Quantum Materials Workshop 2025
양자물성 워크숍 2025

Abstract:
In this talk, I will present real-space lattice relaxation and electronic structure calculations for helical twist multilayer graphene with twist angles of 0.5°, 2°, and 3°, ranging from three to five layers. These angles span regimes where either standard moiré patterns or secondary moiré-of-moiré effects emerge. Depending on the local stacking geometry and twist angle, the system hosts a rich variety of both dispersive and flat bands. I will highlight how real-space relaxation shapes these electronic features and critically assess the validity of the single-moiré approximation at larger twist angles, focusing on its implications for key electronic observables.

Abstract:
Helical trilayer graphene, a structure in which three consecutive graphene layers are twisted by identical angles, has recently emerged as a promising platform for realizing topological flat bands and ideal quantum geometry, offering a fertile ground for exploring interaction-induced topological phases. In this work, we go beyond helical trilayer graphene and extend our analysis to multilayer systems including tetralayer and pentalayer configurations. We first perform comprehensive real-space calculations on helical trilayer, tetralayer, and pentalayer graphene systems, and identify locally periodic single-moiré domains, enabling a tractable continuum model description. Within these reconstructed domains, we analytically derive effective Hamiltonians near the Dirac point using perturbation theory within the first-shell model, capturing the essential features of the electronic structure under the single-moiré approximation. We then evaluate valley Chern numbers and show that they originate from the intrinsic topology encoded in the effective Hamiltonian.

Abstract:
The recent observation of the fractional quantum anomalous Hall effect has triggered tremendous interest in the strongly correlated states of parallel stacked twisted transition metal dichalcogenide (TMD) homobilayer moiré superlattices. These moiré materials have convenient continuum and tight-binding model effective descriptions that provide complimentary approaches to their many-body physics. Here we address how accurately the lattice descriptions reproduce the quantum geometry of the system as obtained from the continuum model description.

Abstract:
Tungsten diselenide (WSe₂), a prominent member of transition metal dichalcogenides (TMDCs), exhibits distinctive valley-dependent electronic properties due to its broken inversion symmetry. Unlike graphene-like systems where tight-binding (TB) models based on pz​ orbitals suffice near high-symmetry points, bilayer WSe₂ requires a more intricate orbital description due to interlayer interactions and orbital complexity. In this work, we develop a simplified TB model for bilayer WSe₂ derived from first-principles density functional theory (DFT) calculations using maximally localized Wannier functions (MLWFs), excluding spin-orbit coupling at this stage. By retaining only a minimal set of nearest-neighbor hopping terms, our model significantly reduces the complexity of the full ab initio Hamiltonian. We validate the model by comparing its band structure to that of full DFT calculations, showing good agreement near the Fermi level. Additionally, symmetry constraints inherent to the bilayer structure allow a further reduction in independent parameters, improving computational efficiency without sacrificing accuracy. This effective model offers a reliable and lightweight framework for studying electronic properties of bilayer WSe₂ and moiré superlattices derived from it.

Abstract:
We investigate strain engineering in single-layer MoS2-Au heterostructures under biaxial and uniaxial tension applied along the zigzag and armchair directions. By systematically varying the strain conditions, we study how different strain configurations influence the electronic and interfacial properties of this 2D material-based system. Under tensile strain, the SBH at the Au/MoS₂ interface decreases and the interfacial binding energy increases, leading to a reduced van der Waals (vdW) gap and enhanced electron tunneling probability. In contrast, compressive strain has the opposite effect, i.e., compressive strain increases the SBH and weakens the interface interaction. The SBH reduction under tensile strain gives rise to enhanced electron transfer from Au to MoS2, resulting in charge redistribution that effectively dopes MoS2 with electrons and shifts its Fermi level closer to the conduction band minimum (CBM). The tunability of SBH and tunneling barriers via strain highlights a viable strategy for optimizing metal–2D semiconductor contacts in nanoelectronics applications.

Abstract:
We present a hypothesis of step-like hysteresis phenomena observed in twisted bilayer graphene (TBG) on WSe₂. Recent studies have revealed that the proximity-induced spin-orbit coupling (SOC) from WSe₂ a wealth of novel electronic phases as a result of interaction-driven spin/valley ordering. However, the step-like hysteresis often observed in two-dimensional magnetic materials, including tBG on WSe₂, remains poorly understood and lacks a comprehensive theoretical explanation. Our calculations employ the self-consistent tight-binding extended Hubbard model(TB+U+V) to simulate the interplay between moiré flat bands in this heterostructure. Our results show that the topologically protected ground states converge differently depending on the initial spin density configuration, suggesting that the step-like hysteresis is due to a multi-level phase transition involving intermediate states.

