I. Souma

Mn 3d partial density-of-states and p-d hybridization in Cd1-xMnxY (Y = S, Se and Te)

Abstract Mn 3 d states in Cd 1- x Mn x Y ( Y = S, Se and Te have been investigated by using synchrotron radiation photoemission. The Mn 3 d partial density-of-states spreads over the whole valence bands as a result of strong p-d hybridization with a prominent peak at 3.3–3.4 eV below the valence band maximum. Degree of hybridization between the majority-spin Mn 3 d and anion p states increases on going from Te to S.

Dynamics of excitonic magnetic polarons in nanostructure diluted magnetic semiconductors

Abstract Exciton dynamics has been studied in quantum wells, quantum dots and quantum wires of diluted magnetic semiconductors by time-resolved photoluminescence spectroscopy. The transient exciton photoluminescence with resonant excitation in Cd 1− x Mn x Te/ZnTe ( x =0.1) multiple quantum wells shows the formation of magnetic polarons in the quantum well excitons. Free and localized states of the excitonic magnetic polarons are distinguished from the difference of the lifetimes and the binding energies. The magnetic polaron formation accompanied with emission or absorption of LO phonons was also observed. Quantum dots of Cd 1− x Mn x Se grown by the self-organization mode showed the luminescence of confined excitons in the dots. The Mn ions in the quantum dots induce large Zeeman shifts and the excitonic magnetic polaron formation in the dots. Quantum wires of Cd 1− x Mn x Se showed polarized exciton emissions parallel to the wires. The polarization was decreased by the external magnetic field perpendicular to the wires. The excitonic magnetic polaron states in the quantum wires are also discussed.

Efficient spin injection into self-assembled quantum dots via LO-phonon-assisted resonant electron tunneling

Spin injection from a diluted magnetic semiconductor quantum well (DMS-QW) into self-assembled quantum dots (QDs) of CdSe is demonstrated via LO-phonon-assisted resonant electron tunneling. The experimental evidence for the spin injection is clearly shown by time-resolved circularly polarized exciton photoluminescence (PL) with the polarization degree up to 40% in QDs. In addition, a type II transition with the lifetime of 3.5ns between electrons in the QDs and heavy holes in the DMS-QW is observed. These PL energies directly indicate that the electron tunneling is resonantly assisted by LO-phonon scattering, which realizes an efficient spin-injection process into QDs.

Magnetoluminescence in quantum dots and quantum wires of II–VI diluted magnetic semiconductors

We report optical properties of quantum dots and quantum wires of diluted magnetic semiconductors. The quantum dots of Cd1−xMnxSe (x=0.03) show the exciton luminescence at around 2.4 eV, which indicates a strong confinement effect of the exciton energy corresponding to the dot size of 4–6 nm. The Zeeman shift of the exciton luminescence was observed with an effective g value of 91, showing a significant exchange interaction of the excitons with the Mn ions in the dots. The exciton luminescence from the quantum wires of Cd1−xMnxSe (x=0.08) shifts by 5.2 meV to the higher energy side with decreasing the wire width from 126 to 26 nm. The high energy shift in the narrow wires indicates the influence of the one-dimensional quantum confinement effect for the exciton states. The effective g value of the exciton in these quantum wires is 100–150. The exciton luminescence from the wires is linearly polarized (up to 80%) parallel to the wire direction at zero field, which indicates one-dimensional properties of the...

Exciton Recombination Processes in Cd1—xMnxSe/ZnSe Quantum Dots under Magnetic Fields

Radiative and non-radiative recombination processes of excitons were studied in quantum dots (QDs) of Cd 1-x Mn x Se, The recombination processes of excitons in the QDs were highly affected by external magnetic fields and the optical excitation power. The short lifetime of QD excitons at zero magnetic field (15 ps) was affected by the Auger recombination of the excitons by transferring the exciton energy to Mn 2+ d electrons in the QDs. The Auger process was suppressed by applying magnetic field in the low power optical excitation of 0.5 W/cm 2 . In higher power excitation of 70 W/cm 2 , the Auger process was not decreased in magnetic field due to the heating of the Mn 2+ ion spins.

