2차원 물질 물리

• Title: Emergent Electronic Phases in Correlated Low-Dimensional Materials: Excitons and Polarons Decoupled from Lattice Symmetry

• Speaker: Prof. Tae-Hwan Kim (Dept. of Physics, POSTECH)

• Date & Time / Venue: May 29th (Thu.), 2025, 4:50 PM / Science Bldg Ⅲ, #111

• Abstract
  :Correlated electron systems often exhibit intertwined structural and electronic instabilities, making it challenging to disentangle their respective roles in emergent quantum phases. In this talk, I will present two recent studies that probe the decoupling of lattice and electronic degrees of freedom in low-dimensional materials, focusing on excitonic insulator behavior in Ta₂NiSe₅ and polaron ordering on cleaved WO₃ surfaces.
  In the first part, I demonstrate that the application of a localized electric field via a low-work-function STM tip induces an insulator-to-seminetal transition in few-layer Ta₂NiSe₅ without altering the low-temperature monoclinic lattice. This decoupling of the electronic and structural transitions provides compelling evidence that the insulating gap is driven by excitonic correlations rather than by lattice distortion alone. Furthermore, layer-dependent STS measurements reveal how electrostatic screening modulates the electronic response, elucidating the interplay between dimensionality and excitonic order.
  In the second part, I discuss the formation of a metallic surface state with robust polaronic characteristics in high-density 2D electron gas formed on the mechanically cleaved WO₃(001) surface. Despite a high electron density, a stable c(2×2) surface reconstruction is observed, stabilized by ordered oxygen vacancies and cooperative lattice distortions. Spectroscopic signatures including peak-dip-hump structures and phonon replica near the Fermi level confirm the presence of strong electron-phonon coupling and ordered polaron states, supported by first-principles calculations. This unexpected resilience of polarons against screening not only challenges the conventional understanding of polaron collapse at high carrier density but also points toward new possibilities for exploring bipolaronic superconductivity or charge-ordered metallic phases.
  Together, these results underscore how precise control over dimensionality, symmetry, and external perturbations allows the realization of nontrivial many-body phases governed by electronic correlations, independent of lattice symmetry breaking.

• Title: Engineering multilayer graphene superlattices as a platform for studying interacting quantum phases

• Speaker: Prof. Joonho Jang (Seoul National University)

• Date & Time / Venue: November 28th (Thu.), 2024, 4:00 PM / Room 104, IBS POSTECH Campus Building

• Abstract
  : A Bilayer of semiconducting 2D electronic systems has long been a versatile platform to study electronic correlation with tunable interlayer tunneling, Coulomb interactions and layer imbalance. In the natural graphite bilayer, Bernal-stacked bilayer graphene (BBG), the Landau level gives rise to an intimate connection between the valley and layer. Adding a moiré superlattice potential enriches the BBG physics with the formation of topological minibands, potentially leading to tunable exotic quantum transports. Further increasing the number of layers is expected to rapidly expand the possible phase space one can explore to tune the interplay between the electronic correlation and band topology.
  In this talk, I will present our recent magneto-transport measurements of a high-quality bilayer graphenehexagonal boron nitride (hBN) heterostructure. The zero-degree alignment between the bilayer graphene and hBN generates a strong moiré superlattice potential for the electrons in BBG and the resulting Landau fan diagram of longitudinal and Hall resistance displays a Hofstadter butterfly pattern with an unprecedented level of detail. Our work demonstrates that the intricate relationship between valley and layer degrees of freedom controls the topology of moiré-induced bands, significantly influencing the energetics of interacting quantum phases in the BBG superlattice. We further observe signatures of field-induced correlated insulators and clear fractional quantization of interaction driven topological quantum phases. Finally, I will discuss the important considerations in utilizing multilayer graphene heterostructures as ideal platforms to study the delicate interplay between topology and electron correlation. In particular, our recent results in helically stacked twisted trilayer graphenes will bepresented as an example.

• Title: Twisted Bilayer Magnets

• Speaker: Prof. Moon Jip Park (Hanyang University)

• Date & Time / Venue: November 6th (Wed.), 2024, 4:00 PM / Physics Seminar Room (Bldg.3, #302)

• Abstract
  : Recent experiments with twisted bilayer materials have provided a versatile platform for the realization of exotic phases of matter. In this talk, I am going to give an introductory talk about twisted bilayer material systems including twisted bilayer graphene. I am going to expand the theory of moire systems to spin systems. Starting from the brief review of twisted bilayer graphene, we develop a concrete theory of twisted bilayer magnetism. We discover a variety of non-collinear magnetic order that has been overlooked in previous theoretical and experimental studies.

• Title: ALD of Metallic Thin Films and Related Materials

• Speaker: Prof. Hyungjun Kim (Yonsei Univ.)

• Date & Time / Venue: October 30th (Wed.), 2024, 4:00 PM / Room 343, IBS POSTECH Campus Building

• Abstract
  : Recently, the exclusive benefits of atomic layer deposition (ALD), including excellent conformality over nanoscale complex structures, high quality of deposited films even at low temperature down to room temperature, atomic scale thickness controllability, and high uniformity over nanoscale structure, make it viable tool for many emerging applications. Among various materials which can be deposited by ALD, I will focus on the preparation of metallic thin films and other related materials such as silicides and chalcogenides. First, metallic thin films deposition using plasma enhanced ALD (PE-ALD) will be presented. Ta/TaN bilayer for Cu metallization will be presented as a first example, which will be followed by ALD of other elemental metal thin films including W and C thin films. For W ALD, the comparison on the growth characteristics and film properties between newly synthesized WC15 precursor and conventional precursor. The application of C thin films including next generation patterning technology as well as battery property enhancement will be presented. Then, ALD of transition metal ALD for contact application will be introduced. The deposition of Co and Ni ALD by both PE- and thermal ALD and Silicide contact preparation by consequential thermal annealing will be presented. Also, high performance device fabrication using semimetal ALD TiSi2 on 2D TMDC will be introduced. Finally, synthesis of noble metal chalcogenides by ALD of noble metals using oxygen as a reactant followed by sulfurization and/or selenization will be presented focusing on the application as a photodetector.

• Title: Quantum light sources in van der Waals materials

• Speaker: Prof. Jieun Lee (Seoul National University)

• Date & Time / Venue: October 30th (Wed.), 2024, 4:00 PM / Physics Seminar Room (Bldg.3, #302)

• Abstract
  : Atomic defects in solid-state materials provide an interesting platform to study quantum information science with applications in quantum sensing, computing and communication. Two-dimensional materials with atomic defects have recently been introduced as emerging systems that can host quantum light sources which are particularly promising for long-distance quantum communication and logic operations at the nanometer scale devices. Interplay between charge, optical, and spin states of defects allows various experimental observations that are critical to understand and manipulate the properties of quantum light emission. In this talk, we introduce electrical engineering and charge state control of quantum light sources in two-dimensional host materials embedded in van der Waals heterostructures. Also, we will discuss our recent efforts to observe quantum light emission in a wider range of van der Waals crystals through tailoring their material properties.

