It is shown that in substitutional alloys, peculiar ordered patterns can result from neighborhood-dependent diffusion activation barriers even when there are no metastable ordered phases. Lattice gases with pure phase separation character are shown to exhibit transient ordered structures that can be retained almost indefinitely, although these structures are not at thermodynamic equilibrium. It is shown that such structures can come about relatively easily by quenching from the high-temperature configurationally random solid solution.

Understanding the structural origin of the functionality in cuprates has attracted tremendous attention over several decades. In particular, probing distortions in the Cu-O bonding is of great importance for exploring the coupling between the charge and the lattice, a key mechanism for superconductivity and other functionality in correlated materials. Here we study a superlattice modulation in the ${\mathrm{Tb}}_2{\mathrm{Cu}}_{0.83}{\mathrm{Pd}}_{0.17}{\mathrm{O}}_4$ “214” material, which possesses no superconductivity itself but has the parent structure of the $R_2{\mathrm{CuO}}_4$ (R = a rare-earth element) superconducting cuprate group. Using transmission electron microscopy (TEM), we find that this superlattice modulation is formed by Cu ion displacements in a direction perpendicular to the Cu-O planes. The superlattice modulation undergoes a reversible electronic smectic-nematic phase transition under electron-beam illumination. With the help of in situ TEM results, our findings imply that the superlattice modulation in this material arises from spatially modulated charge ordering at the Cu sites.

The electronic properties of ${\mathrm{CaCuO}}_2$/${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_3$ (LSMO) superlattices are determined by the electronic structure of the structural units and in particular their interfaces. The electronic structure of LSMO is governed by a metal-insulator transition, which is controlled by the thickness of the units and the sample temperature, resulting in a systematic downward band shift for metallic samples (i.e., thick LSMO units, low temperature). We present a systematic study of the changes in the valence-band structure and screening features in Mn $2p$ and Cu $2p$ core-level spectra. The results show that hybridization of Cu $3d$ orbitals with out-of-plane O $2p$ orbitals can be systematically tuned by controlling the band alignment at the interface via the metal-to-insulator transition of the LSMO units. This opens a new route to rational design of functional interfaces and control of orbital reconstructions.

We investigate the solidification pathways of a population of icosahedral quasicrystals in a liquid through in situ synchrotron x-ray tomography. The wealth of three-dimensional space- and time-resolved data enables us to test the predictions of various models and theories of crystallization on a quasi-crystal-forming alloy. Remarkably, we find the general evolution equation—that accounts for the competing effects of growth, coalescence, and coarsening—fits well our experimental data on surface area concentration. Furthermore, we quantify the orientation selection, screening length, and coarsening rate of the dodecahedra, and compare these results to that of periodic crystals. The latter is a full order of magnitude smaller than that of elemental metal dendrites in the limit of zero volume fraction, a reflection of the low solid-liquid interfacial energy of the icosahedral phase. Our paper provides the critical input data for microstructural models used for integrated computational materials engineering of complex intermetallics, including quasicrystals.

Pressure-induced phase transition in the technologically important tetragonal phase of ${\mathrm{PbTiO}}_3$ has been quite controversial with two entirely different propositions: (1) morphotropic phase boundary-like structural transition with concomitant rotation of the ferroelectric polarization vector and (2) antiferrodistortive (AFD) phase transition followed by emergence of a reentrant ferroelectric phase. We have attempted to address these controversies by enhancing the AFD instability of ${\mathrm{PbTiO}}_3$ through 50% ${\mathrm{BiFeO}}_3$ substitution in a tetragonal composition of the $\left(1\text{−}x\right)\mathrm{PbTi}{\mathrm{O}}_3\text{−}x\mathrm{BiFe}{\mathrm{O}}_3$ solid solution system. Using a high resolution synchrotron x-ray diffraction study of the pressure-induced phase transition in this composition, we present here experimental evidence for the emergence of superlattice reflections at a moderate pressure ${\mathrm{p}}_{\mathrm{c}1}\sim 2.15\mathrm{GPa}$ due to an AFD transition leading to a monoclinic phase in the Cc space group, which permits the rotation of the ferroelectric polarization vector. We also present evidence for a reentrant ferroelectric phase above ${\mathrm{p}}_{\mathrm{c}2}\sim 7\mathrm{GPa}$ in which octahedral tilting provides an efficient mechanism for accommodating volume reduction. The implications of these findings in resolving the existing controversies in ${\mathrm{PbTiO}}_3$ and in providing insight for designing environmentally friendly Pb-free piezoelectric compositions are also discussed.

