Effective theories of quantum liquids (superconductors and superfluids of various types) are derived starting from microscopic models at the absolute zero of temperature. Special care is taken to assure Galilei invariance. The effective theories are employed to investigate the quantum numbers carried by the topological defects present in the phases with spontaneously broken symmetries. Due to topological terms induced by quantum fluctuations, these numbers are sometimes found to be fractional. The zero-temperature effective theories are further used to study the quantum critical behavior of the liquid-to-insulator transition which these systems undergo as the applied magnetic field, the amount of impurities, or the charge carrier density varies. The classical, finite-temperature phase transitions to the normal state are discussed from the point of view of dual theories, where the defects of the original formulation become the elementary excitations. A connection with bosonization is pointed out.
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