Quantum Materials Workshop 2023
양자물성 워크숍 2023

June 21st ~ 23rd, 2023


   This is the 2nd joint workshop of the research teams at UOS, SNU, and SKKU lead by George Jung, Sukbum Chung, Hongki Min and Euyheon Hwang on novel properties of quantum materials. All participants are expected to give a short presentation of each one’s research topic. There will be plenty of room for discussions to find collaboration opportunities.

▶ Speakers

• Li Jinshu (SKKU)
Title: Metal-insulator transition (MIT) in 2D TMD

Since the pioneering observation of metal–insulator transition (MIT) phenomena in 2D Transition Metal Dichalcogenide (TMD) systems [1-7], the underlying theoretical electronic transport properties were investigated comprehensively based on the Boltzmann transport theory, including charged impurity, acoustic phonon, and optical phonon scattering mechanisms. Our theoretical results reveal a metal-insulator transition (MIT) with a characteristic conductivity of e2/h, which are in close agreement with experimentally reported values. We argue that a key feature of the MIT physics is the screened Coulombic disorder arising from the random distribution of charged impurities in the semiconductor structures

*Ref:[1] B. Radisavljevic, A. Kis, Nat. Mater. 2013, 12, 815. [2] X. Chen, Z. Wu, S. Xu, L. Wang, R. Huang, Y. Han, W. Ye, W. Xiong, T. Han, G. Long, Y. Wang, Y. He, Y. Cai, P. Sheng, N. Wang, Nat. Commun. 2015, 6, 6088. [3] B. H. Moon, J. J. Bae, M.-K. Joo, H. Choi, G. H. Han, H. Lim, Y. H. Lee, Nat. Commun. 2018, 9, 2052. [4] B. H. Moon, J. J. Bae, G. H. Han, H. Kim, H. Choi, Y. H. Lee, ACS Nano 2019, 13, 6631. [5] N. R. Pradhan, A. McCreary, D. Rhodes, Z. Lu, S. Feng, E. Manousakis, D. Smirnov, R. Namburu, M. Dubey, A. R. Hight Walker, H. Terrones, M. Terrones, V. Dobrosavljevic, L. Balicas, Nano Lett. 2015, 15, 8377. [6] L. J. Stanley, H.-J. Chuang, Z. Zhou, M. R. Koehler, J. Yan, D. G. Mandrus, D. Popović, ACS Appl. Mater. Interfaces 2021, 13, 10594. [7] Ali, F.; Ali, N.; Taqi, M.; Ngo, T. D.; Lee, M.; Choi, H.; Park, W.-K.; Hwang, E.; Yoo, W. J. Adv. Electron. Mater. 2022, 8 (9), 2200046.

• Seongjoon Kwon (UOS)
Title: Kitaev Spin Interaction in Na2IrO3 / α-RuCl3 Monolayer

Quantum spin liquid is a ground state of a spin system in magnetically frustrated materials. Kitaev propsed the analytically solvable QSL models in anti-ferromagnetic honeycomb lattice so called Kitaev quantum spin liquid(KQSL). The ground state of KQSL could be represented by the Majorana fermions and its Hamiltoian is similar with a graphene model. Na2IrO3 and α-RuCl3 monolayers are candidates of the KQSL materials constructed by the effective spin state of atomic orbitals. So reviewing how to get a Kitaev interaction from these materials and cofirm some conditions for KQSL.

• Koyendrila Debnath (UOS)
Title: Origin of charge density wave (CDW): Fermi surface nesting or electron-phonon couping?

The origin of charge density wave (CDW) has its roots in Peierls distortion, an instability that exists in metallic state of one-dimensional chain of atoms. However, this concept can be extended to reduced dimensional systems due to a particular topology of the Fermi surface (FS). The idea is many electronic states in the FS can be scattered by the nesting wave vector, 𝑞𝐶𝐷𝑊 to other parts on the FS, leading to efficient screening of phonons. This also results in renormalization of the phonon mode at 𝑞𝐶𝐷𝑊 and reconstruction of the lattice at low temperature. We demonstrate this phenomenon in GdTe3, a layered material. On the contrary, we show through our first-principles calculations, that (TaSe4)2I with a strongly anisotropic electronic structure and weakly nested Fermi surface, also hosts CDW. Unlike GdTe3, the intrinsic mechanism of electronic and lattice instabilities (CDW) in (TaSe4)2I is not Fermi surface nesting but dictated by momentum dependent electron-phonon coupling.

*Ref: [1] G. Gruner, Density Waves in Solids (Addison-Wesley, Reading, MA) (1994) [2] MD Johannes and II Mazin. Fermi surface nesting and the origin of charge density waves in metals. Physical Review B, 77(16):165135, 2008. [3] Xuetao Zhu, Yanwei Cao, Jiandi Zhang, EW Plummer, and Jiandong Guo. Classi- fication of charge density waves based on their nature. Proceedings of the National Academy of Sciences, 112(8):2367–2371, 2015.

