A quantum critical point (QCP) is a continuous phase transition that occurs at absolute zero temperature as a non-thermal parameter — pressure, chemical doping, magnetic field, or an interaction strength — is tuned across a critical value. Unlike an ordinary (classical) critical point, where the transition is driven by thermal fluctuations, a QCP is driven by quantum fluctuations required by the Heisenberg uncertainty principle even at . The transition is characterized by critical exponents (conventionally including a correlation-length exponent and a dynamical exponent relating spatial and temporal scaling) that describe how physical quantities diverge as the tuning parameter approaches its critical value.
Although the transition itself is defined at , its influence extends into a “quantum critical fan” at nonzero temperature, where thermodynamic and transport properties can follow anomalous power laws set by the critical exponents rather than the conventional Fermi-liquid behavior expected far from criticality. A widely studied example is the strange metal phenomenon in correlated-electron materials (including the cuprate superconductors and various heavy-fermion compounds): electrical resistivity that grows linearly with temperature down to very low , in contrast to the quadratic temperature dependence a Landau Fermi liquid predicts. Which microscopic mechanism is responsible for a given material’s strange-metal transport — and whether well-defined electronic quasiparticles survive at all near the QCP — remains an active question, with different theoretical approaches sometimes reaching different conclusions for superficially similar observed behavior.