To understand spin interactions in materials of the ${\mathrm{Cu}}_2\mathrm{Sb}$ structure type, inelastic neutron scattering of ${\mathrm{Fe}}_2\mathrm{As}$ single crystals was examined at different temperatures and incident neutron energies. The experimental phonon spectra match well with the simulated phonon spectra obtained from density functional theory (DFT) calculations. The measured magnon spectra were compared to the simulated magnon spectra obtained via linear spin wave theory with the exchange coupling constants calculated using the spin polarized, relativistic Korringa-Kohn-Rostoker method in Zhang et al. [Inorg. Chem. 52, 3013 (2013)]. The simulated magnon spectra broadly agree with the experimental data although the energy values are underestimated along the $K$ direction. Exchange coupling constants between Fe atoms were refined by fits to the experimental magnon spectra, revealing stronger nearest-neighbor Fe1-Fe1 exchange coupling than previously reported. The strength of this exchange coupling is almost an order of magnitude higher than other exchange interactions despite the three-dimensional nature of the phonon interactions. The lack of scattering intensity at energies above 60 meV makes unconstrained determination of the full set of exchange interactions difficult, which may be a fundamental challenge in metallic antiferromagnets.

To improve power efficiency and endurance of magnetic memory technologies, a voltage-controlled mechanism is desirable. The voltage control of magnetic anisotropy (VCMA) effect in MgO stacks is a promising option, however, its strength is too low for memory applications. Replacing the standard MgO layer by an oxide with a higher permittivity $\kappa$ may help improve the VCMA strength. We demonstrate a VCMA effect up to $\xi =75$ fJ/Vm at room temperature in a Co∖Pt bilayer grown on atomic layer deposited (ALD) high-$\kappa {\mathrm{SrTiO}}_3$ (STO). After treating the STO surface with isopropanol, a thin ${\mathrm{CoO}}_x$ interfacial layer is observed, enabling VCMA. Upon cooling down from room temperature to 200 K, the VCMA effect strength increases by a factor of two. This increase is incompatible with the expected Arrhenius temperature dependence for an ionic effect and thus we argue that the observed VCMA effect is electronic. Electronic VCMA is desirable for adequate memory endurance, and hence the approach proposed here has great potential for applications.

The recently discovered ${\mathrm{LaFe}}_2{\mathrm{As}}_2$ superconducting compound, member of the 122 family of iron pnictide superconductors, becomes superconducting below $T_c\approx 13\mathrm{K}$, yet its nominal doping apparently places it in the extreme overdoped limit, where superconductivity should be suppressed. In this work, we investigate the normal state of magneto- and thermoelectric transport and specific heat of this compound. The experimental data are consistent with the presence of highly compensated electron and hole bands, with ∼0.42 electrons per unit cell just above $T_{\mathrm{c}}$, and high effective masses $\sim 3m_0$. The temperature dependence of transport properties strongly resembles that of conventional superconductors, pointing to a key role of electron-phonon coupling. From this evidence, ${\mathrm{LaFe}}_2{\mathrm{As}}_2$ can be regarded as the connecting compound between unconventional and conventional superconductors.

The discovery of a two-dimensional electron system (2DES) at the interfaces of perovskite oxides such as $\mathrm{LaAl}{\mathrm{O}}_3$ and $\mathrm{SrTi}{\mathrm{O}}_3$ has motivated enormous efforts in engineering interfacial functionalities with this type of oxide heterostructures. However, the fundamental origins of the 2DES are still not understood, e.g., the microscopic mechanisms of coexisting interface conductivity and magnetism. Here we report a comprehensive spectroscopic investigation on the depth profile of 2DES-relevant $\mathrm{Ti}3d$ interface carriers using depth- and element-specific techniques like standing-wave excited photoemission and resonant inelastic scattering. We found that one type of Ti $3d$ interface carriers, which give rise to the 2DES are located within three unit cells from the n-type interface in the $\mathrm{SrTi}{\mathrm{O}}_3$ layer. Unexpectedly, another type of interface carriers, which are polarity-induced Ti-on-Al antisite defects, reside in the first three unit cells of the opposing $\mathrm{LaAl}{\mathrm{O}}_3$ layer (∼10 Å). Our findings provide a microscopic picture of how the localized and mobile Ti $3d$ interface carriers distribute across the interface and suggest that the 2DES and 2D magnetism at the $\mathrm{LaAl}{\mathrm{O}}_3/\mathrm{SrTi}{\mathrm{O}}_3$ interface have disparate explanations as originating from different types of interface carriers.

