= Quantum critical point
{wiki}
A quantum critical point is a continuous zero-temperature <quantum phase transition> reached by tuning a nonthermal parameter of a <Hamiltonian>, such as interaction strength or <magnetic field>. The <ground state> changes its long-distance structure, its <correlation length> diverges, and its characteristic excitation energy tends to zero in the thermodynamic limit.
For the <Transverse-field Ising model>, exchange favors aligned spins while the transverse field favors a competing orientation. At the critical ratio the gap closes; the <dynamical critical exponent> relates the diverging time and length scales. A finite sample rounds the singular behavior. Experiments at nonzero <temperature> can access a nearby <quantum-critical regime>, but a finite-temperature anomaly alone does not locate or prove a zero-temperature critical point.
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