String duality and black hole physics: F-string and D-string of type IIB superstring and string theoretical description of black hole radiation

n the first half, SL(2,Z) duality between F-(fundamental) strings and D-(Dirichlet) strings in type IIB theory is studied by considering the effect of string loops. The addition of string world sheet action to the low energy effective action needs some string theoretical explanation since the background fields of the low energy effective action are supposed to be the long range manifestation of a condensate of strings. It is suggested that this may be understood as being due to string loop effects. In the case of type IIB string theory it is rigorously discussed by invoking the Fischler-Susskind effect. The argument provides further justification for SL(2,Z) duality between F-strings and D-strings on the string world sheet.;The rest of the thesis is devoted to the analysis of black hole radiation from the string theory point of view. The emission of a scalar with low energy, from a...