Nanoscale Metallic Grains Research Articles

Predictions of an alternative pathway for grain-boundary driven twinning

The exceptional strength of nanoscale metallic grains is due to strain-accommodation mechanisms that are uncommon in their coarse-grained counterparts, such as deformation twinning. Using an ab initio density functional theory-phase field dislocation dynamics model, we provide evidence of an unconventional kinetic pathway for twin formation in nano-sized crystals involving two grain boundaries. It corresponds to greater favorability because it accommodates strain yet removes debilitating backstresses. A simple map is presented to relate the associated kinetics of nucleation and propagation to intrinsic material defect formation energies.

The coexistence of superconductivity and ferromagnetism in nano-scale metallic grains

A nano-scale metallic grain in which the single-particle dynamics are chaotic is described by the so-called universal Hamiltonian. This Hamiltonian includes a superconducting pairing term and a ferromagnetic exchange term that compete with each other: pairing correlations favor minimal ground-state spin, while the exchange interaction favors maximal spin polarization. Of particular interest is the fluctuation-dominated regime where the bulk pairing gap is comparable with or smaller than the single-particle mean level spacing and the Bardeen–Cooper–Schrieffer theory of superconductivity breaks down. Superconductivity and ferromagnetism can coexist in this regime. We identify signatures of the competition between superconductivity and ferromagnetism in a number of quantities: ground-state spin, conductance fluctuations when the grain is weakly coupled to external leads and the thermodynamic properties of the grain, such as heat capacity and spin susceptibility.

Parity Effects in Nanoscale Metallic Grain Including a Magnetic Impurity

In ultrasmall systems such as nanoscale metallic grains, the discrete energy spectrum together with electron correlations may give dramatic properties at low temperatures. Among others, the Kondo effect has attracted considerable attention recently. In particular, parity effects should emerge depending on the number of total electrons: e.g., the Kondo effect appears (disappears) at low temperatures for an even (odd) number of electrons, yielding a finite (divergent) susceptibility when the Kondo temperature is smaller than the level spacing. More remarkable parity effects were pointed out by Thimm et al. in dynamical as well as transport properties when the mean level spacing is nearly equal to (or even smaller than) the Kondo temperature. They found that the Kondo resonance is discretized into a series of subpeaks with one main peak (two split peaks) at the Fermi level for an even (odd) number of electrons. They claimed that this behavior characterizes the Kondo effect in a metallic grain, which also leads to the oscillation behavior in the conductance. Although such parity effects are naturally expected, it is not clear how a single (double) peak structure is developed in the Kondo resonance depending on whether the electron count is even or odd. More generally we ask which aspect is universal in the parity effects. In this note we revisit this question about the Kondo effect in a metallic grain. Using the self-consistent second-order perturbation expansion, we demonstrate that although the parity effect indeed appears generically, the shape of the spectrum exhibits nonuniversal properties depending on a model system employed, in contrast to the claim of Thimm et al. In the nonequilibrium case, we find the oscillation behavior possessing the parity effect in the differential conductance. We consider a small metallic grain including a magnetic impurity described by the Anderson Hamiltonian,

Spectral statistics on itinerant magnetism in nanoscale metallic grains

Effect of the spectral statistics and level correlation on the itinerant ferromagnetism (FM) in nanoscale metallic grains is presented. The magnetization and spin susceptibility at zero temperature depend only on the spin splitting and the average level spacing. The Stoner criterion implies that the Coulomb interaction is greater than the Fermi energy for the appearance of FM long-range order. The spin wave excitation is found to be different from that of translation-invariant lattice.

Effect of Level Statistics on Local Magnetism in Nanoscale Metallic Grains

Effect of level statistics on local electronic states and local magnetism in nanoscale metallic grains with transition-metal impurity in the ballistic regime is studied. It is shown that the mean occupation of local electron and the local magnetic moment in nanoscale metallic grains with odd conduction-electrons are larger than those with even conduction-electrons. The effect of even-odd parity on the condition for the occurrence of local magnetic moment is also discussed, it is found that the critical value, ρ d (0) U c , for the formation of local moment in nanoscale metallic grains is much smaller than that in bulks. The dependences of the local spin susceptibility on size and the Coulomb interaction are obtained. These results show that the level statistics plays an important role for the local magnetism, it distinguishes the properties of nanoscale metallic grains from those of clusters and bulks.

Local Magnetism in Nanoscale Metallic Grains

Quantum effects on local magnetism in nanoscale metallic grain with transition-metal impurity are studied in the ballistic regime. It is shown that in nanoscale system near the zero temperature, besides the size effect of the energy level, the level statistics and multilevel repulsion effect play important roles for the local electronic states and the formation of the local magnetic moments.

Possible contribution of organometallic species in the solar system ices. Reactivity and spectroscopy

Ionized metals and PAHs have been detected or suspected to be present in cometary and planetary atmospheres. Experimental studies are performed which show that interactions between Fe + and PAHs in the gas phase lead to organometallic complexes [Fe(PAH)] +. This leads to the proposal that metal bonded PAHs should be formed efficiently in these media. Spectral signatures (IR, visible and UV) of similar complexes are measured and their reactivity against common planetary atmospheric molecules (H 2O, N 2, NH 3, CO, C 2H 2, H 2, O 2) are studied. When organometallics condensate on telluric body surfaces, they are expected to colour ices orange or red and one expects the formation of nanoscale metallic grains embedded in a matrix of organic molecules.