$S=1/2$ Heisenberg $J_1\text{−}{J}_2$ chain antiferromagnets have been investigated extensively due to their exotic magnetic states. Here, we report the magnetic behavior of birchite ${\mathrm{Cd}}_2{\mathrm{Cu}}_2{\left({\mathrm{PO}}_4\right)}_2{\mathrm{SO}}_4·5{\mathrm{H}}_2\mathrm{O}$ and its effective spin model. Experimental studies by magnetic susceptibility, magnetization, heat capacity, and $\mu \mathrm{SR}$ measurements indicate the absence of long-range order down to 0.4 K. Theoretical studies reveal that birchite is a model compound for the $J_1\text{−}{J}_2$ antiferromagnetic chain: the intrachain interactions $J_1$ and $J_2$ are antiferromagnetic and their magnitude is about 100 times larger than the interchain interactions. The magnitude of $J_2$ is two to three times larger than that of $J_1$, thus the spin gap is expected to be only a few percent of that of $J_1$. The temperature dependence of the specific heat shows a broad peak at about 1 K ($\simeq 0.036J_1$), which suggests the presence of a spin gap.

Charge transfer from a metal substrate stabilizes honeycomb borophene, whose electron deficit would otherwise spoil the hexagonal order of a $\pi$-bonded two-dimensional (2D) atomic network. However, the coupling between the substrate and the boron overlayer may result in the formation of strong chemical bonds that would compromise the electronic properties of the overlayer. In this paper we present a theoretical study, based on state-of-the-art density-functional and genetic optimization techniques, of the electronic and structural properties of borophene grown on Al(111), with emphasis on the impact of oxygen on the strength of the coupling between the substrate and overlayer. While our results confirm the formation of Al-B bonds, they also predict that oxygen doping reduces charge transfer between aluminum and borophene, thus allowing modulation of their strength and paving the way to engineering the electronic properties of 2D-supported borophene sheets for industrial applications. Our study is completed by a thorough study of the thermodynamic stability of the oxygenated borophene-Al(111) interface.

We introduce the concept of shear band network (SBN) in ductile solids containing soft particles to investigate their irreversible deformation. Drawing on network science tools, we show that the evolution of the SBN explains the material strength at the coarse scale. To gain physical understanding of the activation order of the SBN links during tensile loading, we develop an analytical model based on the continuum theory of irreversible deformation. The results are used to construct an efficient, parameter-free indicator of the material strength that is solely based on the particle size and spatial distribution, thus proving that network science can advance the understanding of strength mechanisms in ductile solids.

Magnetic ion doping in topological insulators (TI's) has emerged to be important both from technological point of view and for experimental verification of exotic fundamental physical concepts. Magnetic ion doping not only opens up the energy gap in surface states of a TI but also changes its bulk band structure significantly. To observe the effect of magnetic ion doping on electronic band structure of TIs density functional theory based ab initio electronic structure calculations have been carried out on 2%, 4% and 6% Mn doped ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ system. To validate the results single crystal of undoped and Mn doped ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ have been grown and have been characterized by angle-resolved photoemission spectroscopy (ARPES) measurements with synchrotron radiation. Most of the results viz., band inversion, gapless surface states and reduction in band gap due to Mn doping, as obtained by theoretical calculations have been verified by ARPES measurements. Additionally, ARPES measurements show opening up of the gapless surface states in Mn doped samples. The samples have also been subjected to x-ray absorption spectroscopy measurements comprising of x-ray near edge structure and extended x-ray absorption fine structure measurements using synchrotron radiation which together conclusively show that Mn ions substitute at Bi sites in ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ lattice.

The band topology of magnetic semimetals is of interest both from the fundamental science point of view and with respect to potential spintronics and memory applications. Unfortunately, only a handful of suitable topological semimetals with magnetic order have been discovered so far. One such family that hosts these characteristics is the antiperovskites, $A_{3}B\mathrm{O}$, a family of 3D Dirac semimetals. The $A={\mathrm{Eu}}^{2+}$ compounds magnetically order with multiple phases as a function of the applied magnetic field. Here, by combining band structure calculations with neutron diffraction and magnetic measurements, we establish the antiperovskite ${\mathrm{Eu}}_3\mathrm{Pb}\mathrm{O}$ as a new topological magnetic semimetal. This topological material exhibits a multitude of different topological phases with ordered Eu moments which can be easily controlled by an external magnetic field. The topological phase diagram of ${\mathrm{Eu}}_3\mathrm{Pb}\mathrm{O}$ includes an antiferromagnetic Dirac phase, as well as ferro- and ferrimagnetic phases with both Weyl points and nodal lines. For each of these phases, we determine the bulk band dispersions, the surface states, and the topological invariants by means of ab initio and tight-binding calculations. Our discovery of these topological phases introduces ${\mathrm{Eu}}_3\mathrm{Pb}\mathrm{O}$ as a new platform to study and manipulate the interplay of band topology, magnetism, and transport.

