Correlative scanning transmission electron microscopy, atom probe tomography, and density functional theory calculations resolve the correlation between elastic strain fields and local impurity concentrations on the atomic scale. The correlative approach is applied to coherent interfaces in a $\kappa$-carbide strengthened low-density steel and establishes a tetragonal distortion of fcc-Fe. An interfacial roughness of $\sim 1\mathrm{nm}$ and a localized carbon concentration gradient extending over $\sim 2–3\mathrm{nm}$ is revealed, which originates from the mechano-chemical coupling between local strain and composition.

Using an ab initio evolutionary structural search, we predict two novel crystalline phases in the H-N-O ternary phase diagram at high pressure, namely, ${\mathrm{NOH}}_4$ and ${\mathrm{HNO}}_3$ (nitric acid). Our calculations show that the $C2/m$ phase of ${\mathrm{NOH}}_4$ becomes stable at 71 GPa, while the $P{2}_1/m$ phase of ${\mathrm{HNO}}_3$ stabilizes at 39 GPa. Both phases remain thermodynamically stable at least up to 150 GPa, the maximum pressure we considered. The $C2/m$ phase of ${\mathrm{NOH}}_4$ contains two O-H layers and one dumbbell cluster layer, formed by two ${\mathrm{NH}}_3$ molecules linked by a N-N covalent bond. The $P{2}_1/m$ phase of ${\mathrm{HNO}}_3$ contains a surprising quasiclover layer formed of H-N-O covalent bonds. Further calculations show that both phases are semiconducting, with band gaps of 6.0 and 2.6 eV for ${\mathrm{NOH}}_4$ and ${\mathrm{NHO}}_3$, respectively. Our calculations also confirm that the compound ${\mathrm{NOH}}_5\left({\mathrm{NH}}_3{\mathrm{H}}_2\mathrm{O}\right)$ becomes unstable at pressures above 122 GPa, decomposing into ${\mathrm{NH}}_3$ and ${\mathrm{H}}_2\mathrm{O}$.

The family of binary compounds composed of magnesium and silicon is rather rich. In addition to the well-known magnesium silicide ${\mathrm{Mg}}_2\mathrm{Si}$, other compounds, including ${\mathrm{MgSi}}_2$, ${\mathrm{Mg}}_4{\mathrm{Si}}_7$, ${\mathrm{Mg}}_5{\mathrm{Si}}_6$, MgSi, and ${\mathrm{Mg}}_9{\mathrm{Si}}_5$, have also been identified and/or proposed in precipitated Al-Mg-Si solid solutions. Nevertheless, computational studies show that only ${\mathrm{Mg}}_2\mathrm{Si}$ is thermodynamically stable at ambient conditions while certain nonzero hydrostatic pressure can stabilize ${\mathrm{Mg}}_9{\mathrm{Si}}_5$ so that it can coexist with ${\mathrm{Mg}}_2\mathrm{Si}$. We conduct a comprehensive search for viable binary compounds of ${\mathrm{Mg}}_x{\mathrm{Si}}_{1-x}$ $(1/3\le x\le 2/3)$, discovering numerous low-energy structures for all the compounds. On one hand, we find that ${\mathrm{MgSi}}_2$, MgSi, and ${\mathrm{Mg}}_9{\mathrm{Si}}_5$ are likely pressure-stabilized materials, while, on the other hand, supporting previous studies, we raise doubt on the existence of ${\mathrm{Mg}}_5{\mathrm{Si}}_6$, and claim that the existence of ${\mathrm{Mg}}_4{\mathrm{Si}}_7$ remains an open question. Therefore, we recommend that (hydrostatic and/or nonhydrostatic) pressure should be explicitly considered when discussing the stability of these solids (and maybe other solids as well) by computations. We also find that ${\mathrm{MgSi}}_2$ can potentially exhibit superconducting behaviors within a wide range of pressure with the critical temperature of up to 7 K.

