De Gennes narrowing in the density fluctuation spectrum of a strange metal

The origin of the strange metal is a long-standing puzzle that some authors believe could signify Planckian dissipation in a many-body system. We recently reported [1,2] momentum-resolved EELS (M-EELS) experiments from the strange metal Bi_2.1Sr_1.9CaCu₂O_8+x indicating that the high-energy (ω > 0.1 eV) charge fluctuation spectrum exhibits a featureless continuum reminiscent of the marginal Fermi liquid (MFL) hypothesis from the late 1980s. However, because of resolution limitations, the low-energy region relevant to transport properties could not be observed. Here we report M-EELS studies of the low-energy region with ω~T using significantly improved momentum and energy resolution. We find that the low-energy charge dynamics exhibit a relaxational form with a momentum-dependent relaxation rate that exhibits a local minimum near the Brillouin zone boundary. This effect, called de Gennes narrowing, is commonly observed in structured liquids where it arises due to steric interactions between rigid objects. This points to the charge transport in strange metals being dominated by charge clusters with a spatial extent of roughly two unit cells.

[1] M. Mitrano et al., Proc. Natl. Acad. Sci. U.S.A. 115, 5392 (2018)

[2] A. A. Husain et al., Phys. Rev. X 9, 041062 (2019)