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Grain boundaries have been shown to dramatically influence the behavior of relatively simple materials such as monatomic metals and binary alloys. The increased chemical complexity associated with multiprincipal element alloys is hypothesized to lead to unique grain boundary phenomena. To explore the relationship between grain boundary structure and chemistry in these materials, hybrid molecular dynamics/Monte Carlo simulations of a faceted $\mathrm{\ensuremath{\Sigma}}11$ $\ensuremath{\langle}110\ensuremath{\rangle}$ tilt boundary, chosen to sample both high- and low-energy boundary configurations, are performed in face-centered cubic (fcc) CrFeCoNiCu and CrFeCoNi equiatomic alloys. Unexpected enrichment of Fe is discovered in the fcc regions adjacent to the interface and found to be correlated with a structurally distinct region of reduced atomic volume. Comparison with the boundary of the same type in monatomic Cu demonstrates that altered near-boundary regions exist in simpler systems as well, with the chemical complexity of the multiprincipal element alloys highlighting its existence and importance.

WTi is used as an adhesive layer in integrated circuit devices. The temperature dependent mechanical properties of WTi are still largely unexplored. In this paper we investigate WTi solid solutions with density functional theory calculations to determine the temperature and concentration dependent behavior of volume and coefficient of thermal expansion. The coefficient of thermal expansion is analyzed in terms of the bulk modulus, heat capacity, and Gr\"uneisen parameter. Furthermore, we gain insight into the bonding of the system via investigation of the electronic structure, phonon density of states, and analysis of the formation energy. Low Ti concentrations lead to strong W-Ti bonding, as manifested in additional high frequency peaks in the phonon density of states. As a consequence, deviations from Vegard's law are found at low Ti concentrations, with a minimum of the lattice constant at about 15 at. % Ti. The CTE as a function of Ti concentration shows a negative trend at low temperatures and Ti concentrations, which is related to a strong decrease of heat capacity. Finally we show that the Debye-Gr\"uneisen model yields results for WTi comparable to the quasiharmonic approach at a fraction of the computational cost.

Relaxation and aging rates of amorphous selenium and its undercooled melt are calculated by molecular dynamics based on an interaction model derived from density functional calculations. After a fast initial relaxation immediately following the quench, the decrease of diffusivity and the simultaneous increase of dynamic heterogeneity follow an exponential law given by defect annihilation. As observed previously in a Lennard-Jones glass, the non-Arrhenius aging of atomic volume and energy is determined by the time dependence of the diffusivity. The aging time is determined for long times by the diffusivity and diverges exponentially with decreasing temperature. Amorphous selenium differs from metallic glasses in that the breaking of covalent bonds is important for the long time decay of the intermediate self-scattering function (ISSF). Surprisingly for the temperatures investigated, aging of the relaxation times of the ISSF and the bond decay follow the aging of the diffusivity.

While binary $R{E}_{2}\mathrm{In}$, where $RE=\mathrm{rare}\phantom{\rule{0.16em}{0ex}}\mathrm{earth}$, have been reported a few decades ago, recent investigations revealed intriguing new physical insights. For instance, the discovery of a nearly ideal first-order ferromagnetic transition in ${\mathrm{Eu}}_{2}\mathrm{In}$ calls for further exploration of structures and properties of $R{E}_{2}\mathrm{In}$, in particular for the least-documented $RE=\mathrm{Eu}$ and Yb cases. Here, we investigate ${\mathrm{Eu}}_{2\text{\ensuremath{-}}x}{\mathrm{Yb}}_{x}\mathrm{In}$ pseudobinaries with nominal values of $x=0.25$, 0.5, 0.75, 1, 1.5, 2 by powder x-ray diffraction (including as function of temperature from 100 to 375 K for ${\mathrm{Yb}}_{2}\mathrm{In}$), magnetization (5--300 K), as well as electrical resistivity (5--300 K) and calorimetric (2--150 K) measurements for ${\mathrm{Yb}}_{2}\mathrm{In}$. Compared to other RE, Yb or Eu always raise challenging questions linked to their valence states. From average atomic volume, Yb is anticipated to be divalent in ${\mathrm{Yb}}_{2}\mathrm{In}$, at least between 100 and 375 K, which is in line with the absence of $4f$ magnetism. In agreement with x-ray diffraction and magnetization data, the resistivity of ${\mathrm{Yb}}_{2}\mathrm{In}$ is rather featureless and typical of a metal. Establishing ${\mathrm{Yb}}_{2}\mathrm{In}$ as a nonmagnetic isostructural reference for ${\mathrm{Eu}}_{2}\mathrm{In}$ allows one to use its heat capacity to revisit that of the latter, and get experimental insights into the exceptional magnetocaloric effect of the compound with Eu. In particular, we show that a third of the total magnetic entropy (${S}_{\mathrm{m}}\ensuremath{\approx}35.6\phantom{\rule{0.16em}{0ex}}\mathrm{J}\phantom{\rule{0.16em}{0ex}}\mathrm{mo}{\mathrm{l}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ at $T=100\phantom{\rule{0.16em}{0ex}}\mathrm{K}$) is concentrated in a 3 K temperature window around the ${T}_{\mathrm{C}}$ of ${\mathrm{Eu}}_{2}\mathrm{In}$. Starting from the ferromagnetic compound ${\mathrm{Eu}}_{2}\mathrm{In}$ $[{T}_{\mathrm{C}}=55.2(5)\phantom{\rule{0.16em}{0ex}}\mathrm{K}]$, we show that Yb substitutions in ${\mathrm{Eu}}_{2\text{\ensuremath{-}}x}{\mathrm{Yb}}_{x}\mathrm{In}$ lead to a decrease in both the Curie temperature [${T}_{\mathrm{C}}=41(2)$ and 32(2) K for $x=0.25$ and 0.5] and magnetic saturation, while weakening the first-order character of the transition as $x$ increases. A significant isothermal entropy change of $5.1(4)\phantom{\rule{0.16em}{0ex}}\mathrm{J}\phantom{\rule{0.16em}{0ex}}\mathrm{mo}{\mathrm{l}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ for $\mathrm{\ensuremath{\Delta}}B=2\phantom{\rule{0.16em}{0ex}}\mathrm{T}$ is found at 44 K in ${\mathrm{Eu}}_{1.75}{\mathrm{Yb}}_{0.25}\mathrm{In}$, demonstrating that the giant magnetocaloric effect of ${\mathrm{Eu}}_{2}\mathrm{In}$ can be tuned to lower temperatures by Yb substitutions.