• Title: Tunneling Probe of 2D and Moire Magnetism

• Speaker: Prof. Adam Wei Tsen (University of Waterloo)

• Date & Time / Venue: October 23th (Wed.), 2024, 4:00 PM / Room 343, IBS POSTECH Campus Building

• Abstract
  : The discoveries of ferromagnetism in single atomic layers have opened a new avenue for two-dimensional (2D) materials research. Not only do they raise fundamental questions regarding the requirements for long-range magnetic order in low-dimensional systems, but they also provide a new platform for the development of spintronic devices. In this talk, I will present a series of studies on the layered ferromagnetic insulator, Crl3, both in the atomically thin limit and in twisted homostructures. By incorporating natural 2D Crl3 as tunnel barriers between graphene electrodes, we are able to achieve extremely large tunnel magnetoresistance and directly observe its spin wave, or magnon, excitation spectrum, from which we can then obtain a simple microscopic Hamiltonian for the monolayer spin system. For twisted Crl3, we observe evidence for two types of moiré magnetic textures that give rise to nonvolatile tunneling magnetoresistance states switchable by magnetic field.

• Title: Atomic-scale thermopower in 2D topological and correlated materials

• Speaker: Prof. Heejun Yang (KAIST)

• Date & Time / Venue: October 17th (Thu.), 2024, 2:00 PM / Room 105, IBS POSTECH Campus Building

• Abstract
  : Thermoelectricity (i.e., thermopower generation) has been investigated mainly on the macroscopic scale despite its origin being linked to materials' local electronic band structure. Recently, the microscopic origins of thermopower have gained attention in the design of novel and efficient thermoelectric devices. In particular, distinct origins for thermopower have been expected with low-dimensional, strongly correlated, and topological materials. In this presentation, I will demonstrate our findings on thermoelectric puddles1,2, phonon puddles3 , and thermal biasing effects to break the atomic lattice symmetry4 in variously stacked graphene and 1T-TaS2 , using our Scanning ThermoElectric Microscopy (SThEM). Based on the sensitive probe of the local density of states’ derivatives in SThEM, harnessing atomic-scale thermopower can be achieved above room temperature, distinguished from conventional low-temperature studies with scanning tunneling microscopy.
  
  References
  1. Nano Letters 19, 61 (2018), Coherent Thermoelectric Power from Graphene Quantum Dots
  2. ACS Nano 15, 5397 (2021), Harnessing Thermoelectric Puddles via the Stacking Order and Electronic Screening in Graphene
  3. Nature Communications 13, 4516 (2022), Atomic-scale thermopower in charge density wave states
  4. Manuscript submitted (2024), Thermal biasing for lattice symmetry breaking and topological edge state imaging

• Title: Quantum Hall Effect in 2DEG Systems: Recent Experimental Results

• Speaker: Prof. Changki Hong (KRISS)

• Date & Time / Venue: October 2nd (Wed.), 2024, 4:00 PM / Physics Seminar Room (Science Bldg Ⅲ, #302)

• Abstract
  : The quantum Hall effect occurs in various types of two-dimensional electron systems, including 2DEG (Two-Dimensional Electron Gas). It is observed when these systems are exposed to strong magnetic fields at low temperatures. This phenomenon has been studied extensively for decades. Among the most interesting outcomes is the fractional quantum Hall effect, an important area in condensed matter physics.
  A particular focus will be on quasiparticles known as anyons, which emerge in fractional quantum Hall states. These anyons show exotic behavior that is neither fermionic nor bosonic, making them a unique subject of research.
  While various new materials have been explored, GaAs/AlGaAs heterostructure 2DEG —the primary focus of this talk— continue to be a foundational system for studying these quantum phenomena. I will cover key experimental techniques, such as interferometry and noise measurements, alongside a discussion of major research papers that highlight the ongoing relevance of anyons in quantum physics.

• Title: Operando TEM investigation of polar domain dynamics in 2D sliding ferroelectrics

• Speaker: Prof. Hyobin YOO (Sogang University)

• Date & Time / Venue: July 2nd (Tue.), 2024, 4:00 PM / Room 343, IBS POSTECH Campus Buliding

• Abstract
  : Control of interlayer stacking angle in two-dimensional (2-D) van der Waals (vdW) heterostructure enables one to engineer the crystal symmetry to imprint novel functionality. By stacking two layers of transition metal dichalcogenides (TMD) with designed twist angle, one can break the inversion symmetry and thereby develop vertical electric polarization. The direction of the electric polarization can be switched electrically, suggesting that the twisted bilayer TMD can host ferroelectricity. Such ferroelectricity reported in twisted bilayer vdW system is distinguished from conventional ferroelectrics in that the lateral sliding of the constituent layers induces vertical electric polarizations.
  Here we employ operando transmission electron microscopy (TEM) to investigate the domain dynamics in 2-D vdW sliding ferroelectrics. Operando TEM technique enables one to examine the structural change in the environment that mimics the electrical device operating condition. We find the domain dynamics in response to vertical electric fields is governed by the consecutive domain wall pinning-depinning process as noted by Barkhausen noises in the polarization hysteresis loop[1] . Moreover, exploiting stroboscopic operando TEM on the vdW ferroelectrics, we directly measured the domain wall velocity which is found to be limited by various disorders present in the specimens[1] . Aberration corrected scanning transmission electron microscopy analysis identifies the microstructural origin for the domain wall pinning, providing structural insight on how to improve the switching speed of the sliding ferroelectrics.
  
  [1] K. Ko et al., Operando electron microscopy investigation of polar domain dynamics in twisted van der Waals homobilayers, Nat. Mater. 22, 992 (2023)

• Title: Tailoring artificial Kondo lattice in van der Waals monolayer crystals

• Speaker: Prof. Ying Shuang FU (Huazhong University of Science & Technology)

• Date & Time / Venue: June 4th (Tue.), 2024, 5:10 PM / Online & Room 105, IBS POSTECH Campus Buliding

• ZOOM Link : https://us06web.zoom.us/j/5351320381?pwd=a0wzLzdMQlp2aGZWaC91Rlo4M1pKQT09

• Abstract
  : The heavy fermion physics is dictated by subtle competing exchange interactions, posing a challenge for their understanding Conventional heavy fermion systems are found in three dimensional f electron materials, which is complicated by the crystal and electronic structures Artificial heavy fermions not only offer new systems that are simple in structure, but also may add desired functionalities In this talk, I will present our recent progress in realizing artificial heavy fermions in van der Waals monolayer crystals or heterostructures that were grown with molecular beam epitaxy Utilizing low temperature spectroscopic imaging scanning tunneling microscopy, we observed switching behavior of correlated gap in monolayer 1 T NbSe 2 showing star of David ( charge density wave ( pattern Upon decreasing Se flux, a new quasi 1 D CDW order emerges as caused by the incorporation of regularly spaced defect lines introduced into the SD motifs Coherent Kondo screening is established along stripes, forming a long sought quasi 1 D Kondo lattice at the monolayer limit We also succeeded in constructing a 2 D Kondo lattice composed of monolayer VSe 2 grown on NbSe 2 which indicates evident signatures of superconducting heavy fermions via proximity effect from the substrate Our study realizes artificial Kondo lattices with tailored dimensionality, establishing connection between the heavy fermion physics and 2 D materials with desired functionalities

• Title: Artificial heavy fermions in a van der Waals heterostructure

• Speaker: Prof. Peter LILJEROTH (Aalto University)