The transition between delocalized and localized electronic configurations is a characteristic property of all small-polaron systems. In this study, we explore a doping approach by which the coexistence between delocalized and localized electronic states may be precisely tuned within extrinsic polaron systems. Through comprehensive ab initio calculations employing hybrid functionals, for which Ti-doped ${\mathrm{SnO}}_2$ is selected as a model system, it is demonstrated how strain and alloying can be utilized to achieve a high degree of control over the activation energetics separating localized and delocalized electronic states. By means of a phenomenological tight-binding analysis of our ab initio results, it is shown that two physical parameters play a dominant role in the associated transition physics: (1) the dopant electronic offset ${\mathrm{\Delta }}_a$, and (2) the dopant bandwidth $W$. Overall, this study presents an exciting route toward tailoring the coexistence between delocalized and localized states with a potentially much greater degree of control than is currently possible in intrinsic materials.

Macroscopic strain experiments have revealed that silicon crystals traversed by parallel, channel-like nanopores functionalized with the artificial muscle polymer polypyrrole (PPy) exhibit large and reversible electrochemomechanical actuation in aqueous electrolytes. On a macroscopic scale these actuation properties are well understood. However, on the microscopical level this system still bears open questions, as to how the electrochemical expansion and contraction of PPy acts on to np-Si pore walls and how the collective motorics of the pore array emerges from the single-nanopore behavior. Here we present synchrotron-based, in operando x-ray diffraction on the evolving electrostrains in epilayers of this material grown on bulk silicon. An analysis of these experiments with micromechanical finite-element simulations, that are based on a full three-dimensional reconstruction of the nanoporous medium by transmission electron microscopy (TEM) tomography, shows that the in-plane mechanical response is dominantly isotropic despite the anisotropic elasticity of the single-crystalline host matrix. However, the structural anisotropy originating from the parallel alignment of the nanopores led to significant differences between the in- and out-of-plane electromechanical response. This response is not describable by a simple two-dimensional arrangement of parallel cylindrical channels. Rather, the simulations highlight that the dendritic shape of the silicon pore walls, including pore connections between the main channels, causes complex, highly inhomogeneous stress-strain fields in the crystalline host. Time-dependent x-ray scattering experiments on the dynamics of the actuator properties hint towards the importance of diffusion limitations, plastic deformation, and creep in the nanoconfined polymer upon (counter)ion adsorption and desorption, the very pore-scale processes causing the macroscopic electroactuation. From a more general perspective, our study demonstrates that the combination of TEM tomography-based micromechanical modeling with high-resolution x-ray scattering experiments provides a powerful approach for in operando analysis of nanoporous composites from the single nanopore up to the porous-medium scale.

The moderate bulk perpendicular magnetic anisotropy (PMA, $K_{\mathrm{u}}\approx 1\mathrm{MJ}/{\mathrm{m}}^3$) and low Gilbert damping (α < 0.01) make $L{1}_0$-FePd a promising candidate for energy-efficient and nonvolatile spintronic devices with large areal densities (down to 5-nm pitch sizes or even lower). Existing applications subject spintronic devices to a wide range of operating temperatures (e.g., −55 to 150 °C). To better address the technological viability of FePd for spintronic applications, it is of utmost importance to evaluate the material performance of $L{1}_0$-FePd (e.g., anisotropy strength and Gilbert damping) at elevated temperatures. In this work, we systematically investigate the effect of buffer layers (Cr/Pt, Cr/Ru, Cr/Rh, Cr/Ir, and Ir) on the PMA and Gilbert damping of $L{1}_0$-FePd from room temperature (RT, 25 °C) to 150 °C using the time-resolved magneto-optical Kerr effect metrology. It is found that the effective anisotropy field ($H_{\mathrm{k},\mathrm{eff}}$) of FePd decreases with the testing temperature ($T_{\mathrm{test}}$) and the ratio of $H_{\mathrm{k},\mathrm{eff}}$(150 °C)/$H_{\mathrm{k},\mathrm{eff}}$(25 °C) is positively correlated to the degree of $L{1}_0$ phase ordering. The Gilbert damping of $L{1}_0$-FePd either increases with $T_{\mathrm{test}}$ or stays nearly constant over the $T_{\mathrm{test}}$ range. We attribute the temperature dependence of Gilbert damping to the spin diffusion length of the metallic buffer layer (λ), presumably through the spin pumping effect. Results of this work provide guidance to tailor $L{1}_0$-FePd properties through buffer layer engineering for applications in spintronic devices over wide operating temperature ranges.

