Fluctuating elastic structures account for the physics of a plethora of systems at low temperatures. In two dimensions these structures often can be described as a lattice of elastic strings. The strings can either be physical observables (magnetic vortices in superconductors, domain walls, steps on crystal surfaces) or they can arise as a more abstract concept from a parameterization of states which do not have obvious string like structures (frustrated spin systems).
I shall discuss two examples of how strings can couple to perturbations. In the presence of {it quenched disorder} thermodynamic quantities are random variables, and I will demonstrate the applicability of the replica approach to the statistical characterization of these variables. Over a wide range of disorder strength good agreement with numerical results for a related random bond dimer model is obtained. The simultaneous effect of {it geometrical frustration and quantum fluctuations} in spin systems can be understood also from the physics of string lattices. I will discuss the application of these concepts to a fully frustrated Ising model in a transversal field.

Institut de Physique Théorique, Université de Cologne