音 賢一

We have investigated the current distribution at quantum Hall effect (QHE) plateau to study mechanisms of the QHE breakdown whose critical current shows sub-linear dependence on the channel width. A Hall-bar device made from GaAs/AlGaAs on a scanning stage is locally illuminated by an infrared laser light to map the QHE breakdown critical current with local photo-excitation. The spatial profile of the critical current reveals the current concentration at the center region of the sample. © 2007 American Institute of Physics.

Magneto-quantum transport properties in AlxGa 1-xAsySb1-y/InAs deep quantum wells (QWs) with different well width (Lw = 15 and 50 nm) have been studied both in low and high magnetic fields. The low-field magnetoconductance has been explained in the light of the two-dimensional weak- localization theory taking account of the effect of Zeeman splitting on spin-orbit scattering of electrons. Analysis of Quantum Hall effect (QHE) in high-field region for Lw = 50 nm indicates the existence of a small number of holes only partially participating with the QHE, suggesting that our QW is a type-II hetero-structure. © 2007 WILEY-VCH Verlag GmbH &amp Co. KGaA.

We measured the magneto-resistance in a two-dimensional electron system in a GaAs/AlGaAs heterostructure under the THz light irradiation. We observed microwave-induced resistance oscillations under a microwave irradiation in GaAs/AlGaAs samples, on which we made three types of Schottky gates. Under a far-infrared irradiation, we observed Shubnikov-de Haas oscillations, and only the oscillation near cyclotron resonance was enhanced. © 2007 American Institute of Physics.

Spatially indirect optical transitions have been studied in ZnSe/BeTe/ZnSe quantum well (QW) structures with a type-H band alignment by means of electric field dependent photoluminescence (PL) measurements. In asymmetric QW structures, which have two kinds of ZnSe layers, two spatially indirect PL peaks were observed, reflecting the difference of confinement energies for electrons. Applying the electric field perpendicular to the QW plane their transition energies are easily modified due to the Stark effect. At a crossover region of these peaks, one strong and sharp peak abruptly appeared, This result indicates the formation of the exciton complexes, in which electrons and holes are correlated over more than two interfaces.

A high resolution spin-flip Raman experiment was carried out in Cd(0.93)ZR(0.07)Te/Cd0.48Zn0.04Mn0.48Te quantum wells at 0.4 K in a magnetic field up to 14 T. The spin-flip Raman scatterings strongly depended on the well width. Resonant excitation of the exciton state in a 4 nm quantum well presented two electron-like spin-flip Raman peaks in addition to multiple Mn2+ spin-flip Raman scatterings. The spin-flip energies are well described by the modified Brillouin functions and are assigned to the electron spin-flip in the localized excitons and the spin-flip of residual electrons, respectively, although the origin of the large energy difference between the two spin-flip processes is a puzzle. On the other hand, in a 9 nm quantum well, we observed a new type of spin-flip excitation instead of the conventional electron spin-flip excitation. The spin-flip energy seemed to be zero up to 4T and then linearly increased with a magnetic field. Furthermore, the spin-flip energy increased with temperature in the temperature range; 0.4 similar to 20 K. These unusual behaviors resemble "softening mode" of spin resonance observed in p-doped ferromagnetic CdMnTe quantum wells.

We have measured the current distribution in GaAs/AlGaAs two-dimensional electron system (2DES) at quantum Hall effect (QHE) regime to investigate the mechanism of QHE breakdown. A Hall-bar device on the scanning stage is locally illuminated by an infrared laser light guided by polarization maintaining optical fiber at QHE condition. We have tried to map the QHE breakdown critical current with local photo-excitation. The mapping of breakdown critical current shows the current concentration at the center region of 2DES channel in the sample where the critical current shows sub-linear dependence on the channel width. We have also tried to measure the Hall potential imaging in the electric current flowing 2DES by means of Pockels effect in GaAs/AlGaAs sample. The experimental techniques of the optical fiber based current distribution imaging in the high magnetic field up to 9 T have been reported in detail. © 2006 IOP Publishing Ltd.

