D-branes and Non-Perturbative Quantum Field Theory: Stringy Instantons   and Strongly Coupled Spintronics
  The non-perturbative dynamics of quantum field theories is studied using theoretical tools inspired by string formalism. Two main lines are developed: the analysis of stringy instantons in a class of four-dimensional N=2 gauge theories and the holographic study of the minimal model for a strongly coupled unbalanced superconductor.   The field theory instanton calculus admits a natural and efficient description in terms of D-brane models. In addition, the string viewpoint offers the possibility of generalizing the ordinary instanton configurations. Even though such generalized, or stringy, instantons would be absent in a purely field-theoretical, low-energy treatment, we demonstrate that they do alter the IR effective description of the brane dynamics by introducing contributions related to the string scale. In the first part of this thesis we compute explicitly the stringy instanton corrections to the effective prepotential in a class of quiver gauge theories.   In the second part of the thesis, we present a detailed analysis of the minimal holographic setup yielding an effective description of a superconductor with two Abelian currents. The model contains a scalar field whose condensation produces a spontaneous symmetry breaking which describes the transition to a superfluid phase. This system has important applications in both QCD and condensed matter physics; moreover, it allows us to study mixed electric-spin transport properties (i.e. spintronics) at strong coupling. 
Daniele Musso
hep-th