Nucleation of P-Branes and Fundamental Strings

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We construct a solution to the low-energy string equations of motion in five dimensions that describes a circular loop of fundamental string exponentially expanding in a background electric H-field. Euclideanising this gives an instanton for the creation of a loop of fundamental string in a background H-field, and we calculate the rate of nucleation. Solutions describing magnetically charged strings and p-branes, where the gauge field comes from Kaluza-Klein reduction on a circle, are also constructed. It is known that a magnetic flux tube in four (reduced) spacetime dimensions is unstable to the pair creation of Kaluza-Klein monopoles. We show that in (4+p) dimensions, magnetic (p+1) ``fluxbranes" are unstable to the nucleation of a magnetically charged spherical p-brane. In ten dimensions the instanton describes the nucleation of a Ramond-Ramond magnetically charged six-brane in type IIA string theory. We also find static solutions describing spherical charged p-branes or fundamental strings held in unstable equilibrium in appropriate background fields. Instabilities of intersecting magnetic fluxbranes are also discussed.