위상 물리

• Title: Correlation and Topology in Rhombohedral Multilayer Graphene

• Speaker: Prof. Youngjoon Choi (POSTECH)

• Date & Time / Venue: September 10th (Wed.), 2025, 4:00 PM / Physics Seminar Room(Science Bldg Ⅲ, #302)

• Abstract
  : Rhombohedral-stacked multilayer graphene provides an exceptional platform for exploring correlated electron phenomena, owing to its flat bands near the Fermi energy and the ability to tune its properties with an external electric field. Its valley-dependent Berry phase further suggests the emergence of topological states when isospin symmetry is broken by electron correlations. In this talk, I will present recent discoveries of novel quantum phases in rhombohedral multilayer graphene, including superconductivity as well as integer and fractional quantum anomalous Hall effects. I will then discuss recent STM measurements that directly visualize intervalley coherent states, offering new insights into the possible origin of superconductivity. These findings open fresh avenues for investigating the interplay between correlation and topology in two-dimensional crystals, with potential implications for realizing topological superconductivity.

• Title: Non-Lorentzian field theory: gravity and topological matter

• Speaker: Dr. Patricio Salgado Rebolledo(APCTP)

• Date & Time / Venue: July 29th (Tue.), 2025,  2:00 PM / #512, APCTP & Online via ZOOM

• ZOOM Link: https://us06web.zoom.us/meeting/register/98YoBRY7QiWL3nVE_Xt4ng

• Abstract
  : Non-Lorentzian field theories and geometries have attracted increasing attention in recent years due to their broad applications in high-energy physics, hydrodynamics, cosmology, and condensed matter systems. In this talk, I will provide an overview of my research, focusing on the applications of non-Lorentzian field theory in the description of gravity and in the construction of effective models for certain topological phases of matter. I will discuss my current research goals and ongoing projects in these areas, and outline my future research directions.

• Title: Toward a Floquet-topological Insulator in Three-terminal Graphene Josephson Junction Driven by Polarized Microwave

• Speaker: Dr. Park Dongsung (POSTECH)

• Date & Time / Venue: July 25th (Fri.), 2025,  1:00 PM / Physics Seminar Room(Science Bldg Ⅲ, #302)

• Abstract
  : 응집계 물리에서 합성 차원은 물질의 고유 특성을 넘어서는 상전이 현상을 가능하게 하며, 비전통적인 방식으로 토폴로지 상을 구현할 수 있도록 합니다. 예컨대, 플로케 공학은 복제 밴드와 비평형 다체 효과 사이의 상호작용을 활용하며 [1,2], 다단자 조셉슨 접합은 초전도 위상의 조합을 통해 비일반적인 안드레브 밴드를 형성할 수 있습니다 [3,4]. 이러한 시스템들은 각각 위상학적 특성을 나타내는 것으로 보고되었지만, 플로케 구동에 따른 발열 문제는 플로케 상과 그 동역학의 실험적 탐구를 어렵게 만들어 왔습니다. 그러나 최근 연구에서는 그래핀 기반 조셉슨 접합이 마이크로파 대역에서 발열 없이 안정적인 플로케-안드레브 밴드를 형성함을 보여주었으며 [5], 이를 통해 플로케와 안드레브 동역학의 상호작용을 통한 새로운 위상상 탐색에 이상적인 플랫폼 가능성을 제시하였습니다. 본 연구에서는 삼단자 그래핀 조셉슨 접합을 기반으로, 단자 간 위상차가 형성하는 안드레브 밴드에 편광된 마이크로파를 인가함으로써, 전적으로 합성 차원 내에서 비자명한 플로케 토폴로지 상을 구현하고자 합니다. 이를 위해 본 연구진이 개발한 마이크로파 제어 기술, 고품질 소자 제작법, 그리고 그래핀 조셉슨 소자의 플로케 이론은, 물질 본연의 성질에 국한되지 않는 차세대 양자소자의 구현 가능성을 제시합니다.

• Title: Transport and thermodynamics in topological magnets

• Speaker: Dr. Kamran Behnia (CNRS & ESPCI)

• Date & Time / Venue: June 16th (Mon.) 4:00 PM / Physics Seminar Room(Science Bldg Ⅲ, #302)

• Abstract
  : Like their electric counterpart, the anomalous Nernst and the anomalous thermal Hall effects are set by the Berry spectrum of the magnetic solid. Correlations between the amplitude of the three anomalous transverse transport coefficients have been observed in numerous topological magnets, such as Mn3X (X=Sn,Ge), Co3Sn2S2, Co2MnGa and YbMnBi2. Onsager reciprocity imposes relations between their amplitude in different configurations.
  Magnetostriction, which refers to a change in the lattice parameter induced by the magnetic field is intimately linked to piezomagnetism. Both arise when the elastic energy is field-dependent. Maxwell relations imply concomitance between linear magnetostriction and piezomagnetism. Experiments on Mn3Sn confirm this expectation.  Finally torque magnetometry emerges as a probe of the angle dependence of magnetic energy. We will review an experiment employing this technique to detect the field-induced distortion of spin texture in Mn3Sn.

• Title: In Search of Majorana Fermions: Topological Superconductors and Quantum Computing

• Speaker: Prof. Sangmo Cheon (Hanyang University)

• Date & Time / Venue: May 29th (Thu.), 2025, 4:00 PM / Room 104(Auditorium), IBS POSTECH Campus Building

• Abstract
  : The quest for topological superconductors (TSCs) and Majorana zero modes (MZMs) lies at the heart of condensed matter physics and quantum computation. MZMs, which are neutral and obey non-Abelian statistics, offer a robust foundation for topological qubits immune to local noise [1]. This talk begins with Kitaev’s chain model—the simplest setting in which MZMs arise via topological phase transitions—and expands to realistic platforms including semiconductor-superconductor hybrid nanowires, 2D topological insulator-superconductor interfaces, etc. Particular focus is placed on FeTe₁₋ₓSeₓ, where topological surface states coexist with s-wave superconductivity, allowing for both vortex-bound and dispersing 1D MZMs along domain walls [2]. Recent advances explore not only external control via magnetic field, pressure, or temperature, but also the role of subsymmetry and crystalline symmetries in stabilizing or manipulating MZMs. Symmetry engineering has thus emerged as a key tool in designing new platforms for Majorana physics [3]. Finally, we focus on Majorana fermions in higher-order topological insulators, particularly MoTe₂. Surface superconductivity, driven by bulk-to-surface proximity-induced p-wave pairing, reveals higher-order topological phases. Analysis of the Bogoliubov-de-Gennes Hamiltonian shows the evolution of third-order topological hinge states into zeroenergy Majorana corner states [4]. We conclude by outlining how these developments connect to quantum computation.
  
