A density-functional theory investigation of the (100) and (110) surfaces of the body-centered cubic (bcc) ${\mathrm{Fe}}_{1-x_{\mathrm{b}}}{\mathrm{Cr}}_{x_{\mathrm{b}}}$ binary alloys, $x_{\mathrm{b}}\le 15$ at.%, is reported. The energies and segregation energies of these surfaces were calculated for chemically homogeneous concentration profiles and for Cr surface contents deviating from the nominal one of the bulk. The implications of these results for the surface alloy phase diagram are discussed. The surface chemistry of $\mathrm{Fe}-\mathrm{Cr}\left(100\right)$ is characterized by a transition from Cr depletion to Cr enrichment in a critical bulk Cr composition window of $6<x_{\mathrm{b}}<9$ at.%. In contrast, such threshold behavior of the surface Cr content is absent for $\mathrm{Fe}-\mathrm{Cr}\left(110\right)$ and a nearly homogeneous Cr concentration profile is energetically favorable. The strongly suppressed surface-layer relaxation at both surfaces is shown to be of magnetic origin. The compressive, magnetic contribution to the surface relaxation stress is found to correlate well with the surface magnetic moment squared at both surface terminations. The stability of the Cr surface magnetic moments against bulk Cr content is clarified based on the surface electronic structure.

We revisited the anisotropy of the heavy-fermion material ${\mathrm{CeCo}}_2{\mathrm{Ga}}_8$ by measuring the electrical resistivity and magnetic susceptibility along all the principal $\mathbf{a},\mathbf{b}$, and $\mathbf{c}$ axes. Resistivity along the $\mathbf{c}$ axis $\left({\rho }_c\right)$ shows clear Kondo coherence below about 17 K, while both ${\rho }_a$ and ${\rho }_b$ remain incoherent down to 2 K. The magnetic anisotropy is well understood within the theoretical frame of crystalline electric field effect in combination with magnetic exchange interactions. We found the anisotropy ratio of these magnetic exchange interactions $|J_{\text{ex}}^c/J_{\text{ex}}^{a,b}|$ reaches a large value of 4–5. We, therefore, firmly demonstrate that ${\mathrm{CeCo}}_2{\mathrm{Ga}}_8$ is a quasi-one-dimensional heavy-fermion compound both electrically and magnetically, and thus provide a realistic example of Kondo chain.

Raman spectroscopy is a widely used, powerful, and nondestructive tool for studying the vibrational properties of bulk and low-dimensional materials. Raman spectra can be simulated using first-principles methods but due to the high computational cost calculations are usually limited only to fairly small unit cells, which makes it difficult to carry out simulations for alloys and defects. Here, we develop an efficient method for simulating Raman spectra of alloys, benchmark it against full density-functional theory calculations, and apply it to several alloys of two-dimensional (2D) transition metal dichalcogenides. In this method, the Raman tensor for the supercell mode is constructed by summing up the Raman tensors of the pristine system weighted by the projections of the supercell vibrational modes to those of the pristine system. This approach is not limited to 2D materials and should be applicable to any crystalline solid with defects and impurities. To efficiently evaluate vibrational modes of very large supercells, we adopt mass approximation, although it is limited to chemically and structurally similar atomic substitutions. To benchmark our method, we first apply it to the ${\mathrm{Mo}}_x{\mathrm{W}}_{(1-x)}{\mathrm{S}}_2$ monolayer in the $\mathrm{H}$ phase where several experimental reports are available for comparison. Second, we consider ${\mathrm{Mo}}_x{\mathrm{W}}_{(1-x)}{\mathrm{Te}}_2$ in the ${\mathrm{T}}^{\prime }$ phase, which has been proposed to be a 2D topological insulator but where experimental results for the monolayer alloy are still missing. We show that the projection scheme also provides a powerful tool for analyzing the origin of the alloy Raman-active modes in terms of the parent system eigenmodes. Finally, we examine the trends in characteristic Raman signatures for dilute concentrations of impurities in ${\mathrm{MoS}}_2$.

