pISSN 0374-4914 eISSN 2289-0041
• ### Research Paper 2020-06-30

#### Spectroscopic Characteristics of Zn$_{1-x}$Mn$_{x}$S Thin Films by Ellipsometry and Absorptance Measurements

Dae Jung KIM, Bong Jin KIM, Duk Hyeon KIM et al.

Abstract : Zn$_{1-x}$Mn$_{x}$S thin films were grown on GaAs(100) substrates by using hot-wall epitaxy. X-ray diffraction patterns revealed the crystal structure of the epilayer. The optical properties were investigated using spectroscopic ellipsometry at 300 K from 3.0 -- 8.7 eV. The obtained data were analyzed to determine the critical points of the pseudodielectric function spectra, $\langle\varepsilon(E)\rangle=\langle\varepsilon_{1}(E)\rangle+\langle\varepsilon_{2}(E)\rangle$, such as $E_{0}/E_{0}+\Delta_{0}$, $E_{1}$, and three $E_{2}(\Sigma,\Delta,\Gamma)$structures. These critical points were determined by fitting analytical line-shapes to numerically calculated derivatives of the pseudodielectric functions. In particular, the transmittance/absorptance measurements were carried out to obtain the optical properties of the Zn$_{1-x}$Mn$_{x}$S epilayers, and the exact optical energy bandgap ($E_{g}$) was determined by comparing these results with the results of the ellipsometry measurements. Also, the energy band structures from the ellipsometry and the absorptance measurements were analyzed as a function of Mn concentration. The results obtained demonstrate that ellipsometry can be efficiently and non-destructively used to examine the energy bandgap of Zn$_{1-x}$Mn$_{x}$S epilayers.

• ### Research Paper 2020-06-30

#### Restoration of Human Activity in the Archaeological site Using Absolute Dating

Myung-Jin KIM*

Abstract : We first used radiocarbon dating, TL/OSL dating and archaeo-magnetic dating to calculate the absolute chronology of various objects excavated from the Gyesil-ri archaeological site at Gongju. After we had summarized the probability density of all absolute chronologies of each object by using the KDE model performed a peak deconvolution with a normal distribution, and found that human activity first occurred at the Gyesil-ri site in 840 $\pm$ 160 AD (1$\sigma$ SD) and continued from 1,250 $\pm$ 160 AD (1$\sigma$ SD) to 1,670 $\pm$ 180 AD (1$\sigma$ SD). Those periods correspond to the Unified Silla, Goryeo dynasty and late Chosun dynasty, respectively. Within the Gyesil-ri site, the objects related to metallic works, including the slag layer in the late Chosun dynasty period were located perpendicular to the contour lines, which means that the metallic works were carried out at higher places in the Gyesil-ri site and that slag, as well as other residuals, were naturally discarded in lower places.

• ### Research Paper 2020-05-29

#### Calculation Method of Total Energy and Atomic Interaction Potential Through Machine Learing Using a Neural Network of Atomic Structure Data

Doukyun KIM, Chul Hong PARK*

Abstract : We examined a machine learning method to extract the atom-pair interaction potential energy in materials. The data for the atomic structures and the corresponding total energies were generated by using the ab-initio molecular dynamic simulation, by which the artificial neural network (ANN) was trained to predict the total energies of the materials. Two ANNs were assigned: one to simulate (i) the dependence of the atom-pair interaction energy on the distance between the nearest atoms and the other to simulate (ii) the angular distortion energy. We found that compared to the true energies the total energies of Si could be successfully predicted with an error of about 1 meV/atom for atomic structures generated at 300 K, and that the dependence of the atomic interaction energy on the distance and the angular distortion energy could be obtained by training an ANN for atomic structures of various volumes.

