音 賢一

Monolayer materials are strongly affected by their potential fluctuations, which are induced by an intrinsic corrugation or the surface roughness of the substrate. We compare the effective exciton-exciton annihilation (EEA) rate constants of monolayer WS2 on substrates with different surface topographies. We show that monolayer WS2 on the substrate with atomically flat terraces displays small effective EEA rate constant deviating from the overall tendency and multiple exciton decay components, which cannot be accounted for by a conventional EEA model. To obtain a correct description, we use a quantized EEA model. The intrinsic EEA rate constant for the flat-terrace substrates determined by this new model is comparable to that of hBN-encapsulated monolayer WS2.

We investigate the electron-phonon coupling in CH3NH3PbX3 lead halide perovskites through the observation of Landau levels and high-order excitons at weak magnetic fields, where the cyclotron energy is significantly smaller than the longitudinal optical phonon energy. The reduced masses of the carriers and the exciton binding energies obtained from these data are clearly influenced by polaron formation. We analyze the field-dependent polaronic and excitonic properties, and show that they can be quantitatively reproduced by the Frohlich large polaron model.

Carrier mobility is one of the most fundamental material parameters of semiconductors and requisite for device applications and interpretation of physical phenomena. We determined the electron and hole mobilities of a CH3NH3PbBr3 single crystal in the high-carrier density regime by combining AC Hall measurements under photoexcitation and two-carrier analysis. Both electron and hole mobilities were significantly enhanced by photo-doping and exceeded 300 cm(2) V-1 s(-1), which are comparable to the electron Hall mobilities of conventional inorganic semiconductors. Our experimental results indicate that charged dislocation scattering dominates the carrier transport at room temperature in the low-carrier density regime.

We exploit an intense terahertz magnetic near field combined with femtosecond laser excitation to break the symmetry of photoinduced spin reorientation paths in ErFeO3. We succeed in aligning macroscopic magnetization reaching up to 80% of total magnetization in the sample to selectable orientations by adjusting the time delay between terahertz and optical pump pulses. The spin dynamics are well reproduced by equations of motion, including time-dependent magnetic potential. We show that the direction of the generated magnetization is determined by the transient direction of spin tilting and the magnetic field at the moment of photoexcitation.

We studied the optical responses of organic-inorganic halide perovskite CH3NH3PbBr3 single crystals doped with heterovalent Bi3+ ions (electron densities up to 2.3 X 10(12) cm(-3)). The Bi doping causes no significant changes in the band gap energy but leads to an enhanced Urbach tail and photoluminescence blue shift. On the basis of the time-resolved photoluminescence measurements, we attribute the PL response to a shorter carrier lifetime induced by Bi doping, which results in a reduced photon recycling effect (i.e., the repeated emission and reabsorption of photons inside the crystal). We discuss the physical relation between Bi concentration and the optical and electric properties of Bi-doped CH3NH3PbBr3 and reveal the unique nature of the Bi3+ ion as a dopant in halide perovskites.

A flake of monolayer graphene was sandwiched between boron nitride sheets. Temperature dependent Shubnikov-de Haas measurements were performed to access how this technique influences the electronic properties of the graphene sample. The maximum mobility found in this configuration was approximately 105 cm(2) Vs(-1). From the phase of the oscillations a Berry phase beta of 1/2 was obtained indicating the presence of Dirac fermions. We obtained Fermi velocities around 0.8x10(6) m s(-1) which imply hopping energies close to 2.5 eV. Furthermore, the carrier lifetime is typically higher than that found in graphene supported by SiO2, reaching values higher than 700 fs.

Applications of high-field terahertz pulses are attractive in physics and terahertz technology. In this study, two applications related to high-intensity terahertz pulses are demonstrated. The field enhancement effect by subwavelength metallic microstructures is utilized for terahertz excitation measurement. The spin precession dynamics in magnetic materials was induced by a terahertz magnetic field. Spin precession was amplified by one order of magnitude in amplitude by the enhanced magnetic terahertz field in orthoferrite ErFeO3 with metal microstructures. The induced spin dynamics was analyzed and explained by LLG-LCR model. Moreover, a detection method for terahertz pulses was developed using a cholesteric liquid crystal at room temperature without any electronic devices. The beam profile of terahertz pulses was visualized and compared to other methods such as the knife edge method using pyroelectric detector and micro-bolometer array. The liquid crystal terahertz imager is very simple and has good applicability as a portable terahertz-sensing card.