A high-resolution resonant Raman spectroscopy system has been developed by using optical fibers and applied to the spin-flip Raman scattering experiment in CdZnTe/CdZnMnTe quantum wells at He-3 temperature in a high magnetic field up to 14 T. Excitation light from an external cavity laser diode or Ti:sapphire laser was introduced into the liquid He-3 chamber by using polarization maintain fiber, and was incident on the sample at the angle of 45 degree through a GRIN collimation lens and a prism to deflect the reflected light from the luminescence/scattering detection pass. Luminescence and scattered light were collected efficiently into the bundled multi-mode fibers through a high NA objective lens and introduced into the slit of a 1.26 m single grating spectrometer equipped with a multi-channel CCD detector and a photon counter. The fiber system provides a convenient way to investigate the spin dynamics in an ideal environments; ultra-low temperature and radiation-free condition. Furthermore, the system is more sensitive than the conventional system utilizing a cryostat with the optical windows where multiple Fresnel loss and the small numerical aperture degrade the sensitivity.

We have studied the optical properties of the modulation n-doped ZnSe/BeTe/ZnSe type-II quantum wells. The reflection spectra have shown typical negatively charged exciton features only in a doped sample. The electron density as well as the mass was determined in high magnetic field (up to 160 T) cyclotron-resonance measurements. Photoluminescence (PL) measurements were performed in pulsed high magnetic fields (up to 42 T) with a Faraday configuration. We found oscillations in both the PL intensity and the peak energy in the doped sample. These oscillations were originated from optical de Haas oscillations. It was found that the observed indirect PL transition occurs via the charged excitons in a type-II quantum configuration.

We have studied the properties of quantum Hall effect (QHE) breakdown where the critical current increases sub-linearly with the channel width (sub-linear type) by using Hall-bar shaped and Corbino geometry samples made from GaAs/AlGaAs heterostructures. The channel width dependence of critical current in Hall-bar and the critical voltage in Corbino geometry shows sub-linear feature. The breakdown current measured in the Hall-bar with the side gate along the sample boundaries is scarcely affected by the side gate voltage. The experimental results support that the sub-linear type QHE breakdown is not "edge effect" but the bulk properties of two-dimensional electron system (2DES). The activation energy of bulk 2DES conductivity in the sub-linear type sample at QHE plateau is highly current dependent. The possible origin of sub-linear type QHE breakdown the localized current distribution in the bulk 2DES and current (electric field) effect on the activation energy in 2DES with small potential fluctuation, is discussed. © 2005 American Institute of Physics.

The linearly polarized PL spectra of the asymmetric superlattice ZnSe / BeTe have been investigated at various exciting light intensities. The ZnSe layers have two kinds of well widths. It has been shown that the typical PL approximately consists of two peak components reflecting in the deference of the recombination energies by size effect, and the polarization degrees of these peak components invert each other under strong excitation. This behavior is probably an important clue that V-shape electron-hole-electron complexes are formed with multi-interlayer correlation at high-density regime.

We have performed magneto-photoluminescence measurements on modulation doped n-type Cd1-xMnxTe/Cd1-yMgyTe single quantum wells. In low magnetic fields (the electrons filling factor v > 1), we observed several photoluminescence lines associated with spin-singlet and spin-triplet charged excitons. In high magnetic fields (V < 1), we observed only one of the spin-triplet charged excitons that are called as a bright type due to characteristic spin states of electrons and holes in dilute magnetic semiconductor quantum wells, where the photoluminescence intensity increased at a fractional value of V(= 2/3). We consider that the screening effect plays a crucial role for this behavior in high magnetic fields.

The cyclotron resonance of two-dimensional electron systems (2DES) of GaAs/AlGaAs heterostructures is investigated in a tilted magnetic field. In this study, the perpendicular (B⊥) and in-plane (B||) magnetic fields are changed systematically. More than two cyclotron resonances caused by B|| are observed. Further, the enhancement of cyclotron mass by B|| and its dependence on B⊥ and electron concentration of 2DES are also investigated in detail. These phenomena are well explained by the resonant subband Landau level coupling model. © 2003 Elsevier B.V. All rights reserved.

We investigate the cyclotron resonance of corrugated lateral superlattices with ultrashort period (12 nm) grown on (7 7 5)B GaAs substrates. In addition to the broad dips of cyclotron resonance compared with those of the unmodulated samples, the magnetic field dependence of observed cyclotron resonance spectra shows the anticross behavior or more complicated behavior. Several possible mechanisms on this behavior are discussed. (C) 2003 Elsevier B.V. All rights reserved.