Molybdenum disulfide nanoribbons with zigzag edges show ferromagnetic and metallic properties based on previous ab initio calculations. The investigation of the role of disorder on the magnetic properties is, however, still lacking due to the computational costs of these methods. In this work we fill this gap by studying the magnetic and electronic properties of several-nanometer-long ${\mathrm{MoS}}_2$ zigzag nanoribbons using tight-binding and Hubbard Hamiltonians. Our results reveal that proper tight-binding parameters for the edge atoms are crucial to obtain quantitatively the metallic states and the magnetic properties of ${\mathrm{MoS}}_2$ nanoribbons. With the help of the fine-tuned parameters, we perform large-scale calculations and predict the spin domain-wall energy along the edges, which is found to be significantly lower compared to that of the zigzag graphene nanoribbons. The tight-binding approach allows us to address the effect of edge disorder on the magnetic properties. Our results open the way for investigating electron-electron effects in realistic-size nanoribbon devices in ${\mathrm{MoS}}_2$ and also provide valuable information for spintronic applications.

We report the anisotropic magnetic properties of single crystals of the triangular lattice magnet $\mathrm{NaBaYb}{\left(\mathrm{B}{\mathrm{O}}_3\right)}_2$. In this material, a layered ${\left[\mathrm{Yb}{\left(\mathrm{B}{\mathrm{O}}_3\right)}_2\right]}_{\infty }$ framework is sandwiched by $\mathrm{N}{\mathrm{a}}^{+}$ or $\mathrm{B}{\mathrm{a}}^{2+}$ layers in alternation. The space group is centrosymmetric, $R\overline{3}m$, with no detectable disorder and $\mathrm{Y}{\mathrm{b}}^{3+}$-based easy axis Kramers doublets forming layered triangular lattice planes. A specific-heat anomaly indicates a second-order phase transition at $T=0.41\left(2\right)\mathrm{K}$ that is suppressed to less than 0.15 K in a 0.1-T magnetic field. With a change in entropy of only 0.6(2)% of $Rln2$ at this transition, $\mathrm{NaBaYb}{\left(\mathrm{B}{\mathrm{O}}_3\right)}_2$, however, retains 94(1)% of $Rln2$ entropy to the lowest temperature accessed ($T=0.15\mathrm{K}$). Strong frustration and the potential for quantum magnetism are implied.

In this theoretical study we scrutinized how the interaction between a magnetic cluster made of three Fe atoms and a ${\mathrm{MoS}}_2$ monolayer adsorbed on Ir(111) reciprocally modifies their structural, electronic, and magnetic properties. From the structural point of view, the sulfur atom directly coordinated by the Fe cluster is pulled out of ${\mathrm{MoS}}_2$ on Ir(111) while this behavior is not present for a freestanding ${\mathrm{MoS}}_2$ single layer. Interestingly, this observation implies that Ir(111) catalyzes such a structural change in ${\mathrm{MoS}}_2$ upon the magnetic cluster adsorption. Besides this, from the magnetic point of view, the sulfur atom lifted from the transition metal dichalcogenide (TMD) monolayer acquires a small magnetic moment and forms together with the Fe atoms a reactive local heterogeneous chalcogene–transition metal magnetic unit that can be employed to functionalize TMD-based hybrid systems.

Using a combination of density functional theory and in situ x-ray photoelectron spectroscopy, we study the atomic and electronic structure of the Eu/EuO interface. Calculations predict that electrons transfer from Eu metal into EuO and induce an unexpected downward band bending at the interface. Accounting for spectral broadening and attenuation of the signal from subsurface layers, the calculated layer-resolved total density of states agrees well with experimental x-ray photoelectron spectroscopy in the valence-band region. The total $4f$ spectrum contains contributions from both Eu and EuO, with the latter component significantly broadened as a result of band bending. This bending and charge transfer originate from Eu Fermi-level pinning at the EuO charge neutrality level, which has been suggested to be located above the conduction-band bottom [Phys. Rev. B 86, 205310 (2012)].

${\mathrm{VO}}_2$ attracts much attention due to its metal-insulator transition. Alloying ${\mathrm{VO}}_2$ with MgO and ${\mathrm{GeO}}_2$ allows the band gap and the transition temperature to be varied. We find that the spin order plays a key role in creating the band gap in the low-temperature $M_1$ phase. For MgO alloying, the alloying fraction $n\left(\mathrm{M}{\mathrm{g}}_n{\mathrm{V}}_{1-n}{\mathrm{O}}_{2-n}\right)$ is varied from 12.5 to 33.3%. The minimum band gap does not change without a structural rearrangement because both band edges of insulating ${\mathrm{VO}}_2$ consist of only V $3d$ states on sixfold-coordinated V sites. A crystal search finds that if the Mg fraction in the alloy is large enough ($>20\%$), fivefold-coordinated V sites can have lower energy than the sixfold sites, and the band gaps are doubled. For ${\mathrm{GeO}}_2$ alloying, the insulating $M_1$ structure reverts to rutile because ${\mathrm{GeO}}_2$ has a rutile phase. The result matches the experimental observation and is very important in guiding ${\mathrm{VO}}_2$'s applications such as smart coating and nonlinear resistor.

