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Quantum phase transition
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<p>In <a href="/facts/Physics/a7aH2y08">physics</a>, a quantum phase transition (QPT) is a <a href="/facts/Phase_transition/eLP2BY8R">phase transition</a> between different <a href="/facts/Quantum_phases/MKmMPkRj">quantum phases</a> (<a href="/facts/Phase_(matter)/NkHXDdN3">phases of matter</a> at <a href="/facts/Absolute_zero/U3UyPl2K">zero temperature</a>). Contrary to classical phase transitions, quantum phase transitions can only be accessed by varying a physical parameter—such as <a href="/facts/Magnetic_field/Ta8Ev588">magnetic field</a> or pressure—at <a href="/facts/Absolute_zero/U3UyPl2K">absolute zero</a> temperature. The transition describes an abrupt change in the <a href="/facts/Ground_state/ML7cfCug">ground state</a> of a many-body system due to its quantum fluctuations. Such a quantum phase transition can be a <a href="/facts/Second-order_phase_transition/eLP2BY8R">second-order phase transition</a>. Quantum phase transitions can also be represented by the <a href="/facts/Topological/2EqW47cU">topological</a> fermion condensation quantum phase transition, see e.g. <a href="/facts/Strongly_correlated_quantum_spin_liquid/cUJx0mq8">strongly correlated quantum spin liquid</a>. In case of <a href="/facts/Three_dimensional/KAqKWUbS">three dimensional</a> <a href="/facts/Fermi_liquid/BubT8nPx">Fermi liquid</a>, this transition transforms the <a href="/facts/Fermi_surface/N6EyILAG">Fermi surface</a> into a Fermi volume. Such a transition can be a <a href="/facts/First-order_phase_transition/eLP2BY8R">first-order phase transition</a>, for it transforms <a href="/facts/Two_dimensional/f4b397W8">two dimensional</a> structure (<a href="/facts/Fermi_surface/N6EyILAG">Fermi surface</a>) into <a href="/facts/Three_dimensional/KAqKWUbS">three dimensional</a>. As a result, the <a href="/facts/Topological_charge/rSa0TewR">topological charge</a> of Fermi liquid changes abruptly, since it takes only one of a discrete set of values. </p>