Quantum phase transition · quantum critical point · non-Fermi liquid fan
A quantum phase transition (QPT) occurs at T=0 as a non-thermal control parameter g (pressure, magnetic field, doping) is tuned through a quantum critical point gc. Unlike classical transitions driven by thermal fluctuations, QPTs are driven by quantum fluctuations that persist at T=0. The quantum critical point controls a "fan" of non-Fermi liquid behavior extending to finite temperatures, where thermodynamic quantities obey power laws set by universal critical exponents ν (correlation length: ξ ∝ |g−gc|^−ν) and z (dynamical: ξ_τ ∝ ξ^z). The scaling hypothesis dictates that all observables are functions of T/|g−gc|^νz alone inside the fan.