BCC transition metals (TMs) exhibit complex temperature and strain-rate dependent plastic deformation behavior controlled by individual crystal lattice defects. Classical empirical and semiempirical interatomic potentials have limited capability in modeling defect properties such as the screw dislocation core structures and Peierls barriers in the BCC structure. Machine learning (ML) potentials, trained on DFT-based datasets, have shown some successes in reproducing dislocation core properties. However, in group VB TMs, the most widely used DFT functionals produce erroneous shear moduli $C_{44}$ which are undesirably transferred to machine-learning interatomic potentials, leaving current ML approaches unsuitable for this important class of metals and alloys. Here, we develop two interatomic potentials for BCC vanadium (V) based on (i) an extension of the partial electron density and screening parameter in the classical semiempirical modified embedded-atom method (XMEAM-V) and (ii) a recent hybrid descriptor in the ML Deep Potential framework (DP-HYB-V). We describe distinct features in these two disparate approaches, including their dataset generation, training procedure, weakness and strength in modeling lattice and defect properties in BCC V. Both XMEAM-V and DP-HYB-V reproduce a broad range of defect properties (vacancy, self-interstitials, surface, dislocation) relevant to plastic deformation and fracture. In particular, XMEAM-V reproduces nearly all mechanical and thermodynamic properties at DFT accuracies and with $C_{44}$ near the experimental value. XMEAM-V also naturally exhibits the anomalous slip at 77 K widely observed in group VB and VIB TMs and outperforms all existing, publically available interatomic potentials for V. The XMEAM thus provides a practical path to developing accurate and efficient interatomic potentials for nonmagnetic BCC TMs and possibly multiprincipal element TM alloys.

Contrary to prior calculations, the Ni-rich ordered structures of the Cr-Ni alloy system are found to be antiferromagnetic under semilocal density-functional theory. The optimization of local magnetic moments significantly increases the driving force for the formation of ${\mathrm{CrNi}}_2$, the only experimentally observed intermetallic phase. This structure's ab initio magnetism appears well described by a Heisenberg Hamiltonian with longitudinal spin fluctuations; itinerant Cr moments are induced only by the strength of exchange interactions. The role of magnetism at temperature is less clear and several scenarios are considered based on a review of experimental literature, specifically a failure of the theory, the existence of an overlooked magnetic phase transition, and the coupling of antiferromagnetism to chemical ordering. Implications for related commercial and high-entropy alloys are discussed for each case.

A method for predicting the ideal strength of metallic glasses based on materials properties—without fitting parameters—is presented, and its accuracy demonstrated for multiple alloys. The theoretical basis for these predictions is the stress-activated transformation of short-range ordered atomic structures into flowing amorphous interfaces with the properties of a supercooled liquid. Our theory for pure metals is extended through a regular solution model in which the enthalpy of fusion is mollified by approximate changes in coordination number between the solid and liquid phases. Additional parameters come from empirical material properties such as density, heat of fusion, and melting temperature.

The formation and migration of charged oxygen vacancies at a ${90}^{\circ }\left(100\right)\left[001\right]$ domain wall in orthorhombic ${\mathrm{CaTiO}}_3$ were studied by means of atomistic simulation techniques. The segregation-energy profiles of an oxygen vacancy across the wall were revealed through careful discrimination to display hyperbolic secant forms, with a domain-wall width consistent with that obtained from a tilt-angle analysis. Equilibrium distributions of oxygen vacancies across the ferroelastic domain wall in a nominally undoped (weakly acceptor-doped) sample were calculated self-consistently (assuming a sech segregation-energy profile) for various temperatures. The calculations indicate a positively charged domain wall, arising from high vacancy concentrations, compensated by negative space-charge layers, in which vacancies are depleted. The activation barriers for oxygen-vacancy migration along the domain wall, determined from nudged-elastic-band calculations, revealed that the long-range diffusion of oxygen vacancies along the wall is neither strongly enhanced nor strongly hindered.