  [1] J. Alicea, New directions in the pursuit of Majorana fermions in solid state systems, Rep. Prog. Phys. 75, 076501 (2012).
  [2] P. Zhang et al., Observation of topological superconductivity on the surface of an iron-based superconductor, Science 360, 182 (2018).
  [3] S.-H. Han, M. Kang, MJ Park, S. Cheon*, Quantized polarization and Majorana fermions beyond tenfold classification, Communications Physics, 7, 243 (2024)
  [4] S Lee, M Kang, DY Kim, J Kim, S Cho, S Cheon*, T Park*, Evidence of surface p-wave superconductivity and higher-order topology in MoTe, Nature communication under review

• Title: Quantum Geometry in Solid-state Physics

• Speaker: Prof. Jun Won Rhim (Ajou University)

• Date & Time / Venue: April. 21st (Mon.), 2025, 4:00 PM / Room 104(Auditorium), IBS POSTECH Campus Building

• Abstract
  : The geometric characteristics, such as topology, of Bloch wavefunctions play crucial roles in the properties of electronic transport. The Hilbert-Schmidt quantum distance, one of the representative geometric quantities, has been shown to be crucial in Landau level structures[1,2] and bulk-boundary correspondence[3,4]. In this talk, I will show that the Hilbert-Schmidt quantum distance is also critical in transport properties, such as thermopower, DC and AC conductivities, of quadratic band-touching systems including the singular flat band systems[5-7]. 

[1] J.-W. Rhim et al., Nature 584, 59 (2020).
[2] Y. Hwang et al., Nature Communications 12, 6433 (2021).
[3] C.-g. Oh et al., Communications Physics 5, 320 (2022).
[4] H. Kim et al., Communications Physics 6, 305 (2023).
[5] C.-g. Oh et al., Advanced Science 11, 2411313 (2024).
[6] C.-g. Oh et al., submitted. (2025).
[7] Y. Kim et al., to be submitted. (2025).

• Title: Topological solitons towards dissipation-less informatics

• Speaker: Prof. Han Woong Yeom (POSTECH)

• Date & Time / Venue: March. 19th (Wed), 2025, 4:00 PM / Physics Seminar Room(Science Bldg Ⅲ, #302)

• Abstract
  : We witness that the silicon device technology, the foundation of current information civilization, is close to its end with the limit of device miniaturization and the explosive growth of the energy consumption in processing information.  While post-Si device architectures such as TMDC devices are under active development, technological and fundamental breakthroughs are requested for devices consuming much less energy. In the physics point of view, it is a challenge to find quasi-particles and their device platforms, that can carry information in higher density and without energy dissipation. While there are a few candidates such as photons, Copper pairs, various quantum Hall currents, spin currents, and excitons, we have proposed topological solitons in 1D and 2D materials as one new direction. In 2013-2017, we made it possible to microscopically observe individual topological solitons in 1D materials for the first time since its discovery in 1979. Until 2022, we developed this research to secure model 1D systems for a few different types of microscopically accessible solitons and to track their motions. Our recent work demonstrates the manipulations of solitons and their interactions, which may open a way toward soliton technology in electronic systems. On the other hand, these works open a research field where one can study structures, electronic states, kinetics, dynamics, and interactions of individual solitons. At the end of the talk, we show that the soliton concepts are helpful in understanding the physics of topological domain walls in complex 2D quantum materials.
  
  References
  T. H. Kim and H. W. Yeom., Phys. Rev. Lett. 109, 246802 (2012).
  S. M. Cheon, S. H. Lee, T. H. Kim, and H. W. Yeom, Science 350, 182 (2015).
  T. H. Kim, S. M. Cheon, and H. W. Yeom, Nature Physics 13, 444 (2017).
  J. W. Park et al., Nature Nanotechnology 17, 244 (2022).
  T. H. Im, and J. W. Park, and Han Woong Yeom, Nature Commun. 14, 5085 (2023).
  T. H. Im, and J. W. Park, and Han Woong Yeom, submitted.

• Title: Theory, prediction and detection for topological and chiral phonons

• Speaker: Prof. Tiantian Zhang (Chinese Academy of Sciences)

• Date & Time / Venue: January 17th (Fri.) 3:00 PM / Physics Seminar Room(Science Bldg Ⅲ, #302)

• Abstract
  : The effective control of phonons not only enhances our understanding of physical processes like thermal conductivity and electroacoustic coupling but also advances potential applications. However, the inherent properties of "zero spin" and electrical neutrality limit phonon manipulation options. Around 2018, the integration of topological band theory into solid phonon spectra and the development of first-principles methods for calculating topological phonons in real materials expanded the scope of phonon modulation [1]. This introduction of "topology" as a degree of freedom in phonon systems paved the way for manipulation. In the first part of my talk, I will discuss theoretical and experimental findings on various topological phonon materials, including FeSi with double-Weyl phonons [1-2], BaPtGe with twofold quadruple Weyl phonons [3-4], and MoB2 with node-line phonons [5].
  In addition to "topology," phonons can exhibit "chirality," with chiral phonons playing a crucial role in scattering processes. Past studies primarily focused on 2D systems with zero group velocities, limiting practical applications in information dissemination [6]. The second part of my talk will extend chiral phonon research to three-dimensional chiral crystal systems [7], confirming the existence of high-velocity chiral phonons at the Brillouin Zone center in α-HgS [8] and elemental Te [9]. The exploration also delves into the relationship between Weyl phonons and chiral phonons [9]. Lastly, a perspective on the physical phenomena triggered by chiral phonons will be provided [10-12].
  