We achieved nearly ideal spin tunneling efficiency η by lowering the interface trap density $D_{\mathrm{it}}$ with a $\mathrm{Si}{\mathrm{O}}_x$ insertion layer in Fe/Mg/amorphous-MgO(1.0–1.5 nm)/plasma-oxidized $\mathrm{Si}{\mathrm{O}}_x(\sim 0.2\mathrm{nm})/n^{+}\text{−}\mathrm{Si}\left(001\right)$ junctions. The spin polarization $P_{\mathrm{S}}$ of tunneling electrons was estimated from three-terminal Hanle signals at 4 K. At the optimum MgO thickness and oxidation time, we obtained $P_{\mathrm{S}}$ which is nearly equal to the spin polarization $P_{\mathrm{FM}}$ of Fe at the Fermi level, that is, $\eta =P_{\mathrm{S}}/P_{\mathrm{FM}}=0.93$. By quantitatively estimating $D_{\mathrm{it}}$ and $P_{\mathrm{S}}$ of various junctions, we show that lowering $D_{\mathrm{it}}$ is crucial to obtain η ≅ 1.

Investigating the competition between ferroelectric ordering and quantum fluctuations is essential for tailoring the desired functionalities of mixed ferroelectric and incipient ferroelectric systems, like, $\left(\mathrm{Ba},\mathrm{Sr}\right)\mathrm{Ti}{\mathrm{O}}_3$ and $\left(\mathrm{Eu},\mathrm{Ba}\right)\mathrm{Ti}{\mathrm{O}}_3$ . Recently, it has been shown that suppression of quantum fluctuations increases ferroelectric ordering in $\left(\mathrm{Eu},\mathrm{Ba}\right)\mathrm{Ti}{\mathrm{O}}_3$ and since these phenomena are coupled to crystallographic phase transitions it is essential to understand the role of phonons. Here, we observe that the unusual temperature dependence of phonons in $\mathrm{BaTi}{\mathrm{O}}_3$ gets suppressed when $\mathrm{B}{\mathrm{a}}^{2+}$ is replaced by $\mathrm{E}{\mathrm{u}}^{2+}$. This manifests in a decrease in the cubic-to-tetragonal (i.e., para-to-ferroelectric) phase transition temperature (by 150 K) and a complete suppression of tetragonality of the lattice (at room temperature by 40% replacement of $\mathrm{B}{\mathrm{a}}^{2+}$ by $\mathrm{E}{\mathrm{u}}^{2+})$. We have quantified the anharmonicity of the phonons and observed that the replacement of $\mathrm{B}{\mathrm{a}}^{2+}$ by $\mathrm{E}{\mathrm{u}}^{2+}$ suppresses it (by 93%) with a resultant lowering of the ferroelectric ordering temperature in the $\mathrm{E}{\mathrm{u}}_x\mathrm{B}{\mathrm{a}}_{1\text{−}x}\mathrm{Ti}{\mathrm{O}}_3$. This suggests that tuning phonon anharmonicity can be an important route to ferroelectric materials.

Lattice dynamics and elastic constants in ${\mathrm{Mg}}_2{\mathrm{Si}}_{1-x}{\mathrm{Sn}}_x$ were investigated using resonant ultrasound spectroscopy, Mössbauer spectroscopy, nuclear inelastic scattering, and inelastic x-ray scattering. Increasing the Sn content $x$ results in smaller elastic constants, lower Sn specific Debye temperature, lower speed of sound, and a softening of acoustic Sn specific phonons. However, close to band convergence at about $x=0.6$, the shear modulus is well below the expected value, which suggests a pronounced connection between band convergence and lattice dynamics in this system. Based on the determined speed of sound and average phonon group velocity, the importance of optical phonons for lattice thermal conductivity is discussed, as the significant reduction in both velocities would yield an implausibly low lattice thermal conductivity of only about 60% of the experimental value. Sn specific thermodynamic quantities calculated from the Sn specific density of phonon states substantiate the general softening of lattice vibrations upon substitution of Si by Sn. A major contribution to the vibrational entropy is thus due to Sn specific vibrational modes. The generalized density of phonon states in ${\mathrm{Mg}}_2{\mathrm{Si}}_{1-x}{\mathrm{Sn}}_x$ derived from inelastic x-ray scattering for one composition shows that vibrational modes related to lightweight Mg and Si set in above 12.5 meV, whereas Sn specific modes are concentrated around 11 meV.

