2020 BK 세미나
2020 BK 세미나 시리즈
12/28 (월): 김 찬 박사 (European X-ray Free-Electron Laser (XFEL) Facility)
12/01 (화): 손창희 교수 (UNIST, 물리학과)
11/30 (월): 김정리 교수 (이화여자대학교 물리학과)
11/26 (목): 남궁곤 교수 (Old Dominion U., Dept of ECE)
11/24 (화): 김재욱 박사 (한국원자력연구원)
11/17 (화): 김헌정 교수 (대구대 물리학과)
11/10 (화): 천세환 박사 (포항가속기연구소 PAL-XFEL)
11/03 (화): 박승룡 교수 (인천대 물리학과)
10/29 (목): 오상현 교수 (U of Minnesota, Dept of EE)
10/27 (화): 진경환 박사 (Center for Artificial Low Dimensional Electronic Systems, IBS)
10/22 (목): 정문석 교수 (성균관대 에너지학과)
10/20 (화): 박혜성 교수 (UNIST, 신소재공학과)
10/15 (목): 서순범 박사 (성균관대 물리학과)
Current Status of Single Particle Imaging Project & 3D Visualization of Phase-Ordering with X-ray Sources
일 시 : 2020년 12월 28일 월요일 11:00
연 사 : 김 찬 박사 (European X-ray Free-Electron Laser (XFEL) Facility)
장 소 : 자연과학관 747호
Single particle imaging (SPI) project with X-ray sources has triggered development of hard X-ray Free-Electron Laser (XFEL) facilities in worldwide. In 2009, the world first angstromwavelength XFEL facility named Linac Coherent Light Source (LCLS) at the SLAC National Accelerator Laboratory was up and running . A few years later, the world’s second and third hard XFELs (SCALA  in Japan and PAL-XFEL  in the Republic of Korea) started their operations. Since 2017, a world best performance XFEL facility, named European XFEL (EuXFEL), has been operational with extreme operating conditions, e.g., a high pulse energy (close to 4mJ/pulse: limited by a radiation safety limit) and a high repetition rate (up to 4.5MHz) . The SPI project has made tremendous progress thanks to the worldwide XFEL facilities although achievable resolution of SPI experiment is still far away from atomic resolution.
In the first part of this seminar, current status and future perspectives of SPI project will be addressed. In the second part of the seminar, 3D visualization of phase-ordering in an Fe-Al alloy by coherent X-ray Bragg ptychography [5,6] will be discussed. This experimental scheme offers the possibility of imaging the phase shift of the radiation scattered by the sample, which is related to the atomic structure such as ordering and lattice strain. Atomic ordering on a sublattice result in the emergence of superlattice reflections that otherwise are forbidden in the bcc structure. The degeneracy of the ordered structures results in domain boundaries that, in addition to the general strain of the lattice, will give a phase shift depending on which reflection is probed. We investigated the lattice strain associated with phase ordered domains by imaging the (002) fundamental and the (001) superlattice reflections of FeAl in the B2 phase . Using the (002) data, the (001) images could be normalized for lattice strain and APDs visualized in 3D for the first time.
Correlated phenomena by relativistic spin-orbit coupling
일 시 : 2020년 12월 1일 화요일 16:00
연 사 : 손 창 희 교수 (UNIST, 물리학과)
장 소 : 온라인 진행
Spins in the presence of relativistic spin orbit coupling can induce new emerging phenomena, such as non-collinear magnetism, metal-insulator transition, topological states, and quantum disordered phases, as well as provide new functionalities for future technologies. One of the key roles the spin-orbit coupling plays in condensed matter is making isotropic Heisenberg spin strongly coupled to charge/orbital/lattice degrees of freedom, generically anisotropic. Here, we will present how strong coupling among spin, charge, lattice, orbital degrees of freedom via spin-orbit coupling can induce new quantum phenomena including Lifshitz-type metal-insulator transition, non-collinear magnetism, two-dimensional ferromagnetism, and flexomagnetic phenomena in 4d and 5d transition metal oxides.
