K. Nishibayashi

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.

Efficient spin relaxation in InGaN/GaN and InGaN/GaMnN quantum wells : An obstacle to spin detection

Transient magneto-optical spectroscopy of InGaNGaN and InGaNGaMnN quantum wells reveals a spin relaxation process with a characteristic time of 50 ps. We show that the observed spin relaxation is m ...

Spin dynamics in a diluted magnetic semiconductor quantum well studied by pump-probe absorption spectroscopy: Magnetic-field-induced suppression of electron-spin relaxation

Exciton spin dynamics is studied in a diluted magnetic semiconductor quantum well of Cd0.95Mn0.05Te by pump-probe absorption spectroscopy under magnetic fields. The time dependences of the saturated absorbance for the higher- and lower-energy spin states of heavy-hole (hh) excitons clarify the following exciton-spin relaxation process in magnetic fields: ultrafast hh-spin relaxation with the formation of dark excitons and subsequent electron-spin relaxation. The electron-spin relaxation due to the s-d exchange mechanism involving Mn spins is suppressed in a high magnetic field by field-induced pinning of the Mn spins.

Spin-injection dynamics in a magnetic double quantum well studied by pump-probe absorption spectroscopy

We study spin-injection dynamics in a magnetic double quantum well by pump-probe absorption spectroscopy. Time dependences of the absorption saturation intensity show a tunneling of spin-polarized carriers from a diluted magnetic well (MW) of Cd0.92Mn0.08Te into a nonmagnetic well (NMW) of CdTe, where the individual tunneling of the electron and heavy hole is elucidated with time constants of 150ps and 1ns. The circular polarization degree of the absorption saturation in NMW increases from 0to70ps, which is caused by the injection of electron spin polarized in MW with a time constant of 54ps.

Magnetic-field-dependent type-II transition induced by resonant electron tunneling from a diluted magnetic semiconductor quantum well into self-assembled quantum dots

Magnetic-field-dependent type-II transition has been observed between electrons in self-assembled quantum dots (QDs) of CdSe and heavy holes (hhs) in a diluted magnetic semiconductor quantum well (DMS-QW) of Zn1−x−yCdxMnySe, originating from an electron tunneling from the DMS-QW into the QDs. The electron tunneling is resonantly assisted by LO-phonon scattering in the QDs. As a result, the type-II emission energy depends on a magnetic field, since the electron energy in the DMS-QW is a function of field strength. The magnetic-field dependence of intensity of the type-II emission suggests a level-crossing field of the hh states between the QD and DMS-QW.

SPIN DYNAMICS IN III-V/II-VI: Mn HETEROVALENT QUANTUM WELLS

We report on the spectroscopic magnetooptical studies of spin dynamics in diluted magnetic semiconductor (DMS) GaAs/AlGaAs/ZnSe/ZnCdMnSe heterovalent double quantum wells (QW). The transients of circularly polarized photoluminescence in an external magnetic field are detected in the structures with different widths of the GaAs QW. The analysis of the data, performed within the rate-equation model, has allowed separate estimations of the spin relaxation rate of localized electrons and holes. The spin flip of the electrons confined in the DMS ZnCdMnSe QW is faster than 20 ps, whereas the spin flip of the heavy hole localized in the GaAs QW is as long as ~9 ns. The long spin flip of the holes is presumably governed by their strong 3-dimensional localization.

Recombination and Spin Dynamics of Excitons in III‐V/II‐VI:Mn Heterovalent Double Quantum Wells

Time‐resolved magneto‐photoluminescence spectroscopy is applied to elucidate dynamics of excitons in GaAs/AlGaAs/ZnSe/ZnCdMnSe heterovalent double quantum wells (QW). An essential slowdown of the recombination rate was observed, resulting from exciton localization due to the compositional disordering at the AlGaAs/ZnSe heterovalent interface. It was found that both exciton lifetime and spin lifetime can be controlled by varying the detuning of the single‐QW electron levels.

Ultrafast Spin Relaxation and Dynamics of Carriers in Diluted Magnetic Semiconductor Quantum Wells

Spin dynamics of carriers in multiple quantum wells of Cd0.95Mn0.05Te has been studied by the transient pump‐probe absorption spectroscopy in magnetic field. The spin‐flip relaxation time of electrons in the quantum wells increases from 7 ps to 18 ps with increasing magnetic field up to 2 T due to the suppression of the exchange mechanism for the spin‐flip relaxation of electrons interacting with Mn ions. In fields higher than 3 T, the electron spin‐flip relaxation time turns to decrease caused by the LO phonon‐related relaxation. The spin‐flip time of holes is less than 200 fs due to the band‐mixing effect in the valence bands.

Spin Dynamics of Type‐II Excitons in Diluted Magnetic Double Quantum Wells

Dynamical properties of the electron and hole spins are demonstrated in double quantum wells composed of diluted magnetic and nonmagnetic semiconductors. Excitonic photoluminescence of the type‐II transition appears for the lower‐lying down‐spin level of the exciton in magnetic field. A type‐I exciton transition is also observed for the opposite up‐spin exciton state. The result shows the spatial spin separation and the spin‐injection dynamics for the heavy‐hole spin states in the double quantum wells.

Ultrafast Exciton Spin Dynamics in Cd1−xMnxTe Quantum Wells

Dynamics of spin‐polarized excitons has been studied in diluted magnetic semiconductor quantum wells by femto‐second pump‐probe spectroscopy in magnetic fields. Transient processes of injection and spin relaxation of excitons are measured in the quantum wells. The injection time and the spin‐flip time of excitons are obtained as 5 ps and 41 ps, respectively.