• Date & Time / Venue: June 4th (Tue.), 2024, 4:00 PM / Online & Room 105, IBS POSTECH Campus Buliding

• ZOOM Link : https://us06web.zoom.us/j/5351320381?pwd=a0wzLzdMQlp2aGZWaC91Rlo4M1pKQT09

• Abstract
  : Van der Waals vdW heterostructures provide unique opportunities for engineering exotic quantum states not found in naturally occurring materials I will highlight this approach by describing our recent results on artificial heavy fermion systems in vdW heterostructures 1 2 We use molecular‐beam epitaxy ( and low‐ temperature scanning tunneling microscopy ( for the sample growth and characterization Building blocks of heavy fermion systems Kondo coupling between a lattice of localized magnetic moments and mobile conduction electrons can be mimicked in a 1 T‐TaS 2 1 H‐TaS 2 heterostructure These results underscore the adaptability of vdW heterostructures in realizing elusive quantum states that cannot be readily found in naturally occurring materials
  
  References
  1. V Vaňo et al Artificial heavy fermions in a van der Waals Heterostructure, Nature 599 582 2021
  2. X Huang et al Doped Mott phase and charge correlations in monolayer 1 T‐NbSe 2 arXiv 2401 08296 2024

• Title: Engineering THz phonon and charge transfer in van der Waals heterostructures

• Speaker: Prof. Yoseob YOON (Northeastern University)

• Date & Time / Venue: May 23rd (Thu.), 2024, 11:00 AM / Room 343, IBS POSTECH Campus Building

• Title: Quantum transport through charge Kondo circuits: Role of electron-electron interactions in Luttinger liquid

• Speaker: Prof. Thi Kim Thanh Nguyen  (Vietnam Academy of Science and Technology)

• Date & Time / Venue: May 21st (Tue.), 2024, 3:00 PM / APCTP #512

• Abstract
  : In this talk, I will briefly introduce our works about the theoretical investigation of the effects of electron-electron interactions in different charge Kondo circuits (CKCs). A one-channel CKC is implemented by a large metallic quantum dot (QD) weakly coupled to an electrode (the source) and strongly coupled to the other one through an almost transparent single-mode quantum point contact (QPC) in the spinless case. The same setup in the spinful case creates a two-channel CKC. In the integer quantum Hall (IQH) charge Kondo implementation, the number of QPCs is equivalent to the number of the orbital channels in the conventional S=1/2 Kondo problem. We investigated two channel Kondo models in these two different setups. The whole system is formed by two- dimensional electron gas (2DEG) in which the QD-QPC structure is characterized by the Luttinger liquid model while the source is described by the Fermi liquid one. The universal temperature scaling laws of the thermoelectric coefficients are computed perturbatively in respect to the reflection amplitude of the QPC using the Abelian bosonization technique. In the first setup, we predict that the relevance of the backscattering g11 process in the electron-electron interactions induces a universal crossover from non-Fermi liquid - two channel Kondo to Fermi liquid - 1 channel Kondo. In the second setup, the universal temperature scaling of the conductance G_O-G [T/T*]^g when the system approaches the two channel Kondo intermediate coupling fixed point allows one to determine the effects of the electron electron-electron interactions in the two- channel charge Kondo-IQH circuits.

• Title: Enigma of two-dimensional melting in a disordered environment

• Speaker: Prof. Amit Ghosal (Indian Institute of Science Education & Research, Kolkata)

• Date & Time / Venue: March 27th (Wed.), 2024, 4:00 - 5:00 PM / Room 105, IBS POSTECH Campus Building

• Abstract
  : We will present the results from our study of melting in a two-dimensional system of classical particles with Gaussian-core interactions in disordered environments. The clean system validates the conventional two-step melting with a hexatic phase intervening between the solid and the liquid. This picture gets significantly modified in disordered environments. Disorder, in the form of a random distribution of pinning centers, forces a hexatic-like low-temperature phase that transits into a liquid at a single melting temperature T_RP. In contrast, pinning centers located at randomly chosen sites of a perfect crystal anchor a solid at low temperatures, which undergoes a direct transition to the liquid at T_CP. Thus, the two-step melting is lost in either case of disorder. We will discuss the characteristics of melting depending on the nature of the impurities. The intriguing dynamical signatures of the system across melting, both in the presence and absence of impurities, will also be discussed.

• Title: Symmetry manipulation of spin currents in van der Waals heterostructures

• Speaker: Dr. Kyoung-Whan Kim (Center for Spintronics, Korea Institute of Science and Technology)

• Date & Time / Venue: January 24th (Wed.), 2023, 1:30 PM / Physics Seminar Room (Science Bldg Ⅲ, #302)

• Abstract
  : The generation of a spin current and its role in magnetization dynamics are central topics in spintronics. Over the last decade, there have been intensive studies on the electrical generation of a spin current through charge-to-spin conversion. In spin-orbit coupled materials under an applied electric field, it is known that a transverse spin current can be generated by the spin Hall effect, providing an effective means to manipulate magnetic states. However, the spin Hall effect is subject to symmetry constraints, hindering magnetic switching without an external magnetic field. In this talk, I will review the symmetry constraints of electrically generated spin current and demonstrate that an effective way to resolve this issue is by exploiting crystalline asymmetry in certain materials, such as WTe2. Unfortunately, the spin-to-charge conversion efficiency in WTe2 is much lower than in conventional transition metals like Pt. Here, we propose a van der Waals heterostructure, WTe2/PtTe2, as an efficient spin source that not only avoids the symmetry constraint but also exhibits a high effective spin Hall conductivity. We introduce a novel spin-to-spin conversion mechanism for the high spin Hall conductivity in the WTe2/PtTe2 multilayer and show that spin-to-spin conversion opens new possibilities in spintronics, which are difficult to be achieved with conventional charge-to-spin conversion mechanism.

• Title: Beginning, present and future of two-dimensional van der Waals magnets

• Speaker: Prof. Je-Geun Park (Seoul National University)

• Date & Time / Venue: November 14th (Tue.), 2023, 2:00 - 4:00 PM / Room 104(Auditorium), IBS POSTECH Campus Building

• Abstract
  : Have you ever thought about how a new physics, small or big, starts? You may, perhaps, many times. Sometimes agonizingly, I am afraid. Yes, we all dream about having something of our own and as a scientist, we have always strived to achieve this dream. But how?

In this talk, I would like to take you through a small but painful journey I dared to take over the last ten years or so. My story is about how I started a then-unknown field of van der Waals magnets. I will tell you about how I started and how it has emerged as one of the most vibrant fields in materials science.

• Title: Second harmonic generation: A symmetry probe for 2D materials

• Speaker: Prof. Shiwei Wu (Fudan University)

• Date & Time / Venue: November 8th (Wed.), 2023, 10:30 AM / Room 104(Auditorium), IBS POSTECH Campus Building

• Abstract
  : Atomically thin two-dimensional materials such as graphene, transition metal dichalcogenide and chromium trihalide monolayers have recently spurred a great of interests due to their unique mechanic, electronic, optical and magnetic properties. And often these properties could be greatly tuned by external stimuli such as electric, magnetic and force field. Individual member in this class of 2D materials is also characteristic in term of different symmetries. Moreover, the symmetries could also be tuned, depending on how monolayers are stacked on one another. These variations in symmetry have given rise to even richer properties among different 2D materials and their homo-/hetero-structures. Therefore, they provide a new playground for nonlinear optics, namely second harmonic generation, because of its sensitivity to symmetries. Vice versa, second harmonic generation becomes a powerful technique to study 2D materials. In this talk, I will present some of our recent results on 2D materials [1-3]. 