The nucleation rate depends on the free-energy barrier and the kinetic factor. While the role of the free energy barrier is a text-book subject, the importance of the kinetic factor is frequently underestimated. In this study, we applied the mean first-passage time method, to obtain the free-energy landscape and kinetic factor directly from the molecular dynamics (MD) simulations of the nucleation of the face-centered cubic (fcc) phase in the pure Ni and the B2 phases in the $\mathrm{N}{\mathrm{i}}_{50}\mathrm{A}{\mathrm{l}}_{50}$ and $\mathrm{C}{\mathrm{u}}_{50}\mathrm{Z}{\mathrm{r}}_{50}$ alloys. The obtained data show that while the free-energy barrier for nucleation is higher in pure Ni the nucleation rate is considerably lower in the $\mathrm{N}{\mathrm{i}}_{50}\mathrm{A}{\mathrm{l}}_{50}$ alloy. This result can be explained by the slow attachment kinetics in the $\mathrm{N}{\mathrm{i}}_{50}\mathrm{A}{\mathrm{l}}_{50}$ alloy, which was related to the ordered nature of the B2 phase. Even smaller fraction of the antisite defects in the $\mathrm{C}{\mathrm{u}}_{50}\mathrm{Z}{\mathrm{r}}_{50}$ alloy leads to such a slow attachment kinetics that the nucleation is never observed for this alloy in the course of the MD simulation. This is consistent with the experimental facts that the $\mathrm{C}{\mathrm{u}}_{50}\mathrm{Z}{\mathrm{r}}_{50}$ alloy is a good glass forming alloy and the $\mathrm{N}{\mathrm{i}}_{50}\mathrm{A}{\mathrm{l}}_{50}$ alloy is not. Thus the present study demonstrates that the atom attachment rate can be the critical factor that controls the nucleation process under certain conditions.

Chromium trihalides are layered and exfoliable semiconductors and exhibit unusual magnetic properties with a surprising temperature dependence of the magnetization. By analyzing the evolution of the magnetocrystalline anisotropy with temperature in chromium iodide $\mathrm{Cr}{\mathrm{I}}_3$, we find it strongly changes from $K_u=300\pm 50\mathrm{kJ}/{\mathrm{m}}^3$ at $5\mathrm{K}$ to $K_u=43\pm 7\mathrm{kJ}/{\mathrm{m}}^3$ at $60\mathrm{K}$, close to the Curie temperature. We draw a direct comparison to $\mathrm{CrB}{\mathrm{r}}_3$, which serves as a reference, and where we find results consistent with literature. In particular, we show that the anisotropy change in the iodide compound is more than 3 times larger than in the bromide. We analyze this temperature dependence using a classical model, showing that the anisotropy constant scales with the magnetization at any given temperature below the Curie temperature, indicating that the temperature dependence can be explained by a dominant uniaxial anisotropy where this scaling results from local spin clusters having thermally induced magnetization directions that deviate from the overall magnetization.

