Kenichi Oto

Optically induced long-lived electron spin coherence in ZnSe∕BeTe type-II quantum wells

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 6.1ns at 1.4K.

Current-direction-dependent commensurate oscillations in GaAs/AlGaAs antidot superlattice

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 (θ) with respect to the current flow direction. At θ = 0°, 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 θ and vanishes at θ = 90°.

Impact of Chemical Doping on Optical Responses in Bismuth-Doped CH3NH3PbBr3 Single Crystals: Carrier Lifetime and Photon Recycling

We studied the optical responses of organic-inorganic halide perovskite CH3NH3PbBr3 single crystals doped with heterovalent Bi3+ ions (electron densities up to 2.3 × 1012 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.

Electric- and magnetic-field effects on radiative recombination in modulation n-doped ZnSe/BeTe type-II quantum wells

We have studied the photoluminescence properties of modulation n-doped ZnSe/BeTe/ZnSe type-II quantum wells. The luminescence spectra and the polarization anisotropy depend strikingly on both the n-doping into the barrier layers and an applied external electric field perpendicular to the layers. These are explained by screening of a built-in electric field with n-doping and Stark effects due to the applied electric field. We find optical Shubinkov–de Hass oscillations in both the photoluminescence intensity and the transition energy under a high magnetic field perpendicular to the well. These features and additional infrared cyclotron-resonance measurements demonstrate that two-dimensional electron gas of a high concentration has been formed in the doped samples. All these present the signature of the charged excitons and also of a correlated excitonic phase of type-II quantum wells.

Optical de Haas oscillations of charged excitons in type-II ZnSe/BeTe quantum wells

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.

Evaluation of Edge Channel Width by Frequency Dependence of Capacitance Minima in Quantum Hall Regime.

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 σ xx of less than 10 -11 S has been obtained accurately by capacitance measurements at the QH plateaus. The relationship between σ xx and capacitance has been discussed.

Spatially direct charged exciton photoluminescence in undoped ZnSe∕BeTe type-II quantum wells

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.

Macroscopic Magnetization Control by Symmetry Breaking of Photoinduced Spin Reorientation with Intense Terahertz Magnetic Near Field

We exploit an intense terahertz magnetic near field combined with femtosecond laser excitation to break the symmetry of photoinduced spin reorientation paths in ErFeO_{3}. 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.

Imaging coherent transport in chemical vapor deposition graphene wide constriction by scanning gate microscopy

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 ∼ 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.

Transport properties and observation of quantum Hall effects of InAs 0.1 Sb 0.9 thin layers sandwiched between Al 0.1 In 0.9 Sb layers

InAs0.1Sb0.9 active layers sandwiched between Al0.1In0.9Sb insulating buffer layers were grown on GaAs (100) 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 500nm. The quantum Hall effects of the InAs0.1Sb0.9 were observed at thicknesses 15, 20, 30, 50, 70, and 100nm. The observation of the quantum Hall effect at thickness more than 50nm strongly suggests the existence of two-dimensional electron gas in the InAs0.1Sb0.9 layer sandwiched between Al0.1In0.9Sb layers.