Photoablation of polymers by pulsed excimer laser radiation is commonly believed to be a controlled layer-by-layer removal process. A mass spectrometer was used to monitor neutral species ejected from polyimide samples in vacuum by 248 nm laser radiation. For fluences close to threshold, the ablation rate starts to drop after the first 100–200 pulses and eventually falls almost to zero. The falloff in etch rate is accompanied by a dramatic slowdown in the product translational energy distributions and by the appearance of conical defects on the sample surface. The number of cones is approximately the same for samples irradiated in air or vacuum for the same number of pulses, proving that ablation debris is not the initiating factor. It is argued that carbon enrichment at the sample surface initiates cone formation by locally shifting the ablation threshold to higher values. In effect, the polymer surface becomes ‘‘radiation hardened.’’
Epitaxial Tl2Ca2Ba2Cu3O10 films on (100) LaAlO3 are prepared by post-annealing 2Tl:2Ca:2Ba:3Cu precursor films at 830–860 °C in ≂0.03–0.15 atm of O2. These films (0.2–1.1 μm thickness) are smooth and shiny to the eye, and have a sharp zero resistance and onset diamagnetic transition temperature of 117–121 K. Transport critical current densities of 1.6×106 A/cm2 at 77 K and ≂105 A/cm2 at 100 K are obtained for a 0.38-μm-thick film in magnetic fields up to 100 Oe. Strong flux pinning at low temperatures is inferred from the weak-field dependence of the critical current density calculated from magnetic hysteresis loops. At 5 K, the best film has a magnetic critical current density of 9×106 A/cm2 in zero field, decreasing gradually to 1.5×106 A/cm2 in a 5-T field.
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