arXiv subjects

Yoonkang Kim

Publications and source records attributed to Yoonkang Kim.

4 recordsLinked to original sources

arXiv2026

Vertical Nb Josephson junctions fabricated by direct metal deposition on both surfaces of freestanding graphene layers

Vertical integration of superconducting electronics requires fabrication strategies that preserve pristine interfaces while accommodating oxidation-sensitive elemental superconductors. However, existing van der Waals-based vertical Josephson junctions largely rely on transfer-based assembly schemes that are incompatible with elemental materials such as niobium (Nb). Here, we introduce a freestanding van der Waals membrane architecture that enables deposition-based fabrication of vertical Josephson junctions through double-sided processing of a single suspended two-dimensional layer. Using multilayer graphene suspended across lithographically defined through-holes in a SiNx membrane, we realize vertical Nb/multilayer graphene/Nb Josephson junctions without ambient exposure of buried interfaces. The resulting devices exhibit clear Josephson coupling, including reproducible supercurrents and a temperature dependence of the critical current consistent with short-junction behaviour. Well-defined magnetic interference patterns governed by the membrane-defined aperture geometry, together with sub-gap features that track a Bardeen-Cooper-Schrieffer (BCS)-like superconducting gap, further confirm the junction quality. This platform establishes a scalable route to vertical superconducting devices based on oxidation-sensitive elemental superconductors and van der Waals materials.

cond-mat.supr-con↗

arXiv2025

Interface-Bound States and Majorana Zero Modes in Lateral Heterostructures of Bi$_2$Se$_3$ and Sb$_2$Te$_3$ with Proximity-Induced Superconductivity

We present a comprehensive investigation into the emergence of interface-bound states, particularly Majorana zero modes (MZMs), in a lateral heterostructure composed of two three-dimensional topological insulators (TIs), Bi$_2$Se$_3$ and Sb$_2$Te$_3$, under the influence of proximity-induced superconductivity from niobium (Nb) contacts. We develop an advanced two-dimensional Dirac model for the topological surface states (TSS), incorporating spatially varying chemical potentials and s-wave superconducting pairing. Using the Bogoliubov-de Gennes (BdG) formalism, we derive analytical solutions for the bound states and compute the local density of states (LDOS) at the interface, revealing zero-energy modes characteristic of MZMs. The topological nature of these states is rigorously analyzed through winding numbers and Pfaffian invariants, and their robustness is explored under various physical perturbations, including gating effects. Our findings highlight the potential of this heterostructure as a platform for topological quantum computing, with detailed predictions for experimental signatures via tunneling spectroscopy.

cond-mat.mes-hall↗

arXiv2025

Topological Surface States of 3D Topological Insulator on Twisted Bilayer Graphene

We present a comprehensive theoretical study of the topological surface states (TSS) of Bi$_2$Se$_3$, a 3D topological insulator, epitaxially grown on twisted bilayer graphene (tBG). The moir\'e potential induced by tBG folds the TSS Dirac cone into the moir\'e Brillouin zone (MBZ), resulting in mini-gap openings, band flattening, and the potential emergence of secondary Dirac points. Using effective field theory, symmetry analysis, and higher-order perturbation theory, we analyze both commensurate and incommensurate twist angles, revealing significant band structure reconstruction in periodic systems and quasi-periodic effects in incommensurate ones. This work provides deep insights into the interplay between topological protection and moir\'e modulation, offering a pathway to engineer novel topological phases.

cond-mat.mes-hall↗

arXiv2025

Stochastic resonance in Schmitt trigger and its application towards weak signal detection

This study explores stochastic resonance (SR) in a Schmitt trigger circuit and its application to weak signal detection. SR, a phenomenon where noise synchronizes with weak signals to enhance detectability, was demonstrated using a custom-designed bi-stable Schmitt trigger system. The circuit's bi-stability was validated through hysteresis curve analysis, confirming its suitability for SR studies. Experimental results revealed SR behavior by analyzing signal-to-noise ratio (SNR) responses to noise amplitude variations. Detection experiments were conducted to determine frequency and amplitude of damping sinusoidal pulses. Frequency detection proved effective, albeit with limitations at low frequencies, while amplitude detection faced challenges due to mathematical complexities. Nonetheless, the study highlights SR's potential for weak signal detection, with proposed enhancements to improve detection accuracy. This work underscores the adaptability of classical SR principles to practical detection systems and suggests future applications in advanced detection technologies, including quantum systems.

eess.SP↗