Hunting for black hole binaries with gravitational waves
일 시 : 2020년 11월 30일 월요일 16:00
연 사 : 김 정 리 교수 (이화여자대학교 물리학과)
장 소 : 자연과학관 746호
The discovery of GW150914 showed some black holes do merge and the coalescence of a binary black hole (BBH) can be "directly observable" with gravitational waves (GWs). Although imaging is not possible with GW observation, masses of individual black holes in a binary can be measured by GW observation with impressive accuracy. With the mass and distance estimates of many BBHs, we can apply the 'standard siren' method to measure a Hubble constant , which was first suggested by Bernard Schutz in 1986. Stellar-mass BBHs are observable with km-scale laser interferometry in the GW frequency range of 10-2000 Hz. Supermassive BBHs would be observable by precision timing of millisecond radio pulsars at nano Hertz frequencies of GW signals. In this talk, I will give an overview on the second Gravitational-Wave Transient Catalog (GWTC-2) published in October, 2020 by the LIGO-Virgo collaboration. If time allows, I will try to briefly touch on the pulsar timing array and their expected constraints for GW background formed by superpositions of supermassive BBHs. Then, I will conclude with prospects for BH astrophysics and "GW cosmology" for the next decades.
How to convert perovskite grain boundaries to be thermally stable?
일 시 : 2020년 11월 26일 목요일 16:00
연 사 : 남 궁 곤 교수 (Old Dominion U., Dept of ECE)
장 소 : 자연과학관 208호
The clean energy revolution is taking place around the world in which we are seeking for reliable, affordable and robust energy sources. Although many researchers recently revolutionized organic-inorganic perovskite solar cells, the long-term stability of perovskite solar cells remains a problem. Typically, the solution-process of perovskite thin films inevitably produce structural imperfections and significant spatial variations of chemical and energetic disorders, particularly at grain boundaries (GBs). Although thermal degradation at GBs in perovskite materials has been extensively studied, how to prevent thermal degradation processes at GB has not yet been fully explored. Herein, in this talk, polymer will be utilized as one way to slow down or prevent thermal degradation of perovskite GBs and its underlying mechanisms will be explained.
Helical magnetic state induced by chemical substitution in a polar antiferromagnet (Ni,Mn)3TeO6
일 시 : 2020년 11월 24일 화요일 16:00
연 사 : 김 재 욱 박사 (한국원자력연구원)
장 소 : 온라인 진행
Non-centrosymmetric structure give rise to various physical properties such as piezoelectricity, weak ferromagnetism, and exotic superconductivity. Recently, modulated magnetic structures, such as helical, cycloidal, or skyrmion, have been observed in various systems. Here, we report the synthesis, physical properties, and structural characterization of the antiferromagnet Ni3-xMnxTeO6 (NMTO, x < 1) with polar and chiral crystal structure. Magnetic susceptibility measurements on a single crystal shows that there exists an intermediate planar anisotropy phase that occurs between the antiferromagnetic ordering temperature (TN) at 74 K and a spin-reorientation transition at 62 K, below which the system exhibits an axial anisotropy. Both transitions are detected in dielectric and electric polarization, demonstrating a strong magnetoelectric coupling. Powder neutron diffraction experiment reveals that the intermediate phase consists of an incommensurate spin-helical state propagating along the c axis. First principle calculation shows that Mn ions chooses to occupy a specific transition-metal site. Thus, NMTO serves as a model system to study the design principle of non-collinear magnetic structure by means of chemical doping.
Chiral anomaly effect in electrical transport of Bi0.96Sb0.04
일 시 : 2020년 11월 17일 화요일 16:00
연 사 : 김 헌 정 교수 (대구대 물리학과)
장 소 : 자연과학관 B117호
As an exotic metallic state, the three-dimensional (3D) Weyl metal has pairs of Weyl nodes in the k space and shows topological surface state known as Fermi arcs and anomalous electrical transport originating from chiral anomaly. Ever since observed in Bi0.96Sb0.04, which becomes a 3D Weyl metal in parallel electric (E) and magnetic (B) fields, the negative longitudinal magnetoresistance had been the only transport signature of this exotic metallic state, also observable in TaAs, Na3Bi, Cd3As2 and so on. As another indisputable fingerprint for existence of chiral anomaly, violation of Ohm’s law was reported in Bi0.96Sb0.04 inthe longitudinal configuration (E//B). In this talk, I will discuss how violation of Ohm’s law becomes direct evidence of chiral anomaly. Violation Ohm’s law in Weyl metal is also expected to open a door to possible invention of a novel nonlinear electronic component at low frequencies.
