  Unlike classical fluids, a quantum Fermi liquid can support a long-lived and
propagating shear sound wave at arbitrarily small wave vectors and frequencies,
reminiscent of the transverse sound in crystals, despite lacking any form of
long-range crystalline order. This mode is expected to be present in moderately
interacting metals where the quasiparticle mass is renormalized to be more than
twice the bare mass in two dimensions (2D), but it has remained undetected
because it is hard to excite since it does not involve charge density
fluctuations, in contrast to the conventional plasma mode. In this work we
propose a strategy to excite and detect this unconventional mode in clean
metallic channels. We show that the shear sound is responsible for the
appearance of sharp dips in the ac conductance of narrow channels at resonant
frequencies matching its dispersion. The liquid resonates while minimizing its
dissipation in an analogous fashion to a sliding crystal. Ultra-clean 2D
materials that can be tuned towards the Wigner crystallization transition such
as silicon metal-oxide-semiconductor field-effect transistors, MgZnO/ZnO,
p-GaAs, and AlAs quantum wells are promising platforms to experimentally
discover the shear sound.