How the exciton fine structure and spectral linewidth in single-walled carbon nanotubes (SWNTs) evolve after ultrafast photoexcitation is critical for both understanding the fundamental optoelectronic properties and the development of nanophotonic functional devices. Yet, studies so far have mostly detected a subset of excitons near the Brillouin-zone center due to symmetry and momentum restrictions of the interband optical probes used, such as photoluminescence and absorption. Here we use ultrafast terahertz spectroscopy to probe the excitonic spectral linewidth associated with internal quantum transition across the entire momentum $K$ space in resonantly excited semiconducting SWNTs. The lowest-lying intraexcitonic transition surprisingly sharpens at both high pump fluence and initial times immediately following the photoexcitation. We attribute these anomalous variations in the spectral linewidth to the role of the dark state $1s\left(g\right)$ that influences the lifetime broadening of the bright state $1s\left(u\right)$, consistent with the temperature-dependent dephasing time with a crossover temperature set by the internal quantum level spacing of excitons.

Based on high-throughput density functional theory calculations, we performed a systematic screening for spin-gapless semiconductors (SGSs) in quaternary Heusler alloys $X{X}^{\prime }YZ$ ($X$, $X^{\prime }$, and $Y$ are transition metal elements except Tc, and $Z$ is one of B, Al, Ga, In, Si, Ge, Sn, Pb, P, As, Sb, and Bi). Following an empirical rule, we focused on compounds with 21, 26, or 28 valence electrons, resulting in 12 000 possible chemical compositions. After systematically evaluating the thermodynamic, mechanical, and dynamical stabilities, we have identified 70 so far unreported SGSs, confirmed by explicit electronic structure calculations with proper magnetic ground states, of which 17 candidates have a distance to the convex hull smaller than 0.10 eV/atom. It is demonstrated that all four types of SGSs can be realized, defined based on the spin characters of the bands around the Fermi energy. Type-II SGSs show promising transport properties for spintronic applications. The effect of spin-orbit coupling is investigated, resulting in large anisotropic magnetoresistance and anomalous Nernst effects.

By performing a critical analysis of the fundamental equations of linear response (LR) formalism in molecules, we explore the interplay between locality of the response density operator and numerical convergence of LR-related quantities. We show that for frequencies below the first ionization potential (IP) of the system, it is possible to express the response density by employing localized states only. Above this threshold energy, such a locality property cannot be achieved. Such considerations may be transposed in terms of the molecule's excited states. We show that not all the system's excitations can be considered on equal footing. There is a discrete sector of excitations, which may also extend above IP, that can be parametrized by observable, localized states, which can be computationally expressed with high precision, provided an adequate level of completeness. We present indicators that can help to quantify such potential observable properties of an excitation, that can be evaluated in any discretization scheme. The remaining excitation modes belong to a continuum spectrum that, on the contrary, is not directly associated to observable properties and can only be effectively represented in a given computational setup. Such considerations are important not only for reproducibility of the results among different computer codes employing diverse formalisms, but also in view of providing a deeper understanding on the impact of models' approximations on the scientific outcomes of the simulation.

In metallic materials subjected to cyclic loading, strain hardening as well as fatigue crack initiation have been linked for a long time with the evolution of dislocation patterns and structures. In particular, the development of low-energy dislocation configurations such as persistent slip bands (PSBs) is considered as a precursor to crack initiation. However, the associated scenarios have been elaborated mainly from postmortem observations capturing only static pictures of dislocation patterns, while the dynamics of the problem has been somewhat overlooked. Here we analyze collective dislocation dynamics during cycling loading of aluminum using acoustic emission (AE). A strong link is revealed between dislocation patterning, cyclic hardening/softening, and the intermittency of plasticity: Plastic intermittency and dislocation avalanches rapidly decay during the initial hardening stage, in conjunction with the reduction of an internal length scale characterizing the dislocation structure. However, in nonannealed samples, a transient softening stage ensues, associated with a brutal reorganization of this structure. These initial stages of cyclic deformation illustrate the competition between two phenomena: collective dislocation dynamics, governed by long-ranged elastic interactions among dislocations, and the emergence of a self-organizing network controlled by short-range interactions and progressively inhibiting collective effects. Later on, the emergence of PSBs is accompanied by a reincrease of the AE intermittent activity. We propose that the associated AE bursts may be the signature of collective and coordinated dislocation motions along PSBs leading to the formation of incipient microcracks.