• Sangwoo Cho (UOS)
Title: edge states in Kitaev quantum spin liquid ribbon

quantum spin liquid는 상호작용하는 양자 스핀으로 이뤄진 물질의 상태로 long-range quantum entanglement, 준입자 분수화 현상이 나타나는 특징이 있다. 그 중에서 Kitaev quantum spin liquid(KQSL)는 정확하게 풀 수 있는 육각격자 모형으로 Majorana fermion이라는 준입자를 도입해서 풀 수 있다. 이 발표에서는 Z2 uniform flux 상태에 있는 ribbons 형태의 KQSL 테두리 상태를 계산한 결과에 대한 내용을 발표할 예정이다. 추가로 테두리 상태의 작용으로 인해 온도의 변화에 더 많은 영향을 받는 국지적 연산자가 어떤 것이 있는지도 계산했다..

• Jiho Jang (SNU)
Title: Theoretical investigation of optical signatures of a single nodal ring in SrAs3

Nodal-line semimetals (NLSMs) are a class of quantum materials where two bands cross on lines in momentum space, with a characteristic frequency-independent flat conductivity. However, the complexity of the nodal line shape and the coexistence of trivial bands at the Fermi energy can make it difficult to study the optical properties of NLSMs. Recently, SrAs3 has been proposed as a promising NLSM candidate. Unlike many other NLSMs, SrAs3 features a single ring-shaped nodal line in momentum space and no trivial bands coexist near the Fermi energy. In this presentation, I will discuss our optical study of a single, elliptical nodal ring in SrAs3 [1]. We calculate the optical conductivity using an effective model Hamiltonian with the aid of density functional theory band calculations. We will show how the nodal-line shape, spin-orbit coupling, and anisotropic velocities along the radial and axial directions affect the optical conductivity. Our results demonstrate excellent agreement between theory and experiment, providing new insights into the optical properties of SrAs3 and other NLSMs.

*Ref: [1] J. Jeon*, J. Jang*, H. Kim, T. Park, D. Kim, S. Moon, J. Kim, J. Shim, H. Min, and E. Choi, Optical transitions of a single nodal ring in SrAs3: radially and axially resolved characterization, arXiv:2303.14973 (2023). (∗ These authors contributed equally to this work.)

• Taehyeok Kim (SNU)
Title: Collective modes in multiband superconductors

*Ref: Changhee Lee, Chiho Yoon, Taehyeok Kim, Suk Bum Chung, and Hongki Min, Phys. Rev. B 104, L241115 (2021).

• Jeonghyeon Suh (SNU)
Title: Magnetotransport by the chiral anomaly in Weyl semimetals

• Taehun Kim (SNU)
Title: Quantum geometric contributions in multiband systems

The geometrical properties of matter have revolutionized our fundamental understanding of various physical phenomena. In condensed matter, the geometry of quantum states plays a crucial role in inter-band transitions, revealing that geometric contributions facilitate superconductivity and superfluidity in flat-band systems. In this talk, we investigate how the effects of multiple bands can be bounded by the positive-definite property of the geometric tensor. Furthermore, we observe that these properties can also be extended to other collective modes, including plasmons.

• Nicolas Leconte (UOS)
Title: Commensuration torques and lubricity in double moire systems.

We study the commensuration torques and layer sliding energetics of alternating twist trilayer graphene (t3G) and twisted bilayer graphene on hexagonal boron nitride (t2G/BN) that have two superposed moir\’e interfaces. Lattice relaxations for typical graphene twist angles of $\sim 1^{\circ}$ in t3G or t2G/BN are found to break the out-of-plane layer mirror symmetry, give rise to layer rotation energy local minima dips of the order of $\sim 10^{-1}$~meV/atom at double moir\’e alignment angles, and have stacking energy minima of comparable magnitude between the next-nearest top-bottom layers. Thus, in t3G the top and bottom layers tend to align when one twisted interface angle is fixed, while in t2G/BN the alignment of the two moir\’e patterns favors in t2G the magic angle near $\theta \simeq 1.12^{\circ}$. Precedence of rotation over sliding during the moir\’e commensuration is confirmed for periodic boundary systems where the sliding energy barriers drop to $\sim 10^{-4}$~meV/atom for physical misalignment of graphene smaller than $\sim 0.03^\circ$. For finite graphene flakes of diameter $D$ we find enhanced friction forces for a wider range of angles $\Delta{\theta}_{\rm FWHM} \sim C/D$ degrees both near the physical alignment angle in t2G and commensurate double moir\’e angles in t3G.