We present a comprehensive study on structural and electronic properties of lanthanum intermetallic compounds (${M}_{x}{\mathrm{La}}_{y}$, $M=\text{Be}$, Mg, Al, Ga, In, Tl, Pb, and Bi) under high pressure. By using a swarm intelligence structure search method combined with first-principles calculations, pressure-induced phase transitions of ${M}_{x}{\mathrm{La}}_{y}$ were investigated, with several new structures predicted. A universal yet intriguing phenomenon was found; that is, all of these compounds will decompose into elemental solids at certain pressures, which is against the general intuition that extreme pressure always stabilizes and densifies materials. Mechanical analysis suggests that this anomalous behavior is associated to the elastic moduli and interatomic interaction in ${M}_{x}{\mathrm{La}}_{y}$, and their changes under extreme pressure. A low bulk modulus and larger atomic volume of La result in a smaller volume for the elemental mixture compared to their compound at high pressures, which leads to an energetically favorable $PV$ work and enthalpy for the elemental mixture. Furthermore, the external pressure tends to weaken the La-$M$ electrostatic interaction in compounds as evidenced by the reduced charge transfer between La and $M$, which in turn modifies the electronegativity of La and $M$ and destabilizes the compounds. Our results shed light on the high-pressure behaviors of La-based intermetallic compounds and provide important guidance for understanding other La-like intermetallic compounds at high pressures.

We calculate the atomic and electronic structures of trirutile-type ${\mathrm{Li}}_{x}{\mathrm{FeF}}_{3}\phantom{\rule{4pt}{0ex}}(x=0,0.25,0.5,0.75,\phantom{\rule{4pt}{0ex}}\text{and}\phantom{\rule{4pt}{0ex}}1)$ by first-principles calculations and evaluate the relative stability among the optimized structures by energy analysis. ${\mathrm{Li}}_{0.5}{\mathrm{FeF}}_{3}$ is more stable than the three-phase coexistence of ${\mathrm{FeF}}_{3},{\mathrm{FeF}}_{2}$, and LiF, whereas the other compositions are unstable. The analyses of the local electron density, local atomic volume, and local atomic configurations show that the formal valence of Fe atoms decreases from trivalent (3+) to divalent (2+) after Li insertion. In addition, we calculate Fe $K$-edge x-ray absorption near-edge structure (XANES) spectra in ${\mathrm{Li}}_{x}{\mathrm{FeF}}_{3}$ and compare them with observed spectra. The calculated XANES spectra agree well with the corresponding observed spectra in areas such as the spectral shape and relative position of the main peaks associated with ${\mathrm{Fe}}^{3+}$ and ${\mathrm{Fe}}^{2+}$. In particular, partial XANES spectra of ${\mathrm{Fe}}^{3+}$ in ${\mathrm{Li}}_{x}{\mathrm{FeF}}_{3}$, for $x=0.25,0.5$, and 0.75, have a specific peak between the main peaks, associated with ${\mathrm{Fe}}^{3+}$ and ${\mathrm{Fe}}^{2+}$. The detailed study reveals that the energy level and intensity ratio of the ${\mathrm{Fe}}^{3+}$ main peaks depend on the adjacent cation site of Fe.

Controlling glass-glass interfaces in metallic nanoglasses is essential for tuning their mechanical properties and in particular, the ability to inhibit severe strain localization through distributed shear band formation. In this paper, molecular dynamics are employed to quantify the structural characteristics of interfaces in a scalable nanoglass model produced through hydrostatic compression of amorphous nanoparticles. Using a framework for distinguishing interfaces from amorphous grains based on the correlation between dilatation and atomic volume distributions, we show that the interfaces in our nanoglass model exhibit a volume fraction of 0.36, a width of approximately 2 nm, excess free volume of 1--2%, and full icosahedral (FI) fraction roughly 30% that of a bulk metallic glass counterpart. While these characteristics are quantitatively unique relative to other nanoglass models (e.g., planar interfaces and Poisson-Voronoi constructions), they are consistent with experimental results reported for nanoglasses consolidated from glassy nanoparticles produced through inert gas condensation. Increasing the consolidation temperature enhanced the FI fraction with a strong bias to the interfaces, thus demonstrating a route for tuning interfacial properties in nanoglasses.

The adsorption geometry, the electronic properties, and the adsorption energy of the prototype organic molecule 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) on a monolayer of hexagonal boron nitride (hBN) grown on the Cu(111) surface were determined experimentally. The perylene core is at a large height of 3.37 $\AA{}$ and only a minute downward displacement of the functional anhydride groups (0.07 \AA{}) occurs, yielding adsorption heights that agree with the sum of the involved van der Waals radii. Thus, already a single hBN layer leads to a decoupled (physisorbed) molecule, contrary to the situation on the bare Cu(111) surface.