Hydrogen is rarely observed as a ligand in hypervalent species, however, we find that high-pressure hydrogenation may stabilize hypervalent hydrogen-rich materials. Focusing on ternary silicon hydrides via lithium doping, we find anions composed of hypervalent silicon with H ligands formed under high pressure. Our results reveal two different hypervalent anions: layered ${\mathrm{SiH}}_5{}^{-}$ and tricapped triangular prismatic ${\mathrm{SiH}}_6{}^{2-}$. These differ from octahedral ${\mathrm{SiH}}_6{}^{2-}$ described in earlier studies. In addition, there are further hydrogen-rich structures, ${\mathrm{Li}}_3{\mathrm{SiH}}_{10}$ and ${\mathrm{Li}}_2{\mathrm{SiH}}_{6+\delta }$, which may be stabilized at high pressure. Our work provides pointers to future investigations on hydrogen-rich materials.

The intrinsic spin Hall conductivity of typical topological insulators ${\mathrm{Sb}}_2{\mathrm{Se}}_3,$ ${\mathrm{Sb}}_2{\mathrm{Te}}_3,$ ${\mathrm{Bi}}_2{\mathrm{Se}}_3,$ and ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ in the bulk form, is calculated from first principles by using density functional theory and the linear response theory in a maximally localized Wannier basis. The results show that there is a finite spin Hall conductivity of 100–200 $(\hslash /2\mathrm{e})(\mathrm{S}/\mathrm{cm})$ in the vicinity of the Fermi energy. Although the resulting values are an order of magnitude smaller than that of heavy metals, they show a comparable spin Hall angle due to their relatively lower longitudinal conductivity. The spin Hall angle for different compounds are then compared to that of recent experiments on topological-insulator/ferromagnet heterostructures. The comparison suggests that the role of the bulk in generating a spin current and consequently a spin torque in magnetization switching applications is comparable to that of the surface including the spin-momentum locked surface states and the Rashba-Edelstein effect at the interface.

Mechanically stable sphere packings are generated in three-dimensional space using the discrete element method, which spans a wide range in structural order, ranging from fully amorphous to quasiordered structures, as characterized by the bond orientational order parameter. As the packing pressure $p$ varies from the marginally rigid limit at the jamming transition ($p\approx 0$) to that of more robust systems ($p\gg 0$), the coordination number $z$ follows a familiar scaling relation with pressure, namely, $\mathrm{\Delta }z=z-z_c\sim p^{1/2}$, where $z_c=2d=6$ ($d=3$ is the spatial dimension). While it has previously been noted that $\mathrm{\Delta }z$ does indeed remain the control parameter for determining the packing properties, here we show how the packing structure plays an influential role on the mechanical (elastic) properties of the packings. Specifically, we find that the elastic (bulk $K$ and shear $G$) moduli, generically referred to as $M$, become functions of both $\mathrm{\Delta }z$ and the structure, to the extent that $M-M_c\sim \mathrm{\Delta }z$. Here, $M_c$ are values of the elastic moduli at the jamming transition, which depend on the structure of the packings. In particular, the zero shear modulus, $G_c=0$, is a special feature of fully amorphous packings, whereas more ordered packings take larger positive values, $G_c>0$. The finite $G_c>0$ in the ordered packings excites acoustic vibrations which add to floppylike modes controlled by $\mathrm{\Delta }z$ and enhance the plateau in the vibrational density of states.

Third bodies are ubiquitous in meso- and nanoscopic friction pairs. So far, few researchers have investigated their effects on structural superlubric contact which is a promising approach in reducing friction and wear. In this work, using molecular dynamics simulations we find that intercalated molecules, a typical type of third body, have multiple effects in graphite contacts. While for commensurate contacts the friction decreases with the coverage of intercalated molecules in general, for an incommensurate interface a complex dependence is observed. With state-of-the-art detailed analysis, we reveal that with coverage increases, for commensurate contacts the change in contact area accounts for the variation in friction. For incommensurate contacts, the resistance due to the direct interaction between intercalated molecules and the slider dominates the energy dissipation. For both kinds of contacts, complex transitions in the structures and kinetic behaviors of intercalated molecules are observed. Our results provide an overall picture for the effects of intercalated molecules on friction in structural superlubric systems.

