마일스톤
번호 Milestone 명 수행기간
시작일 종료일 책임자
1 정온식 단독형 MIT 화재 감지기 2018.02.01 2018.11.30 김현탁 1.1 MIT 감지기 소방산업기술표준 특성평가 2018.02.01 2018.10.31 조성우 1.2 소방산업기술원 형식승인 인증성능시험 통과 2018.06.31 2018.11.30 정순규 2 MIT 센서 이용 연기 감지기 2018.02.01 2018.11.30 김현탁 2.1 소방산업기술표준 특성평가 2018.02.01 2018.10.31 김현탁 2.2 소방산업기술원 형식승인 인증성능시험 통과 2018.06.31 2018.11.30 정순규
마일스톤
번호 Milestone 명 수행기간
책임자
시작일 종료일
3 레퍼런스 레이더 플랫폼 2018.02.01 2018.11.30 구본태
3.1 레이더 RF 프론트엔트 시스템 개발 2018.02.01 2018.8.31 한선호 3.2 레이더 신호처리 플랫폼 개발 2018.06.31 2018.11.30 김덕환
4 레이더 IP 설계 2018.02.01 2018.11.30 구본태
4.1 레이더 RF IP 2018.02.01 2018.11.30 한선호
4.2 레이더 신호처리 알고리즘 IP 2018.02.01 2018.11.30 어익수
제 3 장 연구개발 추진 실적
- Nature Communications vol. 9, 3604 (2018) - 상 위 5% 논문
o MIT 연기 감지기
정 펌웨어 작성 Impurity induced MIT 연구
• Analysis of the diverging effective mass on in YaBa2Cu3O6+x for high-Tc mechanism and pairing symmetry (Int. J. of Modern Phys. B 32, No. 17, 1840031 (2018)) - 이 논문은 Mott MIT 연구와 Impurity-induced MIT와 그 응용이 포함 된 연구논문 (30년 이상 물리문제)
100%
• 강상관 초전도 메카니즘 (30년 이상 물리문제) Intrinsic electronic Raman spectrum and pairing symmetry in cuprate
superconductors (30년 이상 물리문제) :Results in Physics 저널에서 Review 중
100% frequencies, Nature Communications 9, 3604 (2018): 상위 5% 논문
100%
Highly repeatable nanoscale phase coexistence in vanadium dioxide films, Phys. Rev. B 97, 085146
(2018), 미국물리학회 최고논문 100%
<제 2 세부>
: 거리 분해능 = 1m : (PRI=250us, FMCW BW=150MHz, ADC
sampling clock=12.5MHz)
[결과물] FMCW generator IP core
o FMCW generator IP core 설계 및 제작
[결과물] FMCW generator IP core : 구조 – Fractional-N PLL 기반 FMCW
o RF Transmitter IP core (2x1 array) 설 계 및 제작
[결과물] RF Transmitter IP core
- Phase Shifter 2x2 array single chip - 4-채널 RX single chip
0.5km 이상의 드론 Detection 시험 완료 - 탐지각도시험(Azimuth: 90도 / Elevation:
45도) 완료
: RF 하드웨어 플랫폼과 신호처리 하드웨어 플랫폼의 연결 시험
: 필드 성능 시험 완료 (0.5km이상 거리 탐지 성능 확인)
제 2 절 주요 연구결과
1. MIT 기술가. MIT 현상규명
1) Mott MIT와 Impurity-Induced MIT연구
: Int. J. Modern Phys. B 32, 1840031 (2018) [요약]
금속의 자유전자들 사이에서 쿨롱 반발에너지가 매우 큰 금속을 강상관 금속이라고 하고 어떤 임계값을 넘으면 면 부도체 즉 절연체가 된다. 이 절연체를 모트 절연체라고 하며 이 현상을 설명하는 것이 응집물질 물리학의 오랜 물리문제이다. 그런데 30 년이상 물리문제인 고온 초전도 현상을 보이는 물질의 모체 물질은 모트 절연체로 알려져 왔고, 그 모트 절연체에 도핑, 압력, 온도를 가하면 모트 절연체에서 금속으로 전이(Metal-Insulator Transition: MIT)가 일어난다. 그 금속이 저온에서 고온 초전 도 현상을 보인다. 이 메카니즘을 설명하는 것이 30년 이상 미해결 문제이다.
