By means of kinetic Monte Carlo simulations of the self-organized growth of quantum dots in strained semiconductor systems we resolve the seemingly contradictory features of kinetic versus thermodynamic behavior, e.g., with respect to the temperature dependence of the average dot size and their dispersion. We show that the size distribution immediately after deposition is kinetically controlled, with smaller islands for lower temperatures and larger islands for higher temperatures. For longer simulation times the kinetics leads to equilibration, and a crossover effect between the size distributions occurs, which is in good agreement with the predictions of thermodynamics.
Charge transfer between the Cu-0 chains and the Cu-02 planes has been studied by far infrared reAectivity (FIR) and Raman scattering (RS) measurements on YBa2Cu306F~compounds with 0(y & 1.46. Analysis of the reAectivity spectra by the four parameter semiquantum model shows the occurrence of the insulator/metallic transition for y =0. 76, in agreement with the dc conductivity. The co«-coTo frequency splittings related to the intralayer modes involving the atomic vibrations of the BaO and Cu02 planes decrease, whereas the co«-coTo splittings related to the interlayer modes, involving the atomic vibrations of the Cu2-04-Cul bonding, increase. We discuss three possible reasons for the change of the LO-TO splitting with increased fluorine doping: {i) charge transfer between ions which changes effective ionic charges; (ii) effect of lattice disorder induced by the fluorine, and (iii) the effect of the plasma in ceramics. We show that the charge transfer assumption is more than likely and is consistent with both FIR and RS investigations. Raman spectra exhibit frequency hardening of the 04-Cu2 modes and frequency softening of the 02-03 modes with increased fIuorine doping. The former points out the increase of the Coulomb interactions between ions within the interlayer bonds and the latter, their decrease and screening inside the CuO& planes. The phonon line shape of the B&g mode related to the Cu02 planes changes with the fluorine doping. The phonon line shape depends on the electronic polarization operator (EPO) which has been related to the free-carrier density of the Cu02 planes. The increase of the EPO points out the enhancement of the free-carrier density in the Cu02 planes. The charge transfer is revealed by the increase of the ionic Coulomb interactions within the Cul-04-Cu2 interlayer bonds, and their decrease and screening by the free carriers enhancement inside the Cu02 planes as fluorine doping increases. I. INTRDDUCTIQNSince the discovery of YBCO high-T, superconductors, many Raman and infrared measurements have been performed on YBa2Cu306+ compounds with variable oxygen content. 'The main objective was to study, by a phonon analysis, the tetragonal/orthorhombic (T/O) and the insulator/metallic (I/M) phase transition which simultaneously occur with superconductivity at x =0.35.The existence of a charge transfer between CuO chains and Cu02 planes, via the BaO planes, is a current interpretation for the increase of the hole concentration in the CuO2 planes and the I/M transition.Optical conductivity measurements show that the mobile carrier spectral weight is redistributed from the charge-transfer band region of the insulating YBa2Cu306 to low frequencies as the doping is increasing. ' The low-frequency conductivity associated with the Cu02 planes exhibits a nonDrude response. On the contrary, the interplanar optical response of highly doped YBCO is characterized by a Drude-like behavior. ' ' " Neutron-powder-diffraction measurements show a signi6cant change in the Cul-04-Cu2 and Cu2-Cu2 bond lengths near x =...
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