arXiv subjects

Kyung-Mo Kim

Publications and source records attributed to Kyung-Mo Kim.

4 recordsLinked to original sources

arXiv2026

Controlling Intertwined Electronic Orders in FeSe with Exfoliation

Controlling intertwined electronic orders in two-dimensional superconductors offers an effective route to answering fundamental questions and engineering new quantum devices. However, tuning the balance between competing orders typically requires complex chemistry, strain, or interface engineering. Here, we show that a pristine alternative is the dimensional reduction of the unconventional superconductor FeSe. Exfoliation suppresses the bulk electronic nematic response and switches the superconducting symmetry from bulk s-wave to d-wave-dominant. Transport, electron microscopy, and Raman spectroscopy establish the substantial weakening of nematic order in thin flakes. To probe superconductivity, we perform angle-dependent Andreev reflection spectroscopy on pristine crystal edges. As the junction's orientation is rotated, the spectra evolve from zero-energy bound states to coherence peaks. The injection angle, field, and temperature dependence, along with theoretical modeling, confirm that exfoliation switches the superconducting symmetry. Our results suggest a versatile superconducting platform for engineering quantum orders and provide fresh insights into the underlying pairing mechanisms.

cond-mat.supr-con↗

arXiv2026

Scalable, Simple, and Versatile Encapsulation of 2D Materials and Devices

Air-sensitive 2D materials present a fundamental challenge for device integration. Encapsulation is often required to preserve intrinsic properties, yet conventional protection strategies often fail for thicker layers and complicate fabrication. Here, we demonstrate that electron-beam (e-beam) evaporated aluminum oxide ($\mathrm{AlO}_x$) serves as both an effective encapsulation layer and a platform for direct device fabrication. Unlike transfer-based approaches, this scalable method is compatible with thicker flakes and full device or wafer coverage. It requires no stacking procedures and enables contacts without post-encapsulation etching. Using rare-earth tritellurides ($\mathrm{RTe}_3$, R = La, Er), semimetallic $\mathrm{WTe}_2$, and superconducting $\mathrm{FeTe}_x\mathrm{Se}_{1-x}$, we show that $\mathrm{AlO}_x$ suppresses oxidation and preserves intrinsic optical and electronic properties. We establish substrate-dependent optimization of encapsulation across a range of flake thicknesses, demonstrate that ultrathin $\mathrm{AlO}_x$ preserves $\mathrm{WTe}_2$'s plasmonic response and maintains superconducting performance in $\mathrm{FeTe}_x\mathrm{Se}_{1-x}$. Thus we overcome the longstanding tradeoff between encapsulation and straightforward device fabrication in fragile quantum materials.

cond-mat.mes-hall↗

arXiv2024

Uncovering the Hidden Ferroaxial Density Wave as the Origin of the Axial Higgs Mode in RTe$_3$

The recent discovery of an axial amplitude (Higgs) mode in the long-studied charge density wave (CDW) systems GdTe$_3$ and LaTe$_3$ suggests a heretofore unidentified hidden order. A theoretical study proposed that the axial Higgs results from a hidden ferroaxial component of the CDW, which could arise from non-trivial orbital texture. Here, we report extensive experimental studies on ErTe$_3$ and HoTe$_3$ that possess a high-temperature CDW similar to other RTe$_3$ (R = rare earth), along with an additional low-temperature CDW with an orthogonal ordering vector. Combining Raman spectroscopy with large-angle convergent beam electron diffraction (LACBED), rotational anisotropy second-harmonic generation (RA-SHG), and muon-spin relaxation ($μ$SR), we provide unambiguous evidence that the high-temperature CDW breaks translation, rotation, and all vertical and diagonal mirror symmetries, but not time-reversal or inversion. In contrast, the low-temperature CDW only additionally breaks translation symmetry. Simultaneously, Raman scattering shows the high-temperature CDW produces an axial Higgs mode while the low-temperature mode is scalar. The weak monoclinic structural distortion and clear axial response in Raman and SHG are consistent with a ferroaxial phase in RTe$_3$ driven by coupled orbital and charge orders. Thus, our study provides a new standard for uncovering unconventional orders and confirms the power of Higgs modes to reveal them.

cond-mat.str-el↗

arXiv2024

La$_2$O$_3$Mn$_2$Se$_2$: a correlated insulating layered d-wave altermagnet

Altermagnets represent a new class of magnetic phases without net magnetization that are invariant under a combination of rotation and time reversal. Unlike conventional collinear antiferromagnets (AFM), altermagnets could lead to new correlated states and important material properties deriving from their non-relativistic spin-split band structure. Indeed, they are the magnetic analogue of unconventional superconductors and can yield spin polarized electrical currents in the absence of external magnetic fields, making them promising candidates for next-generation spintronics. Here, we report altermagnetism in the correlated insulator, magnetically-ordered tetragonal oxychalcogenide, La$_2$O$_3$Mn$_2$Se$_2$. Symmetry analysis reveals a $\mathit{d}_{x^2 - y^2}$-wave type spin momentum locking, which is supported by density functional theory (DFT) calculations. Magnetic measurements confirm the AFM transition below $\sim$166 K while neutron pair distribution function analysis reveals a 2D short-range magnetic order that persists above the Néel temperature. Single crystals are grown and characterized using X-ray diffraction, optical and electron microscopy, and microRaman spectroscopy to confirm the crystal structure, stoichiometry, and uniformity.

cond-mat.mtrl-sci↗