Resistance measurements of a quantum ring were carried out under irradiation of microwave. The irradiation of microwave affects Aharonov-Bohm oscillation. The broad peak was also observed in the condition the radius of the quantum ring agrees with the classical cyclotron radius of electrons. This can be interpreted as a photon absorption by the electrons in a quantum ring. © 2003 Elsevier B.V. All rights reserved.

The cyclotron resonance of laterally modulated two-dimensional electron systems grown on (775)B-oriented GaAs substrates is measured. The lateral period of this structure is ultrashort, 12 nm. Observed cyclotron resonance spectra show the anticrossing behavior or more complicated behavior. It is found that the behavior cannot be explained by the Landau bands formation and the resonant subband-Landau level coupling that is induced by the potential modulation. A possible explanation for the behavior is the effect of electron-electron interactions introduced by breakdown of translational invariance. (C) 2003 Elsevier Ltd. All rights reserved.

The in-plane magnetic field dependence of cyclotron resonance was measured both in single and double layer two-dimensional electron system (2DES). The influence of the electron density and perpendicular magnetic field on the increment of cyclotron mass by in-plane magnetic field were also studied. It was observed that in-plane dependence of cyclotraon mass in single-layer 2DES changes with carrier density and perpendicular magnetic field in the low electron density.

We investigate different types of the breakdown of the quantum Hall effect (QHE), which are characterized by the sample width dependence of the critical current for the breakdown. The distinction of two different types of breakdown is thought to be the properties of the two-dimensional electron gas (2DEG) wafers, but it is not related directly to the 2DEG mobility and electron density determined by Hall measurement at low magnetic field. We observe the switching between linear and sublinear dependence of the QHE breakdown by varying temperature, gate voltage, and by illumination condition. The type of QHE breakdown is determined by the extent of potential fluctuations in the 2DEG, which are characterized by the random potential from ionized donors and by screening effects, comparing with those resulting from the thermal broadening of the electron distribution. The result of magnetocapacitance between 2DEG and Schottky gate also suggests that the 2DEG homogeneity play an important role in the breakdown of the QHE. © 2002 Elsevier Science B.V. All rights reserved.

The in-plane magnetic field (B∥) dependence of cyclotron resonance is investigated in both single and double layer two-dimensional electron systems by Fourier spectrometry in tilted magnetic fields. The observed B∥ dependence of the cyclotron mass (mC) is complicated and varies with perpendicular magnetic field in the double layer system. The splitting of mC by B∥ around filling factor 2 is observed in the single layer system. © 2001 Elsevier Science B.V. All rights reserved.

We have investigated carrier relaxation dynamics in single CdSe/ZnSe quantum dot (QB) by time-resolved micro-photoluminescence (PL). The discrete sharp lines, originated from individual QD states, exhibit various rise and decay time constants. The decay times of the sharp lines from ground states and excited states are estimated to be 700 similar to 800 psec and 400 similar to 500 psec, respectively, and the rise times of the ground states become longer compared with those of the excited states. These results are in contrast to successive change of the rise and decay times observed in time-resolved macro-PL with varying the detection wavelength. The quasi-continuum higher states with much shorter decay times are clearly observed over the discrete states of the QDs.

The in-plane magnetic field (B∥) dependence of cyclotron mass (mC) in two-dimensional electron system has been measured. In addition to the increment of mC with B∥, the split of mC by B∥ is observed near the filling factor of Landau level ν = 2. Some possible origins on this splitting are discussed. © 2001 Elsevier Science B.V.

We have investigated the width of the compressible strips in the edge state by magnetocapacitance between two-dimensional electron gas and a metal gate. The detail of Landau level filling factor dependence of the width is measured around the quantum Hall plateaux, and the influence of confinement potential on the width has been studied by tuning the side gate voltage. It is confirmed that the width is mainly governed by the depletion region at the sample boundary the spatially gentle confinement potential forms wider compressible and incompressible strips. The nonlocal resistance has been also studied in the samples with tunable confinement potential by side gate, which supports the result of the capacitance measurement. © 2001 Elsevier Science B.V.

