= Non-Fermi liquid
A <non-Fermi liquid> is an interacting fermionic state whose low-energy behavior is not described by the long-lived <quasiparticles> of a <Fermi liquid>. In the conventional <Fermi liquid> picture, excitations close enough to the <Fermi surface> become sharp compared with their excitation <energy>. If that organizing principle fails, extending the same quasiparticle approximation toward zero <energy> can give the wrong response.
The term describes a failure of one framework, rather than one unique replacement theory. A one-dimensional <Luttinger liquid> provides a controlled example: its long-wavelength excitations are collective density modes, and its correlation functions need not have the free-fermion form. An anomalous finite-<temperature> transport curve alone does not establish a particular zero-<temperature> <non-Fermi liquid>. Spectral and thermodynamic measurements must be interpreted alongside the relevant dimensionality, interactions and accessible <energy> range.
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