• Prathap Kumar Jharapla (UOS)
Title: Electronic structure and Landau level spectrum of lattice-relaxed twisted triple-bilayer graphene

We study the electronic structure and landau level spectrum of twisted triple bilayer graphene (t3BG) based on atomistic calculations for three possible configurations AB/AB/AB, AB/BA/AB and AB/AB/BA denoting the specific stacking of each bilayer where the top and bottom bilayers are rotated with respect to the middle bilayer. By simulating the atomic relaxation process and using an accurate real-space TB model, we aim to gain insights into the unique electronic characteristics of this system. We further predict which structures are likely to be energetically stable.

• Kyungjin Shin (SNU)
Title: Effective theory of biased alternating-twist multilayer graphene

Recently, the emergence of alternating-twist multilayer graphene (ATMG), where the twist angle alternates \pm\theta/2 between the interface as a generalization of twisted bilayer graphene, has been studied intensively as an ideal platform to investigate the correlated electron phenomena. In particular, ATMG has attracted much attention due to its robust superconductivity observed from bilayer to pentalayer samples. In this talk, we present theoretical study of energy and optical absorption spectra of ATMG under a perpendicular electric field [1]. Using the first-order degenerate-state perturbation theory treating the interlayer potential as a perturbation, we analytically investigate the low-energy effective Hamiltonian and its energy spectrum up to pentalayer. We also present general rules for constructing effective Hamiltonian of biased ATMG with arbitrary number of layers. Moreover, we calculate the optical conductivity of ATMG with and without the interlayer potential difference, revealing that a step-like feature arises from the splitting of Dirac nodes by the applied electric field.

*Ref:[1] Kyungjin Shin, Yunsu Jang, Jiseon Shin, Jeil Jung, and Hongki Min, Electronic structure of biased alternating twist multilayer graphene, arXiv:2212.14541 (2022), accepted by Phys. Rev. B.

• Fengping Li (UOS)
Title: Moiré flat bands and interfacial charge polarization in lattice relaxed twisted bilayer hexagonal boron nitride

We simulate the electronic properties and the interfacial polarization of twisted bilayer hexagonal boron nitride by including the relaxation effects explicitly using an exact exchange and random phase approximation parametrized molecular dynamics force field in combination with a newly parametrized two-center approximation tight-binding model. The resulting lattice reconstruction induces a larger band gap than the one from the rigid structure while maximum localization of the flat band below 1° can be linked to the size of the deformed stacking domain regions. The band gap can be further engineered by applying an electric field leading to deformation of the moiré superlattice. The real space local density distribution indicates that interfacial charge polarization is sizeable for the parallel moiré stacking configuration between layers while it is negligible for the anti-parallel configuration under both relaxation effects and external electric fields.

• Youngju Park (UOS)
Title: Electric field effects on spectral functions in twisted monolayer-bilayer graphene

We explored the spectral function of twisted monolayer-bilayer graphene (tMBG) in the presence of external electric fields based on real-space tight-binding (TB) model approach informed by density functional theory (DFT) [1]. Thanks to intrinsic inversion symmetry breaking, it leads to asymmetric behavior depending on the direction of the electric fields. We found the similarity between the spectral functions of tMBG and those of either twisted bilayer graphene (tBG) or twisted double bilayer graphene (tDBG), which can be obtained by a finite external electric field. Also, we observe the spiral shaped spectral weights for constant energy cuts in tBG, tMBG, and tDBG. We show that the interplay of intra- and inter-sublattice interlayer tunneling plays a key role for the spiral shaped spectral functions using both numerical and analytic ways. We expect our results to provide meaningful guides when interpreting possible experiments.

*Ref: Nicolas Leconte et al., Phys. Rev. B 106, 115410 (2022).

• Shaifullah Md (UOS)
Title: Electronic structure calculations of lattice-relaxed hBN-encapsulated tBG

tBG deposited on hBN has been shown to give rise to the quantum anomalous Hall effect under certain conditions. One of these conditions is the level of commensuration between the moires formed by GBN and tBG. As encapsulation with an additional layer of hBN has been shown to be a viable route to possibly enhance the low energy effects stemming from a single moire in single layer graphene deposited on hBN, we study here the effect of this so-called supermoire h-BN encapsulation effect on tBG for different commensurate twist angles, where our first set of results focuses on magic angle tBG. We approach this problem by performing first the real-space lattice minimizations of the supermoire structures and then use these structures as input for accurate tight-binding calculations.

