Positron confinement in ultrafine embedded particles: Quantum-dot-like state in an Fe-Cu alloy

A quantum-dot-like positron state is demonstrated in the ``defect-free'' particles using coincidence Doppler broadening of positron annihilation radiation. The wave functions of all positrons are entirely confined spatially in all three directions within the embedded nanosize and subnanosize Cu particles in a dilute Fe-Cu alloy. With use of this probe, the ultrafine particles are revealed to have nearly the same electron momentum distribution as bulk Cu, to be free from Fe atoms and defects, three dimensional, and to have no open-volume defects at the interfaces which can trap the positron. These successes indicate that this positron state promises to be a powerful tool for the studies of mesoscopic systems in metals and semiconductors.