Stacking-dependent interlayer interactions are important for understanding the structural and electronic properties in incommensurable two-dimensional material assemblies where long-range moir\'e patterns arise due to small lattice constant mismatch or twist angles. Here we study the stacking-dependent interlayer coupling energies between graphene (G) and hexagonal boron nitride (BN) homo- and heterostructures using high-level random-phase approximation (RPA) ab initio calculations. Our results show that although total binding energies within LDA and RPA differ substantially by a factor of 200%--400%, the energy differences as a function of stacking configuration yield nearly constant values with variations smaller than 20%, meaning that LDA estimates are quite reliable. We produce phenomenological fits to these energy differences, which allows us to calculate various properties of interest including interlayer spacing, sliding energetics, pressure gradients, and elastic coefficients to high accuracy. The importance of long-range interactions (captured by RPA but not LDA) on various properties is also discussed. Parametrizations for all fits are provided.

The critical properties of the single-crystalline van der Waals bonded ferromagnet ${\mathrm{Fe}}_{3\ensuremath{-}x}{\mathrm{GeTe}}_{2}$ were investigated by bulk dc magnetization around the paramagnetic to ferromagnetic (FM) phase transition. The ${\mathrm{Fe}}_{3\ensuremath{-}x}{\mathrm{GeTe}}_{2}$ single crystals grown by self-flux method with Fe deficiency $x\ensuremath{\approx}0.36$ exhibit bulk FM ordering below ${T}_{c}=152$ K. The M\"ossbauer spectroscopy was used to provide information on defects and local atomic environment in such crystals. Critical exponents $\ensuremath{\beta}=0.372(4)$ with a critical temperature ${T}_{c}=151.25(5)$ K and $\ensuremath{\gamma}=1.265(15)$ with ${T}_{c}=151.17(12)$ K are obtained by the Kouvel-Fisher method, whereas $\ensuremath{\delta}=4.50(1)$ is obtained by a critical isotherm analysis at ${T}_{c}=151$ K. These critical exponents obey the Widom scaling relation $\ensuremath{\delta}=1+\ensuremath{\gamma}/\ensuremath{\beta}$, indicating self-consistency of the obtained values. With these critical exponents the isotherm $M(H)$ curves below and above the critical temperatures collapse into two independent universal branches, obeying the single scaling equation $m={f}_{\ifmmode\pm\else\textpm\fi{}}(h)$, where $m$ and $h$ are renormalized magnetization and field, respectively. The exponents determined in this study are close to those calculated from the results of the renormalization group approach for a heuristic model of three-dimensional Heisenberg ($d=3,n=3$) spins coupled with the attractive long-range interactions between spins that decay as $J(r)\ensuremath{\approx}{r}^{\ensuremath{-}(3+\ensuremath{\sigma})}$ with $\ensuremath{\sigma}=1.89$.

The van der Waals coefficient ${C}_{6}(\ensuremath{\theta};nlJM)$ of two like Rydberg atoms in their identical Rydberg states $|nlJM\ensuremath{\rangle}$ is resolved into four irreducible components called scalar ${R}_{ss}$, axial (vector) ${R}_{aa}$, scalar-tensor ${R}_{sT}={R}_{Ts}$, and tensor-tensor ${R}_{TT}$ parts in analogy with the components of dipole polarizabilities. The irreducible components determine the dependence of ${C}_{6}(\ensuremath{\theta};nlJM)$ on the angle $\ensuremath{\theta}$ between the interatomic and the quantization axes of atoms. The spectral resolution for the biatomic Green's function with account of the most contributing terms is used for evaluating the components ${R}_{\ensuremath{\alpha}\ensuremath{\beta}}$ of atoms in their Rydberg series of doublet states of the low angular momenta ($^{2}S, ^{2}P, ^{2}D, ^{2}F$). The polynomial presentations in powers of the Rydberg-state principal quantum number $n$ taking into account the asymptotic dependence ${C}_{6}(\ensuremath{\theta};nlJM)\ensuremath{\propto}{n}^{11}$ are derived for simplified evaluations of irreducible components. Numerical values of the polynomial coefficients are determined for Rb atoms in their $n{\phantom{\rule{0.222222em}{0ex}}}^{2}{S}_{1/2}, n{\phantom{\rule{0.222222em}{0ex}}}^{2}{P}_{1/2,3/2}, n{\phantom{\rule{0.222222em}{0ex}}}^{2}{D}_{3/2,5/2}$, and $n{\phantom{\rule{0.222222em}{0ex}}}^{2}{F}_{5/2,7/2}$ Rydberg states of arbitrary high $n$. The transformation of the van der Waals interaction law $\ensuremath{-}{C}_{6}/{R}^{6}$ into the dipole-dipole law ${C}_{3}/{R}^{3}$ in the case of close dipole-connected two-atomic states (the F\"orster resonance) is considered and the dependencies on the magnetic quantum numbers $M$ and on the angle $\ensuremath{\theta}$ of the constant ${C}_{3}(\ensuremath{\theta};nlJM)$ are determined together with the ranges of interatomic distances $R$, where the transformation appears.

Density functional theory in principle predicts correct ground-state properties for all materials. However, the rocksalt structure of MnO has been obtained only by the high-level random phase approximation and diffusion Monte Carlo (DMC). Here, we propose and test for MnO, FeO, CoO, and NiO that a semilocal density functional can solve this problem by properly including both self-interaction and van der Waals corrections. The importance of the latter was previously unanticipated. The MnO structural energy difference from SCAN+$r\text{VV10+}U$ agrees well with that from DMC. Here SCAN is a recent semilocal exchange-correlation functional, $r\text{VV10}$ is the revised Vydrov-Van Voorhis long-range van der Waals correction, and the onsite Hubbard $U$ is taken from linear response.

The Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation to the exchange-correlation energy is widely used in materials science. For many solids it somewhat overestimates lattice constants and underestimates cohesive energies. What attraction among the ions is missing in PBE? For ionic solids and heavy metals, a detailed analysis shows that the answer is the van der Waals (vdW) interaction arising from electric charge fluctuations on different ions. Our vdW correction to PBE is a short-range damped and long-range screened pairwise interaction among the ions. We find that the three-body interactions are small due to screening, and that the vdW correction stabilizes the bcc structure of cesium halides over the rocksalt structure.