• Thaikun Kanyamon (SKKU)
Title: TBD

• Yiyang Wang (UOS)
Title: Preliminary Study on Lattice Relaxation Modeling of Non-Euclidean Surface Twist Structures for Second Harmonic Generation

Second harmonic generation (SHG) is a crucial optical process extensively employed in materials science and optical device research. Non-Euclidean surface twist structures have a significant impact on SHG response, yet the understanding of their influence mechanisms remains limited. This report presents a preliminary study on simulating the effects of non-Euclidean surfaces using lattice relaxation. We have successfully replicated the one-dimensional and two-dimensional lattice relaxation models described in the literature. By reproducing these models, we demonstrate the potential influence of non-Euclidean surface twist structures on SHG and discuss avenues for further research. Although our work is in progress and incomplete, this study lays the groundwork for a deeper understanding of the optical properties of non-Euclidean surface twist structures and provides valuable insights for future investigations.

• Jinyoung Byun (SKKU)
Title: Strain effects on the electronic structures of CdSe quantum dots

• Dongkyu Lee (UOS)
Title: Accurate Extended Hubbard Tight-Binding Models of Rhombohedral Stacked Graphene Employing the ACBN0-functional DFT+U+V

Rhombohedral graphene exhibits a flat band along with localized low-energy states. When low-energy states are strongly localized their structure becomes ‘correlated’ and acquires a marked many-body character. However, LDA(GGA) fails to capture such local interactions, and this applies to tight-binding models based on DFT as well. Although there are some many-body correction methods such as DFT+U+V or DFT+GW, they are limited by reliance on empirical parameters or a significant increase in computational cost. We report realistic estimates of on-site Hubbard U and inter-site Hubbard V for rhombohedral graphene based on the Agapito-Curtarolo-Buongiorno-Nardelli pseudohybrid functional method[1,2]. Moreover, we propose an improved extended Hubbard model that addresses double-counting issue based on DFT+U+V calculations.

*Ref: [1] Lee, Sang-Hoon et al., Physical Review Research, 2(4) (2020). [2] Agapito, Luis A., et al. Physical Review X 5.1 (2015).

• Xinbiao Wang (SKKU)
Title: The electronic and optical properties of AgGa1–xInxS2 mixed alloys and structure prediction of Ag-Ga-S chemical system.

The electrical and optical properties of AgGa1–xInxS2 semiconductors have been investigated from first principles within the density functional theory(DFT). A slight bowing of band gaps as a function of Ga/In content was revealed. Indium composition fluctuation (clustering) is simulated by different distributions of In atoms and it is shown that it slightly influences the band gaps, which means it is very robust to tune the energy gap by doping In atoms. In addition, the effects of composition x on absorption, real part of optical conductivity, and dielectric function were calculated. The change in optical properties caused by doping is anisotropic due to the crystal is uniaxial with the optical axis directed along the z direction. A semiconductor AgGaS6 monolayer is predicted by using the global minimum structure search method of swarm intelligence.Based on first-principles calculations,we proved that ABX6 monolayer (A=Ag,Cu,B=Al,Ga,In,X=S,Se,Te) are stable.

• Jisoo Woo (UOS)
Title: Electronic flat bands in twisted mono-bi-mono graphene

Thin 2D materials layered in many different ways give rise to various electronic properties. In this paper, we derived an effective continuum model Hamiltonian for twisted monolayer-bilayer- monolayer graphene(tMBMG) and bilayer-monolayer-bilayer graphene(tBMBG) to find the attrac- tive conditions that lead to a nearly flat band in the band structure at different angles and with a per- pendicular magnetic field. In addition, we calculated effective Coulomb potential and valley Chern numbers relevant to the electronic properties for the phase diagram. Considering many stackings of tMBMG and tBMBG under different conditions, we visualized the band structures ,normalized Den- sity/Local Density Of States(DOS/LDOS) and the strong electron-electron interaction(U/W) for the flat band. In case of ABBC(θ=0.75◦, ∆=0.054eV) for tMBMG , ABBBC(θ=1.71◦, ∆=−0.053eV) and ABBAC(θ=1.18◦, ∆=−0.023eV) for tBMBG, We observed a promising condition for the flat band, characterized by a nearly zero band dispersion and a high density of states, indicating potential for novel electronic and optical properties

• Zhang Qi (SKKU)
Title: Electronic transport properties of ferroelectric field effect transistors

Ferroelectric materials field-effect transistors (FETs) is an important topic in current materials science research. Ferroelectric materials possess unique properties when applied in electronic devices, such as reversible electric field control and non-volatile storage capabilities. In this study, we investigate the electronic transport properties of ferroelectric material FETs using Boltzmann transport theory. We consider the influence of the polarization of ferroelectric materials depending on electric field and field-induced charge redistribution on the electronic transport. Our computational results demonstrate that ferroelectric material FETs exhibit distinct characteristics compared to traditional transistors such as field-controlled conductivity. These findings provide important theoretical guidance for the design and optimization of ferroelectric material FETs, contributing to the advancement and application of ferroelectric devices.

▶ Organizers

• Prof. George J. Jung (UOS)
• Prof. Hongki Min (SNU)
• Prof. Euyheon Hwang (SKKU)
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