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We propose a fast method for high-throughput screening of potential superconducting materials. The method is based on calculating metallic screening of zone-center phonon modes, which provides an accurate estimate for the electron-phonon coupling strength. This method is complementary to the recently proposed Rigid Muffin Tin (RMT) method, which amounts to integrating the electron-phonon coupling over the entire Brillouin zone (as opposed to the zone center), but in a relatively inferior approximation. We illustrate the use of this method by applying it to MgB$_\text{2}$, where the high-temperature superconductivity is known to be driven largely by the zone-center modes, and compare it to a sister compound AlB$_\text{2}$. We further illustrate the usage of this descriptor by screening a large number of binary hydrides, for which accurate first-principle calculations of electron-phonon coupling have been recently published. Together with the RMT descriptor, this method opens a way to perform initial high-throughput screening in search of conventional superconductors via machine learning or data mining.

The effect of electronic Coulomb interaction on the phase formation of fcc Ce lattice is investigated by full ab initio calculations without adjustable Coulomb $U$ and $J$ parameters using the Gutzwiller wavefunction-based correlation matrix renormalization theory (CMRT). Its total energy and pressure as a function of volume agree reasonably well with existing $\text{DFT}+\text{Gutzwiller}$ calculations and experiments, indicating correct capture of electronic correlation and screening effects within the CMRT formalism. A stable phase is found in line with the experimental $\ensuremath{\alpha}$-Ce phase, and a lurking phase is identified supposedly linked with the experimental $\ensuremath{\gamma}$-Ce phase. A criterion based on the local $4f$ electron charge fluctuation is introduced to confirm the distinct electronic correlation natures of both phases.

Electron-phonon interactions are fundamentally important physical processes responsible for many key discoveries in condensed matter physics and material sciences. Herein, by exploiting the scattering-type scanning near-field optical microscope (s-SNOM) excited with a femtosecond infrared (IR) laser, we explored the strong coupling between IR phonons in few-layer graphene (FLG) with ultrahot electrons, which are heated up by the intense laser field enhanced by the s-SNOM tip. More specifically, we found that the intensity of the phonon resonance can be tuned systematically by varying the laser power that controls the electron temperature. Furthermore, the high spatial resolution of s-SNOM allows us to map the local phonon characteristics at sharp boundaries and nanostructures. Our findings offer insights into the intriguing physics behind the electron-phonon interactions in nonequilibrium conditions and open a pathway for manipulating phonons with optical means.

The complex interplay between energetic and kinetic factors that governs the phase and morphology selections can originate at the earliest stage of crystallization in the amorphous parent phases. Because of the extreme difficulties in capturing the microscopic nucleation process, a detailed picture of how initial disordered structures affect the transformation pathway remains unclear. Here, we report the experimental observation of widely varying phase selection and grain size evolution during the devitrification of a homogeneous melt-spun glassy ribbon. Two different crystalline phases $\ensuremath{\theta}\text{\ensuremath{-}}{\mathrm{Al}}_{5}\mathrm{Sm}$ and $\ensuremath{\varepsilon}\text{\ensuremath{-}}{\mathrm{Al}}_{60}{\mathrm{Sm}}_{11}$ are found to form in the different regions of the same metallic glass (MG) ribbon during the devitrification. The grain size of the $\ensuremath{\varepsilon}\text{\ensuremath{-}}{\mathrm{Al}}_{60}{\mathrm{Sm}}_{11}$ phase shows a strong spatial heterogeneity. The coarse-grained $\ensuremath{\varepsilon}\text{\ensuremath{-}}{\mathrm{Al}}_{60}{\mathrm{Sm}}_{11}$ phase coupled with the small volume fraction of the $\ensuremath{\theta}\text{\ensuremath{-}}{\mathrm{Al}}_{5}\mathrm{Sm}$ phase is preferably formed close to the wheel side of the melt-spun ribbon. Combining experimental characterization and computational simulations, we show that phase selection and microstructure evolution can be traced back to different types and populations of atomic clusters that serve as precursors for the nucleation of different crystalline phases. Inhomogeneous cooling rates cause different structure orders across the glass sample during the quenching process. Our findings provide direct insight into the effect of structural order on the crystallization pathways during the devitrification of MG. It also opens an avenue to study the detailed nucleation process at the atomic level using the MG as a platform and suggests the opportunity of microstructure and property design via controlling the cooling process.

