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The structures and magnetic properties of Mn clusters containing 13, 15, 19 and 23 atoms have been predicted from ab initio electronic structure calculations. The lowest energy structures of the 13-, 19- and 23-atom clusters are icosahedral and have ordered spin configurations in which blocks of ferromagnetically coupled spins are antiferromagnetically coupled with each other. The 15-atom cluster is body-centered cubic with antiferromagnetic coupling. The net moments are small and lie in the range of (formula presented) however, the average local atomic moments have high values close to (formula presented) These results are discussed in light of recent Stern-Gerlach experiments. {\textcopyright} 2002 The American Physical Society.

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Physical Review B - Condensed Matter and Materials Physics

Atomic structures and magnetic behavior of Mn clusters

We investigate the statics and dynamics of spatial phase segregation process of a mixture of fermion atoms in a harmonic trap using the density functional theory. The kinetic energy of the fermion gas is written in terms of the density and its gradients. Several cases have been studied by neglecting the gradient terms (the Thomas-Fermi limit) which are then compared with the Monte-Carlo results using the full gradient corrected kinetic energy. A linear instability analysis has been performed using the random-phase approximation. Near the onset of instability, the fastest unstable mode for spinodal decomposition is found to occur at q = 0. However, in the strong coupling limit, many more modes with q ≈ KF decay with comparable time scales.

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European Physical Journal D

Statics and dynamics of phase segregation in multicomponent fermion gas

Metal ([Formula presented])-encapsulated caged clusters of Ge are studied using the ab initio pseudopotential plane-wave method and the generalized gradient approximation for the exchange-correlation energy. Depending upon the size of the [Formula presented] atom, we find Frank-Kasper polyhedral [Formula presented] for [Formula presented], Zr, Hf and capped decahedral or cubic [Formula presented] and [Formula presented] clusters for several [Formula presented] atoms. The growth behavior differs from the one found in [Formula presented] clusters. The highest-occupied–lowest-unoccupied molecular orbital gaps are, however, similarly large or even higher in some cases. [Formula presented] and [Formula presented] are magnetic. The weak interaction between the clusters makes such species attractive for cluster assembled materials. {\textcopyright} 2002 The American Physical Society.

Physical Review Letters

Metal-Encapsulated Caged Clusters of Germanium with Large Gaps and Different Growth Behavior than Silicon

Vertical and adiabatic ionization potentials of small tin clusters having upto 20 atoms are numerically estimated using ab initio quantum mechanical methods. The ground state geometries and energetics of neutral and singly positively charged clusters are obtained using ultrasoft pseudopotential plane wave method with generalized gradient approximations. The calculated ionization potentials (IPs) show good agreement with the experimental results. The IPs of Sn7 and Sn10 show local maxima that corroborate the magic behavior of these clusters as found in the mass abundance spectra and photoionization experiments. A comparative study is also made to understand the effect of different exchange correlation functionals on the IPs using Gaussian molecular orbital methods. {\textcopyright} 2002 Published by Elsevier Science B.V.

Chemical Physics Letters

Ionization potentials of small tin clusters: First principles calculations

Recently, sharply reduced magnetic moments were measured in Mn13 and Mn19 clusters with values of about 0.5 $\mu$B/atom, while other clusters had moments of 1 to 1.4$\mu$B/atom. It was postulated that this sharply reduced magnetic moment results from the icosahedral growth sequence. We confirm the stability of the icosahedral structures. The icosahedral Mn13 and Mn19 clusters have several nearly degenerate low energy states, each with a low magnetic moment. It is shown that the mixture of ferromagnetic and antiferromagnetic coupling is responsible for the low net magnetic moments.

Materials Transactions

Anomalous magnetic moments of Mn13 and Mn19 clusters

Studies of the atomic and electronic structures of Pd clusters with 2-23, 55 and 147 atoms have been carried out using the ultrasoft pseudopotential plane wave method and the spin-polarized generalized gradient approximation for the exchange-correlation energy. We find predominantly an icosahedral growth in these clusters and size dependent oscillatory magnetic moments that are not confined just to the surface atoms. Atomically closed shell 13-, 55- and 147-atom clusters have large moments of (formula presented) (formula presented) and (formula presented) respectively. But cubic (formula presented) isomer has a low moment of (formula presented) only, indicating the importance of the icosahedral structure in the development of magnetism in Pd clusters. The evolutions of the (formula presented) hybridization, the magnetic moment and the electronic structure are discussed as a function of the cluster size. The magnetic energy is found to be small. Further studies on the adsorption of H and O show that though both an increase as well as a decrease of the moment are possible, often there is a reduction in the magnetic moments of Pd clusters. A hydrogen induced metal to nonmetal transition is predicted in (formula presented) and (formula presented) clusters. {\textcopyright} 2002 The American Physical Society.

