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Speed Control of Vehicles with Variable Valve Lift Engine by Nonlinear MPC
Akihiro Murayama,Masaki Yamakita 제어로봇시스템학회 2009 제어로봇시스템학회 국제학술대회 논문집 Vol.2009 No.8
In this study, we purpose a control system of engine torque for a V6 sparkignition engine with avariable valve lift system. We apply an onlinear receding horizon control to the SICE benchmark problem where we assume that the control inputs are throttle angle, variable valve lift and ignition timing. Moreover, a fuel injection control is also developed by estimating the amount of air in the engine. We propose a vehicle speed tracking control by defining time derivative of input as new input and dealing with input constraint by transformation of input variables. Finally, we show some numerical simulation result stoverify the proposed methods.
Temperature dependence of the radiative recombination time in laterally coupled GaAs quantum dots
Kim, Heedae,Murayama, Akihiro,Kim, Jongsu,Song, Jindong Elsevier 2018 APPLIED SURFACE SCIENCE - Vol.457 No.-
<P><B>Abstract</B></P> <P>We performed temperature dependent photoluminescence measurements of exciton states in GaAs laterally coupled quantum dots. The temperature dependence of the radiative decay time was used to investigate the density of states to study the intra-relaxation of thermally excited exciton states. We analyzed the density of states in laterally coupled quantum dots by observing the radiative decay time of exciton states as a function of temperature, where the decay time of exciton photoluminescence was calibrated with respect to photoluminescence intensity to extract the radiative decay time. In the case of laterally coupled quantum dots, a one-dimensional structure can be formed by elongation along the direction of coupling between two quantum dots. However, this assumption is not valid when the size in the uncoupled direction is comparable to that in the coupled direction. From experimental measurements, the power order of the temperature dependence was found to be 0.8, which corresponds to a quasi-one-dimensional system (∼T<SUP>α=0.8</SUP>), which is in between a two-dimensional (α = 1) and a one-dimensional (α = 0.5) density of states. This is due to several factors including a large size along the uncoupled direction, an extended wave function owing to optical coupling, and the presence of dark states.</P> <P><B>Highlights</B></P> <P> <UL> <LI> The dimensionality of laterally coupled quantum dots is reported based on the measurement of temperature dependent radiative decay time. </LI> <LI> A quasi-one-dimensional density of states is observed with respect to the power order of the temperature dependence. </LI> <LI> Time-integrated photoluminescence spectral intensities are measured at various temperatures in laterally coupled quantum dots. </LI> </UL> </P>
박성호,김종수,Junich Takayama,Akihiro Murayama,김광석 한국물리학회 2018 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.73 No.3
We have performed time-resolved photoluminescence of dilute GaAs/AlGaAs quantum rings for increasing excitation and temperature. Although a large distance between quantum rings (200 ± 50 nm) prevents a ring-to-ring carrier transport, a decay time difference over the wide spectrum and its change became significant with increased excitation and temperature. Those results were explained in terms of an internal relaxation of the fine levels in a quantum ring structure. As carrier injection is increased, the carrier relaxation of high energy levels becomes enhanced possibly via the state-filling effect and Auger scattering. We also found that the population decay of low energy levels slows down due to a carrier feeding, but the net population of intermediate levels become nearly constant due to a balanced carrier transfer in between the high and low energy levels. For increased temperature, we observed a photoluminescence quenching at the high energy levels as a consequence of a thermal escape from the shallow potential depth.
Polarization anisotropy of exciton in self-assembled elliptical InP/InGaP quantum dots
김인홍,Takayuki Kiba,Akihiro Murayama,송진동,김광석 한국물리학회 2015 Current Applied Physics Vol.15 No.6
Optical anisotropy of self-assembled elliptical InP quantum dots has been investigated in terms of the polarization dependence of excitons. Although large size inhomogeneity is present, two kinds of characteristic quantum dots, which are classified into large and small quantum dots, were found in terms of the polarization anisotropy.We have confirmed that the large quantum dots are more pronounced in the polarization anisotropy, where the degree of linear polarization for the large quantum dots is significantly larger (~60%) than that for the small ones (~36%). The effective shape of quantum dots is also estimated by using the size dependence of oscillator strength, which is in agreement with the AFM image. We also suggest that the anisotropy of exciton oscillator strength can be modified via the dipole edipole interaction between nearest exciton dipoles.
Lee, Woojin,Kim, Minwoo,Yang, Hanyi,Kyhm, Kwangseuk,Murayama, Akihiro,Kheng, Kuntheak,Mariette, Henri,Dang, Le Si Optical Society of Korea 2018 Current Optics and Photonics Vol.2 No.6
We have investigated the dimensional nature of center-of-mass exciton confinement states in a CdMnTe/CdTe/CdMnTe heterostructure, where the CdTe well is too wide (144 nm) to confine both electrons and holes but able to confine whole excitons in the center-of-mass coordinate. Fine multiple photoluminescence spectra with a few meV separation were observed at 6 K. From the thickness dependence of the transition rate, they were attributed to even numbered center-of-mass exciton confinement states (N = 2, 4, 6, ${\cdots}$, 18). Dimensionality of the center-of-mass exciton confinement states was also investigated in terms of temperature dependence of radiative decay time. At low temperatures (${\leq}12K$), we found that the ground state excitons are likely localized possibly due to the barrier interface fluctuation, resulting in a constant decay time (~350 ps). With increased temperature (${\geq}12K$), localized excitons are thermally released, giving rise to a linear temperature dependence of radiative decay time as an evidence of two-dimensional nature.
Kyhm, Kwangseuk,Kim, Jihoon,Yang, Ho-Soon,Je, Koo-Chul,Murayama, Akihiro American Scientific Publishers 2012 Journal of Nanoscience and Nanotechnology Vol.12 No.3
<P>The ultrafast spin dynamics of the bright exciton in CdSe/ZnS nanocrystal quantum dots has been investigated using a circularly polarized pump-probe experiment. A remarkably fast spin flip (-500 fs) of the bright exciton was observed at 4 K, which is attributed to the anisotropic electron-hole exchange interaction and the random positioning of nanocrystal quantum dots. In the presence of an applied magnetic field (5 T), the exciton spin parallel to the external magnetic field was favored due to Zeeman splitting. We found that this imbalance can possibly be suppressed by the state-blocking and the mixing of the 1(L) and 1(U) states in asymmetric quantum dots.</P>