Rapid progress in noisy intermediate-scale quantum (NISQ) computing technology has led to the development of novel resource-efficient hybrid quantum-classical algorithms, such as the variational quantum eigensolver (VQE), that can address open challenges in quantum chemistry, physics and material science. Proof-of-principle quantum chemistry simulations for small molecules have been demonstrated on NISQ devices. While several approaches have been theoretically proposed for correlated materials, NISQ simulations of interacting periodic models on current quantum devices have not yet been demonstrated. Here, we develop a hybrid quantum-classical simulation framework for correlated electron systems based on the Gutzwiller variational embedding approach. We implement this framework on Rigetti quantum processing units (QPUs) and apply it to the periodic Anderson model, which describes a correlated heavy electron band hybridizing with non-interacting conduction electrons. Our simulation results quantitatively reproduce the known ground state quantum phase diagram including metallic, Kondo and Mott insulating phases. This is the first fully self-consistent hybrid quantum-classical simulation of an infinite correlated lattice model executed on QPUs, demonstrating that the Gutzwiller hybrid quantum-classical embedding framework is a powerful approach to simulate correlated materials on NISQ hardware. This benchmark study also puts forth a concrete pathway towards practical quantum advantage on NISQ devices., Comment: 14 pages, 5 figures

We studied the electronic structure of ${\mathrm{PtPb}}_{4}$ using laser angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT) calculations. This material is closely related to ${\mathrm{PtSn}}_{4}$, which exhibits exotic topological properties such as Dirac node arcs. The Fermi surface (FS) of ${\mathrm{PtPb}}_{4}$ consists of two electron pockets at the center of the Brillouin zone (BZ) and several hole pockets around the zone boundaries. Our ARPES data reveal significant Rashba splitting at the $\mathrm{\ensuremath{\Gamma}}$ point, in agreement with DFT calculations. The presence of Rashba splitting may render this material of potential interest for spintronic applications.

Manipulating the electronic properties of graphene has been a subject of great interest since it can aid material design to extend the applications of graphene to many different areas. In this paper, we systematically investigate the effect of lead (Pb) intercalation on the structural and electronic properties of epitaxial graphene on the SiC(0001) substrate. We show that the band structure of Pb-intercalated few-layer graphene can be effectively tuned through changing intercalation conditions, such as coverage, location of Pb, and the initial number of graphene layers. Lead intercalation at the interface between the buffer layer (BL) and the SiC substrate decouples the BL from the substrate and transforms the BL into a $p$-doped graphene layer. We also show that Pb atoms tend to donate electrons to neighboring layers, leading to an $n$-doping graphene layer and a small gap in the Dirac cone under a sufficiently high Pb coverage. This paper provides useful guidance for manipulating the electronic properties of graphene layers on the SiC substrate.

We analyze several methods of obtaining the accurate relativistic total energy (TE) variations using traditional perturbation theories (PTs) and proposed coupling constant integration (CCI) methods. For this purpose, we perform benchmark calculations within the density functional theory taking the spin-orbit coupling (SOC) and its derivative as a perturbation. The TE change due to SOC addition obtained from both PTs and CCI is shown to reach the accuracy of fully self-consistent TE calculations. Similar accuracy is also obtained even for the magnetocrystalline anisotropy energy (MAE). The real advantage of the proposed methods is to use PTs and CCI methods in those electronic structure methods where accurate total energies currently cannot be obtained with required accuracy. Correspondingly, we demonstrate the applicability of suggested methods for calculations of MAE in different magnetic materials using a dynamic mean-field method. All suggested PTs and CCI methods also provide convenient site, orbital, and spin decompositions of the TE variation, creating a powerful way to analyze microscopic physics in strongly correlated materials.

An open-access database specialized for magnetic compounds, as well as for magnetic clusters, is developed with a focus on magnets free from rare earths. Data-intensive methods are used to facilitate the theoretical and experimental design and discovery of new magnetic materials. The utility of the datasets for computational screening, machine-learning modeling, and experimental fabrication is discussed.

We report the laser-induced solid-state transformation between a periodic ``approximant'' and quasicrystal in the Al-Cr system during rapid quenching. Dynamic transmission electron microscopy allows us to capture in situ the dendritic growth of the metastable quasicrystals. The formation of dendrites during solid-state transformation is a rare phenomenon, which we attribute to the structural similarity between the two intermetallics. Through ab initio molecular dynamics simulations, we identify the dominant structural motif to be a 13-atom icosahedral cluster transcending the phases of matter.

We propose a scheme of decomposition of the total relativistic energy in solids to intra- and interatomic contributions. The method is based on a site variation of such fundamental constant as the speed of light. As a practical illustration of the method, we tested such decomposition in the case of a spin-orbit interaction variation for the decomposition of the magnetic anisotropy energy (MAE) in CoPt. We further studied the ${\ensuremath{\alpha}}^{\ensuremath{''}}\text{\ensuremath{-}}{\mathrm{Fe}}_{16}{\mathrm{N}}_{2}$ magnet doped by Bi, Sb, Co, and Pt atoms. It was found that the addition of Pt atoms can enhance the MAE by as much as five times while Bi and Sb substitutions double the total MAE. Using the proposed technique, we demonstrate the spatial distribution of these enhancements. Our studies also suggest that Sb, Pt, and Co substitutions could be synthesized by experiments.

The correlation between structure and glass formability in glassy systems is a long-standing puzzle. To solve this puzzle, many descriptors based on the short-range order (SRO) have been proposed. Here we show that the SRO, however, offers little help in explaining the glass formability and stability; instead it is the formation of medium-range order that stabilizes the glass against crystallization by suppressing the atomic rearrangement and compositional change. Our results provide a perspective for understanding the correlation between structure and stability in glasses.

