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Magneto-Photoluminescence and Spin Dynamics of Self-Assembled CdSe Quantum Dots in Zn1-xMnxSe
EunsoonOh,S.M.Soh,이중욱,K.J.Yee,J.C.Woo,H.S.Jeon,김대식,S.Lee,J.K.Furdyna,H.C.Ri,H.S.Chany,S.H.Park 한국물리학회 2003 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.42 No.6
We have studied photoluminescence spectra of CdSe self-assembled quantum dots (QD's) embedded in Zn$_{1-x}$Mn$_x$Se ($x$ = 0.05 and 0.15) with and without an external magnetic field. Under a magnetic field, the photoluminescence from CdSe QD's was circularly polarized under light excitation above the bandgap energy of ZnMnSe, indicating carrier spin polarization in CdSe QD's. The time-resolved photoluminescence study showed that spin-flip occured very rapidly as photocarriers transfered from the ZnMnSe to the CdSe QD's to reach statistical equilibrium of the spin states in the CdSe QD's. The Zeeman splitting in the CdSe QD's was several meV, still larger than the thermal energy at 5 K. These results suggest that the spin-polarization in CdSe QD's is mainly due to the Zeeman splitting of carriers in the CdSe quantum dots.
Ga1-xMnxAs의 시분해 Differential Reflectivity 연구
김신,오은순,이중욱,김대식,이상훈,J. K. Furdyna 한국물리학회 2006 새물리 Vol.52 No.2
We investigated the time-resolved differential reflection in GaMnAs by using a pump-probe method at various wavelengths. At room-temperature, the differential reflectivity $\Delta$R in Ga$_{0.97}$Mn$_{0.03}$As was found to be dominated by the negative component for photon energies higher than its bandgap energy, E$_g$. On the other hand, $\Delta$R in Ga$_{0.95}$Mn$_{0.05}$As was positive for E $>$ E$_g$, similar to observations in semi-insulating GaAs. The negative $\Delta$R component from Ga$_{0.97}$Mn$_{0.03}$As appear to originate mainly from induced absorption associated with deep level defects whereas the positive $\Delta$R component is due to free carrier absorption and bandgap renormalization. We measured the transmission spectra of these samples and found that the absorption edge in Ga$_{0.95}$Mn$_{0.05}$As was somewhat broader than it was in Ga$_{0.97}$Mn$_{0.03}$As. This could be due to the larger potential fluctuation for larger Mn concentrations. For photon energies very near the absorption edge, a negative differential reflectivity signal was obtained at 10 K. The signal was significantly weaker for Ga$_{0.95}$Mn$_{0.05}$As, probably due to the absorption bleaching associated with the band filling effect. Molecular Beam Epitaxy (MBE)로 성장한 Ga$_{1-x}$Mn$_x$As 시료를 pump-probe 방법을 사용하여 다양한 파장에서의 시분해 differential reflectivity를 측정하였다. Ga$_{1-x}$Mn$_x$As 시료의 free carrier absorption, bandgap renormalization, 그리고 deep level defect에 의한 induced absorption 등의 효과를 분석하였다. Ga$_{0.97}$Mn$_{0.03}$As 시료의 경우 deep level defect에 의한 induced absorption 효과가 지배적인 것으로 생각되는데, 이는 저온 성장한 GaAs에서 나타난 arsenic antisites deep level donor 등의 defect에 의한 영향과 유사하였다. 반면 defect의 영향이 상대적으로 적은 Ga$_{0.95}$Mn$_{0.05}$As 시료의 경우는 semi-insulating GaAs 시료의 신호와 더 유사하게 free carrier, bandgap renormalization에 의한 신호가 지배적이었다. 또한, 높은 Mn concentration에 의한 absorption edge broadening을 transmission spectrum으로 확인할 수 있었다.
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.
Eect of p-Type Buer Layer on the Properties of GaMnAs Ferromagnetic Semiconductors
Y. J. Yoon,S. J. Chung,H. J. Lee,S. Lee,S.Y. An,X. Liu,J. K. Furdyna 한국물리학회 2004 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.45 No.3
We have studied the magnetic and transport properties of Ga1 epilayers grown either on undoped or on p-type doped GaAs buer layer. The temperature dependence of the resistivity at zero magnetic eld reveals that the Curie temperature (TC) in the Ga1 layer grown on p-type buer shows a larger coercive eld and much slower decay of remanent magnetization than the layer grown on undoped buer. This robust magnetic behavior observed in the doped sample is discussed in terms of the increase of free carrier concentration in the system arising from p-type doping in the buer layer.
Elecrtinic Raman scattering in Cd_1-x C0_xSe
Bartholomew, D. U.,Suh, E. K.,Remdas, A. K.,Rodriguez, S.,Debeska, U.,Furdyna, J. K. 全北大學校 基礎科學硏究所 1991 基礎科學 Vol.14 No.1
The Raman spectrum of the diluted magnetic semiconductor Cd_(1-x)Co_xSe(x=0.035 and 0.082) exhibits a line(W_PM) associated with the transition between the Zeeman-split orbital Γ_2 ground-state levels of Co^2+ and is characterized by g=2.32. Below 20 K, this Raman peak shifts to higher frequencies, signaling the onset of magnetic ordering. At low temperatures, spin-flip Raman scattering from electrons bound to shallow donors(W_SF) is observed. The Raman shift as a function of magnetic field and temperature shows a large s-d exchange interaction(αN_O=320meV) and clear evidence of a bound magnetic polaron. Evidence of magnetic ordering at low temperatures, indicated by the temperature dependence of W_PM, and the large antiferromagnetic temperature(T_AF)required to fit the temperature and magnetic-field dependence of W_SF, indicate a significantly larger Co^2+-Co^2+ antiferromagnetic coupling than is the case for Mn^2+-Mn^2+ in, for example, Cd_(1-x)Mn_xSe.