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Inter-dot spin exchange interaction in coupled II–VI semiconductor quantum dots
Lee, S.,Dobrowolska, M.,Furdyna, J. K. WILEY-VCH Verlag 2006 Physica status solidi. PSS. B, Basic solid state p Vol.243 No.4
<P>We have performed polarization selective photoluminescence (PL) experiment on a series of self-assembled quantum dots (QDs) in the form of single- and double-QD layer systems. The double layers were formed from diluted magnetic semiconductor (DMS) (either CdMnSe or CdZnMnSe) and non-DMS (either CdSe or CdZnSe) layers, separated by non-DMS ZnSe barriers. The peaks from DMS and non-DMS QD layers are clearly resolved in the PL spectra taken at zero magnetic field. When a magnetic field is applied to the double layer QD (DLQD) system, the intensities of the circularly polarized PL peaks corresponding to the non-DMS (i.e., CdSe and CdZnSe) layers exhibit significant changes, reflecting correspondingly large changes in the degrees of spin polarization of the non-DMS QDs. This enhancement of spin polarization observed in the double-layer QD structures is interpreted in terms of anti-parallel spin interaction between carriers localized in the coupled QD pairs. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)</P>
Spin Phenomena of CdZnSe Self-assembled Quantum Dots Investigated by Magneto-photoluminescence
Yungjun Kim,M. Dobrowolska,J. K. Furdyna,이상훈 한국물리학회 2009 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.55 No.1
Magneto-photoluminescence (PL) experiments were carried out on a self-assembled CdZnSe/ZnSe quantum dot (QD) system. The intensities of the PL peak exhibited significant differences for the two circular polarizations when a magnetic field was applied. A simple two-level rate equation model, including the spin flip time τs and the exciton recombination time τr , was used to analyze the observed degree of the polarization in the polarization selective PL measurements. Specifically, the magnetic field dependence of the polarization was fitted by treating τs/τr as a fitting parameter in the model. The best fitting was obtained with the value of τs/τr ~ σ, indicating that the spin relaxation time is much longer than the exciton recombination time. Such polarization selective PL measurements were further performed at several different temperatures, which showed a systematic decrease in the degree of polarization with increasing temperature. Magneto-photoluminescence (PL) experiments were carried out on a self-assembled CdZnSe/ZnSe quantum dot (QD) system. The intensities of the PL peak exhibited significant differences for the two circular polarizations when a magnetic field was applied. A simple two-level rate equation model, including the spin flip time τs and the exciton recombination time τr , was used to analyze the observed degree of the polarization in the polarization selective PL measurements. Specifically, the magnetic field dependence of the polarization was fitted by treating τs/τr as a fitting parameter in the model. The best fitting was obtained with the value of τs/τr ~ σ, indicating that the spin relaxation time is much longer than the exciton recombination time. Such polarization selective PL measurements were further performed at several different temperatures, which showed a systematic decrease in the degree of polarization with increasing temperature.
Carrier relaxation processes in magnetic semiconductor quantum dot systems
이상훈,J. K. Furdyna,M. Dobrowolska 한국물리학회 2005 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.47 No.4
The energy relaxation process of dSe/ZnMnSe quantum dots (QDs) was investigated using polarization-selective magneto-photoluminescence (PL). Peaks from the Mn2+ internal transition, the CdSe QDs, and the ZnMnSe barrier were observed in the system. The CdSe QD peak was relatively weak while the intensity of the Mn2+ transition was strong in the spectrum taken at zero magnetic field. However, the peaks intensities changed significantly when a magnetic field was applied, and the situation became reversed (i.e., the CdSe QD peak become stronger than the peak due to the Mn2+ internal transition, specifically for + polarization). The opposite intensity behavior was observed in the peak from the Mn2+ ions and in the PL of the CdSe QDs and revealed that these two carrier capture centers competed with each other for carriers excited in the ZnMnSe barrier. The variation in the Mn2+ internal transition with the magnetic field was well described by using a model developed under an angular momentum selection rule. This suggests that competition between the two energy transfer processes in the CdSe/ZnMnSe QD system is responsible for the observed PL intensity behavior.
