Josephson current through a multilevel quantum dot with strong Coulomb repulsion is studied by means of a Hartree-Fock approximation. We pay attention to the offdiagonal Cooper pair tunneling process in which an up-spin electron and a down-spin electron are transferred to the opposite bank through different single-particle levels in the dot. It is shown that, if the offdiagonal tunneling process is relevant, the system can behave as a π junction even when the dot is nonmagnetic without a localized spin. It is also found that π-junction behavior is enhanced with increasing the number of the dot levels. On the other hand, if the offdiagonal tunneling process is completely negligible, the multilevel structure suppresses π-junction behavior. These results indicate that we should take the multilevel effects into account in analyzing the Josephson current through a correlated quantum dot.Progress in semiconductor technology has made it possible to fabricate an SNS (S: superconductor, N: normal conductor) junction with a highly controllable N segment made by two-dimensional electron gas. 1) In these systems, which enable us to study mesoscopic effects on the Josephson current, electron-electron interactions in the N segment are thought to play only a minor role. However, if the N segment becomes small enough to be considered a quantum dot, we need to take account of strong Coulomb interaction. In some situations, the correlated quantum dot can be taken for an Anderson impurity. 2)The influence of magnetic impurities on the Josephson current has been considered for many years. Shiba and Soda 3) calculated the Josephson current through an insulating barrier containing paramagnetic impurities and showed that the spin-flip tunneling processes contribute negatively to the Josephson current compared with the nonmagnetic tunneling processes. Bulaevskii et al. 4) pointed out that, if the spin-flip processes predominate the nonmagnetic processes, the Josephson coupling energy is minimized at ϕ = π, where ϕ is the phase difference of the two superconductors, and the corresponding critical current becomes negative. They named this anomalous junction the π junction. In practical situations, however, since the direct tunneling processes not passing through the impurities predominate the indirect spin-flip processes, it appears difficult to expect a π junction in this system.From the viewpoint of realizing a π junction, a superconductor-quantum-dot-superconductor system is a promising candidate. In this system, electron transport is governed by only indirect processes. Previously, the Josephson current through a correlated quantum dot has been studied only in the limit where the dot has a spin-degenerate single level, 5-7) although an available quantum dot has a multilevel structure. In this letter, we study the Josephson current through a multilevel quantum dot with strong Coulomb repulsion by means of a Hartree-Fock approximation (HFA). We shall elucidate the significant role of the offdiagonal Cooper pair tunneling process in wh...

A new pixel circuit for "clamped inverter driving" was developed. Since a PMOS inverter is applied instead of the conventional CMOS for the pixel circuit, the circuit is simplified and is composed of only three TFTs. A simulation of gray-scale characteristics verified that this pixel circuit can compensate variation in V th and carrier mobility. A 2.5-inch OLED display based on the new pixel circuit was fabricated by LTPS processing and achieves 6-bit gray-scale and good uniformity. IntroductionSeveral designs of pixel circuits for organic light-emitting-diode (OLED) displays have been reported [1][2][3]. Regarding the driving scheme for OLED displays, it is known that the voltageprogramming method has advantages of high-speed writing and compatibility with existing driver ICs. However, this scheme has one problem that must be solved; namely, V th and carrier mobility of the TFTs in the pixel circuit must be compensated effectively. At SID'02 and '03, we proposed a "clamped inverter driving" method, which uses sweep-signal comparison to control the duration of light emission [4,5]. This method is a kind of voltage programming. It compensates V th and carrier mobility of a TFT effectively and achieves not only luminous characteristics of 6-bit gray-scale and good uniformity but also gamma correction and peak luminance. The pixel circuit for this driving method, however, requires more than four TFTs. Accordingly, in the present work, we designed a simplified pixel circuit with only three TFTs. Such a pixel circuit composed of fewer TFTs has advantages of high aperture ratio and high yield ratio. We applied it to a 2.5-inch OLED display and evaluated the display characteristics of the OLED panel. Figure 1 shows the configuration of the pixel circuit for the previously developed "clamped inverter driving" scheme, which controls the brightness of the OLED by means of changing the length of the "ON" and "OFF" states of the inverter. TFT T1 and TFT T2 compose the CMOS inverter, and TFT T3 and TFT T4 -Fig. 1 Configuration of previous pixel circuit Fig. 2 Timing chart for OLED driving V data =High Writing mode Illumination mode V data =Low V data =High V data =Low OLED ON OFF ON OFF Vdata Bright Dark Data-line voltage Sweep signal Data signal OLED T4 T3 T1 T2 Illumination line Reset line Data line C1 CMOS inverter Clamped inverter driving method

Innovative pixel driving for active matrix OLED displays is described. A full-color display with a luminous deviation of less than 1.6%, which means a 6-bit or 64-level gray-scale accuracy, is shown. The driving is preferable for full-color video with gamma correction.

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