작성자(김현탁)는 이 문제에 대해서 모트 MIT를 BR(Brinkman-Rice picture)픽쳐를 확장하여 Impurity-induced (Hole-driven) MIT를 발견하고 그것을 기반으로 30년 이상의 물리문제를 설명한다. 그것에 대한 Impurity-driven MIT를 간략하게 설명한다.
<그림 1> Drawing of Eq. (1) in the extended BR picture [1,2].
impurity doped insulator. The metal-insulator transition (MIT) is shown at red dot line between doped insulator and metal specifying excitation. nc º NcµDris the doping concentration for the MIT. The quantum critical point is given at the transition point.(c) The antiferromagnetic Mott insulator with the Mott gap of Ucis assumed at U/Uc=1 in the BR picture (Fig. 2c), as denoted by black dot in Fig. 2b. Fig. 2dshows a band structure of an impurity-doped Mott insulator (red dot in Fig.2b) with both the main Hubbard bands for direct transition and an impurity (or extrinsic semiconductor) band for indirect transition. UB – upper Hubbard band, LB – lower Hubbard band[40], EF–Fermilevel,Δdirect– energy gap for direct transition, Δact– activation energy for indirect transition, Ω – thermal phonon. Impurity concentration Ntot=Ncis proportional to Dr=Ntot/ntotintheEBRpicture,wherentotis the carrier density in the main Hubbard band. The IMT (or MIT) is indirect between Fig. 2dand Fig. 2e, which names the indirect Mott MIT [5]. For a strongly correlated insulator of VO2, the IMT criterion, Ntot=Nc,is 0.018% [4].
[1] H T Kim, Physica C 341-348 (2000) 259.
[2] H. T. Kim, http://arxiv.org/abs/cond-mat/0110112.
[3] H. T. Kim, B. G. Chae, D. H. Youn, S. L. Maeng, G. Kim, K. Y. Kang, Y. S. Lim, New J. Phys. 6 (2004) 52.
[4 M. Kang and S. W. Kim, J W Ryu, J. Appl. Phys. 118 (2015) 035105.
[5] H. T. Kim, M. Kim, A. Sohn, T. Slusar, G. Seo, H. Cheong, D W Kim, J. Phys.: Condens. Matter 28 (2016) 085602 (2016).
Impurity-driven Insulator-to-Metal Transition in VO2
How is a relation between impurities and the insulator-to-metal transition (IMT) explained in strongly correlated systems ? A representative strongly correlated Mott insulator VO2(3d1) has the direct gap (DdirectµVdirect) of 0.6 eV and the indirect gap (activation energy) of Dact/2µVindirect»0.15 eV coming from impurity indirect band (see Fig. 4c in [1]). At Tc, Ddirect=Dact=0 is satisfied and the IMT occurs. The metal carriers near core region can be trapped when the critical onsite Coulomb repulsion Ucbetween carriers exists; the metal become a Mott insulator. Then, a potential energy for the Mott insulator can be defined as
Vg=(Vdirect+Uc)+Vindirect,= -(2/3)EF(1 + e(Ntem(T)/ntot)) +Uc,
= -(2/3)EF(1 + e(Ntot/ntot) (1-exp(-Dact/kBT))) +Uc, --- (1)
where Vdirect= -(2/3)EFcomes from the screened Coulomb pseudopotential at K=0. Dr=Ntot/ntot»0.018% is defined as the critical impurity doping quantity [2], where ntotis the bound charge density in the direct d-band and Ntotis the bound charge density in the impurity indirect band (see Fig. 4c in [1]). Vindirect=-(2/3)EFe(Ntem(T)/ntot)is calculated by the Tayler-series expansion of the chemical potential μ when impurity carriers in metal exist, where Ntem(T)=Ntot(1-exp(-Dact/kBT)) is defined.