We report a synthesis method, and structural and magnetic characterization, for the fluoroperovskite ${\mathrm{NaFeF}}_3$. We have developed a wet-chemical method that allows preparation of large volumes of air-sensitive fluoroperovskites with high purity. ${\mathrm{NaFeF}}_3$ has a Néel temperature ($T_N$) of 90 K and a Weiss constant ($\theta$) of −124 K, corresponding to dominant antiferromagnetic interactions. Below $T_N$, a slight difference is observed between zero-field and field-cooled samples, indicating spin canting and weak ferromagnetism. AC magnetometry supports that weak ferromagnetism is inherent to ${\mathrm{NaFeF}}_3$ and not due to an impurity. From powder neutron diffraction data, we describe the magnetic structure precisely as a weakly canted G-type (magnetic space group $P{n}^{\prime }m{a}^{\prime }$). A ferromagnetic component is allowed in $P{n}^{\prime }m{a}^{\prime }$, however, this component is too small to be confirmed on the basis of powder neutron diffraction data and may be absent in zero magnetic fields.

Polycrystalline members of the ${\text{GaV}}_4{\text{S}}_{8-y}{\text{Se}}_y$ family of materials with small levels of substitution between $0\le y\le 0.5$ and $7.5\le y\le 8$ have been synthesized in order to investigate their magnetic and structural properties. Substitutions to the skyrmion hosting parent compounds ${\text{GaV}}_4{\text{S}}_8$ and ${\text{GaV}}_4{\text{Se}}_8$ are found to suppress the temperature of the cubic-to-rhombohedral structural phase transition that occurs in both end compounds and to create a temperature region around the transition where there is a coexistence of these two phases. Similarly, the magnitude of the magnetization and temperature of the magnetic transition are both suppressed in all substituted compounds until a glassy-like magnetic state is realized. There is evidence from the ac susceptibility data that skyrmion lattices with similar dynamics to those in ${\text{GaV}}_4{\text{S}}_8$ and ${\text{GaV}}_4{\text{Se}}_8$ are present in compounds with very low levels of substitution, $0<y<0.2$ and $7.8<y<8$, however, these states vanish at higher levels of substitution. The magnetic properties of these substituted materials are affected by the substitution altering exchange pathways and resulting in the creation of increasingly disordered magnetic states.

It was recently reported that a continuous electric current is a powerful control parameter to trigger changes in the electronic structure and metal–insulator transitions (MITs) in ${\mathrm{Ca}}_2{\mathrm{RuO}}_4$. However, the spatial evolution of the MIT and the implications of the unavoidable Joule heating have not been clarified yet, often hindered by the difficulty to assess the local sample temperature. In this work, we perform infrared thermal imaging on single-crystal ${\mathrm{Ca}}_2{\mathrm{RuO}}_4$ while controlling the MIT by electric current. The change in emissivity at the phase transition allows us to monitor the gradual formation and expansion of the metallic phase upon increasing current. Our local temperature measurements indicate that, within our experimental resolution, the MIT always occurs at the same local transition temperatures, irrespective of whether it is driven by temperature or current. Our results highlight the importance of local heating, phase coexistence, and microscale inhomogeneity when studying strongly correlated materials under the flow of electric current.

Strain in metal(100) heteroepitaxial thin-film growth can lead to a variety of effects such as the formation of stacking faults (SFs). To gain a better understanding we have carried out temperature-accelerated dynamics (TAD) simulations of the submonolayer growth of Cu islands on a biaxially strained Cu substrate at 200 K. In the case of 4% compressive strain we find that SF formation occurs with a morphology very similar to the structures found experimentally in Cu/Ni(100) growth. We also find that islands play a key role by lowering the barrier for vacancy formation. In particular, once two substrate vacancies are formed and diffuse to form the appropriate configuration this leads to the formation of a SF in both the substrate and island. While the activation barrier for SF formation is very high, due to the presence of a large number of low-frequency vibrational modes, the saddle-point entropy is large while the corresponding Vineyard prefactor is more than 10 orders of magnitude larger than is typical of atomic processes in fcc metals. Similarly, an analysis of the entropy of the SF state indicates that the reverse prefactor is much smaller, while the free energy of this state is lower than that of the initial state.