We introduce Gutzwiller conjugate gradient minimization (GCGM) theory, an ab initio quantum many-body theory for computing the ground-state properties of infinite systems. GCGM uses the Gutzwiller wave function but does not use the commonly adopted Gutzwiller approximation (GA), which is a major source of inaccuracy. Instead, the theory uses an approximation that is based on the occupation probability of the on-site configurations, rather than approximations that decouple the site-site correlations as used in the GA. We test the theory in the one-dimensional and two-dimensional Hubbard models at various electron densities and find that GCGM reproduces energies and double occupancies in reasonable agreement with benchmark data at a very small computational cost.

The development of long-range ferromagnetic (FM) order in dilute magnetic topological insulators can induce dissipationless electronic surface transport via the quantum anomalous Hall effect. We measure the magnetic excitations in a prototypical magnetic topological crystalline insulator, ${\mathrm{Sn}}_{0.95}{\mathrm{Mn}}_{0.05}\mathrm{Te}$, using inelastic neutron scattering. Neutron diffraction and magnetization data indicate that our ${\mathrm{Sn}}_{0.95}{\mathrm{Mn}}_{0.05}\mathrm{Te}$ sample has no FM long-range order above a temperature of 2 K. However, we observe slow, collective FM fluctuations $(l70\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{eV})$, indicating proximity to FM order. We also find a series of sharp peaks originating from local excitations of antiferromagnetically (AF) coupled and isolated Mn-Mn dimers with ${J}_{\mathrm{AF}}=460\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{eV}$. The simultaneous presence of collective and localized components in the magnetic spectra highlight different roles for substituted Mn ions, with competition between FM order and the formation of AF-coupled Mn-Mn dimers.

By performing extensive molecular dynamics simulations, we investigate the deformation behavior in $\mathrm{A}{\mathrm{l}}_{90}\mathrm{S}{\mathrm{m}}_{10}$ and $\mathrm{C}{\mathrm{u}}_{64.5}\mathrm{Z}{\mathrm{r}}_{35.5}$ alloys after elongated isothermal annealing in the vicinity of the glass-transition temperature (${T}_{g}$). Different microstructural response to the annealing process was observed: $\mathrm{A}{\mathrm{l}}_{90}\mathrm{S}{\mathrm{m}}_{10}$ maintains the glassy structure with improved energetic stability, enhanced short-range order (SRO), and a more pronounced spatial network that extends beyond the first atomic shell, while $\mathrm{C}{\mathrm{u}}_{64.5}\mathrm{Z}{\mathrm{r}}_{35.5}$ forms nanocrystalline Laves $\mathrm{C}{\mathrm{u}}_{2}\mathrm{Zr}$ phases. Shear banding occurs in both annealed systems under shear loading. For $\mathrm{A}{\mathrm{l}}_{90}\mathrm{S}{\mathrm{m}}_{10}$, the spatial network formed by the local clusters characterizing the SRO of the system is significantly weakened but remains appreciable in the shear band. In contrast, the crystalline ordering in the $\mathrm{C}{\mathrm{u}}_{64.5}\mathrm{Z}{\mathrm{r}}_{35.5}$ is completely destroyed during shear banding. Consequently, while displaying higher yield strength, the annealed $\mathrm{C}{\mathrm{u}}_{64.5}\mathrm{Z}{\mathrm{r}}_{35.5}$ sample appears to be less ductile. By carefully examining the effect of microstructures on the structural ordering in the shear band and the consequent mechanical response, our work contributes to a better understanding of the deformation mechanism of amorphous alloys as compared with that in crystalline materials.

Interatomic potentials based on neural-network machine learning (ML) approach to address the long-standing challenge of accuracy versus efficiency in molecular-dynamics simulations have recently attracted a great deal of interest. Here, utilizing Pd-Si system as a prototype, we extend the development of neural-network ML potentials to compounds exhibiting various types of bonding characteristics. The ML potential is trained by fitting to the energies and forces of both liquid and crystal structures first-principles calculations based on density-functional theory (DFT). We show that the generated ML potential captures the structural features and motifs in $\mathrm{P}{\mathrm{d}}_{82}\mathrm{S}{\mathrm{i}}_{18}$ and $\mathrm{P}{\mathrm{d}}_{75}\mathrm{S}{\mathrm{i}}_{25}$ liquids more accurately than the existing interatomic potential based on embedded-atom method (EAM). The ML potential also describes the solid-liquid interface of these systems very well. Moreover, while the existing EAM potential fails to describe the relative energies of various crystalline structures and predict wrong ground-state structures at $\mathrm{P}{\mathrm{d}}_{3}\mathrm{Si}$ and $\mathrm{P}{\mathrm{d}}_{9}\mathrm{S}{\mathrm{i}}_{2}$ composition, the developed ML potential predicts correctly the ground-state structures from genetic algorithm search. The efficient ML potential with DFT accuracy from our study will provide a promising scheme for accurate atomistic simulations of structures and dynamics of complex Pd-Si system.

