Search

Alarge superconducting machine, JT-60SA has been constructed to provide major contributions to the ITER program and DEMO design. For the success of the ITER project and fusion reactor, understanding and development of plasma controllability in ITER and DEMO relevant higher beta regimes are essential. JT-60SA has focused the program on the plasma controllability for scenario development and risk mitigation in ITER as well as on investigating DEMO relevant regimes. This paper summarizes the high research priorities and strategy for the JT-60SA project. Recent works on simulation studies to prepare the plasma physics and control experiments are presented, such as plasma breakdown and equilibrium controls, hybrid and steady-state scenario development, and risk mitigation techniques. Contributions of JT-60SA to ITER and DEMOhave been clarified through those studies. Peer Reviewed Article escrit per 127 autors/autores: M Yoshida, G Giruzzi, N Aiba, J F Artaud, J Ayllon-Guerola, L Balbinot, OBeeke, E Belonohy, P Bettini, W Bin, A Bierwage, T Bolzonella, M Bonotto, CBoulbe, J Buermans, M Chernyshova, S Coda, R Coelho, S Davis, C Day, GDeTommasi, M Dibon, A Ejiri, G Falchetto, A Fassina, B Faugeras, L Figini, M Fukumoto, S Futatani, K Galazka, J Garcia, M Garcia-Muñoz, L Garzotti, L Giacomelli, L Giudicotti, S Hall, N Hayashi, C Hoa, M Honda, K Hoshino, M Iafrati, A Iantchenko, S Ide, S Iio, R Imazawa, S Inoue, A Isayama, E Joffrin, K Kamiya, Y Ko, M Kobayashi, T Kobayashi, G Kocsis, A Kovacsik, T Kurki-Suonio, B Lacroix, P Lang, Ph Lauber, A Louzguiti, E de la Luna, G Marchiori, M Mattei, A Matsuyama, S Mazzi, A Mele, F Michel, Y Miyata, J Morales, P Moreau, A Moro, T Nakano, M Nakata, E Narita, R Neu, S Nicollet, M Nocente, S Nowak, F P Orsitto, V Ostuni, Y Ohtani, N Oyama, R Pasqualotto, B Pégourié, E Perelli, L Pigatto, C Piccinni, A Pironti, P Platania, B Ploeckl, D Ricci, P Roussel, G Rubino, R Sano, K Särkimäki, K Shinohara, S Soare, C Sozzi, S Sumida, T Suzuki, Y Suzuki, T Szabolics, T Szepesi, Y Takase, M Takech, N Tamura, K Tanaka, H Tanaka, M Tardocchi, A Terakado, H Tojo, T Tokuzawa, A Torre, N Tsujii, H Tsutsui, Y Ueda, H Urano, M Valisa, M Vallar, J Vega, F Villone, T Wakatsuki, T Wauters, M Wischmeier, S Yamoto, L Zani

We investigated how to control the nano-pinhole structure of a confined-current-path current-perpendicular-to-plane giant magnetoresistive (CCP-CPP GMR) sensor by inducing pulse current. We found an improved MR ratio with reduced sensor resistance by applying an optimal pulse current. This was assumed to be due to the enlarged nano-pinhole area and reduced pinhole resistance. When excessive pulse current was applied, the MR ratio and sensor resistance decreased because the CPP-GMR sensor degraded thermally.

Recently, high-temperature superconductor (HTS) cables have been widely studied because of their compactness and high power capacity compared to conventional copper cables. In HTS cables, AC loss is an important issue since large losses reduce the efficiency of the power line. Among HTS cables, tri-axial cable is under intensive investigation recently, since it has a smaller amount of HTS tapes, small leakage fields and small heat loss in leak when compared with the three single-phase cables. For realizing high current capacity, more than one layer is required for each phase; therefore AC loss of the multi-layer tri-axial HTS cable should be carefully examined. In the tri-axial cable, different phase currents produce the out-of-phase magnetic fields on the other phase layers. In case of multi-layer arrangement, net magnetic fields on layer surfaces may exceed the penetration field of the HTS tape. Therefore in this paper, we analyze the AC loss of a tri-axial HTS cable which is composed of two layers per phase. Here, we treat the tri-axial cable which consists of two different longitudinal segments and thus satisfies balanced phase and homogeneous current distribution condition by controlling twist pitch and length of separate segments.