Transition-metal nanoparticles adsorbed on graphene are of great interest due to the unique catalytic and magnetic properties resulting from nanoparticles-graphene interactions. Comparison between the physical properties of such systems and those of the same nanoparticles in the gas phase is especially important. Here we report a systematic density functional investigation of the structural, energetic, and magnetic properties of small ${\mathrm{Ni}}_{n}$, ${\mathrm{Pd}}_{n}$, and ${\mathrm{Pt}}_{n}$ clusters, comprising from $n=1$ to 6 atoms, in the gas phase and adsorbed on a graphene monolayer. Our results show that the $\mathrm{Ni}$ adatom binds to the graphene hollow site, with $\ensuremath{-}1.47\text{\ensuremath{-}}\mathrm{meV}$ adsorption energy, while $\mathrm{Pd}$ and $\mathrm{Pt}$ prefer the bridge sites, with $\ensuremath{-}1.14$- and $\ensuremath{-}1.62\text{\ensuremath{-}}\mathrm{meV}$ adsorption energies, respectively. This difference is determined by a competition between quantum and classical forces. ${\mathrm{Ni}}_{2}$ and ${\mathrm{Pt}}_{2}$ dimers bind perpendicularly on hollow and bridge sites, respectively, while ${\mathrm{Pd}}_{2}$ lies parallel to the graphene sheet, with each adatom on a bridge site. For larger ${\mathrm{TM}}_{n}$ ($\mathrm{TM}$ = $\mathrm{Ni}$, $\mathrm{Pd}$, $\mathrm{Pt}$; $n=3--6$) clusters, either two or three atoms bind to bridge graphene sites. In almost all cases the adsorbed clusters retain their gas-phase structures. The exceptions are ${\mathrm{Ni}}_{5}$ and ${\mathrm{Pt}}_{4}$, which acquire more compact structures with effective coordination number 12 and $19%$ larger than in the gas phase, respectively. As the number of atoms grows, the cluster binds more weakly to the graphene, while its binding energy mounts up. Van der Waals corrections to the plain density functional theory (DFT) total energy raise the adsorption energy, but leave the cluster structure unchanged, in the gas phase or upon adsorption. Bader charge analysis shows that adsorption causes minor charge redistribution: the $\mathrm{TM}$ atoms bound to $\mathrm{C}$ atoms become positively charged, while the remaining metal atoms acquire negative charge. We have derived an approximate analytical expression for the local densities of states for the $d$ orbitals of $\mathrm{Ni}$, $\mathrm{Pd}$, and $\mathrm{Pt}$ adatoms, on the basis of an extended Anderson-Newns model. Comparison with the DFT local densities of states for adsorption at hollow sites has identified interference among the wave functions responsible for the binding of distinct $d$ levels to the $\mathrm{C}$ atoms. No such interference has become visible for adsorption at bridge sites.

We report on the five-body repulsive and attractive van der Waals interactions between the strongly dipole-dipole coupled Rydberg states. Compared to four-body van der Waals interactions, five-body van der Waals interactions show more energy levels and more potential wells caused by avoided crossings. This research bridges the few-body physics and many-body physics. Other disciplines, such as chemistry, biology, and medical fields, will also benefit from better understanding van der Waals interactions.

For metallic glasses, ``flexibility volume'' has recently been introduced as a property-revealing indicator of the structural state the glass is in. This parameter incorporates the atomic volume and the vibrational mean-square displacement, to combine both static structure and dynamics information. Flexibility volume was shown to quantitatively correlate with the properties of metallic glasses [J. Ding et al., Nat. Commun. 7, 13733 (2016)]. However, it remains to be examined if this parameter is useful for other types of glasses with bonding characteristics, atomic packing structures, as well as properties that are distinctly different from metallic glasses. In this paper, we tackle this issue through systematic molecular-dynamics simulations of amorphous silicon $(a$-Si) models produced with different cooling rates, as $a$-Si is a prototypical covalently bonded network glass whose structure and properties cannot be characterized using structural parameters such as free volume used for metallic and polymeric glasses. Specifically, we demonstrate a quantitative prediction of the shear modulus of $a$-Si from the flexibility for atomic motion. This flexibility volume descriptor, when evaluated on the atomic scale, is shown to also correlate well with local packing, as well as with the propensity for thermal relaxations and shear transformations, providing a metric to map out and explain the structural and mechanical heterogeneity in the amorphous material. This case study of a model of covalently bonded network $a$-Si, together with our earlier demonstration for metallic glasses, points to the universality of flexibility volume as an indicator of the structure state to link with properties, applicable across amorphous materials with different chemical bonding and atomic packing structures.

The theory of the long-range interaction of metastable excited atomic states with ground-state atoms is analyzed. We show that the long-range interaction is essentially modified when quasi-degenerate states are available for virtual transitions. A discrepancy in the literature regarding the van der Waals coefficient C_6(2S;1S) describing the interaction of metastable atomic hydrogen (2S state) with a ground-state hydrogen atom is resolved. In the the van der Waals range a_0 > hbar c/L, we find an oscillatory tail with a negligible interaction energy below 10^(-36) Hz. Dirac-delta perturbations to the interaction are also evaluated and results are given for all asymptotic distance ranges; these effects describe the hyperfine modification of the interaction, or, expressed differently, the shift of the hydrogen 2S hyperfine frequency due to interactions with neighboring 1S atoms. The 2S hyperfine frequency has recently been measured very accurately in atomic beam experiments.

