Quantum Materials Workshop 2023
양자물성 워크숍 2023

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.

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.

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.

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..

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.)

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

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.

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.

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.

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.

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.

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).

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.