${\mathrm{SrTiO}}_3$ is an incipient ferroelectric on the verge of a polar instability, which is avoided at low temperatures by quantum fluctuations. Within this unusual quantum paraelectric phase, superconductivity persists despite extremely dilute carrier densities. Ferroelectric fluctuations have been suspected to play a role in the origin of superconductivity by contributing to electron pairing. To investigate this possibility, we used optical second harmonic generation to measure the doping and temperature dependence of the ferroelectric order parameter in compressively strained ${\mathrm{SrTiO}}_3$ thin films. At low temperatures, we uncover a spontaneous out-of-plane ferroelectric polarization with an onset that correlates perfectly with normal-state electrical resistivity anomalies. These anomalies have previously been associated with an enhancement of the superconducting critical temperature in doped ${\mathrm{SrTiO}}_3$ films, directly linking the ferroelectric and superconducting phases. We develop a long-range mean-field Ising model of the ferroelectric phase transition to interpret the data and extract the relevant energy scales in the system. Our results support a long-suspected connection between ferroelectricity and superconductivity in ${\mathrm{SrTiO}}_3$, but call into question the role played by ferroelectric fluctuations.

Using a first-principles approach, we analyze the impact of DX centers formed by S, Se, and Te dopant atoms on the thermal conductivity of GaAs. Our results are in good agreement with experiments and unveil the physics behind the drastically different effect of each kind of defect. We establish a causal chain linking the electronic structure of the dopants to the thermal conductivity of the bulk solid, a macroscopic transport coefficient. Specifically, the presence of lone pairs leads to the formation of structurally asymmetric DX centers that cause resonant scattering of incident phonons. The effect of such resonances is magnified when they affect the part of the spectrum most relevant for thermal transport. We show that, in the vibrational spectrum of the perturbed system, they take the form of modes that are localized around the defect but still extended enough to couple with incident phonons. Finally, we illustrate the connection between flat adjacent minima in the energy landscape and resonant phonon scattering through detailed analyses of the energy landscape of the defective structures.

We use density-functional theory plus dynamical mean-field theory to demonstrate the emergence of a metallic layer at the interface between the two Mott insulators ${\mathrm{LaTiO}}_3$ and ${\mathrm{LaVO}}_3$. The metallic layer is due to charge transfer across the interface, which alters the valence state of the transition-metal cations close to the interface. Somewhat counterintuitively, the charge is transferred from the Ti cations with formal $d^1$ electron configuration to the the V cations with formal $d^2$ configuration, thereby increasing the occupation difference of the $t_{2g}$ states. This can be understood as a result of a gradual transition of the charge-transfer energy, or electronegativity, across the interface. The spatial extension of the metallic layer, in particular toward the ${\mathrm{LaTiO}}_3$ side, can be controlled by epitaxial strain, with tensile strain leading to a localization within a thickness of only two unit cells. Our results open up a route for creating a tunable quasi-two-dimensional electron gas in materials with strong electronic correlations.

We study shallow and deep levels in carbon-doped hexagonal boron nitride crystals precipitated from a molten metal solution in a high-temperature furnace. Reflectance and photoluminescence under deep ultraviolet excitation are complemented by spatially resolved experiments by means of a scanning confocal micro-photoluminescence setup operating in the ultraviolet. Isotopically controlled carbon doping does not induce any energy shift of the well-known deep-level emission at 4.1 eV. Our detailed characterization in a series of carbon-doped crystals reveals that the incorporation of carbon during the growth process results in a distinct class of shallow and deep levels in hexagonal boron nitride, calling into question the exact role of carbon in the growth of hexagonal boron nitride and its direct or indirect influence on the formation of the crystal defects.