One-step first principles molecular-dynamics (FPMD) simulations were carried out to investigate structural transformations in wurtzite-type AlN under compression and decompression. The hexagonal transformation path $P{6}_{3}mc\text{−}P{6}_3/mmc\text{−}Cmmm\text{−}Fm\overline{3}m$ was identified during compression at $P=50$ GPa and $T=500$ K. The inversion of this path of transformation was established at the decompression of compressed rock-salt AlN at 1500 K. The plausible origin of these transformations is established. The occurrence of intermediate and metastable phases was predicted by taking into account their dynamical and elastic instability, and for this purpose, phonon spectra and elastic constant calculations were performed. FPMD simulations were used to generate amorphous AlN and to investigate its behavior under pressure. The sequence of phase transitions: low-density amorphous—high-density amorphous—rock-salt type was revealed at $P=40$ GPa, and the thermodynamic and mechanical properties of crystalline and amorphous AlN were compared. It was found that the one-step FPMD simulations is a viable scheme to predict phase transitions in crystalline and amorphous AlN with higher precision compared with other FPMD approaches with step-wise pressure changes.

Metal oxide/water interfaces play a crucial role in many electrochemical and photocatalytic processes, such as photoelectrochemical water splitting, the creation of fuel from sunlight, and electrochemical $\mathrm{C}{\mathrm{O}}_2$ reduction. First-principles electronic structure calculations can reveal unique insights into these processes, such as the role of the alignment of the oxide electronic energy levels with those of liquid water. An essential prerequisite for the success of such calculations is the ability to predict accurate structural models of these interfaces, which in turn requires careful experimental validation. Here we report a general, quantitative validation protocol for first-principles molecular dynamics simulations of oxide/aqueous interfaces. The approach makes direct comparisons of interfacial x-ray reflectivity (XR) signals from experimental measurements and those obtained from ab initio simulations with semilocal and van der Waals functionals. The protocol is demonstrated here for the case of the ${\mathrm{Al}}_2{\mathrm{O}}_3\left(001\right)$/water interface, one of the simplest oxide/water interfaces. We discuss the technical requirements needed for validation, including the choice of the density functional, the simulation cell size, and the optimal choice of the thermodynamic ensemble. Our results establish a general paradigm for the validation of structural models and interactions at solid/water interfaces derived from first-principles simulations. While there is qualitative agreement between the simulated structures and the experimental best-fit structure, direct comparisons of simulated and measured XR intensities show quantitative discrepancies that derive from both bulk regions (i.e., alumina and water) as well as the interfacial region, highlighting the need for accurate density functionals to properly describe interfacial interactions. Our results show that XR data are sensitive not only to the atomic structure (i.e., the atom locations) but also to the electron-density distributions in both the substrate and at the interface.

We describe a systematic approach to building ab initio tight-binding models and apply this to hexagonal metals Mg and Zr. Our models contain three approximations to plane-wave density functional theory (DFT): (i) we use a small basis set, (ii) we approximate self-consistency, and (iii) we approximate many-center exchange and correlation effects. We test a range of approximations for many-center exchange-correlation and self-consistency to gauge the accuracy of each in isolation. This systematic approach also allows us to combine multiple approximations in the optimal manner for our final tight-binding models. Furthermore, the breakdown of errors into those from individual approximations is expected to be a useful guide for which approximations to include in other tight-binding models. We attempt to correct any remaining errors in our models by fitting a pair potential. Our final tight-binding model for Mg shows excellent agreement with plane-wave results for a wide range of properties (e.g., errors below 10% for self-interstitial formation energies and below 3% for equilibrium volumes) and is expected to be highly transferable due to the minimal amount of fitting. Calculations with our Zr model also show good agreement with plane-wave results (e.g., errors below 2% for equilibrium volumes) except for properties where self-consistency is important, such as self-interstitial formation energies. However, for these properties we are able to generate a tight-binding model which shows excellent agreement with non-self-consistent DFT with a small basis set (i.e., many-center effects are captured accurately by our approximations). As we understand the source of remaining errors in our Zr model we are able to outline the methods required to build upon it to describe the remaining properties with greater accuracy.

