arXiv subjects

Young-Jae Choi

Publications and source records attributed to Young-Jae Choi.

3 recordsLinked to original sources

arXiv2024

Emergent Quantum Phenomena of Noncentrosymmetric Charge-Density Wave in 1T-Transition Metal Dichalcogenides

1T-transition metal dichalcogenides (TMD) have been an exciting platform for exploring the intertwinement of charge density waves and strong correlation phenomena. While the David star structure has been conventionally considered as the underlying charge order in the literature, recent scanning tunneling probe experiments on several monolayer 1T-TMD materials have motivated a new, alternative structure, namely the anion-centered David star structure. In this Letter, we show that this novel anion-centered David star structure manifestly breaks inversion symmetry, resulting in flat bands with pronounced Rashba spin-orbit couplings. These distinctive features unlock novel possibilities and functionalities for 1T-TMDs, including the giant spin Hall effect, the emergence of Chern bands, and spin liquid that spontaneously breaks crystalline rotational symmetry. Our findings establish promising avenues for exploring emerging quantum phenomena of monolayer 1T-TMDs with this novel noncentrosymmetric structure.

cond-mat.str-el↗

arXiv2024

Study of vacancy ordering and the boson peak in metastable cubic Ge-Sb-Te using machine learning potentials

The mechanism of the vacancy ordering in metastable cubic Ge-Sb-Te (c-GST) that underlies the ultrafast phase-change dynamics and prominent thermoelectric properties remains elusive. Achieving a comprehensive understanding of the vacancy-ordering process at an atomic level is challenging because of enormous computational demands required to simulate disordered structures on large temporal and spatial scales. In this study, we investigate the vacancy ordering in c-GST by performing large-scale molecular dynamics simulations using machine learning potentials. The initial c-GST structure with randomly distributed vacancies rearranges to develop a semi-ordered cubic structure with layer-like ordered vacancies after annealing at 700~K for 100~ns. The vacancy ordering significantly affects the lattice dynamical properties of c-GST. In the initial structure with fully disordered vacancies, we observe a boson peak, usually associated with amorphous solids, that consists of localized modes at $\sim$0.575~THz. The boson peak modes are highly localized around specific atomic arrangements of straight vacancy-Te-vacancy trios. As vacancies become ordered, the boson peak disappears and the Debye-Waller thermal \textit{B} factor of Te decreases substantially. This finding indicates that the c-GST undergoes a transition from amorphous-like to crystalline-like solid state by thermal annealing in low-frequency dynamics.

cond-mat.mtrl-sci↗

arXiv2022

Divergent phonon angular momentum driven by temperature and strain

The phonon angular momentum (PAM) may exhibit exotic temperature dependence as it is sensitive to the phonon lifetime. Constant phonon-lifetime approximation fails to depict such behavior. Here, we study the PAM of AlN, GaN, and graphene-like boron nitride (g-BN) monolayer with full consideration of phonon lifetime using first-principles calculations. We show that wurtzite AlN and GaN acquire divergent PAM at low temperatures from their lowest-lying phonon branches. The g-BN monolayer, on the other hand, does not have finite PAM at equilibrium structure. Rather it shows intriguing strain-dependence in PAM; the compressive strain greater than the critical size generates divergent PAM at low temperatures due to the divergent lifetime of TA phonons. As PAM couples with rotational excitations in solids associated with charge, spin, or electromagnetic fields, our study demonstrates a possibility of mechanical and thermal engineering of such excitations.

cond-mat.mtrl-sci↗