While several studies confirmed that machine-learned potentials (MLPs) can provide accurate free energies for determining phase stabilities, the abilities of MLPs for efficiently constructing a full phase diagram of multicomponent systems are yet to be established. In this work, by employing neural network interatomic potentials (NNPs), we demonstrate construction of the MgO-CaO eutectic phase diagram with temperatures up to 3400 K, which includes liquid phases. The NNP is trained over trajectories of various solid and liquid phases at several compositions that are calculated within the density functional theory (DFT). For the exchange-correlation energy among electrons, we compare the PBE and SCAN functionals. The phase boundaries such as solidus, solvus, and liquidus are determined by free energy calculations based on the thermodynamic integration or semigrand ensemble methods, and salient features in the phase diagram such as solubility limit and eutectic points are well reproduced. In particular, the phase diagram produced by the SCAN-NNP closely follows the experimental data, exhibiting both eutectic composition and temperature within the measurements. On a rough estimate, the whole procedure is more than 1000 times faster than pure-DFT based approaches. We believe that this work paves the way to fully ab initio calculation of phase diagrams.

A large magnetocaloric effect with its maximum near the boiling point of natural gas occurs in a rare-earth intermetallic compound ${\mathrm{Nd}}_2\mathrm{In}$. While behaviors of physical properties indicate that paramagnetic-ferromagnetic transformation supporting the large magnetocaloric effect is first order in nature, temperature-dependent crystallographic study reveals no changes in lattice symmetry and lack of discontinuities either in phase volume or lattice parameters. The borderline first-order nature of phase transformation in ${\mathrm{Nd}}_2\mathrm{In}$ is markedly different from conventional first-order magnetic transitions occurring in other members of the family – isostructural ${\mathrm{Pr}}_2\mathrm{In}$ and nonisostructural ${\mathrm{Eu}}_2\mathrm{In}$.

The realization of the stable structure of Heusler compounds and the study of different properties is an important step for their potential application in spintronics and magnetoelectronic devices. In this paper, using the plane-wave pseudopotential method within the framework of density functional theory (DFT), we investigate 25 quaternary Heusler compounds for their electronic, magnetic, and mechanical properties. The Open Quantum Materials Database (OQMD) is used to screen a large number of compounds to narrow down the possible synthesizable materials. The convex-hull distance and elastic constants are exploited to confirm the thermodynamic and mechanical stability of the compounds. The careful study of the different structures suggests that five of the compounds crystallize in a type-1 structure whereas 20 compounds adopt type-3 structure. The possible explanation for the observed behavior is made by invoking electronegativity arguments and through the study of individual spin magnetic moments in different structures. The compounds with diverse electronic and magnetic properties such as half-metallicity, spin gapless semiconducting behavior, and nonmagnetic semiconducting property have been identified.

The synthesis, structure, and properties of single crystalline ${\mathrm{Mn}}_{1.05}\mathrm{Bi}$ in the polar space group $Fdd2$ are reported. ${\mathrm{Mn}}_{1.05}\mathrm{Bi}$ is isostructural to the previously reported ${\mathrm{Mn}}_{1.05}{\mathrm{Rh}}_{0.02}\mathrm{Bi}$, with both ordered interstitials and ordered vacancies of Mn leading to Kagome-like layers. The ordering of the interstitials breaks inversion symmetry and forces the material into a polar space group. DC magnetization reveals ferromagnetic properties with a huge magnetic anisotropy, with the magnetization pinned along the $a$ axis (the stacking axis), and multiple magnetic transitions which retain this anisotropy. AC measurements confirm these transitions and show very sluggish spin dynamics along the $a$ axis, with a very large temperature-dependent out-of-phase response. Heat-capacity measurements reveal the presence of Schottky defects, and resistivity measurement confirms the transitions and reveal the material to be dominated by magnetic scattering. Overall, ${\mathrm{Mn}}_{1.05}\mathrm{Bi}$ shows magnetic properties markedly different from hexagonal, NiAs-type MnBi, driven by ordered interstitials and vacancies of Mn, stabilizing a likely complex magnetic structure with strongly temperature-dependent spin dynamics. This is supported by density-functional theory calculations, which suggest a strongly anisotropic noncollinear ground state driven by Kagome layers and asymmetric Mn environments.

