Despite numerous studies on the glass transition and the related relaxation dynamics, the physical mechanism of activation of multiscale relaxation events under various external stimuli in amorphous materials is still unclear. In this study, by combining traditional differential scanning calorimetry (DSC) and Flash DSC with heating rates spanning over five orders of magnitude, the thermodynamic responses have been systematically studied for several fragile and strong metallic glasses (MGs). A common endothermic event, before the glass transition, is detected when the heating rate increases above a critical value for fragile MGs. This endothermic event is verified to represent the activation of secondary $\beta$ relaxation (Johari-Goldstein relaxation), which is commonly found in amorphous materials. For fragile MGs, with an increase in fragility, the critical heating rate to separate $\beta$ relaxation from $\alpha$ relaxation decreases. In contrast, the $\beta$ relaxation does not appear within the current experimental heating rate limit for strong glass systems. Finally, based on the potential energy landscape model and the flow unit model of the heterogeneous structure for MGs, a pathway is proposed for the fragile and strong MGs to understand the physical mechanism for the separation of $\beta$ relaxation from $\alpha$ relaxation via ultrafast heating. This study clearly demonstrates that the Flash DSC with a wide heating rate range is an effective tool to study the relaxation dynamics in amorphous materials.

While GeSn alloys with high Sn content constitute direct group-IV semiconductors, their growth on Si remains challenging. The deposition of a few monolayers of pure Sn on Ge and their overgrowth with Ge using molecular beam epitaxy can be a means of obtaining Sn-rich quantum wells with very high Sn content while maintaining high crystal quality. Here, we provide structural and compositional information on such structures with very high accuracy. Based on our characterization results we theoretically predict transition energies and compare them with experimental results from photoluminescence measurements. Our results constitute the groundwork for tuning the molecular beam epitaxy based growth of Sn-rich quantum wells and dots for applications in electronic and optoelectronic devices.

The influence of structural disorder on the thermal transport in the colusite $\mathrm{C}{\mathrm{u}}_{26}{\mathrm{V}}_2\mathrm{S}{\mathrm{n}}_6{\mathrm{S}}_{32}$ has been investigated by means of low-temperature thermal conductivity and specific heat measurements (2–300 K), ${}^{119}\mathrm{Sn}$ Mössbauer spectroscopy and temperature-dependent powder inelastic neutron scattering (INS). Variations in the high-temperature synthesis conditions act as a key parameter for tuning the degree of disorder in colusite compounds. Intriguingly, we find that all synthesized samples are disordered, the degree of which varies with the synthesis conditions used. Mössbauer data clearly evidence that Sn atoms do not solely occupy the $6c$ site of the crystal lattice but are present on possibly both the Cu and V sites, leading to a random distribution of these three cations within the unit cell. Increasing the disorder in these materials tends to lead to a smearing out of the main features in the phonon density of states measured by INS. Although the evolution of the inelastic signal upon warming is well described by a quasiharmonic approximation, elastic properties calculations indicate large average Grüneisen parameters, consistent with those determined experimentally from thermodynamic data. Increasing the level of disorder results in a decreased average Grüneisen parameter suggesting that the lowered lattice thermal conductivity is not driven by enhanced anharmonicity. These results provide experimental evidence to support that the remarkable changeover in the lattice thermal conductivity from crystalline to glasslike is solely driven by enhanced disorder accompanied by local lattice distortions.

Understanding and quantifying mass-transport properties in multicomponent alloys poses the issues of (1) harvesting the atomic scale data, (2) setting up and solving a complex mathematical problem, and (3) identifying the underlying physics. This Rapid Communication demonstrates that the kinetic cluster expansion formalism associated with the open-source code kineclue provides the means to solve the last two issues. The efficiency of this framework is illustrated on the study of the temperature-composition dependence of the flux coupling properties in Fe(C), Fe(O), and Fe(C,O) alloys, with a particular emphasis on the synergetic effects between alloying elements.