The atomic-level structures of liquids and glasses are amorphous, lacking long-range order. We characterize the atomic structures by integrating radial distribution functions (RDF) from molecular dynamics (MD) simulations for several metallic liquids and glasses: Cu46Zr54, Ni80Al20, Ni33.3Zr66.7, and Pd82Si18. Resulting cumulative coordination numbers (CN) show that metallic liquids have a dimension of d = 2.55 +/- 0.06 from the center atom to the first coordination shell and metallic glasses have d = 2.71 +/- 0.04, both less than 3. Between the first and second coordination shells, both phases crossover to a dimension of d = 3, as for a crystal. Observations from discrete atom center-of-mass position counting are corroborated by continuously counting Cu glass- and liquid-phase atoms on an artificial grid, which accounts for the occupied atomic volume. Results from Cu grid analysis show short-range d = 2.65 for Cu liquid and d = 2.76 for Cu glass. Cu grid structures crossover to d = 3 at {\xi}~8 {\AA} (~3 atomic diameters). We study the evolution of local structural dimensions during quenching and discuss its correlation with the glass transition phenomenon., Comment: 15 pages, 6 figures in main text

We present a comprehensive density-functional theory study of total energy and structural properties of As, Sb, and Bi in their A7 ground-state structure and in the bcc, fcc, and simple cubic (sc) modifications. We also investigate continuous structural transitions between these structures. The electronic structures and total energies are calculated both within the generalized gradient approximation (GGA) and local-density approximation (LDA) to the exchange-correlation energy as well as with and without inclusion of the spin-orbit coupling (SOC). The total energies of deformed structures are displayed in contour plots as functions of selected structural parameters and/or atomic volume; these plots are then used for understanding and interpreting structural parameters of As, Sb and Bi thin films on various substrates. Our calculated values of lattice parameters for (0001) thin films of Bi on Si(111) and Ge(111) substrates agree very well with available experimental data. In analogy with that, we suggest to investigate (0001) thin films of As on Ti(0001), Co(0001), Zn(0001) and Rh(111) substrates, of Sb on C(0001), Zn(0001), Al(111), Ag(111) and Au(111) substrates and of Bi on Co(0001), Al(111), Rh(111), Ba(111) and Pb(111) substrates. For these cases, we also predict the lattice parameters of the films. A large part of our results are theoretical predictions which may motivate experimentalists for a deeper study of these systems.

We theoretically address grazing incidence fast atom diffraction (GIFAD) for H atoms impinging on a LiF(001) surface. Our model combines a description of the H-LiF(001) interaction obtained from density functional theory calculations with a semiquantum treatment of the dynamics. We analyze simulated diffraction patterns in terms of the incidence channel, the impact energy associated with the motion normal to the surface, and the relevance of van der Waals (vdW) interactions. We then contrast our simulations with experimental patterns for different incidence conditions. Our most important finding is that for normal energies lower than 0.5 eV and incidence along the (100) channel, the inclusion of vdW interactions in our potential energy surface yields a greatly improved accord between simulations and experiments. This agreement strongly suggests a non-negligible role of vdW interactions in H-on-LiF(001) GIFAD in the low-to-intermediate normal energy regime. Fil: Bocan, Gisela Anahí. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina Fil: Miraglia, Jorge Esteban. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina Fil: Gravielle, Maria Silvia. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; Argentina

Long-range van der Waals (vdW) interaction is critically important for intermolecular interactions in molecular complexes and solids. However, accurate modeling of vdW coefficients presents a great challenge for nanostructures, in particular for fullerene clusters, which have huge vdW coefficients but also display very strong nonadditivity. In this work, we calculate the coefficients between fullerenes, fullerene and sodium clusters, and fullerene and alkali atoms with the hollow-sphere model within the modified single-frequency approximation (MSFA). In the MSFA, we assume that the electron density is uniform in a molecule and that only valence electrons in the outmost subshell of atoms contribute. The input to the model is the static multipole polarizability, which provides a sharp cutoff for the plasmon contribution outside the effective vdW radius. We find that the model can generate ${C}_{6}$ in excellent agreement with expensive wave-function-based ab initio calculations, with a mean absolute relative error of only $3%$, without suffering size-dependent error. We show that the nonadditivities of the coefficients ${C}_{6}$ between fullerenes and ${\mathrm{C}}_{60}$ and sodium clusters ${\mathrm{Na}}_{n}$ revealed by the model agree remarkably well with those based on the accurate reference values. The great flexibility, simplicity, and high accuracy make the model particularly suitable for the study of the nonadditivity of vdW coefficients between nanostructures, advancing the development of better vdW corrections to conventional density functional theory.

We consider the the van der Waals force involving excited atoms in general environments, constituted by magnetodielectric bodies. We develop a dynamical approach studying the dynamics of the atoms and the field, mutually coupled. When only one atom is excited, our dynamical theory suggests that for large distances the van der Waals force acting on the ground-state atom is monotonic, while the force acting in the excited atom is spatially oscillating. We show how this latter force can be related to the known oscillating Casimir--Polder force on an excited atom near a (ground-state) body. Our force also reveals a population-induced dynamics: for times much larger that the atomic lifetime the atoms will decay to their ground-states leading to the van der Waals interaction between ground-state atoms., Comment: 19 pages, 4 figures

Acene molecular crystals are of current interest in organic optoelectronics, both as active materials and for exploring and understanding new phenomena. Phonon scattering can be an important facilitator and dissipation mechanism in charge separation and carrier transport processes. Here, we carry out density functional theory (DFT) calculations of the structure and the full phonon dispersion of crystalline naphthalene, a well-characterized acene crystal for which detailed neutron-diffraction measurements, as well as infrared and Raman spectroscopy, are available. We evaluate the performance, relative to experiments, of DFT within the local density approximation (LDA); the generalized gradient approximation of Perdew, Burke, and Ernzerhof (PBE); and a recent van der Waals--corrected nonlocal correlation (vdW-DF-cx) functional. We find that the vdW-DF-cx functional accurately predicts lattice parameters of naphthalene within 1%. Intermolecular and intramolecular phonon frequencies across the Brillouin zone are reproduced within 7.8% and 1%, respectively. As expected, LDA (PBE) underestimates (overestimates) the lattice parameters and overestimates (underestimates) phonon frequencies, demonstrating their shortcomings for predictive calculations of weakly bound materials. If the unit cell is fixed to the experimental lattice parameters, PBE is shown to lead to improved phonon frequencies. Our study provides a detailed understanding of the phonon spectrum of naphthalene, and highlights the importance of including van der Waals dispersion interactions in predictive calculations of lattice parameters and phonon frequencies of molecular crystals and related organic materials.