We use a scanning gate microscopy to perturb coherent transport in chemical vapor deposition (CVD) graphene wide constriction. Particularly, we observe conductance oscillations in the wide constriction region (W similar to 800 nm) characterized by spatial conductance variations, which imply formation of the nanometer-scale ring structure due to the merged domains and intrinsic grain boundaries. Moreover, additional hot charges from high current can suppress the coherent transport, suggesting that the hot carriers with a wide spreading kinetic energy could easily tunnel merged domains and intrinsic grain boundaries in CVD-grown graphene due to the heating effect, a great advantage for applications in graphene-based interference-type nano-electronics. (C) 2016 AIP Publishing LLC.

We demonstrate enhancement of the spin precession of orthoferrite ErFeO3 using the magnetic near-field produced by a split-ring resonator (SRR), using the terahertz pump-optical Faraday probe measurement. The precession amplitude was enhanced by ∼8 times when the resonance frequency of spin precession was close to the magnetic resonance of SRR. The time evolution of spin precession was successfully reproduced by a coupled spin- and SRR-resonance model mediated by the magnetic near-field. It is suggested that optimization of the metamaterial structure would further increase the enhancement factor, leading to the nonlinear control of spin dynamics using terahertz radiation.

Terahertz metamaterial technique enables a sophisticated electromagnetic excitation of materials. Here, we demonstrate resonant excitation of spin precession motion of canted antiferromagnet ErFeO3 using the magnetic near-field produced by split ring resonator (SRR). When the resonance frequency of spin precession was near the magnetic resonance mode of SRR, we observed substantial increase of precession amplitude. The time evolution of spin motion can be successfully reproduced by a model that takes into account an interactive coupling between spin- and SRR-resonance mediated by magnetic near-field. Results indicate that bidirectional coupling between spin and SRR is essential in understanding the system.

We have developed a frequency-domain terahertz spectrometer based on homebuilt 1 mu m band external cavity diode lasers, for high resolution spectroscopy. Our spectrometer is digitally controlled to a resolution of 10 MHz, and uses InGaAs/GaAs photoconductive antennas. We have obtained a spectrum in the range 0.02 THz to 2.5 THz, which exceeds the conventional temperature tuning range of a distributed feedback diode laser. We achieved a signal-to-noise ratio of up to 80 dB at around 0.05 THz, and 20 dB at around 2.0 THz. We observed water vapor spectra in the atmosphere with a frequency step of 0.6 GHz in the region between 1.0 THz and 2.0 THz. We have demonstrated that our 1 mu m-band frequency-domain terahertz spectrometer is competitive when compared with existing 800 nm- and 1.5 mu m-band systems. (C) 2013 AIP Publishing LLC.

We have investigated the magneto-transport around filling factor nu=0 in monolayer graphene sheet. Zero Hall plateau of Hall conductivity and minimum of longitudinal conductivity have been clearly observed despite the highly fluctuated behaviour of magneto-resistance above 10 T. These phenomena can be understood by the transport of counter-propagating electron and hole edge states at the charge neutrality point. We tried to explain the origin of remarkable fluctuations in magneto-resistance by the energy relaxations among the counter-propagating edge channels.

Spin polarization measurement is important for the study of a variety of spin states in quantum Hall system. Kerr rotation is proportional to spin polarization, so we developed a high sensitive measurement of Kerr rotation by using homodyne detection and a variety of modulation techniques. Furthermore, we developed Kerr rotation spectra measurement system base on the multi-channel homodyne detection, which enables the assignment of the optical transitions and semi-quantitative estimation of spin polarization by integrating the spectra over the specific optical transition. The spin polarization presents a rapid spin depolarization on both sides of v = 1 due to Skyrmionic excitation. However the top of spin polarization presents a narrow flat region. Furthermore the spin polarization around v = 3 also shows a rapid spin depolarization which suggest the existence of Skyrmion at higher odd filling factor.

