacancy clusters were studied on the basis of mechanical relaxation experiments in Fe-C alloys with a variety of carbon concentrations, providing at the same time the conditions for vacancies to be out of equilibrium. The observed relaxation peaks were assigned to the dissolution of different carbon-vacancy clusters, according to their activation energies and relaxation strengths, which are found to be in the correct energy range according to density functional calculations available from the literature.

The structural phase transitions of $M{\mathrm{F}}_3$ ($M$ = Al, Cr, V, Fe, Ti, Sc) metal trifluorides are studied within a simple Landau theory consisting of tilts of rigid $M{\mathrm{F}}_6$ octahedra associated with soft antiferrodistortive optic modes that are coupled to long-wavelength strain generating acoustic phonons. We calculate the temperature and pressure dependence of several quantities such as the spontaneous distortions, volume expansion, and shear strains as well as $T-P$ phase diagrams. By contrasting our model to experiments we quantify the deviations from mean-field behavior and find that the tilt fluctuations of the $M{\mathrm{F}}_6$ octahedra increase with metal cation size. We apply our model to predict giant barocaloric effects in Sc-substituted ${\mathrm{TiF}}_3$ of up to about $15{\mathrm{J}\mathrm{K}}^{-1}{\mathrm{kg}}^{-1}$ for modest hydrostatic compressions of $0.2\mathrm{GPa}$. The effect extends over a wide temperature range of over $140\mathrm{K}$ (including room temperature) due to a large predicted rate, $d{T}_c/dP=723\mathrm{K}{\mathrm{GPa}}^{-1}$, which exceeds those of typical barocaloric materials. Our results suggest that open lattice frameworks such as the trifluorides are an attractive platform to search for giant barocaloric effects.

The interaction of metallic plasmonic nanostructures and organic semiconductor thin films plays a crucial role in engineering light harvesting and energy transfer processes, e.g., for optoelectronic applications. Plasmonic resonances of the metal structures can be used to increase the light emission or absorption of organic molecules. Here small molecules are employed since they can form organic layers with a defined crystalline order and orientation of the transition dipole. Extinction measurements combined with numerical simulations of a hybrid system consisting of a gold nanowire grating and a thin film of diindenoperylene (DIP) are reported. The experimental results are compared to the simulations and indicate an enhanced absorption in the wavelength region corresponding to the transition from the highest occupied molecular orbital to the lowest unoccupied molecular orbital of DIP. This enhancement is found to be related to the localized field enhancement near the individual nanostructures as well as to grating-induced effects. Notably, the hybrid system also exhibits parallel lattice resonances, which have recently been discussed for two-dimensional (2D) gold nanostructure arrays. In this study a hybrid plasmonic-organic small molecule system exhibiting these modes is investigated. The results for this model system show a way to modify the optical properties of plasmonic nanostructures by collective effects to achieve stronger light-matter interaction in a wide range of hybrid plasmonic systems.

The different but related chemical behaviors of Pb(II) oxides compared to Sn(II) oxides, and the existence of known alkali/alkali-earth metal Sn(II) ternary phases, suggest that there should be additional ternary Pb(II) oxide phases. Here, we report structure searches on the ternary alkaline-earth metal Pb(II) oxides leading to four new phases. These are two ternary Pb(II) oxides, ${\mathrm{SrPb}}_2{\mathrm{O}}_3$ and ${\mathrm{BaPb}}_2{\mathrm{O}}_3$, which have larger chemical potential stability ranges compared with the corresponding Sn(II) oxides, and additionally two other ternary Pb(II) oxides, ${\mathrm{CaPb}}_2{\mathrm{O}}_3$ and ${\mathrm{BaPbO}}_2$, for which there are no corresponding Sn(II) oxides. Those Pb(II) oxides are stabilized by Pb-rich conditions. These structures follow the Zintl behavior and consist of basic structural motifs of ${\left({\mathrm{PbO}}_3\right)}^{4-}$ anionic units separated and stabilized by the alkaline-earth metal ions. They show wide band gaps ranging from 2.86 to 3.12 eV, and two compounds $({\mathrm{CaPb}}_2{\mathrm{O}}_3$ and ${\mathrm{SrPb}}_2{\mathrm{O}}_3)$ show rather light hole effective masses (around $2m_0)$. The valence band maxima of these compounds have a Pb-$6s$/O-$2p$ antibonding character, which may lead to $p$-type defect (or doping) tolerant behavior. This suggests alkaline-earth metal Pb(II) oxides may be potential $p$-type transparent conducting oxides.