The photodegradation of CdSe quantum dots (QDs) is investigaitd by micro-photoluminescence (PL) measurements. With increasing exposure time of laser, the PL intensity of the QDs gradually decreases up to approximate to 25 s and remains constant after approximate to 25 s. In contrast, the PL intensity of ZnCdSe quantum well (QWL) keeps decreasing. In micro-FL spectra, some sharp lines corresponding to single QD states disappear after the photodegradation, which would be due to defects such as dislocations induced in the QDs. All these results suggest that the dislocations induced by photodegradation in one dot do not propagate to other dots. This is very promising for CdSe QD lasers.

We investigate the breakdown of the quantum Hall effect (QHE) and observe a remarkable change in dependence of the critical current for the breakdown in sample width. Both linear and sub-linear dependences of the critical current on sample width are obtained for the same sample, depending on temperature, way of illumination, and gate bias voltage. The experimental results of the breakdown of the QHE are explained by comparing electron density fluctuations, which are characterized by the random potential from the ionized donors and by screening effects, with the thermal broadening of the electron energy distribution. It is suggested that the electron density fluctuations play an important role in the breakdown of the QHE. © 2002 Elsevier Science Ltd. All rights reserved.

The in-plane magnetic field (B-parallel to) dependence of cyclotron effective mass (m(c)) in a double-layer two dimensional electron system has been investigated by far-infrared cyclotron resonance spectroscopy. At a relatively small perpendicular magnetic field (B-perpendicular to = 5.0 T), with increasing B-parallel to, m(c) of the symmetric subband increases, while that of the antisymmetric subband decrease and vanishes above B-parallel to = 4.0 T. These behaviors can be qualitatively explained by the semi-classical calculation. For B-perpendicular to > 5.0 T, we have observed the complicated B-parallel to dependence of rn that contains more than two kinds of m(c)'s.

The magnetic electron focusing effect (MEFE) and Weiss oscillations (WO) on two dimensional electron gas (2DGE) in the magnetic fields parallel to the 2DEG plane were studied using ballistic electron transport. The results confirmed that the cyclotron trajectory was distorted from isotropic circle to assymetric loop in high magnetic fields. The distortion modulus from the MEFE experiment shows that the super-linear dependence on parallel magnetic field and the extent of the distortion was in agreement with the theoretical calculations. The distortion modulus estimated from WO was observed to be much smaller than that from the MEFE. © 2001 Elsevier Science B.V.

We have measured the capacitance between adjacent edge channels (CM) in the GaAs/AlGaAs two-dimensional electron gas in the quantum Hall plateau to investigate the structure of edge channels. The value of CM is mainly determined by the ratio of edge channel width (compressible region) to the separation between them (incompressible region) and the length of the edge channels. However, the energy relaxation between the edge channels reduces the effective length of the adjacent edge channels. Only at ν = 3 quantum Hall plateau, it is able to measure CM to investigate the width of incompressible region, since the energy relaxation between the ν = 2 and 3 channel is small enough.

Far-infrared magneto-absorption of double-layer two-dimensional electron system has been measured by Fourier transform spectrometer in tilted magnetic fields. We have measured the dependence of cyclotron mass (m(C)) on the parallel (B-parallel to) and perpendicular (B-perpendicular to) magnetic fields systematically. The obtained B-parallel to dependence of m(C) is complicated and varies with B-perpendicular to, which cannot be explained by serai-classical theory. (C) 2000 Elsevier Science B.V. All rights reserved.

In a vertically coupled dot, it is expected that generalized Kohn theorem is invalid. Then electron-electron interaction can be observed in far-infrared absorption. This interaction depends on the coupling between dots. We fabricated a vertically aligned double-dot array using a double heterostructure to investigate the coupling. The strength of the coupling depends on the confine potential of the dot. In far-infrared magneto-optical absorption measurements under pulsed photoexcitation, we observed peak shift of asymmetry states and increasing of absorption intensity of symmetry states of the dot. This is a result of the confined potential modulation by light. This indicates the possibility of optical control of the coupling.

Variation in the temperature dependence of the resistance and magnetic-field dependence of negative magnetoresistance (MR) with increasing gate voltage has been studied on an n-channel poly-Si MOS inversion layer. We find a crossover from strongly-localized (hopping) to weakly-localized (diffusive) regime in two dimensions around the sheet resistance of ≈a6 kΩ with increasing gate voltage. In both regimes, the negative MR due to orbital motion is observed and explained by the quantum interference effects.