In order to understand why carbon dioxide (CO2) and methane (CH4) molecules interact differently with surfaces, we investigate the Casimir-Polder energy of a linearly polarizable CO2 molecule and an isotropically polarizable CH4 molecule in front of an atomically thin gold film and an amorphous silica slab. We quantitatively analyze how the anisotropy in the polarizability of the molecule influences the van der Waals contribution to the binding energy of the molecule.

I propose a van der Waals density functional (vdW-DF) that improves upon the description of energetics and geometries of molecules, solids, and adsorption systems over the original vdW-DF. The functional is based on the nonlocal correlation for the second version of the vdW-DF [Lee et al., Phys. Rev. B 82, 081101(R) (2010)] and an exchange functional that recovers the second-order gradient expansion approximation in the slowly varying limit, while reproducing the large density gradient behavior proposed by Becke [J. Chem. Phys. 85, 7184 (1986)]. A systematic assessment of the proposed functional is presented, which demonstrates the applicability of the proposed vdW-DF to a wide range of systems.

We report the first observation of a weak ferromagnetic state of Cr in Cr/Ru(0001) superlattices, based on magnetic hysteresis and corroborated by x-ray magnetic circular dichroism at the CrL(2,3) edges. In situ reflection high-energy electron diffraction, x-ray diffraction, and Cr K-edge polarized x-ray absorption investigations have shown that the Cr layers thinner than 8 angstroms adopt a slightly distorted hcp structure, accompanied by a large atomic volume expansion of up to 14% compared to the bcc packing volume. The expanded hcp structure clearly induces the observed ferromagnetism, in agreement with theory.

High-resolution and magnetic neutron-diffraction measurements were performed to investigate the crystal and magnetic structures of bilayered FeOCl and polyaniline-intercalated FeOCl. A quasi-two-dimensional crystallographic structure, where charge neutral $({\mathrm{Fe}}_{2}{\mathrm{O}}_{2}{\mathrm{Cl}}_{2}{)}_{n}$ lamellas are weakly linked via Van der Waals interactions, has made FeOCl a good host for accommodating guest molecules. A three-dimensional long-range ordering of the Fe spins in FeOCl develops below 80 K, with a magnetic unit cell 28 times the size of the nuclear one. A spiral magnetic structure was obtained, reflecting the competition between antiferromagnetic superexchange coupling and ferromagnetic direct exchange coupling. Polyaniline intercalation interrupts the magnetic correlations between the neighboring bilayers, rendering a coupled-bilayer quasi-two-dimensional magnetic order for the Fe spins in $({\mathrm{C}}_{6}{\mathrm{D}}_{4}\mathrm{ND}{)}_{0.16}\mathrm{FeOCl}.$ No significant change on the ordering temperature of the Fe spins by the intercalation reaction was observed.

It is demonstrated that excited van der Waals systems can relax by electron emission via a novel interatomic mechanism. The process is analyzed by means of extensive ab initio calculations of potential energy surfaces and electronic decay rates. The electronic emission, taking place on the same time scale as the motion of the atomic nuclei, is accompanied by interesting dynamical effects amenable to experimental observations. These effects arise as a consequence of the weak chemical bond in van der Waals clusters and the Coulomb repulsion pattern originating from electron emission.

Through quantum-beat spectroscopy collision relaxation of a high vibrational level of SO2 at 44 877.52 cm(-1) is characterized. This is a first measurement of collision relaxation for a single, highly excited vibrational level. The deduced relaxation cross section of this excited level by Ar is 216 A(2), 5 times the area of the hard sphere, and by an ambient temperature SO2 molecule is 969 A(2), almost 20 times the hard sphere. These cross sections indicate that relaxation collisions of highly vibrationally excited molecules have effective distances much longer than van der Waals radii and involve mechanisms qualitatively different from lower excitations.

Microscopic calculations of the pitch of cholesteric liquid crystals are based on a few types of interactions between molecules: (1) short-range repulsive, (2) direct Coulomb, and (3) long-range van der Waals interactions. Recently, it was shown that first two types cannot be treated in the frame of mean-field approximation. Here we show that, contrary to common belief, an accurate evaluation of the intermolecular dispersion forces contributing to chiral ordering requires consideration of biaxial correlations between molecules which are neglected in the mean-field approximation. We found that in the presence of biaxial correlations chiral interactions depend very weakly on the anisotropy of the local (i.e., atomic) polarizability. Instead, the chiral interaction between two molecules is dominated by the character of biaxial correlations, the isotropic part of local polarizability of one molecule, and a chiral parameter of the other molecule, which characterizes the chiral molecular geometry and is similar to that found previously for steric interactions. Disciplines Physics | Quantum Physics This journal article is available at ScholarlyCommons: http://repository.upenn.edu/physics_papers/317 van der Waals interactions in cholesteric liquid crystals S. A. Issaenko* and A. B. Harris Department of Statistics, Wharton Business School, University of Pennsylvania, Philadelphia, Pennsyllvania 19104 Department of Physics, University of Pennsylvania, Philadelphia, Pennsylvania 19104 ~Received 19 April 1999! Microscopic calculations of the pitch of cholesteric liquid crystals are based on a few types of interactions between molecules: (1) short-range repulsive, (2) direct Coulomb, and (3) long-range van der Waals interactions. Recently, it was shown that first two types cannot be treated in the frame of mean-field approximation. Here we show that, contrary to common belief, an accurate evaluation of the intermolecular dispersion forces contributing to chiral ordering requires consideration of biaxial correlations between molecules which are neglected in the mean-field approximation. We found that in the presence of biaxial correlations chiral interactions depend very weakly on the anisotropy of the local ~i.e., atomic! polarizability. Instead, the chiral interaction between two molecules is dominated by the character of biaxial correlations, the isotropic part of local polarizability of one molecule, and a chiral parameter of the other molecule, which characterizes the chiral molecular geometry and is similar to that found previously for steric interactions. PACS number~s!: 61.30.Cz