The interaction between a ${}^1/{}_2\left[\overline{1}10\right]\left(111\right)$ edge dislocation and a (001) Guinier-Preston (GP) zone in dilute Al-Cu alloys is studied via atomistic modeling. In stark contrast to the previously reported Orowan looping mechanism where the GP zone remains intact after yield, we discover a competing mechanism where the dislocation cuts the GP zone into two pieces. We identify the key atomic process triggering the cutting mechanism and calculate its activation barrier at various strains. In further conjunction with the transition state theory, the occurrence probability of a trigger event is mapped out over a broad range of $T\text{—}\dot{\varepsilon }$ parameter space. The predictions of the so-constructed mechanism map are validated by parallel MD simulations. The implications of our findings regarding the discrepancies between the existing age hardening model and experiments are also discussed.

The Organic Materials Database (OMDB) is an open database hosting about 22 000 electronic band structures, density of states, and other properties for stable and previously synthesized three-dimensional organic crystals. The web interface of the OMDB offers various search tools for the identification of novel functional materials such as band structure pattern matching and density of states similarity search. In this work, the OMDB is extended to include magnetic excitation properties. For inelastic neutron scattering, we focus on the dynamic structure factor $S(\mathbf{q},\omega )$ which contains information on the excitation modes of the material. We introduce a new dataset containing atomic magnetic moments and Heisenberg exchange parameters for which we calculate the spin wave spectra and dynamic structure factor with linear spin wave theory and atomistic spin dynamics. We thus develop the materials informatics tools to identify novel functional organic and metalorganic magnets.

The ability to design crystal lattices with specific sublattice geometries has long been sought after especially in the context of harnessing magnetic frustration to elicit emergent physics. One approach which has seen some success recently is to design magnetic sublattices chemically through the use of nonmagnetic linker ligands as scaffolding for the magnetic ions. Here we report the magnetic properties of one such family of materials, the transition metal ($\mathit{TM}$) selenite hydrates with chemical formula $T{M}_3{\left({\mathrm{SeO}}_3\right)}_3{\mathrm{H}}_2\mathrm{O}$. These materials link highly distorted $\mathit{TM}{\mathrm{O}}_6$ octahedra via nonmagnetic [${\mathrm{SeO}}_3{]}^{2+}$ linkers. Studying members with $\mathit{TM}$ = Mn, Co, and Ni we use magnetic susceptibility and neutron powder diffraction to identify antiferromagnetic order in all three compounds with moderate frustration indexes. A comparison of the magnetic structures suggests that while the overall structure of the ${\mathrm{SeO}}_3$ scaffolding remains unchanged the $\mathit{TM}$ effects changes in the magnetic properties through tuning the internal bonding parameters and the single-ion physics. Changing from Mn to Co is found to be particularly consequential manifesting changes in both the ordered-moment direction and in the direction of the ordering vector. Field-dependent measurements of the susceptibility and heat capacity reveal metamagnetic transitions in both ${\mathrm{Mn}}_3{\left({\mathrm{SeO}}_3\right)}_3{\mathrm{H}}_2\mathrm{O}$ and ${\mathrm{Co}}_3{\left({\mathrm{SeO}}_3\right)}_3{\mathrm{H}}_2\mathrm{O}$ indicating nearby magnetic ground states accessible under relatively small applied fields. Density functional theory calculations broadly confirm these results, showing both a sensitivity of the magnetic structure to the $\mathit{TM}$ and its local environment. Although no spin liquid behavior is achieved, these results suggest the fruitfulness of such synthesis philosophies and encourage future work to engender higher frustration in these materials via doping, field, pressure, or larger linker ligands.