Superconductors with the A15 structure are prototypical type-II s-wave superconductors which have generated considerable interest in early superconducting material history. However, the topological nature of the electronic structure remains unnoticed so far. Here, using first-principles band structure calculations based on density functional theory, we show that the A15 superconductors (Ta$_3$Sb, Ta$_3$Sn, and Ta$_3$Pb) have nontrivial band topology in the bulk electronic band structures, leading to the formation of topological surface states near the Fermi energy. Due to the bulk superconductivity, the proximity effect in the topological surface states would induce topological superconductivity even without heterostructure of a topological insulator and an s-wave superconductor. Our results indicate that the A15 superconductors are promising candidates for the realization of the topological superconductivity and the Majorana fermion., 11 pages, 4 figures

The onset of ferromagnetism in cobalt-tin alloys is investigated experimentally and theoretically. The $\mathrm{C}{\mathrm{o}}_{1+x}\mathrm{Sn}$ alloys were prepared by rapid quenching from the melt and form a modified hexagonal NiAs-type crystal structure for $0.45\ensuremath{\le}x\ensuremath{\le}1$. The magnetic behavior is described analytically and by density-functional theory using supercells and the coherent-potential approximation. The excess of Co concentration $x$, which enters the interstitial $2d$ sites in the hypothetical NiAs-ordered parent alloy CoSn, yields a Griffiths-like phase and, above a quantum critical point $({x}_{c}\ensuremath{\approx}0.65)$, a quantum phase transition to ferromagnetic order. Quantum critical exponents are determined on the paramagnetic and ferromagnetic sides of the transition and related to the nature of the magnetism in itinerant systems with different types of chemical disorder.

We report terahertz (THz) electrodynamics of a moderately clean A15 superconductor (SC) following ultrafast excitation to manipulate quasiparticle (QP) transport. In the Martensitic normal state, we observe a photo enhancement in the THz conductivity using optical pulses, while the opposite is observed for the THz pump. This demonstrates wavelength-selective nonthermal control of conductivity distinct from sample heating. The photo enhancement persists up to an additional critical temperature, above the SC one, from a competing electronic order. In the SC state, the fluence dependence of pair-breaking kinetics together with an analytic model provides an implication for a ``one photon to one Cooper pair'' nonresonant energy transfer during the 35-fs laser pulse; i.e., the fitted photon energy $\ensuremath{\hbar}{\ensuremath{\omega}}_{}$ absorption to create QPs set by $2{\mathrm{\ensuremath{\Delta}}}_{SC}/\ensuremath{\hbar}\ensuremath{\omega}=0.33%$. This is more than one order of magnitude smaller than in previously studied BCS SCs, which we attribute to strong electron-phonon coupling and possible influence of phonon condensation.

The 1144 iron arsenide (e.g. CaKFe4As4) has recently been discovered and inspired a tide of search for superconductors. Such far, the discovered compounds are confined to iron arsenides (ABFe4As4), where A and B are either alkali metals or alkaline earth elements. In this work, we propose two directions in searching 1144 structures: (i) using tri-valence cations for A; (ii) substituting the transition metal, e.g. replacing Fe by Co. Following the two directions, we employ density functional theory to study stability and electronic structures of 1144 pnictides of various tri-valence cations (La, Y, In, Tl, Sm and Gd), as well as cobalt arsenides. For LaAFe4As4, the 1144 phase can be stabilized in three systems: LaKFe4As4, LaRbFe4As4 and LaCsFe4As4, which show quasi-two-dimensional semi-metal features similar to the iron pnictide superconductors: hole-type Fermi surface at Gama point and electron-type Fermi surface at M point in B.Z. In addition, LaKFe4As4 feature an extra bubble shaped Fermi surface sheets, distinct from the other two peers. Y does not support any 1144 phase within our search. For In and Tl, substitute Fe by Co and two unknown compounds of the 122 phase are stabilized: InCo2As2 and TlCo2As2. The two cobalt arsenides have Fermi surfaces of similar topology as iron arsenides, but the Fermi surfaces are all electron-type, showing potentials to be undiscovered superconductors. Stable 1144 phases are also found in InKCo4As4 and InRbCo4As4. For Sm and Gd, most 1144 and 122 iron arsenides are found unstable.

We investigate the magnetocrystalline anisotropy of ${\mathrm{YCo}}_{5}$ and ${\mathrm{ZrCo}}_{5}$ compounds from first-principles calculations using a real-space formalism of pseudopotentials. We study not only the experimentally observed phases but also other possible crystal structures in order to examine the impact of structural differences on the magnetic properties. Our results indicate that it may be difficult to enhance magnetocrystalline anisotropy constant ${K}_{1}$ and saturation magnetization ${M}_{s}$ simultaneously in ${\mathrm{YCo}}_{5}$ compounds in a hexagonal crystal family. We find that ${\mathrm{ZrCo}}_{5}$ compounds have moderate ${K}_{1}$ and sufficient ${M}_{s}$, which can be much better than those of conventional ferrite magnets. We expect that ${\mathrm{ZrCo}}_{5}$ compounds, consisting of affordable elements, are promising rare-earth-free materials that will be useful for permanent magnet applications.

