For noninteracting fermions at zero temperature, the Fermi surface is the momentum-space boundary between occupied and unoccupied states. Its shape can be a circle, several pockets or a more complicated surface, depending on the dispersion and filling.
In an interacting Fermi liquid, occupation is not simply zero or one on either side. Instead the momentum distribution has a discontinuity with quasiparticle weight , and low-energy Landau quasiparticles define the Fermi surface. This distinguishes a sharp quasiparticle boundary from an ideal-gas step distribution. The geometry controls low-energy phase space, transport and collective response. Quantum oscillations constrain extremal orbit areas, while momentum-resolved spectral measurements can probe excitations near the chemical potential; the interpretation requires the relevant interaction and measurement model.
The Fermi surface is a concept in solid-state physics that describes the collection of energy states occupied by electrons in a metal or a semiconductor at absolute zero temperature. It is a critical concept in understanding the electronic properties of materials, particularly in relation to their conductivity and other physical behaviors. In more detail, the Fermi surface is defined in the context of the Fermi energy, which is the highest energy level occupied by electrons at absolute zero.
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