Icosahedral growth, magnetic behavior and adsorbate-induced metal-nonmetal transition in palladium clusters

It is well known that (formula presented) bonding in carbon can result in stable cage structures, but pure Si clusters with similar cage structures are unstable. Using first-principles calculations, we show that a dodecahedral cage of silicon can be stabilized dynamically as well as energetically by doping with Ba, Sr, Ca, Zr and Pb atoms to create structures of silicon similar to that of the smallest carbon fullerene. The stability and bonding in such cages shed light on Si clathrates in which (formula presented) is the basic building block of the structure. Moreover, the charge distributions and highest-occupied-lowest-unoccupied molecular orbital gaps for these cage structures can be tuned by changing the metal atom. This allows additional freedom for the design of nanomaterials involving Si. {\textcopyright} 2002 The American Physical Society.

First-principles calculations of metal stabilized (formula presented) cages

TRANSACTIONS-MATERIALS RESEARCH SOCIETY OF JAPAN

Phase Separation in Fermion Droplets

Results of first-principles calculations on pure and doped aluminum clusters are analyzed using the electron localization function (ELF) to obtain a real-space representation of the electronic shell structure. Our results provide a quantitative analysis of the bonding nature and localization of charge in jelliumlike metal clusters and show that similar to atoms, ELF reproduces the electronic shell structure of clusters in real space. {\textcopyright} 2001 The American Physical Society.

Real-space representation of electron localization and shell structure in jelliumlike clusters

Ab-initio calculations are performed to study interaction of hydrogen on aluminum clusters. The results show large binding energies for a single hydrogen atom on Al7 and Al13 and two hydrogen atoms on Al6. The highest occupied-lowest unoccupied molecular orbital (HOMO-LUMO) gaps of Al7 and Al13 become dramatically large by absorbing hydrogen. These large binding energies and HOMO-LUMO gaps make Al7H and Al13H clusters magic(s). The atomic structures and binding energies of Al6H2 and Al8H2 are used to explain the reason why hydrogen is likely to be dissociated only on Al6. These results are discussed in the light of available experimental data and are in very good agreement with them.

Bonding character of hydrogen in aluminum clusters

Ab initio electronic structure calculations on Snn (n≤20) clusters using ultrasoft pseudopotentials and generalized gradient approximation for the exchange-correlation energy show the binding energies of clusters with n≥10 to be only about 11% less than the calculated bulk value. This is likely to be responsible for the recently reported [Phys. Rev. Lett. 85, 2530 (2000)] higher melting temperatures of these clusters than the bulk value. The growth behavior is found to differ from the one known for Si and Ge clusters at n≥8 but 10- and 18- to 20-atom clusters are similar. The calculated lowest energy fragmentation products are in excellent agreement with experiments and suggest that the lowest energy structures, obtained here, are close to the global minima.

Small clusters of tin: Atomic structures, energetics and fragmentation behavior

Cubic Caged Clusters of Silicon Kumar, Vijay; Kawazoe, Yoshiyuki

We report metal-encapsulated caged clusters of silicon from ab initio pseudopotential plane wave calculations using generalized gradient approximation for the exchange-correlation energy. Depending upon the size of the metal (M) atom, silicon forms fullerenelike M@Si16, M = Hf, Zr and cubic M@Si14 M = Fe, Ru, Os, caged clusters. The embedding energy of the M atom is ≈ 12 eV due to strong M − Si interactions that make the cage compact. Bonding in these clusters is predominantly covalent and the highest-occupied–lowest-unoccupied molecular orbital gap is ≈ 1.5 eV. However, an exceptionally large gap (2.35 eV) is obtained for Ti@Si16 Frank-Kasper polyhedron. Interaction between these clusters is weak, making them attractive for cluster-assembled materials. {\textcopyright} 2001 American Physical Society.

Metal-encapsulated fullerenelike and cubic caged clusters of silicon

Scripta Materialia

Icosahedral growth and non-metal-metal transition in strontium clusters

We present results of the electronic structure and stability of calcium coverings on C60 using a linear combination of atomic orbitals and the local density functional approximation. Calculations on CaxC60 with x=12, 20 and 32 show partial charge transfer from calcium atoms to C60 and hybridization of the calcium and fullerene states. This leads to (i) a large binding energy of Ca on C60 which decreases with an increasing coverage and (ii) formation of a metallic shell on C60. The large abundance of the Ca32C60 complex is shown to be due to geometric factors in agreement with experiments. {\textcopyright} 2001 Elsevier Science B.V.