We developed a novel system for a high sensitive measurement of Kerr rotation spectra, and Kerr rotation spectra of quantum Hall system in AlGaAs/GaAs 17nm quantum well were measured. Spin polarization of quantum Hall states was obtained by integrating the spectra, since Kerr rotation spectra reflect spin population of electrons. The spin polarization decreased rapidly on both sides of nu = 1, which is ascribed to Skyrmion effect. However, the spin polarization present a flat region around nu = 1 which means a quantum Hall ferromagnet is not fully spin polarized.

Spin relaxation of two-dimensional electrons in a GaAs/AlGaAs quantum well was studied by time-resolved Kerr rotation measurements using a two-color pump and probe technique. In quantum Hall ferromagnets, the spin-wave relaxation is strongly influenced by the photogenerated Skyrmion and anti-Skyrmion pairs. By tuning the pump and probe lights to the lowest optical transition, an intrinsic filling factor dependence of spin relaxation is obtained without photogeneration of Skyrmions. The relaxation time of the spin wave presents a sharp peak at odd filling factors, accompanied by dips on both sides of it. The peculiar filling factor dependence of the spin-wave relaxation around quantum Hall ferromagnets can be explained by the interaction between the spin wave and Skyrmion. Observation of a similar feature around v = 1, 3, and 5 may suggest the existence of Skyrmions around higher odd filling factors.

We have measured the magneto-capacitance of front-gated capacitor made of Kish graphite to investigate the electronic properties of surface graphene sheets on bulk graphite. The low-field Shubnikov-de Haas oscillations in magneto-capacitance are almost similar with ones of the resistance measurement, which are related to Dirac fermion-like carriers around the H-point such as ones at the K-point in monolayer graphene. At high magnetic fields, the magneto-capacitance oscillations, which may be affected by the surface state on the graphite flake, show slightly different patterns from those in the magneto-resistance. (C) 2009 Elsevier B.V. All rights reserved.

Low cost and attractive equipment for use in experimental higher physics education has been desired by teachers, especially those in developing world. A system of novel experimental apparatus named Personal Desk Lab (PDL) was developed [1]. Each apparatus is miniaturized to one-fifth of the conventional one or less, and built on a steel plate; therefore, it is portable and can be used on a classroom desk. Each set is constructed by some parts divided according to their functions, and some of the parts are used in a number of experiments; which saves material, cost, and storage space. All parts are designed to be easy to make, maintain and repair. Almost all apparatus are battery driven [2]. After ICPE 2006, we have improved the system continuously; consequently, the experimental themes cover the field of mechanics, electromagnetism and optics. The number of these themes in use exceeds ten. The performance of PDL has been tested at Chiba University, Japan and Royal University of Phnom Penh (RUPP), Cambodia. In Chiba University, physics education with PDL is currently conducted individually to 80(max.) students in a classroom at the same time, and to more than 900 students per year. Experiment class using PDL in Cambodia started on October 2008 with 120 students of physics department, RUPP. They were divided into three classes, and conducted four experimental themes in pairs. The advantages confirmed from the practices at two universities are as follows: (1) the use of PDL arouses learner's interest, promotes their deep understanding extensively, and inspires to learn further; and (2) costs for introduction and running of PDL system are fairly small compared to the traditional one. Furthermore, the instruction for distant learners having PDL on each hand was conducted successfully through internet.

The high-resolution spin-flip Raman-scattering experiments were carried out on high-quality CdTe quantum wells below 0.5 K in high magnetic fields up to 14 T. An obvious exciton spin-flip Raman-scattering signal was observed in each quantum well under the Voigt configuration. The exciton exchange splitting in each quantum well was precisely determined. © 2009 The American Physical Society.