In this paper, we present a coupled experimental/theoretical investigation of pressure effect on the ferromagnetism of LaCrGe3 and LaCrSb3 compounds. The magnetic, electronic, elastic and mechanical properties of LaCrGe3 and LaCrSb3 at ambient condition are studied by first-principles density functional theory calculations. The pressure dependences of the magnetic properties of LaCrGe3 and LaCrSb3 are also investigated. The ferromagnetism in LaCrGe3 is rather fragile with a ferro- to paramagnetic transition at a relatively small pressure (around 7 GPa from our calculations, and 2 GPa in experiments). The key parameter controlling the magnetic properties of LaCrGe3 is found to be the proximity of the Cr DOS to the Fermi surface, a proximity that is strongly correlated to the distance between Cr atoms along the c-axis, suggesting that there would be a simple way to suppress magnetism in systems with one dimensional arrangement of magnetic atoms. By contrast, the ferromagnetism in LaCrSb3 is not fragile. Our calculation results are consistent with our experimental results and demonstrate the feasibility of using first-principles calculations to aid experimental explorations in screening for materials with fragile magnetism.

The multimode Jahn-Teller (JT) effect in a bulk system of a neutral nitrogen-vacancy ($\mathrm{N}{\mathrm{V}}^{0}$) center in diamond is investigated via first-principles density-functional-theory calculations and the intrinsic distortion path (IDP) method. The adiabatic potential energy surface of the electronic ground state of the $\mathrm{N}{\mathrm{V}}^{0}$ center is calculated based on the local spin-density approximation. Our calculations confirm the presence of the dynamic Jahn-Teller effect in the ground $^{2}E$ state of the $\mathrm{N}{\mathrm{V}}^{0}$ center. Within the harmonic approximation, the IDP method provides the reactive path of JT distortion from unstable high-symmetry geometry to stable low-symmetry energy minimum geometry, and it describes the active normal modes participating in the distortion. We find that there is more than one vibrational mode contributing to the distortion, and their contributions change along the IDP. Several vibrational modes with large contributions to JT distortion, especially those modes close to 44 meV, are clearly observed as the phonon sideband in photoluminescence spectra in a series of experiments, indicating that the dynamic Jahn-Teller effect plays an important role in the optical transition of the $\mathrm{N}{\mathrm{V}}^{0}$ center.

A series of iron arsenides (e.g., ${\text{CaRbFe}}_{4}{\text{As}}_{4}, {\text{SrCsFe}}_{4}{\text{As}}_{4}$) have been discovered recently, and have provoked a rise in superconductor searches in a different phase, known as the 1144 phase. For the presence of various chemical substitutions, it is believed that more 1144 compounds remain to be discovered. In this work, we perform general model analysis as well as scenario calculation on a basis of density functional theory to investigate phase stability in a variety of compounds. We predict that the 1144-type phase could be stabilized in ${\text{EuKFe}}_{4}{\text{As}}_{4}, {\text{EuRbFe}}_{4}{\text{As}}_{4}, {\text{EuCsFe}}_{4}{\text{As}}_{4}, {\text{CaCsFe}}_{4}{\text{P}}_{4}, {\text{SrCsFe}}_{4}{\text{P}}_{4}, {\text{BaCsFe}}_{4}{\text{P}}_{4}, {\text{InCaFe}}_{4}{\text{As}}_{4}, {\text{InSrFe}}_{4}{\text{As}}_{4}$, etc. Remarkably, it involves rare earths, trivalence elements (e.g., indium) and iron phosphides, which greatly expands the range of its existence and suggests a promising prospect for experimental synthesis. In addition, we find that the formation of many random doping compounds (e.g., ${\text{Ba}}_{0.5}{\text{Cs}}_{0.5}{\text{Fe}}_{2}{\text{As}}_{2}, {\text{Ba}}_{0.5}{\text{Rb}}_{0.5}{\text{Fe}}_{2}{\text{As}}_{2}$) is driven by entropy and could be annealed to a 1144-type phase. Eventually, we plot a phase diagram about two structural factors $\mathrm{\ensuremath{\Delta}}a$ and $\mathrm{\ensuremath{\Delta}}c$, giving a bird's-eye view of stability of various 1144 compounds.

The atomic structure of $\mathrm{N}{\mathrm{i}}_{64.5}\mathrm{Z}{\mathrm{r}}_{35.5}$ metallic glass has been investigated by molecular dynamics (MD) simulations. The calculated structure factors from the MD glassy sample at room temperature agree well with the x-ray diffraction (XRD) and neutron diffraction (ND) experimental data. Using the pairwise cluster alignment and clique analysis methods, we show that there are three types of dominant short-range order (SRO) motifs around Ni atoms in the glass sample of $\mathrm{N}{\mathrm{i}}_{64.5}\mathrm{Z}{\mathrm{r}}_{35.5}$, i.e., mixed-icosahedron(ICO)-cube, intertwined-cube, and icosahedronlike clusters. Furthermore, chemical order and medium-range order (MRO) analysis show that the mixed-ICO-cube and intertwined-cube clusters exhibit the characteristics of the crystalline B2 phase. Our simulation results suggest that the weak glass-forming ability (GFA) of $\mathrm{N}{\mathrm{i}}_{64.5}\mathrm{Z}{\mathrm{r}}_{35.5}$ can be attributed to the competition between the glass forming ICO SRO and the crystalline mixed-ICO-cube and intertwined-cube motifs.