An active learning procedure called deep potential generator (DP-GEN) is proposed for the construction of accurate and transferable machine learning-based models of the potential energy surface (PES) for the molecular modeling of materials. This procedure consists of three main components: exploration, generation of accurate reference data, and training. Application to the sample systems of Al, Mg, and Al-Mg alloys demonstrates that DP-GEN can produce uniformly accurate PES models with a minimal number of reference data.

Electrides, with their excess electrons distributed in crystal cavities playing the role of anions, exhibit a variety of unique electronic and magnetic properties. In this work, we employ the first-principles crystal structure prediction to identify a new prototype of $A_{3}B$ electride in which both interlayer spacings and intralayer vacancies provide channels to accommodate the excess electrons in the crystal. This $A_{3}B$ type of structure is calculated to be thermodynamically stable for two alkaline metals oxides $({\mathrm{Rb}}_3\mathrm{O}$ and ${\mathrm{K}}_3\mathrm{O})$. Remarkably, the unique feature of multiple types of cavities makes the spatial arrangement of anionic electrons highly flexible via elastic strain engineering and chemical substitution, in contrast to the previously reported electrides characterized by a single topology of interstitial electrons. More importantly, our first-principles calculations reveal that ${\mathrm{Rb}}_3\mathrm{O}$ is a topological Dirac nodal line semimetal, which is induced by the band inversion at the general electronic $k$ momentums in the Brillouin zone associated with the intersitial electric charges. The discovery of flexible electride in combining with topological electronic properties opens an avenue for electride design and shows great promises in electronic device applications.

Giant zero-field cooling exchange-bias-like behavior with $H_{\mathrm{EB}}=3.49\mathrm{kOe}$ was found in an antiperovskite $\mathrm{M}{\mathrm{n}}_3\mathrm{C}{\mathrm{o}}_{0.61}\mathrm{M}{\mathrm{n}}_{0.39}\mathrm{N}$ compound. The magnetic structure of $\mathrm{M}{\mathrm{n}}_3\mathrm{C}{\mathrm{o}}_{0.61}\mathrm{M}{\mathrm{n}}_{0.39}\mathrm{N}$ was resolved to be ferrimagentic ordering composed of canted ${\mathrm{\Gamma }}^{5\mathrm{g}}$ antiferromagnetic (AFM) and ferromagnetic (FM) along the [111] direction by the neutron diffraction technique. The exchange coupling model was proposed together with the first principles calculation for further understanding this exchange-bias-like behavior. It was found that the ferromagnetic exchange interaction between FM and the canted ${\mathrm{\Gamma }}^{5\mathrm{g}}$ AFM play an important role in the particular exchange-bias-like behavior. The exchange coupling constructed in the lattice is distinct from the interactions between collinear AFM and FM in conventional exchange bias system. In addition to the enhanced horizontal shift, hysteresis loops obtained after FC cooling also exhibited vertical shift. The macroscopic vertical shift of the magnetization is ascribed to the increase of the magnetic moment of canted ${\mathrm{\Gamma }}^{5\mathrm{g}}$ spins along the external magnetic field. This finding will promote the development of advanced magnetic devices.

Reversible hydrogen incorporation was recently attested [Lu et al., Nature (London) 546, 124 (2017)] in ${\mathrm{SrCoO}}_{2.5}$, the brownmillerite phase (BM) of strontium cobalt oxide (SCO), opening new avenues in catalysis and energy applications. However, existing theoretical studies of BM-SCO are insufficient, and that of ${\mathrm{HSrCoO}}_{2.5}$, the newly reported hydrogenated SCO (H-SCO) phase, is especially scarce. In this work, we demonstrate how the electron-counting model can be used in understanding the phases, particularly in explaining the stability of the oxygen-vacancy channels, and in examining the Co valance problem. Using density-functional theoretical methods, we analyze the crystalline, electronic, and magnetic structures of BM- and H-SCO. Based on our structure search, we discovered stable phases with large band gaps ($>1$ eV) for both BM-SCO and H-SCO, agreeing better with experiments on the electronic structures. Our calculations also indicate limited charge transfer from H to O, which may explain the special stability of the H-SCO phase and the reversibility of H incorporation observed in experiments. Contrary to the initial study, our calculations also suggest intrinsic antiferromagnetism of H-SCO, showing how the measured ferromagnetism has possible roots in hole doping.

