arXiv subjects

Sungjong Woo

Publications and source records attributed to Sungjong Woo.

10 recordsLinked to original sources

arXiv2026

Unfolding Bloch States in Disordered Systems

In crystalline solids, disorder breaks translational symmetry and obscures k-resolved Bloch states, limiting an accurate description of wavefunction-based observables. In this work, we present a method that unfolds not only the band structures but also the corresponding Bloch states in disordered systems, going beyond conventional band-unfolding techniques. As a prototype application, we study defective graphene and demonstrate the capabilities by capturing key wavefunction-level responses, including disorder-driven redistribution of Berry curvature.

cond-mat.mtrl-sci↗

arXiv2024

Engineering high Chern number insulators

The concept of Chern insulators is one of the most important buliding block of topological physics, enabling the quantum Hall effect without external magnetic fields. The construction of Chern insulators has been typically through an guess-and-confirm approach, which can be inefficient and unpredictable. In this paper, we introduce a systematic method to directly construct two-dimensional Chern insulators that can provide any nontrivial Chern number. Our method is built upon the one-dimensional Rice-Mele model, which is well known for its adjustable polarization properties, providing a reliable framework for manipulation. By extending this model into two dimensions, we are able to engineer lattice structures that demonstrate predetermined topological quantities effectively. This research not only contributes the development of Chern insulators but also paves the way for designing a variety of lattice structures with significant topological implications, potentially impacting quantum computing and materials science. With this approach, we are to shed light on the pathways for designing more complex and functional topological phases in synthetic materials.

cond-mat.mes-hall↗

arXiv2024

Probing Intrinsic Elastic Properties of Multilayer Graphene -- a New Mechanical Constant

We present measurements on in-plane Young's modulus and the Gr\"{u}neisen parameter of multilayer graphene with varying number of layers, obtained through {\it in situ} bulge tests. Accurate determination of their elastic parameters poses a significant experimental challenge due to the substantial differences in mechanical behavior between intra- and inter-layers. To address this, we develop a novel theoretical model with first-principles calculations to investigate thickness-dependent incomplete strain transfer between the layers. Our findings show that the experimentally measured elastic constants, which deviate from computed intrinsic values, fail to fully capture ideal mechanical couplings between layers. As a solution, we propose a new mechanical modulus that integrates the Gr\"{u}neisen parameter and in-plane Young's modulus, providing a more reliable representation of their mechanical properties, independent of unavoidable interlayer effects.

cond-mat.mtrl-sci↗

arXiv2020

Topological edge states in bowtie ladders with different cutting edges

We have studied topological edge states in bowtie ladders with various edge truncations. The symmetric bowtie ladder, which comprises two trivial Su-Schrieffer-Heeger (SSH) lattices, exhibits an insulator-metal transition with trivial insulating states. On the other hand, the lattice can be transformed into an extended SSH lattice depending on the edge shapes with non-trivial insulating states in that the winding number is non-zero. The winding numbers are permutationally designated in the phase diagram depending on the choice of unit cell. The topological edge states are affected by the shape of the edge and the corresponding winding number. We also studied general bowtie ladder models with richer phase diagrams using the characteristics of the localization length of the edge states showing state bifurcation.

cond-mat.mes-hall↗

arXiv2019

Symmetry-Protected Degeneracies in the Electronic Band Structure of Oxidized Black Phosphorous

We explore the oxidation of a single layer of black phosphorous using ab initio density functional theory calculation. We search for the equilibrium structures of phosphorene oxides, PO$_x$ with various oxygen concentrations $x$ ($0{\le}x{\le}1$). By evaluating the formation energies with diverse configurations and their vibrational properties for each of various $x$ values, we identify a series of stable oxidized structures with $x$ and confirm that the oxidation occurs naturally. We also find that oxidation makes some modes from the P-O bonds P-P bonds IR-active implying that the infrared spectra can be used to determine the degree of oxidation of phosphorene. Our electronic structure calculation reveals that the fully oxidized phosphorene (PO) has a direct band gap of 0.83 eV similar to the pristine phosphorene. Intriguingly, the PO possesses two nonsymmorphic symmetries with the inversion symmetry broken, guaranteeing a symmetry-protected band structures including the band degeneracy and four-fold degenerate Dirac points. Our results may provide a significant insight into the intriguing relations between symmetry of lattice and band topology of electrons.