The stress in a read head in a hard disk drive was investigated by using finite element method (FEM) calculations. We compared the structurally induced effect of stress in a current-in-plane (CIP) structure and in a current-perpendicular-to-plane (CPP) structure. We found that electrodes in the CIP structure produced a large amount of in-plane anisotropic stress at the pinned layer. However, anisotropic stress induced in the CPP structure by the sensor-side material such as hard bias was less than that in the CIP structure. This was because the CPP structure did not have thick electrodes on the sensor side.

We investigated the effect of the misorientation angle of an r-plane sapphire substrate on a-plane GaN grown by metalorganic vapor phase epitaxy. The misorientation angle was changed systematically in the direction of the [0001] c-axis or [1100] m-axis (denoted θ1) and in the direction of the [1120] a-axis (denoted θ2). Results show that the surface morphology and crystalline quality are very sensitive to θ1 misorientation angle. We successfully grew an a-plane GaN layer with a pit-free surface by optimizing the misorientation angle of the r-plane sapphire substrate. We found that the r-plane sapphire substrate with a θ1 misorientation angle between -0.53 and -0.28° is very effective for growing a-plane GaN with a pit-free surface. Moreover, a θ1 misoriention angle between -0.53 and -0.28° improves crystalline quality in the GaN c-axis direction, but causes a slight increase in tilt mosaicity in the GaN m-axis direction. In contrast, a θ2 misorientation angle between -0.55 and +0.52° produces no differences in the surface morphology and crystalline quality of a-plane GaN.

In JFT-2M, the applicability of reduced activation ferritic steel to a fusion demonstration reactor has been demonstrated mainly concerning the ferromagnetic effect on plasma production, control, stability and confinement. The compatibility of the close ferritic wall with high-normalized beta plasmas has been investigated mainly because the demonstration reactor will utilize the wall stabilization effect. Due to improvements in the operation scenario and the modification of the guard limiter configuration, the region showing good compatibility was extended to the closer wall position (the wall position was normalized to the plasma minor radius; rwall/a~ 1.25) and the higher normalized beta (βN ~ 3.5), compared with the previous works with the relatively far wall position and stronger magnetic field (Tsuzuki K. et al 2003 Nucl. Fusion. 43 1288). A reduction in the growth rate of the instability was also observed in the close wall case, which presumably corresponds to the wall stabilization effect, similar to a resistive wall without ferromagnetism.

The relationship between the magnetic field and the effective mass m* of the two-dimensional electron gas in the triangular quantum well at the heterointerface of modulation-doped Al0.15Ga0.85N/GaN heterostructures is investigated by means of temperature-dependent Shubnikov-de Haas measurements at low temperatures and high magnetic fields. The values of the m* have strong dependence on the magnetic field, which is thought to be attributed to conduction-band nonparabolicity of GaN. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We have carried out time-integrated and spectrally-resolved four-wave mixing (FWM) measurements to study the effects of built-in strain within GaN films on their exciton structures and on their coherent dynamics. Precise values for exciton energies, linewidths, and oscillation periods of quantum beats were presented. Remarkable phase shifts of the quantum beats were observed in the GaN films on a-sapphire, suggesting the exciton-exciton interaction caused by the reduction of the crystal symmetry.

The electrostatic and density fluctuation are measured simultaneously with a heavy ion beam probe. The electrostatic fluctuation with the geodesic acoustic mode (GAM) frequency is observed in L-mode plasmas and not in H-mode plasmas. The poloidal and radial structure is consistent with the GAM. So the fluctuation is concluded to be the GAM.The amplitude of the GAM changes in the radial direction; it is small near the separatrix, has a maximum at 3 cm inside the separatrix and decreases again to 5 cm inside the separatrix.The GAM and the temporal behaviour of the ambient density fluctuation show a significant coherence, and the phase of modulation of the ambient density fluctuation tends to delay the potential oscillation of the GAM. It is clearly verified that the GAM affects ambient fluctuation and also the local particle transport through modulation of the amplitude of the ambient fluctuation.