A current induced by the van der Waals interaction between two parallel two-dimensional metals is reinterpreted in the quasiparticle picture. It is shown that in the presence of a dissipation, this current vanishes due to an effect of the van der Waals interaction on scattering processes.

A structural phase transition and the equation of state of the simple-hexagonal (sh) phase, $P\ensuremath{-}V,$ of phosphorus were investigated at pressures up to 280 GPa using monochromatic synchrotron x-ray diffraction. The $P\ensuremath{-}V$ phase, which was stable above 137 GPa, has shown a structural transition to a higher-pressure phase, $P\ensuremath{-}VI,$ above 262 GPa, while the $c/a$ ratio of the sh phase slightly increased with increasing pressure from 0.948 to 0.956. The structure of the $P\ensuremath{-}VI$ phase has been proposed to be bcc. The bulk modulus ${(B}_{0})$ and the atomic volume at atmospheric pressure ${(V}_{0})$ of the sh phase were estimated to be 84.1 GPa and 14.2 ${\mathrm{\AA{}}}^{3}$, with a fixed value of the pressure derivative of the bulk modulus ${(B}_{0}^{\ensuremath{'}}).$

Monovacancies for seven bcc d-transition metals V, Cr, Fe, Nb, Mo, Ta, and W have been studied in detail from first-principles calculations. A full-potential, linear muffin-tin-orbital (FP-LMTO) method has been used in conjunction with both the local-density approximation (LDA) and the generalized-gradient approximation (GGA) to calculate volume-relaxed vacancy formation energies in all seven metals. A complementary ab initio pseudopotential (PP) method has been used to calculate both volume- and structure-relaxed LDA formation energies and formation volumes in V, Nb, Mo, Ta, and W. Fully relaxed PP geometries have also been applied to FP-LMTO LDA and GGA calculations. From these results, the following clear trends and conclusions emerge: (i) for the same fully relaxed geometry, FP-LMTO-LDA and PP-LDA formation energies are nearly identical; (ii) the lowest calculated formation energies are within or close to experimental error bars for all bcc metals except Cr, and the overall agreement with experiment is better for the $4d$ and $5d$ metals than the $3d$ metals; (iii) GGA and LDA formation energies are very similar for the $4d$ and $5d$ metals but for the $3d$ metals, and especially Fe, GGA performs better; (iv) volume- and structural-relaxation contributions lower the calculated formation energy by 0.1--0.5 eV, and improve agreement with experiment; (v) fully relaxed LDA formation volumes are in the narrow range $(0.45--0.62){\ensuremath{\Omega}}_{0},$ where ${\ensuremath{\Omega}}_{0}$ is the equilibrium atomic volume; and (vi) the dominant structural effects are an approximate 5% inward relaxation of the first near-neighbor shell for group-V metals and a corresponding 1% inward relaxation for group-VI metals, with the exception of Mo, for which the second-shell atoms also relax inward by about 1%.

The van der Waals (vdW) interaction between thin metallic films varies with separation as the separation to a fractional power. This is in contrast to the usual integer-power separation dependence ...

We explore a situation where the van der Waals long-range atom-surface interaction is repulsive. This repulsion originates in a resonant coupling between a virtual emission at 12.15 mm of a Cs 6D3 2 atom and a virtual excitation of a surface polariton in sapphire. The experimental evidence is based upon the analysis of the spectroscopic response of Cs* in the near-infrared range with a technique that probes a distance range 100 nm away from the sapphire surface. We also demonstrate the critical dependence of atom-surface forces on the sapphire crystal orientation.

The paper reports on calculations of the flow properties in low-temperature He free-jet expansions by means of the Lennard-Jones potential and the recently appearing He-He van der Waals potential based on perturbation theory. The calculated speed ratios are compared with both the values we measured in the 20‐80 K source temperature range and data which were already reported. The comparison points out a better agreement of our experimental results with the speed ratio values obtained by the recent potential whenever the parallel temperature of the beam decreases below 10 22 K. In the same temperature range the light He-cluster size is analyzed. @S1050-2947~99!01004-5# PACS number~s!: 34.20.Cf, 36.40.2c

We report the first microscopic (RPA) calculation of the van der Waals interaction between two self-consistent jellium metal slabs---from asymptotic separations down to full contact. We also present the first test of a recently proposed seamless van der Waals density functional, which reproduces the RPA results satisfactorily at all separations.

The surface tension and the Tolman length have been presented as series in terms of the interface curvature $c.$ The expansion has been limited by a linear term on $c$ for the Tolman length and a square one for the surface tension. In the framework of the van der Waals capillarity theory the expansion coefficients have been expressed in terms of the planar interface characteristics. The coefficients asymptotic behavior has been derived in the vicinity of the liquid-vapor critical point. For the van der Waals fluid, the results of expansions have been compared with the data of direct numerical calculations. A wide curvature interval of the suitability of derived formulas has been stated. This fact allows the use of them in the homogeneous nucleation theory to calculate the critical nucleus formation work.