Theory and stochastic lattice-gas modeling is developed for the formation of intercalated metal islands in the gallery between the top layer and the underlying layer at the surface of layered materials. Our model for this process involves deposition of atoms, some fraction of which then enter the gallery through well-separated pointlike defects in the top layer. Subsequently, these atoms diffuse within the subsurface gallery leading to nucleation and growth of intercalated islands nearby the defect point source. For the case of a single point defect, continuum diffusion equation analysis provides insight into the nucleation kinetics. However, complementary tailored lattice-gas modeling produces a more comprehensive and quantitative characterization. We analyze the large spread in nucleation times and positions relative to the defect for the first nucleated island. We also consider the formation of subsequent islands and the evolution of island growth shapes. The shapes reflect in part our natural adoption of a hexagonal close-packed island structure. Motivation and support for the model is provided by scanning tunneling microscopy observations of the formation of intercalated metal islands in highly-ordered pyrolytic graphite at higher temperatures.

We report thermoelectric (TE) properties of topological surface Dirac states (TSDS) in three-dimensional topological insulators (3D-TIs) purely isolated from the bulk by employing single-crystal $\mathrm{B}{\mathrm{i}}_{2-x}\mathrm{S}{\mathrm{b}}_x\mathrm{T}{\mathrm{e}}_{3-y}\mathrm{S}{\mathrm{e}}_y$ films epitaxially grown in the ultrathin limit. Two intrinsic nontrivial topological surface states, a metallic TSDS (m-TSDS) and a gap-opened semiconducting topological state (g-TSDS), are successfully observed by electrical transport, and important TE parameters [electrical conductivity (σ), thermal conductivity (κ), and thermopower ($S$)] are accurately determined. Pure m-TSDS gives $S=-44\mu \mathrm{V}{\mathrm{K}}^{-1}$, which is an order of magnitude higher than those of the conventional metals and the value is enhanced to $-212\mu \mathrm{V}{\mathrm{K}}^{-1}$ for g-TSDS. It is clearly shown that the semiclassical Boltzmann transport equation (SBTE) in the framework of constant relaxation time (τ) most frequently used for conventional analysis cannot be valid in 3D-TIs and strong energy dependent relaxation time τ($E$) beyond the Born approximation is essential for making intrinsic interpretations. Although σ is protected on the m-TSDS, κ is greatly influenced by the disorder on the topological surface, giving a dissimilar effect between topologically protected electronic conduction and phonon transport.

Ab initio density-functional theory simulations of scandium and aluminum nitride (${\mathrm{Al}}_{1-x}{\mathrm{Sc}}_x\mathrm{N}$) alloys have been carried out to investigate the impact of alloy configuration on their piezoelectric activity. The $d_{33,f}$ piezoelectric strain coefficient has been calculated for six ordered III-V configurations for ${\mathrm{Al}}_{0.5}{\mathrm{Sc}}_{0.5}\mathrm{N}$ alloys—including chalcopyritelike and CuPt-like structures. It varies from 9.3 to $41.4\mathrm{pm}{\mathrm{V}}^{-1}$, while its value for a randomized configuration is 28.3 $\mathrm{pm}{\mathrm{V}}^{-1}$. The alloy configurations have an impact on the degree of competition between near-neighbor coordination of the parent compounds (AlN, ScN) which is known to be responsible for local structural instabilities. This in turn affects (i) the strain sensitivity along the $c$ axis of the wurtzite internal parameter $u$ for Al and Sc sites and (ii) the stiffness of the material, both identified as key contributors to $d_{33,f}$. These findings suggest that spontaneous ordering of the alloy during film deposition or post-treatments should be considered as a way to optimize the piezoelectric activity of ${\mathrm{Al}}_{1-x}{\mathrm{Sc}}_x\mathrm{N}$ alloys.

