arXiv subjects

Keun Hyuk Ahn

Publications and source records attributed to Keun Hyuk Ahn.

5 recordsLinked to original sources

arXiv2025

Flat Midgap Topological Surface and Hypersurface Bands without Parameter Tuning

The Su-Schrieffer-Heeger model is extended to the three and higher dimensional systems. Nearly or absolutely flat midgap surface and hypersurface bands are predicted based on the topological analysis, which do not require fine tuning of parameters. By adding the on-site Coulomb interaction for the three dimensional systems, we computationally show that the large difference in the band widths between the surface and the bulk leads to the strongly correlated phenomena, specifically magnetism, confined only on the surface. Possible experimental realizations in solid state materials and metamaterials are discussed.

cond-mat.str-el↗

arXiv2022

Observation of Phase Controllable Majorana-like Bound States in Metamaterial-based Kitaev Chain Analogues

We experimentally demonstrate that Majorana-like bound states (MLBSs) can occur in quasi-one-dimensional metamaterials, analogous to Majorana zero modes (MZM) in the Kitaev chain. In a mechanical spinner ladder system, we observe a topological phase transition and spectral-gap-protected edge MLBSs. We characterize the decaying and oscillatory nature of these MLBS pairs and their phase-dependent hybridization. It is shown that the hybridization can be tuned to yield the analogue of parity switching in MZMs, a key element of topological qubits. We find strong agreements with theory.

cond-mat.mtrl-sci↗

arXiv2020

Observation of Flat Frequency Bands at Open Edges and Antiphase Boundary Seams in Topological Mechanical Metamaterials

Motivated by the recent theoretical studies on a two-dimensional (2D) chiral Hamiltonian based on the Su-Schrieffer-Heeger chains, we experimentally and computationally demonstrate that topological flat frequency bands can occur at open edges of 2D planar metamaterials and at antiphase boundary seams of ring-shaped or tubular metamaterials. Specifically, using mechanical systems made of magnetically coupled spinners, we reveal that the presence of the edge or seam bands that are flat in the entire projected reciprocal space follows the predictions based on topological winding numbers. The edge-to-edge distance sensitively controls the flatness of the edge bands and the localization of excitations. The analogue of the fractional charge state is also observed. Possible realizations of flat bands in a large class of metamaterials, including photonic crystals and electronic metamaterials, are discussed.

cond-mat.mes-hall↗

arXiv2019

Flat Energy Bands within Antiphase and Twin Boundaries and at Open Edges in Topological Materials

A model for two-dimensional electronic, photonic, and mechanical metamaterial systems is presented, which has flat one-dimensional zero-mode energy bands and stable localized states of a topological origin confined within twin boundaries, antiphase boundaries, and at open edges. Topological origins of these flat bands are analyzed for an electronic system as a specific example, using a two-dimensional extension of the Su-Schrieffer-Heeger Hamiltonian with alternating shift of the chains. It is demonstrated that the slow group velocities of the localized flat band states are sensitively controlled by the distance between the boundaries and the propagation can be guided through designed paths of these boundaries. We also discuss how to realize this model in metamaterials.

cond-mat.mes-hall↗

arXiv2018

Photoinduced Nonequilibrium Dynamics in Charge Ordered Materials

We study the nonequilibrium dynamics of photoinduced phase transitions in charge ordered (CO) systems with a strong electron-lattice interaction and analyze the interplay between electrons, periodic lattice distortions, and a phonon thermal reservoir. Simulations based on a tight-binding Hamiltonian and Boltzmann equations reveal partially decoupled oscillations of the electronic order parameter and the periodic lattice distortion during CO melting, which becomes more energy efficient with lower photon energy. The cooling rate of the electron system correlates with the CO gap dynamics, responsible for an order of magnitude decrease in the cooling rate upon the gap reopening. We also find that the time-dependent frequency of coherent oscillation reflects the dynamics of the energy landscape, such as transition between single-well and double-well, which sensitively depends on the photon energy and the pump fluence. The results demonstrate the intricate nonequilibrium dynamics in CO materials.

cond-mat.str-el↗