It is known that the zircon-type orthovanadates $\mathrm{RV}{\mathrm{O}}_4$ show promise in many different applications as catalysts and optical materials. In this work, we demonstrate that the $\mathrm{TbV}{\mathrm{O}}_4$ compound can be also used as magnetic refrigerant in efficient and ecofriendly cryocoolers due to its strong magnetocaloric effect at low-temperature regime. The application of a relatively low magnetic field of 2 T along the easy magnetization axis ($a$) gives rise to a maximum entropy change of about 20 J/kg K at 4 K. More interestingly, under sufficiently high magnetic fields, the isothermal entropy change $-\mathrm{\Delta }{S}_{\mathrm{T}}$ remains approximately constant over a wide temperature range which is highly appreciated from a practical point of view. In the magnetic field change of 7 T, $-\mathrm{\Delta }{S}_{\mathrm{T}}$ that reaches roughly 22 J/kg K remains practically unchanged between 0 and 34 K leading to an outstanding refrigerant capacity of about 823 J/kg. On the other hand, the lowering of crystallographic symmetry from the tetragonal to the orthorhombic structure occurring close to 33 K as confirmed by Raman scattering data results in a strong magnetic anisotropy. Accordingly, strong thermal effects can be also obtained simply by spinning the $\mathrm{TbV}{\mathrm{O}}_4$ single crystals between their hard and easy orientations in constant magnetic fields instead of the standard magnetization-demagnetization process. Such rotating magnetocaloric effects would open the way for the implementation of $\mathrm{TbV}{\mathrm{O}}_4$ in a new generation of compact and simplified magnetic refrigerators that can be dedicated to the liquefaction of hydrogen and helium.

Combining crystal structure search and first-principles calculations, we report a series of two-dimensional (2D) metal borides including orthorhombic (ort-)$M{\mathrm{B}}_6$ $(M=\mathrm{Mg},\mathrm{Ca},\mathrm{Ti},\mathrm{Y})$ and hexagonal (hex-)$M{\mathrm{B}}_6$ $(M=\mathrm{Mg},\mathrm{Ca},\mathrm{Sc},\mathrm{Ti})$. Then, we investigate their geometrical structures, bonding properties, electronic structures, mechanical properties, phonon dispersions, thermal stability, dynamic stability, charge density wave (CDW) phase transition, electron-phonon coupling (EPC), superconducting properties, and so on. Our ab initio molecular dynamics simulation results show that these $M{\mathrm{B}}_6$ can maintain their original configurations up to about 1000 or 700 K (only for hex-${\mathrm{MgB}}_6$), indicating their excellent thermal stability. All their elastic constants satisfy the Born mechanically stable criteria and no imaginary frequencies are observed in their phonon dispersions. Interestingly, there may exist a CDW phase transition for ort-${\mathrm{TiB}}_6$ from type-I to type-II $2\times 1$ supercell structure and for ort-${\mathrm{YB}}_6$ from type-I to type-III $2\times 1$ supercell structure. Besides, these 2D $M{\mathrm{B}}_6$ are all predicted to be intrinsic phonon-mediated superconductors. By analytically solving the McMillan-Allen-Dynes formula derived from the microscopic theory of Bardeen, Cooper, and Schrieffer, we obtain the superconducting transition temperature ($T_{\mathrm{c}}$) for these materials, which are in the range of 1.4–22.6 K. Among our studied $M{\mathrm{B}}_6$, the highest $T_{\mathrm{c}}$ (22.6 K) appears in hex-${\mathrm{CaB}}_6$, whose EPC constant $\left(\lambda \right)$ is 0.87. By applying tensile/compressive strains on ort-/hex-${\mathrm{CaB}}_6$, we find that the compressive strain can obviously soften the acoustic-phonon branch and enhance the EPC as well as $T_{\mathrm{c}}$. The $T_{\mathrm{c}}$ of the hex-${\mathrm{CaB}}_6$ can be increased from 22.6 to 28.4 K under compressive strain of 3%. These findings enrich the database of 2D superconductors and should stimulate experimental synthesizing and characterizing of 2D superconducting metal borides.

Doping at the Sr site in ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ is predicted to be a possible route to high-temperature superconductivity, which has not yet been experimentally achieved. We have made a comprehensive investigation on the interplay among the Ir-O-Ir bond angle, carrier density, and magnetic and transport properties of rare-earth-doped ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ by choosing Pr and Ce as the dopants. We find that compared with Pr doping, Ce doping introduces more effective charge carriers into the ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ matrix, leading to a more rapid suppression of its magnetic ordering. A metallic-like behavior has been observed in heavily Ce-doped samples. The evolutions of magnetic and transport behaviors are found to be less relevant to the distorted Ir-O-Ir bond angle. The present results suggest that the charge carrier density could be a crucial factor in determining the physical properties of rare-earth-doped ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ compounds.