The high-resolution spin-flip Raman-scattering experiments were carried out on high-quality CdTe quantum wells below 0.5 K in high magnetic fields up to 14 T. An obvious exciton spin-flip Raman-scattering signal was observed in each quantum well under the Voigt configuration. The exciton exchange splitting in each quantum well was precisely determined.

A spatially direct photoluminescence (PL) spectrum associated with type-I interband transition of a ZnSe layer in undoped ZnSe/BeTe type-II quantum structures was investigated by varying the photo-excitation density. For a sample with a narrower ZnSe layer both PL intensity and linewidth of the trion show a superlinear increase with increasing excitation density in comparison to that with a wider ZnSe layer. The results are explained by the effective increase of the electron concentration and an enhanced dephasing rate of the trion resulting from the electron-trion scattering. The effective increase of the electron concentration in the ZnSe layer is considered to be originating from both the greater transfer rate of the hole into the BeTe layer due to the narrower ZnSe layer and the efficient spatially indirect transition PL of the complex states through the interface. With decreasing excitation density, no indication of any shift in peak energy density was observed indicating that the studied structures are of excellent quality.

We have investigated the magneto-transport properties in InAs/AlGaSbAs quantum well (QW), where the lattice mismatch is less than 0.5%. In tilted magnetic fields, the spin-resolved subband-Landau-level coupling has been clearly observed in the magneto-resistance due to the large g-factor of this QW. An anomaly in the Hall effect has been observed at high magnetic fields of the filling factor being less than unity, which is caused by the coexistence of electrons and holes in the system.

The dynamics between electron and nuclear spins in quantum Hall regime is investigated by a time-resolved Kerr rotation (TRKR) spectroscopy carried down in tilted-field geometry. The spin-flip energy varies almost linearly with the applied magnetic field under the nuclear spin depolarizing radio frequency field and the helicity modulation of circularly polarized pump, while the spin-flip energy obtained under a circularly polarized pump deviates from linearly behavior. This deviation is ascribed to the Overhauser shift due to dynamic nuclear spin polarization (DNP). The DNP depends markedly on the filling factor and present peaks at even filling factors. These results can be well explained by a simple model based on the detailed balance between electron and nuclear spins.

The electron spin polarization images in a single layer GaAs/AlGaAs quantum well in the quantum Hall effect (QHE) regime have been observed by using an optical fiber-based high sensitive Kerr rotation microscope. The observed Kerr signal image at the Landau level filling factor nu = 1 reveals the small electron density fluctuation less than 1% through the degree of electron spin polarization. We have also observed the Kerr rotation images in the current flowing QHE devices. At nu = 1 QHE plateau, the winding strip of just nu = 1 incompressible state, which contributes the dissipationless electron transport, has been clearly observed in the bulk region of the devices. The highly current dependent Kerr images have valuable information for studying current density distribution and the distribution of spin polarization in the QHE regime. The experimental results and techniques of the Kerr imaging in the QHE regime have been reported in detail.

InAs0.1Sb0.9 active layers sandwiched between Al0.1In0.9Sb insulating buffer layers were grown on GaAs (1 0 0) substrates by molecular beam epitaxy. The basic transport properties at room temperature and quantum Hall effects at low temperature of the InAs0.1Sb0.9 were studied as a function of InAs0.1Sb0.9 thickness. The electron mobility of the InAs0.1Sb0.9 active layers had a very high value and very small thickness dependence at less than 500 nm. The quantum Hall effects of the InAs0.1Sb0.9 were observed at thicknesses 15, 20, 30, 50, 70, and 100 nm. The observation of the quantum Hall effect at thickness more than 50 nm strongly suggests the existence of two-dimensional electron gas in the InAs0.1Sb0.9 layer sandwiched between Al0.1In0.9Sb layers. (C) 2008 Elsevier Ltd. All rights reserved.