  

References：
[1]. T. Zhang, et al., Phys. Rev. Lett., 120, 016401 (2018)
[2]. H. Miao, T. Zhang, et al., Phys. Rev. Lett., 121, 035302 (2018)
[3]. T. Zhang, et al., Phys. Rev. B, 102 , 125148 (2020)
[4]. H. Miao, T. Zhang, et al., Phys. Rev. B, 103, 184301 (2021)
[5] T. Zhang, et al., Phys. Rev. Lett., 123, 245302 (2019)
[6] H. Zhu, et al., Science, 359, 579-582 (2018)
[7] T. Zhang and S. Murakami, Phys. Rev. B , 4, L012024 (2022)
[8] K. Ishito, … T. Zhang, … et al., Nat. Phys. , 19, 142-142 (2023)
[9] T. Zhang, et al., Nano Lett., 23, 7561–7567 (2023)
[10] S. Zhang, K. Luo and T. Zhang, npj Comput. Mater. 10, 264 (2024)
[11] M. Che, et al., arXiv:2411.03754 (2024)
[12] R. Yang, et al., arXiv:2410.21775 (2024)

• Title: Tuning Nonequilibrium to Topological Matter

• Speaker: Prof. Kwon Park (KIAS)

• Date & Time / Venue: March 27th (Wed) 4:00 - 5:30 PM / Physics Seminar Room(Science Bldg Ⅲ, #302)

• Abstract
  : Topological matter is one of the most characteristic examples of quantum matter. While various classes of topological matter have been investigated, including topological insulators, Dirac/Weyl semimetals, nodal-line semimetals, and so on, the vast majority of topological matter is still confined within the framework of equilibrium physics. In this talk, I would like to discuss the possibility of creating a new form of topological matter in nonequilibrium by periodically driving topologically trivial systems.

• Title: New flat band materials platform based on line-graph lattice geometry

• Speaker: Dr. Mingu Kang (Cornell Univ.)

• Date & Time / Venue: January 25st (Tue.) , 2024, 2:00 PM / Physics Seminar Room(Science Bldg Ⅲ, #302)

• Abstract
  : Flat band materials based on Moiré technology have achieved tremendous success over the past few years, realizing a rich array of exotic phases of matter at the intersection of strong electronic interaction and nontrivial quantum geometry. However, the temperature scale of Moiré flat band physics has been fundamentally limited to low temperatures due to the large Moiré length scale and low flat band electron density. In this talk, I will discuss a new design principle of flat band materials based on line-graph lattice geometries. Taking the kagome lattice and the pyrochlore lattice as representative examples of 2D and 3D line-graph lattices, respectively, I will demonstrate the experimental realization of line-graph flat bands in kagome metal CoSn and pyrochlore metal CaNi2. Once these kagome and pyrochlore flat bands are tuned to the Fermi level by chemical substitution, we observed the emergence of flat band magnetism and superconductivity, respectively. A comparison of the temperature scales of these emergent phenomena in Moiré and line-graph flat bands reveals that the line-graph flat band systems offer promising opportunities to realize flat band physics at significantly higher temperature scales. I note that the kagome and pyrochlore lattices are merely examples within the much wider line-graph, split-graph, and bipartite lattice families – this emphasizes that there are a plethora of other geometric motifs to be explored and potential flat band materials to be discovered in the future in this new area of research.

• Title: Terahertz Spectroscopy of Quantum Materials

• Speaker: Prof. Jae Hoon Kim (Department of Physics, Yonsei University)

• Date & Time / Venue: November 21st (Tue.) , 2023, 3:00 PM / Room 104(Auditorium), IBS POSTECH Campus Building

• Abstract
  : Quantum materials at the forefront of condensed matter research require access to extremely low frequencies of the electromagnetic spectrum beyond the infrared. The terahertz region lying between the infrared and the microwave frequencies has emerged as a fertile ground for exploring novel effects and phenomena in quantum materials. Recent developments in femtosecond solid-state lasers brought an unexpected opportunity to implement terahertz time-domain spectroscopy (THz-TDS). Here, the fundamental principle of standard THz-TDS is introduced along with immediate applications to research on quantum materials. Case studies, including superconductors, topological insulators, quantum spin liquids (QSLs), and two-dimensional (2D) magnets, will be presented.

• Title: Anomalous quasi particles in a ferromagnetic kagome metal

• Speaker: Prof. Soh, Yeong Ah (Paul Scherrer Institute, 5232 Villigen, Switzerland)

• Date & Time / Venue: November 14th (Tue.) , 2023, 11:00 AM / Lecture Room (Bldg.3, #115)

• Abstract
  : It is widely thought that flat-bands as well as Dirac crossings can be supported in a kagomé layer. One model kagome system is Fe3Sn2, a ferromagnet with a high (~640 K) Curie temperature, which undergoes a first order spin reorientation around 120 K(1, 2).  Our density functional theory (DFT) calculations predict Weyl nodes near the Fermi level EF and electron pockets at the zone center(3).  Magnetotransport of Fe3Sn2 displays anomalous behaviour at temperatures below 80 K, where the spin reorientation is complete, such as tunability of the carrier density via magnetization(4) and a 3-fold antisymmetric planar Hall effect(5). We report here on the use of micro-focused ARPES.  With higher spatial-resolving capability, we discover a sharp band, which cannot be reproduced by our DFT calculation suggesting that its origin is from strong correlations, a view reinforced by our additional finding that it and its neighbouring bands display marginal Fermi liquid behaviour (6). 

References: 

1. N. Kumar, Y. Soh, Y. Wang, Y. Xiong, Magnetotransport as a diagnostic of spin reorientation: Kagome ferromagnet as a case study. Physical Review B 100, 214420 (2019). 

2. K. Heritage et al., Images of a first order spin-reorientation phase transition in a metallic kagomeferromagnet. Adv. Funct. Mater. 30, 1909163 (2020). 

3. M. Yao et al., Switchable Weyl nodes in topological Kagome ferromagnet Fe3Sn2. ArXiv e-prints. 2018. 

4. N. Kumar, Y. Soh, Y. Wang, J. Li, X. Y., Tuning the electronic band structure in a kagome ferromagnetic metal via magnetization. Phys Rev B 106, 045120 (2022). 

5. N. Kumar, Y. Soh, Y. Wang, J. Li, Y. Xiong, Anomalour Planar Hall Effect in a kagomeferromagnet. arXiv:2005.14237,  (2020). 

6. S. A. Ekahana et al., Anomalous quasiparticles in the zone center electron pocket of the kagomé ferromagnet Fe3Sn2. arXiv2206.13750,  (2022), accepted by Nature.

