Masakatsu Hirasawa

Nonmetal-metal-nonmetal transition and large negative magnetoresistance in (Ga, Mn)As/GaAs

Abstract We have studied magnetic and transport properties of a series of Ga 1−x Mn x As GaAs samples with different Mn concentrations (x = 0.015−0.071. For Mn content higher than about 0.02, carrier(hole)-induced ferromagnetism is observed. Samples with x = 0.035 and 0.043 behave as ferromagnetic dirty metals. With further increase of Mn content above x ∼ 0.05, the zero-field resistivity turns a semiconducting temperature dependence. Very large negative magnetoresistance is observed in non-metallic samples near the metal-nonmetal transitions both in the low and the high Mn content regimes.

Low-temperature conduction and giant negative magnetoresistance in III–V-based diluted magnetic semiconductor: (Ga, Mn)As/GaAs

Abstract We have studied the transport and magnetic properties of Ga1−xMnxAs/GaAs with different Mn content (0.015 0.02. With incresing Mn concentration, the system undergoes an insulator–metal–insulator transition. Low temperature hopping conduction on the insulator side of the metal–insulator boundaries exhibits giant negative magnetoresistance.

Giant Negative Magnetoresistance of (Ga,Mn)As/GaAs in the Vicinity of a Metal–Insulator Transition

We have studied the transport and magnetic properties of Ga1—xMnxAs/GaAs with different Mn content (0.015 < x < 0.071). In these magnetic semiconductors, the exchange interaction between the carriers and the localized magnetic moments leads to unique transport properties. In this paper, we discuss an insulator–metal–insulator transition and a giant negative magnetoresistance observed in the vicinity of the metal–insulator transitions.

Coulomb blockade in arrays of quantum dots

We report current–voltage (I–V) characteristics of two-dimensional arrays of quantum dots fabricated from a GaAs/AlGaAs heterostructure. We designed the arrays for the self-capacitance of the dots to be comparatively low, in order to observe long-range interaction between the dots while keeping the tunability of the coupling by the gate voltage. The observed I–V characteristics showed no sign of charge injection threshold voltage scaled by the size of the array, which is theoretically predicted for disordered arrays without long-range Coulomb interaction. Instead, charge soliton injection in a much lower voltage region is observed indicating long-range interaction.

Angular dependent magnetoresistance oscillation in GaAs/AlGaAs superlattice

Abstract Vertical transport in GaAs/AlGaAs semiconductor superlattice has been studied as a function of magnetic field angle. A series of resistance peaks corresponding to the angular dependent magnetoresistance oscillation (ADMRO) effect were investigated. The present samples have higher carrier densities and wider well widths than previously studied. This allowed us to observe higher order ADMRO peaks. Another resistance peak is found at θ=90°(B‖layer). The origin of this peak structure is discussed in terms of occurrence of closed orbits in some part of a weakly corrugated cylindrical Fermi surface for this field orientation.

Amplitude spectra of molecular vibration modes in phthalocyanine: comparison with Raman excitation profile

We performed for the first time a comparative study of vibrational-amplitude profile with the Raman-excitation profile, and focused the difference between them, which provides important information about the phase related to the Raman process.

Real-time spectroscopy of charge-transfer excitation in phthalocyanine tin dichloride

Charge-transfer (CT)-excited state in phthalocyanine tin dichloride is observed by real-time vibrational spectroscopy with a 6-fs pulse laser. The main absorption (Q-band) has no obvious CT character in the primary stage of the photocarrier generation.