References: 

[1] Zeyuan Sun et al., Giant nonreciprocal second harmonic generation from antiferromagnetic bilayer CrI3. Nature 572, 497 (2019). 

[2] Yu Zhang et al., Doping-induced second harmonic generation in centrosymmetric graphene from quadrupole response. Physical Review Letters 122, 047401 (2019). 

[3] Yuwei Shan et al., Stacking-symmetry governed second harmonic generation in graphene trilayers. Science Advances 4, eaat0074 (2018)

• Title: Twisted two-dimensional materials

• Speaker: Prof. Choi, Hyoung Joon (Yonsei Univ.)

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

• Abstract
  : Twisted layers of two-dimensional materials such as graphene, black phosphorus, and transition-metal dichalcogenides have emerged as a new platform of novel electronic structures. Since they have large moiré supercells typically, it becomes a challenge to perform accurate theoretical calculations of their electronic structures. Here, we present atomistic approaches to atomic and phononic structures of twisted graphene layers [1-3] and twisted transition-metal dichalcogenide layers [4-6], and discuss their atomic, electronic, and phononic structures [1-6]. In addition, we discuss peculiar electronic structures of twisted black phosphorus layers [7]. 

[1] Young Woo Choi and HyoungJoon Choi, Phys. Rev. B 98, 241412 (2018). 

[2] Young Woo Choi and HyoungJoon Choi, Phys. Rev. B 100, 201402 (2019). 

[3] Young Woo Choi and HyoungJoon Choi, Phys. Rev. Lett. 127, 167001 (2021). 

[4] Soo Yeon Lim et al., ACS Nano 17, 13938 (2023). 

[5] Siwon Oh, Han-gyu Kim, Jungcheol Kim, HuiseokJeong, HyoungJoon Choi, and Hyeonsik Cheong, submitted. 

[6] HuiseokJeong, Han-gyu Kim, and HyoungJoon Choi, in preparation. 

[7] Taesik Nam, Han-gyu Kim, and HyoungJoon Choi, in preparation.

• Title: Correlated Phases in Two-dimensional Twisted Moiré Materials

• Speaker: Prof. Oh, Myungchul (POSTECH)

• Date & Time / Venue: September 20th (Wed.), 2023, 4:00 PM / Physics Seminar Room(Science Bldg Ⅲ, #302)

• Abstract
  : In a flat band system, interactions between electrons become dominent due to the suppressed kinetic energy and many-body effect begins to occur. The recent breakthrough in engineering the band structure by creating a moiré superlattice in a twisted two-dimensional system has paved the way for the exploration of numerous strongly correlated quantum phenomena that emerge from the symmetry broken many-body ground state, such as correlated insulators, non-trivial topological phases, and unconventional superconductors [1-4]. In this talk, I will discuss the moiré superlattice flat band engineering in twisted two-dimensional van der Waals heterostructure and the correlated phases on the moiré superlattice systems, and describe underlying many-body physics in those phases. I will also highlight the novel scanning probe microscopy technique that has enhanced our understanding of the microscopic electronic structures of their ground state[2,4,5]. At the end of this talk, I will discuss on future prospects of twisted two-dimensional systems putting recent research progress together.

• Title: Plasmonic nanofocusing for nonlinear optical nano-spectroscopy of two-dimensional materials

• Speaker: Prof. VasilyKravtsov (ITMO University)

• Date & Time / Venue: June 23nd (Fri.), 2023, 10:00 AM / Physics Seminar Room(Science Bldg Ⅲ, #302)

• Abstract
  : Optical properties of many materials and macroscopic systems are defined by ultrafast dynamics of electronic, vibrational, and spin excitations localized on the nanoscale. Harnessing these excitations for material engineering, optical computing, and control of chemical reactions has been a long-standing goal in science and technology. However, it is challenging due to the lack of spectroscopic techniques that can resolve processes simultaneously on the nanometer spatial and femtosecond temporal scales. We develop a novel type of near-field ultrafast microscopy based on the concept of adiabatic plasmonic nanofocusing. Simultaneous spatio-temporal resolution on nanometer and femtosecond scale is achieved by using a nonlinear optical response induced by ultrafast surface plasmon polaritons nanofocused on a metal tip. We apply this technique to investigate nonlinear optical response and its dynamics in two-dimensional materials. In graphene, we reveal 5-6 fs dephasing times due to strong electron-electron interaction and nonlocal response on the scale of 100-300 nm. In monolayer WSe2, we reveal coherent electron dynamics with dephasing times in 5-60 fs range and its spatial heterogeneity on 50-100 nm scale.

• Title: 2D material transistors : Common interest Between physics and the semiconductor industry 

• Speaker: Prof. Dongseok Suh (Ewha Womans Univ.) 

• Date & Time / Venue: May 22nd (Mon.), 2023, 4:00 PM / Room #104 - Auditorium IBS POSTECH Campus(New building) 

• Abstract
  : Transistors are essential components in electronic devices, and the semiconductor industry is constantly seeking ways to scale down transistors to achieve higher performance and power efficiency. However, traditional transistor technologies face challenges in further scaling due to physical limitations. 2D materials, such as graphene and transition metal dichalcogenides (TMDs), offer unique properties that make them promising candidates for next-generation transistors. They can be combined to form heterostructures, enabling the creation of complex electronic devices with tailored properties. Integrating 2D materials into transistor designs is seen as a potential solution to the scaling challenge. Transistors provide an excellent platform for studying 2D materials from a physics perspective. By adjusting the gate voltage, the carrier concentration in the channel region of the transistor can be controlled, providing a means to examine the electronic properties and explore the unique phenomena exhibited by 2D materials as well as their heterostructures. Additionally, 2D material itself is the surface of the transistor channel that is highly sensitive to the surrounding conditions. Such sensitivity allows researchers to study the interaction between 2D materials and their environment. In this presentation, we will discuss the experimental approaches from a physics viewpoint involving 2D material transistors in contact with functional materials exhibiting ferroelectricity, ferromagnetism, or other quantum behaviors, which gives a novel tool to detect the phase transition of functional materials. Furthermore, we will showcase our endeavors to address the industrial-level interest in developing ferroelectric-FETs and negative-capacitance FETs using 2D materials.

• References
  [1] “Charge carrier modulation in graphene on ferroelectric single-crystal substrates”, NPG Asia Materials 14 (1), 58 (2022)
  [2] “Distinctive Photo‐Induced Memory Effect in Heterostructure of 2D Van Der Waals Materials and Lanthanum Aluminate”, Advanced Optical Materials 10 (16), 2200124 (2022).
  [3] “Two-dimensional ferromagnetism detected by proximity-coupled quantum Hall effect of graphene”, npj Quantum Materials 7 (1), 27 (2022)
  [4] “Quantum Conductance Probing of Oxygen Vacancies in SrTiO3 Epitaxial Thin Film using Graphene”, Advanced Materials 29 (18), 1700071 (2017)
  [5] “Voltage Scaling of Graphene Device on SrTiO3 Epitaxial Thin Film”, Nano Letters 16 (3), 1754-1759 (2016) 

• Title: Operando electron microscopy investigation of polar domain dynamics in twisted van der Waals homobilayers 

• Speaker: Prof. Hyobin Yoo (Sogang Univ.) 