The nature and strength of the bonding forces between two II-passivated Si surfaces are studied with the density-functional theory, using an approach based on recent theoretical advances in understanding of van der Waals forces between two surfaces. Contrary to previous suggestions of van der Waals attraction between H overlayers, we find that the attraction is mainly due to long-range van der Waals interactions between the Si substrates, while the equilibrium separation is determined by short-range repulsion between occupied Si-H orbitals. Estimated bonding energies and Si-H frequency shifts are in qualitative agreement with experiment. [S0163-1829(98)06448-0].

The strength and effect of surface van der Waals forces on the shape of multiwalled and single-walled carbon nanotubes is investigated using atomic-force microscopy, continuum mechanics, and molecular-mechanics simulations. Our calculations show that depending on the tube diameter and number of shells, the van der Waals interaction between nanotubes and a substrate results in high binding energies, which has also been determined experimentally. Nanotubes on a substrate may consequently experience radial and axial deformations, which significantly modify the idealized geometry of free nanotubes. These findings have implications for electronic transport and the tribological properties of adsorbed nanotubes.

The importance of the long-range Lifshitz--van der Waals interaction energy between condensed bodies is well known. However, its implementation for interacting bodies that are highly irregular and separated by distances varying from contact to micrometers has received little attention. As part of a study of collisions of irregular aerosol particles, an approach based on the Lifshitz theory of van der Waals interaction has been developed to compute the interaction energy between a sphere and an aggregate of spheres at all separations. In the first part of this study, the iterated sum-over-dipole interactions between pairs of approximately spherical molecular clusters are compared with the Lifshitz and Lifshitz-Hamaker interaction energies for continuum spheres of radii equal to those of the clusters' circumscribed spheres and of the same masses as the clusters. The Lifshitz energy is shown to converge to the iterated dipolar energy for quasispherical molecular clusters for sufficiently large separations, while the energy calculated by using the Lifshitz-Hamaker approach does not. Next, the interaction energies between a contacting pair of these molecular clusters and a third cluster in different relative positions are calculated first by coupling all molecules in the three-cluster system and second by ignoring the interactions between the molecules of the adhering clusters. The error calculated by this omission is shown to be very small, and is an indication of the error in computing the long-range interaction energy between a pair of interacting spheres and a third sphere as a simple sum over the Lifshitz energies between individual, condensed-matter spheres. This Lifshitz energy calculation is then combined with the short-separation, nonsingular van der Waals energy calculation of Lu, Marlow, and Arunachalam, to provide an integrated picture of the van der Waals energy from large separations to contact.

The determination of the atomic structure of a Mn monolayer deposited at room temperature on Ag(001) was achieved by using surface extended x-ray absorption fine structure. Keeping in mind the previous results of x-ray photoelectron diffraction, several structural models involving the two topmost layers of the Ag substrate were tested in ab initio polarization-dependent XAFS calculations. Among these models, the most consistent one with the experimental data is made of a mixture of two different local environments that correspond to two different structures: first an inverted layer of Mn atoms substituting Ag ones underneath the topmost plane in fcc Ag; second a ${\mathrm{Ag}}_{0.5}{\mathrm{Mn}}_{0.5}$ surface alloy confined to the two topmost layers of Ag substrate. This latter arrangement corresponds to the observed $c(2\ifmmode\times\else\texttimes\fi{}2)$ low-energy electron diffraction long-range order. Accurate values for the Mn-Ag and Mn-Mn first interatomic distances were derived from standard analysis and found to be $2.86\ifmmode\pm\else\textpm\fi{}0.02$ and $2.88\ifmmode\pm\else\textpm\fi{}0.02\AA{},$ respectively. This definitely establishes that, in this surface alloy, Mn adopts an unusually large atomic volume, essentially the same as Ag. This finding is consistent with a high spin state of the Mn similar to the one in dilute Ag-based alloys or in the atomic ${}^{6}{S}_{5/2}$ ground state.

The pressure effect of self-diffusion in the high-temperature bcc phase of zirconium has been studied with high accuracy using the radiotracer technique. Activation volumes of $0.184\ifmmode\pm\else\textpm\fi{}0.016\ensuremath{\Omega}$ (\ensuremath{\Omega}: atomic volume) at 1423 K and $0.213\ifmmode\pm\else\textpm\fi{}0.014\ensuremath{\Omega}$ at 1273 K were obtained. Simultaneously, the activation volumes of the ${}^{95}\mathrm{Nb}$ solute diffusion were measured which amount to $0.161\ifmmode\pm\else\textpm\fi{}0.014\ensuremath{\Omega}$ at 1423 K and $0.193\ifmmode\pm\else\textpm\fi{}0.023\ensuremath{\Omega}$ at 1273 K. The small activation volumes indicate a strong relaxation of the vacancy. The large relaxation volume reflects the inherent weakness of the bcc lattice towards a shear in $〈111〉$ direction. The results provide an explanation for the absence of positron trapping in group-IV transition metals.

We describe a measurement of the long-range van der Waals interaction $V=\ensuremath{-}{C}_{6}{/r}^{6}$ between free neutral sodium atoms and clusters $({\mathrm{Na}}_{n},1l~nl~20)$ and ${\mathrm{C}}_{60}$ fullerenes. The ${C}_{6}$ coefficients were derived from absolute integral cross sections for low-energy scattering of a supersonic beam of metal particles by ${\mathrm{C}}_{60}$ vapor. Their values are extremely high, ranging from $\ensuremath{\sim}{10}^{4}\mathrm{a}.\mathrm{u}.$ up to more than ${10}^{5}\mathrm{a}.\mathrm{u}.,$ corresponding to center-of-mass scattering cross sections of up to $\ensuremath{\sim}6000{\mathrm{\AA{}}}^{2}$ (which exceeds the hard-sphere dimensions of the particles by a factor of 20). These experimental values of ${C}_{6}$ are in excellent quantitative agreement with the predictions of the London theory of dispersion forces based on atom and cluster spectral properties. The strength of the long-range potential is due to high cluster polarizabilities.