Ferroelectric materials hold great potential for alternative memories and computing, but several challenges need to be overcome before bringing the ideas to applications. In this context, the recently discovered link between electric polarization and spin textures in some classes of ferroelectrics expands the perspectives of the design of devices that could simultaneously benefit from ferroelectric and spintronic properties. Here, we explore the concept of nonvolatile ferroelectric control of charge-to-spin conversion in semimetallic ${\mathrm{WTe}}_2$, which may provide a way for nondestructive readout of the polar state. Based on the first-principles simulations, we show that the Rashba-Edelstein effect (REE) that converts electric currents into spin accumulation switches its sign upon the reversal of the electric polarization. The numerical values of REE, calculated for the first time for both bulk and bilayer ${\mathrm{WTe}}_2$, demonstrate that the conversion is sizable, and may remain large even at room temperature. The ferroelectric control of spin transport in nonmagnetic materials provides functionalities similar to multiferroics and allows the design of memories or logic-in-memory devices that combine ferroelectric writing of information at low power with the spin-orbit readout of state.

We investigate the thermodynamic, magnetic, and electrical transport properties of a magnetic semimetal EuZnGe using single crystals grown from Eu-Zn flux in sealed tantalum tubes. Magnetic properties are found to be isotropic in the paramagnetic state while we observe an enhancement of in-plane magnetic susceptibility at the temperature near $T^{*}=11.3\mathrm{K}$, suggesting an easy-plane anisotropy at low temperatures. Magnetic transition temperature is lower than $T^{*}$ as specific heat shows a peak at $T_{\text{N}}=7.6\mathrm{K}$. We reveal the magnetic modulation along the $c$ axis by resonant x-ray scattering at Eu $L_2$ edge, which suggests competing magnetic interactions among Eu triangular-lattice layers. We observe a double-peak structure in the intensity profile along (0, 0, $L$) below $T_{\text{N}}$, which is mainly composed of a dominant helical modulation with $\mathit{q}\sim$ (0, 0, 0.4) coexisting with a secondary contribution from $\mathit{q}\sim$ (0, 0, 0.5). We reproduce the intensity profile with a random mixture of five- and four-sublattice helices with spin rotation skipping due to hexagonal in-plane anisotropy. The metallic conductivity is highly anisotropic with the ratio ${\rho }_{zz}/{\rho }_{xx}$ exceeding 10 over the entire temperature range and additionally exhibits a sharp enhancement of ${\rho }_{zz}$ at $T_{\text{N}}$ giving rise to ${\rho }_{zz}/{\rho }_{xx}\sim 50$, suggesting a coupling between out-of-plane electron conduction and the spiral magnetic modulations. In-plane magnetic field induces a spin-flop-like transition, where the $q=0.4$ peak disappears and an incommensurate peak of approximately $q_{\text{ICM}}\sim$ 0.47 emerges, while the $q=0.5$ modulation retains a finite intensity. This transition correlates with nonmonotonic magnetoresistance and Hall resistivity, suggesting a significant interplay between electrons and spin structures through Ruderman-Kittel-Kasuya-Yosida interaction.