• Date & Time / Venue: May 8th (Mon.), 2023, 4:00 PM / Physics Seminar Room(Science Bldg Ⅲ, #302)

• Abstract
  : Conventional antiferroelectric (AFE) materials with atomic scale anti-aligned electric dipoles exhibit a linear dielectric response to the small electric fields. Under a strong electric field, the AFEs undergo a transition to a ferroelectric (FE) phase where the dielectric response becomes hysteretic. Moiré superlattice formed in the twisted stacks of noncentrosymmetric van der Waals (vdW) crystals exhibits an array of triangular domains with antialigned electric dipoles that alternate in moiré length scale. In this moiré domain-antiferroelectic (MDAF) arrangement, the distribution of electric dipoles is clearly distinguished from that of recently reported 2- dimensional ferroelectrics (FEs), suggesting dissimilar domain dynamics in MDAF and FE states. Here we performed operando transmission electron microscopy (TEM) investigation on twisted bilayer transition metal dichalcogenides that enables real-time observation of the polar domain response to applied electric fields. We find that the topological protection provided by the domain wall network (DWN) in the MDAFs, results in linear dielectric response to the small electric field but prevents the MDAF to FE transition even at large electric field. As one decreases the twist angle, however, MDAF to FE transition occurs when the hysteretic domain wall motion becomes permissible due to the disappearance of the topologically protected DWN. In this FE phase, we find the domain dynamics in response to vertical electric fields is governed by the consecutive domain wall pinning-depinning process. Aberration corrected scanning TEM (STEM) analysis identifies the microstructural origin for the domain wall pinning, providing structural insight on how to improve the switching speed of vdW FE. 

• Title: Oxidation physics and metaltronics using ultra-flat copper thin film

• Speaker: Prof. Se-Young Jeong (Pusan. Univ.)

• Date & Time / Venue: May 3rd (Wed.), 2023, 4:00 ~ 5:30 PM / Physics Seminar Room(Science Bldg Ⅲ, #302)

• Abstract
  : Due to the lattice mismatch with the substrate, the crystallinity of thin films and the electron motion is hindered by numerous grain boundaries, raising the question of whether the known physical properties, including electron transport in metal thin films, hold true. However, the use of Atomic Sputtering Epitaxy (ASE) represents a pivotal step in uncovering the hidden physical properties of metal. By growing thin films like homoepitaxy, despite heteroepitaxy, a longer mean free path and coherence length of electrons is achieved, thereby unlocking diverse physical phenomena. This seminar introduces a groundbreaking approach to thin film growth, involving the use of extended atomic distance mismatch (EADM) to circumvent the challenges posed by lattice mismatches. Additionally, the novel concept of oxidation physics, treating oxidation as a vector quantity, is explored, opening up a range of possibilities. The revolutionary notion of metaltronics, which seeks to introduce semiconductor properties into metal, is also presented, offering new applications for metals beyond traditional use as electrodes. Specifically, this seminar delves into the potential of copper, a metal we believed we knew all too well.

• Title: Steady Floquet–Andreev states in graphene Josephson junctions

• Speaker: Sein Park (POSTECH) 

• Date & Time / Venue: March 31th (Fri) , 2023, 10:30 AM / Physics Seminar Room(Science Bldg Ⅲ, #302)

• Abstract
  : Engineering quantum states through light–matter interaction has created a paradigm in condensed-matter physics. A representative example is the Floquet–Bloch state, which is generated by time-periodically driving the Bloch wavefunctions in crystals. Previous attempts to realize such states in condensed-matter systems have been limited by the transient nature of the Floquet states produced by optical pulses1,2,3, which masks the universal properties of non-equilibrium physics. Here we report the generation of steady Floquet–Andreev states in graphene Josephson junctions by continuous microwave application and direct measurement of their spectra by superconducting tunnelling spectroscopy. We present quantitative analysis of the spectral characteristics of the Floquet–Andreev states while varying the phase difference of the superconductors, the temperature, the microwave frequency and the power. The oscillations of the Floquet–Andreev-state spectrum with phase difference agreed with our theoretical calculations. Moreover, we confirmed the steady nature of the Floquet–Andreev states by establishing a sum rule of tunnelling conductance4, and analysed the spectral density of Floquet states depending on Floquet interaction strength. This study provides a basis for understanding and engineering non-equilibrium quantum states in nanodevices.

• Title: Quantum Hall superfluid in twisted bilayer/double bilayer graphene 

• Speaker: Prof. Youngwook Kim (DGIST) 

• Date & Time / Venue: March 27th (Mon.) , 2023, 10:30 AM / Physics Seminar Room(Science Bldg Ⅲ, #302)

• Abstract
  : We introduce a novel two-dimensional electronic system with ultrastrong interlayer interactions, namely twisted bilayer graphene with a large twist angle, as an ideal ground for realizing interlayer-coherent excitonic condensates. In these systems, subnanometer atomic separation between the layers allows significant interlayer interactions, while interlayer electron tunneling is geometrically suppressed due to the large twist angle. By fully exploiting these two features we demonstrate that a sequence of odd-integer quantum Hall states with interlayer coherence appears at the second Landau level (N = 1). Notably the energy gaps for these states are of order 1 K, which is several orders of magnitude greater than those in GaAs. Furthermore, a variety of quantum Hall phase transitions are observed experimentally. All the experimental observations are largely consistent with our phenomenological model calculations. Hence, we establish that a large twist angle system is an excellent platform for high-temperature excitonic condensation.
  We also observed similar states in a stack of two decoupled graphene bilayers. Indeed, such a Bose-Einstein condensate is observed for half filling in each bilayer sheet when the partially filled level has orbital index 1, whereas it is absent for partially filled levels with orbital index 0. The application of asymmetric top and bottom gate voltages enables to influence the orbital nature of the electronic states of the graphene bilayers and to navigate in an orbital mixed space. The latter hosts an even denominator fractional quantum Hall state at total filling -3/2. Our observations suggest a unique edge construction involving both electrons and chiral p-wave composite fermions. 

• Title: Mattermorphosis 

• Speaker: Prof. Thomas Heine (Chair of Theoretical Chemistry, TU Dresden) 

• Date & Time / Venue: March 7th (Tue.) , 2023, 4:00 PM / Physics Seminar Room(Science Bldg Ⅲ, #302)

• Abstract
  : The properties of a material are defined by their composition and structure. This principle of chemistry is wellknown for decades and the basis for the development of molecules and materials. In my talk, I will show some factors that define materials properties beyond their intrinsic characteristics: In the first part, I will show how the edge structure in graphene nanoribbons determines the band gap (including metallic state!) and electronic topology of these all-carbon materials. In the second, I will discuss superlattice states in transition metal dichalcogenide bilayers, which can form flat bands, Dirac or kagome features depending on the twist angle. In the final part, I will discuss how the underlying lattice topology can define the properties of a 2D framework material, such as a 2D polymer, covalent-organic framework or metal-organic framework. Some of the introductory literature are given below (if you don’t have access, they are also available on arXiv or chemRXiv. 

