= Holographic D.C. conductivity
Holographic D.C. conductivity describes the zero-frequency electric response of a boundary system using a bulk <gauge field>. In a clean normal state at nonzero <charge density>, the electric current generally overlaps with conserved <momentum>. A constant <electric field> then continuously accelerates that <momentum>, producing a zero-frequency delta function rather than an ordinary finite D.C. response.
Introducing relaxation of <momentum> can broaden this contribution and yield finite conductivity when no other protected dissipationless channel remains. That qualification matters: a <superfluid> can retain a delta function even when translations are broken. One must specify the phase, conserved quantities and frequency limit before interpreting a bulk calculation as a measured D.C. conductivity.
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