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Résumé |
The strange metal is thought to represent a universal limit on quantum entanglement in a many-body system, though this hypothesis remains unproven. In 1989, theorists proposed that universality might manifest in strange metals as scale-invariant behavior in the dynamic charge susceptibility, $\chi''(q, \omega)$, at finite momentum, $q$. In this talk I will describe the first measurements of $\chi''(q, \omega)$ in the thermal energy regime, $\hbar\omega \sim k_B T$, obtained via momentum-resolved inelastic electron scattering from the strange metal Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$. The susceptibility is independent of momentum and follows the universal scaling form $\chi''(q, \omega) = T^{-\nu} f(\omega/T)$, with exponent $\nu = 0.94$. Further, the response shows conformal invariance, meaning the dynamics behave as if they occur on a circle of radius $1/T$ in imaginary time, characterized by conformal dimension $\Delta = 0.06$. These findings suggest that the strange metal is a universal phenomenon potentially compatible with holographic theories of matter in which conformal invariance plays a central role. |
