Journal of the Korean Physical Society最新文献

Amorphous Ga₂O₃ thin films were deposited on c-plane sapphire substrates by RF magnetron sputtering under varying oxygen partial pressures and crystallized into the monoclinic β-phase via post-annealing at 1000 °C in an oxygen atmosphere. Atomic force microscopy revealed that films grown under low oxygen conditions exhibited surface cracks, while those deposited at higher oxygen pressures showed smooth, crack-free surfaces. Raman spectroscopy showed a progressive increase in the intensities of Ga–O vibrational modes around 200 and 415 cm⁻¹ with increasing oxygen partial pressure, indicating enhanced structural ordering. Optical transmittance measurements revealed a non-linear change in bandgap energy from 4.8 to 4.95 eV, likely due to variations in defect concentration and stoichiometry. The refractive index remained nearly constant at ~ 1.76, suggesting low film density or structural inhomogeneity. These results suggest that β-Ga₂O₃ thin films fabricated under different oxygen partial pressures may still exhibit variations in microstructure and optical properties even after post-annealing.

Korea 4th Generation Synchrotron Radiation Accelerator(Korea-4GSR), aim to achieve a 4 GeV ultra-low emittance beam with a current of up to 400 mA to enhance beam brightness. However, lowering the beam emittance increases the electron density within the bunch, which in turn reduces the beam lifetime due to Touschek scattering and intra-beam scattering. Therefore, to reduce the electron density within the bunch, we considered the application of a normal conducting 3rd harmonic cavity with a 1.5 GHz operating frequency and proceeded with prototype development. The developed harmonic cavity was designed based on the Spanish ALBA model, with design modifications for cooling and mechanical parts. In this presentation, the design, fabrication, and low-power test results of the harmonic cavity were described. With future performance validation and improvements, it is anticipated that this cavity could be utilized not only in the Korea-4GSR but also in other accelerators with similar specifications.

We perform head-on collision simulations of compact dark matter subhalos using distinct numerical methods for fuzzy dark matter (FDM) and cold dark matter (CDM) models. For FDM, we solve the Schrödinger–Poisson equations with a pseudo-spectral solver, while for CDM, we utilize a smoothed particle hydrodynamics N-body code. Our results show that velocity decrease of subhalos is significantly greater in FDM model than in CDM, particularly at lower initial velocities, attributed to gravitational cooling—a unique mechanism of stabilizing in FDM with dissipating kinetic energy. This stark contrast in energy dissipation between two DM models suggests that FDM may offer valuable insights into understanding the dynamic behaviors of DM during galaxy cluster collisions, such as those observed in the Bullet cluster and Abell 520. These findings strongly suggest that FDM is not only capable of explaining these complex astrophysical phenomena but also serves as a compelling alternative to the traditional CDM model, offering resolutions to longstanding discrepancies in DM behavior.

Trapped interfacial bubbles in van der Waals (vdW) heterostructures degrade electronic performance but also present opportunities for local strain engineering. We develop a comprehensive thermomechanical model to investigate how localized continuous-wave (CW) laser heating can be used to manipulate such bubbles in hBN/graphene/hBN stacks. By integrating finite-difference heat conduction simulations with analytical models of bubble energetics, we quantify laser-induced temperature distributions and the conditions required for bubble migration. Our model incorporates optical absorption in both the encapsulated 2D layers and silicon substrate, vertical and lateral heat spreading, and interfacial thermal resistance. A temperature-based criterion for bubble migration is derived from the energy balance between internal pressure buildup and adhesion-limited motion. We show that focused laser power (40 mW) can raise local temperatures to 800 K, sufficient to overcome adhesion barriers for bubbles above a critical size. Notably, our analysis reveals that while the migration direction depends on a competition between internal pressure and adhesion energy gradients, the temperature dependence of adhesion typically dominates, driving bubbles toward colder regions under realistic conditions. These results offer a predictive framework for optothermal interface engineering, enabling spatially controlled bubble removal, cleanliness enhancement, and programmable strain in 2D material devices.

We demonstrate that strong local electric fields generated by a scanning tunneling microscope (STM) tip induce nanoscale structural modifications beneath the surface of bilayer (textrm{Ta}_{2}textrm{NiSe}_{5}). Applying voltage pulses with a positive sample bias leads to the formation of depressions at the pulse sites and protrusions at laterally displaced locations along the crystal’s chain direction. Bias-dependent STM imaging and scanning tunneling spectroscopy reveal no measurable change in the surface electronic structure, indicating that the observed height variations originate from structural, rather than electronic, effects. We propose that the electric field drives anisotropic atomic migration within the van der Waals gap, resulting in reversible and directional reconfiguration of subsurface layers. Sequential pulsing further confirms the dynamic nature of this process, allowing local depressions and protrusions to be erased or repositioned. Our findings introduce a mechanism for electric-field-induced subsurface patterning in layered materials, with potential applications in reconfigurable nanoscale devices in van der Waals materials.

