Quantum antiferromagnets in a magnetic field

Low-dimensional antiferromagnets in an external magnetic field provide an i deal illustration of the physics of quantum phase transitions. This theoret ical analysis is motivated by the two-leg spin ladder geometry, which has b ean the subject of much experimental study in the material CuHpCl. The non- linear sigma model is used to characterise the quantum phases of the system , and the bond-operator description to discuss excitation spectra and quant um phase transitions between ground states.