This paper presents results of bispectral analysis applied to floating potential fluctuations in the edge region of ohmically heated plasmas in the JAERI Fusion Torus-2 Modified (JFT-2M) tokamak. Inside the outermost surface of plasmas, coherent mode fluctuations (CMs) in floating potential were observed around the frequency of the geodesic acoustic mode. The squared bicoherence shows significant nonlinear couplings between the CMs and background fluctuations. The biphase at the frequency of the CMs is localized around π, while that at frequencies of background fluctuations distributes in a wide range. The total bicoherence at the frequency of the CMs is proportional to the squared amplitude of the CMs. These observations are consistent with the theoretical prediction on the drift wave-zonal flow systems. Interpretation of the absolute value of the total bicoherence is also discussed.

The characteristics of geodesic–acoustic-mode (GAM) are investigated through direct and simultaneous measurement of electrostatic and density fluctuations with a heavy ion beam probe.The amplitude of the GAM changes in relation to the radial position; it is small near the separatrix, reaches a local maximum at 3 cm inside the separatrix and then decreases again to 5 cm inside the separatrix. The frequency is constant in the range, though the predicted GAM frequency varies according to the temperature gradient. The correlation length is about 6 cm and comparable to the structure of the amplitude of the GAM. The results indicate the GAM has a radial structure which reflects the local condition at about 3 m inside the separatrix.The phase relation between the GAM oscillation indicates that the GAM is a radial propagating wave.The interaction between the GAM and the ambient density fluctuation is shown by the high coherence between the GAM oscillation and the temporal behaviour of the ambient density fluctuation. Moreover, the phase relation between the electric field fluctuation of the GAM ( ) and the amplitude of the density fluctuation indicates that the modulation of the ambient density fluctuation delays the . The causality between the GAM and the modulation of the density fluctuation is revealed.

We have developed a new terahertz (THz) wave detector using an Nb-based superconducting tunnel junction (STJ). The STJ consists of a Nb/Al/AlO x /Al/Nb structure on a single-crystal LiNbO 3 substrate. Using LiNbO 3 as a substrate, an irradiating THz wave is converted to phonons in LiNbO 3 , which has a relatively high absorption coefficient. The phonons propagating in the substrate reach the Nb base electrode of the STJ and break Cooper pairs. As a result, the THz wave can be detected as tunneling current signals in the STJ. THz waves from a widely tunable THz-wave parametric oscillator (TPO) were irradiated on to the STJ, which acted as a THz wave detector. The STJs were cooled to an operating temperature of 0.4 K using a 3 He cryostat. We have observed STJ signals related to THz wave irradiations. In this paper, fabrication processes and characteristics of THz wave detectors using STJs are also discussed.

We have proposed and been developing a new broad-band terahertz (THz) radiation detector using a Nb-based superconducting tunnel junction (STJ). The STJ's were fabricated on LiNbO/sub 3/ and LiTaO/sub 3/ mono-crystal substrates. We radiated monochromatic THz pulses in the range 1-2 THz with the pulse repetition rate of 49 Hz, and successfully detected the corresponding periodic signals via a charge-sensitive preamplifier. The current signal obtained by the derivative of the output signal gave the time response of about 15-20 microseconds. The frequency response was compared with that of a conventional pyro-electric sensor. We found their frequency response showed the similar shape in frequency, demonstrating the flat response of the STJ detector in 1-2 THz. These results show that the detector can be used as a broad-band THz and far-infrared radiation detector above the frequency corresponding to the energy gap of the superconductor used for the base electrode.

Quantum and transport scattering times related to different subbands in modulation-doped Al0.22Ga0.78N/GaN single quantum wells (SQW) have been studied by magnetotransport measurements. The quantum scattering time related to the first and the second subbands is determined to be 0.078 ps and 0.088 ps, respectively, at 1.5 K. Results indicate that the scatterings from heterointerface and ionized donors at both side of GaN well are all important in limiting quantum scattering time. By using mobility spectrum technique the transport scattering time related to the first subband is determined to be 0.128 ps. Transport-to-quantum scattering time ratio in the first subband is 1.6. It is concluded the large angle scatterings play a very important role in limiting quantum scattering time in this SQW.