In the present work, the physical properties and hydrogen storage capacity of Sr-based perovskites, XSrH₃, where (X = N, Cl, and Mg), are investigated. Every compound has thermal stability and is dynamic in structure. For NSrH₃, ClSrH₃, and MgSrH₃, the corresponding symmetry lattice parameters are 3.701 Å, 3.800 Å, and 3.7328 Å. All compounds exhibit thermodynamic stability, as indicated by the estimated negative formation energy. It has been discovered that Sr-based perovskite compounds exhibit dynamic stability through phonon dispersion analysis. They have been shown to be both mechanically and elastically stable using the mechanical elastic calculation. The bulk modulus, shear modulus, and Poisson’s ratio can all be found using the calculated acquired elastic constant. It was found that all substances are elastically anisotropic and brittle. The examination of the band gap reveals that both exhibit metallic behavior. All substances exhibit their highest levels of conductivity and absorption in the UV region of their optical characteristics.

The DC/RF performance of a T-gate AlN/GaN heterojunction MOS-HEMT (AGMH) device on SiC substrate with a Hafnium-based high-k gate dielectric material is thoroughly and meticulously investigated in this article. The research investigation looks at how different gate lengths affect important device metrics, such as cut-off frequency (f_T), intrinsic capacitances (C_GD & C_GS), G_M (transconductance), and I_D (drain current). The proposed AGMH device with L_G of 40 nm, t_b of 3 nm, t_ox of 3 nm, L_GS of 250 nm, & L_GD of 400 nm exhibited I_D-max (maximum I_D) of 2.221 A/mm, G_M-peak (peak transconductance) of 505.5 mS/mm, & f_T-max (maximum f_T) of 256 GHz. The remarkable DC/RF performance results from strong carrier confinement & minimized leakage current (I_DL) enabled by scaling down the device parameters. This makes them a desirable option for RF power electronics and microwave (µw) applications in future generations, with a great deal of room for performance and efficiency gains.

In this work, for the first time, an ensembled machine learning-based Hybrid Stacking approach is presented for small-signal behavioral modeling of high electron mobility transistors (HEMT). The device under test (DUT) is AlGaN/InGaN/GaN HEMT on a silicon carbide (SiC) substrate characterized at frequencies up to 50 GHz under room temperature. The stacking model was developed and trained on technology computer-aided design (TCAD)-generated data using four input parameters. It focuses on representing the device’s input–output behavior without delving deeply into the underlying physics. It can handle complex, nonlinear relationships and provide insights into device performance across varying conditions. The model’s predicted and simulated S-parameters show excellent agreement across the entire frequency range. The model demonstrated exceptional accuracy in both interpolation and extrapolation tests, achieving a mean absolute error (MAE) of 3.55E(-)03, mean squared error (MSE) of 5.20E(-)5, and root mean square error (RMSE) of 5.298E(-)03. The R-squared and explained variance scores were approximately 0.99 and 0.998, respectively. By precisely capturing the dependability of S-parameters on bias points and operating conditions, the proposed methodology highlights its potential to reduce barriers to adopting machine learning techniques in semiconductor research. This approach enhances the understanding of GaN HEMT performance and encourages the exploration of advanced ML models for broader applications in device analysis and optimization.

This study investigates the effect of rotation on viscoelastic fluid convection using the Voigt model in a porous medium under thermal nonequilibrium conditions. The analysis considers three boundary conditions with different combinations of free and rigid surfaces. The Darcy–Brinkman model is used to characterize the porous medium, and the Coriolis term is incorporated into the momentum equation to account for rotational effects. A dual-temperature model represents thermal non-equilibrium. Stability analysis is performed using both nonlinear (energy method) and linear (normal mode) approaches. The formulated eigenvalue problems are solved using single-term Galerkin method, from which explicit expressions for the Rayleigh number are derived. The critical Rayleigh number is then obtained by minimizing these expressions with respect to the wavenumber. The results establish stability thresholds and identify the key factors influencing the onset of both stationary and oscillatory convection. The global stability analysis confirms identical Rayleigh numbers for both approaches. Increasing the viscoelastic parameter (lambda) stabilizes oscillatory convection, leading to its disappearance beyond (lambda >1.3) (free–free), (lambda >0.26) (rigid–free), and (lambda >0.13) (rigid–rigid) boundary conditions, while keeping other parameters fixed, with a corresponding reduction in the wave number ranges.