[1] Structure-Imposed Electronic Topology in Cove-Edged Graphene Nanoribbons. F. M. Arnold, T.-J. Liu, A. Kuc, T. Heine, Phys. Rev. Lett. 129 (2022) 216401
[2] Topological two-dimensional polymers. M. A. Springer, T.-J. Liu, A. Kuc, T. Heine, Chem. Soc. Rev. 49 (2020) 2007-2019
[3] 2D Conjugated Polymers: Exploiting Topological Properties for the Rational Design of Metal-Free Photocatalysts. Y. Jing, X. Zhu, S. Maier, T. Heine, Trends Chem. 4 (9) (2022) 792-806 

• Title: Topotactic redox engineering toward new/functional materials

• Speaker: Woo Jin Kim (SLAC)

• Date & Time / Venue: February 23th (Thu.) , 2023, 4:00 PM / Physics Seminar Room(Science Bldg Ⅲ, #302)

• Abstract
  : Due to their multiple oxidation states, transition metal oxides can be existing in various forms with different structure symmetries. By selectively introducing or removing oxygen from specific sites in a crystal lattice, it is possible to manipulate the material's electronic structure, leading to the emergence of quantum phenomena, such as colossal magnetoresistance and superconductivity [1,2]. Topotactic redox reactions allow for this reversible reduction and oxidation process to occur without altering the relative structure of the cations of the material. In this talk, I will discuss a quasi-two-dimensional material system that is stabilized through topotactic reduction, where the Jahn-Teller effect results in a beautiful pattern of distortions. In the second half of the talk, I will introduce the use of mechanochemistry on freestanding oxide membranes to greatly reduce activation energy for redox reactions, which can be useful for creating new and clean functional oxides.

[1] R. von Helmolt et al., Phys. Rev. Lett. 71, 2331–2333 (1993).

[2] D. Li et al., Nature 572, 624–627 (2019).

• *Supported by Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, under contract No. DE-AC02-76SF00515.

• Title: Edge transport in gapped bilayer graphene

• Speaker: 정현우 (이길호 교수님 연구실)

• Date & Time / Venue: January 31th (Tue.), 2023, 4:30 PM / Physics Seminar Room(Science Bldg Ⅲ, #302)

• 
  

• Abstract
  : Dissipationless topologically protected states have been extensively studied in various materials because of its applicability to quantum information technology. Graphene family (mono, bi and twisted graphene) with broken inversion symmetry have shown anomalous nonlocal resistance, which are claimed to be topological nature by several studies. However, the origin of the nonlocal resistance has been strongly debated. With our transport data from the gapped bilayer graphene device, I will discuss which process increases the nonlocal resistance greatly that it appears anomalous.  

1. 연사: 정윤장 박사 (UMD)

2. 일시: 2022. 12. 8(목), 오후 4시

3. 장소: 공학3동 302호 (세미나실)

4. 제목: Ultra-high-quality quantum materials

Quantum materials can loosely be defined as those used for the investigation of quantum phenomena such as entanglement, many-body physics, or topological behavior. While the physical properties of the materials themselves obviously play an essential role in determining whether a material can be classified as a quantum material or not, oftentimes what is equally important is the quality of the material. This is because typically, quantum phenomena are inherently delicate and hence quite susceptible to any perturbative species within the system that could cause a scattering event.

A hallmark example that demonstrates the impact of material quality on quantum science is the two-dimensional electron system (2DES) hosted in GaAs/AlAs based quantum wells.

Over the past few decades, a plethora of quantum phases such as Wigner solids, odd- andeven-denominator fractional quantum Hall states (FQHSs), and stripe/nematic phases have materialized in these 2DESs as sample qualityprogressed. In this talk, I will discuss how to fabricateultra-high-quality AlAs and GaAs 2DESs via molecular beam epitaxy. The samples presented in this work have the highest reported electron mobility in their corresponding systems, with the GaAs 2DESs going a step further and holding the world record for electronmobility in any system ever measured by mankind. Such levels of sample cleanliness have an immediate impact on the study of quantum many-body physics, as demonstrated by the emergence of several new interaction-driven phases. While GaAs and AlAs are discussed as examples here, similar design and growth principles can be applied to other group IV, III-V, and II-VI material systems to expand the list of ultra-high-quality quantum materials. 

IBS-CALDES SEMINAR (Zoom)

▶Zoom Link : https://us06web.zoom.us/j/83170412906?pwd=WFozcndEK1VRMURERnI1MkExL0UvZz09

▶ID :  831 7041 2906  /  PW : 603934

Electrons Innovate Materials:Quantum Alchemy in “2D materials”, “Semiconductor” and “Metal”

Department of Energy Science, Center for Electride Materials, Sungkyunkwan University, Korea

Sung Wng Kim

kimsungwng@skku.edu

In this talk, I would like to introduce the exotic material, electrides from their history and basics to recent research, with particular focus on two-dimensional electrides.

Electride, which is regarded as a new emergent quantum material, is ionic crystal in which electrons serve as anions. The physical properties of electrides are determined by the topology of cavities or channels which confine anionic electrons. The most representative property is a low work function based on the anionic electrons. Recently, it was demonstrated that the intralayer space of 2D layered materials can be the confining sites for anionic electrons, showing a freedom in degree of localization. This new 2-dimensional electrides have provided fundamental difference in electronic structure from the 2-dimensional electron gas systems in topology and physical properties. It will be highlighted that the diverse magnetism based on two-dimensionally confined anionic electrons can be possible in electrides even without magnetic elements. Further, the water- and acid-stable 2D electrides enabling a persistent electrocatalytic reaction such as HER and ORR will be introduced as practical applications of electrides. As perspectives, pure 2D electron phase on the 2-dimensional electride and novel metal surface created by electrides will be introduced and discussed in the context of new “electron physics”.

      Mesoscopic structures in ultrathin silica films

Kristen M. Burson,1 Hyun Jin Yang,2 Daniel S. Wall,1 Thomas Marsh,1 Zechao Yang,2 David Kuhness,2 Matthias Brinker, 2 Leonard Gura,2 Markus Heyde,2 Wolf-Dieter Schneider,2 and Hans-Joachim Freund2

1) Hamilton College, 198 College Hill Road, Clinton, NY, 13323 USA

2)Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany,

(corresponding author W.-D. Schneider, e-mail: wolf-dieter.schneider@epfl.ch) 

Silica films can be prepared in both, crystalline and vitreous forms as well as of mixtures between them. In the past, the atomic-scale structure of this film system has been in the center of interest. However, at a larger scale, mesoscale structures like holes and substrate steps can play an important role for confined space reactions and other applications of silica films. In the present investigation we report on mesoscale structures in silica films grown on Ru(0001) in ultra-high vacuum, and probed with scanning tunneling microscopy (STM). We find that silica films can exhibit coexisting phases of monolayer, zigzag, and bilayer structures. These coexisting phases were observed to be influenced by holes in the film structure and also by atomic-scale substrate steps. Specifically, film regions bordering holes in silica bilayer films exhibit vitreous character, even in regions with predominant crystalline film structure.