Spin Relaxation Time of CdZnSe/ZnSe Self-Assembled Quantum Dots in a Magnetic Field
이상훈,J. K. Furdyna,M. Dobrowolska 한국물리학회 2006 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.49 No.1
We have performed polarization-resolved magneto-photoluminescence (PL) experiments on a selfassembled CdZnSe quantum dot (QD) system to investigate the influence of an external magnetic field on the spin relaxation time of excitons in QDs. When the QDs are excited with circularly polarized light, the PL emissions at phonon resonance energies exhibit a noticeable degree of circular polarization (33 % and 6 % for 1-LO and 2-LO resonances, respectively) even at zero magnetic field while the PL at non-resonant positions shows zero circular polarization. The degree of polarization increases significantly when an external magnetic field is applied. Based on a simple two-level rate equation model, including a spin-flip time s and an exciton recombination time r, we are able to extract the magnetic field dependence of the ratio s/r. This ratio increases monotonically with the field and eventually becomes larger than 1, indicating that at high magnetic fields, spin relaxation times can exceed exciton recombination times.
이상훈,J. K. Furdyna,M. Dobrowolska 한국물리학회 2007 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.50 No.3
The magneto-optical properties of quantum-dot (QD) and quantum-well (QW) coupled structures are investigated using magneto-photoluminescence (PL). The coupled system consists of a self-assembled non-magnetic CdSe quantum-dot layer and a 7-nm ZnCdMnSe diluted- magnetic-semiconductor (DMS) quantum well (QW) layer. We have observed two well-separated PL peaks, one corresponding to emission from the CdSe QD layer and the other to the emission for ZnCdMnSe QW layer. The temperature dependence of the PL intensity clearly demonstrates the characteristics of zero- and two-dimensional structures for the CdSe QDs and the ZnCdMnSe QW, respectively. The significant PL energy shift and intensity variation observed for the DMS QW in a magnetic field can be understood based on the giant Zeeman splitting of the band edges of the DMSs. Furthermore, we have observed a type-II transition between the conduction band of the DMS QW and the valence band of non-DMS QD, which enables us to determine the band alignment of the coupled systems.
Optical Properties of Self-Assembled Quantum Dots in Single and Double-Layer Configurations
이상훈,H. C. Kim,Y. J. Cho,M. Dobrowolska,J. K. Furdyna 한국물리학회 2008 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.53 No.5
We have performed photoluminescence (PL) experiments on CdSe and CdZnSe self-assembled quantum dot (QD) systems in single- and double-layer geometries. The QDs were formed by using molecular beam epitaxy to deposit three monolayers (ML) of either CdSe or CdZnSe. The ZnSe barrier thickness separating the QD layers in the double layer geometry was 20 monolayers (MLs). Strong photoluminescence (PL) peaks were observed for CdSe and CdZnSe QDs in both single- and double-layer QD systems. The intensities of the PL peaks from the QDs exhibited a systematic decrease with increasing temperature. The temperature dependence of the integrated PL revealed that the activation energy of CdSe QDs for PL quenching was significantly different between the two geometries. The observed difference in the activation energies of the CdSe QDs was discussed based on the effective carrier delocalization via neighboring higher-energy CdZnSe QDs in the double-layer QD system.
Multi-level States of GaMnAsP Single Layer Induced by Spin-orbit Torque
Kyung Jae Lee,Seongjin Park,Phunvira Chongthanaphisut,Sanghoon Lee,X. Liu,M. Dobrowolska,J. K. Furdyna 한국자기학회 2021 한국자기학회 학술연구발표회 논문개요집 Vol.31 No.1
We have investigated spin-orbit torque (SOT) induced switching of a 25 nm GaMnAsP single layer with perpendicular magnetic anisotropy (PMA) grown by molecular beam epitaxy (MBE). We have fabricated Hall device along [100] crystal direction, in which Rashba-type and Dresselhaus-type spin-orbit induced (SOI) fields are perpendicular to each other. The SOT induced magnetization switching was carried out with a current density of ~1.9×10<sup>6</sup> A/cm<sup>2</sup> under in-plane bias field of 500 Oe at 55 K. We have achieved a reproducible and tunable multi-level states of minor-loop by performing current scan ranging from 8.0×10<sup>5</sup> A/cm<sup>2</sup> to 1.9×10<sup>6</sup> A/cm<sup>2</sup> while monitoring Hall resistance. This multi-level states can also be produced by applying pulsed current with duration of 160 ms. The results suggest that the system can be used as a memristors for neuromorphic computing.