We have successfully obtained GaN self-assembled quantum dots (QDs) with high density (>2×10 11 cm −2 ) by the Stranski–Krastanow growth mode in metalorganic chemical vapor deposition. The density of the QDs can be controlled between 10 8 and 10 11 cm −2 by changing an amount of GaN nominally deposited. The photoluminescence peaks from both the QDs and the wetting layer are clearly observed even in uncapped QD structures at room temperature.

We propose an ultra-thin ( ∼ 3 nm) InGaN-channel high electron mobility transistor (HEMT) structure with two-dimensional electron gas (2DEG) mobility of over 1000 cm 2 / V s at room temperature (RT). This structure suppresses alloy-disorder scattering in the channel because part of the 2DEG penetrates into the GaN buffer layer. All the AlGaN/InGaN/GaN heterostructures were grown on a (0 0 0 1) sapphire substrate using two-flow atmospheric metalorganic chemical vapor deposition (MOCVD). As a result, we achieved 2DEG mobility of 1110 cm 2 / V s with a sheet carrier density of 1.31 × 10 13 cm - 2 at RT, which is the highest mobility reported to date for an InGaN-channel structure. Excellent electron confinement was also observed compared to a conventional AlGaN/GaN heterostructure, despite the use of a very thin InGaN-channel. We also investigated the relationship between the density of the 2DEG and strain relaxation in InGaN-channel structures. Strain relaxation in the InGaN layer was enhanced by an increase in thickness with the result that the negative polarization charge drastically decreased at the InGaN/GaN interface. Thus, the potential at the interface dropped and the 2DEG density increased.

Results of coherent edge fluctuation measurements using three diagnostics (a reciprocating Langmuir probe, a two channel O-mode reflectometer, and fast magnetic probes) in H-mode discharges on JFT-2M are presented. In discharges in which a high recycling steady (HRS) H-mode phase is obtained through a transient phase with slightly enhanced Dα intensity, two types of coherent fluctuations are observed. The higher frequency mode (around 300 kHz) is the high frequency mode (HFM) observed in the HRS H-mode (Kamiya K et al 2003 9th IAEA Tech. Meeting H-mode Workshop Topic B-14). The lower frequency mode has a frequency of around 80 kHz. The HFM is detected by a Langmuir probe over a wide region in the SOL, as well as by the reflectometer and magnetic probes. However, the HFM is not detected by the higher frequency (38 GHz) channel of the reflectometer after the HRS transition, suggesting that the HFM is not located deeply inside the plasma. The 80 kHz mode is detected by both channels of the reflectometer and by a Langmuir probe, but not by magnetic probes, suggesting that it is an electrostatic mode. In contrast to the HFM, the 80 kHz mode is detected by the Langmuir probe only near the separatrix during the transient phase, which leads to either the HRS phase or the ELMy phase, and is similar to the fluctuations reported in Shinohara K et al (1998 J. Plasma Fusion Res. 74 607).

Spin splitting of the two-dimentional electron gas (2DEG) in modulation-doped Al0.22Ga0.78N/GaN heterostructures has been studied by means of magnetotransport measurements at low temperatures and high magnetic fields. The spin splitting is observed in Shubnikov–de Haas oscillations at a magnetic field higher than 5.4 T and a temperature of 1.4 K. The effective g factor g* is enhanced due to the exchange interaction of the 2DEG at high densities. The ratio of the transverse effective g factor g⊥* and the longitudinal effective g factor g∥* is g⊥*/g∥*=2.6 indicating the large difference between g∥* and g⊥*. It is demonstrated that the anisotropy of the g* is due to the strong polarization-induced electric field at the heterointerface.

Multiple-layer stacked GaN quantum dots (QDs) with intense photoluminescence (PL) have been grown by the Stranski-Krastanow growth mode in metalorganic chemical vapor deposition. Scanning transmission electron microscopy (STEM) analysis shows that the vertical aligned QDs are formed, which results from a strain field induced by buried islands. We have also investigated PL spectra at room temperature. The PL intensity increases with increasing number of the stacked layer. This indicates that the carriers can be injected into each layer of GaN quantum dots.