• ### Research Paper 2020-05-29

#### Magnetic Resonance Study of $^{7}$Li and $^{39}$K Nuclei in Li$_{3}$K(SO$_{4}$)$_{2}$ Single Crystals

Tae Ho YEOM*

Abstract : Li$_{3}$K(SO$_{4}$)$_{2}$ single crystals were grown and the lattice constants were determined by using X-ray diffraction. The nuclear magnetic resonances (NMR) in the $^{7}$Li and the $^{39}$K nucleus in Li$_{3}$K(SO$_{4}$)$_{2}$ single crystals were investigated by using Fourier transform NMR (FT-NMR) spectrometry. The quadrupole interaction of $^{7}$Li nuclei decreases as the temperature is increased. $T_{1}$, the spin-lattice relaxation time, for $^{7}$Li and $^{39}$K in Li$_{3}$K(SO$_{4}$)$_{2}$ crystal was analyzed in the temperature range of 180 K -- 430 K. The $T_{1}$ values for the $^{7}$Li and the $^{39}$K nuclei in the crystals decrease linearly as temperature is increased. The mechanism of the spin-lattice relaxation for both the $^{7}$Li and the $^{39}$K nuclei in Li$_{3}$K(SO$_{4}$)$_{2}$ single crystals seems to be a single-phonon direct process, not a Raman process. $E_{a}$, the activation energies for the molecular motions of the $^{7}$Li and the $^{39}$K nuclei surrounded by oxygen atoms were found to be 3.08 $\pm$ 0.19 kJ/mol ($\omega_{0}$/2$\pi$ = 77.27 MHz) and 2.52 $\pm$ 0.19 kJ/mol ($\omega_{0}$/2$\pi$ = 155.51 MHz) for $^{7}$Li nucleus and 1.36 $\pm$ 0.08 kJ/mol (18.67 MHz) for the $^{39}$K nucleus.

• ### Research Paper 2020-04-29

#### Temperature Dependence of Magnetic Anisotropy in Co/Pt film on VO$_{2}$ with a Structural Transition

Won Jun JUNG, Santosh GHIMIRE, Joonghoe DHO*

Abstract : Fifty-nanometer thick (100) and (101) VO$_{2}$ films were synthesized on C-plane and R-plane Al$_{2}$O$_{3}$ substrates using pulsed laser deposition. X-ray diffraction measurements suggested that the VO$_{2}$ films with a root-mean-square roughness of 0.58 -- 1.08 nm had good crystalline properties. The temperature dependence of the resistance for the VO$_{2}$ films displayed a metal-insulator transition behavior at T$_{\text{MI}}$= 325 -- 350 K. Co/Pt multilayers with perpendicular magnetic anisotropy were deposited on (100) and (101) VO$_{2}$ film by using DC sputtering, and their magnetic properties were monitored by using magneto-optic Kerr effect measurements. Although the VO$_{2}$ has a structural transition from monoclinic to tetragonal at the T$_{\text{MI}}$, the (100) VO$_{2}$ plane seems to have opposite changes in the two in-plane lattice constants, so no net strain is induced in the neighboring Co/Pt film. On the other hand, the (101) VO$_{2}$ with net changes in both in-plane lattice constants at the T$_{\text{MI}}$ presumably provides interfacial stress to the neighboring Co/Pt film thus, it induces a net magnetoelastic effect, which results in an increases in the coercive fields of ~ 5 Oe along the easy axis and ~200 Oe along the hard axis.

## Current Issue

June, 2020 | Volume 70, No. 6
• ### Research Paper 2020-06-30

#### Spectroscopic Characteristics of Zn$_{1-x}$Mn$_{x}$S Thin Films by Ellipsometry and Absorptance Measurements

Dae Jung KIM, Bong Jin KIM, Duk Hyeon KIM et al.

Abstract : Zn$_{1-x}$Mn$_{x}$S thin films were grown on GaAs(100) substrates by using hot-wall epitaxy. X-ray diffraction patterns revealed the crystal structure of the epilayer. The optical properties were investigated using spectroscopic ellipsometry at 300 K from 3.0 -- 8.7 eV. The obtained data were analyzed to determine the critical points of the pseudodielectric function spectra, $\langle\varepsilon(E)\rangle=\langle\varepsilon_{1}(E)\rangle+\langle\varepsilon_{2}(E)\rangle$, such as $E_{0}/E_{0}+\Delta_{0}$, $E_{1}$, and three $E_{2}(\Sigma,\Delta,\Gamma)$structures. These critical points were determined by fitting analytical line-shapes to numerically calculated derivatives of the pseudodielectric functions. In particular, the transmittance/absorptance measurements were carried out to obtain the optical properties of the Zn$_{1-x}$Mn$_{x}$S epilayers, and the exact optical energy bandgap ($E_{g}$) was determined by comparing these results with the results of the ellipsometry measurements. Also, the energy band structures from the ellipsometry and the absorptance measurements were analyzed as a function of Mn concentration. The results obtained demonstrate that ellipsometry can be efficiently and non-destructively used to examine the energy bandgap of Zn$_{1-x}$Mn$_{x}$S epilayers.

• ### Research Paper 2020-06-30

#### Effects of Electrospinning Nozzle size and Voltage on Polyvinylpyrrolidone Fiber Structure Formation

In-Bo SHIM, Taejoon KOUH*

Abstract : The formation of a polyvinylpyrrolidone fiber via electrospinning is investigated. Especially, the effects of the nozzle size and voltage during electrospinning are studied. For the experiment, three different nozzle sizes are used for fiber production under electrospinning at voltages of 5, 10, and 15 kV. A careful examination of the scanning electron micrographs reveals that both are important factors in determining the dimension and the uniformity of the electrospun fibers. A smaller nozzle size with a higher voltage is preferable for forming a fiber structure with a smaller diameter, down to sub-microns. Also, a high electrospinning voltage tends to lead to the uniform formation of the fiber structure, as indicated by the coefficient of variation in the fiber diameter distribution. The study presented here can help to guide and optimize the electrospinning process for the controlled production of fiber structure.