Metallic delafossite materials (e.g., $\mathrm{PdCo}{\mathrm{O}}_2, \mathrm{PtCo}{\mathrm{O}}_2$), have attracted much recent attention due to record-high oxide conductivities, the origins of which remain unclear. Relatively little attention has been paid to their related thermal properties, however. We address this here via wide temperature range experimental studies of the crystal structure, thermal expansion, and specific heat of single-crystal $\mathrm{PdCo}{\mathrm{O}}_2$, combined with density-functional theory (DFT) calculations of the electronic and phononic densities of states, and thus thermal properties. $\mathrm{PdCo}{\mathrm{O}}_2$ is shown to retain the $R\overline{3}m$ space group from 12 to 1000 K, exhibiting a- and c-axis thermal expansion in good quantitative agreement with DFT-based lattice dynamics calculations. The Co-O bond lengths additionally elucidate the stability of the low-spin state of the nominally ${\mathrm{Co}}^{3+}$ ions, which is a notable difference between the edge-shared Co-O octahedra in $\mathrm{PdCo}{\mathrm{O}}_2$ and the corner-shared octahedra in Co-based perovskites. Measurements of specific heat from 1.9 to 400 K provide accurate values for the Debye temperature and Sommerfeld coefficient, the phononic part being describable via a combined Debye-Einstein approach (accounting for high-frequency oxygen-related optical phonons), with excess intermediate-temperature specific heat due to a prominent low-energy peak in the phonon density of states. Most significantly, all electronic and phononic contributions to the specific heat are shown to be remarkably closely reproduced by DFT-based calculations, establishing quantitative understanding of key thermal properties of the metallic delafossite $\mathrm{PdCo}{\mathrm{O}}_2$.

Normal grain growth in multiphase polycrystalline systems includes sluggish evolution of minor-phase grains and relatively faster growth of the major-phase grains. Which of these two factors predominantly affects the growth kinetics of the overall microstructure remains an open question. Critical insights to answer this question are offered in this work, through data science techniques, by cumulatively analyzing the influence of the evolution rate of all constituent phase grains on the simulated growth kinetics of duplex and triplex microstructures. Moreover, predictive models relating volume fraction of the constituent phases to the growth kinetics of multiphase systems are developed and are extensively validated. The simulated data set encompassing a large area of interest and feature-rich information, which lends itself to the current analyses, is built through a suitable data-acquisition strategy, and a thermodynamically consistent multiphase-field approach. Statistical analyses of 14 triplex and 5 duplex microstructures, included in the data set, unravel that the grain-growth kinetics in both multiphase systems is primarily governed by the evolution rate of phase(s) occupying larger volume fraction. This effect is observed in spite of the sluggish growth exhibited by the minor-phase grains. Furthermore, the predictive models, developed using regression and feed-forward neural network, indicate a nonlinear, but proportional, relation between the phase fractions and kinetic coefficients that quantify the grain-growth rate of multiphase systems.

Structural, electronic, and thermodynamic properties of native defects in GaS and GaSe monolayers are investigated by means of accurate ab initio calculations. Based on their charge transition levels we assess the influence of the studied defects on the electrical properties of the monolayers. Specifically, we show that native defects do not behave as shallow dopants and their presence cannot account for the experimentally observed intrinsic doping. In addition, we predict that native defects are efficient compensation and recombination centers. Besides pointing out their detrimental nature, we also calculate the corresponding finite-temperature formation energies and provide a window of growth conditions able to reduce the concentration of all relevant native defects.

Two-dimensional materials have great potential for applications as high-performance electronic devices and efficient thermal rectificators. Among them, pristine phosphorene, a single layer of black phosphorus, has shown promising properties such as ultrahigh charge mobility, a tunable band gap, and mechanical flexibility. However, the introduction of extended structural defects such as grain boundaries (GBs) has, in general, a detrimental influence on the electronic and thermal transport properties by causing additional scattering events. In this computational study, based on a combination of a density-functional parametrized tight-binding approach with the Landauer theory of quantum transport, we show that applying a strain can help to partially counteract this effect. We exemplify this by addressing the electronic and phononic transmission of two specific grain boundaries containing $5|7$ (GB1) and $4|8$ (GB2) defects, respectively. Under uniaxial strain, the electronic band gaps can be reduced for both types of GB, while the respective thermal conductance is only weakly affected despite rather strong changes in the frequency-resolved phonon transmission. The combination of both effects mainly produces an increase of about a factor of 2 in the thermoelectric figure of merit $ZT$ for a GB2 system. Hence, our results provide insights into the manipulation of transport properties as well as the generation of potential thermoelectric materials based on phosphorene.