Recently, all-$d$-metal Ni(Co)-Mn-Ti Heusler systems have become the research hotspot due to their magnetoresponsive properties associated with a tunable first-order magnetostructural transformation (MST) and excellent mechanical stability for potential applications. However, the presence of large thermal hysteresis acts as an obstacle to the cyclic operation of this novel material. In this present paper, we investigate a large reversible magnetocaloric effect (MCE) and magnetoresistance (MR) in ${\mathrm{Ni}}_{37-x}{\mathrm{Co}}_{13+x}{\mathrm{Mn}}_{34.5}{\mathrm{Ti}}_{15.5}$ all-$d$-metal Heusler alloys that undergo a first-order MST accompanied by a large magnetization change between ferromagnetic austenite and antiferromagnetic martensite phases. Tuning the small at.% of Co doping in ${\mathrm{Ni}}_{37-x}{\mathrm{Co}}_{13+x}{\mathrm{Mn}}_{34.5}{\mathrm{Ti}}_{15.5}$ systems, we achieved an optimum composition with $x$ = 1 where low thermal hysteresis of $\sim 4.7$ K, a narrow transformation interval of $\sim 11.2$ K, and improved sensitivity of transformation temperature $\sim 2.8$ K/T is observed. In addition, the origin of small hysteresis is studied based on geometric compatibility between cubic austenite and monoclinic martensite phases, calculated from the powder x-ray diffraction data. These optimized parameters lead to a reversible magnetic field-induced inverse martensitic transition under the field cycling, yielding a large reversible magnetic entropy change ($\mathrm{\Delta }{\mathrm{S}}_M$) of $\sim 17.78$ J ${\mathrm{kg}}^{-1}{\mathrm{K}}^{-1}$ at 277 K in a field change of 5 T. Moreover, a large reversible magnetoresistance (MR) of $\sim 14\%$ out of 32.6% is also obtained under 5-T magnetic field for $x$ = 1 sample in all-$d$-metal Heusler alloys. These reversible magnetoresponsive properties are comparable to other Ni-Mn-based Heusler alloys and have not been reported so far in the all-$d$-metal Heusler system. Therefore our present work demonstrates a pathway to design new cyclically stable multifunctional materials in all-$d$-metal Heusler systems for solid-state cooling devices and magnetic recording applications.

Ironmaking processes using steel scrap are becoming increasingly important in the effort to realize a decarbonized society, but their utilization is limited by the surface hot shortness related to cracking at paramagnetic γ-Fe grain boundaries (GBs) due to tramp elements such as Cu and Sn. To understand its electronic origin, GB embrittlement due to Cu and Sn was investigated by first-principles computational tensile tests (FPCTTs) on paramagnetic γ-Fe GBs. The paramagnetic γ-Fe GB was modeled as the $\mathrm{\Sigma }5\left(310\right)\mathrm{GB}$ in the antiferromagnetic double-layer (AFMD) configuration. Both Cu and Sn reduce the fracture stress, fracture energy, and fracture strain of the γ-Fe GB. The effect of Sn was more pronounced than that of Cu, which agrees well with experimental results on surface hot shortness. Crystal orbital Hamiltonian population (COHP) analysis of the electronic structure during the tensile processes revealed that the atomic orbitals of Cu are more localized than those of Fe in γ-Fe, resulting in the breaking of Cu–Fe bonds at a lower strain and stress, which leads to GB embrittlement. In addition to this effect, Sn increases the bond lengths between adjacent Fe atoms and weakens the Fe–Fe bonds, causing significant GB embrittlement. The obtained knowledge contributes to the elucidation of the mechanism of the surface hot shortness. Furthermore, the FPCTT using the γ-Fe GBs in the AFMD configuration is useful to investigate the effects of various solute elements on paramagnetic γ-Fe GBs, and is expected to be utilized to improve material properties governed by GB fracture.

In recent years, liquid metal dealloying has emerged as a promising material processing method to generate micro- and nanoscale bicontinuous or porous structures. Most previous studies focused on the experimental characterization of the dealloying process and on the properties of the dealloyed materials, leaving the theoretical study incomplete to fully understand the fundamental mechanisms of the liquid metal dealloying process. In this paper, we use theoretical models and phase-field simulations to clarify the kinetics and pattern formation during liquid metal dealloying. Our investigation starts from a theoretical analysis of the 1D dissolution of a binary precursor alloy, which reveals that the 1D dissolution process involves two regimes. In the first regime, due to the low solubility of one of the elements in the melt, it accumulates at the solid-liquid interface, which reduces the dissolution kinetics. In the second regime, the interface kinetics reaches a stationary regime where both elements of the precursor alloy dissolve into the melt. Previous works revealed that in the early dealloying stage, the dealloying front is destabilized by an interfacial spinodal decomposition, which triggers the formation of interconnected ligaments. We extend this line of work by proposing a linear stability analysis able to predict the initial length scale of the ligaments formed in the initial stage of the dealloying. Combining this analysis with the 1D dissolution model proposed here enables us to better understand the initial conditions (composition of the precursor alloy and the melt) leading to a planar dissolution without interface destabilization. Finally, we report a strong influence of solid-state diffusion on dealloying. Although the solid-state diffusivity is four to five orders of magnitude smaller than in the liquid phase, it is found to affect both dissolution kinetics and ligament morphologies.