cond-mat.mtrl-sci↗

arXiv2019

Emergent localized states at the interface of a twofold $\mathcal{PT}$-symmetric lattice

We consider the role of non-triviality resulting from a non-Hermitian Hamiltonian that conserves twofold PT-symmetry assembled by interconnections between a PT-symmetric lattice and its time reversal partner. Twofold PT-symmetry in the lattice produces additional surface exceptional points that play the role of new critical points, along with the bulk exceptional point. We show that there are two distinct regimes possessing symmetry-protected localized states, of which localization lengths are robust against external gain and loss. The states are demonstrated by numerical calculation of a quasi-1D ladder lattice and a 2D bilayered square lattice.

cond-mat.mes-hall↗

arXiv2018

Splitting of conductance resonance through a magnetic quantum dot in graphene

We report a dual resonance feature in ballistic conductance through a quantum Hall graphene nanoribbon with a magnetic quantum dot. Such a magnetic quantum dot localizes Dirac fermions exhibiting anisotropic eigenenergy spectra with broken time-reversal symmetry. Interplay between the localized states and quantum Hall edge states is found to be two-fold, showing Breit-Wigner and Fano resonances, which is reminiscent of a double quantum dot system. By fitting the numerical results with the Fano-Breit-Wigner lineshape from the double quantum dot model, we demonstrate that the two-fold resonance is due to the valley mixing that comes from the coupling of the magnetic quantum dot with quantum Hall edge channels; an effective double quantum dot system emerges from a single magnetic quantum dot in virtue of the valley degree of freedom. It is further confirmed that the coupling is weaker for the Fano resonance and stronger for the Breit-Wigner resonace.

cond-mat.mes-hall↗

arXiv2017

Strain-shear coupling in bilayer MoS2

Layered materials such as graphite and transition metal dichalcogenides have extremely anisotropic mechanical properties owing to orders of magnitude difference between in-plane and out-of-plane interatomic interaction strengths. Although effects of mechanical perturbations on either intra- or inter-layer interactions have been extensively investigated, mutual correlations between them have rarely been addressed. Here we show that layered materials have an inevitable coupling between in-plane uniaxial strain and interlayer shear. Because of this, the uniaxial in-plane strain induces an anomalous splitting of the degenerate interlayer shear phonon modes such that the split shear mode along the tensile strain is not softened but hardened contrary to the case of intralayer phonon modes. We confirm the effect by measuring Raman shifts of shear modes of bilayer MoS2 under strain. Moreover, by analyzing the splitting, we obtain an unexplored off-diagonal elastic constant, demonstrating that Raman spectroscopy can determine almost all mechanical constants of layered materials.

cond-mat.mtrl-sci↗

arXiv2015

Poisson's Ratio of Layered Two-dimensional Crystals

We present first-principles calculations of elastic properties of multilayered two-dimensional crystals such as graphene, h-BN and 2H-MoS2 which shows that their Poisson's ratios along out-of-plane direction are negative, near zero and positive, respectively, spanning all possibilities for sign of the ratios. While the in-plane Poisson's ratios are all positive regardless of their disparate electronic and structural properties, the characteristic interlayer interactions as well as layer stacking structures are shown to determine the sign of their out-of-plane ratios. Thorough investigation of elastic properties as a function of the number of layers for each system is also provided, highlighting their intertwined nature between elastic and electronic properties.

cond-mat.mtrl-sci↗

arXiv2011

Electronic topological transition in sliding bilayer graphene

We demonstrate theoretically that the topology of energy bands and Fermi surface in bilayer graphene undergoes a very sensitive transition when extremely tiny lateral interlayer shift occurs in arbitrary directions. The phenomenon originates from a generation of effective non-Abelian vector potential in Dirac Hamiltonian by the sliding motions. The characteristics of the transition such as pair annihilations of massless Dirac fermions are dictated by the sliding direction owing to a unique interplay between the effective non-Abelian gauge fields and Berry's phases associated with massless electrons. The transition manifests itself in various measurable quantities such as anomalous density of states, minimal conductivity, and distinct Landau level spectrum.

cond-mat.mes-hall↗