• Title: Exploring the 3D Nano and Atomic World: Coherent Diffractive Imaging and Atomic Electron Tomography

• Speaker: Jianwei (John) Miao (UCLA, Department of Physics and Astronomy)

• Date & Time / Venue: September 15th (Fri.) , 2023, 2:00 ~ 3:30 PM / Physics Seminar Room(Science Bldg Ⅲ, #302)

• Abstract
  : The discovery and analysis of X-ray diffraction from crystals by von Laue and Braggs in 1912 marked the birth of crystallography. Over the last century, crystallography has been fundamental to the development of many fields of science. However, many samples in physics, chemistry, materials science, nanoscience, geology, and biology are non- crystalline, and thus their 3D structures are not accessible by traditional crystallography. Overcoming this hurdle has required the development of new structure determination methods. In this talk, I will present two methods that can go beyond crystallography: coherent diffractive imaging (CDI) and atomic electron tomography (AET). In CDI, the diffraction pattern of a non-crystalline sample or a nanocrystal is first measured and then directly phased to obtain an image. The well-known phase problem is solved by combining the oversampling method with iterative algorithms. In the first part of the talk, I will briefly discuss the principle of CDI and highlight the direct observation of 3D topological magnetic monopoles and their interactions in a ferromagnetic meta-lattice. In the second part of the talk, I will present a general tomographic method, termed AET, for 3D structure determination of crystal defects and disordered materials at the single atomic level. By combining advanced electron microscopes with powerful computational algorithms, AET has been used to determine the 3D atomic structure of amorphous solids and quantitatively characterize short- and medium-range order. We observe that, although the 3D atomic packing of short-range order is geometrically disordered, some short-range-order structures connect to form crystal-like superclusters and give rise to medium-range order. As coherent X-ray sources and powerful electron microscopes are under rapid development around the world, we expect that CDI and AET will find broad applications in condensed matter physics, materials science, chemistry, nanoscience and technology.

• Title: Exploring the 3D Nano and Atomic World: Coherent Diffractive Imaging and Atomic Electron Tomography

• Speaker: Jianwei (John) Miao (UCLA, Department of Physics and Astronomy)

• Date & Time / Venue: September 15th (Fri.) , 2023, 2:00 ~ 3:30 PM / Physics Seminar Room(Science Bldg Ⅲ, #302)

• Abstract
  : The discovery and analysis of X-ray diffraction from crystals by von Laue and Braggs in 1912 marked the birth of crystallography. Over the last century, crystallography has been fundamental to the development of many fields of science. However, many samples in physics, chemistry, materials science, nanoscience, geology, and biology are non- crystalline, and thus their 3D structures are not accessible by traditional crystallography. Overcoming this hurdle has required the development of new structure determination methods. In this talk, I will present two methods that can go beyond crystallography: coherent diffractive imaging (CDI) and atomic electron tomography (AET). In CDI, the diffraction pattern of a non-crystalline sample or a nanocrystal is first measured and then directly phased to obtain an image. The well-known phase problem is solved by combining the oversampling method with iterative algorithms. In the first part of the talk, I will briefly discuss the principle of CDI and highlight the direct observation of 3D topological magnetic monopoles and their interactions in a ferromagnetic meta-lattice. In the second part of the talk, I will present a general tomographic method, termed AET, for 3D structure determination of crystal defects and disordered materials at the single atomic level. By combining advanced electron microscopes with powerful computational algorithms, AET has been used to determine the 3D atomic structure of amorphous solids and quantitatively characterize short- and medium-range order. We observe that, although the 3D atomic packing of short-range order is geometrically disordered, some short-range-order structures connect to form crystal-like superclusters and give rise to medium-range order. As coherent X-ray sources and powerful electron microscopes are under rapid development around the world, we expect that CDI and AET will find broad applications in condensed matter physics, materials science, chemistry, nanoscience and technology.

• Title: Topological spin-texture in the pseudogap phase of a high-Tc superconductor 

• Speaker: Zechao WANG (Tsinghua University) 

• Date & Time / Venue: April 24th (Mon.) , 2023, 4:00 PM / Online (Zoom)

• Abstract
  : An outstanding challenge in condensed matter physics research over the past three decades is to understand the pseudogap (PG) phenomenon of the high-transition-temperature copper-oxides. A variety of experiments have indicated a symmetry-broken state below the characteristic temperature T*. Among them, while the optical study5 has indicated the mesoscopic domains to be small, all these experiments lack nanometer-scale spatial resolution, and the microscopic order parameter has so far remained elusive. Here, we report the first direct observation of topological spin-texture in an underdoped cuprate, YBa2Cu3O6.5, in the PG state, using Lorentz transmission electron microscopy. The spin-texture features vortex-like magnetization density in the CuO2 sheets, with a relatively large length scale of about 100 nm. We identify the phase diagram region in which the topological spin-texture exists and demonstrate the ortho-II oxygen order and suitable sample thickness to be crucial for its observation by our technique. We furthermore discuss an intriguing interplay observed among the topological spin-texture, PG state, charge order, and superconductivity[1].
  
  [1]. Zechao Wang, Ke Pei, Liting Yang, Chendi Yang, Guanyu Chen, Xuebing Zhao, Chao Wang, Zhengwang Liu, Yuan Li*, Renchao Che*, Jing Zhu*. Topological spin-texture in the pseudogap phase of a high-Tc superconductor. Nature, 615, 405-410. (2023). 

• Title: Binary-state nanophotonics with non-Hermitian and topological Hamiltonians

• Speaker: Prof. Jae Woong Yoon (Hanyang Univ.)

• Date & Time / Venue: March 29th (Wed.) , 2023, 4:00 ~ 5:30 PM / Physics Seminar Room(Science Bldg Ⅲ, #302)

• Abstract
  : Non-Hermitian and topological physics have been developed from the quantum field theory and solid-state physics. Their key concepts are presently far-reaching to wide ranges of different physics realms as they suggest extra degrees of freedom for controlling wave phenomena in unprecedented ways. Towards this end, photonic nanostructures have provided fertile grounds because of their powerful capability to construct desired interaction potentials and possibility for practical applications as well. In this talk,I provide our recent advances in non-Hermitian and topological photonics. In particular, I focus on waveguide and diffraction grating structures where intricate non-Hermitian and topological potential distributions can be coded in remarkably simple geometric parameters. I explain mathematical methods to relate potential distributions to geometries, fundamental phenomena in binary non-Hermitian and topological systems, and their applications to practical devices such as optical isolators, intensity/phase modulators, broadband couplers, and beam shapers. I'll discuss remaining challenges and future directions in the end.

