arXiv subjects

Eunjip Choi

Publications and source records attributed to Eunjip Choi.

3 recordsLinked to original sources

arXiv2025

Unveiling three types of fermions in a nodal ring topological semimetal through magneto-optical transitions

We investigate the quasiparticles of a single nodal ring semimetal SrAs$_3$ through axis-resolved magneto-optical measurements. We observe three types of Landau levels scaling as $\varepsilon \sim \sqrt{B}$, $\varepsilon \sim B^{2/3}$, and $\varepsilon \sim B$ 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.

cond-mat.mtrl-sci↗

arXiv2023

Optical transitions of a single nodal ring in SrAs$_3$: radially and axially resolved characterization

SrAs$_3$ is a unique nodal-line semimetal that contains only a single nodal ring in the Brillouin zone, uninterrupted by any trivial bands near the Fermi energy. We performed axis-resolved optical reflection measurements on SrAs$_3$ and observed that the optical conductivity exhibits flat absorption up to 129 meV in both the radial and axial directions, confirming the robustness of the universal power-law behavior of the nodal ring. Furthermore, in conjunction with model and first-principles calculations, the axis-resolved optical conductivity unveiled fundamental properties beyond the flat absorption, including the overlap energy of the topological bands, the spin-orbit coupling gap along the nodal ring, and the geometric properties of the nodal ring such as the average ring radius, ring ellipticity, and velocity anisotropy. In addition, our temperature-dependent measurements revealed a spectral weight transfer between intraband and interband transitions, indicating a possible violation of the optical sum rule within the measured energy range.

cond-mat.str-el↗

arXiv2023

Interaction of in-plane Drude carrier with c-axis phonon in $\rm PdCoO_2$

We performed polarized reflection and transmission measurements on the layered conducting oxide $\rm PdCoO_2$ thin films. For the ab-plane, an optical peak near $Ω$ $\approx$ 750 cm$^{-1}$ drives the scattering rate $γ^{*}(ω)$ and effective mass $m^{*}(ω)$ of the Drude carrier to increase and decrease respectively for $ω$ $\geqq$ $Ω$. For the c-axis, a longitudinal optical phonon (LO) is present at $Ω$ as evidenced by a peak in the loss function Im[$-1/\varepsilon_{c}(ω)$]. Further polarized measurements in different light propagation (q) and electric field (E) configurations indicate that the Peak at $Ω$ results from an electron-phonon coupling of the ab-plane carrier with the c-LO phonon, which leads to the frequency-dependent $γ^{*}(ω)$ and $m^{*}(ω)$. This unusual interaction was previously reported in high-temperature superconductors (HTSC) between a non-Drude, mid-infrared band and a c-LO. On the contrary, it is the Drude carrier that couples in $\rm PdCoO_2$. The coupling between the ab-plane Drude carrier and c-LO suggests that the c-LO phonon may play a significant role in the characteristic ab-plane electronic properties of $\rm PdCoO_2$ including the ultra-high dc-conductivity, phonon-drag, and hydrodynamic electron transport.

cond-mat.mtrl-sci↗