Area-dependent memristive devices such as $\mathrm{Al}/{\mathrm{Pr}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_3$ (PCMO) stacks are highly interesting candidates for synapses in neuromorphic circuits due to their gradual switching properties, their reduced variability and the possibility to tune the resistance with the device area. However, due to the complexity of the different processes taking place, the electronic and ionic transport in theses devices is so far only poorly understood and physical compact models to simulate their behavior are missing so far. We developed a mathematical description of the dynamics of theses devices based on a simple two-resistor model that reproduces the device behavior very well. Based on x-ray photoelectron spectroscopy and impedance spectroscopy we assign the two resistors to the ${\mathrm{AlO}}_x$ layer and a depletion zone at the ${\mathrm{Pr}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_3$ layer, respectively. We assign the parameters used within the mathematical model to physical parameters and make use of them in order to explain the dynamics of the switching processes during the SET and RESET process in different voltage regimes. For both poly- and single crystalline PCMO thin film devices, oxygen migration between the ${\mathrm{AlO}}_x$ and the PCMO depletion zone is responsible for the resistance change. However, the dynamics differ significantly due to the increased mobility of oxygen vacancies with increasing defect density in the case of the polycrystalline samples. Moreover, we observe volatile subloops in our current-voltage curves, which vanish within millisecond time scale. Based on our two-resistor model and the band diagram derived from spectroscopic measurements, we assign these subloops to the injection of electrons into traps within the ${\mathrm{AlO}}_x$ barrier.

A general method was developed to intercalate metals under layered materials through a controlled density of sputtered defects. The method has already been applied to study a range of metals intercalated under graphite, and different types of morphologies were realized. In the current work, we extend the method to the study of intercalation under ${\mathrm{MoS}}_2$, noting that work on this system is rather limited. We use Cu as the prototype metal for comparison with Cu intercalation under graphite. Although the growth conditions needed for intercalation under graphite and ${\mathrm{MoS}}_2$ are similar, the type of intercalated phases is very different. Each Cu island that nucleates on top of ${\mathrm{MoS}}_2$ during Cu deposition provides material that is transferred below ${\mathrm{MoS}}_2$ through sputtered defects under the island base; this transfer results in a uniform intercalated Cu “carpet” morphology that extends over the mesoscale. On the contrary, Cu intercalation under graphite results in well separated, compact islands formed by monomer detachment from small Cu islands on top and transfer below through defects far from the islands. The structural techniques (scanning electron microscopy and atomic force microscopy) and spectroscopic techniques (x-ray photoelectron spectroscopy and energy-dispersive spectroscopy) are used for the characterization of the intercalated Cu layer.

We report hyperspectral microscopy in ${sp}^2$-bonded boron nitride polytypes with a simultaneous inspection of the photoluminescence signal in the UV-C and UV-B spectral ranges. By comparing two different polytypes extracted from the same polycrystalline sample, we reveal that the well-known “4 eV defect” does not emit at the same energy for polytypes in the $AB$ and $A{A}^{\prime }$ stackings. The zero-phonon line is either at 4.14 or 4.16 eV for the noncentrosymmetric $AB$ stacking, instead of the usual 4.09 eV energy in the $A{A}^{\prime }$ stacking. Our results open the way for different characterization methods of the stacking order in ${sp}^2$-bonded boron nitride, and bring alternative inputs for the elucidation of the atomistic configuration of points defects by advanced ab initio calculations.