• Title: Exploring light-induced phenomena in condensed matter systems with ab initio approach

• Speaker: Prof. DongbinShin (GIST)

• Date & Time / Venue: February 20th (Mon) , 2023, 2:00 PM / Physics Seminar Room(Science Bldg Ⅲ, #302)

• Abstract
  : Light-induced phase transitions in condensed matter systems have attracted significant attention due to their potential applications and unprecedented physical phenomena. Recent studies have demonstrated light-induced topological phase transitions in materials such as WTe2 and ZrTe5 [1-2], which have been explained by lattice dynamics caused by excited electronic structures [3]. Additionally, light-induced ferroelectric transitions are observed in quantum paraelectric SrTiO3 through mid-infrared and terahertz lights [4-5], with theoretical evidence suggesting the unique properties of the quantum paraelectric phase is the origin of this terahertz field-induced ferroelectricity [6]. This seminar will delve into the theoretical explanations behind recent experimental observations, including light-induced magnetic momentum under time-reversal symmetric conditions in NbAs2, real-time energy-orbital resolved dynamics in Mxene induced by light, and light-induced ferroelectricity in SrTiO3. 

• Title: Topological Phases of Matter and Their Classification 

• Speaker: Minyoung You  (APCTP)

• Date & Time / Venue: February 9th (Thu.), 2023, 10:00 AM, 2:00 PM, 4:30 PM / #512, APCTP & Online via ZOOM 

• Abstract
  : I will give an overview of topological phases of matter and their classification. We will discuss different types of topological phases: intrinsic topological orders and SPT orders, and invertible and non-invertible phases; and give examples of each. We will also discuss the connection to topological quantum field theory (TQFT). - 1+1d: Matrix Product States and classification of 1+1d gapped systems; 1+1d TQFT and Frobenius algebras; SPT order and projective representations (group cohomology); fermionic invertible topological order and the Kitaev chain - 2+1d: Intrinsic topological order and anyons (example: toric code); 2+1d TQFT and modular tensor categories; adding fermions and symmetry - Brief comments on 3+1d topological orders; fractonic phases and their properties.

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Topological generation and control of spin-triplet supercurrents in chiral antiferromagnetic Josephson junctions

Kun-Rok Jeon

Department of Physics, Chung-Ang University

The proximity-coupling of a chiral non-collinear antiferromagnet (AFM) [1,2] with a singlet superconductor allows spin-unpolarized singlet Cooper pairs to be converted into spin-polarized triplet pairs, thereby enabling non-dissipative, long-range spin correlations [3,4]. The mechanism of this conversion derives from fictitious magnetic fields that are created by a non-zero Berry phase [5] in AFMs with noncollinear atomic-scale spin arrangements [1,2]. In the first part of my talk, I would like to describe our recent achievement of long-ranged lateral Josephson supercurrents through an epitaxial thin film of the triangular chiral AFM Mn3Ge [6]. The Josephson supercurrents in this chiral AFM decay by approximately one to two orders of magnitude slower than would be expected for singlet pair correlations [3,4] and their response to an external magnetic field reflects a clear spatial quantum interference. Given the long-range supercurrents present in both singleand mixed-phase Mn3Ge, but absent in a collinear AFM IrMn, our results pave a way for the topological generation of spin-polarized triplet pairs [3,4] via Berry phase engineering [5] of the chiral AFMs. Spin-triplet supercurrent spin-valves [7-9] - switching the spin-polarized triplet supercurrents on and off via a magnetic-field-controlled noncollinearity between the spin-mixer and spin-rotator magnetizations in ferromagnetic Josephson junctions - are of practical importance for the realization of superconducting spintronic logic circuits [3]. In the second part, I would like to present our recent demonstration of an antiferromagnetic equivalent of such spin-triplet supercurrent spin-valves [7-9] in chiral antiferromagnetic Josephson junctions and a direct current superconducting quantum interference device (dc SQUID), where non-collinear atomic-scale spin arrangements in the topological chiral antiferromagnet Mn3Ge [1,2] with fictitious magnetic fields (i.e. Berry curvature)8 accommodate triplet Cooper pairing over a long distance (> 150 nm) [6]. We theoretically reproduce the observed supercurrent spin-valve behaviours under a tiny magnetic field (< 2 mT) for current-biased junctions and a dc SQUID, possessing hysteretic field interference of the Josephson critical current, based on the magneticfield-modulated antiferromagnetic texture which alters the Berry curvature [6,10]. This provides a topological route for controlling the pair amplitude of spin triplets in the single chiral antiferromagnet [10].

References

[1] Nature 527, 212 (2015),[2] Sci. Adv. 2, e1501870 (2016), [3] Nat. Phys. 11, 307 (2015), [4] Rep. Prog. Phys. 78, 104501 (2015), [5] Rev. Mod. Phys. 82, 1959 (2010), [6] Nat. Mater. 20, 1358 (2021), [7] Phys. Rev. B 76, 060504(R) (2007), [8] Nat. Comm. 5, 4771 (2014), [9] Phys. Rev. Lett. 116, 077001 (2016), [10] Nat. Comm. 12, 572 (2021), [11] Submitted (2021).

Topological states in aperiodic systems

Huaqing Huang

School of Physics, Peking University

Despite the rapid progress in the field of topological states, most of the topological systems studied up to now are based on crystalline materials. Recently, the study of topological states was extended to aperiodic systems, such as quasicrystals and amorphous systems. In this talk, I will first briefly review crystalline topological states and the structural characteristic of quasicrystals. I will then discuss the generalization of topological states to quasicrystals which are characterized by newly defined real-space topological invariants. Furthermore, a generic orbital design of higher-order quasicrystalline insulators and an effective model for fractional topological corner modes in quasicrystals will also be discussed. In addition, I will discuss the possibility of structural amorphization-induced topological states.

 References:

[1] H. Huang and F. Liu, Phys. Rev. Lett. 121, 126401 (2018)

[2] H. Huang and F. Liu, Phys. Rev. B 98, 125130 (2018)

[3] C. Wang, et al. Phys. Rev. Lett. 129, 056403 (2022)

[4] H. Huang, et al. Nano Letters 16, 7056–7062 (2021)

[5] C. Wang, et al. Phys. Rev. Lett. 128, 056401 (2022) 

Symmetry Protected Topological Order in Open Quantum Systems

Dr. Alex Turzillo (Perimeter Institute for Theoretical Physics)

We systematically investigate the robustness of symmetry protected topological (SPT) order in open quantum systems by studying the evolution of string order parameters and other probes under noisy channels. We find that one-dimensional SPT order is robust against noisy couplings to the environment that satisfy a strong symmetry condition, while it is destabilized by noise that satisfies only a weak symmetry condition, which generalizes the notion of symmetry for closed systems. We also discuss "transmutation" of SPT phases into other SPT phases of equal or lesser complexity, under noisy channels that satisfy twisted versions of the strong symmetry condition.

