Circuit-Level Model of Phase-Locked Spin-Torque Oscillators

Ahn, Sora,Lim, Hyein,Kim, Miryeon,Shin, Hyungsoon,Lee, Seungjun IOP Publishing 2013 Japanese journal of applied physics Vol.52 No.4

<P>Spin-torque oscillators (STOs) are new oscillating devices based on spintronics technology with many advantageous features, i.e., nanoscale size, high tunability, and compatibility with standard silicon processing. Recent research has shown that two electrically connected STOs may operate as a single device when specific conditions are met. To overcome the limitation of the small output power of STOs, the phase-locking behavior of multiple STOs is hereby extensively investigated. In this paper, we present a circuit-level model of two coupled STOs considering the interaction between them such that it can represent the phase-locking behavior of multiple STOs. In our model, the characteristics of each STO are defined first as functions of applied DC current and external magnetic field. Then, the phase-locking condition is examined to determine the properties of the two coupled STOs on the basis of a theoretical model. The analytic model of two coupled STOs is written in Verilog-A hardware description language. The behavior of the proposed model is verified by circuit-level simulation using HSPICE with CMOS circuits including a current-mirror circuit and differential amplifiers. Simulation results with various CMOS circuits have confirmed the effectiveness of our model. (C) 2013 The Japan Society of Applied Physics</P>

Analytic Model of Spin-Torque Oscillators (STO) for Circuit-Level Simulation

Ahn, Sora,Lim, Hyein,Shin, Hyungsoon,Lee, Seungjun The Institute of Electronics and Information Engin 2013 Journal of semiconductor technology and science Vol.13 No.1

Spin-torque oscillators (STO) is a new device that can be used as a tunable microwave source in various wireless devices. Spin-transfer torque effect in magnetic multilayered nanostructure can induce precession of magnetization when bias current and external magnetic field are properly applied, and a microwave signal is generated from that precession. We proposed a semi-empirical circuit-level model of an STO in previous work. In this paper, we present a refined STO model which gives more accuracy by considering physical phenomena in the calculation of effective field. Characteristics of the STO are expressed as functions of external magnetic field and bias current in Verilog-A HDL such that they can be simulated with circuit-level simulators such as Hspice. The simulation results are in good agreement with the experimental data.

Analytic Model of Spin-Torque Oscillators (STO) for Circuit-Level Simulation

Sora Ahn,Hyein Lim,Hyungsoon Shin,Seungjun Lee 대한전자공학회 2013 Journal of semiconductor technology and science Vol.13 No.1

Spin-torque oscillators (STO) is a new device that can be used as a tunable microwave source in various wireless devices. Spin-transfer torque effect in magnetic multilayered nanostructure can induce precession of magnetization when bias current and external magnetic field are properly applied, and a microwave signal is generated from that precession. We proposed a semi-empirical circuit-level model of an STO in previous work. In this paper, we present a refined STO model which gives more accuracy by considering physical phenomena in the calculation of effective field. Characteristics of the STO are expressed as functions of external magnetic field and bias current in Verilog-A HDL such that they can be simulated with circuit-level simulators such as Hspice. The simulation results are in good agreement with the experimental data.

Study on the mechanisms of seizure-like events suppression effect by electrical stimulation using a microelectrode array

Ahn, Sora,Jo, Sumin,Jun, Sang Beom,Lee, Hyang Woon,Lee, Seungjun Lippincott Williams & Wilkins 2017 NeuroReport Vol.28 No.9

<P>In this paper, we studied the mechanisms underlying the suppression of seizure-like events (SLEs) by electrical stimulation. We conducted an in-vitro experiment using entorhinal cortex combined hippocampal slices and two convulsant drugs, bicuculline and 4-aminopyridine, to induce spontaneous SLEs. We used a microelectrode array to observe network dynamics over the entire hippocampal area simultaneously, including regions far from the stimulation site. We stimulated the entorhinal cortex region, which has been determined to be a focus of SLEs by Granger causality analysis of multichannel time series data, by an external electrode. In bicuculline application, electrical stimulation showed a marked suppression effect, even though the sizes of the effective region differed. In 4-aminopyridine application, however, stimulation under the same conditions did not suppress the activities in similar to 80% of SLEs. The suppression effect was more remarkable in the areas surrounding the stimulation site in both cases. Our experimental results could support the neuronal depolarization blockade mechanism by accumulation of extracellular potassium ions, which is one of the most convincing mechanisms to understand seizure suppression phenomena because of electrical stimulation. Computer simulation using a neuronal network model also confirmed the mechanism. Copyright (C) 2017 Wolters Kluwer Health, Inc. All rights reserved.</P>