Topological materials have been discovered so far largely by searching for existing compounds in crystallographic databases, but there are potentially new topological materials with desirable features that have not been synthesized. One of the desirable features is high tunability resulting from the band inversion with a very small direct band gap, which can be tuned by changes in pressure or strain to induce a topological phase transition. Using density-functional theory (DFT) calculations, we have studied the septuple layered $A{B}_2{X}_4$ series compounds, where $A=$ (Ge, Sn and Pb), $B=$ (As, Sb and Bi), and $X=$ (Se and Te). With the DFT thermodynamic stability validated by the already-reported compounds in these series, we predict stable Se compounds, which are not found in crystallographic database. Among them, we find that $\mathrm{Ge}{\mathrm{Bi}}_2{\mathrm{Se}}_4$ and $\mathrm{Ge}{\mathrm{Sb}}_2{\mathrm{Se}}_4$ having a small direct band gap at the $Z$ point are very close to a strong topological insulator, which can be tuned by a moderate pressure to induce the band inversion. Importantly, the topological features with the small direct band gap are well isolated in both momentum and energy windows, which offers high tunability for studying the topological phase transition.

Using first-principles density-functional theory calculations, we investigate the pressure-induced quantum phase transition (QPT) from the charge density wave (CDW) to the pristine phase in the layered kagome metal ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ consisting of three-atom-thick $\mathrm{Sb}\text{−}{\mathrm{V}}_3\mathrm{Sb}\text{−}\mathrm{Sb}$ and one-atom-thick Cs layers. The CDW structure having the formation of trimeric and hexameric V atoms with buckled Sb honeycomb layers features an increase in the lattice parameter along the $c$ axis, compared with its counterpart pristine structure having the ideal ${\mathrm{V}}_3\mathrm{Sb}$ kagome and planar Sb honeycomb layers. Consequently, as pressure increases, the relatively smaller volume of the pristine phase contributes to reducing the enthalpy difference between the CDW and pristine phases, yielding a pressure-induced QPT at a critical pressure $P_c$ of $\approx 2$ GPa. Furthermore, we find that (i) the superconducting transition temperature $T_c$ increases around $P_c$ due to a phonon softening associated with the periodic lattice distortion of V trimers and hexamers and that (ii) above $P_c$, optical phonon modes are hardened with increasing pressure, leading to monotonic decreases in the electron-phonon coupling constant and $T_c$. Our findings not only demonstrate that the uniaxial strain along the $c$ axis plays an important role in the QPT observed in ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ but also provide an explanation for the observed superconductivity around $P_c$ in terms of a phonon-mediated superconducting mechanism.

Ferroelectric $\mathrm{BaTi}{\mathrm{O}}_3$ thin films epitaxially integrated on Si are an emergent platform for fabricating integrated electro-optical modulators using the linear electro-optic effect for applications in silicon photonics. These devices hold great promise for optical neuromorphic and quantum computing. Understanding the domain morphology of such films is essential for building ultrafast, ultralow-power electro-optic modulators. However, the domain morphology of the film is marked by significant complexity and our knowledge of it and its relation to the electro-optic response in epitaxial thin films is limited. In this paper, we use a phase-field model implemented within the finite-element method to map domain morphologies. The corresponding electro-optic response is also discussed.

The fracture surface of slow and continuous crack propagation during environmental stress cracking of a semicrystalline polyethylene exhibits isotropic roughness exponents at the local scale but resolved across the macroscopic fracture surface a clear position dependence is found. The spatially resolved roughness exponent admits values in the range between 0.1 and 0.4, demonstrating nontrivial exponents in the small length-scale regime. Instead, they vary across the fracture surface according to the stress-state distribution, which suggests that the exponents are intimately linked to the locally dominating dissipation processes during craze cracking.

