K. Kayanuma

Efficient spin depolarization in ZnCdSe spin detector: an important factor limiting optical spin injection efficiency in ZnMnSe∕ZnCdSe spin light-emitting structures

Spin depolarization of a ZnCdSe quantum-well spin detector (SD) in ZnMnSe∕ZnCdSe light-emitting quantum structures is investigated by cw and time-resolved optical orientation spectroscopy. It is shown that spin depolarization is governed by three distinct spin relaxation processes with the corresponding polarization decay times of 850, 30, and <10ps. The dominant and the fastest process is attributed to spin relaxation accompanying energy relaxation of hot excitons (and hot carriers) within the SD, providing evidence that it can be an important source of spin loss, leading to the observed limited efficiency of optical spin injection in the structures.

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 ...

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.

Exciton Spin Injection in CdTe/Cd1–xMnxTe Double Quantum Wells

Spin injection processes of excitons were studied by time-resolved photoluminescence in double quantum wells consisting of Cd 0.95 Mn 0.05 Te and CdTe layers. Selective excitation of the magnetic and non-magnetic wells confirms the dominant spin injection process from the magnetic well to the non-magnetic well. The exciton spin injection from the magnetic quantum well of Cd 0.95 Mn 0.05 Te arises with an injection time of 10 ps and induces a transient spin polarization with the degree of 0.20 in the non-magnetic well of CdTe. The exciton spin relaxation time in the CdTe well was found to be sufficiently longer than the exciton lifetime of 38 ps.

Spin transport dynamics of excitons in CdTe/Cd1−xMnxTe quantum wells

Transport properties of spin-polarized excitons were studied in the double quantum well system composed of Cd0.95Mn0.05Te and CdTe wells. Circular polarization degrees of the time resolved exciton photoluminescence in magnetic field showed that the spin-polarized excitons diffused from the magnetic quantum well and injected to the non-magnetic quantum well by conserving their spins. The spin-polarized excitons injected into the nonmagnetic well reaches 18% of the nonmagnetic well excitons. From the circular polarization degree and the lifetime of the magnetic quantum well excitons, the spin relaxation time of the excitons in the Cd0.95Mn0.05Te well was determined as 275–10 ps depending on the magnetic field strength.

Magnetic-field-induced level crossing and the spin dynamics of excitons in Zn1-xMnxTe/ZnTe quantum wells

Magnetic-field-induced level crossing and the spin dynamics of excitons in a Zn1−xMnxTe/ZnTe single quantum well are studied. The circularly-polarized photoluminescence (PL) shows that the down spin branch of the Zn1−xMnxTe exciton overlaps with both the up and down spin branches of the ZnTe exciton at a crossing field (Hc) of 4 T, due to the giant Zeeman shift of Zn1−xMnxTe. The PL intensities and lifetimes in each layer become gradually equal toward Hc, which shows the mixing of wavefunctions of the excitons generated in each layer. Above Hc, each branch of the spin-polarized exciton separates again. The lifetimes of the spin-polarized exciton PL reflect the spin-flip relaxation in ZnTe and the spin mixing between Zn1−xMnxTe and ZnTe layers.

Excitonic Properties and Dynamics in Diluted Magnetic Semiconductor Quantum Nanostructures

Nanostructures of diluted magnetic semiconductors are fabricated by molecular beam epitaxy and electron beam lithography. Excitons in these nanostructures show the low dimensional confinement effects and the spin interaction with magnetic ions. The magneto-optical properties and formation and relaxation dynamics of excitons are studied by ultrafast time-resolved spectroscopy.

Spin relaxation times of exciton states in ZnCdSe/ZnSe low dimensional heterostructures

Abstract Using the pulsed excitation with a femtosecond laser and streak-camera detection we have studied the processes of spin relaxation in the system of localized excitons in low dimensional superlattices formed by the insertion of CdSe submonolayers into the ZnSe matrices. Polarization of exciton luminescence in the magnetic field arises due to the thermal redistribution of population in the system of splitted exciton spin sublevels. The distribution of polarization across the PL band countour is governed by the complicated interplay of energy and spin relaxation processes. The study of PL decay kinetics in polarized light allows us to determine with the help of a simple two-level model the times of exciton spin relaxation. Spin relaxation times strongly decrease with the increase of magnetic field indicating the phonon assisted mechanism of spin relaxation. The detailed analysis of the dependence of spin relaxation times on the exciton localization energy is not possible with the model used due to the complicated character of exciton population kinetics for deeply localized states. Deeply localized states are to a great extent populated through the tunnelling relaxation of excitons with higher energies. The transfer of polarization in the processes of exciton energy relaxation has been clearly observed in the study of spectrally resolved decay kinetics in polarized light.

Dynamical spin separation induced by spin-dependent type-II band alignment in a diluted magnetic double quantum well

Dynamical spin separation is demonstrated in a magnetic double quantum well composed of a diluted magnetic semiconductor well (MW) and a nonmagnetic well (NMW). Excitonic photoluminescence of the type-II transition between an electron in the NMW and a down-spin heavy hole (hh) in the MW is induced with an increase in the magnetic field. A type-I transition involving an up-spin hh inside the NMW is simultaneously observed. Therefore, the hh spins are concluded to be spatially separated, which is sustained by a low hh-spin injection time of 500ps from the NMW to the MW.

Optical Study of Spin Injection Dynamics in Double Quantum Wells of II‐VI Diluted Magnetic Semiconductors

The spin injection process is studied in double quantum wells of ZnMnSe and ZnCdSe with a tunneling barrier by circularly‐polarized transient photoluminescence spectroscopy. The result shows two types of spin injections in the time ranges of 30 and 1000 ps. The observed spin injection processes are quantitatively interpreted by the individual tunneling of electrons and holes based on the rate equation analysis.