Zero temperature phase transitions in spin-ladders: Phase diagram and dynamical studies of Cu-2(C5H12N2)(2)Cl-4

In a magnetic field, spin-ladders undergo two zero-temperature phase transi tions at the critical fields H-c1 and H-c2. An experimental review of stati c and dynamical properties of spin-ladders close to these critical points i s presented. The scaling functions, universal to all quantum critical point s in one-dimension, are extracted from (a) the thermodynamic quantities (ma gnetization) and (b) the dynamical functions (NMR relaxation). A simple map ping of strongly coupled spin ladders in a magnetic field on the exactly so lvable XXZ model enables to make detailed fits and gives an overall underst anding of a broad class of quantum magnets in their gapless phase (between H-c1 and H-c2). In this phase, the low temperature divergence of the NMR re laxation demonstrates its Luttinger liquid nature as well as the novel quan tum critical regime at higher temperature. The general behavior close these quantum critical points can be tied to known models of quantum magnetism.