Metallic coverings of calcium on C60

Recently there has been experimental evidence for structural transformations in single wall carbon nanotube bundles (SWCNTB) under pressure. We have performed ab initio electronic structure calculations on (10, 10) and (17, 0) SWCNTBs using the generalized gradient approximation for the exchange-correlation potential. When the cell shape is fixed to an hexagonal one, we find flattening of the nanotube walls in both the cases at low pressures. At higher pressures, the flattened walls buckle and ultimately cross-link. However, relaxation of the cell shape leads to a reversible first order transition of the (10, 10) SWCNTB lattice from near hexagonal to monoclinic. These results are discussed in the light of the available experimental results.

AIP Conference Proceedings

Structural transformations in single wall carbon nanotube bundles

We report high pressure Raman studies on single wall carbon nanotube bundles under hydrostatic conditions using two different pressure transmitting media, alcohol mixture and pure water. The radial and tangential modes show a blue shift when SWNT bundle is immersed in the liquids at ambient pressures. The pressure dependence of the radial modes is the same in both liquids. However, the pressure derivatives d$\omega$/dP of the tangential modes are slightly higher for the water medium. Raman results are compared with studies under non-hydrostatic conditions and with recent high-pressure X-ray studies. It is seen that the mode frequencies of the recovered sample after pressure cycling from 26 GPa are downshifted by ∼7-10 cm-1 as compared to the starting sample.

Physica Status Solidi (B) Basic Research

Pressure effects on single wall carbon nanotube bundles

Using ab initio pseudopotential plane wave method and generalized gradient approximation for the exchange and correlation energy, we report the finding of high spin ground state of (formula presented) cluster in accordance with the Hund's rule of maximum spin at half-filling. It has perfect icosahedral symmetry and a magnetic moment of (formula presented) forming an open shell superatom. Further studies of its interaction with an Al atom have led to an electronically closed shell magic cluster with 1.68 eV highest occupied–lowest unoccupied molecular orbital (HOMO-LUMO) gap such that the added Al atom is incorporated within the cage around the Cu atom. {\textcopyright} 2001 The American Physical Society.

Hund's rule in metal clusters: Prediction of high magnetic moment state of (formula presented) from first-principles calculations

Ab initio molecular-dynamics studies on SrN (N = 2–35, 55 and 147) clusters show d electron induced icosahedral growth. There is a gradual increase in the hybridization between the sp and d states as well as the metallization of clusters as the size grows. For Sr147, the highest occupied and lowest unoccupied molecular orbital gap becomes vanishingly small. The nonmetal-metal transition is found to occur more prominently along the close-packed directions from the center to the 12 vertices with an oscillatory behavior of the bond length. However, in other directions, there is a monotonic expansion as one goes from the center to the surface of the cluster. This is in contrast to contraction known in other metal clusters but agrees with the interlayer expansion found on the surfaces of divalent metals. {\textcopyright} 2001 The American Physical Society.

Evolution of electronic states and abnormal multishell relaxations in strontium clusters

Most stable structures and physical properties are studied for silicon-doped (Formula presented) and (Formula presented) clusters using the ab initio molecular-dynamics method within the framework of a plane-wave pseudopotential approach and the local density as well as the generalized gradient approximations. The lowest energy structures of the undoped clusters are found to be Jahn-Teller distorted icosahedron, double icosahedron and decahedron, respectively. Substitutional doping with a Si impurity makes these clusters electronically closed shell and leads to a large gain in the binding energy, which decreases with an increase in the cluster size in a nonmonotonic way. The heat of solution of a Si atom in clusters is found to be exothermic as compared to endothermic behavior in bulk aluminum. Further, a Si impurity is found to stabilize the (Formula presented) cluster in cuboctahedral structure. However, a capped icosahedron as well as a double icosahedron are found to be nearly degenerate with about 1.77 eV higher binding energy. For (Formula presented) the decahedral isomer has the lowest energy with a highest-occupied lowest-unoccupied molecular-orbital gap of 0.82 eV. It is also found to be very stable when heated at 700 K. Similar results are likely to hold in the case of doping with germanium. We discuss the significance of these results for the understanding of the stability of silicon-doped quasicrystals. {\textcopyright} 2000 The American Physical Society.