Ising spin glasses in a magnetic field

Ground states of the three-dimensional Edwards-Anderson spin glass are comp uted in the presence of an external magnetic field. Our algorithm is suffic iently powerful for us to treat systems with up to 600 spins. We consider h ow the ground state changes as the field is increased; the statistical anal ysis of these changes indicates that the spin glass phase at zero temperatu re does not survive in the presence of any finite field. This is in agreeme nt with the droplet model or scaling predictions, but in disagreement with the mean field picture. For comparison, we also investigate a dilute mean f ield spin glass model where an Almeida-Thouless line is clearly present. [S 0031-9007(99)09316-3].