https://scholar.dgist.ac.kr/handle/20.500.11750/11797 Fri, 24 Apr 2026 19:51:55 GMT 2026-04-24T19:51:55Z https://scholar.dgist.ac.kr/handle/20.500.11750/60208 Title: From Synthesis to Superconductivity: Hands-On Learning of Quantum Materials through YBCO Doping Author(s): Park, Jaemun; Cho, Beopgil; Rhee, Taeseong; Park, Keeseong Abstract: In the advancement of quantum materials research, consistent follow-up content in science education becomes paramount. In this article, we report how the synthesis and characterization of the high-temperature superconductor, yttrium barium copper oxide (YBCO), can effectively address this concern. Our exploration encompasses the comparison between pristine YBCO samples and La-, Ce-, and Fe-doped variants to examine the emergence and suppression of superconductivity. Magnetic levitation experiments under liquid nitrogen conditions revealed the presence of the flux pinning effect in La- and Ce-doped samples, whereas it was absent in Fe-doped samples, consistent with temperature-dependent resistance measurements. X-ray diffraction analysis confirmed that La atoms successfully substituted for the Y site, while Ce atoms remain incompatible with the YBCO phase under our synthesis conditions. In contrast, when Fe atoms replace the Cu site, they suppress the superconductivity and transform the crystal structure from orthorhombic to tetragonal. We suggest that our study not only provides insights into the physical properties of YBCO through introducing various dopants but also serves as a benchmark for educators developing analogous or extended experiments. Sat, 31 Jan 2026 15:00:00 GMT https://scholar.dgist.ac.kr/handle/20.500.11750/60208 2026-01-31T15:00:00Z https://scholar.dgist.ac.kr/handle/20.500.11750/59993 Title: Accurate Assessments of the Electronic Structures of Ultrathin PtSe2: Bandgap Quantification and Critical Thickness for the Metal-Semiconductor Transition Author(s): Kim, Hansung; Cha, Janghwan; Seo, Jong Hyeok; Cho, Beopgil; Park, Jaemun; Park, Keeseong; Watanabe, Kenji; Taniguchi, Takashi; Ha, Dong Han; Kwon, Jihwan; Seo, Sunae; Kim, Yong-Sung; Jung, Suyong Abstract: Ultrathin PtSe2, a member of the group-10 transition metal dichalcogenides, has emerged as a promising two-dimensional material due to its layer-dependent, tunable bandgap. Notably, a unique semiconductor-to-metal transition is predicted as the layer number of this material increases; however, pinpointing the exact critical thickness for this transition and reliably quantifying the energy gaps of the semiconducting layers remain formidable challenges. In this work, all-van der Waals assembled multiprobe schemes and planar tunnel junctions are employed to systematically investigate the thickness-sensitive charge transport properties and energy gaps of ultrathin PtSe2 films. Temperature-dependent measurements reveal that PtSe2 exhibits semiconducting behavior from monolayer to five layers, with a transition to a semimetallic state at six layers. Furthermore, using electron tunneling spectroscopy, we accurately quantify the energy gaps of monolayer, bilayer, and trilayer PtSe2 and identifies that PtSe2 in monolayer form behaves as an n-type semiconductor but intriguingly transitions to a p-type semiconductor in bilayer form. First-principles calculations highlight the importance of correctly evaluating interlayer distances to select the appropriate density functional theory functional, enabling reliable predictions of the critical thickness of ultrathin PtSe2 for the semiconductor-to-metal transition and corresponding electronic structures. Thu, 31 Jul 2025 15:00:00 GMT https://scholar.dgist.ac.kr/handle/20.500.11750/59993 2025-07-31T15:00:00Z https://scholar.dgist.ac.kr/handle/20.500.11750/59039 Title: Evidence for orbital Fulde-Ferrell-Larkin-Ovchinnikov state in the bulk limit of 2H-NbSe2 Author(s): Cho, Chang-woo; Lortz, Timothee T.; Lo, Kwan To; Ng, Cheuk Yin; Chui, Shek Hei; Allan, Abdel Rahman; Abdel-Hafiez, Mahmoud; Park, Jaemun; Cho, Beopgil; Park, Keeseong; Yuan, Noah F. Q.; Lortz, Rolf Abstract: The Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state is an unusual superconducting phase that survives beyond the Pauli paramagnetic limit through spatial modulation of the order parameter. An even more exotic variant-the orbital FFLO state-was recently reported in thin flakes of 2H-NbSe2, involving the interplay of Ising spin-orbit coupling and orbital pair breaking. Here, we report thermodynamic signatures consistent with an orbital FFLO state in bulk 2H-NbSe2, based on high-resolution magnetization and torque measurements under strictly parallel to the NbSe2 basal plane. In the magnetic phase diagram, a crossover to a first-order transition appears above 3 T and disappears with slight field misalignment, indicating field-angle dependent Pauli-limited behavior. Additionally, we observe a reversible step-like anomaly within the superconducting state, and a pronounced six-fold in-plane modulation of the upper critical field above this phase transition. These results suggest that the orbital FFLO state is likely realized even in the bulk limit of 2H-NbSe2. Thu, 31 Jul 2025 15:00:00 GMT https://scholar.dgist.ac.kr/handle/20.500.11750/59039 2025-07-31T15:00:00Z https://scholar.dgist.ac.kr/handle/20.500.11750/58375 Title: Nominal Kagome Antiferromagnetic Mn3Sn: Effects of excess Mn and its novel synthesis method Author(s): Park, Jaemun; Kim, Woo-Yong; Cho, Beopgil; Choi, Woojae; Kwon, Yong Seung; Seo, Jungpil; Park, Keeseong Abstract: The antiferromagnetic (AFM) Weyl semimetal Mn3Sn has attracted significant interest due to its intriguing topological and transport properties. However, the reproducibility of experimental results has been limited, potentially stemming from the thermodynamically stable Mn3+xSn1-x phase, where excess Mn substitutes at Sn sites and alters its intrinsic helical ordering. In this study, we present a Bi flux-assisted recrystallization method for synthesizing high-quality nominal Mn3Sn single crystals. Our approach yields stoichiometric and homogeneous samples with the largest residual resistivity ratio (RRR > 23) and sharper magnetic phase transitions, confirming their high purity. While the triangular AFM phase at room temperature is independent of sample quality, the helical magnetic ordering exhibits strong quality dependence, with additional helical phases emerging between 250 K and 280 K. At low temperatures, the system retains a semimetallic nature, as evidenced by the lower Sommerfeld coefficient (gamma), differential conductance (dI/dV) spectra, and magnetoresistance measurements. These findings highlight the interplay between chemical composition and magnetic phase transitions in Mn3Sn and establish a direct link between its helical ordering and electronic structure tuning. Our results not only provide a pathway for producing high-quality Mn3Sn single crystals but also offer a valuable platform for exploring unresolved aspects of its helical phases and potential applications in AFM spintronics. Sat, 31 May 2025 15:00:00 GMT https://scholar.dgist.ac.kr/handle/20.500.11750/58375 2025-05-31T15:00:00Z