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内容記述 |
In this thesis, we apply a new path-integral theory to the many impurities Holstein <br />model to calculate the photoemission spectra (PES) of a doped simple cubic lattice. <br />We mean to investigate the two intrinsic attributes of electrons: itineracy and <br />localization, which is able to clarify the co-existence of a Fermi edge and the step-like <br />multi-phonon structure, observed recently in the PES experiment of the boron-doped <br />diamond (BDD). Focusing on the area close to the Fermi level, a simple cubic lattice <br />structure is adopted instead of the real diamond one to simulate the various valence <br />band nature of BDD. This can simplify the problem without losing the key point. <br /> For the phonon effect becomes significant just after the doping, we take into <br />account the electron-phonon (e-ph) coupling only at the doped sites. However, the <br />phonon effect is then not very noticeable after averaging all the sites, so we also <br />calculate the spectrum of doped sites only, beside the whole system spectrum. From <br />the classical Monte Carlo (MC) computation, the emergence of a clear Fermi edge is <br />seen. increasing the doping ratio, the impurity band expands upto the top of valence <br />band, and fills the small semiconductor gap gradually. Thus, the sample undergoes a <br />semiconductor-metal transition. Electrons can move freely from one impurity atom to <br />another one through those intermediate carbon atoms. In quantum MC simulations, <br />the lattice Green's function is calculated by the path-integral theory to reproduce the <br />spectral function. From the whole system spectra, the phase transition is confirmed on <br />the increase of the dopant concentration. The satellite structure is observed in the <br />doped sites spectrum, even within lightly doped sample. This structure has not been <br />found in classical MC cases, obviously coming from the phonon quantum character in <br />the e-ph coupling. Increasing the coupling constant, a second phonon peak also <br />presents corresponding to the double-phonon, even multi-phonon scattering process. <br />Because of the stronger coupling, a clear Fermi edge also appears although the doping <br />rate is low. The co-existence of a Fermi edge and the step-like multi-phonon satellite <br />structure is reproduced completely, which can be interpreted from the two basic <br />properties of electrons: itineracy and localization. <br /> At the same time, our method, which can distinguish the spectra of different <br />components in the material as we have done in calculating the spectra for the whole <br />system and the doped sites only respectively, is very useful-to study the doped <br />systems, and to explain the resonant PES experiments. |
要旨・審査要旨 (199.8 kB)