The driving force dependence of the surface velocity and the average height of faceted merged steps, the terrace-surface slope, and the elementary step velocity are studied using the Monte Carlo method in the nonequilibrium steady state. The Monte Carlo study is based on a lattice model, the restricted solid-on-solid model with point-contact-type step-step attraction (p-RSOS model). The main focus of this paper is a change of the “kink density” on the vicinal surface. The temperature is selected to be in the step-faceting zone [N. Akutsu, AIP Adv. 6, 035301 (2016)] where the vicinal surface is surrounded by the (001) terrace and the (111) faceted step at equilibrium. Long time simulations are performed at this temperature to obtain steady states for the different driving forces that influence the growth/recession of the surface. A Wulff figure of the p-RSOS model is produced through the anomalous surface tension calculated using the density-matrix renormalization group method. The characteristics of the faceted macrostep profile at equilibrium are classified with respect to the connectivity of the surface tension. This surface tension connectivity also leads to a faceting diagram, where the separated areas are, respectively, classified as a Gruber-Mullins-Pokrovsky-Talapov zone, step droplet zone, and step-faceting zone. Although the p-RSOS model is a simplified model, the model shows a wide variety of dynamics in the step-faceting zone. There are four characteristic driving forces: $\mathrm{\Delta }{\mu }_y,\mathrm{\Delta }{\mu }_f,\mathrm{\Delta }{\mu }_{co}$, and $\mathrm{\Delta }{\mu }_R$. For the absolute value of the driving force, $\left|\mathrm{\Delta }\mu \right|$ is smaller than $\mathrm{Max[}\mathrm{\Delta }{\mu }_y,\mathrm{\Delta }{\mu }_f]$, the step attachment-detachments are inhibited, and the vicinal surface consists of (001) terraces and the (111) side surfaces of the faceted macrosteps. For $\mathrm{Max[}\mathrm{\Delta }{\mu }_y,\mathrm{\Delta }{\mu }_f]<|\mathrm{\Delta }\mu |<\mathrm{\Delta }{\mu }_{co}$, the surface grows/recedes intermittently through the two-dimensional (2D) heterogeneous nucleation at the facet edge of the macrostep. For $\mathrm{\Delta }{\mu }_{co}<\left|\mathrm{\Delta }\mu \right|<\mathrm{\Delta }{\mu }_R$, the surface grows/recedes with the successive attachment-detachment of steps to/from a macrostep. When $\left|\mathrm{\Delta }\mu \right|$ exceeds $\mathrm{\Delta }{\mu }_R$, the macrostep vanishes and the surface roughens kinetically. Classical 2D heterogeneous multinucleation was determined to be valid with slight modifications based on the Monte Carlo results of the step velocity and the change in the surface slope of the “terrace.” The finite-size effects were also determined to be distinctive near equilibrium.

Based on the work of Gritsenko et al. (GLLB) [Phys. Rev. A 51, 1944 (1995)], the method of Kuisma et al. [Phys. Rev. B 82, 115106 (2010)] to calculate the band gap in solids was shown to be much more accurate than the common local density approximation (LDA) and generalized gradient approximation (GGA). The main feature of the GLLB-SC potential (SC stands for solid and correlation) is to lead to a nonzero derivative discontinuity that can be conveniently calculated and then added to the Kohn-Sham band gap for a comparison with the experimental band gap. In this work, a thorough comparison of GLLB-SC with other methods, e.g., the modified Becke-Johnson (mBJ) potential [Tran and Blaha, Phys. Rev. Lett. 102, 226401 (2009)], for electronic, magnetic, and density-related properties is presented. It is shown that for the band gap, GLLB-SC does not perform as well as mBJ for systems with a small band gap and strongly correlated systems, but is on average of similar accuracy as hybrid functionals. The results on itinerant metals indicate that GLLB-SC overestimates significantly the magnetic moment (much more than mBJ does), but leads to excellent results for the electric field gradient, for which mBJ is in general not recommended. In the aim of improving the results, variants of the GLLB-SC potential are also tested.

We report extensive structural and spectroscopic characterization of four mixed stack charge-transfer (ms-CT) crystals formed by the electron donor 3,$3^{\prime },5,5^{\prime }$-tetramethylbenzidine (TMB) with Chloranil (CA), Bromanil (BA), 2,5-difluoro-tetracyanoquinodimethane $\left({\mathrm{TCNQF}}_2\right)$, and tetrafluoro-tetracyanoquinodimethane $\left({\mathrm{TCNQF}}_4\right)$. Together with the separately studied TMB-TCNQ [Phys. Rev. B 95, 024101 (2017)] the TMB-acceptor series spans a wide range of degree of CT, from about 0.14 to 0.91, crossing the neutral-ionic interface, yet retaining similar packing and donor-acceptor CT integrals. First principle calculations of key phenomenological parameters allow us to get insight into the factors determining the degree of CT and other relevant physical properties.

