Island Size Selectivity during 2D Ag Island Coarsening on Ag (111)

We report on early stages of submonolayer Ag island coarsening on Ag(111) surface at room temperature ($300$ K) carried out using realistic kinetic Monte Carlo (KMC) simulations. We find that during early stages, coarsening proceeds as a sequence of selected island sizes creating peaks and valleys in the island size distribution. We find that island-size selectivity is due to formation of kinetically stable islands for certain sizes because of adatom detachment/attachment processes and large activation barrier for kink detachment. In addition, we find that the ratio of number of adatom attachment to detachment processes to be independent of parameters of initial configuration and also on the initial shapes of the islands confirming that island-size selectivity is independent of initial conditions.These simulations were carried out using a very large database of processes identified by their local environment and whose activation barriers were calculated using the embedded-atom method

Structural, Vibrational and Thermodynamic Properties of AgnCu34-n Nanoparticles

We report results of a systematic study of structural, vibrational and thermodynamical properties of 34-atom bimetallic nanoparticles from the AgnCu34-n family using model interaction potentials as derived from the embedded atom method and in the harmonic approximation of lattice dynamics. Systematic trends in the bond length and dynamical properties can be explained largely on arguments based on local coordination and elemental environment. Thus increase in the number of silver atoms in a given neighborhood introduces a monotonic increase in bond length while increase of the copper content does the reverse. Moreover, based on bond lengths of the lowest coordinated (6 and 8) copper atoms with their nearest neighbors (Cu atoms), we find that the nanoparticles divide into two groups with average bond length either close to (~ 2.58 A) or smaller (~ 2.48 A) than that in bulk copper, accompanied by characteristic features in their vibrational density of states. For the entire set of nanoparticles, vibrational modes are found above the bulk bands of copper/silver. Furthermore, a blue shift in the high frequency end with increasing number of copper atoms in the nanoparticles is traced to a shrinkage of bond lengths from bulk values. The vibrational densities of states at the low frequency end of the spectrum scale linearly with frequency as for single element nanoparticles, however, the effect is more pronounced for these nanoalloys. The Debye temperature was found to be about one third of that of the bulk for pure copper and silver nanoparticles with a non-linear increase with increasing number of copper atoms in the nanoalloys.Comment: 37 pages, 12 figure

Structure, Dynamics and Themodynamics of a metal chiral surface: Cu(532)

The structure, vibrational dynamics and thermodynamics of a chiral surface, Cu(532), has been calculated using a local approach and the harmonic approximation, with interatomic potentials based on the embedded atom method. The relaxation of atomic positions to the optimum configuration results in a complex relaxation pattern with strong contractions in the bond length of atoms near the kink and the step site and an equivalently large expansion near the least under-coordinated surface atoms. The low coordination of the atoms on the surface affects substantially the vibrational dynamics and thermodynamics of this system. The local vibrational density of states show a deviation from the bulk behavior that persist down to the 10th layer resulting in a substantial contribution of the vibrational entropy to the excess free energy amounting to about 90 meV per unit cell at 300K

Diffusion of the Cu monomer and dimer on Ag(111): Molecular dynamics simulations and density functional theory calculations

We present results of molecular dynamics (MD) simulations and density functional theory (DFT) calculations of the diffusion of Cu adatom and dimer on Ag(111). We have used potentials generated by the embedded-atom method for the MD simulations and pseudopotentials derived from the projected-augmented-wave method for the DFT calculations. The MD simulations (at three different temperatures: 300, 500, and 700 K) show that the diffusivity has an Arrhenius behavior. The effective energy barriers obtained from the Arrhenius plots are in excellent agreement with those extracted from scanning tunneling microscopy experiments. While the diffusion barrier for Cu monomers on Ag(111) is higher than that reported (both in experiment and theory) for Cu(111), the reverse holds for dimers [which, for Cu(111), has so far only been theoretically assessed]. In comparing our MD result with those for Cu islets on Cu(111), we conclude that the higher barriers for Cu monomers on Ag(111) results from the comparatively large Ag-Ag bond length, whereas for Cu dimers on Ag(111) the diffusivity is taken over and boosted by the competition in optimization of the Cu-Cu dimer bond and the five nearest-neighbor Cu-Ag bonds. Our DFT calculations confirm the relatively large barriers for the Cu monomer on Ag(111)-69 and 75 meV-compared to those on Cu(111) and hint a rationale for them. In the case of the Cu dimer, the relatively long Ag-Ag bond length makes available a diffusion route whose highest relevant energy barrier is only 72 meV and which is not favorable on Cu(111). This process, together with another involving an energy barrier of 83 meV, establishes the possibility of low-barrier intercell diffusion by purely zigzag mechanisms