2012년 봄 가뭄 분석

안소라 ( Sora Ahn ),김대일 ( Daeil Kim ),이관호 ( Gwanho Lee ),김성준 ( Soengjoon Kim ) 한국농공학회 2012 한국농공학회 학술대회초록집 Vol.2012 No.-

기후변화에 의해 가뭄은 찾아오는 주기가 짧아지고, 지속기간이 더 길어지면서 그 강도도 더욱 심해질 것으로 전망되고 있다. 이는 과거에 우리가 알고 있던 가뭄으로 농업용수를 개발하면 이제는 필요로 하는 충분한 농업용수의 공급을 감당할 수 없다는 것을 의미한다. 또한 그동안 10년 빈도 수준의 가뭄에 대응하도록 개발해 왔던 저수지, 양수장 등의 주요 수리시설들도 앞으로 다가올 가뭄에는 취약할 수밖에 없다. 실제로 올 봄에는 기상 관측이 시작된 이래로 최악의 극심한 가뭄이 발생되었다. 가뭄이 가장 극심했던 6월의 강수량은 74.7mm로 평년(158.6mm)의 50% 수준으로 기록되었고, 6월말 전국 농업용 저수지 저수율은 39%로 평년 60.9% 대비 21.9%가 낮은 수치이며, 전년대비로는 36.5%가 낮은 기록이었다. 특히 강수량이 적었던 경기, 충남, 전북지역의 저수율이 낮아서 가뭄이 지속된 것으로 분석되었다. 이에 본 연구에서는 가뭄상황을 평가하고 가뭄의 시간적, 공간적인 규모를 파악하기 위해 가뭄지수 및 위성영상을 활용하여 2012년 봄 가뭄 상황을 분석하였다. 먼저 SPI(Standardized Precipitation Index)와 WADI(Water supply Drought Index)를 이용하여 봄 가뭄을 분석한 결과 SPI와 WADI 모두 5월 말부터 약간씩 가뭄이 진행되어 6월 말에 아주 극심한 가뭄으로 진행되는 것을 알 수 있었다. 기상학적 가뭄지수인 SPI의 경우 6월 31일 전국평균 34mm가량의 비로 시작되어 전국적으로 내린 비로인해 7월 중순 이후에는 가뭄이 거의 해갈되었지만, 용수공급을 고려한 가뭄지수인 WADI의 경우에는 7월 말인 현재까지도 농업용저수지 유역에는 극심한 가뭄 나타내었다. 이처럼 농업용저수지의 가뭄은 7월에 내린 비로는 바로 해갈되지 않고 계속 극심한 가뭄으로 표현되는 것을 알 수 있다. 두 번째로 MODIS 위성영상의 NDVI(Normalized Difference Vegetation Index)와 EVI(Enhanced Vegetation Index)를 이용해 한반도 봄 가뭄을 분석하였다. NDVI와 EVI 각각 2012년 올해의 값과 과거 극심한 가뭄으로 기록되는 해(2001, 2008, 2009년)를 제외한 평년의 값을 비교분석하였다. 그 결과 올 봄 4월말∼5월말 50년 만의 가뭄이 발생한 북한 신의주, 평양, 서해안 지역의 가뭄을 확인할 수 있었고, 6월에 경기, 충남, 전남, 전북지역과 경북지역 일부가 연한 녹색으로 식생 활력도가 낮은 것으로 분석되어 실제 가뭄이 발생된 지역과 동일하게 가뭄이 발생했음을 알 수 있었다. 특히 6월에 극심한 가뭄을 겪은 충청도 지방을 살펴본 결과 NDVI의 경우 충남, 충북지역 모두 평년과 비교했을 때 가뭄이 가장 극심했던 6월말의 경우 한눈에도 확연히 식생 활력도가 떨어짐을 알 수 있었으며, EVI의 경우에도 식생 활력도가 떨어진 지역을 많이 나타내었다.

