The far-infrared and infrared reflection spectra of TI,Ba,Ca,Cu,O,, superconducting film are reported in this paper. It is found there are three sharp reflection edges in both the superconducting and normal states, their positions being near % 150, % 350 and % 600 cm-'. We attribute these reflection spectra to the Drude free carriers and a wide mid-infrared absorption band. When the incident angle increases from 7" to 30" , some additional structures appear in the reflection spectra, which are the A,, phonon modes vibrating along c-axis. PACC: 7400; 7475; 7830Ye Hong-juan et al. Vol. 1 at zero frequency, and some mid-infrared absorption whose origin is not yet clear. There are three reflection edges in both superconducting and normal states, which do not shift. This behavior is similar to that for other oxide superconductors. How to interpret the mid-infrared absorption and lack of shift of reflection edges requires further investigation.

Two separate aspects of beams of rectangular cross section have been considered, namely the maximum space-charge-limited current which can be passed through rectangular slot apertures, and the maximum current density which can be obtained in collimated and focused beams. Both these limiting effects must be considered when attempting to maximize the current in beams of rectangular cross section. In both cases it has been found that the maximum current density is approximately the same for doubly focused rectangular beams, for singly focused ribbon beams, and for axially symmetric beams.

Recent work suggests that fundamental and Dirichlet strings, and their (p, q) bound states, may be observed as cosmic strings. The evolution of cosmic string networks, and therefore their observational signals, depends on what happens when two strings collide. We study this in string perturbation theory for collisions between all possible pairs of strings; different cases involve sphere, disk, and annulus amplitudes. The result also depends on the details of compactification; the dependence on ratios of scales is only logarithmic, but this is still numerically important. We study a range of models and parameters, and find that in most cases these strings can be distinguished from cosmic strings that arise as gauge theory solitons.

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