In this paper we report successful synthesis and magnetic properties of $\left(\mathrm{Ca},\mathrm{Na}\right){\left(\mathrm{Zn},\mathrm{Mn}\right)}_2\mathrm{S}{\mathrm{b}}_2$ as a ferromagnetic dilute magnetic semiconductor (DMS). In this DMS material the concentration of magnetic moments can be controlled independently from the concentration of electric charge carriers that are required for mediating magnetic interactions between these moments. This feature allows us to separately investigate the effect of carriers and of spins on the ferromagnetic properties of this DMS alloy, and particularly of its critical ferromagnetic behavior. We use a modified Arrott plot analysis to establish critical exponents β, γ, and δ for this alloy. We find that at low Mn concentrations $(<10$ at. %), it is governed by short-range three-dimensional (3D)-Ising-like behavior, with experimental values of β, γ, and δ very close to theoretical 3D-Ising values of 0.325, 1.24, and 4.815. However, as the Mn concentration increases, this DMS material exhibits a mixed-phase behavior, with γ retaining its 3D-Ising characteristics, but β crossing over to longer-range mean-field behavior.

Some metal complexes and clusters can crystalize into structures with high rotational symmetry in the asymmetric unit cell. For such systems, magic-angle-spinning nuclear magnetic resonance (NMR) is particularly appropriate for detecting these symmetries and the corresponding local atomic coordination. In this paper, we study three related metal-atom clusters: ${\mathrm{LiAlH}}_4, {\mathrm{LiAlPh}}_4$, and the $\left[\mathrm{Li}{\left({\mathrm{Bu}}_2\mathrm{O}\right)}_3\right]\left[{\mathrm{Li}}_4{\mathrm{Al}}_5{\mathrm{Ph}}_{12}\right]$ cluster, which we recently synthesized. For the two trivalent aluminum species, we compare NMR parameters obtained from experiment and density-functional theory (DFT) calculations, including chemical shielding, nuclear quadrupolar interactions, and the Al-C1 $J$-coupling constant of ${\mathrm{LiAlPh}}_4$. The NMR parameters were extracted from measurements at two very different magnetic fields, 11.7 T and 2.35 T. The low field measurements were crucial for both the direct spectra observation of the Al-C1 $J$-coupling and the precise determination of the Al quadrupolar interaction in ${\mathrm{LiAlPh}}_4$. We also present DFT-derived NMR parameters for the larger $\left[\mathrm{Li}{\left({\mathrm{Bu}}_2\mathrm{O}\right)}_3\right]\left[{\mathrm{Li}}_4{\mathrm{Al}}_5{\mathrm{Ph}}_{12}\right]$ cluster.

Perovskite chromium oxyfluoride $\mathrm{SrCr}{\mathrm{O}}_{2.6}{\mathrm{F}}_{0.4}$ epitaxial thin films were fabricated via low-temperature topotactic fluorination of $\mathrm{SrCr}{\mathrm{O}}_{2.8}$ precursor with polyvinylidene fluoride as a fluorine source. The obtained $\mathrm{SrCr}{\mathrm{O}}_{2.6}{\mathrm{F}}_{0.4}$ thin film had a different chemical composition than that of bulk polycrystalline $\mathrm{SrCr}{\mathrm{O}}_{2.8}{\mathrm{F}}_{0.2}$, possibly due to the higher reactivity of such thin film. Both the precursor and fluorinated thin films containing $\mathrm{C}{\mathrm{r}}^{3.6+}$ exhibited insulating behavior with band gaps of ∼0.6 eV, in contrast to metallic $\mathrm{SrCr}{\mathrm{O}}_3$ with $\mathrm{C}{\mathrm{r}}^{4+}$ at 300 K. The experimentally observed valence and conduction bands of the $\mathrm{SrCr}{\mathrm{O}}_{2.8}$, $\mathrm{SrCr}{\mathrm{O}}_3$, and $\mathrm{SrCr}{\mathrm{O}}_{2.6}{\mathrm{F}}_{0.4}$ thin films suggested that the spectral weight transfer from the coherent part to the incoherent part with the reduction of Cr valences was caused by strong electron correlation effects. This study offers a fundamental understanding of the fluorine doping effects on a crystalline structure and the correlated electronic performance of chromium oxides.

