Spin pumping (SP) and the spin Seebeck effect (SSE), two of the most common methods for generating a pure spin current from ferromagnetic insulators, are considered to share similar physical mechanisms. However, a systematic study of the fundamental difference of their working principle is missing. In this Rapid Communication, we present experimental evidence of the contrast in a pure spin current generated by SP and SSE, based on results from yttrium iron garnet (YIG) with various crystalline properties. It is shown that while the SP-induced spin current could be two-orders-of-magnitude different between the polycrystalline and epitaxial films, the SSE-excited spin current is surprisingly insensitive to the different crystal structures. Our results clearly distinguish the coherent mechanism of SP from the noncoherent mechanism of the SSE. Consequently, the robust SSE voltage against poor crystallinity proves that the SSE is a powerful tool to explore pure spin current physics, and suggests that polycrystalline YIG films are a promising candidate for spin caloritronic applications.

Electrical conductivity and high dielectric constant are in principle self-excluding, which makes the terms insulator and dielectric usually synonymous. This is certainly true when the electrical carriers are electrons, but not necessarily in a material where ions are extremely mobile, electronic conduction is negligible, and the charge transfer at the interface is immaterial. Here we demonstrate in a perovskite-derived structure containing five-coordinated Ti atoms, a colossal dielectric constant (up to ${\text{10}}^9)$ together with very high ionic conduction ${\text{10}}^{-3}\text{S}{\text{cm}}^{-1}$ at room temperature. Coupled investigations of $I\text{−}V$ and dielectric constant behavior allow us to demonstrate that, due to ion migration and accumulation, this material behaves like a giant dipole, exhibiting colossal electrical polarization (of the order of $\text{0.1}\text{C}{\text{cm}}^{-2})$. Therefore it may be considered as a “ferro-ionet” and is extremely promising in terms of applications.

Although copper and bismuth do not form any compounds at ambient conditions, two intermetallics, $\mathrm{CuBi}$ and ${\mathrm{Cu}}_{11}{\mathrm{Bi}}_7$, were recently synthesized at high pressures. Here we report on the discovery of additional copper-bismuth phases at elevated pressures with high densities from ab initio calculations. In particular, a ${\mathrm{Cu}}_2\mathrm{Bi}$ compound is found to be thermodynamically stable at pressures above 59 GPa, crystallizing in the cubic Laves structure. In strong contrast to ${\mathrm{Cu}}_{11}{\mathrm{Bi}}_7$ and CuBi, cubic ${\mathrm{Cu}}_2\mathrm{Bi}$ does not exhibit any voids or channels. Since the bismuth lone pairs in cubic ${\mathrm{Cu}}_2\mathrm{Bi}$ are stereochemically inactive, the constituent elements can be closely packed and a high density of 10.52 g/${\mathrm{cm}}^3$ at 0 GPa is achieved. The moderate electron-phonon coupling of $\lambda =0.68$ leads to a superconducting temperature of 2 K, which exceeds the values observed both in ${\mathrm{Cu}}_{11}{\mathrm{Bi}}_7$ and CuBi, as well as in elemental Cu and Bi.

Recent progress in materials informatics has opened up the possibility of a new approach to accessing properties of materials in which one assays the aggregate properties of a large set of materials within the same class in addition to a detailed investigation of each compound in that class. Here we present a large scale investigation of electronic properties and correlated magnetism in Ce-based compounds accompanied by a systematic study of the electronic structure and $4f$-hybridization function of a large body of Ce compounds. We systematically study the electronic structure and $4f$-hybridization function of a large body of Ce compounds with the goal of elucidating the nature of the $4f$ states and their interrelation with the measured Kondo energy in these compounds. The hybridization function has been analyzed for more than 350 data sets (being part of the IMS database) of cubic Ce compounds using electronic structure theory that relies on a full-potential approach. We demonstrate that the strength of the hybridization function, evaluated in this way, allows us to draw precise conclusions about the degree of localization of the $4f$ states in these compounds. The theoretical results are entirely consistent with all experimental information, relevant to the degree of $4f$ localization for all investigated materials. Furthermore, a more detailed analysis of the electronic structure and the hybridization function allows us to make precise statements about Kondo correlations in these systems. The calculated hybridization functions, together with the corresponding density of states, reproduce the expected exponential behavior of the observed Kondo temperatures and prove a consistent trend in real materials. This trend allows us to predict which systems may be correctly identified as Kondo systems. A strong anticorrelation between the size of the hybridization function and the volume of the systems has been observed. The information entropy for this set of systems is about 0.42. Our approach demonstrates the predictive power of materials informatics when a large number of materials is used to establish significant trends. This predictive power can be used to design new materials with desired properties. The applicability of this approach for other correlated electron systems is discussed.