Effect of dipolar interactions on the magnetization of a cubic array of nanomagnets

We investigated the effect of intermolecular dipolar interactions on a cubic 3D ensemble of 5X5X4=100 nanomagnets, each with spin $S = 5$. We employed the Landau-Lifshitz-Gilbert equation to solve for the magnetization $M(B)$ curves for several values of the damping constant $\alpha$, the induction sweep rate, the lattice constant $a$, the temperature $T$, and the magnetic anisotropy field $H_A$. We find that the smaller the $\alpha$, the stronger the maximum induction required to produce hysteresis. The shape of the hysteresis loops also depends on the damping constant. We find further that the system magnetizes and demagnetizes at decreasing magnetic field strengths with decreasing sweep rates, resulting in smaller hysteresis loops. Variations of $a$ within realistic values (1.5 nm - 2.5 nm) show that the dipolar interaction plays an important role in the magnetic hysteresis by controlling the relaxation process. The $T$ dependencies of $\alpha$ and of $M$ are presented and discussed with regard to recent experimental data on nanomagnets. $H_A$ enhances the size of the hysteresis loops for external fields parallel to the anisotropy axis, but decreases it for perpendicular external fields. Finally, we reproduce and test an $M(B)$ curve for a 2D-system [M. Kayali and W. Saslow, Phys. Rev. B {\bf 70}, 174404 (2004)]. We show that its hysteretic behavior is only weakly dependent on the shape anisotropy field and the sweep rate, but depends sensitively upon the dipolar interactions. Although in 3D systems, dipole-dipole interactions generally diminish the hysteresis, in 2D systems, they strongly enhance it. For both square 2D and rectangular 3D lattices with ${\bm B}||(\hat{\bm x}+\hat{\bm y})$, dipole-dipole interactions can cause large jumps in the magnetization.Comment: 15 pages 14 figures, submitted to Phys. Rev.

First principles calculations of the electronic and geometric structure of Ag27Cu7Ag_{27}Cu_{7} nanoalloy

\emph{Ab initio} calculations of the structure and electronic density of states (DOS) of the perfect core-shell $Ag_{27}Cu_{7}$ nanoalloy attest to its $D_{5h}$ symmetry and confirm that it has only 6 non-equivalent (2 $Cu$ and 4 $Ag$) atoms. Analysis of bond-length, average formation energy, heat of formation of $Ag_{27}Cu_{7}$ and $L1_2$ $Ag-Cu$ alloys provide an explanation for the relative stability of the former with respect to the other nanoalloys in the same family. The HOMO-LUMO gap is found to be 0.77 eV, in agreement with previous results. Analysis of the DOS of $Ag_{27}Cu_{7}$, $L1_2$ $Ag-Cu$ alloys and related systems provides insight into the effects of low coordination, contraction/expansion and the presence of foreign atoms on the DOS of $Cu$ and $Ag$. While some characteristics of the DOS are reminiscent of those of the phonon-stable $L1_2$ $Ag-Cu$ alloys, the $Cu$ and $Ag$ states hybridize significantly in $Ag_{27}Cu_{7}$, compensating the $d$-band narrowing that each atom undergoes and hindering the dip in the DOS found in the bulk alloys. Charge density plots of $Ag_{27}Cu_{7}$ provide further insights into the relative strengths of the various interatomic bonds. Our results for the electronic and geometric structure of this nanoalloy can be explained in terms of length and strength hierarchies of the bonds, which may have implications also for the stability of alloy in any phase or size.Comment: 16 figure

Time-dependent density-matrix functional theory for biexcitonic phenomena

We formulate a time-dependent density-matrix functional theory (TDDMFT) approach for higher-order correlation effects like biexcitons in optical processes in solids based on the reduced two-particle density-matrix formalism within the normal orbital representation. A TDDMFT version of the Schr\"odinger equation for biexcitons in terms of one- and two-body reduced density matrices is derived, which leads to finite biexcitonic binding energies already with an adiabatic approximation. Biexcitonic binding energies for several bulk semiconductors are calculated using a contact biexciton model

Temperature-dependent properties of 147-and 309-atom iron-gold nanoclusters

The properties of several Au-N and AuN-xFex nanoclusters are obtained by means of classical molecular dynamics calculations. In particular we study the configurations Au-147, Au134Fe13, Au-309, and Au254Fe55, which correspond to icosahedral magic numbers, for both the gold and the iron. We investigate the melting and freezing processes, atomic diffusion, hardness, vibration spectra, and specific heat of these nanoclusters. All the data obtained point toward the stability of the AuN-xFex system, with the gold atoms on the outside of the iron core