We report the observation of long-lived excitons in GaN/AlN self-assembled quantum dots at the temperature of 3.5K. The PL decay time strongly increases and reaches μs order with increasing size of the QDs. This strongly suggests that the PL decay is almost free from non-radiative processes and dominated by radiative recombination processes in the GaN QDs. The increase of the PL decay time with increasing size of the QDs originates from the reduction of the oscillator strength due to the strong built-in electric field in the GaN/AlN heterostructures.

We present the first observation of intersubband transition (ISBT) from the ground state (el) to the third subband (e3) in GaN/AlGaN quantum wells (QWs). A clear peak of the ISBT el-e3 was observed in GaN/Al 0.56 Ga 0.44 N QWs. This demonstrates that a strong built-in field due to the piezoelectric and spontaneous polarization effects exists in the structure. The polarization field results in a symmetry breaking of a QW, and thus ISBT el-e3, which in a symmetrical square well is normally forbidden by parity. The band-to-band photoluminescence emission at energies below the GaN band gap also shows the existence of a polarization field.

In continuation to the steady state detection of below-gap states by combining a below-gap excitation (BGE) light with an above-gap excitation (AGE) light, we have studied the time-response of both donor-acceptor pair luminescence (DAP) and yellow luminescence (YL) in undoped GaN after switching off the BGE light. An extremely slow recovery of YL with a time constant of up to 28 s was observed, which is interpreted as a relaxation process of electrons from a below-gap state chosen by a BGE energy of 1.17 eV. Trap parameters were quantitatively determined by studying the intensity change and subsequent recovery time of DAP as a function of AGE and BGE power densities. The BGE energy dependence of the intensity change and subsequent recovery time of DAP and YL implies an energy distribution of traps in the band-gap of GaN. The combination of time-resolved PL measurement under two-wavelength excitation scheme was shown to be powerful for understanding the carrier recombination dynamics including YL among below-gap states.

We have investigated intersubband absorption of GaN/Al 0.58 Ga 0.42 N multiple quantum well structures of high quality, which were grown by low-pressure metal organic chemical vapor deposition (MOCVD). The crystalline quality and uniformity of the AlGaN barriers were improved by suppressing gas-phase parasitic reaction between NH 3 and TMAl. This, in turn, produced a very sharp intersubband absorption spectrum with a spectral linewidth as narrow as 35 meV.

In addition to detection of nonradiative recombination centers by a below-gap excitation (BGE) technique at steady state, we have studied the time-response of both donor-acceptor pair luminescence and yellow luminescence (YL) in GaN after switching off the BGE light. An extremely slow recovery of YL with a time constant of up to 28 s was observed, which is interpreted as a relaxation process of electrons from the state chosen by a BGE energy of 1.17 eV. The dependence of recovery time on the BGE energy and on the above-gap and below-gap excitation power densities became clear for the first time; it reflects the carrier recombination dynamics including YL among below-gap states.

This paper presents the 2.4 GHz front-end and the first downconversion section of a fully integrated low-IF receiver. The dual-conversion receiver and rejects the image repeatably by 60 dB using integrated polyphase filters without calibration or tuning. The gain of the RF mixer and IF amplifier is switchable to slide the available dynamic range of the following stages, based on the conditions of the input signal. The front-end and downconversion sections drain 35 mA on average from a 3.3 V supply. Minimum cascade NF is 7.2 dB, and maximum cascade IIP3 is −3.4 dBm.

An IF strip for a wireless receiver supports variable baud rate by changing filter bandwidth. Sliding and step adaptive dynamic range are both used at IF to minimize the power consumption at the prevailing channel conditions. A combination of VGA and PGA is developed for 64-QAM. The circuit drains an average of 16 mA from 3.3 V, with peak supply current of 73 mA. The differential input noise is as low as 3.9 nV/adHz, while maximum IIP3 is +22 dBm with respect to 100 ohms.

Absence of nonradiative recombination (NRR) centers inside GaAs wells and at GaAs/Al 0.2 Ga 0.8 As hetero-interfaces in a Si modulation-doped GaAs/Al 0.2 Ga 0.8 As multiple quantum well (MQW) structure became clear for the first time by an improved two-wavelength excited photoluminescence (PL). The NRR parameters of modulation and uniform-doped MQWs were determined self-consistently by combining the analysis of the PL intensity change due to the below-gap excitation with the internal quantum efficiency and the recombination lifetime. These results showed the superiority of modulation-doping scheme over that of uniform-doping for light emitting devices.