We report a combination of x-ray and neutron diffraction studies, Mössbauer spectroscopy, and muon spin relaxation (${\mu }^{+}\mathrm{SR}$) measurements to probe the structure and magnetic properties of the semiconducting $\beta -\mathrm{C}{\mathrm{e}}_2{\mathrm{O}}_2\mathrm{FeS}{\mathrm{e}}_2$ oxychalcogenide. We report a structural description in space group $Pna{2}_1$ which is consistent with diffraction data and second harmonic generation measurements and reveal an order-disorder transition on one Fe site at $T_{\mathrm{OD}}\approx 330\mathrm{K}$. Susceptibility measurements, Mössbauer, and ${\mu }^{+}\mathrm{SR}$ reveal antiferromagnetic ordering below $T_{\mathrm{N}}=86\mathrm{K}$ and more complex short range order above this temperature. 12 K neutron diffraction data reveal a modulated magnetic structure with $\mathbf{q}=0.444{\mathbf{b}}_{\mathrm{N}}{}^{*}$.

A mechanism of anomalous tension-compression (T-C) asymmetry in ultrafine-grained (UFG) metals is proposed using large-scale atomistic simulations and dislocation theory. Unlike coarse-grained metals, UFG Al exhibits remarkable T-C asymmetry of the yield stress. The atomistic simulations reveal that the yield event is not related to intragranular dislocations but caused by dislocation nucleation from the grain boundaries (GBs). The dislocation core structure associated with the stacking fault energy in Al is strongly affected by the external stress compared with Cu; specifically, high tensile stress stabilizes the dissociation into partial dislocations. These dislocations are more likely to be nucleated from GBs and form deformation twins from an energetic viewpoint. The mechanism, which is different from well-known mechanisms for nanocrystalline and amorphous metals, is unique to high-strength UFG metals and can explain the difference in T-C asymmetry between UFG Cu and Al.

We demonstrate the use of selective thermal evaporation to separate and purify small molecular weight polymers into highly monodisperse polymers over an extended range of polymerization index. By exploiting the calculated dependence of polymer vapor pressure on polymerization index $N$ and temperature $T$, we can isolate individual components $(N$-mers) of an initially polydisperse mixture. To demonstrate this ability, we consider polystyrene samples of $M_w=600$ g/mol and $M_w=890$ g/mol with narrow molecular weight distributions, as well as a $M_w=1200$ g/mol sample with a broader distribution. In each case we are able to separate the sample into milligram quantities of many different components. Using this technique, we have been able to isolate $N$-mers from 3 to 13. We use differential scanning calorimetry to measure the $T_g$ values of these components, and find that the components have the same $T_g$ values independent of the $M_w$ or polydispersity of the sample they originate from. We find that even initially narrow molecular weight distributions have many different components whose $T_g$ values can differ by more than 50 K. Calculations suggest the isolated components have $M_w/M_n$ values less than 1.001 and through a second iteration of the process could become as low as 1.000 003. The measured $T_g$ values for the $N$-mers as well as large $N$ polymers are well described by a simple relation derived from the Fox equation for the $T_g$ of mixtures.