Developing accurate and computationally efficient methods to calculate the electronic structure and total energy of correlated-electron materials has been a very challenging task in condensed matter physics and materials science. Recently, we have developed a correlation matrix renormalization (CMR) method which does not assume any empirical Coulomb interaction $U$ parameters and does not have double counting problems in the ground-state total energy calculation. The CMR method has been demonstrated to be accurate in describing both the bonding and bond breaking behaviors of molecules. In this study, we extend the CMR method to the treatment of electron correlations in periodic solid systems. Using a linear hydrogen chain as a benchmark system, we show that the results from the CMR method compare very well with those obtained recently by accurate quantum Monte Carlo (QMC) calculations. We also study the equation of states of three-dimensional crystalline phases of atomic hydrogen. We show that the results from the CMR method agree much better with the available QMC data in comparison with those from density functional theory and Hartree-Fock calculations.

Using a real-space implementation of pseudopotentials within the density-functional theory, we show that the magnetization of a ${\mathrm{Co}}_{3}\mathrm{N}$ cluster with a recently discovered atomic structure is significantly affected by nitrogen dopants. In a ${\mathrm{Co}}_{3}\mathrm{N}$ cluster with the hexagonal $P{6}_{3}/mmc$ structure, N dopants promote spin polarization for the Co-$3d$ electrons and the dopants themselves make additional contributions to net magnetic moment. These two factors enhance the total magnetic moment of a hexagonal ${\mathrm{Co}}_{3}\mathrm{N}$ cluster, which can be as strong as bulk iron. In contrast, N dopants in a ${\mathrm{Co}}_{3}\mathrm{N}$ cluster with the rhombohedral $R\overline{3}c$ structure degrade magnetic moment and the dopants are magnetically ``inert,'' which results in lower total magnetic moments in rhombohedral ${\mathrm{Co}}_{3}\mathrm{N}$ clusters. These changes in magnetic moment originate from differences in the orbital hybridization between the Co-$3d$ and N-$2p$ states. We also examine how the magnetization of a ${\mathrm{Co}}_{3}\mathrm{N}$ cluster depends on a N content. We find that the total magnetic moment of a hexagonal ${\mathrm{Co}}_{3}{\mathrm{N}}_{1+x}$ cluster with $\ensuremath{-}0.15\ensuremath{\le}x\ensuremath{\le}0.15$ is tunable and can be enhanced further by controlling the amount of nitrogen dopants.

Alnico alloy is a very promising candidate for rare-earth-free, high-performance permanent magnets, especially for high-temperature and levitation applications---including motors for electric vehicles. Engineering better magnets, and motors, is stymied by a lack of knowledge of alnico's nanoscale structure. Using advanced simulation methods, this study investigates the dependence of chemical composition and ordering of magnetic and matrix phases of alnico on annealing temperature. A surprising phase forms when the system is annealed at low temperatures, offering a possible route to improving alnico's performance.

Intriguing topological phases may appear in both insulating and semimetallic states. Topological insulators exhibit topologically nontrivial band inversion, while topological Dirac/Weyl semimetals show ``relativistic'' linear band crossings. Here, we report an unusual topological state of ${\mathrm{Pt}}_{3}\mathrm{Sn}$, where the two topological features appear simultaneously. Based on first-principles calculations, we show that ${\mathrm{Pt}}_{3}\mathrm{Sn}$ is a three-dimensional weak topological semimetal with topologically nontrivial band inversion between the valence and conduction bands, where the band structure also possesses type-II Dirac points at the boundary of two electron pockets. The formation of the Dirac points can be understood in terms of the representations of relevant symmetry groups and the compatibility relations. The topological surface states appear in accordance with the nontrivial bulk band topology. The unique coexistence of the two distinct topological features in ${\mathrm{Pt}}_{3}\mathrm{Sn}$ enlarges the material scope in topological physics, and is potentially useful for spintronics.

Theory and stochastic lattice-gas modeling is developed for the formation of intercalated metal islands in the gallery between the top layer and the underlying layer at the surface of layered materials. Our model for this process involves deposition of atoms, some fraction of which then enter the gallery through well-separated pointlike defects in the top layer. Subsequently, these atoms diffuse within the subsurface gallery leading to nucleation and growth of intercalated islands nearby the defect point source. For the case of a single point defect, continuum diffusion equation analysis provides insight into the nucleation kinetics. However, complementary tailored lattice-gas modeling produces a more comprehensive and quantitative characterization. We analyze the large spread in nucleation times and positions relative to the defect for the first nucleated island. We also consider the formation of subsequent islands and the evolution of island growth shapes. The shapes reflect in part our natural adoption of a hexagonal close-packed island structure. Motivation and support for the model is provided by scanning tunneling microscopy observations of the formation of intercalated metal islands in highly-ordered pyrolytic graphite at higher temperatures.