First-principles studies of the structural and electronic properties of rigid carbon nanofoam

Sora Park,Kritsada Kittimanapun,Jeung-Sun Ahn,David Tomanek,Young-Kyun Kwon 한국진공학회 2010 한국진공학회 학술발표회초록집 Vol.38 No.-

Rational design of the S,N-heteroacene-based nonfullerene by introducing the fluorine atom for efficient high-performance organic solar cells

Sora Oh,Mufarah Amjad,Taek Ahn,Sang Kyu Lee 대한화학회 2023 Bulletin of the Korean Chemical Society Vol.44 No.5

Abstract Fluorination has been established as an effective structural modification strategy to influence properties and has attracted attention in organic solar cells. Here, we design and synthesize three fused-ring electron acceptors based on ladder-type pentacyclic heteroacenes as the electron-rich unit and 1,1-dicyanomethylene-3-indanones with 0–2 fluorine substituents as the electron-deficient units. Among these three molecules, two fluorine substituents nonfullerene acceptor, DTP-2F exhibit strong absorption with high extinction coefficients and deep HOMO and LUMO level as compared with DTP-0F and DTP-1F. Device studies show that fluorinated nonfullerene acceptors (NFAs) lead to reduced VOC but increased JSC and FF, and therefore, the ultimate influence to efficiency depends on the compensation of VOC loss and gains of JSC and FF. Furthermore, fluorination affect to film morphology and miscibility between donor and acceptor. The fluorination of NFAs is one of the best design method of the donor and acceptor materials for efficient highperformance organic solar cell.

Simple and Versatile Non-Fullerene Acceptor Based on Benzothiadiazole and Rhodanine for Organic Solar Cells

Ahn, Jongho,Oh, Sora,Lee, HyunKyung,Lee, Sangjun,Song, Chang Eun,Lee, Hang Ken,Lee, Sang Kyu,So, Won-Wook,Moon, Sang-Jin,Lim, Eunhee,Shin, Won Suk,Lee, Jong-Cheol American Chemical Society 2019 ACS APPLIED MATERIALS & INTERFACES Vol.11 No.33

<P>Most non-fullerene acceptors (NFAs) are designed in a complex planar molecular conformation containing fused aromatic rings in high-efficiency organic solar cells (OSCs). To obtain the final molecules, however, numerous synthetic steps are necessary. In this work, a novel simple-structured NFA containing alkoxy-substituted benzothiadiazole and a rhodanine end group (BTDT2R) is designed and synthesized. We also investigate the photovoltaic properties of BTDT2R-based OSCs employing representative polymer donors (wide band gap and high-crystalline P3HT, medium band gap and semicrystalline PPDT2FBT, and narrow band gap and low-crystalline PTB7-Th) to compare the performance capabilities of fullerene acceptor-based OSCs, which are well matched with various polymer donors. OSCs based on P3HT:BTDT2R, PPDT2FBT:BTDT2R, and PTB7-Th:BTDT2R achieved efficiency as high as 5.09, 6.90, and 8.19%, respectively. Importantly, photoactive films incorporating different forms of optical and molecular ordering characteristics exhibit favorable morphologies by means of solvent vapor annealing. This work suggests that the new n-type organic semiconductor developed here is highly promising as a universal NFA that can be paired with various polymer donors with different optical and crystalline properties.</P> [FIG OMISSION]</BR>

Density Functional Study of the Structural and Electronic Properties of Various Fullerene Derivatives

Sora Park,Jeung Sun Ahn,Young-Kyun Kwon 한국진공학회 2009 한국진공학회 학술발표회초록집 Vol.37 No.-

Electronic Structures and Vibrational Spectra of Various Fullerene Structures and their Adducts

Sora Park,Jeung Sun Ahn,Young-Kyun Kwon 한국진공학회 2009 한국진공학회 학술발표회초록집 Vol.36 No.-