In the present study we have demonstrated epitaxial stabilization of the metastable magnetically hard $\varepsilon \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$ phase on top of a thin MgO(111) buffer layer grown onto the GaN (0001) surface. The primary purpose to introduce a 4-nm-thick buffer layer of MgO in between ${\mathrm{Fe}}_2{\mathrm{O}}_3$ and GaN was to stop thermal migration of Ga into the iron oxide layer. Though such migration and successive formation of the orthorhombic ${\mathrm{GaFeO}}_3$ was supposed earlier to be a potential trigger of the nucleation of the isostructural $\varepsilon \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$, the present work demonstrates that the growth of single crystalline uniform films of epsilon ferrite by pulsed laser deposition is possible even on the MgO capped GaN. The structural properties of the 60-nm-thick ${\mathrm{Fe}}_2{\mathrm{O}}_3$ layer on MgO/GaN were probed by electron and x-ray diffraction, both suggesting that the growth of $\varepsilon \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$ is preceded by formation of a thin layer of $\gamma \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$. The presence of the magnetically hard epsilon ferrite was independently confirmed by temperature dependent magnetometry measurements. The depth-resolved x-ray and polarized neutron reflectometry reveal that the 10 nm iron oxide layer at the interface has a lower density and a higher magnetization than the main volume of the $\varepsilon \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$ film. The density and magnetic moment depth profiles derived from fitting the reflectometry data are in a good agreement with the presence of the magnetically degraded $\gamma \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$ transition layer between MgO and $\varepsilon \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$. The natural occurrence of the interface between magnetoelectric $\varepsilon$- and spin caloritronic $\gamma$-iron oxide phases can enable further opportunities to design novel all-oxide-on-semiconductor devices.

This work presents a study of the oxyborate ${\mathrm{Ni}}_5\mathrm{Ti}{\left({\mathrm{O}}_2{\mathrm{BO}}_3\right)}_2$ using single-crystal x-ray diffraction and magnetic and specific-heat measurements. This material has in its crystalline structure subunits in the form of three-legged ladders. Despite the magnetic ions occupying these low-dimensional structures, we find that this system has long-range magnetic order at low temperatures. The analysis of the magnetization and specific-heat data shows that the process of establishing this order is complex and occurs in several stages upon lowering the temperature. Initially at 92 K a partial magnetic ordering of the Ni moments in one of the two ladders sets in. As the system is further cooled, weakly coupled clusters of spins order, and, finally, at 78 K the whole system of spins in the two ladders are magnetically ordered. Since the Ti ions are found in the nonmagnetic valence state ${\mathrm{Ti}}^{4+}$, the magnetic order is exclusively due to the ${\mathrm{Ni}}^{2+}$ ions with spin $S=1$. We discuss the ferrimagnetic nature of the long-range order, which is consistent with our knowledge of the magnetic interactions, the atomic distances, and bond angles obtained from the x-ray data.

The Pd- and Pt-based $AB{\mathrm{O}}_2$ delafossites are a unique class of layered, triangular oxides with two-dimensional electronic structure and a large conductivity that rivals the noble metals. Here, we report successful growth of the metallic delafossite $\mathrm{PdCo}{\mathrm{O}}_2$ by molecular beam epitaxy (MBE). The key challenge is controlling the oxidation of Pd in the MBE environment where phase segregation is driven by the reduction of $\mathrm{PdCo}{\mathrm{O}}_2$ to cobalt oxide and metallic palladium. This is overcome by combining low-temperature (300 °C) atomic layer-by-layer MBE growth in the presence of reactive atomic oxygen with a postgrowth high-temperature anneal. Thickness dependence (5–265 nm) reveals that in the thin regime (<75 nm), the resistivity scales inversely with thickness, likely dominated by surface scattering; for thicker films, the resistivity approaches the values reported for the best bulk crystals at room temperature, but the low-temperature resistivity is limited by structural twins. This work shows that the combination of MBE growth and a postgrowth anneal provides a route to creating high-quality films in this interesting family of layered, triangular oxides.

The varying degrees of short- and long-range order exhibited by multicomponent solids complicates a first-principles calculation of nondilute diffusion coefficients. Temporal and spatial variations in the local degree of order affect the migration barriers of individual hops and can result in strong correlations between successive hops that ultimately affect macroscopic transport coefficients. Here we report on a first-principles study of diffusion in Ni-rich Ni-Al alloys. We used cluster expansion Hamiltonians to describe the energies of the end states and migration barriers of each hop. Kinetic Monte Carlo simulations were performed to calculate macroscopic transport coefficients. Variations in the degree of ordering are shown to play a significant role in affecting diffusion coefficients. While Al has a higher mobility than Ni in the disordered Ni-rich fcc solid solution, it becomes significantly less mobile in the ordered $L{1}_2{\gamma }^{\prime }$ phase due to strong thermodynamic tendencies that keep Al trapped to disconnected sublattice sites.