Co3Si was recently reported to exhibit remarkable magnetic properties in the nanoparticle form [Appl. Phys. Lett. 108, 152406 (2016)], yet better understanding of this material is to be promoted. Here we report a study on the crystal structures of Co3Si using adaptive genetic algorithm, and discuss its electronic and magnetic properties from first-principles calculations. Several competing phases of Co3Si have been revealed from our calculations. We show that the hexagonal Co3Si structure reported in experiments has lower energy in non-magnetic state than ferromagnetic state at zero temperature. The ferromagnetic state of the hexagonal structure is dynamically unstable with imaginary phonon modes and transforms to a new orthorhombic structure, which is confirmed by our structure searches to have the lowest energy for both Co3Si and Co3Ge. Magnetic properties of the experimental hexagonal structure and the lowest-energy structures obtained from our structure searches are investigated in detail.

Using scanning tunneling microscopy, we characterize a surface carbide that forms when Dy is deposited on the basal plane of graphite. To form carbide islands on terraces, Dy is first deposited at 650--800 K, which forms large metallic islands. Upon annealing at 1000 K, these clusters convert to carbide. Deposition directly at 1000 K is ineffective because nucleation on terraces is inhibited. Reaction is signaled by the fact that each carbide cluster is partially or totally surrounded by an etch pit. The etch pit is one carbon layer deep for most carbide clusters. Carbide clusters are also identifiable by striations on their surfaces. Based on mass balance, and assuming that only the surface layer of carbon is involved in the reaction, the carbide has stoichiometry $\mathrm{D}{\mathrm{y}}_{2}\mathrm{C}$. This is Dy-rich compared with the most common bulk carbide $\mathrm{Dy}{\mathrm{C}}_{2}$, which may reflect limited surface carbon transport to the carbide.

Structures and magnetic properties of Fe16-xCoxN2 are studied using adaptive genetic algorithm and first-principles calculations. We show that substituting Fe by Co in Fe16N2 with Co/Fe ratio smaller than 1 can greatly improve the magnetic anisotropy of the material. The magnetocrystalline anisotropy energy from first-principles calculations reaches 3.18 MJ/m3 (245.6 {\mu}eV per metal atom) for Fe12Co4N2, much larger than that of Fe16N2 and is one of the largest among the reported rare-earth free magnets. From our systematic crystal structure searches, we show that there is a structure transition from tetragonal Fe16N2 to cubic Co16N2 in Fe16-xCoxN2 as the Co concentration increases, which can be well explained by electron counting analysis. Different magnetic properties between the Fe-rich (x 8) Fe16-xCoxN2 is closely related to the structural transition.

The temperature-pressure phase diagram of the ferromagnet LaCrGe_{3} is determined for the first time from a combination of magnetization, muon-spin-rotation, and electrical resistivity measurements. The ferromagnetic phase is suppressed near 2.1 GPa, but quantum criticality is avoided by the appearance of a magnetic phase, likely modulated, AFM_{Q}. Our density functional theory total energy calculations suggest a near degeneracy of antiferromagnetic states with small magnetic wave vectors Q allowing for the potential of an ordering wave vector evolving from Q=0 to finite Q, as expected from the most recent theories on ferromagnetic quantum criticality. Our findings show that LaCrGe_{3} is a very simple example to study this scenario of avoided ferromagnetic quantum criticality and will inspire further study on this material and other itinerant ferromagnets.

We employ a real-space pseudopotential method to compute the structural energies of a prototypical metal-semiconductor interface. Specifically, we examine a Pb(111) film overlaid on a Si(111) substrate as a function of the metal thickness. For each layer of Pb, we fully relax the atomic coordinates and determine the lowest-energy structure. Owing to the lattice mismatch between the Pb and Si crystal structures, we consider a large supercell containing up to 1505 atoms for the largest system. Systems of this size remain challenging for most current computational approaches and require algorithms specifically designed for highly parallel computational platforms. We examine the structural properties of the interface with respect to the thickness of the metal overlayer, e.g., the corrugation of the profile of the Pb overlayer. The combined influence of the Si substrate and quantum confinement results in a rich profile for a transition between a thin overlayer (less than a few monolayers), where the corrugation is strong, and the bulk region (more than a half-dozen layers), where the overlaid Pb film is atomically flat. This work proves the feasibility of handling systems with such a level of complexity.

We develop a new implementation of the Gutzwiller approximation in combination with the local density approximation, which enables us to study complex 4f and 5f systems beyond the reach of previous approaches. We calculate from first principles the zero-temperature phase diagram and electronic structure of Pr and Pu, finding good agreement with the experiments. Our study of Pr indicates that its pressure-induced volume-collapse transition would not occur without change of lattice structure-contrarily to Ce. Our study of Pu shows that the most important effect originating the differentiation between the equilibrium densities of its allotropes is the competition between the Peierls effect and the Madelung interaction and not the dependence of the electron correlations on the lattice structure.

