Conformally invariant charge fluctuations in a strange metal

The hallmarks of the strange metal phase are a linear resistivity that violates the Mott-Ioffe-Regel limit and a scattering rate, constrained by the Planckian limit, determined only by fundamental constants. Using momentum-resolved EELS (M-EELS), we previously observed that in the high-energy region (E > 200 meV), the dynamic charge susceptibility of the strange metal Bi_2.1Sr_1.9CaCu₂O_8+x (Bi-2212, T_c=91 K) exhibits a featureless continuum, strikingly similar to Marginal Fermi Liquid (MFL) hypothesis [1-3]. In this work, we present new M-EELS results on the low-energy region of the spectrum (E ~ k_BT). Above T_c, the density fluctuations are scale-invariant and mostly momentum-independent. Further, the charge fluctuations exhibit conformal symmetry with a conformal dimension Δ = 0.05, significantly deviating from the MFL hypothesis, which corresponds to Δ = 0.5. Our findings suggest that the strange metal phase might be understood as a zero-dimensional object exhibiting quantum critical dynamics.

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

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

[3] J. Chen et al., Phys. Rev. B 109, 045108 (2024)