Investigating the unknown structure of the complex cubic phase, previously observed to crystallize from melt-spun amorphous Al–10 at.% Sm alloy, we determine the structure in full site-occupancy detail, highlighting several critical structural features that govern the far-from-equilibrium phase selection pathway. Using an efficient genetic algorithm combining molecular dynamics, density functional theory, and x-ray diffraction, the structure is clearly identified as body-centered cubic $Im\overline{3}m$ (No. 229) with ∼140 atoms per cubic unit cell and a lattice parameter of 1.4 nm. The complex structure is further refined to elucidate the detailed site occupancy, revealing full Sm occupancy on 6b sites and split Sm/Al occupancy on 16f sites. Based on the refined site occupancy associated with the experimentally observed phase, we term this phase $\varepsilon -\mathrm{A}{\mathrm{l}}_{60}\mathrm{S}{\mathrm{m}}_{11}$(bcc), corresponding to the limiting situation when all 16f sites are occupied by Sm. However, it should be recognized that the range of solubility enabled by split occupancy at Sm sites is an important feature in phase selection under experimental conditions, permitting an avenue for transition with little or no chemical partitioning. Our analysis shows that the $\varepsilon -\mathrm{A}{\mathrm{l}}_{60}\mathrm{S}{\mathrm{m}}_{11}$(bcc) exhibits a “3-6-6-1” first-shell packing around Sm centers on 16f sites, the same dominant motif exhibited by the undercooled liquid. The coincident motif supports the notion that liquid/glass ordering at high undercooling may give rise to topological invariants between the noncrystalline and crystalline states that provide kinetic pathways to metastable phases that are not accessible during near-equilibrium processing.

Electronic structure calculations are used to examine the magnetic properties of ${\mathrm{Fe}}_2\mathrm{P}$-based alloys and the mechanisms through which the Curie temperature and magnetocrystalline anisotropy can be optimized for specific applications. It is found that at elevated temperatures the magnetic interaction in pure ${\mathrm{Fe}}_2\mathrm{P}$ develops a pronounced two-dimensional character due to the suppression of the magnetization in one of the sublattices, but the interlayer coupling is very sensitive to band filling and structural distortions. This feature suggests a natural explanation for the observed sharp enhancement of the Curie temperature by alloying with multiple elements, such as Co, Ni, Si, and B. The magnetocrystalline anisotropy is also tunable by electron doping, reaching a maximum near the electron count of pure ${\mathrm{Fe}}_2\mathrm{P}$. These findings enable the optimization of the alloy content, suggesting coalloying of ${\mathrm{Fe}}_2\mathrm{P}$ with Co (or Ni) and Si as a strategy for maximizing the magnetocrystalline anisotropy at and above room temperature.

Dirac points in two-dimensional (2D) materials have been a fascinating subject of research, with graphene as the most prominent example. However, the Dirac points in existing 2D materials, including graphene, are vulnerable against spin-orbit coupling (SOC). Here, based on first-principles calculations and theoretical analysis, we propose a new family of stable 2D materials, the HfGeTe-family monolayers, which host so-called spin-orbit Dirac points (SDPs) close to the Fermi level. These Dirac points are special in that they are formed only under significant SOC, hence they are intrinsically robust against SOC. We show that the existence of a pair of SDPs are dictated by the nonsymmorphic space group symmetry of the system, which are very robust under various types of lattice strains. The energy, the dispersion, and the valley occupation around the Dirac points can be effectively tuned by strain. We construct a low-energy effective model to characterize the Dirac fermions around the SDPs. Furthermore, we find that the material is simultaneously a 2D ${\mathbb{Z}}_2$ topological metal, which possesses nontrivial ${\mathbb{Z}}_2$ invariant in the bulk and spin-helical edge states on the boundary. From the calculated exfoliation energies and mechanical properties, we show that these materials can be readily obtained in experiment from the existing bulk materials. Our result reveals HfGeTe-family monolayers as a promising platform for exploring spin-orbit Dirac fermions and topological phases in two-dimensions.

The optical properties of congruent lithium niobate are analyzed from first principles. The dielectric function of the material is calculated within time-dependent density-functional theory. The effects of isolated intrinsic defects and defect pairs, including the ${{\mathrm{Nb}}_{\mathrm{Li}}}^{4+}$ antisite and the ${{\mathrm{Nb}}_{\mathrm{Li}}}^{4+}-{{\mathrm{Nb}}_{\mathrm{Nb}}}^{4+}$ pair, commonly addressed as a bound polaron and bipolaron, respectively, are discussed in detail. In addition, we present further possible realizations of polaronic and bipolaronic systems. The absorption feature around 1.64 eV, ascribed to small bound polarons [O. F. Schirmer et al., J. Phys. Condens. Matter 21, 123201 (2009)], is nicely reproduced within these models. Among the investigated defects, we find that the presence of bipolarons at bound interstitial-vacancy pairs ${\mathrm{Nb}}_{\mathrm{V}}-{\mathrm{V}}_{\mathrm{Li}}$ can best explain the experimentally observed broad absorption band at 2.5 eV. Our results provide a microscopic model for the observed optical spectra and suggest that, besides ${\mathrm{Nb}}_{\mathrm{Li}}$ antisites and Nb and Li vacancies, Nb interstitials are also formed in congruent lithium-niobate samples.

