arXiv subjects

Kyungjin Shin

Publications and source records attributed to Kyungjin Shin.

3 recordsLinked to original sources

arXiv2026

Supermoiré domain-resolved effective Hamiltonians and valley topology in helical multilayer graphene

Extending moiré graphene beyond twisted bilayers, helical trilayer graphene has shown topological bands and correlated states with reshaped moiré periodicity. Here we develop a theoretical framework for helical multilayer graphene to investigate its supermoiré relaxation and low-energy electronic structure. Using real-space lattice calculations, we find that relaxation reconstructs the system into locally periodic single-moiré domains, which provide the basis for a continuum description. Within each reconstructed domain, downfolding the first-shell model yields effective Hamiltonians near the Dirac points that reveal how the low-energy spectrum decomposes into folded Dirac sectors. We further evaluate the valley Chern numbers encoded in these effective Hamiltonians, obtaining domain-dependent and gate-tunable topological responses consistent with the lattice calculations. Our results establish a domain-resolved organizing principle for thicker helical graphene stacks, in which folded Dirac sectors partition the low-energy spectrum, while local stacking families determine the corresponding band character and topological response.

cond-mat.mes-hall↗

arXiv2024

Sliding-dependent electronic structures of alternating-twist tetralayer graphene

We study the electronic structure of alternating-twist tetralayer graphene, especially near its magic angle $θ= 1.75^\circ$, for different AA, AB, and SP sliding geometries at their middle interface that divides two twisted bilayer graphenes. This sliding dependence is shown for the bandwidths, band gaps, and $K$-valley Chern numbers of the lowest-energy valence and conduction bands as a function of twist angle and interlayer potential difference. Our analysis reveals that the AA sliding is most favorable for narrow bands and gaps, and the AB sliding is most prone to developing finite valley Chern numbers. We further analyze the linear longitudinal optical absorptions as a function of photon energy and the absorption map in the moiré Brillouin zone for specific transition energies. A self-consistent Hartree calculation reveals that the AA system's electronic structure is the most sensitive to variations in carrier density.

cond-mat.mes-hall↗

arXiv2023

Electronic structure of biased alternating-twist multilayer graphene

We theoretically study the energy and optical absorption spectra of alternating twist multilayer graphene (ATMG) under a perpendicular electric field. We obtain analytically the low-energy effective Hamiltonian of ATMG up to pentalayer in the presence of the interlayer bias by means of first-order degenerate-state perturbation theory, and present general rules for constructing the effective Hamiltonian for an arbitrary number of layers. Our analytical results agree to an excellent degree of accuracy with the numerical calculations for twist angles $θ\gtrsim 2.2^{\circ}$ that are larger than the typical range of magic angles. We also calculate the optical conductivity of ATMG and determine its characteristic optical spectrum, which is tunable by the interlayer bias. When the interlayer potential difference is applied between consecutive layers of ATMG, the Dirac cones at the two moiré Brillouin zone corners $\bar{K}$ and $\bar{K}'$ acquire different Fermi velocities, generally smaller than that of monolayer graphene, and the cones split proportionally in energy resulting in a step-like feature in the optical conductivity.

cond-mat.mes-hall↗