Hydrogen (H) is the simplest ambipolar element, being recognized as one of the most important and ubiquitous defects in the field of semiconductor physics. One of the few means that have been applied to obtain experimental information about the local electronic state of isolated H is the use of muon (Mu) as pseudo-H. Here, we introduce a model that would make a paradigm shift to the microscopic understanding of Mu-related defects by providing resolution to serious discrepancies between the implications from implanted-Mu studies and theoretical predictions on the electronic state of H from ab initio density functional theory calculations in oxide semiconductors that have hindered the integration of both Mu and H knowledge.

We give an overview of the old and new topic, “charge symmetry”, a basic concept in nuclear physics. Old data implied that this symmetry might be broken largely for Λ hypernuclei. The state-of-the-art experimental techniques we developed, the decay π spectroscopy of electro-produced hypernuclei and the γ-ray spectroscopy of hypernuclei with a Ge detector array, have established the existence of a large charge symmetry breaking in ⁴_ΛH and ⁴_ΛHe hypernuclei.

We establish a Bloch band theory in a non-Hermitian system which is a nonequilibrium system described by a non-Hermitian Hamiltonian. Under an open boundary condition, our Bloch band theory can determine the Brillouin zone for a complex Bloch wave number reproducing energy bands in the thermodynamic limit. This Brillouin zone is called the generalized Brillouin zone. The generalized Brillouin zone tells us various physics in a non-Hermitian system. As an example, we can show the bulk-edge correspondence between a topological invariant defined from the generalized Brillouin zone and existence of topological edge states.

We demonstrated a vertical transistor, where organic molecules behave as quantum dots. A noteworthy aspect of the device is that the transistor channel consists of a double tunnel junction based on a metal-insulator-semiconductor (MIS) structure, where C₆₀ molecules are isolated from each other and are then embedded in the insulating layer of the MIS structure. The transistor thus permits to evaluate quantum transport induced by the individual molecules even in a macroscopic device. We found that the tunnel transport was explained by an orthodox theory that is widely used for single-carrier transport. The simulated drain current-drain voltage curves and differential conductance (dI_d /dV_d) curves well reproduced those obtained experimentally. Of importance is that the intervals of the dI_d /dV_d peaks derived from the degenerate molecular orbitals correspond to the charging energy of single or a few C₆₀ molecules. Furthermore, the single-carrier tunneling was still observed even at room temperature. Our proposed device therefore has the potential to integrate attractive molecular functions into current Si devices, and to deliver unique device operations unobtainable with inorganic quantum dots.

Non-linear correlated charge motions driven by a 6-femtosecond nearly single-cycle strong electric field of >10 megavolts/cm are observed as a stimulated emission and an unconventional second harmonic generation (SHG) in a layered organic superconductor. These non-linear charge motions before an increase in the electron temperature are enhanced by critical fluctuations near the superconducting transition temperature. The stimulated emission is attributed to a synchronized petahertz charge oscillation. On the other hand, the unconventional SHG is induced by charge acceleration that is confirmed by carrier envelope phase dependence of the SHG. Quantum many-body analyses clarify the detailed mechanisms.