The combination of charge and spin degrees of freedom with electronic correlations in condensed matter systems leads to a rich array of phenomena, such as magnetism, superconductivity, and novel conduction mechanisms. While such phenomena are observed in bulk materials, a richer array of behaviors becomes possible when these degrees of freedom are controlled in atomically layered heterostructures, where one can constrain dimensionality and impose interfacial boundary conditions. Here, we unlock a host of unique, hidden electronic and magnetic phase transitions in $\mathrm{NdNi}{\mathrm{O}}_3$ while approaching the two-dimensional (2D) limit, resulting from the differing influences of dimensional confinement and interfacial coupling. Most notably, we discover a phase in fully 2D, single-layer $\mathrm{NdNi}{\mathrm{O}}_3$, in which all signatures of the bulk magnetic and charge ordering are found to vanish. In addition, for quasi-two-dimensional layers down to a thickness of two unit cells, bulk-type ordering persists but separates from the onset of insulating behavior in a manner distinct from that found in the bulk or thin-film nickelates. Using resonant x-ray spectroscopies, first-principles theory, and model calculations, we propose that the single-layer phase suppression results from an alternative mechanism of interfacial electronic reconstruction based on ionicity differences across the interface, while the phase separation in multilayer $\mathrm{NdNi}{\mathrm{O}}_3$ emerges due to enhanced 2D fluctuations. These findings provide insights into the intertwined mechanisms of charge and spin ordering in strongly correlated systems in reduced dimensions and illustrate the ability to use atomic layering to access hidden phases.

Electronic charge rearrangements at interfaces between organic molecules and solid surfaces play a key role in a wide range of applications in catalysis, light-emitting diodes, single-molecule junctions, molecular sensors and switches, and photovoltaics. It is common to utilize electrostatics and Pauli pushback to control the interface electronic properties, while the ubiquitous van der Waals (vdW) interactions are often considered to have a negligible direct contribution (beyond the obvious structural relaxation). Here, we apply a fully self-consistent Tkatchenko-Scheffler vdW density functional to demonstrate that the weak vdW interactions can induce sizable charge rearrangements at hybrid metal/organic systems (HMOS). The complex vdW correlation potential smears out the interfacial electronic density, thereby reducing the charge transfer in HMOS, changes the interface work functions by up to 0.2 eV, and increases the interface dipole moment by up to 0.3 Debye. Our results suggest that vdW interactions should be considered as an additional control parameter in the design of hybrid interfaces with the desired electronic properties.

Lattice constant and cohesive energy are basic properties in the design of materials and devices. However, due to neglect of long-range van der Waals (vdW) interactions, density functional approximations (DFAs) often yield unusually large errors for ionic solids and heavy metals. Here, we propose a model for the dynamically screened vdW correction, including the leading order as well as higher-order contributions. The striking feature of this model is that important screening effects and higher-order contributions are properly considered and that its contribution to the short-range part is removed by a novel damping function for the avoidance of double counting. As a result, the model dramatically reduces the error of the DFA-GGA in lattice constant and cohesive energy. We also find that the three-body interactions are small, due to the screening effects. These observations greatly improve our fundamental understanding of vdW interactions and enhance the applicability of efficient semilocal DFAs.

${\mathrm{Pr}}_4{\mathrm{Ni}}_3{\mathrm{O}}_8$ is an overdoped analog of hole-doped layered cuprates. Here we show via ab initio calculations that Ce-doped ${\mathrm{Pr}}_4{\mathrm{Ni}}_3{\mathrm{O}}_8$ $\left({\mathrm{Pr}}_3{\mathrm{CeNi}}_3{\mathrm{O}}_8\right)$ has the same electronic structure as the antiferromagnetic insulating phase of parent cuprates. We find that substantial Ce doping should be thermodynamically stable and that other 4+ cations would yield a similar antiferromagnetic insulating state, arguing this configuration is robust for layered nickelates of low-enough valence. The analogies with cuprates at different $d$ fillings suggest that intermediate Ce-doping concentrations near 1/8 should be an appropriate place to search for superconductivity in these low-valence Ni oxides.