When Vg=0 at Eq. (1), Uc=-(Vdirect+Vindirect)is given.
Then, Uc= (2/3)EF(1 + e(Ntot/ntot) (1-exp(-Dact/kBT))),
= (2/3)C(ntot+Ntot)2/3(1 + e(Ntot/ntot) (1-exp(-Dact/kBT))) is expressed in terms of Ntot, where EF=C(ntot+Ntot)2/3isdefinedandCis a proportional constant.
At the IMT, since Dact=0is givenand Ntotis excited,
Uc=(2/3)C(ntot+Ntot)2/3is reduced as U=(2/3)C(ntot)2/3<Uc.
Then, the correlated Mott insulator becomes metal by the breakdown of Uc-->Uinduced by excitation of Ntotfrom bound state to conduction band. The IMT can be switched by the doping (excitation; Dact»0, Ntotgoes to conduction band, so Ntot=0) and the de-doping (de-excitation;Dact»0.15, Ntotis bound from conduction band to indirect band) of Dr=Ntot/ntotto the bound state, by applying external parameters such as heat, pressure, doping etc. The Mott insulator with the metallic electronic structure is formed by bounding the carriers of ntotin the metal state trapped by the impurity carrier density Ntot; this is an impurity-driven IMT and can be applied to all strongly correlated systems.
[1] Hyun-Tak Kim, Minjung Kim, Ahrum Sohn, Tetiana Slusar, Giwan Seo, Hyeonsik Cheongand Dong-Wook Kim, J.
Phys.:Condens. Matter 28 (2016) 085602.
[2] Hyun-Tak Kim, Byung-Gyu Chae, Doo-Hyeb Youn, Sung-Lyul Maeng, Gyungock Kim, Kwang-Yong Kang, Yong-Sik Lim,New J. Phys. 6 (2004) 52.
Diagram explaining the high-Tc mechanism for the formation of the node gap
The dx2-y2 electronic structure can be formed when the metal-insulator transition(MIT) occurs at the node in an isotropic pseudogap structure (bluedashedring). The small pink circles in the large pink circle are regarded as the nodal Fermi points made by the d-wave MIT near doping xc. The pink circle is the Fermi surface formed by increased doping. The red-dashed arrow indicates that bound charges in the pseudogap potential at the node are excited to the Fermi energy, due to the d-wave MIT (conceptual indication). The small green circles in the large green circle are superconducting intrinsic gaps at the node when the nodal Fermi points become a superconductor (pink circle -> green circle) [1,2]. The green ring is the isotropic superconducting s-wave-like gap resulting from the Fermi arc at optimal doping. If the superconducting energy gap has dx2-y2-wave-pairing symmetry, the d-wave-MIT should occur at the anti-node. However, this research does not support the d-wave pairing symmetry. The constant maximum carrier density at the nodal Fermi point (or velocity) was first disclosed [1].
<그림 2> A mechanism of the node gap formation
metal-insulator transition from the pseudogap insulator to metal at node.) [2] H. T. Kim, B. J. Kim, K. Y. Kang, Physica C460-462 (2007) 943.
2) Highly Repeatable Nanoscale Phase Coexistence in Vanadium Dioxide Films
(Phys. Rev. B 97, 085146 (2018)): ETRI-Wiliam & Marry U. 국제공동연구 [요약]
강상관 모트 절연체는 불균일한 것이 특징이다. 그런데 그 불균일한 물질로 소자를 만들면 재현성이 떨어질 수 있기 때문에 이 것이 문제가 되어 왔다. 모트 절연체 VO2 물질로 나노급으로 불균일성을 조사한 결과, 그 불균일한 정도는 나모급 정도로 작았 다. 또 소자를 만들어서 온도를 올리면서 MIT 현상을 일으키고 다시 온도를 낮추면서 측정한 결과 같은 나노급 이미지를 보여 주었다. 그래서 비록 나노급 불균일성이 있다고 해도 소자에서 재현성이 있음을 처음으로 밝혔다.