■ Title: Light-induced phenomena in condensed matter system from ab initio approach

■ Speaker: Dr. Dongbin Shin(GIST)

■ Zoom Link: https://us06web.zoom.us/j/89484098328?pwd=WG1aa3FSMzNZNE5ZUTVwRTJRVDllUT09

■ Zoom meeting ID & PW :  ID: 894 8409 8328  /  PW: 071582

■ Date & Time : DEC. 7.(Wed.), 3:00pm ~

■ Abstract:

Recent studies on light-matter interaction have attracted attention by showing unprecedented physical phenomena and the possibility of application devices. For example, light-induced topological phase transitions in WTe2 and ZrTe5 are experimentally demonstrated. A follow-up theoretical study explained that this topological phase transition originates from the lattice distortion induced by exciting electronic structure. Also, light-induced ferroelectric transitions in quantum paraelectric SrTiO3 are experimentally demonstrated by applying mid-infrared and terahertz lights. It is theoretically proved that the unique property of the quantum paraelectric phase could lead to terahertz field-induced ferroelectricity. These results indicate that light can control the phase of the material with various microscopic mechanisms and suggest the possibility of brand-new optical control devices. This seminar will introduce recent studies on light-induced phase transition in condensed matter systems and related microscopic mechanisms from ab inito approach. 

New twist in familiar spin models 

Prof. Haruki Watanabe (Univ. of Tokyo)

2022. Dec. 7th (Wed) 4:15 pm~

3-301 Seminar Room

The transverse-field Ising model and Kitaev toric code model are canonical examples of spin models featuring a symmetry breaking of Z2 symmetry and a Z2 topological order, respectively. Nowadays these pedagogical models are covered in many undergraduate / graduate level lectures and textbooks [1]. Recently, we found a new family of extesions of these spin models to N-level spins, which show nontrivial system-size dependence [2]. For example, our ZN toric code realizes a topological order without ground state degeneracy on torus for a sequence of system size. We will discuss general lessons we can learn from these examples.

[1] e.g., Hal Tasaki, Physics and mathematics of quantum many-body systems, Graduate Texts in Physics (Springer, 2020).

[2] HW, Meng Cheng, Yohei Fuji, Ground state degeneracy on torus in a family of ZN toric code, arxiv:2211.00299.

IBS-CALDES SEMINAR (Zoom)

▶Zoom Link : https://us06web.zoom.us/j/89975791050?pwd=T0swSHJzbWZyMjZGUUJqUnBCU05Idz09

▶ID :  899 7579 1050  /  PW : 746176

Material design with the van der Waals stacking of bismuth-halide chains realizing a higher-order topological insulator

Takeshi Kondo

Institute for Solid State Physics, The University of Tokyo, Japan

Materials with quantum spin Hall insulator layers weakly coupled and stacked on top of each other form Z2 weak topological insulator (WTIs) [1,2], the sides of which are topologically non-trivial and flow a highly directional, non-dissipative spin current.  The same concept holds for higher-order topological insulators (HOTIs), which are similarly constructed from stacking quantum spin Hall insulators but, in this case, yield topologically protected one-dimensional helical hinge states. HOTI is a new class of topological insulators predicted in compounds previously thought to as trivial insulators under the Z2 criterion by expanding the topological categorization to the Z4 topological index. The material first proposed to be in the higher-order topological phase is Bulk bismuth [3], which is, however, a semimetal and cannot be made insulating by a simple parameter tuning like carrier doping. The experimental realization of a higher-order topological insulator in a 3D material has been anticipated in materials science. If achieved, it will allow for exploring many quantum phenomena, such as spin currents around hinges and quantized conductance under external fields.

In my talk, I will introduce that the quasi-1D bismuth halides Bi4X4 (where X is either I or Br) provide an excellent platform to realize various topological phases that can be selected by different ways the chain layers are stacked. Bi4I4 with single-layered chains per unit cell consisting of A-stacking develops a WTI state, where quasi-1D topological surface states are realized on the crystal side surface. In contrast, a simple insulator phase is formed in Bi4I4, where the chains adopt a double-layered structure consisting of AA’-stacking. Bi4Br4 is a HOTI candidate in the form of double layers, one of which is flipped by 180 degrees in the unit cell (AB-stacking). Using this material design concept and angle-resolved photoemission spectroscopy, I will demonstrate that Bi4Br4 is a higher-order topological insulator in its three-dimensional bulk state [4].

[1] R. Noguchi et al., Nature 566, 518 (2019).

[2] P. Zhang et al., Nature Communications 12, 406 (2021).

[3] F. Schindler et al., Nature Physics 14, 918 (2018).

[4] R. Noguchi et al., Nature Materials 20, 473 (2021).

Transition metal dichalcogenides (TMDCs) have been an important research subject, due to their unique electrical and optical properties, for the last decade or so. TMDCs have two distinct electronic band extrema with different electron spin states, +K and –K valleys. Valley polarization was demonstrated by creating excitons at a specific valley using circularly polarized light of proper helicity. Furthermore, valley-polarized exciton transport, driven by thermal or chemical potential gradients, can exhibit valley-dependent transverse motion originating from the Berry curvature difference in +K and –K valleys, which is known as exciton Hall effect. In this talk, I will present survey on valley polarization as well as exciton Hall effect and recent work on them in our lab.

Title:Tunable electron topology and correlation in few-layer rhombohedral graphene

Speaker: Prof. Long Ju(Department of Physics, MIT, longju@mit.edu​)

Date & Time: July 25 (mon.) 2:00 pm ~ 3:20 pm

Venue : Science Bldg., 3-302(공학3동 302호 강의실)

Abstract:

Graphene has been a model solid state system where novel quantum phenomena emerge from the interplay between symmetry, band topology and reduced dimensionality. In particular, few layer graphene with the rhombohedral stacking order has a unique bandstructure with an electrically tunable bandgap and a valley-dependent Berry phase. These features result in unusual electrical and optical properties, for which optical spectroscopy/microscopy are powerful characterization tools. In this talk, I will first show our experimental demonstration of the topological valley transport at AB/BA stacking domain walls in bilayer graphene. These domain walls are 1D conducting channels that feature the quantum valley Hall edge states. Next, I will present our efforts on probing the orbital magnetism of electrons through studying excitons in bandgap-tuned bilayer graphene. Due to the electron pseudospin and Berry curvature effects, these excitons obey unusual valley-dependent optical selection rules and a large valley g-factor of 20 in magnetic field. Finally, I will show our recent work on probing strong electron correlation in ABC trilayer graphene and its implications for correlation-driven topological phenomena.​