X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and density functional theory calculations were used to study the electronic structure of three lanthanide scandates: $\mathrm{GdSc}{\mathrm{O}}_3,\mathrm{TbSc}{\mathrm{O}}_3$, and $\mathrm{DySc}{\mathrm{O}}_3$. X-ray photoelectron spectra simulated from first-principles calculations using a combination of on-site hybrid and $\mathrm{GGA}+U$ methods were found to be in good agreement with experimental x-ray photoelectron spectra. The hybrid method was used to model the ground state electronic structure and the $\mathrm{GGA}+U$ method accounted for the shift of valence state energies due to photoelectron emission via a Slater-Janak transition state approach. From these results, the lanthanide scandate valence bands were determined to be composed of $\mathrm{Ln}4f,\mathrm{O}2p$, and $\mathrm{Sc}3d$ states, in agreement with previous work. However, contrary to previous work the minority $\mathrm{Ln}4f$ states were found to be located closer to, and in some cases at, the valence band maximum. This suggests that minority $\mathrm{Ln}4f$ electrons may play a larger role in lanthanide scandate properties than previously thought.

Phonon dispersion of titanite ${\mathrm{CaTiSiO}}_5$ has been calculated using the variational density functional perturbation theory. The experimentally known out-of-center distortion of the Ti atom is confirmed. The distortion is associated with a $B_u$ mode that is unstable for wave vectors normal to the octahedral chain direction of the $C2/c$ aristotype structure. The layer of wave vectors with imaginary mode frequencies also comprises the Brillouin zone boundary point $Y$ $(0,1,0)$, which is critical for the transition to the $P{2}_1/c$ ground-state structure. The phonon branch equivalent to the imaginary branch of the titanite aristotype is found to be stable in malayaite ${\mathrm{CaSnSiO}}_5$. The unstable phonon mode in titanite leads to the formation of transoriented short and long Ti-O1 bonds. The Ti as well as the connecting O1 atom exhibit strongly anomalous Born effective charges along the octahedral chain direction [001], indicative of the strong covalency in this direction. Accordingly and in contrast to malayaite, LO-TO splitting is very large in titanite. In the $C2/c$ phase of titanite, the Ti-O1-Ti distortion chain is disordered with respect to neighboring distortion chains, as all chain configurations are equally unstable along the phonon branch. This result is in agreement with diffuse x-ray scattering in layers normal to the chain direction that is observed at temperatures close to the $P{2}_1/c$ to $C2/c$ transition temperature and above. The resulting dynamic chains of correlated Ti displacements are expected to order in two dimensions to yield the $P{2}_1/c$ ground-state structure of titanite.

We have investigated structural, transport, and optical properties of tensile strained ${\left(\mathrm{S}{\mathrm{r}}_{1-x}\mathrm{L}{\mathrm{a}}_x\right)}_3\mathrm{I}{\mathrm{r}}_2{\mathrm{O}}_7$ ($x=0$, 0.025, 0.05) epitaxial thin films. While high-$T_{\mathrm{c}}$ superconductivity is predicted theoretically in the system, we have observed that all of the samples remain insulating with finite optical gap energies and Mott variable-range hopping characteristics in transport. Cross-sectional scanning transmission electron microscopy indicates that structural defects such as stacking faults appear in this system. The insulating behavior of the La-doped $\mathrm{S}{\mathrm{r}}_3\mathrm{I}{\mathrm{r}}_2{\mathrm{O}}_7$ thin films is presumably due to disorder-induced localization and ineffective electron doping of La, which brings to light the intriguing difference between epitaxial thin films and bulk single crystals of the iridates.