The anomalous Nernst effect (ANE)—the generation of a transverse electric voltage by a longitudinal heat current in conducting ferromagnets or antiferromagnets—is an appealing approach for thermoelectric power generation in spin caloritronics. The ANE in antiferromagnets is particularly convenient for the fabrication of highly efficient and densely integrated thermopiles as lateral configurations of thermoelectric modules increase the coverage of heat source without suffering from the stray fields that are intrinsic to ferromagnets. In this work, using first-principles calculations together with a group theory analysis, we systematically investigate the spin-order-dependent ANE in noncollinear antiferromagnetic Mn-based antiperovskite nitrides ${\mathrm{Mn}}_{3}X\mathrm{N}(X=\text{Ga}$, Zn, Ag, and Ni). The ANE in ${\mathrm{Mn}}_{3}X\mathrm{N}$ is forbidden by symmetry in the $R1$ phase but amounts to its maximum value in the $R3$ phase. Among all ${\mathrm{Mn}}_{3}X\mathrm{N}$ compounds, ${\mathrm{Mn}}_3\mathrm{NiN}$ presents the most significant anomalous Nernst conductivity of $1.80\mathrm{A}{\mathrm{K}}^{-1}{\mathrm{m}}^{-1}$ at 200 K, which can be further enhanced if strain, electric, or magnetic fields are applied. The ANE in ${\mathrm{Mn}}_3\mathrm{NiN}$, being one order of magnitude larger than that in the famous ${\mathrm{Mn}}_3\mathrm{Sn}$, is the largest one discovered in antiferromagnets so far. The giant ANE in ${\mathrm{Mn}}_3\mathrm{NiN}$ originates from the sharp slope of the anomalous Hall conductivity at the Fermi energy, which can be understood well from the Mott relation. Our findings provide a host material for realizing antiferromagnetic spin caloritronics that promises exciting applications in energy conversion and information processing.

Use of amorphous silicon ($a$-Si) and hydrogenated amorphous silicon ($a$-Si:H) in photovoltaics has been limited by light-induced degradation (the Staebler-Wronski effect) and low hole mobilities, and voids have been implicated in both problems. Accurately modeling the void microstructure is critical to theoretically understanding the cause of these issues. Previous methods of modeling voids have involved removing atoms according to an a priori idea of void structure and/or using computationally expensive molecular dynamics. We propose a new fast and unbiased approach based on the established and efficient Wooten-Winer-Weaire Monte Carlo method, by using a range of fixed densities to generate equilibrium structures of $a$-Si and $a$-Si:H that maintain 4-coordination. We find a smooth evolution in bond lengths, bond angles, and bond angle deviations $\mathrm{\Delta }\theta$ as the density is changed around the equilibrium value of $4.9\times {10}^{22}$ $\mathrm{atoms}/{\mathrm{cm}}^3$. However, a significant change occurs at densities below $4.3\times {10}^{22}$ $\mathrm{atoms}/{\mathrm{cm}}^3$, where voids begin to form to relieve tensile stress, akin to a cavitation process in liquids. We find both small voids (radius $\sim 3$ Å) and larger ones (up to 7 Å), which compare well with available experimental data. The voids have an influence on atomic structure up to 4 Å beyond the void surface and are associated with decreasing structural order, measured by $\mathrm{\Delta }\theta$. We also observe an increasing medium-range dihedral order with increasing density. Our method allows fast generation of statistical ensembles, resembles a physical process during experimental deposition, and provides a set of void structures for further studies of their effects on degradation, hole mobility, two-level systems, thermal transport, and elastic properties. The basic concept of generating voids at low density is applicable to other amorphous materials.