The most characteristic functional property of antiferroelectric materials is the possibility to induce a phase transition from a nonpolar to a polar phase by an electric field. Here, we investigate the effect of this field-induced phase transition on the birefringence change of $\mathrm{Pb}{\mathrm{Zr}}_{0.95}{\mathrm{Ti}}_{0.05}{\mathrm{O}}_3$. We use a transparent polycrystalline $\mathrm{Pb}{\mathrm{Zr}}_{0.95}{\mathrm{Ti}}_{0.05}{\mathrm{O}}_3$ film grown on ${\mathrm{PbTiO}}_3/{\mathrm{HfO}}_2/{\mathrm{SiO}}_2$ with interdigitated electrodes to directly investigate changes in birefringence in a simple transmission geometry. In spite of the polycrystalline nature of the film and its moderate thickness, the field-induced transition produces a sizable effect observable under a polarized microscope. The film in its polar phase is found to behave like a homogeneous birefringent medium. The time evolution of this field-induced birefringence provides information about irreversibilities in the antiferroelectric switching process and its slow dynamics. The change in birefringence has two main contributions: One that responds briskly and a slower one that rises and saturates over a period of as long as 30 min. Possible origins for this long saturation and relaxation times are discussed.

It has long been known that the addition of small quantities (“doping”) of so-called reactive elements (REs) such as Y, Zr, and Hf to high-temperature ${\text{Al}}_2{\text{O}}_3$ scale-forming alloys improves oxidation resistance. The presence of reactive elements at grain boundaries lowers the growth rate of the $\alpha \text{−}{\text{Al}}_2{\text{O}}_3$ scales, but the cause of the reduced scale growth kinetics is not fully understood. Explanations based on steric effects and explanations based on reducing the grain boundary electronic conductivity have been proposed. We have used density functional theory to study the structural and electronic properties of two $\mathrm{\Sigma }7$ bicrystal grain boundaries containing Y, Hf, and Zr substitutional defects on Al sites. The presence of RE substitutional defects plays a minimal direct role in reducing the density of electronic states near the valence-band maximum. However, ${\mathrm{Hf}}^{4+}$ or ${\mathrm{Zr}}^{4+}$ substitutions at the grain boundary repel the positively charged oxygen vacancy ${\text{V}}_{\text{O}}^{2+}$. As ${\text{V}}_{\text{O}}^{2+}$ contributes a defect state above the valence-band maximum but below the Fermi energy, this indirectly lowers the density of current-carrying holes and thus the electronic conductivity of the grain boundary. Replacing ${\mathrm{Al}}^{3+}$ ions with ${\mathrm{Hf}}^{4+}$ or ${\mathrm{Zr}}^{4+}$ ions also makes the grain boundary positively charged, further reducing the hole density.

A simulation approach of field evaporation is presented. The model combines classical electrostatics with molecular dynamics simulations. Unlike previous atomic-level simulation approaches, our method does not rely on an evaporation criterion based on thermal activation theory, instead, electric-field-induced forces on atoms are explicitly calculated and added to the interatomic forces. Atoms then simply move according to the laws of classical molecular dynamics and are “evaporated” when the external force overcomes interatomic bonding. This approach thus makes no ad hoc assumptions concerning evaporation fields and criteria, which makes the simulation fully physics-based and “ab initio” apart from the interatomic potential. As proof of principle, we perform simulations to determine material dependent critical voltages which allow assessing the evaporation fields and the corresponding steady-state tip shapes in different metals. We also extract critical evaporation fields in elemental metals and sublimation energies in a high entropy alloy to have a more direct comparison with tabulated values. In contrast to previous approaches, we show that our method is able to successfully reproduce the enhanced zone lines observed in experimental field desorption patterns. We also demonstrate the need for careful selection of the interatomic potential by a comparative study for the example of Cu-Ni alloys.

The dielectrically consistent reference interaction site model (DRISM) is one of the methods used to solve the well-known drawback of the reference interaction site model (RISM) theory: That it underestimates the dielectric constant of the solutions. Recently, Nishihara and Otani developed the first-principles effective screening medium (ESM) method combined with RISM theory, called ESM-RISM, to simulate physical properties at the electrode/electrolyte interface [Phys. Rev. B 96, 115429 (2017)]. In this study, we combined DRISM with the ESM-RISM framework to increase the accuracy of electrochemical interface simulations, which was applied to the Pt(111)/aqueous water interface as a benchmark calculation. The electrochemical properties at the interface, such as the potential of zero charge, standard hydrogen electrode potential, and double-layer capacitance, all reasonably agree with previous experiments. The thickness of the contact layer was found to correlate well with double-layer capacitance. We believe that the present method is a useful tool to better model the physical properties at electrochemical interfaces.