Tunneling of spin polarized excitons in double quantum wells of Cd1−xMnxTe and CdTe

Abstract Spin-polarized tunneling of excitons has been studied by the transient exciton luminescence in magnetic field in the double quantum well system. The double quantum well was composed of the wells of diluted magnetic semiconductor and nonmagnetic semiconductor. The spin-polarized excitons tunnel from the magnetic quantum well to the nonmagnetic quantum wells through the barrier, and their spins are conserved. The tunneling time of the exciton spins was determined from the circular polarization degree of the photoluminescence of the quantum well excitons.

Submicron scale hybrid structures of diluted magnetic semiconductor quantum wells with ferromagnetic Co wires

Submicron scale hybrid structures of diluted magnetic semiconductor (DMS) quantum wells (QWs) with ferromagnetic Co wires have been developed for the purpose of applying microscopic magnetic fields to the DMS quantum structures. The Zn1−x−yCdxMnySe QW was made into wires with the width down to 0.1 μm and each wire was sandwiched between the Co wires. The magneto-optical properties of the hybrid structures were studied by spin–flip light scattering of paramagnetic Mn ions in the DMS QW in addition to ferromagnetic surface spin-wave scattering of the Co wires, where the spin–flip energy is highly sensitive to the magnetic field HCo flux applied from the Co wires. The application of uniform magnetic fields HCo flux⩾0.25 T from the Co wires to the DMS QW has been attained with the Co-wire spacing of 0.2 μm. In addition, the distribution of HCo flux in the DMS QW is quantitatively evaluated from the spectral shape of the spin–flip scattering. Moreover, the application of HCo flux to the DMS QW is switched on a...

Spin-flip rate of excitonic magnetic polarons in Cd1−xMnxTe/Cd1−yMgyTe quantum wells

The lifetime of excitons in Cd1−xMnxTe/Cd1−yMgyTe single quantum wells was investigated by time-resolved photoluminescence spectroscopy. Spin-flips between dark and bright excitons remarkably change the excitonic lifetime yielding a two-exponential decay in the photoluminescence intensity. The temperature dependence of the two decay times shows that the spin-flip rate depends strongly on the formation of excitonic magnetic polarons. By forming polarons, the spin-flip rate from dark excitons to bright excitons is reduced effectively by the spin- barriers of the polarized Mn ions, which extends significantly the luminescence decay time.

Dynamics of photoexcited carriers in molecular beam epitaxy grown semimagnetic epilayers in magnetic fields

Time-resolved photoluminescence has been measured on molecular beam epitaxy grown Cd1−xMnxTe epilayer samples. The photoluminescence intensity of magnetic polarons exhibits a very long emission tail. The observed phenomenon is attributed to the conversion of the dark excitons to dipole allowed transitions by a spin-flip process. The magnetic field dependence of the two-exponential decay property is discussed within the frame of giant Zeeman splitting, the spin-flip scattering, and the formation of the magnetic polarons.

Excitonic spin states and radiative recombinations in Cd1−xMnxTe/Cd1−yMgyTe spin superlattices

Abstract The formation of a spin superlattice structure was realized in Cd0.95Mn0.05Te/Cd0.95Mg0.05Te under a magnetic field. The experimental evidence of the spin superlattice structure was found from time-resolved polarized magneto-photoluminescence measurements, in the Zeeman splittings and the exciton magnetic polaron formation. The Zeeman splitting directly indicates the spatial separation of spin-up and spin-down electrons and heavy holes, which are localized in the Cd0.95Mg0.05Te and Cd0.95Mn0.05Te layers, respectively. The formation of exciton magnetic polarons under a magnetic field can be attributed to the spin-dependent confinement where a large polaron binding energy of 3.5 meV has been observed in 2.5 T to be enhanced by the field-induced localization of the excitons.