It is generally believed that in first-order phase transitions in materials with imperfections, the formation of phase domains must be affected to some extent by stochastic (probabilistic) processes. The stochasticity would lead to unreliable performance in nanoscale devices that have the potential to exploit the transformation of physical properties in a phase transition. Here we show that stochasticity at nanometer length scales is completely suppressed in the thermally driven metal-insulator transition (MIT) in sputtered vanadium dioxide (VO2) films. The nucleation and growth of domain patterns of metallic and insulating phases occur in a strikingly reproducible way. The completely deterministic nature of domain formation and growth in films with imperfections is a fundamental and unexpected finding about the kinetics of this material. Moreover, it opens the door for realizing reliable nanoscale devices based on the MIT in VO2 and similar phase-change materials
[본문 설명]
It is generally believed that in first-order phase transitions in materials with imperfections, the formation of phase domains must be affected to some extent by stochastic processes. The stochasticity would lead to unreliable performance in nanoscale devices that have the potential to exploit the transformation of physical properties in a phase transition. Here we show that stochasticity at nanometer length scales is completely suppressed in the thermally driven metal-insulator transition (MIT) in sputtered vanadium dioxide (VO2) films. The nucleation and growth of domain patterns of metallic and insulating phases occur in a strikingly reproducible way. The completely deterministic nature of domain formation and growth in films with imperfections is a fundamental and unexpected finding about the kinetics of this material. Moreover, it opens the door for realizing reliable nanoscale devices based on the MIT in VO2 and similar phase-change materials. To investigate the above mentioned, we image the patterns formed by coexisting metallic and insulating domains in the thermally driven MIT in a sputtered VO2 film using the technique of scattering-type scanning near-field infrared microscopy (S-SNIM). Fig. 6 shows the S-SNIM images obtained in the same spatial region on heating or cooling run through the phase coexistence regime of VO2 sample. It can be seen that the patterns are static and stable in time. Moreover, phase domains nucleate and grow reproducibly in separate thermal runs through the MIT. Deterministic factors that are “quenched,” or frozen into the film at the time of growth, alone dictate nucleation and domain patterns.
<그림 3> Near-field infrared amplitude images of the same region at different temperatures are displayed. Higher infrared amplitude corresponds to metallic regions, while lower signals correspond to insulating regions. The signals are normalized to the average signal of the completely insulating 329 K image (not shown). (a) and (b) show separate heating runs. (c) and (d) show separate cooling runs.
Figure 3(b)-(h) shows that the nucleation occurs at precisely the same location in each unidirectional, monotonic temperature excursion across the MIT in a given time interval with a probability (P_N) proportional to e^((-∆G_local^*)⁄(k_B T)) [Fig. 7(h)].
This pertains to homogeneous nucleation, which occurs in the homogeneous bulk, as well as heterogeneous nucleation, which occurs at an imperfection such as an atomic defect or a grain boundary. In heterogeneous nucleation, the free energy of forming the critical nucleus can be altered significantly, thereby reducing the barrier to nucleation. Nevertheless, as long as a barrier to nucleation exists, the process is expected to be inherently stochastic. Thus, it is quite surprising that stochastic nucleation is not observed in our experiments.
Thus, we have shown that phase-domain nucleation and propagation are completely deterministic processes in the thermally driven MIT in a sputtered VO2 film. In such films, quenched disorder can be used to reliably control the spatial distribution and propagation of phase domains. Interestingly, nanoscale spatial inhomogeneity in the ultrafast optically driven MIT suggests that our conclusion can be generalized to the MIT driven by optical pulses. Our work provides a platform for further meaningful exploration into reliable nanoscale VO2 electronic and photonic devices.
<그림 4> (a) Macroscopic thermal hysteresis loop measured via infrared transmission through the film-substrate system (upper panel) and dc resistance of the film (lower panel). (b)–(e): S-SNIM images demonstrating the nucleation sites on heating (b) and (c), and cooling (d) and (e) of the same area as shown in Near-field infrared amplitude images. (f), (g) Local hysteresis width (∆T_c) and local phase-equilibrium temperature (T_c^avg) respectively, for the area shown in (b)–(e). White (blue) circles in (b)–(g) serve to guide the eye to some of the nucleation sites which occur on heating (cooling). (h) Schematic of the free-energy landscape for a domain of characteristic linear dimension r for different types of nucleation sites. Here we make the distinction between heterogeneous nucleation and “barrierless nucleation.” Barrierless nucleation is a special case of heterogeneous nucleation where, unlike the more general case, the nucleation barrier is completely removed, and nucleation occurs deterministically.
