The theory of phase transitions driven by thermal fluctuations is one of the centerpieces of condensed matter physics. Even at zero temperature however, quantum fluctuations can destroy order and drive a phase transition that is tuned by some non-thermal parameter (doping, magnetic field, pressure, strain, etc). Experiments cannot access absolute zero temperature, but the presence of the quantum phase transition creates a region of parameter space where quantum and thermal fluctuations compete. An appropriate theoretical description has not yet been established for this region, but various experiments have found strong deviations from the standard (and highly successful) theory of metals, Landau’s theory of the Fermi liquid.
Quantum Criticality can also be an avenue towards understanding other phases of matter. To date, the exact mechanism of high-temperature superconductivity has not been completely understood. The fact that superconductivity emerges in the close vicinity of QCP for different unconventional SCs raises the question if there is a more fundamental relation between quantum critical fluctuations and this ordering phenomenon.