Cyclotron resonance measurement of the double-layer two-dimensional electron system in tilted magnetic fields is made with far-infrared Fourier transform spectroscopy. Parallel magnetic field (B∥) dependence of the cyclotron mass is obtained by setting B∥ and the perpendicular magnetic field (B⊥) independently. We find a complicated behaviour of the cyclotron mass in tilted magnetic fields which is not explained by semiclassical theory.

Far-infrared magnetooptical absorption measurement of a quantum dotarray is carried out. In contrast to the starting two-dimensionalelectron gas, the shift of the resonance peak of a dot array structurecan be clearly observed under pulsed photoexcitation.From the delay time variation, change of the peak shift is quiteslow. This peak shift does not depend simply on the change in thecarrier concentration by photoexcitation. The photoexcited carrierchanges the charge states around the dot. The peak shift can beinterpreted by the potential modulation due to photoexcitation.

We have measured the capacitance between adjacent edge channels (CM) in the GaAs/AlGaAs two dimensional electron gas under the quantum Hall plateau condition to investigate the structure of edge channels. The value of CM is determined by the energy relaxation length of adjacent edge channels and the ratio of edge channel width to the separation between them. The separation lengths between the adjacent edge channels at v=2 and 4 have been estimated from the information of both CM and the total width of edge channels.

The cyclotron resonance of the double-layer two-dimensional electron system in tilted magnetic fields is investigated with the far-infrared Fourier spectrometer. We measure the parallel magnetic field (B∥) dependence of cyclotron mass due to the distortion of the 'Fermi loop' by B∥.

We have evaluated the width of edge channels in the fractional and integer quantum-Hall (QH) regime by the magnetocapacitance measurement between a gate and two-dimensional electron system in GaAs/AlGaAs heterostructures. The frequency dependence of the magnetocapacitance between 100 Hz and 1 MHz has been measured at temperatures between 35 mK and 0.4 K. At the fractional QH regime, the total width of the edge channels is found to be 2.5 and 9 mu m at 35 mK at the QH plateaux of filling factor v = 1/3 and 2/3, respectively. In the integer QH regime, the temperature dependence of the edge channel width is almost flat in the range of 35 mK-0.4 K. At the QH plateau of v = 1 due to spin-splitting Landau level, the width of total edge channel is almost the same value with that at v = 2. The width at v = 3 is also comparable to that at v = 4, although the residual bulk conductivity of v = 3 is an order of magnitude larger than that of v = 4. (C) 1998 Elsevier Science B.V. All rights reserved.

The magnetocapacitance between a two-dimensional electron system (2DES) and the gate electrode on it has been measured in the fractional quantum Hall effect regime. By measuring the edge-length dependence of the magnetocapacitance, we show that the conducting region (the edge state) is formed along the sample boundary. The width of edge state (We) and the bulk conductivity of the 2DES are estimated from the frequency dependence of the magnetocapacitance. The estimated We is much wider than that expected from the one-electron approximation picture in the integer quantum Hall regime. © 1998 American Physical Society.

Negative magnetoresistance (MR) due to orbital motion in the two-dimensional variable-range hopping (2D-VRH) has been studied near the crossover to diffusive regime on n-channel poly-Si thin film transistors (TFTs) with varying gate voltage. Dependences of the observed negative MR on temperature and magnetic field agree, in the particular regime of Efros-Shklovskii VRH, with the theoretical predictions based on the quantum interference in VRH as ∼ T-3/2 B2 in low magnetic fields and as ∼ T-3/4 B in moderate fields, even when the localization length ξ is larger than the hopping length Rh (Rh ≳ 0.44ξ).

The low-temperature transport in high-mobility poly-Si TFT fabricated using laser annealing of amorphous Si Film has been studied in the two-dimensional variable-range hopping (VRH) regime near the crossover between the weak and strong localization. The resistance follows the Efros-Shklovskii (ES) VRH as In R α T-1/2 below ∼ 20 K in the presence of a Coulomb gap in the density of states. The negative magnetoresistance ΔR/R varies below ∼ 4 K as ∼T-3/2B2 in low-magnetic fields and as ∼T-3/4B in moderate fields both in agreement with the theoretical predictions based on quantum interference.