The spin coherence of photoexcited electrons in non-doped ZnSe/BeTe type-II quantum wells was investigated by a time-resolved Kerr rotation (TRKR) technique using resonant excitation of either excitons or negatively charged excitons (trions) in magnetic fields at 1.4 K. The TRKR signal arising from the Lamor precession of photo-excited electron spins in ZnSe layers was observed and enhanced at the trion resonance, where the spin relaxation time reached up to several nanoseconds. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

We have investigated transport properties in InAs/AlGaSbAs and InAsSb/AlInSb quantum wells (QW) in the quantum Hall regime. The carrier density in InAs QW is tuned by using a front gate bias, and the contour plot of resistance as a function of perpendicular magnetic field and gate voltage reveals the spin-resolved subband-Landau-level coupling in tilted magnetic fields. An anomalous transport occurred by the coexistence of electrons and holes in the system has been observed both in InAs and InAsSb QWs.

We have measured the current distribution in GaAs/AlGaAs two-dimensional electron system at quantum Hall effect regime, where the critical current shows sub-linear dependence on the channel width. An optical-fiber-based potential imaging system by Pockels effect is improved by using a twin-optical bridge detector which reduces retardation noise in the optical fiber. We have also mapped the resistance change by the local photo-excitation in the QHE regime. Both the observed images show the evidence of current concentration at center region of the channel, and the extent of current concentration increases with increasing the current.

A Time-resolved Kerr rotation spectroscopy under selective excitation by narrow spectrum pump beam is applied to high-mobility two dimensional (2D) electrons. The large non-oscillating Kerr signal is ascribed to the formation of Skyrmions by photoexcited carrier, and is confirmed by the selective excitation in TRKR measurement. The collapse of the spin coherence (dip of T-2*) near v = 1 may be related by the formation of Skyrmions and anti-Skyrmiosn pair under photoexcitation.

We report results of the reflection (type-I, in ZnSe layer) and photoluminescence (PL) spectra (type-II) measurements performed on n-doped ZnSe/BeTe/ZnSe type-II quantum well structures at low temperature (5-10 K). The reflection spectra show a typical charged exciton (X-) feature. The PL spectra show an asymmetry of the peak, which is a characteristic feature for a charged exciton type transition. The dependence of the PL peak energy on the in-plane magnetic field is considered to be due to the magnetic-field-induced displacement of interlayer negatively charged exciton dispersion in momentum space.

Time-resolved Kerr rotation spectroscopy under the radio frequency field to depolarize dynamic nuclear polarization reveals the intrinsic spin-relaxation time (T(2)(*)) and g factor of two-dimensional electrons in a quantum Hall system. Out-of-plane magnetic field increases the spin coherence drastically through the Landau level quantization. T(2)(*) is enhanced strongly around odd filling factors where a quantum Hall ferromagnet is formed. Collapse of spin coherence and appearance of an anomalous Kerr signal observed around nu=1 are discussed in the relation to the formation of Skyrmions.

The spin coherence of photoexcited electrons in ZnSe/BeTe type-II quantum wells has been investigated by the time-resolved Kerr rotation technique. Fast and efficient escape of photoexcited holes from the ZnSe layers to the BeTe layers suppresses the electron-hole recombination and their exchange interaction. This effect leads to the formation of dense electrons in ZnSe layers and long electron spin dephasing time reaching a value of 6.1 ns at 1.4 K. (C) 2008 American Institute of Physics.

Photoluminescence (PL) spectra occurred as a spatially direct optical transition inside of the ZnSe layer in undoped ZnSe/BeTe/ZnSe type-II quantum structures have been studied. We have found that the charged exciton transition was observed at the lower energy side of the exciton transition in the spatially direct PL. The formation of the charged exciton was attributed to the accumulated electrons in the ZnSe layer after the photoexcitation accompanied by the holes being escaped from this well and injected into the BeTe layer. (c) 2008 American Institute of Physics.

