Techniques arising from string theory can be used to study assemblies of strongly-interacting fermions. Via these `holographic dualities', various strongly-coupled many body systems are solved using an auxiliary theory of gravity. Simple holographic realizations of finite density exhibit spectral functions with sharp Fermi surfaces, of a form distinct from the Landau theory. Whether or not there exist long-lived quasiparticles depends on parameters of the gravity theory; for some values, the system is a marginal Fermi liquid. The contribution to the conductivity from these Fermi surfaces has an interesting temperature

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