• ### Research Paper 2020-06-30

#### Optical Properties of 2H Phase WTe$_{2}$ Flakes

Yonghee JO, Dae Hyun JUNG, TaeWan KIM* et al.

Abstract : Tungsten ditelluride (WTe$_{2}$) has attracted increasing attention because of its unique properties, such as its relatively narrow bandgap and large carrier transport, which depend on its phases. A mechanically exfoliated ultrathin WTe$_{2}$ flake was transferred on to a SiO$_{2}$/Si substrate. A transferred WTe$_{2}$ flake with a thickness of 10 -- 40 nm was determined by using Atomic force microscopy (AFM). We systematically investigated the thickness-dependent optical properties of the WTe$_{2}$ flakes. The dominant Raman $\mathbf{A}_{1}^{3}, \mathbf{A}_{1}^{4}, \mathbf{A}_{1}^{7} and \mathbf{A}_{1}^{9}$ peaks were observed. Strong photoluminescence emission peaks with a band gap of 1.83 eV were observed for flakes with thicknesses of 10 nm, possibly because of a radiative transition. We note that the radiative transition efficiency was enhanced with decreasing number of WTe$_{2}$ flake layers.

• ### Research Paper 2020-06-30

#### Restoration of Human Activity in the Archaeological site Using Absolute Dating

Myung-Jin KIM*

Abstract : We first used radiocarbon dating, TL/OSL dating and archaeo-magnetic dating to calculate the absolute chronology of various objects excavated from the Gyesil-ri archaeological site at Gongju. After we had summarized the probability density of all absolute chronologies of each object by using the KDE model performed a peak deconvolution with a normal distribution, and found that human activity first occurred at the Gyesil-ri site in 840 $\pm$ 160 AD (1$\sigma$ SD) and continued from 1,250 $\pm$ 160 AD (1$\sigma$ SD) to 1,670 $\pm$ 180 AD (1$\sigma$ SD). Those periods correspond to the Unified Silla, Goryeo dynasty and late Chosun dynasty, respectively. Within the Gyesil-ri site, the objects related to metallic works, including the slag layer in the late Chosun dynasty period were located perpendicular to the contour lines, which means that the metallic works were carried out at higher places in the Gyesil-ri site and that slag, as well as other residuals, were naturally discarded in lower places.

• ### Research Paper 2020-06-30

#### Study of Content Addressable Memory in a Spiking Neural Network Model

Myoung Won CHO*

Abstract : The implementation of neural network content-addressable memory (CAM) relates to the attractor dynamics in the firing process. If some firing patterns are carved as fixed-point attractors, one of them can be recalled, depending on initial firing states. Many studies following the landmark achievement by Hopfield, have suggested an algorithm based on an analogy between the asymptotic dynamics of neural networks and the equilibrium properties of magnetic systems. However, the firing process of biological neurons, which progresses depending on minute spiking timings, has distinctive properties of attractor dynamics with those of the classical neuron models. We here study the characteristics of fixed-point attractors in a spiking neural network model preserving the attribute of spiking-timing-dependent interactions between neurons. We show that the attractor dynamics in the model relate to the phase-locking dynamics in a biological neural network when the memorized firing states are orthogonal to one another. We also introduce how non-orthogonal firing states can be memorized in the model and how one of them can be recalled, depending on the initial states, by applying the Hopfield network.

## Current Issue

• ### Gravitational Waves and Tidal Deformability of Neutron Stars

Young-Min KIM, Kyujin KWAK, Yeunhwan LIM, Chang Ho HYUN, Chang-Hwan LEE*

New Phys.: Sae Mulli 2018; 68(7): 707-717
https://doi.org/10.3938/NPSM.68.707

• ### Theory of the Noise Spectral Density for Micro-Cantilevers Under Resonant Frequency Control

Manhee LEE*

New Phys.: Sae Mulli 2018; 68(7): 718-723
https://doi.org/10.3938/NPSM.68.718

• ### Crystal Structure and Surface Characteristics of ZnO Thin Films Deposited by Using RF Magnetron Sputtering

Bong Ju LEE, Myung Bok LEE*

New Phys.: Sae Mulli 2018; 68(7): 724-730
https://doi.org/10.3938/NPSM.68.724