Contact person: Prof.  Jonghwan Kim(054-279-2137,  jonghwankim@postech.ac.kr )

v  Title: Orbital texture effects in electronic transport

v  Speaker: Dr. Kyoung-Whan Kim (KIST)

v  Location: #302, Science Building III & Online (ZOOM)

v  ZOOM: ID 956 2694 3727 / PWD 949221

v  Date & Time: Jun. 8th (Wed.), 1:00 PM (~ 2 hour long)

v  ZOOM URL: https://postech-ac-kr.zoom.us/j/95626943727?pwd=WUhZT2RHUkxMTllhd0drYjNtNDlDdz09

The discovery of the quantum Hall effect has led to three Nobel prizes and the booming field of topological phases of quantum matter. The quantum Hall effect is usually observed in 2D. It has been a long-standing challenge to realize a quantum Hall effect in 3D. We predict a new mechanism of 3D quantum Hall effect in topological semimetals [1-2]. Topological semimetals, which host topologically-protected surface states, known as the Fermi arcs. The Fermi arcs at two opposite surfaces can form a 2D electron gas that supports a 3D quantum Hall effect. Possible signatures are observed in the topological Dirac semimetal Cd3As2 [e.g., Faxian Xiu et al., Nature 565, 331 (2019)]. This 3D quantum Hall give an example of (d-2)-dimensional boundary states in higher-order topological phases of matter [3]. On the other hand, the charge-density-wave mechanism of the 3D quantum Hall effect has been observed recently in ZrTe5 [Liyuan Zhang et al., Nature 569, 537 (2019)]. We develop a theory for the CDW mechanism of the 3D quantum Hall effect [4] and coexisting metainsulator transition [5]. The theories can capture the main features in the experiment. More importantly, it poses a rare case, in which one magnetic field can induce an order-parameter phase transition in one dimension but a topological phase transition in other two dimensions. 

References: 

[1] C. M. Wang et al., PRL 119, 136806 (2017). 

[2] Hai-Zhou Lu, National Science Review 6, 208 (2019). 

[3] Rui Chen et al., PRL 127, 066801 (2021). 

[4] Fang Qin et al., PRL 125, 206601 (2020). 

[5] Peng-Lu Zhao et al., PRL 127, 046602 (2021). 

■ ZOOM Webinar

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v  Title: Real-time dynamics of topological order in Dirac semiconductors

v   Speaker: Prof. JaeDong Lee (DGIST)

v   ZOOM ID: 948 064 9013 / Password: 123456

v   Date & Time: May 25th (Wed.) 4:00 pm ~

v  Zoom URL: https://us02web.zoom.us/j/9480649013?pwd=dmVqaWZ3bHozbjZXRjd0RllvVVVkdz09

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The second-order optical nonlinearity has been a focus of basic research and technological development for decades as it is both a probe of inversion symmetry breaking in media and the basis for generating coherent light from far-infrared to ultraviolet wavelengths. Here, we focus on the relation between band geometry and topology with nonlinear optics. In my first part of my talk, I will present the discovery of giant second harmonic generation in the polar Weyl semimetal TaAs[1,2]. In the second part, I will talk about nonlinear THz emission spectroscopy on chiral topological semimetals. A remarkable example occurs when chiral semimetals with topologically protected band degeneracies are illuminated with circularly polarized light. Under the right conditions, the part of the generated photocurrent that switches sign upon reversal of the light's polarization, known as the circular photogalvanic effect (CPGE), is predicted to depend only on fundamental constants. The conditions to observe quantization are non-universal, and depend on material parameters and the incident frequency. In my talk, I will discuss nonlinear terahertz emission spectroscopy with tunable photon energy from 0.2 eV - 1.1 eV in the chiral topological semimetals CoSi [3,4] and RhSi[5]. Particularly, we identify a large longitudinal photocurrent in CoSi, which is much larger than the photocurrent in any chiral crystal reported in the literature. I will discuss how quantized CPGE can be reached in these two compounds and the experimental status.

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IBS-CALDES SEMINAR (Zoom)

▶Zoom Link : https://us06web.zoom.us/j/89675857723?pwd=S3k1MG9vNWdOOHZ0aFJRWDl4V1hVQT09

▶ID :  896 7585 7723  /  PW : 807323

Interaction Driven Quantum Anomalous Hall Phases in Moiré Materials

Two-dimensional moiré superlattices have emerged as an ideal system to study the many-body interactions and correlated states. Recently, the quantum anomalous Hall phase was observed in MoTe2/WSe2 heterobilayers at half-filling (one hole per moiré unit cell) [Nature 600, 641 (2021)]. However, the mechanism behind the emergence of the topological phase is not known.  In this work, we propose that the topologically nontrivial phase can be induced by the pseudo-magnetic fields caused by lattice relaxation.

We point out that a periodically modulated pseudo-magnetic field breaks the intra-valley time-reversal symmetry and induce non-zero Chern numbers at each valley. At half-filling, the strong Coulomb interactions lift the valley degeneracy and induce a valley-polarized state, where the quantum anomalous Hall effect can be observed. Our theory identifies a new mechanism to achieve topologically nontrivial states and provides a basis for the study of other strongly correlated phases [1].

With new experimental data available concerning the quantum anomalous Hall states in MoTe2/WSe2, we point out that the observed state can also be a topological valley coherent state which is a new state of matter which had not been discovered before.

In this talk, we will also discuss related interaction driven, time-reversal breaking phases observed in gate-defined Josephson junction in twisted bilayer graphene [2,3].

References:

1. Phys. Rev. Lett. 128, 026402 (2022)].

2. arXiv:2110.01067

3. arXiv:2202.05663

v  Title: Recent progress in the study of topological phases of matter

v  Speaker: Prof. Bohm-Jung Yang (SNU)

v  ZOOM ID: 948 064 9013 / Password: 123456

v  Date & Time: Apr. 27th (Wed.) 4:00 pm ~

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v  Title: Design synthetic topological matter with atoms and lights in a quantum simulator

v   Speaker: Prof. Gyu-Boong Jo (HKUST)

v   ZOOM ID: 948 064 9013 / Password: 123456

v   Date & Time: Mar. 23rd (Wed.) 4:00 pm ~

Zoom URL: https://us02web.zoom.us/j/9480649013?pwd=dmVqaWZ3bHozbjZXRjd0RllvVVVkdz09

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Wave function geometry and anomalous Landau levels of flat bands 

Prof. Bohm Jung Yang (Seoul National Univ, Korea)

