Takeshi J. Inagaki

Photoluminescence spectrum of highly excited single CdS nanocrystals studied by a scanning near-field optical microscope

Many-body effects in strongly photoexcited single CdS nanocrystals embedded in Al2O3 matrices have been studied by a scanning near-field optical microscope at low temperatures. Under weak excitations, we find a sharp photoluminescence (PL) band originating from the exciton recombination. With an increase of the excitation laser energy, the PL intensity increases and the broad PL band is superimposed on the exciton-related sharp PL band. From a theoretical analysis, it was concluded that the broadening of the PL band is due to electron-hole pairs in CdS nanocrystals. The many-body effects of correlated exciton systems in CdS nanocrystals will be discussed.

Many-body theory of pump-probe spectra for highly excited semiconductors

We present a unified theory for pump-probe spectra in highly excited semiconductors, which is applicable throughout the whole density regime including the high-density electron-hole BCS state and the low-density excitonic Bose-Einstein condensate (BEC). The analysis is based on the BCS-like pairing theory combined with the Bethe-Salpeter (BS) equation, which first enables us to incorporate the state-filling effect, the band-gap renormalization and the strong/weak electron-hole pair correlations in a unified manner. We show that the electron-hole BCS state is distinctly stabilized by the intense pump-light, and this result strongly suggests that the macroscopic quantum state can be observed under the strong photoexcitation. The calculated spectra considerably deviate from results given by the BCS-like mean field theory and the simple BS equation without electron-hole pair correlation especially in the intermediate density states between the electron-hole BCS state and the excitonic BEC state. In particular, we find the sharp stimulated emission and absorption lines which originate from the optical transition accompanied by the collective phase fluctuation mode in the electron-hole BCS state. From the pump-probe spectral viewpoint, we show that this fluctuation mode changes to the exciton mode with decreasing carrier density

Photoluminescence properties of highly excited CdSe quantum dots

Abstract Many-body effects were studied in highly photoexcited CdSe quantum dots (QDs). The size of the QDs was larger than the exciton Bohr radius in bulk CdSe crystals, so that the excitons are confined in the CdSe QDs. With increasing excitation density, the photoluminescence (PL) intensity increased linearly and was then saturated. In the intensity saturation regime, the blueshift and the broadening of the PL spectrum were observed. From a theoretical analysis including both exciton–exciton and carrier–carrier interactions, it was concluded that the PL intensity saturation originates from the deformation of the exciton wave function due to the quasi-Fermi-level generation.

Theory of the luminescence spectra of high-density electron-hole systems : crossover from excitonic Bose-Einstein condensation to electron-hole BCS state

Abstract We present a unified theory of luminescence spectra for highly excited semiconductors, which is applicable both to the electron–hole BCS state and to the exciton Bose–Einstein condensate. The crossover behavior between electron–hole BCS state and exciton Bose–Einstein condensate clearly manifests itself in the calculated luminescence spectra. The analysis is based on the Bethe–Salpeter equation combined with the generalized random-phase-approximation, which enables us to consider the multiple Coulomb scattering and the quantum fluctuation associated with the center-of-mass motion of electron–hole pairs. In the crossover regime, the calculated spectra are essentially different from results obtained by the BCS-like mean-field theory and the interacting Boson model. In particular, it is found that the broad spectrum, arising from the recombination of electron–hole BCS state, splits into the P- and P 2 -luminescence bands with decreasing particle density. The dependence of these bands on the carrier density is in good agreement with experiments for highly excited semiconductors.

Photon pairing and the strong coupling phase of massive quantum electrodynamics Cooper equation

Through the Cooper equation of the photon pairing, the instability of the normal vacuum of the Quantum Electrodynamics with the massive electron is studied. Using the low energy effective Lagrangian, the normal vacuum is shown to be unstable against the condensation of the photon pairs above the critical value of the fine structure constant. These agree with the previous results obtained by the Bethe-Salpeter equation. The presence of the weak electric external field enhances the instability thus lowering the critical value. This can be a basis for the explanation of the anomalous GSI e+e− events.

Photon pairing in massless quantum electrodynamics

Abstract Instability of quantum electrodynamics with a massless electron due to photon pairing is studied. Through the box diagram, the photon-photon interaction turns out to be attractive and the two-photon bound state becomes tachyonic. The critical coupling for this phenomenon is zero. The influence of photon pairing on the chiral symmetry breaking is also discussed, which suggests that the critical coupling for the chiral symmetry breaking is also zero.

Dynamics of Photoinduced Phase Transition from a Dimer Mott Insulator to a Metal

We theoretically investigate the electron–phonon dynamics induced by photoexcitation in dimer Mott insulators of the form κ-(BEDT-TTF)2X [BEDT-TTF: bis(ethylenedithio)-tetrathiafulvalene, X: a counteranion]. We adopt the quarter-filled extended Hubbard Hamiltonian coupled with the dimer-bond stretching phonon modes. Utilizing the wave function where electron wave functions in the ground-state and photoexcited-state manifolds are coupled with different phonon wave functions, we solve the time-dependent Schrodinger equation numerically with a small initial disorder in the lattice. In the case where each of the dominant peaks of the interdimer charge transfer excited states is excited, the electron wave function is almost unchanged when the electron–phonon coupling constant s is smaller than the threshold value sTH. When \(s \gtrsim s_{\text{TH}}\) holds, there is an incubation time, and the decoherence of dimer-bond oscillations and the transitions to metallic states proceed simultaneously after the incubat...

Effects of Effective Mass Anisotropy and Effective Mass Difference in Highly Photoexcited Semiconductors

We analyze the effects of effective mass anisotropy and the effective mass difference between electrons and holes on the electron–hole ( e – h ) BCS state in highly excited semiconductors at variou...

Enhancement of the Electron–Hole BCS Order by Energy Band Anisotropy in Highly Photoexcited Semiconductors

We discuss the enhancement effects of the electron–hole BCS order due to the effective-mass anisotropy in highly photoexcited semiconductors. The present study provides us with a new possibility to experimentally observe a macroscopic quantum state by applying the uniaxial stress to semiconducting materials with high density electrons and holes. It is quantitatively shown that this enhancement due to the stress induced mass anisotropy can exceed the reduction of the order arising from the temperature effect and to from the difference in valence- and conduction-band structures.

Correlation Strength Crossover of Auger Recombination in One-Dimensional Mott Insulators

We theoretically investigate the relaxation of photogenerated charge carriers in one-dimensional Mott insulators. We adopt the Pariser–Parr–Pople model, and numerically calculate the time development of the nonequilibrium state excited by a weak light pulse. We investigate the dependence of the dynamics on the Coulomb correlation strength in the case where the binding effect between photogenerated opposite charges is significant. In the strong-correlation region (\(U \gg W\)), where U is the onsite Coulomb interaction energy and W is the bandwidth, the Auger recombination where two bound holon–doublon pairs decay into one unbound holon–doublon pair dominates the decay dynamics. From the strong- to intermediate (\(U \simeq W\))-correlation regions, the Auger recombination is always dominant, but the charge carriers involved in the process cannot be described by a holon and a doublon in the intermediate-correlation region. In the crossover region between these two regions, the Auger coefficient is strongly ...