Shock front dynamics in the pulsed laser deposition of YBa₂Cu₃O_7−x

Abstract

The pulsed laser deposition of YBa2Cu3O7−x targets by excimer laser at fluences of 4–10 J cm−2 in low pressure oxygen backgrounds yields emissive plumes with kinetic energies of 50–200 eV, driving the formation of a shock front with Mach numbers of M = 10–50. The propagation of the shock front is independent of atomic species and adequately characterized by the Sedov–Taylor shock model if the dimensionality of the plume is allowed to deviate from ideal spherical expansion. The ideal efficiency of energy conversion from laser pulse to shock expansion is nearly unity at 1 Torr, but decreases rapidly at lower pressures, where the plume expands beyond the laser footprint during ablation. The low oxygen background pressures, 100–1000 mTorr, typically employed for the production of superconducting films is sufficient for the generation of a strong shock front with shock thickness of 5 mm to less than 0.4 mm, but too low to develop three-dimensional flow. Indeed, dimensionality of the expansion ranges from n = 0.8 to 2.4 over the background oxygen pressure range of 25–1000 mTorr. Shock strength is proportional to the Mach number and inversely dependent on pressure, indicating a thickness limited to approximately the collision mean free path.

Electronic State Distributions of YBa2Cu3O7-x Laser-Ablated Plumes

“…Since the plume typically travels at high Mach numbers (M on the order of 10) compared to the background gas, [68] the boundary between the plume and background gas (referred to as the 'contact front' [50]) becomes a shock front. In addition, the compression of the background gas by the moving plume leads to creation of a shock wave in the background gas.…”

“…[84] This model has been very successful at modelling YBCO plumes at pressures in the hundreds of mTorr, using the value of n=3 for a 3-D spherical shock, however the best value of n for modeling plume expansion is pressure dependent, with the value of n approaching the 3-D ideal at higher pressures, and dropping to n=0, (essentially a free expansion) at pressures at 25 mTorr. [68] One deficiency of the blast wave model is that it predicts the plume will expand indefinitely. At higher pressures and propagation distances, a drag model may be used to account for the eventual stopping of the plume.…”

“…Work by Geohegan [36] and Phelps [68] has shown the drag model to be superior for modeling plume motion at higher pressures and propagation distances. Unlike the blast wave theory, it provides an estimate for the initial plume velocity.…”

“…And while blast wave theory in most cases is a better model of plume front motion, drag theory is superior at modeling motion of the position of peak intensity. [68] Using the wavefront velocities derived from position vs time data, the gas temperature and pressure in the shocked region may be determined using relations derived from the conservation laws for mass and momentum as well as the ideal gas law: [58] ρ S = ρ g (1 + 1/γ S ) (2.18)…”

“…Time-of-flight measurements have included optical [25,26,36,68] and ion probe time-of-flight measurements [29,38], which have been very successful at characterizing the kinetic energy of ablation plumes according to free streaming, drag, or shock models, depending on the stage of the plume evolution. [36] Spectroscopic techniques have included absorption [37] and emission spectroscopy [8-10, 27-29, 91], as well as laser induced fluorescence.…”