Investigations of magnetic ground and excited states emerging from spin-orbit coupling in strongly correlated electron system
일 시 : 2020년 11월 10일 화요일 16:00
연 사 : 천 세 환 박사 (포항가속기연구소 PAL-XFEL)
장 소 : 자연과학관 747호
Spin-orbit coupling is a relativistic interaction between spin and orbital motion of a particle. In strongly correlated electron system, the coupling between an electron’s spin and orbital degrees of freedom is responsible for unconventional magnetic phenomena such as magnetoelectric (ME) effects and topological magnetic orders. Hexaferrites feature a giant magnetoelectricity at room temperature enabling control of substantial electric and magnetic polarizations by magnetic and electric fields, respectively. Apart from these static effects, the hexaferrites host a novel dynamic ME effect, i.e. electromagnon – a magnon excited by modulating electric dipole. I will discuss investigation of both static and dynamic ME effects in Y-type and Z-type hexaferrites via multiple experimental techniques such as electric/magnetic transport and terahertz optical spectroscopy measurements. Magnetic ground states of honeycomb and pyrochlore iridates involve topological magnetic orders such as quantum spin liquid and Weyl semimetal states, respectively, constructed with spin-orbit entangled Jeff = 1/2 state. Their relevance to the topological orders are manifested in the magnetically excited states. Resonant inelastic x-ray scattering (RIXS) serves as an essential probe for the excited states allowing one to identify momentum dependence of the excitations. The RIXS studies of honeycomb Na2IrO3 and Li2IrO3, and pyrochlore Eu2Ir2O7 will be presented. Lastly, I will introduce progress in study of ultrafast magnetic dynamics using PAL-XFEL. X-ray free electron laser (XFEL) is a new generation x-ray source that has key features of femtoseconds pulse and high brightness. The capability of optical laser pump and XFEL probe experiment allows one to access in the time domain the intriguing magnon excitations. Our observation of coherent magnons in hexaferrites and iridates demonstrate a new capacity of magnetic spectroscopy opening a gateway to ultrafast magnetic dynamics in magnetic materials.
Experimental observation of the Berry curvature by angle- resolved photoemission
일 시 : 2020년 11월 03일 화요일 16:00
연 사 : 박 승 룡 교수 (인천대 물리학과 )
장 소 : 자연과학관 208호
Angle-resolved photoemission spectroscopy (ARPES) has been now widely used to map electronic band dispersions in momentum space in various materials such as 2D transition metal dichalcogenides (TMD). ARPES is being developed in a couple of ways to obtain more informations of solids. Circular dichroism (CD) in ARPES is one of them to obtain the information of wave function character such as the Berry curvature. In this talk, basic principles of ARPES will be briefly introduced. After that, recent results on observation of the Berry curvature in a TMD, 2H-WSe2 by using CD-ARPES will be discussed. [1, 2]
Nanogap Resonators for Extreme Polaritonics
일 시 : 2020년 10월 29일 목요일 16:00
연 사 : 오 상 현 교수 (U of Minnesota, Dept of EE )
장 소 : 자연과학관 747호
I will present experimental results obtained from gold nano-coax resonators for applications in mid-infrared sensing, molecule trapping, and silicon photonics waveguide integration.
In the second part, I will describe a new device structure – image polariton resonator – that can be coupled with graphene, hexagonal boron nitride, or other 2D materials for extreme field confinement via ultra-confined polaritons.