3) Imaging the Nanoscale Phase Separation in Vanadium Dioxide Thin Films at Terahertz Frequencies
(Nature Communications 9, 3604 (2018)) : ETRI-Columbia 국제 공동연구
[요약]
20nm의 측정영역 초고속 펨토초 측정 기술로 대표적인 MIT 물질인 VO2에서 TeraHertz (THz) 와 Near Infrared (NIR) 두 영역에서 온도를 올리면서 MIT를 측정한 결과 THz 영역에서 나노급 불균일성이 존재함을 실험적으로 보이고 Dynamic MeanField Theory으로 그 정당성을 밝혔다.
Vanadium dioxide (VO2) is a material that undergoes an insulator–metal transition upon heating above 340 K. It remains debated as to whether this electronic transition is driven by a corresponding structural transition or by strong electron–electron
correlations. Here, we use apertureless scattering near-field optical microscopy to compare nanoscale images of the transition in VO2 thin films acquired at both mid-infrared and terahertz frequencies, using a home-built terahertz near-field microscope. We observe a much more gradual transition when THz frequencies are utilized as a probe, in contrast to the assumptions of a classical first-order phase transition. We discuss these results in light of dynamical mean-field theory calculations of the dimer Hubbard model recently applied to VO2, which account for a continuous temperature dependence of the optical response of the VO2 in the insulating state. In Fig. 5, we show the key experimental data of this work, temperature-dependent images at THz and MIR frequencies of the near-field response of a 100 nm thick VO2 film grown on sapphire. The top row of images are taken at THz frequencies, and the images in the bottom row are taken in the MIR with a commercial scattering near-field optical microscopy (SNOM) using a 10 μm CO2 laser source. The MIR images reveal that upon heating, the VO2 sample
<그림 5> SNOM images of the VO2 IMT. The images are taken at THz (top row) and MIR (bottom row) frequencies during a heating cycle. In all images, the signal at every temperature is normalized to the average signal obtained on gold (bright red region in the upper right or right of the image, for THz and MIR respectively). The dashed yellow line denotes the boundary between VO2 and gold regions. The THz and MIR data are S2 and S3, which is the detected signal demodulated at the second and third harmonic of the tip tapping frequency, respectively. Low near-field signal (blue) is measured in insulating regions, while high near-field signal (red) corresponds to a metallic state. The scales are different for the THz and MIR images to highlight the transition from insulator to metal in both cases. Scale bar, 2 μm.
insulating to metallic signal levels through the same temperature region. A histogram representation of the pixel intensity in each image, as shown in Fig. 5, elucidates this distinction.
At temperatures in the middle of the area-averaged transition, the MIR histograms are bimodal. There is an abrupt change in MIR near-field signal between metallic and insulating domains, represented by the separation between the two peaks in the histograms. The pixels in the THz images, on the other hand, are distributed according to a single Gaussian at all temperatures;
there is no clear separation in THz near-field signal between insulating and metallic domains. We plot the mean of each THz histogram as a function of temperature in Fig. 9c as circles, connected by a dashed line as a guide to the eye. The error in the parameter estimation of the fitted histogram is dominated by random noise in the THz-SNOM measurement, which is approximately 5% of the metallic near-field signal level. The THz near-field signal appears to evolve continuously with temperature.
Thus, using a novel THz-SNOM with 130 nm spatial resolution, we find that the nature of the domain formation through the phase transition in VO2 thin films appears homogeneous and continuous at THz frequencies. Moreover, MIR near-field images
Thus, using a novel THz-SNOM with 130 nm spatial resolution, we find that the nature of the domain formation through the phase transition in VO2 thin films appears homogeneous and continuous at THz frequencies. Moreover, MIR near-field images