Drude in D major

We study holographic momentum relaxation in the limit of a large number of spacetime dimensions d. for an axion model we find that momentum conservation is restored as d becomes large. to compensate we scale the strength of the sources with d so that momentum is relaxed even at infinite d. we analytically obtain the quasi-normal modes which control electric and heat transport, and give their frequencies in a 1/d expansion. we also obtain the ac thermal conductivity as an expansion in 1/d, which at leading order takes drude form. to order 1/d our analytical result provides a reasonable approximation to the ac conductivity even at d = 4, establishing large d as a practical method in this context. as a further application, we discuss the signature of the transition from coherent to incoherent behaviour known to exist in the system for finite d.