Discrimination of non-radiative recombination centers in Si-doped GaAs/AlGaAs quantum well structures has been achieved by means of a two wavelength excited photoluminescence technique. In a crystal grown at 700°C, traps were found neither in GaAs wells nor in Al 0.2 Ga 0.8 As barriers, but inside external Al 0.4 Ga 0.6 As layers which originally were left out of the doping region. These traps were classified as a pair: one originating in a lattice native defect or a residual impurity; and the other by Si atoms (possibly a DX center) diffused from the doped region. In contrast, traps were found even inside the GaAs wells in case of samples grown at 600°C, reflecting deteriorated crystal quality.

A superconducting magnetic shield of high-Tc superconductor Bi(Pb)-Sr-Ca-Cu-Ox has been constructed whose diameter is 65 cm and length is 160 cm. We have successfully observed magnetic fields from somatosensory-evoked human brains in the superconducting magnetic shield by stimulating the median nerves of patients by current pulses. We made a 64-channel whole-head SQUID magnetometer of superconductor/normal metal/superconductor (SNS) junctions which do not show low-frequency telegraph noise. The sensitivities of the dc SQUID mesoscopic SNS junctions are around 5 fT Hz-1/2 even in rather unfavourable surroundings. The magnetic shield can reduce a magnetic field by around -80 dB or a factor of 10-4 even at as low a frequency as 0.05 Hz. Therefore SQUIDs of SNS junctions and a superconducting magnetic shield are a good combination.

A superconducting magnetic shield of high-Tc superconductor Bi(Pb)SrCaCuOx has been constructed whose diameter is 65 cm and length is 160 cm respectively. We have successfully observed somatosensory evoked magnetic fields coming out from human brains in the superconducting magnetic shield stimulating the median nerves of patients by current pulses. We made a 64-channel whole-head SQUID magnetometer of SNS (Superconductor/Normal metal/Superconductor) junctions. The sensitivities of the dc-SQUID of mesoscopic SNS junctions are around 5 fT//spl radic/(Hz) even in rather unfavorable surroundings. The magnetic shield can reduce a magnetic field to around -80 dB or 10/sup -4/ even at as low a frequency as 0.05 Hz.

We have previously demonstrated that a spectrally-resolved four-wave mixing highlights the anisotropically polarized excitons in the uniaxially strained GaN films [1], where the enhancement of sensitivity for an uniaxial strain was achieved by nonlinear response of the exciton polarization. In this work, we have applied this technique to the GaN films on various substrates including isotropic ones, and have examined the sensitivity of the technique. The results show distinct polarizations and energy variations originating from the uniaxial strain, the magnitude of which depends on the sample and its position. The minimum changes of the polarized FWM intensity and exchange energy splittings correspond to a uniaxial strain of 5.0 × 10–5, which currently gives a lower resolution limit of this technique. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The spatial distribution of below-gap states in undoped GaAs/AlGaAs quantum-well structures and their effect on the luminescence intensity has been determined by a spectroscopic measurement of two-wavelength excited photoluminescence. Tuning the photon energy of above-gap excitation revealed that these below-gap states were formed neither in the GaAs well layers nor at the GaAs/AlGaAs hetero-interface, but in the AlGaAs barrier layers. The properties of below-gap states in AlGaAs were found to be strongly dependent on growth temperature.

The magnetic properties and resistivity of electroplated FeNiP film were investigated. The resistivity of FeNiP film increased and its magnetic flux density decreased with the addition of phosphorus. The optimum Fe29Ni64P7 film showed higher resistivity (85μΩcm) than Fe55Ni45 film (45 μΩcm). The magnetic flux density, saturation magnetostriction, anisotropic field, and coercivity of Fe29Ni64P7 film were about 1.0T, 50×10-7, 15 Oe, and lower than 1.0 Oe, respectively. The frequency characteristic and thermal stability of FeNiP film were also investigated. The permeability of FeNiP film began to decrease in a higher-frequency region than that of FeNi film. The resistivity of FeNiP film decreased as a result of annealing at 200°C, where as that of FeNi film was unchanged after annealing at temperatures up to 300°C.