The design of next generation $\beta$-type titanium implants requires detailed knowledge of the relevant stable and metastable phases at temperatures where metallurgical heat treatments can be performed. Recently, a standard specification for surgical implant applications was established for Mo-Ti alloys. However, the thermodynamic properties of this binary system are not well known and two conflicting descriptions of the $\beta$-phase stability have been presented in the literature. In this study, we use ab initio calculations to investigate the Mo-Ti phase diagram. These calculations predict that the $\beta$ phase is stable over a wide concentration range, in qualitative agreement with one of the reported phase diagrams. In addition, they predict stoichiometric compounds, stable at temperatures below $300{}^{\circ }\mathrm{C}$, which have not yet been detected by experiments. The resulting solvus, which defines the transition to the $\beta$-phase solid solution, therefore occurs at lower temperatures and is more complex than previously anticipated.

Pressure-induced superconductivity and structural phase transitions in phosphorus (P) are studied by resistivity measurements under pressures up to 170 GPa and by fully ab initio crystal structure exploration and superconductivity calculations up to 350 GPa. Two distinct superconducting transition temperature $\left(T_C\right)$ vs pressure $\left(P\right)$ trends at low pressure have been reported more than 30 years ago, and we are able to devise a consistent explanation founded on thermodynamically metastable phases of black phosphorus. Our experimental and theoretical results form a single, consistent picture which not only provides a clear understanding of elemental P under pressure but also sheds light on the longstanding and unsolved anomalous superconductivity trends. Moreover, at higher pressures we predict a similar scenario of multiple metastable structures which coexist beyond their thermodynamical stability range. We observe that all the metastable structures systematically exhibit larger transition temperatures than the ground-state structures, indicating that the exploration of metastable phases represents a promising route to design materials with improved superconducting properties.

We report the growth of ${\mathrm{PbTiO}}_3$ thin films by molecular-beam epitaxy utilizing continuous codeposition. In addition to the requirements from thermodynamics, whether the resulting film is single-phase ${\mathrm{PbTiO}}_3$ or not at a particular temperature depends strongly on the film growth rate and the incident fluxes of all species, including titanium. We develop a simple theory for the kinetics of lead oxidation on the growing film surface and find that it qualitatively explains the manner in which the adsorption-controlled growth window of ${\mathrm{PbTiO}}_3$ depends on lead flux, oxidant flux, and titanium flux. We successfully apply the kinetic theory to the dependence of the growth of ${\mathrm{BiFeO}}_3$ on oxidant type and surmise that the theory may be generally applicable to the adsorption-controlled growth of complex oxides by MBE.

The stacking effect on the $E_{2g}^1$ and $A_{1g}$ vibrational modes of mechanically exfoliated ${\mathrm{MoS}}_2$ samples supported on ${\mathrm{SiO}}_2$ was investigated by nonresonant Raman spectroscopy at high-pressure conditions. The split of $E_{2g}^1$ and $A_{1g}$ modes were observed for single- and bilayered samples whereas such splitting disappears for higher numbers of layers. The differences on the $E_{2g}^1$ pressure coefficients allowed us to interpret the observed splitting as due to the presence of two types of regions corresponding to a high and a low conformation of ${\mathrm{MoS}}_2$ to the substrate roughness. The difference in the pressure coefficient appears then as due to the biaxial stress introduced via the substrate compression. Such effects were not observed for the $A_{1g}$ mode due to its vibration symmetry. This out-of-plane vibration is mainly affected by the normal stress that corresponds to pressure transmitted by the pressure-transmitting medium.

The electronic and optical properties of the paradigmatic F4TCNQ-doped pentacene in the low-doping limit are investigated by a combination of state-of-the-art many-body ab initio methods accounting for environmental screening effects, and a carefully parametrized model Hamiltonian. We demonstrate that while the acceptor level lies very deep in the gap, the inclusion of electron-hole interactions strongly stabilizes dopant-semiconductor charge transfer states and, together with spin statistics and structural relaxation effects, rationalize the possibility for room-temperature dopant ionization. Our findings reconcile available experimental data, shedding light on the partial vs. full charge transfer scenario discussed in the literature, and question the relevance of the standard classification in shallow or deep impurity levels prevailing for inorganic semiconductors.