Abstract:
Recent studies have shown that gapped monolayer graphene and transition-metal dichalcogenides under continuous circularly polarized light develop valley imbalance that yields chiral edge plasmons. In this work we study chiral edge plasmon modes in bilayer graphene under optical pumping and perpendicular electrostatic gating. Under this configuration, we find that the non-reciprocity of the chiral edge plasmons is significantly enhanced and broadly gate-tunable, highlighting bilayer graphene as a versatile platform for controllable non-reciprocal plasmonic devices.

Abstract:
We study the plasmon properties of N-layer 2D electron systems including interlayer tunneling effects. By solving the Kac–Murdock–Szegő matrix problem, we analytically derive the plasmon dispersions in the long-wavelength limit, demonstrating that N-1 undamped out-of-phase modes exist along with one in-phase classical plasmon mode. Our analysis reveals that the out-of-phase modes develop energy gaps depending on the tunneling amplitude and are governed by specific interband transitions, whereas the in-phase mode remains qualitatively unaffected by tunneling and is determined by intraband effects. We further investigate how bulk plasmon properties emerge in the limit of infinitely many layers. These findings can be applied to general coupled layered structures.

Abstract:
We investigate the impact of electron-hole puddles on two-dimensional (2D) carrier transport in ultrathin field-effect transistors, focusing on the role of charged impurities inevitably present in the device environment. We quantify potential fluctuations induced by residual electric charges and analyze their effects on transport properties. Conductivity is calculated by using both Boltzmann transport theory and self-consistent effective-medium theory, which show excellent agreement with experimental data from inhomogeneous PtSe₂ systems. Our findings show that electron-hole puddles lead to reduced mobility, threshold gate voltage shifts, and enhanced minimum conductivity, and these effects become particularly pronounced in systems with thin conducting channels. Although charged impurities give a little effect on transport at high carrier densities, they play a crucial role near band edges, where variations in the local potential create inhomogeneous puddles, leading to a mixed carrier transport regime over a specific range of gate voltages. Additionally, we identify a transition from inhomogeneous to homogeneous transport with increasing layer thickness. This study provides a comprehensive framework applicable to various 2D material systems, offering valuable insights for optimizing ultrathin electronic devices.

Abstract:
The temperature-dependent metal–insulator transition (MIT) is a compelling phenomenon arising from the complex interplay among topology, disorder, and many-body interactions. This transition is characterized by a crossover from metallic behavior (dρ/dT > 0) at low temperatures to insulating behavior (dρ/dT < 0) at higher temperatures. In Bi₀.₉₆Sb₀.₀₄ thin films, a monotonic metallic temperature dependence is observed for thicknesses d > 30 nm, while films with d < 30 nm exhibit a clear MIT. Notably, a topological phase transition from a Dirac to a Weyl semimetal also emerges below this critical thickness. By incorporating charged impurity scattering and acoustic phonon scattering into our analysis, we develop a comprehensive theoretical framework to account for the observed MIT behavior in two-dimensional Dirac semimetals (2D DSMs). The suppression of the MIT with increasing film thickness is attributed to a dimensional crossover from quasi-2D to 3D, revealing the fundamental physical mechanisms governing the transport properties of these topological systems.

Abstract:
Since the discovery of superconductivity in copper-oxide high-temperature superconductors, a key theoretical question has been whether the single-band Hubbard model can qualitatively explain the unconventional superconductivity. Specifically, we ask: does the extended Hubbard model have a d-wave superconducting ground state for both hole and electron doping, and is the pairing stronger for holes? In this talk, we address this question using the density matrix renormalization group method and newly developed quantum Monte Carlo simulations.

