Saroj K. Nayak

Anomalous magnetism in small Mn clusters

Abstract Unlike any other transition metal clusters, and in sharp contrast to its bulk behavior, Mn clusters containing up to 5 atoms are found to retain their atomic magnetic moments. Mn 2 , Mn 3 , Mn 4 and Mn 5 clusters in their ground states are coupled ferromagnetically and carry a moment of 10, 15, 20 and 25 μ B , respectively. Several low-lying excited states have also been identified. These results are based on all electron first-principles self-consistent calculations using density functional theory and generalized gradient approximation. Predictive capability of this approach is established by comparing the calculated energetics, electronic and magnetic properties of Mn 2 + with experiment.

Atomic and electronic structure of neutral and charged SinOm clusters

Using molecular orbital approach and the generalized gradient approximation in the density functional theory, we have calculated the equilibrium geometries, binding energies, ionization potentials, and vertical and adiabatic electron affinities of SinOm clusters (n⩽6,m⩽12). The calculations were carried out using both Gaussian and numerical form for the atomic basis functions. Both procedures yield very similar results. The bonding in SinOm clusters is characterized by a significant charge transfer between the Si and O atoms and is stronger than in conventional semiconductor clusters. The bond distances are much less sensitive to cluster size than seen for metallic clusters. Similarly, calculated energy gaps between the highest occupied and lowest unoccupied molecular orbital (HOMO-LUMO) of (SiO2)n clusters increase with size while the reverse is the norm in most clusters. The HOMO-LUMO gap decreases as the oxygen content of a SinOm cluster is lowered eventually approaching the visible range. The photolum...

Equilibrium geometries, electronic structure and magnetic properties of small manganese clusters

The equilibrium geometries, electronic structure and magnetic properties of small Mn clusters consisting of up to five atoms have been calculated self-consistently using first principles molecular orbital theory. The electron-electron interaction has been accounted for using the local spin density and generalized gradient approximation to the density functional theory. The atomic orbitals forming the molecular orbital have been represented separately by Gaussian and numerical basis sets. Two different computer codes (Gaussian 94 and DMOL) were used to check the numerical consistency of our calculations. is found to be a weakly bound van der Waals molecule and its binding energy depends sensitively on the choice of basis set as well as the form of the exchange-correlation potential. The binding energies are less sensitive to these approximations in larger clusters. The binding improves with cluster size, but remains significantly lower than those in other transition metal clusters. The equilibrium geometries are fairly compact and symmetric although other isomers with distorted geometries and with nearly the same energy as that of the ground state do exist for . The clusters also exhibit a variety of low-lying spin multiplicities, but the ground state spin configuration is ferromagnetic with a magnetic moment of . This not only contrasts with its bulk behaviour which is antiferromagnetic, but also differs from the behaviour in other transition-metal clusters where the magnetic moments/atom are always less than the free-atom value. The results are compared with available experiments on matrix isolated Mn clusters.

Thermodynamics of small nickel clusters

The thermal stabilities and dynamics of small nickel clusters consisting of 7 to 23 atoms have been studied extensively by a molecular dynamics method based on a tight-binding many-body potential. Physical properties such as the caloric curve, melting temperature, and bond-length fluctuations are computed. The simulation indicates that the clusters undergo solid-liquid phase change with the increase in internal energy. The melting temperature is found to be a non-monotonic function of cluster size with some clusters exhibiting pre-melting behaviour. Considerable depression in the melting point of small clusters has been observed - the highest melting temperature for small clusters is found to be almost half the value for the bulk nickel. The results are discussed in the light of recent nanocalorimetric experiments on small finite particles.

Structure and properties of Ni7 cluster isomers

Abstract First principles calculations based on molecular orbital theory as well as molecular dynamics simulation using a many body interatomic potential reveal the existence of two nearly degenerate isomeric forms of Ni 7 -a capped octahedron and a pentagonal bipyramid. Contrary to expectations, the magnetic moments of the two isomers are the same. Although the reactivity of the isomers with N 2 is different, it is argued that the current experimental studies of N 2 saturation coverage cannot confirm the coexistence of the two isomers. Further experiments are necessary to achieve this goal.

Observation of a spin-protected high-energy isomer of Al4N− cluster

Photoelectron spectroscopy experiments combined with first-principles calculations based on generalized gradient approximation to the density functional theory show that an Al4N− anion cluster lying 0.97 eV above its ground-state structure can exist not because it is protected by an energy barrier but because it has a different spin multiplicity. Its existence is established by comparing the calculated vertical electron detachment energies with the experimental photo-detachment spectra. The electron affinity of Al3N, on the other hand, is anomalously low showing the characteristic of a closed-shell molecule and providing the signature of a monovalent Al.

Dynamics and instabilities near the glass transition: From clusters to crystals

Molecular dynamics simulation has been used to explore the evolution, kinetics, and dynamics of a liquid–glass transition in clusters and bulk matter. We demonstrate a dynamical indicator that characterizes the onset of the glass transition in clusters and is consistent with other indicators of glass transitions in bulk systems. This criterion, based on changes in chaotic behavior as measured by the largest Liapunov exponent, reveals aspects of the microscopic processes associated with the phase change from liquid to glass, and provides a connection between the thermodynamic and dynamical behavior of systems and their multidimensional potential surfaces.

Structural isomerism of niobium clusters and its role on reactivity

Abstract Structures and energetics of Nb clusters are studied using a many-body potential based on the tight binding formulation and molecular dynamics simulations. Local and global minima are searched using steepest descent and conjugate gradient methods. Sequences of minima are thus ordered by energy revealing energetically degenerate isomers. While Nbn clusters for n ≤ 7 do not exhibit any structural isomerism, larger Nbn clusters do. In particular, Nb11 has two isomers with almost the same binding energy but completely different geometries. The role of geometry on reactivity is discussed and compared with recent experiments.

Magnetic bistability of supported Mn clusters

Using the KKR Greens function method and the local spin density approximation of the density functional theory, we have studied ultrasmall Mn clusters on an Ag(001) substrate. Our results show that supported Mn clusters not only possess a large magnetic moment per atom, but also exhibit magnetic bistability, making it possible to probe quantum tunneling in mesoscopic systems. We discuss our results in the light of recent experiments on supported clusters.