The present characterization of mesoscale  structures in Ru-supported ultrathin silica films provides a scale-up of the former atomic-scale investigations with implications for catalysis and chemistry in confined space. Specifically, the transition from the amorphous to the crystalline phase near film holes is an exciting observation which may be expected to have implications for structural control of materials.

This project has received funding from the European Research Council (ERC) under the European Unions Horizon 2020 Research and Innovation Program (Grant Agreement No. 669179).

Title :   Halide Perovskite Single Crystals : Growth, Characterization, and Stability for Optoelectronic Applications 

abstract : 

Reported perovskite devices based on polycrystalline thin films suffer immensely from poor stability and high trap density owing to grain boundaries limiting their performance. Perovskite single crystal structures have been recently explored to construct stable devices and reduce the trap density compared to their thin-film counterpart. We present a novel method of growing sizable CH3NH3PbX3 (X = I, Br, and Cl) single crystals based on the high solubility characteristic of hybrid perovskites at low temperature within inverse temperature crystallization. Photogenerated charge transport is critical to further improving the performance of perovskite-based optical devices. In this study, characteristics of photodetectors has been studied with a variety of electrodes including Au, Ag, and TiO2. The transport mechanism of the photogenerated carriers in each device is elucidated by analysis of the current density-voltage curves obtained under dark or illuminated conditions at a visible wavelength of 640 nm or a near-ultraviolet wavelength of 405 nm. The photocurrent under the super-bandgap illumination is improved under the hole-device of p-type iodide and bromide crystals and the electron-device of n-type chloride crystals. The defect-assisted excitation under sub-bandgap illumination is associated with generation of the holes with the defects located near the conduction band or the electrons with the valence band in each perovskite materials. Furthermore, the photodetection responsivity exhibits strong enhancement of about 10 times or more by selection of the proper electrode. The present results suggest a promising strategy for designing efficient photodetectors using photogenerated electrons or holes and trap-assisted photocarriers that contribute to photocurrent enhancement. 

Y. Cho, H. R. Jung, and W. Jo, “Halide Perovskite Single Crystals: Growth, Characterization, and Stability for Optoelectronic Applications”, Nanoscale (2022) 14, 9248 

Title :   Halide Perovskite Single Crystals : Growth, Characterization, and Stability for Optoelectronic Applications 

abstract : 

Reported perovskite devices based on polycrystalline thin films suffer immensely from poor stability and high trap density owing to grain boundaries limiting their performance. Perovskite single crystal structures have been recently explored to construct stable devices and reduce the trap density compared to their thin-film counterpart. We present a novel method of growing sizable CH3NH3PbX3 (X = I, Br, and Cl) single crystals based on the high solubility characteristic of hybrid perovskites at low temperature within inverse temperature crystallization. Photogenerated charge transport is critical to further improving the performance of perovskite-based optical devices. In this study, characteristics of photodetectors has been studied with a variety of electrodes including Au, Ag, and TiO2. The transport mechanism of the photogenerated carriers in each device is elucidated by analysis of the current density-voltage curves obtained under dark or illuminated conditions at a visible wavelength of 640 nm or a near-ultraviolet wavelength of 405 nm. The photocurrent under the super-bandgap illumination is improved under the hole-device of p-type iodide and bromide crystals and the electron-device of n-type chloride crystals. The defect-assisted excitation under sub-bandgap illumination is associated with generation of the holes with the defects located near the conduction band or the electrons with the valence band in each perovskite materials. Furthermore, the photodetection responsivity exhibits strong enhancement of about 10 times or more by selection of the proper electrode. The present results suggest a promising strategy for designing efficient photodetectors using photogenerated electrons or holes and trap-assisted photocarriers that contribute to photocurrent enhancement. 

Y. Cho, H. R. Jung, and W. Jo, “Halide Perovskite Single Crystals: Growth, Characterization, and Stability for Optoelectronic Applications”, Nanoscale (2022) 14, 9248 

Speaker: Prof. Se Kwon Kim (KAIST)

Time: Sep 21 (Wed) 4 pm  

Place: Physics Seminar Room (Science Bldg. III, #302)

Title: Quantum Spintronics

abstract : 

Recent advancements in spintronic techniques originally developed for spin-based devices now enable us to study fundamental spin physics of various quantum materials with unprecedented spin-current control and measurement, opening a new area of theoretical and experimental investigation of quantum systems. In this talk, we will introduce this emerging research area of spin transport in quantum materials which is fueled by the global interest in quantum information science. As examples, we will discuss our discovery of magnonic topological insulators realized by 2D magnets [1-3], which shows how spintronic techniques can be used for probing elusive quantum materials, and our prediction of long-range spin transport mediated by a vortex liquid in superconductors [4], which shows that quantum materials can provide novel platforms for efficient spin-transport devices. We will conclude the talk by offering a future outlook on quantum spintronics. 

[1] S. K. Kim, H. Ochoa, R. Zarzuela, and Y. Tserkovnyak, “Realization of the Haldane-Kane-Mele Model in a System of Localized Spins,” Phys. Rev. Lett. 117, 227201 (2016) [2] G. Go, S. K. Kim, and K.-J. Lee, "Topological Magnon-Phonon Hybrid Excitations in Two-Dimensional Ferromagnets with Tunable Chern Numbers," Phys. Rev. Lett. 123, 237207 (2019) [3] S. Zhang, G. Go, K.-J. Lee, S. K. Kim, "SU(3) Topology of Magnon-Phonon Hybridization in 2D Antiferromagnets," Phys. Rev. Lett. 124, 147204 (2020) [4] S. K. Kim, R. Myers, and Y. Tserkovnyak, "Nonlocal Spin Transport Mediated by a Vortex Liquid in Superconductors," Phys. Rev. Lett. 121, 187203 (2018)

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: What can van der Waals materials bring to quantum technologies?

v Speaker: Dr. Kin Chung Fong (Raytheon BBN Tech., USA)

v Place: Physics Lecture Room (Science Bldg. III, #111)

v ZOOM: ID 925 5317 1007 / PWD 123456

v Date & Time: May 19th (Thu.), 11:00 AM ~

v Zoom https://postech-ac-kr.zoom.us/j/92553171007?pwd=dFM0TW44OFlOdjU4bzJaVVlCUXI0UT09

v  Title: Operando transmission electron microscopy investigation

on domain dynamics in two dimensional ferroelectric materials

v  Speaker: Prof. Hyobin Yoo (Sogang Univ.)

v  ZOOM ID: 948 064 9013 / Password: 123456

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

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

* When entering the meeting, please rename your profile as your full name (affiliation)

2021-2 물성물리학특론 III : 2차원물질연구의동향 (PHYS703-01)

매주 화요일/목요일 오후 5시-6시 15분

9/7, 9/9, 9/14, 9/16 Introduction : Physics in two dimensional materials (Prof. Gil Young Cho)

9/23, 9/28, 9/30, 10/5 Structure of two dimensional materials (Prof. Si Young Choi)

10/7, 10/12, 10/14, 10/19 Optical properties of two dimensional materials (Prof. Jonghwan Kim)

10/21, 10/26, 10/28 (no lecture) 

11/2, 11/4, 11/9, 11/11 Superconductivity in two dimensional materials (Prof. Gil-Ho Lee)