Abstract:
On the recent report of a field-induced first order transition in the superconducting state of CeRh2As2, which is a possible indication of a parity-switching transition of the superconductor, the microscopic physics is still under investigation. However, if two competing pairing channels of opposite parities do exist, a particle-particle collective mode referred to as the Bardasis-Schrieffer (BS) mode should generically exist below the pair-breaking continuum. The BS mode of the CeRh2As2 superconductor can couple to the light, as it arises from a pairing channel with the parity opposite to that of the superconducting condensate. Here, by using a generic model Hamiltonian we carry out a qualitative investigation on the excitation energy of the BS mode with respect to the out-of-plane magnetic fields and its contribution to the optical conductivity. Our findings indicate that the distinct coupling between the BS mode and the light can serve as evidence for the competing odd-parity channels of CeRh2As2 and other locally non-centrosymmetric superconductors.

Abstract:
CeRh₂As₂ has emerged as a promising platform for unconventional superconductivity due to its unique electronic structure and locally non-symmorphic crystal symmetry. A type-II van Hove singularity near the Fermi level enhances electronic correlations, while the crystal symmetry allows the coexistence of even- and odd-parity pairing states, leading to a complex superconducting phase diagram. Notably, the system exhibits strong anisotropy under external magnetic fields, showing distinct superconducting transitions depending on field orientation. In this work, we employ the fluctuation-exchange (FLEX) approximation to compute the self-energy, susceptibility, and vertex corrections. Based on these, we solve the linearized Eliashberg equation to analyze and compare possible superconducting channels, elucidating the role of spin fluctuations in this anisotropic, correlated system.

Abstract:
We investigate the quasiparticles of a single nodal ring semimetal SrAs3 through axis-resolved magneto-optical measurements. We observe three types of Landau levels scaling as , , and that correspond to Dirac, semi-Dirac, and classical fermions, respectively. Through theoretical analysis, we identify the distinct origins of these three types of fermions present within the nodal ring. In particular, semi-Dirac fermions—a novel type of fermion that can give rise to a range of unique quantum phenomena—emerge from the endpoints of the nodal ring where the energy band disperses linearly along one direction and quadratically along the perpendicular direction, a feature not achievable in nodal point or line structures. The capacity of the nodal ring to simultaneously host multiple fermion types, including semi-Dirac fermions, establishes it as a valuable platform to expand the understanding of topological semimetals.

Abstract:
TWe study the modulation of zero-line modes (ZLMs) in bilayer graphene nanoribbons under external electric fields, focusing on structures with central AA and SP stacking domains that define key stacking boundaries. Both zigzag and armchair edge terminations are analyzed. In zigzag nanoribbons, an in-plane perpendicular electric field strongly modifies the band structure, while a vertical field has little effect. This anisotropic response results from directional coupling between the in-plane field and localized electronic states at the stacking boundaries. Additionally, the domain size and ribbon length significantly influence charge distribution and ZLM confinement. Charge density analysis near the Fermi level reveals that stacking configuration and external fields can be used to control ZLM formation and localization. These results highlight the potential of ZLMs as robust, topologically protected conduction channels, offering promising applications in low-dissipation electronics, quantum interconnects, and valleytronic devices requiring electrically tunable transport pathways.

Abstract:
Floquet theory has recently attracted significant attention in the study of two-dimensional materials. When a static Hamiltonian is perturbed by a time-periodic external potential, the system evolves into Floquet states, which are usually understood as the temporal analog of Bloch states in spatially periodic systems. In this talk, I will explain how Floquet theory is applied to 2D materials. In specific, I will explicitly demonstrate that linearly/circularly polarized light can drive a topological phase transition and the quantum Hall effect as well. Finally, I will discuss my recent work on few-layer black phosphorus, focusing on the theoretical issues, and also the remaining open questions such as how electron density is distributed in periodically driven systems, and how this influences their conductivity.

Abstract:
Twisted van der Waals heterostructures have led to emerging layer-dependent correlated physics in moiré potentials. While optoelectronic controls over interlayer electronic coupling have been reported, the concomitant interlayer vibration has not yet been controlled. Here, we report experimental evidence of ultrafast optical control over the amplitude and oscillation period of interlayer breathing phonons in WSe2/WS2 heterobilayers. Femtosecond optical excitation above the Mott density in gate-tuned devices shows as large as 10% changes of stiffening and softening amplitude of coherent phonons. A theoretical model, incorporating both Buckingham and Hartree energies, is presented to elucidate the impact of charge-separated carriers generated by photoexcitation on phonon dynamics. This work, therefore, provides insights for extending optoelectronic engineering into the coherent phonons in moiré systems.