Two-dimensional transition metal dichalcogenides (TMDs) can adopt one of several possible structures, with the most common being the trigonal prismatic and octahedral symmetry phases. Since the structure determines the electronic properties, being able to predict phase preferences of TMDs from just the knowledge of the constituent atoms is highly desired, but has remained a long-standing problem. In this study, we applied high-throughput quantum mechanical computations with machine learning algorithms to solve this old problem. Our analysis provides insights into determining physiochemical factors that dictate the phase preference of a TMD, identifying and going beyond the attributes considered by earlier researchers in predicting crystal structures. A knowledge of these underlying physiochemical factors not only helps us to rationalize, but also to accurately predict structural preferences. We show that machine learning algorithms are powerful tools that can be used not only to find new materials with targeted properties, but also to find connections between elemental attributes and the target property/properties that were not previously obvious.

Iridates with the $5d^4$ electronic configuration have attracted recent interest due to reports of magnetically ordered ground states despite longstanding expectations that their strong spin-orbit coupling would generate a $J=0$ electronic ground state for each ${\mathrm{Ir}}^{5+}$ ion. The major focus of prior research has been on the double perovskite iridates ${\mathrm{Ba}}_2{\mathrm{YIrO}}_6$ and ${\mathrm{Sr}}_2{\mathrm{YIrO}}_6$, where the nature of the ground states (i.e., ordered vs nonmagnetic) is still controversial. Here, we present neutron powder diffraction, high-energy-resolution fluorescence-detected x-ray absorption spectroscopy (HERFD-XAS), resonant inelastic x-ray scattering (RIXS), magnetic susceptibility, and muon spin relaxation data on the related double perovskite iridates ${\mathrm{Ba}}_2{\mathrm{LuIrO}}_6, {\mathrm{Sr}}_2{\mathrm{LuIrO}}_6, {\mathrm{Ba}}_2{\mathrm{ScIrO}}_6$, and ${\mathrm{Sr}}_2{\mathrm{ScIrO}}_6$ that enable us to gain a general understanding of the electronic and magnetic properties for this family of materials. Our HERFD-XAS and RIXS measurements establish $J=0$ electronic ground states for the ${\mathrm{Ir}}^{5+}$ ions in all cases, with similar values for Hund's coupling $J_{\mathrm{H}}$ and the spin-orbit coupling constant ${\lambda }_{\mathrm{SOC}}$. Our bulk susceptibility and muon spin relaxation data find no evidence for long-range magnetic order or spin freezing, but they do exhibit weak magnetic signals that are consistent with extrinsic local moments. Our results indicate that the large ${\lambda }_{\mathrm{SOC}}$ is the key driving force behind the electronic and magnetic ground states realized in the $5d^4$ double perovskite iridates, which agrees well with conventional wisdom.

High-$k$ dielectrics ${\mathrm{Al}}_2{\mathrm{O}}_3$ and ${\mathrm{HfO}}_2$ are widely used in combination with Si to produce electronic components and solar cells. A negative fixed charge is known to appear at Si/high-$k$ material interface after high-temperature annealing, yet the formation mechanism of the negative charge is poorly understood. In this work, we investigate the parameters affecting the charge formation to provide a better understanding and control of the charge in deposition and postprocessing steps. We observe negative charge formation in the interface after annealing by capacitance-voltage measurement. We demonstrate the effects of annealing temperature and high-$k$ film thickness on the charge formation. We further discuss the mechanism of the charge formation and present results supporting one recently suggested mechanism explaining the negative charge as an acceptor state in the interface. We observe a structural modification of the interface produced by annealing with photoelectron spectroscopy as a core-level shift in ${\mathrm{SiO}}_2$ binding energy. We investigate the electrostatic potential and electron distribution in the interface by ab initio calculations to understand the effect of the structural modification on the electronic structure of the interface.