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2-Dimensional van der Waals Antiferromagnets Studies with Optical Spectroscopy
Hyeonsik Cheong 한국자기학회 2021 한국자기학회 학술연구발표회 논문개요집 Vol.31 No.2
Two-dimensional magnetic van der Waals materials are attracting much interest recently. Magnetism in low dimensional systems is an interesting topic for the fundamental physics, and atomically thin magnetic materials are promising candidates for novel spintronic devices. Antiferromagnetic 2-dimensional materials are particularly interesting both for fundamental physics and also for antiferromagnetism-based spintronic devices. However, traditional research tools to probe antiferromagnetic ordering such as neutron scattering cannot be employed for atomically thin materials due to the small sample volume. Although magneto-optical Kerr effect measurements can be used to monitor the magnetic ordering in ferromagnetic materials, the lack of net magnetization precludes the use of the Kerr effect in probing antiferromagnetic ordering. Optical techniques such as Raman spectroscopy or second-harmonic generation (SHG) are becoming increasingly important for the study of antiferromagnetic 2-dimensional materials. Raman spectroscopy, for example, can be an invaluable tool to probe the magnetic transition in antiferromagnetic van der Waals materials that show spectroscopic features that correlate with magnetic ordering. Furthermore, recent spectroscopic studies revealed a novel coherent state in some of these materials stabilized by the antiferromagnetic ordering. In this presentation, I will review recent achievements in the study of antiferromagnetism in 2 dimensions using optical spectroscopy.
In - situ Raman Spectroscopy of Amorphous Hydrous RuO₂ Thin Films
Hyeonsik Cheong,Bo Young Jung,Se-Hee Lee 한국진공학회(ASCT) 2003 Applied Science and Convergence Technology Vol.12 No.S1
Amorphous hydrous ruthenium oxide thin films have attracted much interest owing to the possibility of using this material in electrochemical supercapacitors. Recently, it was found that this material is also electrochromic: during the charging/discharging cycle, the optical transmittance of the thin film is modulated. The physical and chemical origin of this phenomenon is not fully understood yet. In this work, we performed in-situ Raman spectroscopy measurements on amorphous hydrous ruthenium oxide thin films during the charging/discharging cycles. Unambiguous changes in the Raman spectrum were observed as protons were injected or extracted from the thin film. When the samples were annealed to reduce the water content, there is a consistent trend in the Raman spectrum. The origins of the Raman features and their relation to the electrochromic and/or supercapacitor characteristics is discussed.
Photoluminescence Up - conversion in GaAs / AlGaAs Heterostructures
Hyeonsik M. Cheong 한국진공학회(ASCT) 2002 Journal of Korean Vacuum Science & Technology Vol.6 No.2
Photoluminescence up-conversion in semiconductor heterostructures is a phenomenon in which luminescence occurs at energies higher than that of the excitation photons. It has been observed in many semiconductor heterostructure systems, including InP/AnALAs, CdTe/CdMgTe, GaAs/ordered-(Al)GaInP, GaAs/AlGaAs, and InAs/GaAs. In this work, GaAs/AlGaAs heterostructures are used as a model system to study the mechanism of the up-conversion process. This system is ideal for testing different models because the band offsets are quite well documented. Different heterostructures are designed to study the effect of disorder on the up-converted luminescence efficiency. In order to study the roles of different types of carriers, the effect of doping was investigated. It was found that the up-converted luminescence is significantly enhanced by p-type doping of the higher-band-gap material.
Lim, Hyunjin,Park, Sooyoun,Cheong, Hyeonsik,Choi, Hyun-Min,Kim, Young Chool Elsevier 2010 Diamond and related materials Vol.19 No.10
<P><B>Abstract</B></P><P>We have performed low-temperature (8K) photoluminescence (PL) measurements on 71 natural and 12 high-pressure-and-high-temperature (HPHT)-treated type IIa diamonds. The GR1, NV<SUP>0</SUP>, NV<SUP>−</SUP>, H4, and H3 defect center PL signals are compared. Some distinct differences in the PL lineshape, intensity, and appearance of side-band PL signals are observed. Furthermore, we processed 6 of the natural diamond samples with the HPHT treatment to investigate the effect of the treatment on the PL spectrum. By systematically analyzing the differences in the PL spectra, we developed a scheme to discriminate natural and HPHT-treated diamonds with 99% validity.</P>
Exploring the SERS background using a sandwiched graphene monolayer with gap-plasmon junctions
Park, Won-Hwa,Cheong, Hyeonsik IOP 2016 Journal of Physics. D, Applied Physics Vol.49 No.10
<P>We examine the origin of the surface-enhanced Raman scattering (SERS) background mediating with a graphene monolayer at Au nanoparticle-Au thin film junctions. We find the degree of the asymmetric shape of the radial breathing like mode (RBLM) peak at 150 cm<SUP>−1</SUP> is strongly correlated with the accompanied SERS background intensities. The SERS investigation with the out-of-plane modes (300~700 cm<SUP>−1</SUP>) of graphene gives significant clues that the SERS background, the degree of the tilted formation from the analysis of the transverse acoustic peaks, and the spectral shape of the RBLM peak are strongly correlated with each other, indicating that electron–phonon coupling plays a crucial role in increasing the SERS background.</P>