2021. 11. 29 (월) 오전 10시

Online via Zoom (https://us02web.zoom.us/j/5967951037?pwd=UEhQeU1VZ1JoVEdFamlUQUZaZ2tOZz09)

 Semiclassical quantization of electronic states under magnetic field describes not only the Landau level spectrum but also the geometric responses of metals under a magnetic field. However, it is unclear whether this semiclassical idea is valid in dispersionless flat-band systems, in which an infinite number of degenerate semiclassical orbits are allowed. In this talk, I am going to show that the semiclassical quantization rule breaks down for a class of flat bands including singular flat bands [1-5] and isolated flat bands [6]. The Landau levels of such a flat band develop in the empty region in which no electronic states exist in the absence of a magnetic field. The total energy spread of the Landau levels of flat bands is determined by the quantum geometry of the relevant Bloch states, which is characterized by their Hilbert–Schmidt quantum distance and fidelity tensors. The results indicate that flat band systems are promising platforms for the direct measurement of the quantum geometry of wavefunctions in condensed matter.

[1] J. W. Rhim and B. -J. Yang, “Classification of flat bands according to the band-crossing singularity of Bloch wave functions”, PRB 99, 045107 (2019)

[2] J. W. Rhim, K. Kim, B. -J. Yang, “Quantum distance and anomalous Landau levels of flat bands”, Nature 584, 59-63 (2020)

[3] Y. Hwang, J. Jung, J. W. Rhim, B. -J. Yang, “Wave function geometry of band crossing points in two-dimensions”, PRB 103, L241102 (2021)

[4] Y. Hwang, J. W. Rhim, B. -J. Yang, “Flat bands with band crossing enforced by symmetry representation”, PRB 104, L081104 (2021); PRB 104, 085144 (2021)

[5] J. W. Rhim and B. -J. Yang, “Singular flat bands”, Advances in Physics X, 6:1, 1901606 (2021)

[6] Y. Hwang, J.-W. Rhim, B.-J. Yang, “Geometric characterization of anomalous Landau levels of isolated flat bands”, Nature Communications 12, 6433 (2021)

The unique properties of nanostructured topological matter

Prof. Kornelius Nielsch (IFW Dresden)

2021. 11. 26. (금) 오후 4시

Online via ZOOM (https://us06web.zoom.us/meeting/register/tZ0rceqsqTkpGdPkz33qmrKTj5U1rQ5pSX43)

In this lecture, we debate about the interconnection between thermoelectric performance and topological insulator nature of chalcogenide-type materials [1,2]. While topological surface states seem to play positive role in the thermoelectric transport in (nanograined) bulk material, it will be shown that they contribute to the transport in nanostructures due to their high surface-to-volume ratio. By tuning the charge carrier concentration, a crossover between a surface-state-dominant and a Fuchs-Sondheimer transport regime is observed in ALD grown Sb2Te3 layers[3]. Magnetic ordering on the surface of a topological insulator nanowire could enable room-temperature topological quantum and spintronic devices. In general, we will discuss about the challenges to synthesize topological insulators which exhibit an insulating transport behavior at least at low temperatures. 

Furthermore, the magneto-thermoelectric transport of Weyl semimetal like HfTe5 and NbP will be presented[4]. We have measured the chiral magnetoresistance in the Weyl semimetal NbP and detected signatures of the mixed axial-gravitational anomaly in the transport experiments. This work has stimulated a scientific discussion about the application of theoretical models from High Energy Physics in the area of thermoelectric materials. Many of these so-called quantum materials have been analyzed for thermoelectric applications in previous decades. An outlook will be given about magnetic doping of topological insulators and Weyl semimetals.

[1] C. Bauer, I, Veremchuk, C. Kunze, A. Benad, V. M Dzhagan, D. Haubold, D. Pohl, G. Schierning, K. Nielsch, V. Lesnyak, A. Eychmüller, Heterostructured Bismuth Telluride Selenide Nanosheets for Enhanced Thermoelectric Performance, Small Science 1, 2000021 (2021).

[2] B. Hamdou, J. Kimling, A. Dorn, E. Pippel, R. Rostek, P. Woias, K. Nielsch. Thermoelectric Characterization of Bismuth Telluride Nanowires, Synthesized Via Catalytic Growth and Post-Annealing, Adv. Mater. 25, 239 (2013).

[3] N.F. Hinsche, S. Zastrow, J. Gooth, L. Pudewill, R. Zierold, F. Rittweger, T. Rauch, J. Henk, K. Nielsch, I. Mertig. Impact of the Topological Surface State on the Thermoelectric Transport in Sb2Te3 Thin Films, ACS Nano 9, 4406 (2015).

[4] J. Gooth, A. C. Niemann, T. Meng, A. G. Grushin, K. Landsteiner, B. Gotsmann, F. Menges, M. Schmidt, C. Shekhar, V. Sueß, R. Huehne, B. Rellinghaus, C. Felser, B. Yan, K. Nielsch, Experimental signatures of the mixed axial-gravitational anomaly in the Weyl semimetal NbP, Nature 54, 24–327 (2017).

 Quantum Geometry of Light-Matter Interactions: New Approach to Multi-Band Physics

1. 연사: 안준영 박사 (Harvard University)

2. 일시: 2021. 11. 15(월), 오후 4시

3. 장소: 공학3동 302호 (세미나실)

Multi-band effects are ubiquitous phenomena in quantum materials – consisting of topological and correlated materials. Topological materials are intrinsically multi-band phases of matter that are formed by band inversion. Many correlated materials also feature strong multi-band mixing, because they feature small intra-band kinetic energy, which relatively enhances both correlation and multi-band effects. In this talk, I will introduce quantum geometry as a unifying principle to characterize multi-band effects in topological and correlated materials. After arguing that quantum geometric quantities characterize multi-band effects, I will reveal the quantum geometric meaning of resonant optical transitions [1,2]. This result provides a systematic way to study multi-band effects in quantum materials through light-matter interactions. I will apply this idea to the long-standing puzzle about the spectral weight transfer in high-temperature superconductors [3]. Finally, I will discuss some future directions for studying multi-band physics of quantum materials.

[1] J. Ahn, G. Y. Gyo, N. Nagaosa, and A. Vishwanath, arXiv:2103.01241, Nat. Phys. (accepted).

[2] J. Ahn, G. Y. Gyo, and N. Nagaosa, Phys. Rev. X 10, 041041 (2020).

[3] J. Ahn and N. Nagaosa, Phys. Rev. B 104, L100501 (2021).