Low-temperature magneto-transport properties of AlxGa1-xAsySb1-y/InAs deep quantum wells (QWs) with different well widths (Lw=15-500 nm) and their dimensionality were studied. The Hall data were analyzed by means of the two-carrier model: coexistence of electrons and holes for Lw=15-150 nm and existence of two types of electrons with different mobility for Lw=300 and 500 nm. The profile of perpendicular magnetoresistance (MR), ρxx(B⊥) for the two-dimensional electrons (2DEs) in narrow QWs (Lw=15 and 50 nm) can be divided into the following four regimes: (1) the negative MR due to the weak localization (WL) in extremely low B-fields, (2) the crossing of ρxx for different temperatures at Bc=1/μe in moderate B-fields due to the orbital effect on the electron-electron interaction, (3) the Shubnikov-de Haas oscillations and (4) the quantum Hall effect in high-B fields. As for the in-plane MR except in the very low-field region characterized by the WL, Δρxx(B∥)/ρ0 always starts from a negative one essentially independent of T, which was explained by the classical boundary scattering at the wall of the barrier in quasi-ballistic regime. The quantum Hall resistance of the electron-hole system has also been discussed. © 2007 Elsevier B.V. All rights reserved.

Both negative and positive persistent photoconductivity (NPPC and PPPC) effects on the low-temperature transport properties were studied in the AlxGa1-xAsySb1-y/InAs quantum wells with a well width of Lw=15 and 50 nm. The irradiation of light with λ&lt 1.0 μm causes the NPPC, while the light with λ=1.6 μm causes the PPPC. The Hall coefficient RH(B) in the dark or after illumination considerably increases with increasing B, indicating the coexistence of majority electrons and minority holes and allowing to analyze it by means of the two-carrier model. The PPPC effect leads to an increase in both density (n and p) and mobility of electrons and holes, while the NPPC inversely leads to their decrease, which was confirmed by the PPC effects on the SdH oscillations and the Hall resistance in the regime of quantum Hall effect in high magnetic fields. We have found the experimental relation, pμn3, so that the minority holes increases (decreases) much more rapidly than the majority electrons increases (decreases) in the PPPC (NPPC). © 2007 Elsevier B.V. All rights reserved.

Spin-flip excitations in non-doped Cd(0.93)ZN(0.07)Te/Cd0.48Zn0.04Mn0.48Te quantum wells have been comprehensively studied by spin-flip Raman seattering (SFRS) spectroscopy and time-resolved Keff rotation (TRKR) spectroscopy. In 4 nm quantum well, two spin-flip Raman peaks were observed in addition to the multiple Mn2+ spin-flip scatterings. The spin-flip energies are isotropic against the magnetic field direction and well described by modified Brillouin functions. Based on the circumstantial analysis, they are assigned to the spin-flip of residual electrons and the electron spin-flip in the localized exciton, respectively, even though the large energy difference between the two electron spin-flip processes is a puzzle. While, in 9 rim quantum well a strange spin-flip excitation was observed together with a very weak Mn2+ spin-flip scattering. The spin-flip energy changed strangely up to the magnetic field 4T, and then linearly increased with field (lg*l=1.15). A high-resolution TRKR spectroscopy revealed an unusual temperature dependence, which resembled "softening mode" of spin resonance observed in p-doped ferromagnetic CdMnTe quantum wells. However, these behaviors are well understood by an "inverted spin configuration", which results from a negative g*-factor and a very weak s-d interaction between the electrons and the manganese ions in the barrier.

We have developed an optical-fiber-based Hall potential imaging system using the Pockels effect in GaAs/AlGaAs two dimensional electron systems (2DES) in the quantum Hall regime to investigate current distributions. The mapping of the Hall potential shows the current concentration at the bulk region of 2DES samples, where the critical current depends on the channel width sub-linearly. We report in detail the experimental techniques of the imaging system operating in high magnetic fields.

Weak localization has been studied in an AlxGa 1-xAsySb1-y/InAs quantum well (QW) by means of tuning the carrier density n via the persistent photoconductivity effect in order to explore the origin of spin-orbit interaction (SOI) in this system. The magnetoconductance in extremely weak 5-fields has been fitted by taking account of the spin-Zeeman effect on the SOI arising from the spin splitting in asymmetric systems. The SO field (Bso) deduced has been found to be independent of n, thereby confirming the dominant role of the Rashba field as the cause of the SOI. © 2007 American Institute of Physics.