We observe a band-to-band photoluminescence (PL) of the well layers in GaAs/AlGaAs quantum well structures (hνP = 1.56 eV) under below-gap excitation (BGE) with a Nd:YAG laser (hνB = 1.17 eV) at 77 K. The origin of the up-conversion luminescence was inside the epitaxially grown well layers and is different from those reported in GaAs substrates. A detailed study of a two-wavelength excited PL was carried out by changing the density of both the BGE and the above-gap excitation (AGE) by a He–Ne laser (hνA = 1.96 eV) individually. The up-conversion process corresponds to the increase in the PL intensity due to the BGE in two-wavelength excited PL, which reveals the mechanism of a cascade excitation via a below-gap state in quantum wells for the first time. A rate- equation analysis explained the measured BGE density dependence of the up-conversion luminescence.

A modified jacketing method, which uses a preform produced by suction casting and a tapered jacketing tube for fiber drawing, was developed to fabricate low-loss, high-Δn fluoride single-mode fibers for optical amplifiers. A fabricated 500 m long ZrF4-based fiber had a constant core diameter and an almost constant mean strength of ~435 MPa over its whole length. Low transmission losses below 20 dB/km were achieved in rare earth-doped ZrF4-based single-mode fibers with a Δn of 3.7%. An InF3-based single-mode fiber with a Δn of 3.6%, a loss of 114 dB/km at 1.23 μm and a signal gain coefficient of 0.18 dB/mW at 1.30 μm was fabricated using this jacketing method.

We have developed a phononmediated radiation detector using Nb-based superconducting tunnel junctions (STJs). The STJs are fabricated on a substrate as an absorber of the radiation, and a substantial amount of emergent phonons with an energy above the energy gap are detected. The performance of the detector depends on the collection efficiency of the phonons. A selection of an appropriate substrate is required for optimizing the detector performance. We fabricated Nb-based STJs on a Al/sub 2/O/sub 3/ and a LiNbO/sub 3/ substrates, and compared the pulse heights of substrate phonon events due to X-ray irradiation to the substrates. We found that the maximum pulse height of the phonon events for the LiNbO/sub 3/ substrate was about twice of that for the Al/sub 2/O/sub 3/ substrate, so that these spectra which was obtained from two STJs showed different shapes, especially the peak due to phonon events.

We have fabricated Nb-based superconducting tunnel junctions (STJs) with Al trapping layers using the photosensitive polyimide insulation layer for X-ray detector. It is possible to simplify the fabrication process of the STJs, because the photosensitive polyimide is used as the insulation layer instead of conventional inorganic insulation films without an etching process in a vacuum chamber. The polyimide insulation layer expects to be stable for thermal cycling between 0.3 K and room temperature. The preliminary experiments of 5.9 keV radiation shows the energy resolution of 86 eV.

Weak anti-localization of the two-dimensional electron gas (2DEG) in a modulation-doped Al0.22Ga0.78N∕GaN single heterostructure has been investigated through magnetoresistance measurements at low temperatures. The elastic scattering time τe, dephasing time τϕ and spin-orbit scattering time τso at various temperatures are obtained. When the second subband in the triangular quantum well at the heterointerface is occupied by the 2DEG, the weak anti-localization is observed clearly, which is thought to be due to the strong spin-orbit effect induced by the intersubband scattering. The spin-orbit effect and the intersubband scattering become stronger with increasing temperature.

We have studied the photoluminescence (PL) of a lightly Se-doped GaAs/AlGaAs quantum well under both above-gap and below-gap excitation (AGE and BGE, respectively) and observed a saturation of the PL quenching with increasing the BGE power for the first time. The PL quenching saturation was attributed to the trap-filling effect of electrons in one of below-gap states. An additional condition of the trap-filling regime in the rate-equation analysis enabled us to determine recombination parameters of two interacting below-gap states quantitatively. A numerically obtained result explained the measured dependence of the PL quenching on the BGE power fairly well.