The interaction of hydrogen (H) with dislocations in tungsten (W) must be understood in order to model the mechanical response of future plasma-facing materials for fusion applications. Here, hybrid quantum mechanics/molecular mechanics (QM/MM) simulations are employed to study the $\langle 111\rangle$ screw dislocation glide in W in the presence of H, using the virtual work principle to obtain energy barriers for dislocation glide, H segregation, and pipe diffusion. We provide a convincing validation of the QM/MM approach against full DFT energy-based methods. This is possible because the compact core and relatively weak elastic fields of $\langle 111\rangle$ screw dislocations allow them to be contained in periodic DFT supercells. We also show that H segregation stabilizes the split-core structure while leaving the Peierls barrier almost unchanged. Furthermore, we find an energy barrier of less than 0.05 eV for pipe diffusion of H along dislocation cores. Our quantum-accurate calculations provide important reference data for the construction of larger-scale material models.

Layered materials can usually grow without strain on top of distinct substrates if the only interaction between them is due to van der Waals forces. In such a scenario it would be expected that the heterointerface made up of weak bounds would not affect the overlayed material significantly for several large lattice-mismatched systems. Here we have studied the first stages of the heteroepitaxial growth of layered bismuth telluride topological insulator on top of highly oriented pyrolitic graphite (HOPG) by molecular beam epitaxy. Samples were investigated by atomic force microscopy (AFM), synchrotron x-ray diffraction (XRD), and micro-Raman spectroscopy. AFM images show hexagonal/triangular flat islands with exposed HOPG areas for the low coverage regime, and the lattice parameter of these $\mathrm{B}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_3$ structures were measured by XRD. The existence of pseudomorphic strain at the initial $\mathrm{B}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_3$ layers was retrieved by both XRD and Raman spectroscopy. We have found evidence that $\mathrm{B}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_3$ layers near the interface are subject to an in-plane compressive strain, leading to a pseudomorphic out-of-plane lattice expansion. Furthermore, the presence of $\mathrm{B}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_3$ islands locally distorts the topmost layer of HOPG, resulting in tensile strain which was measured by Raman spectroscopy. The observed relaxation of 0.1–0.2% for each van der Waals gap is used to calculate elastic constants of $\mathrm{B}{\mathrm{i}}_2$ bilayers, which are crucial building blocks for the formation of other $\mathrm{B}{\mathrm{i}}_x\mathrm{T}{\mathrm{e}}_y$ topological insulator compounds. Finally, the impact of such a strain in $\mathrm{B}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_3$ electronic structure was investigated by density functional theory calculations. The results show that the band structure of this strained material remains unchanged at the center of the Brillouin zone, confirming the robustness of surface states, but it is consistently affected at the M and K zone edges.

The integration of oxides on Si remains challenging, which largely hampers the practical applications of oxide-based electronic devices with superior performance. Recently, $\mathrm{LaMn}{\mathrm{O}}_3/\mathrm{SrTi}{\mathrm{O}}_3$ (LMO/STO) heterostructures have gained renewed interest for the debating origin of the ferromagnetic-insulating ground state as well as for their spin-filter applications. Here we report on the structural and magnetic properties of high-quality LMO/STO heterostructures grown on silicon. The chemical abruptness across the interface was investigated by atomic-resolution scanning transmission electron microscopy. The difference in the thermal expansion coefficients between LMO and Si imposed a large biaxial tensile strain to the LMO film, resulting in a tetragonal structure with $c/a\sim 0.983$. Consequently, we observed a significantly suppressed ferromagnetism along with an enhanced coercive field, as compared to the less distorted LMO film ($c/a\sim 1.004$) grown on STO single crystal. The results are discussed in terms of tensile-strain enhanced antiferromagnetic instabilities. Moreover, the ferromagnetism of LMO on Si sharply disappeared below a thickness of 5 unit cells, in agreement with the LMO/STO case, pointing to a robust critical behavior irrespective of the strain state. Our results demonstrate that the growth of oxide films on Si can be a promising way to study the tensile-strain effects in correlated oxides, and also pave the way towards the integration of multifunctional oxides on Si with atomic-layer control.

