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The morphology and growth of interfacial intermetallic compound (IMC) between Sn-3.0Ag-0.5Cu solder alloy and Cu substrate metal of solder joint is reported. The IMC morphology and IMC thickness layer were observed at three different porosities of porous Cu interlayer. The results revealed that during soldering process, Cu6Sn5 compound with scallop like morphology was formed at the interface of both the solder alloy and Cu substrate and at solder alloy and porous Cu interlayer. By adding porous Cu interlayer at the solder joint, the IMC thickness increased with increasing soldering temperature and the number of pores in porous Cu interlayer. The effect of porosity on increasing the IMC layer was also due to the slower cooling rate during solidification of molten solder.

The half-filled Hubbard model on the honeycomb lattice is investigated by numerically exact large-scale quantum Monte Carlo simulations for lattice sizes up to 2592 sites. By performing careful finite-size scaling for the spin-spin correlation functions, calculated with a high degree of accuracy, we find that the ground state is antiferromagnetically long-range ordered at U/t = 4, where U is the on-site Hubbard interaction and t is the nearest neighbor hopping. Our result is in sharp contrast to a recent report [Meng et al., Nature 464, 847 (2010)], where instead strong evidence of spin liquid behavior is found.

The spin nanotube has attracted a lot of interest as an intersection between the strongly correlated electronics and the nanoscience. Among spin nanotubes we focus on the S = 1/2 three-leg spin tube, because it has the largest quantum fluctuation and the strongest frustration. Using the numerical exact diagonalization of the finite-cluster Heisenberg model, we reveal that two singlet ground states are degenerated in the spin gap phase of the spin tube. It is also found that the spin gap is characterized by the quantized Berry phase.

We theoretically investigate the possibility of a Mott insulating state in a class of molecular conductors where the composition ratio between two molecular species is incommensurate to each other, i.e., incommensurate molecular conductors. Although the band-filling becomes irrational and then metallic behavior would be expected, the on-site Coulomb repulsion among the valence electrons together with the potential from the closed-shell ions supplying the carriers can give rise to an 'incommensurate Mott insulator', which is characterized by the carriers being localized with a period incommensurate to the underlying lattice. First, we numerically confirm the existence of this insulating state in a simplified one-dimensional model by a clear opening of the charge gap within quantum Monte Carlo simulation. We also find an incommensurate antiferromagnetic insulating state as mean-field solutions in two-dimensional models, adopting the crystal structures of the actual materials showing insulating behavior experimentally, (MDT-TS)(AuI2)0.441 and κ-(BEDT-TTF)4X (X = Hg2.78Cl8 and Hg2.89Br8), which correspond to the incommensurate Mott insulating states.