We present theoretical calculations for the temperature dependence of both the static and the dynamic structure of liquid lithium. Our approach is based on the neutral pseudoatom method to obtain the screening valence electron density around a Li+ ion as well as the effective interatomic pair potential, and on the variational modified hypernetted chain integral equation theory of liquids to obtain the liquid static structure. Then, the dynamic structure is calculated in the viscoelastic approximation. This combination results in a whole theory free of adjustable parameters. The results obtained show rather good agreement with the available simulation and experimental data.

We developed a helmet-sized superconducting magnetic shield for a high-Tc Bi-Pb-Sr-Ca-Cu-O superconductor. The shield is 32 cm in diameter, 60 cm in depth, and 2.5 cm in thickness. It was experimentally confirmed that the container can reduce a Weak magnetic field to -100 dB or 1/100000. The shielding effect of the superconducting container does not decrease even at frequencies as low as 0.2 Hz. Using equipment consisting of a superconducting container, a cryostat to keep the container at liquid nitrogen temperature, and an rf SQUID gradiometer, we successfully detected the magnetic fields of a human brain. We detected somatosensory magnetic fields by stimulating the tibial nerves of the patient with current pulses.

Magnetic shielding was shown to be improved by superimposing a magnet cylinder over a superconducting cylinder. Examples of such configurations are soft iron and electromagnetic steel cylinders over a copper-oxide superconducting cylinder. The value of the magnetic shielded field for the superimposed cylinder was found to be several times more than that of single superconducting cylinders such as YBCO or a BPSCCO cylinder. Experimental results reveal good characteristics which include the effects of the excitation magnetic field on the inner magnetic field within both the single superconducting and superimposed cylinders, the temporal change of the trapped magnetic field within the superconducting cylinder, the distribution of the magnetic shielded field along the axial direction of the superimposed cylinders, and the relationship between the length of the magnetic cylinder and the shielding effect. >

A superconducting quantum interference device (SQUID) magnetometer has successfully detected magnetic fields emitted from a human brain in a helmet-size superconducting magnetic shield of the high-T/sub c/ superconductor Bi-Sr-Ca-Cu-O/sub x /(BSCCO). The sensitivities of the RF-SQUID and the DC-SQUID of quasi-planar Josephson junctions-superconducting weak links are 20 fT/ square root Hz and 4 fT/ square root Hz, respectively. The magnetic shield can reduce a magnetic field to -120 dB or 10/sup -6/ even at as low a frequency as 0.2 Hz. The authors have detected somatosensory evoked magnetic fields stimulating the tibial nerves of the patient by current pulses. >

We have developed a very large magnetic shielding vessel made from high-Tc Bi-Pb-Sr-Ca-Cu-O superconductor and have experimentally confirmed that this vessel can reduce a weak magnetic field to -100 dB or 1/105. The size of this vessel is 32 cm in diameter, 64 cm in depth and 2.5 cm in thickness. The shielding effect of the vessel does not decrease even at a low frequency like 0.2 Hz. The high attenuation ratio in the low frequency range indicates that magnetic shielding using a high-Tc superconductor is promising for neuromagnetic measurements with SQUID.

The effects of the notch tip radius on acoustic emission (AE) characteristics have been studied for short-glass-fibre reinforced poly(ethylene terephthalate) (SGFR-PET) tensile specimens. The adopted notch tip radii (ϱ) were nominally zero (0) and 2.3 mm. The larger ϱ caused an increase in the load corresponding to the beginning of AE and an increase in the maximum load and fracture displacement. AE peak amplitudes (V p ) showed ϱ dependence of the damage formation and fracture mechanism of the composite. For ϱ = 0, V p increased drastically and then decreased with increases in load. However, for ϱ = 2.3 mm V p increased fairly monotonically. In a higher amplitude range, V p exhibited a minimum at the initiation of a main crack for both radii. Frequency analysis suggested that AE sources were not dependent on ϱ, and AE events for the higher V p corresponded to fibre breakages. Reflected and transmitted optical microscopy combined with ultrasonic C-scan revealed that fibre breakages ahead of the notch tip led to initiation of a main crack, which agreed with the results of V p above. When the main crack propagated, V p increased rapidly again for both radii, indicating additional fibre breakage. As a consequence, AE characteristics representing the damage formation mechanism in short-fibre-reinforced thermoplastic were found to be considerably influenced by the notch tip radius.