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Time-Resolved Photoluminescence in CdSe/CdZnSe and CdSe/CdMnSe Coupled Quantum Dot Structures
오은순,D. G. Choi,J. H. Park,Jacek Furdyna,이상훈,김신,T. K. Lee 한국물리학회 2006 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.49 No.2I
We studied the time-resolved photoluminescence (PL) from coupled quantum dot (QD) systems (CdSe/CdZnSe and CdSe/CdMnSe) and the PL decay as functions of the magnetic field and the temperature. In the CdSe/CdMnSe system, the PL decay of the + component exhibited biexponential behavior under external fields, which indicated that a fraction of the CdSe QDs were not coupled to the CdMnSe QDs. The PL lifetimes of these structures were found to be more significantly affected by non-radiative recombination, rather than by the vertical coupling between the QDs.
Control of Coherent Phonon Decay in GaAs by Using a Secondary Pump Pulse
Kang-Jeon Han,이기주,Dong-Wook Jang,Jacek Furdyna,Ji-Hee Kim,Xinyu Liu,Yong-Sik Lim 한국물리학회 2007 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.50 No.3
We report on the control of coherent longitudinal-optical phonon decay in GaAs by using an additional carrier pump in femtosecond pump-probe measurements. A suppression of the coherent phonon amplitude via the carrier-phonon interaction, which is launched by using photo-excitation of the carrier pump, is demonstrated. Phonon decay controls were achieved by adjusting the intensity and the relative timing of the carrier pump pulse.
Spin-Polarized Photoreflectance in Ferromagnetic GaMnAs
Ji-Hee Kim,이기주,Dong-Wook Jang,Jacek Furdyna,Kang-Jeon Han,Xinyu Liu 한국물리학회 2007 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.50 No.3
We report on the spin-polarized photoreflectance in a ferromagnetic GaMnAs layer. The transient photoreflectance results show a circular polarization dependence of the probe beam in quasi-degenerate pump-probe measurements. The circular polarization dependence, which follows the magnetization as we increase the sample temperature, can be understood from the strong spin exchange coupling between conduction or valence band electrons and localized Mn ion spins.
Decimal Tunneling Magnetoresistance States in Fe/GaAlAs/GaMnAs Magnetic Tunnel Junction
Taehee Yoo,Sanghoon Lee,Xinyu Liu,Furdyna, Jacek K.,Dong Uk Lee,Eun Kyu Kim IEEE 2014 IEEE transactions on magnetics Vol.50 No.11
<P>We report the realization of ten stable tunneling magnetoresistance (TMR) states in a single device. To achieve ten resistance states, we have used a magnetic tunnel junction (MTJ) structure that consists of two magnetic layers, which are Fe and GaMnAs ferromagnetic layers. Owing to the two in-plane magnetic easy axes that result from strong cubic anisotropies in both Fe and GaMnAs layers, noncollinear magnetic configurations between two magnetic layers were realized, in addition to the parallel and antiparallel configurations. Such noncollinear magnetic configurations provide stable intermediate TMR values between two extreme values corresponding parallel and antiparallel configurations. The number of stable TMR values was further increased by forming multidomain structures in the MTJ structure. We demonstrate that we can obtain up to ten stable TMR values, and they can be controlled by applying the appropriate magnetic field sequences.</P>
Anisotropic AC Magnetic Susceptibility in (Ga,Mn)As Films
Xiang Li,Sining Dong,Taehee Yoo,Xinyu Liu,Sanghoon Lee,Furdyna, Jacek K.,Dobrowolska, Margaret IEEE 2015 IEEE transactions on magnetics Vol.51 No.11
<P>We report a systematic investigation of ac magnetic susceptibility in (Ga,Mn)As films as a function of temperature and magnetic field, carried out in parallel with dc magnetization measurements. The temperature dependence of ac susceptibility χ<SUB>ac</SUB> shows anisotropic behavior: 1) a single peak in χ<SUB>ac</SUB> is observed close to T for the [11̅0] orientation of the driving ac field; 2) a single peak is also seen close to 22 K for the field along [110]; and 3) peaks at both these temperatures are observed for the field applied along [100]. A detailed analysis of the ac and dc data unambiguously indicates that the peak near T is related to the paramagnetic-to-ferromagnetic phase transition, with the ferromagnetic domains nucleating with their easy axes aligned with the [11̅0] direction, providing a clear picture of uniaxial domain behavior near T . The peak near 22 K, on the other hand, is related to the onset of biaxial domain structure in (Ga,Mn)As induced by the competition between uniaxial and cubic anisotropy. More specifically, the ac susceptibility peak near T<SUB>C</SUB> involves 180° magnetization flips along the [11̅0] easy axis of the domains, while the peak near 22 K originates from magnetization wobbling between two biaxial easy axes separated by a small angle.</P>
Antiferromagnetic Interlayer Exchange Coupling in Ferromagnetic GaMnAs/GaAs:Be Multilayers
Lee, Hakjoon,Lee, Sangyeop,Choi, Seonghoon,Yoo, Taehee,Lee, Sanghoon,Liu, Xinyu,Furdyna, Jacek K. IEEE 2015 IEEE transactions on magnetics Vol.51 No.11
<P>Interlayer exchange coupling (IEC) between the GaMnAs layers in GaMnAs/GaAs:Be multilayer systems has been investigated using magnetotransport experiments. The observation of a stable antiparallel magnetization alignment state from the systems indicates the presence of antiferromagnetic (AFM) IEC between the GaMnAs layers. The transitions between parallel and antiparallel alignments of GaMnAs magnetic layers in the system were carefully investigated by measuring resistance change with increasing temperature under various bias magnetic fields. From the dependence of the transition temperature on bias fields, we have estimated the magnitude of AFM IEC and its temperature behavior of our GaMnAs/GaAs:Be multilayers.</P>
Park, Jun Hong,Vishwanath, Suresh,Wolf, Steven,Zhang, Kehao,Kwak, Iljo,Edmonds, Mary,Breeden, Michael,Liu, Xinyu,Dobrowolska, Margaret,Furdyna, Jacek,Robinson, Joshua A.,Xing, Huili Grace,Kummel, Andr American Chemical Society 2017 ACS APPLIED MATERIALS & INTERFACES Vol.9 No.34
<P>To fabricate practical devices based on semiconducting two-dimensional (2D) materials, the source, channel, and drain materials are exposed to ambient air. However, the response of layered 2D materials to air has not been fully elucidated at the molecular level. In the present report, the effects of air exposure on transition metal dichalcogenides (TMD) and metal dichalcogenides (MD) are studied using ultrahigh-vacuum scanning tunneling microscopy (STM). The effects of a 1-day ambient air exposure on MBE-grown WSe2, chemical vapor deposition (CVD)-grown MoS2, and MBE SnSe2 are compared. Both MBE grown WSe2 and CVD-grown MoS2 display a selective air exposure response at the step edges, consistent with oxidation on WSe2 and adsorption of hydrocarbon on MoS2, while the terraces and domain/grain boundaries of both TMDs are nearly inert to ambient air. Conversely, MBE-grown SnSe2, an MD, is not stable in ambient air. After exposure in ambient air for 1 day, the entire surface of SnSe2 is decomposed to SnOx and SeOx as seen with X-ray photoelectron spectroscopy. Since the oxidation enthalpy of all three materials is similar, the data is consistent with greater oxidation of SnSe2 being driven by the weak bonding of SnSe2.</P>