11/16, 11/18, 11/23, 11/25 Magnetism in two dimensional materials (Prof. Jun Sung Kim)

11/30, 12/2, 12/7, 12/9 Electron correlation in two dimensional materials (Prof. Dae Su Lee)

12/14, 12/16 (no lecture, term-paper writing) 

12/21 or 12/23 (Exam)

Mechanisms of the bottom-up synthesis of van der Waals materials

Speaker: Prof. Feng Ding, Dept. of Material Science and Engineering, UNIST / CMCM, IBS

15:00 PM, November 12(Fri), 2021

Zoom Link:  https://us02web.zoom.us/j/86731687471?pwd=dStXTkJqS2ZEaUl3TUhjWWtpK2pVQT09

ID: 867 3168 7471 / PW: gdjSN3

Many emerging technologies call for the controllable fabrication of various van der Waals (vdW) materials. Applications in the semiconducting industry require the synthesis of high-quality vdW materials with a very large, wafer-scale area. To achieve this, a bottom up synthesis is essential. In this tutorial lecture, I will briefly introduce some parts of more than ten years of our research on mechanisms of the bottom-up synthesis of vdW materials, namely:

i. The intrinsic, weak vdW interaction between a vdW material and the substrate resulting in a new paradigm of materials epitaxy, which is beyond the existing theories of crystal growth.[1-2] The development of a theory of vdW materials growth is essential for the controllable synthesis of vdW materials such as large vdW single crystals with desired thickness and twisting angles between neighboring layers as well as for their heteroepitaxial superlattices.

ii. The first example is the mechanism of CVD growth of graphene that includes vdW interaction between graphene and various substrates [3], the mode of graphene CVD growth[4],the alignment of graphene on various substrates [4], and two possible routes towards the synthesis of wafer-scale single crystalline graphene films [5-7].

iii. The second example is the growth mechanism for hexagonal boronnitride(hBN). With the help of our theory [8],we are now able to grow wafer-scale hBN with proper thicknesses[9].

iv. A general theory of vdW materials’ epitaxy proposed later [10]. Based on a simple analysis, we found that substrates with low symmetry are preferred for the epitaxial growth of various vdW single crystals. The theory is now broadly used to guide the controllable growth of various 2D materials, such asTMDs. [11-12]

v. Besides that, I will introduce our experimental efforts on the preparation of high index substrates and on the growth of high-quality graphene which are both assisted by our theoretical studies.[13]

I will summarize these achievements, the challenges of the exciting research topics, and the future of vdWmaterials growth.

References:

[1].Chem. Rev.2021, 121, 11, 6321; [2]Adv. Mater. 2019, 31, 1801583; [3]J. Phys. Chem. Lett. 2012, 3, 2822; [4]J. Phys. Chem. Lett. 2014, 5, 3093; [5] Adv. Mater. 2015, 27, 1376; [6]Nat. Mater. 2016, 15, 43; [7]Sci. Bull. 2017, 62, 1074; [8]Nanoscale 2017, 9, 3561; [9]Nature 2019, 570, 91; [10]Nat. Commun. 2020, 11, 5862; [11]Nat. Nano., 2021, in press; [12]Adv. Funct. Mater. 2021, 31, 2102138; [13]Nature 2020, 581, 406.

Quantum light from 2D moiré heterostructures

1. 연사: 백현준 박사 (KIST)

2. 일시: 2021. 11. 11(목), 오후 4시

3. 장소: 공학3동 302호 (세미나실)

Van der Waals heterostructures can be designed to confine electrons and holes in unique ways. One remarkable approach is to vertically stack two atomically thin layers of transition metal dichalcogenide (TMD) semiconductors. The relative twist or lattice mismatch between the two layers leads to moiré pattern formation, which modulates the electronic band structure according to the atomic registry. Single-particle wave packets can be trapped in the moiré-induced potential pockets with three-fold symmetry, leading to the formation of trapped interlayer excitons. This can create uniform high-density arrays of quantum emitters or topological bands whose properties can be manipulated by electric or strain fields. In this talk, I will explain photoluminescence emission of moiré confined excitons in MoSe2/WSe2 heterobilayer [1,2]. Polarization and magnetic-field dependence of moiré confined excitons will be explained. Furthermore, interesting properties of moiré excitons like antibunching, large Stark shift, and doping dependence will be presented [2,3]. Finally, correlated states, such as Mott insulating states and Wigner crystals, observed from moiré heterostructures will be presented.

[1] M. Brotons-Gisbert, H. Baek, A. Molina-Sánchez, A. Campbell, D. Scerri, D. White, K. Watanabe, T. Taniguchi, C. Bonato, and B. D. Gerardot, “Spin-layer locking of interlayer excitons trapped in moiré potentials”, Nature Materials 19, 630 (2020)

[2] H. Baek, M. Brotons-Gisbert, Z. X. Koong, A. Campbell, M. Rambach, K. Watanabe, T. Taniguchi, and B. D. Gerardot, “Highly energy-tunable quantum light from moiré-trapped excitons”, Science Advances 6, eaba8526 (2020)

[3] H. Baek, M. Brotons-Gisbert, A. Campbell, K. Watanabe, T. Taniguchi, B. D. Gerardot, “Optical read-out of Coulomb staircases in a moiré superlattice via trapped interlayer trions”, Nature Nanotechnology, online published (2021)

Structure, functions, dynamics, and interactions are the basic properties to systematically understand physical systems existing in nature. In particular, there have been many scientific adventures to understand light-matter interactions, yet in the classical regime at the microscale due to the diffraction-limited optical resolution. Recently, plasmonic nano-cavity enables to induce light-matter interactions and tip-enhanced nano-spectroscopy enables to probe them at the nanoscale [1-3]. However, these two approaches have developed independently with their own weaknesses so far. In this talk, I provide a novel concept of “tip-enhanced cavity-spectroscopy (TECS)” overcoming the limitations of previous approaches to induce, probe, and dynamically control ultrastrong light-matter interactions in the quantum tunneling regime [4, 5]. Furthermore, I provide several new directions of nano-spectroscopy and -imaging, which have not been thought in the near-field optics community before. First, we exploit extremely high tip-pressure (~GPa scale) to directly modify the lattice structure and electronic properties of materials [6, 7]. Second, we dynamically control the near-field polarization by adopting adaptive optics technique to near-field optics [8]. Third, we develop conductive TECS to modify electrical properties of materials by directly flowing an electric current through the cavity junction. In addition, in the last part of this talk, I present specific research directions of our group in the fields of cavity quantum optics, plexciton condensate, quantum molecular physics, and quantum nonlinear optics, which can be enabled through the TECS approach.

[1] Park, K.-D. et al., Nature Nanotechnology 13, 59 (2018).

[2] Park, K.-D. et al., Science Advances 5, eaav5931 (2019).

[3] Kang, M. et al., Nature Communications (2022) In press.

[4] Lee, H. et al., Advanced Functional Materials 31, 2102893 (2021).

[5] Lee, H. and Koo, Y. et al., Science Advances 8, eabm5236 (2022).

[6] Koo, Y. et al., Advanced Materials 33, 2008234 (2021).

[7] Lee, H. et al., ACS Nano 15, 9057 (2021).

[8] Lee, D. Y. et al., Nature Communications 12, 3465 (2021).