Shi Chen

Dislocations in third-generation semiconductor gallium nitride (GaN) have always been a subject of intense study. Here, we investigate the core structures and electronic properties of prismatic edge dislocations in wurtzite GaN using a combination of the discrete Peierls theory and first-principles calculations. We identify four primary analytical core configurations, some of which exhibit reconstruction. Stable glide dislocations are found to be dangling-bond-free, whereas shuffle dislocations typically possess dangling bonds yet exhibit limited electronic activity. Different shuffle-type cores show similar electronic properties, consistent with their structural similarities. The intermediate states during glide dislocation motion may significantly influence GaN’s electronic behavior. This work validates the accuracy of our combined theoretical and computational approach for atomic-scale dislocation characterization and establishes a foundation for dislocation engineering in high-performance GaN devices.

Tantalum is extensively used in inertial confinement fusion research for targets in radiation transport experiments, hohlraums in magnetized fusion experiments, and lining foams for hohlraums to suppress wall motions. To comprehend the physical processes associated with these applications, detailed information regarding the ionization composition and electrical conductivity of tantalum plasma across a wide range of densities and temperatures is essential. In this study, we calculate the densities of ionization species and the electrical conductivity of partially ionized, nonideal tantalum plasma based on a simplified theoretical model that accounts for high ionization states up to the atomic number of the element and the lowering of ionization energies. A comparison of the ionization compositions between tantalum and copper plasmas highlights the significant role of ionization energies in determining species populations. Additionally, the average electron–neutral momentum transfer cross-section significantly influences the electrical conductivity calculations, and calibration with experimental measurements offers a method for estimating this atomic parameter. The impact of electrical conductivity in the intermediate-density range on the laser absorption coefficient is discussed using the Drude model.

The laser ablation pressure is often estimated by an exponential scaling law, which is determined by integral averaging of 1–2 ns short pulses. Here, we report a direct, continuous measurement of ablation pressure of a 3–10 ns ramp 351-nm laser pulse. Aluminum is deposited on LiF window, ablation pressure is inferred from the particle velocity of interface using the backward integration method, combined with incident ramp shaped laser intensity, and the real-time laser-driven ablation pressure as a function of incident laser power density on an aluminum target is obtained. For the same ablation pressure scale, when intensity is higher than 1TW/cm2, the surface illuminated intensity of laser should be modified with the cosine of incident angle, which agrees well with the modified Manheimer model [Scheiner and Schmitt, Phys. Plasmas 26(2), 024502 (2019)]. On the other hand, the incidence angle has the least effect when the intensity is below 0.1 TW/cm2. In the range of 0.1–1 TW/cm2, a significant loss in ablation pressure is observed, deviating notably from the exponential scaling relationship. This characteristic makes the laser direct drive ramp loading technique unfriendly to elastoplastic and phase transition problems.

Ion evaporation from a conductive meniscus has been of significant interest in the theoretical investigation of electro-hydrodynamics and application exploration across various fields. This study focuses on developing a fluid-fluid methodology for the dynamical simulation of a conductive meniscus undergoing ion evaporation and uncovering the interaction between the plume and meniscus using the ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate as a case study. In the fluid-fluid model, we propose a simplified fluid plume model to acquire the charge distribution in free space, and validate it against a particle plume model and a full fluid plume model. The meniscus evolution is described by expanding the leaky dielectric model to account for charge conservation in the liquid as well as self-heating and inhomogeneous physical properties. The arbitrary Lagrangian–Eulerian method is used to track the sharp liquid–vacuum interface. Dynamic simulation with the simplified fluid plume model is more than 150 times faster than that with the full fluid plume model. The electrohydrodynamic process of the meniscus evolving to form a droplet is analyzed, with a detailed discussion on the space charge effect caused by evaporated ions. Results indicate that neglecting the space charge effect during conical meniscus formation leads to a singular meniscus tip. Instead, the reverse electric field induced by the space charge suppresses this singularity, assisting the conical meniscus to produce a jet. Additionally, the high-throughput ion evaporation significantly enlarges the diameter of droplet formed on the conical meniscus due to the reverse electric field of space charge.

In ultra-short pulse laser machining and micro/surface processing, accurate simulation of laser ablation is important for understanding laser-target interaction and improving ablation performance, but it remains challenging. This work aims to develop a numerical model to improve the accuracy of the ablation depth calculation. A grid deformation scheme is proposed based on energy conservation and considering contributions to instant material removal from both the electron and lattice subsystems. By incorporating this scheme with the two-temperature model (TTM), a reasonable prediction of the instant target surface profile during laser ablation has been achieved. In the case of single-pulse femtosecond laser ablation of Ti6Al4V, the calculated ablation depth ranges from 0.06 to 0.56 μm for laser energy from 1.0 to 10.0 μJ. For single-pulse picosecond laser ablation of stainless steel, as laser energy increases from 6.0 to 18.5 μJ, the predicted ablation crater deepens accordingly from 40 to 87 nm. In addition, for multi-pulse picosecond laser ablation of stainless steel, a linear dependence of the ablation depth on the pulse number is observed up to a depth of about 803 nm at 6.0 μJ and 20 pulses. In all the above-mentioned cases, the calculation results are in better agreement with experimental measurements than conventional TTM or other material removal schemes, validating the accuracy of the proposed model.