Interaction between K adatoms on graphene is investigated by first-principles calculations based on density function theory and analytical analyses based on the $\mathbf{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{p}$ perturbation theory. The calculation shows that there is a strong repulsion between K adatoms. The main origin of this strong repulsion is not from the dipole-dipole interaction as suggested for K adatoms on graphite surface, but comes from the screened Coulomb interaction. Potassium adatom on graphene donates its $s$ electron and becomes ${\mathrm{K}}^{+}$. The positively charged K adatom induces electron density oscillation on graphene which is responsible for the screened Coulomb repulsion between the K adatoms.

We observe that Cu clusters grow on surface terraces of graphite as a result of physical vapor deposition in ultrahigh vacuum. We show that the observation is incompatible with a variety of models incorporating homogeneous nucleation and calculations of atomic-scale energetics. An alternative explanation, ion-mediated heterogeneous nucleation, is proposed and validated, both with theory and experiment. This serves as a case study in identifying when and whether the simple, common observation of metal clusters on carbon-rich surfaces can be interpreted in terms of homogeneous nucleation. We describe a general approach for making system-specific and laboratory-specific predictions.

Adsorption of Fe atom and ${\mathrm{Fe}}_{n}$ ($n=2\ensuremath{-}7$) clusters on graphene was studied by first-principles calculation and chemical bonding analysis. Various adsorption configurations, i.e., Fe clusters placed at different lateral positions and with different orientations with respect to the graphene lattice, have been optimized to locate the ground-state geometry of the Fe clusters on graphene. The calculation shows that Fe on graphene prefers a three-dimensional growth mode with a critical size of two atoms. The calculation also shows that ${\mathrm{Fe}}_{n}$ clusters on graphene exhibit ferromagnetic order and large magnetic moment. However, due to charge redistribution caused by the interaction with graphene, some ${\mathrm{Fe}}_{n}$ clusters on graphene have smaller magnetic moments compared to the corresponding free-standing clusters. Adsorption of ${\mathrm{Fe}}_{n}$ clusters also induces magnetic moments in graphene, and the induced magnetic moment on each carbon atom in graphene is correlated with the distortion of the graphene lattice.

We generalized the commonly used Gutzwiller approximation for calculating the electronic structure and total energy of strongly correlated electron systems. In our method, the evaluation of one-body and two-body density matrix elements of the Hamiltonian is simplified using a renormalization approximation to achieve better scaling of the computational effort as a function of system size. To achieve a clear presentation of the concept and methodology, we describe the detailed formalism for a finite hydrogen system with minimal basis set. We applied the correlation matrix renormalization approximation approach to a H${}_{2}$ dimer and H${}_{8}$ cubic fragment with minimal basis sets, as well as a H${}_{2}$ molecule with a large basis set. The results compare favorably with sophisticated quantum chemical calculations. We believe our approach can serve as an alternative way to build up the exchange-correlation energy functional for an improved density functional theory description of systems with strong electron correlations.

When the thickness of ultrathin metal films approaches the nanometer scale comparable to the coherence length of the electrons, significant effects on the structure stability and the electronic properties of the metal films emerge due to electron confinement and quantization of the allowed electronic states in the direction perpendicular to the film. Using first-principles calculations, we showed that such quantum size effects can induce oscillatory electrostatic potential and thus alternating electric field on the surface of the wedge-shaped Pb(111) films. The alternating electric field has significant influence on surface reactivity, leading to selective even- or odd-layer adsorption preference depending on the charge state of the adatoms, consistent with the odd-layer preference of higher Mg coverage on wedge-shaped Pb(111) films, as observed in experiment.

We present zero-temperature first-principles calculations of elemental cerium and we compute its pressure-volume phase diagram within a theoretical framework able to describe simultaneously both the α and the γ phases. A surprising result revealed by our study is the presence of a clear signature of the transition at zero temperature and that this signature can be observed if and only if the spin-orbit coupling is taken into account. Our calculations indicate that the transition line in the pressure-temperature phase diagram of this material has a low-T critical point at negative pressures, placed very close to zero temperature. This suggests that cerium is very close to being "quantum critical," in agreement with recent experiments.

Self-organized islands of uniform heights can form at low temperatures on metal/semiconductor systems as a result of quantum size effects, i.e., the occupation of discrete electron energy levels in the film. We compare the growth mode on two different substrates [Si(111)- (7x7) vs Si(111)- Pb(sqrt[3]xsqrt[3] )] with spot profile analysis low-energy electron diffraction. For the same growth conditions (of coverage and temperature) 7-step islands are the most stable islands on the (7x7) phase, while 5-step (but larger islands) are the most stable islands on the (sqrt[3]xsqrt[3] ). A theoretical calculation suggests that the height selection on the two interfaces can be attributed to the amount of charge transfer at the interface.

We have performed extensive first-principles calculations to study the binding and diffusion of a Ge ad-dimer on the Si(001) surface. The diffusion of a Ge ad-dimer along the lowest-energy-barrier pathway is found to be piecewise but with strong correlation, and its energy barrier is 0.77 eV. Such a correlated piecewise diffusion pathway is also found to be favored by a Si ad-dimer on Si(001), with an energy barrier of 1.02 eV in excellent agreement with experimental measurements. (c) 2000 The American Physical Society.