Experimental Study on Quantum Coherence in Solids

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We have observed, for the first time, clear sign of the Fano effect, which is a consequence of phase-shift and interference in a combined system of a quantum dot and a quantum ring. The observation leads to new knowledge of quantum coherence, e.g., the nature of Fano parameter. We have also applied this effect to investigate various problems on quantum coherence. For example, we have clearly observed electrostatic control of quantum phase and found a new theoretical method to treat thermal broadening of the quantum phase, which is applied to analyze experimental results. Especially, we have given a partial answer to a long standing problem of "in-phase Coulomb peaks". We further found the existence of strongly coupled states in quantum dots and multi-level transport through an ordinal resonant tunneling and co-tunneling through a strongly coupled state results in Fano interference in a quantum dot without any outer reference arm. External magnetic field causes focusing of strongly coupled states and smooth transition to outmost edge states is observed.We have investigated the relation between the spin states in quantum dots and the quantum coherence by a quantum ring structure. We have found that the spin scattering causes quantum decoherence through the formation of quantum entangled states between a local spin and a spin of traversing electron.We have shown that coherent control of electronic path through magnetic decoration on two-dimensional electrons. We have found a new quantum oscillation in magnetoresistance of quantum anti-dot systems.