Using inelastic neutron-scattering spectroscopy, we have identified twin-split scattering spectra in hydrogen-terminated nanocrystalline silicon. We show that this duality originates from the cooperative motion of hydrogen and silicon. Our formalism for the inelastic neutron-scattering spectrum, which is derived from the Hermite orthogonal wave functions in terms of the normal coordinates, elucidates the physical origin of the observed quantum twin spectra and predicts the possible occurrence of this behavior in other materials such as metal hydrides.

Hierarchical self-assembly consisting of local associations of simple building blocks for the formation of complex structures widely exists in nature, while the essential role of local assembly remains unknown. In this work, by using computer simulations, we study a simple model system consisting of associating colloidal hemispheres crystallizing into face-centered-cubic crystals comprised of spherical dimers of hemispheres, focusing on the effect of dimer formation on the hierarchical crystallization. We found that besides assisting the crystal nucleation because of increasing the symmetry of building blocks, the association between hemispheres can also induce both reentrant melting and reentrant crystallization depending on the range of interaction. Especially when the interaction is highly sticky, we observe a novel reentrant crystallization of identical crystals, which melt only in a certain temperature range. This offers another axis in fabricating responsive crystalline materials by tuning the fluctuation of local association.

We study the temperature dependence of the Rashba-split bands in the bismuth tellurohalides $\mathrm{BiTe}X$ $(X=\text{I},\text{Br},\text{Cl})$ from first principles. We find that increasing temperature reduces the Rashba splitting, with the largest effect observed in BiTeI with a reduction of the Rashba parameter of 40% when temperature increases from 0 to 300 K. These results highlight the inadequacy of previous interpretations of the observed Rashba splitting in terms of static-lattice calculations alone. Notably, we find the opposite trend, a strengthening of the Rashba splitting with rising temperature, in the pressure-stabilized topological-insulator phase of BiTeI. We propose that the opposite trends with temperature on either side of the topological phase transition could be an experimental signature for identifying it. The predicted temperature dependence is consistent with optical conductivity measurements, and should also be observable using photoemission spectroscopy, which could provide further insights into the nature of spin splitting and topology in the bismuth tellurohalides.

The synthesis and characterization of Li-doped boron nitride is reported where the sample is in the dilute limit with a stoichiometry of ${\mathrm{Li}}_{0.01}({\mathrm{BN})}_3$. The diffusion of atomic Li dominates above 150 K, with ${\mathrm{D}}_{\mathrm{Li}}\sim {10}^{-10}{\mathrm{cm}}^2{\mathrm{s}}^{-1}$, where the Li diffusion rate increases with temperature resulting in activated behavior with an energy scale of 27 meV. Below 150 K the Li diffusion freezes out and the sample enters a magnetic state at approximately 70 K that shows evidence for being glassy in nature. It is believed that this is due to the freezing of Li atoms within the lattice which then involves partial electron injection into the BN layers, and this provides a mechanism for magnetic exchange, resulting in a divergence of the magnetic susceptibility. Our work shows the promise of this material for future study, where there is currently much interest in Li base compounds for energy storage.

Under the generic term “Stranski-Krastanov” are grouped two different growth mechanisms of SiGe quantum dots. They result from the self-organized Asaro-Tiller-Grinfel'd (ATG) instability at low strain, while at high strain, from a stochastic nucleation. While these regimes are well known, we elucidate here the origin of the transition between these two pathways thanks to a joint theoretical and experimental work. Nucleation is described within the master equation framework. By comparing the time scales for ATG instability development and three-dimensional (3D) nucleation onset, we demonstrate that the transition between these two regimes is simply explained by the crossover between their divergent evolutions. Nucleation exhibits a strong exponential deviation at low strain while ATG behaves only algebraically. The associated time scale varies with $exp(1/x^4)$ for nucleation, while it only behaves as $1/x^8$ for the ATG instability. Consequently, at high (low) strain, nucleation (instability) occurs faster and inhibits the alternate evolution. It is then this different kinetic evolution which explains the transition from one regime to the other. Such a kinetic view of the transition between these two 3D growth regimes was not provided before. The crossover between nucleation and ATG instability is found to occur both experimentally and theoretically at a Ge composition around 50% in the experimental conditions used here. Varying the experimental conditions and/or the system parameters does not allow us to suppress the transition. This means that the SiGe quantum dots always grow via ATG instability at low strain and nucleation at high strain. This result is important for the self-organization of quantum dots.