Topological superconducting phase in high-Tc superconductor MgB2 with Dirac–nodal-line fermions
일 시 : 2020년 10월 27일 화요일 16:00
연 사 : 진 경 환 박사 (Center for Artificial Low Dimensional Electronic Systems, IBS)
장 소 : 자연과학관 747호
Topological superconductors are an intriguing and elusive quantum phase, characterized by topologically protected gapless surface/edge states residing in a bulk superconducting gap, which hosts Majorana fermions. Unfortunately, all currently known topological superconductors have a very low transition temperature, limiting experimental measurements of Majorana fermions. Here we discover the existence of a topological Dirac–nodal-line state in a well-known conventional high-temperature superconductor, MgB2. First-principles calculations show that the Dirac–nodal-line structure exhibits a unique one-dimensional dispersive Dirac–nodal line, protected by both spatial-inversion and time-reversal symmetry, which connects the electron and hole Dirac states. Most importantly, we show that the topological superconducting phase can be realized with a conventional s-wave superconducting gap, evidenced by the topological edge mode of the MgB2 thin films showing chiral edge states. Our discovery may enable the experimental measurement of Majorana fermions at high temperature.
Unveiling the Physics of Emerging Materials with Novel Spectroscopy
일 시 : 2020년 10월 22일 목요일 16:00
연 사 : 정 문 석 교수(성균관대 에너지학과)
장 소 : 자연과학관 747호
Since the discovery of new nanomaterials such as carbon nanotubes and graphene, due to their unique properties, researchers have tried a lot to find another new material that can outperform conventional materials. Recently, research on new materials such as transition metal dichalcogenides and tellurene, which are two-dimensional semiconductors, has been actively conducted. In addition, halide perovskite, which is used in photovoltaic devices, is also receiving a lot of attention. Research on such a new substance begins with finding a new physics hidden in it, and up to presenting applicability using the found physics. Our study aims to peek into new physics using novel spectroscopic equipment in emerging materials, and to present a perspective on applicability. In the presentation, I will introduce the research on emerging materials with tip enhanced Raman spectroscopy and IR nanoscopy. In addition, deep learning applied correlation spectroscopy approach for finding unknown physics will be provided.
Low-dimensional Nanomaterials into Functional Devices
일 시 : 2020년 10월 20일 화요일 16:00
연 사 : 박 혜 성 교수(UNIST, 신소재공학과)
장 소 : 자연과학관 747호
Low-dimensional nanomaterials have attracted widespread attention in various fields of research endeavors owing to their unique and desirable physical properties. We have synthesized various kinds of high-quality low-dimensional materials using bottom-up (chemical vapor deposition, ligand exchange approach, solution phase deposition, and hydrothermal reaction) or top-down method (mechanical and liquid phase exfoliations). Furthermore, we modulated the physical and optical properties of low-dimensional materials through heterostructure formation, phase transition, vacancy formation, functionalization, and chemical composition control, and demonstrated several applications of property-modulated low-dimensional nanomaterials including ultrasensitive chemical sensor and electrochemical water splitting with outstanding stability and performance. We also investigated promising possibility of low-dimensional nanomaterials in various components of photovoltaics including transparent conducting electrode, charge transporting layer, and interfacial buffer layer. The low-dimensional nanomaterials enhanced the performance and flexibility of functional devices through the enhanced charge transfer efficiency, mechanical robustness, and light absorption capability.
Study of the relation between magnetism, superconductivity and quantum criticality on 4f-electron compounds under extreme conditions
일 시 : 2020년 10월 15일 목요일 16:00
연 사 : 서 순 범 박사 (성균관대 물리학과)
장 소 : 자연과학관 747호
In condensed matter physics, novel quantum states of matter may emerge near zero temperature, including unconventional superconductivity, complex magnetic order, and non-Fermi liquid behavior. In order to create predictive understanding of these new phases unexpected by conventional theories, proper materials and control parameters are necessary. Here we demonstrate methods of investigating new quantum states of matter in f-electron systems using a variety of experimental techniques under extreme conditions of low temperature, hydrostatic pressure, and high magnetic field. We discuss results that reveal quantum states near the quantum critical point (QCP) of an f-electron compound at which a quantum phase transition is driven by quantum fluctuations. In particular, doping effects on the pressure induced antiferromagnetic QCP in CeRhIn5 have been studied to probe the relationship between QCP and superconducting phase. In addition, we discuss results that exhibit novel quantum states in various Ce based compounds under pressure, including nematic phase coupled with magnetism and pseudogap phase near a superconducting phase, akin to high-Tc superconductors. In particular, recently discovered nematic state in CeAuSb2 will be discussed through the experimental results as well as theoretical model.