Local thermal expansions and lattice strains in the Elinvar alloy ${\mathrm{Fe}}_{49.66}{\mathrm{Ni}}_{42.38}{\mathrm{Cr}}_{5.49}{\mathrm{Ti}}_{2.47}$ (Ni Span C) and the stainless steel $\text{SUS}304{\mathrm{Fe}}_{71.98}{\mathrm{Ni}}_{9.07}{\mathrm{Cr}}_{18.09}{\mathrm{Mn}}_{0.86}$ (AISI304) were investigated by the temperature-dependent Cr, Fe, and Ni $K$-edge extended x-ray absorption fine-structure (EXAFS) measurements, combined with the path-integral effective classical potential Monte Carlo (PIECP MC) theoretical simulations. From the EXAFS analysis of the Elinvar alloy, the local thermal expansion around Fe is found to be considerably smaller than the ones around Ni and Cr. This observation can be understood simply because Fe in the Elinvar alloy exhibit an incomplete Invar-like effect. Moreover, in both the Elinvar and SUS304 alloys, the local thermal expansions and the lattice strains around Cr are found to be larger than those around Fe and Ni. From the PIECP MC simulations of both the alloys, the first-nearest neighbor Cr-Fe pair shows extraordinarily large thermal expansion, while the Cr-Cr pair exhibits quite small or even negative thermal expansion. These findings consequently indicate that the lattice strains in both the Elinvar and SUS304 alloys are concentrated predominantly on the Cr atoms. Although the role of Cr in stainless steel has been known to inhibit corrosion by the formation of surface chromium oxide, the present investigation may interestingly suggest that the Cr atoms in the bulk play a hidden new role of absorbing inevitable lattice strains in the alloys.

We report the spontaneous decay of a soft, optical phonon in a solid. Using neutron spectroscopy, we find that specific phonon lifetimes in the relaxor ${\mathrm{PbMg}}_{1/3}{\mathrm{Nb}}_{2/3}{\mathrm{O}}_3$ are anomalously short within well-defined ranges of energy and momentum. This behavior is independent of ferroelectric order and occurs when the optical phonon with a specific energy and momentum can kinematically decay into two acoustic phonons with lower phase velocity. We interpret the well-known relaxor “waterfall” effect as a form of quasiparticle decay analogous to that previously reported in quantum spin liquids and quantum fluids.

We report single-crystal growth, electrical resistivity $\rho$, anisotropic magnetic susceptibility $\chi$, and heat capacity $C_p$ measurements on the one-dimensional spin-chain ruthenate ${\mathrm{Na}}_2{\mathrm{RuO}}_4$. We observe variable range hopping (VRH) behavior in $\rho \left(\mathit{T}\right)$. The magnetic susceptibility with magnetic field perpendicular (${\chi }_{\perp }$) and parallel (${\chi }_{\parallel }$) to the spin chains is reported. The magnetic properties are anisotropic with ${\chi }_{\perp }>{\chi }_{\parallel }$ in the temperature range of measurements $\mathit{T}\approx 2-305$ K with ${\chi }_{\perp }/{\chi }_{\parallel }\approx 1.4$ at 305 K. From an analysis of the $\chi \left(\mathit{T}\right)$ data we attempt to estimate the anisotropy in the $g$ factor and Van Vleck paramagnetic contribution. An anomaly in $\chi \left(\mathit{T}\right)$ and a corresponding step-like anomaly in $C_p$ at ${\mathit{T}}_N=37$ K confirms long-range antiferromagnetic ordering. This temperature is an order of magnitude smaller than the Weiss temperature $\theta \approx -250$ K and points to suppression of long-range magnetic order due to low dimensionality. A fit of the experimental $\chi \left(\mathit{T}\right)$ by a one-dimensional spin-chain model gave an estimate of the intrachain exchange interaction $2J\approx -85$ K and the magnitude of the interchain coupling $|2J_{\perp }|\approx 3$ K.

