Study of Atomic and Electronic Structures of Nano-Particles in the Early Stages of Their Formation Using Self-Searching Positrons

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Embedded (sub-)nano particles in materials play important roles in controlling the physical and mechanical properties of the host materials. Thus it is very indispensable to clarify the evolution processes of the nano particles as well as their atomic and electronic structures. This study aims to establish a novel approach to these issues with use of the latest positron-annihilation experimental and theoretical methods, such as coincidence Doppler broadening (CDB), two-dimensional angular correlation of annihilation radiation (2D-ACAR) and corresponding first-principles calculations.First we have revealed the formation process of Cu (bcc) particles in thermally aged Fe-Cu alloys by detailed aging experiments; the evolution of Cu aggregation in different aging conditions has been investigated by the CDB method. Second we have obtained detailed aging-induced changes in the 2D-ACAR momentum distribution, especially around Fermi cut-off momentum region. Larger smearing around the Fermi momentum is observed for the small Cu particles with sub-nano sizes. This observation strongly suggests a new and unique method for detecting sub-nano size Cu particles in Fe. At this stage we have succeeded to observe the nano particles larger than 0.5 nm.We also have measured 2D-ACAR spectrum of aging-induced particles in the single crystals of Al alloys: Ag_2Al and fcc Zn particles. Combining the first-principles calculations, we obtained detailed Fermi surfaces of the embedded particles.