https://scholar.dgist.ac.kr/handle/20.500.11750/11797 2026-04-04T15:18:27Z 2026-04-04T15:18:27Z 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 https://scholar.dgist.ac.kr/handle/20.500.11750/59993 2026-02-10T02:10:29Z 2025-07-31T15:00:00Z

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.

2025-07-31T15:00:00Z 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 https://scholar.dgist.ac.kr/handle/20.500.11750/59039 2026-02-09T18:01:08Z 2025-07-31T15:00:00Z

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.

2025-07-31T15:00:00Z Park, Jaemun Kim, Woo-Yong Cho, Beopgil Choi, Woojae Kwon, Yong Seung Seo, Jungpil Park, Keeseong https://scholar.dgist.ac.kr/handle/20.500.11750/58375 2025-12-18T02:41:42Z 2025-05-31T15:00:00Z

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.

2025-05-31T15:00:00Z Cho, Beopgil Park, Jaemun Choi, Woojae Park, Keeseong Kwon, Yong Seung https://scholar.dgist.ac.kr/handle/20.500.11750/58359 2025-07-25T04:26:10Z 2025-09-30T15:00:00Z

Title: Thermally activated flux flow of vortex liquid and suppression of vortex glass in Ca10(Pt4As8)(Fe2As2)5 Author(s): Cho, Beopgil; Park, Jaemun; Choi, Woojae; Park, Keeseong; Kwon, Yong Seung Abstract: To investigate the presence and role of magnetic fluctuations below the superconducting transition temperature in Ca10(Pt4As8)(Fe2As2)5 superconductors, which exhibit static non-magnetic behavior and linear temperature-dependent electrical resistivity in the normal state, we conducted detailed measurements of electrical resistivity under magnetic fields applied along the c-axis (H//c) and the ab-plane (H//ab) to analyze the vortex creep behavior. For H//c, vortex liquid and low-temperature vortex glass phases were observed, similar to those in typical iron pnictide superconductors. In contrast, for H//ab, no vortex glass phase was observed at low temperatures, but instead, a vortex liquid phase extended over a wide temperature range was observed. This behavior has never been reported in iron pnictide systems. This suppression of the vortex glass phase is attributed to the presence of antiferromagnetic fluctuations within the superconducting dome. © 2025 Elsevier Ltd

2025-09-30T15:00:00Z