We present an ab initio algorithm for quantum dynamics simulations that reformulates the traditional “curse of dimensionality” that plagues all state-of-the-art techniques for solving the time-dependent Schrödinger equation. Using a stochastic wave-function ansatz that is based on a set of interacting single-particle conditional wave functions, we show that the difficulty of the problem becomes dominated by the number of trajectories needed to describe the process, rather than simply the number of degrees of freedom involved. This highly parallelizable technique achieves quantitative accuracy for situations in which mean-field theory drastically fails to capture qualitative aspects of the dynamics, such as quantum decoherence or the reduced nuclear probability density, using orders of magnitude fewer trajectories than a mean-field simulation. We illustrate the performance of this method for two fundamental nonequilibrium processes: a photoexcited proton-coupled electron transfer problem, and nonequilibrium dynamics in a cavity bound electron-photon system in the ultrastrong-coupling regime.

Electronic states of a free-standing hydrogenated monolayer boron (HB) sheet were studied via soft x-ray spectroscopies at the B $K$-shell absorption edge and first-principles calculations. The HB sheet is semimetallic with electron and hole pockets at the $Y$ and $\mathrm{\Gamma }$ points, respectively. The electron band results from the B-H-B bonds formed during synthesis from a ${\mathrm{MgB}}_2$ crystal, while the hole band is kept through the process and originates from a honeycomb lattice boron layer or borophene in ${\mathrm{MgB}}_2$. Our results suggest that the HB sheet is a promising two-dimensional material for realizing new boron-based or superconducting nanodevices.

Stabilizing atomically dispersed catalytic metal species at surfaces is a significant challenge for obtaining high-performance single atom catalysts. This is because of the strong tendency for the dispersed metal atoms to agglomerate. We propose that dislocations can provide a strong anchor for stabilizing single atoms. A ½[110]{100} edge dislocation in Cu doped ceria, $\mathrm{Cu}\text{−}\mathrm{Ce}{\mathrm{O}}_2$, is investigated as a model system with density functional theory. The defect formation energies are found to be lower at the dislocation core, with a large segregation energy ranging within 0.8–2.5 eV depending on the site and species at the dislocation core. The high segregation energy indicates that the edge dislocations can enrich Cu defects in an atomically sized area and, thus, have a potential to strongly anchor single atom species at surfaces. Moreover, the edge dislocation also stabilizes reduced cation species, Cu (1+) and Ce (3+). The more reduced dislocation core can offer high concentration of oxygen vacancy as well as in-gap electronic states which provide more reactivity for surface reactions.

We explore two methods for single-crystal growth of the theoretically proposed magnetic Weyl semimetals $R\mathrm{AlGe}$ ($R$ = Pr, Ce), which prove that a floating-zone technique, being both crucible- and flux-free, is crucial to obtain perfectly stoichiometric $R\mathrm{AlGe}$ crystals. In contrast, the crystals grown by a flux-growth technique tend to be Al-rich. We further present both structural and elemental analyses, along with bulk magnetization and electrical resistivity data on the crystals prepared by the floating-zone technique. Both systems with the intended 1:1:1 stoichiometry crystallize in the anticipated polar $I{4}_{1}md$ (No. 109) space group, although neither displays the theoretically expected ferromagnetic ground state. Instead PrAlGe displays a spin-glass-like transition below 16 K with an easy $c$ axis and CeAlGe has an easy-$ab$-plane antiferromagnetic order below 5 K. The grown crystals provide an ideal platform for microscopic studies of the magnetic field-tunable correlation physics involving magnetism and topological Weyl nodes.