The results of low-temperature transport properties on high-mobility poly-Si thin film transistors (TFTs) have been examined by measurement of the resistance as a function of temperature, magnetoresistance (MR) and Hall effect. It has been shown that the transition between the two-dimensional (2D) weak and strong localization (WL and SL) occurs around the sheet resistance R = πℏ/e2 (≃ 13 kΩ), where (1/2)kFl ∼ 1, in the band-tail states, caused by potential fluctuations in the inversion layer of TFTs. In the barely SL regime, R at low temperatures is found to obey the Efros-Shklovskii variable-range hopping (VRH) law in the presence of the Coulomb gap in the density of states. In both regimes of WL and SL, a negative MR has been observed and interpreted by the quantum interference effect in each regime.

We have investigated the bulk conductivity (σxx) of two-dimensional electron gas in GaAs/AlGaAs heterostructure in the quantum Hall regime by the magnetocapacitance measurement with Corbino geometry sample. The σxx at the Hall plateau down to 10-11 S is measured, and the temperature dependence of σxx is discussed. The experimental characteristic of the capacitance measurement is also discussed.

In order to investigate the influence of light illumination on the superconductivity proximity effect between superconductor and photoconducting semiconductor, we fabricate mesoscopic devices from a pair of Pb, In, Au multilayer superconducting films on indium-doped Pb1- xSn xTe (x=0.25) photoconducting semiconductor single crystal. We measure the superconducting characteristic of these devices under illumination. In the devices with the gap length of 0.2-0.3 µ m between a pair of the superconductor films, the hysteresis loop in the current-voltage curve is observed, which is a typical characteristic of the Josephson junction. Furthermore the critical current (the point at which resistance increases rapidly) increases with increasing illumination intensity.

We investigate the relation between commensurability oscillations and current how direction in lateral two-dimensional anisotropic antidot lattices. The anisotropy is introduced as one-directional fluctuation or expansion of the lattice spacing. The fluctuation of the antidot location suppresses the peaks of commensurability oscillations. The suppression is dominated by the fluctuation perpendicular to the current how direction. In one-directional expanded lattices, the peaks appear at the magnetic fields which correspond to the cyclotron motions with the diameter of the periods perpendicular to the current. These results suggest that the commensurability oscillations are determined only by the period along the direction perpendicular to the current.

The frequency dependence of the capacitance between two-dimensional electron systems (2DESs) and the gate has been studied at the quantum Hall (QH) plateaus. The capacitance minima at the QH plateaus are highly dependent on the area of edge channels at high frequencies. The residual contribution of bulk channel to the capacitance has been estimated, and the widths of edge channels have been evaluated. The diagonal conductivity σxx of less than 10-11 S has been obtained accurately by capacitance measurements at the QH plateaus. The relationship between σxx and capacitance has been discussed.

To explore commensurate oscillations in antidot lattice, the dependence on the antidot array number (N) perpendicular to the current flow direction is studied. With decreasing N, the peaks of the oscillations become small. Even at N = 1, the peaks do not vanish. The relationship between the oscillations and the current flow direction is investigated in rectangular antidot lattices which are rotated at five different angles (theta) with respect to the current flow direction. At theta = 0 degrees, the shorter side of the cell is perpendicular to the current flow direction. The main peak magnetic field of the oscillations is determined by the period of the shorter side of the cell. The peak height decreases with increasing theta and vanishes at theta = 90 degrees.

The magnetic electron focusing effect (MEFE) is investigated using a device with two extra probes connected by a gate-controlled byway channel. The focusing peak height is inversely proportional to the byway channel resistance. The results show that the focusing peaks are caused by the current through the byway. It is found that, at most, about 9% of the total current flows in the byway when the focusing effect occurs. Moreover, a countercurrent flows in the byway when the focusing effect does not occur.

Angular distribution of emitted electrons is investigated by using the magnetic electron focusing effect in a device with probes obliquely or vertically connected to a two dimensional electron gas, and by using low field magnetoresistance of the other device with two pairs of an emitter wire and a collector point contact. It is found that the distribution strongly depends on the structure just at the outlet of the wire rather than the wire direction.