A wide band-gap perovskite oxide $\mathrm{BaSn}{\mathrm{O}}_3$ is attracting much attention due to its high electron mobility and oxygen stability. On the other hand, $\mathrm{BaHf}{\mathrm{O}}_3$ was recently reported to be an effective high-$k$ gate oxide. Here, we investigate the band gap and mobility of solid solutions of $\mathrm{BaS}{\mathrm{n}}_{1-x}\mathrm{H}{\mathrm{f}}_x{\mathrm{O}}_3$ ($x=0-1$) (BSHO) as a basis to build advanced perovskite oxide heterostructures. All the films were epitaxially grown on MgO substrates using pulsed laser deposition. Density functional theory calculations confirmed that Hf substitution does not create midgap states while increasing the band gap. From x-ray diffraction and optical transmittance measurements, the lattice constants and the band-gap values are significantly modified by Hf substitution. We also measured the transport properties of $n$-type La-doped BSHO films $\left[\left(\mathrm{Ba},\mathrm{La}\right)\left(\mathrm{Sn},\mathrm{Hf}\right){\mathrm{O}}_3\right]$, investigating the feasibility of modulation doping in the $\mathrm{BaSn}{\mathrm{O}}_3/\mathrm{BSHO}$ heterostructures. The Hall measurement data revealed that, as the Hf content increases, the activation rate of the La dopant decreases and the scattering rate of the electrons sharply increases. These properties of BSHO films may be useful for applications in various heterostructures based on the $\mathrm{BaSn}{\mathrm{O}}_3$ system.

Two-dimensional (2D) honeycomb lattices beyond graphene promise new physical properties such as quantum spin Hall effect. While there have been claims of growth of such lattices (silicene, germanene, stanene), their existence needs further support and their preparation and characterization remain a difficult challenge. Our findings suggest that two distinct phases associated with germanene, the analog of graphene made of germanium (Ge) instead of carbon, can be grown on Ag(111) as observed by scanning tunneling microscopy, low-energy electron diffraction, and angle-resolved photoemission spectroscopy. One such germanene exhibits an atom-resolved alternatively buckled full honeycomb lattice, which is tensile strained and partially commensurate with the substrate to form a striped phase (SP). The other, a quasifreestanding phase (QP), is also consistent with a honeycomb lattice with a lattice constant incommensurate with the substrate but very close to the theoretical value for freestanding germanene. The SP, with a lower atomic density, can be driven into the QP and coexist with the QP by additional Ge deposition. Band mapping and first-principles calculations with proposed SP and QP models reveal an interface state exists only in the SP but the characteristic σ band of freestanding germanene emerges only in the QP—this leads to an important conclusion that adlayer-substrate commensurability plays a key role to affect the electronic structure of germanene. The evolution of the dual germanene phases manifests the competitive formation of Ge-Ge covalent and Ge-Ag interfacial bonds.

Mn and Fe-doped GaN are widely studied prototype systems for hole-mediated magnetic semiconductors. The nature of the hole states around the Mn and Fe impurities, however, remains under debate. Our self-interaction corrected density-functional calculations show that the charge neutral $\mathrm{Mn}{}^0$ and positively charged ${\mathrm{Fe}}^{+}$ impurities have symmetry-broken $d^5+h$ ground states, in which the hole is trapped by one of the surrounding N atoms in a small polaron state. We further show that both systems also have a variety of other $d^5+h$ configurations, including symmetric, delocalized states, which may be stabilized by axial strain. This finding opens a pathway to promote long-range hole-mediated magnetic interactions by strain engineering and clarifies why highly strained thin-films samples often exhibit anomalous magnetic properties.

Spin-orbit coupling can lead to exotic states of matter and unexpected behavior of the system properties. In this paper, we investigate the influence of spin-orbit coupling induced by proximity effects on a monolayer of superconductor (with $s$-wave or $d$-wave pairing) placed on an insulating bulk. We show that the critical temperatures $T_c$ of the superconducting states can be tuned by the spin-orbit coupling both in the case of on-site and intersite pairing. Moreover, we discuss a possibility of changing the location of the maximal $T_c$ from the half filling into the underdoped or overdoped regimes.