Holography and defect conformal field theories

We develop both the gravity and field theory sides of the Karch-Randall conjecture that the near-horizon description of a certain D5-D3 brane configuration in string theory, realized as AdS(5)xS(5) bisected by an AdS(4)xS(2) "brane," is dual to N=4 super Yang-Mills theory in R-4 coupled to an R-3 defect. We propose a complete Lagrangian for the field theory dual, a novel "defect superconformal field theory" wherein a subset of the fields of <N=4 SYM theory interacts with a d=3 SU(N) fundamental hypermultiplet on the defect preserving conformal invariance and 8 supercharges. The Kaluza-Klein reduction of wrapped D5 modes on AdS(4)xS(2) leads to towers of short representations of OSp(4\4), and we construct the map to a set of dual gauge-invariant defect operators O-3 possessing integer conformal dimensions. Gravity calculations of < O-4 > and < O4O3 > are given. Spacetime and N dependence matches expectations from DCFT, while the behavior as functions of lambda=g(2)N at strong and weak coupling is generically different. We comment on a class of correlators for which a nonrenormalization theorem may still exist. Partial evidence for the conformality of the quantum theory is given, including a complete argument for the special case of a U(1) gauge group. Some weak coupling arguments which illuminate the duality are presented.