Tuning magnetic properties of perovskite thin films is a central topic of recent studies because of its fundamental significance. In this work, we demonstrated the modification of the magnetism of $\mathrm{L}{\mathrm{a}}_{0.9}\mathrm{C}{\mathrm{a}}_{0.1}\mathrm{Co}{\mathrm{O}}_3$ (LCCO) thin films by introducing a stripelike superstructure in a controllable manner using electron beam irradiation (EBI) in a transmission electron microscope. The microstructure, electronic structure, strain change, and origin of magnetism of the LCCO thin films were studied in detail using aberration-corrected scanning transmission electron microscopy, electron energy loss spectroscopy, and ab initio calculations based on density functional theory. The results indicate that the EBI-induced unit cell volume expansion accompanies the formation of oxygen vacancies and leads to the spin state transition of Co ions. The low spin state of $\mathrm{C}{\mathrm{o}}^{4+}$ ions depress the stripelike superstructure, while higher spin states of Co ions with lower valences are conductive to the formation of “dark stripes”. Our work clarifies the origin of magnetism of epitaxial LCCO thin films, benefiting a comprehensive understanding of correlated physics in cobalt oxide thin films.

Polar compensation can play an important role in the determination of interfacial electronic and magnetic properties in oxide heterostructures. Using x-ray absorption spectroscopy, x-ray magnetic circular dichroism, bulk magnetometry, and transport measurements, we find that interfacial charge redistribution via polar compensation is essential for explaining the evolution of interfacial ferromagnetism in ${\mathrm{LaNiO}}_3/{\mathrm{CaMnO}}_3$ superlattices as a function of ${\mathrm{LaNiO}}_3$ layer thickness. In insulating superlattices (four unit cells or less of ${\mathrm{LaNiO}}_3$), magnetism is dominated by Ni–Mn superexchange, while itinerant electron-based Mn–Mn double exchange plays a role in thicker metallic superlattices. X-ray magnetic circular dichroism and resonant x-ray scattering show that Ni–Mn superexchange contributes to the magnetization even in metallic superlattices. This Ni–Mn superexchange interaction can be explained in terms of polar compensation at the ${\mathrm{LaNiO}}_3–{\mathrm{CaMnO}}_3$ interface. These results highlight the different mechanisms responsible for interfacial ferromagnetism and the importance of understanding compensation due to polar mismatch at oxide-based interfaces when engineering magnetic properties.

Grain boundaries (GBs) in crystalline materials are traditionally described by five crystallographic angles, which are assumed to fully define the GB structure and energy. It has recently been realized that variations in the atomic density $\lambda$ in the GB region can drastically alter the GB structure and cause transformations between different GB phases. Here we extend the previous studies of Cu $\mathrm{\Sigma }5$ GBs by computing the structures and energies of a set of [001] symmetrical tilt GBs over the entire angular range by allowing arbitrary variations in $\lambda$. The results confirm the existence of stable and metastable phases in all GBs studied here. There are three types of structural units that can describe all GB structures obtained in this work. The work demonstrates that $\lambda$ should be added to the description of GBs as an extra thermodynamic parameter that helps predict the GB phases and transformations among them.

The influence of Sb content, substrate type, and cap layers on the quantum anomalous Hall effect observed in V-doped (Bi,Sb)${}_2{\mathrm{Te}}_3$ magnetic topological insulators is investigated. Thin layers showing excellent quantization are reproducibly deposited by molecular beam epitaxy at growth conditions affecting a compromise between controlled layer properties and high crystalline quality. The Sb content can be reliably determined from the in-plane lattice constant measured by x-ray diffraction, even in thin layers. This is the main layer parameter to be optimized in order to approach charge neutrality. Within a narrow range at about 80% Sb content, the Hall resistivity shows a maximum of about $10\mathrm{k}\mathrm{\Omega }$ at 4 K and quantizes at mK temperatures. Under these conditions, thin layers grown on Si(111) or InP(111) and with or without a Te cap exhibit quantization. The quantization persists independently of the interfaces between cap, layer, and substrate, the limited crystalline quality, and the degradation of the layer proving the robustness of the quantum anomalous Hall effect.