We explore the thickness dependence of the converse piezoelectric coefficient ($d_{33}$) in epitaxial thin films of ${\mathrm{BaTiO}}_3$ (BTO) grown on (001) ${\mathrm{SrTiO}}_3$ substrates. Piezoresponse force microscope was performed using an atomic force microscope equipped with an interferometric displacement sensor allowing direct quantification of electromechanical coupling coefficients in BTO free from unwanted background contributions. We find that 80-nm-thick films exhibit a $d_{33}$ of $\sim 20.5\mathrm{pm}/\mathrm{V}$, but as the thickness is reduced, the $d_{33}$ reduces to less than 2 pm/V for a 10 nm film. To explain the atomistic origin of the effect, we performed molecular dynamics simulations with a recently developed ab initio-derived reactive force field, constructed using the ReaxFF framework. Simulations predict that under applied electric fields thin films of ${\mathrm{BaTiO}}_3$ show an increasing thickness, with compressive strain, of the region screening the depolarization-field. This study confirms quantitatively the drop in piezoelectric performance in BTO ultrathin films and again highlights the importance of the screening mechanisms when films approach the ultrathin limits in dictating the functional behaviors.

Chiral magnets are of fundamental interest and have important technological ramifications. The origin of chiral magnets lies in the Dzyaloshinskii-Moriya interaction (DMI), an interaction whose experimental and theoretical determination is laborious. We derive an expression that identifies the electric dipole moment as descriptor for the systematic design of chiral magnetic multilayers. Using density functional theory calculations, we determine the DMI of (111)-oriented metallic ferromagnetic $Z/\mathrm{Co}/\mathrm{Pt}$ multilayers of ultrathin films. The nonmagnetic layer $Z$ determines the DMI at the Co-Pt interface. The results validate the electric and magnetic dipole moments as excellent descriptors. We found a linear relation between the electric dipole moment of Pt, the Allen electronegativity of $Z$, and the contribution of Pt to the total DMI.

We examine the charge density wave (CDW) properties of $1T-{\mathrm{VSe}}_2$ crystals grown by chemical vapor transport (CVT) under varying conditions. Specifically, we find that upon lowering the growth temperature ($T_g<{630}^{\circ }\mathrm{C}$), there is a significant increase in both the CDW transition temperature and the residual resistance ratio (RRR) obtained from electrical transport measurements. Using x-ray photoelectron spectroscopy, we correlate the observed CDW properties with stoichiometry and the nature of defects. In addition, we have optimized a method to grow ultrahigh-purity $1T-{\mathrm{VSe}}_2$ crystals with a CDW transition temperature $T_{\mathrm{CDW}}=(112.7\pm 0.8)$ K and maximum residual resistance ratio $\mathrm{RRR}\approx 49$, which is the highest reported thus far. This work highlights the sensitivity of the CDW in $1T-{\mathrm{VSe}}_2$ to defects and overall stoichiometry and the importance of controlling the crystal growth conditions of strongly correlated transition metal dichalcogenides.

First-principles lattice dynamics calculations were performed to reveal an atomic-level origin of excess vibrational entropies at grain boundaries (GBs) in MgO. Fourteen symmetric tilt GBs with various structural units were systematically examined. The excess vibrational entropies were found to vary depending on the individual GBs, and as a result, the relative thermodynamic stability of the GBs studied changed with temperature. The excess GB volumes were less correlated with the excess vibrational entropies. By contrast, classifying ions in terms of their coordination numbers, bond-length changes at GBs were well correlated with atom-projected excess vibrational entropies for both Mg and O ions. Bond-length changes at GBs, which originate from changes in bond strength, are therefore a critical, well-defined descriptor for evaluating excess vibrational entropies and thereby excess free energies in MgO.