A momentum-gapped collective mode in Nature: Pines' Demon in Sr₂RuO₄

In certain multiband metals, electrons in different bands can oscillate in anti-phase under the influence of an electric field. This results in the formation of a neutral mode termed the Pines’ demon, a collective mode comprising two species of charged particles collaborating to sustain the collective motion's neutrality—an acoustic plasmon mode in three-dimensional metals.

The theoretical prediction of this phenomenon dates back to 1956 by D. Pines¹, yet its experimental validation materialized only recently² in Sr₂RuO₄. This work explores the Pines’ demon in Sr₂RuO₄, incorporating momentum-relaxation effects and showing that they are crucial to undersand the physics of this previously unseen phenomenon.

While conventional approaches such as RPA and beyond-RPA corrections in both bulk and surface models predict the existence of an acoustic plasmon, they predict a linear dispersion intersecting the origin different from the experimentally observed dispersion reported₂. Our analysis reveals that the inclusion of momentum-relaxation effects results in a modification of the linear acoustic plasmon for any 3D system hosting acoustic plasmons, leading to a universal phenomenon of momentum-gapping in any 3D system with momentum relaxation. These results rule out many-body interactions or surface effects as the origin of the experimental measurements of the Pines’ demon in Sr₂RuO₄. Furthermore, the predicted and observed acoustic plasmon dispersion constitutes the first detection of a momentum-gapped dispersion in a 3D electronic system.

[1] Pines, D, Can. J. Phys. 34, 1379–1394 (1956).

[2] Husain, A. A., Huang, E. W., Mitrano, M., Rak, M. S., Rubeck, S. I., Guo, X., Yang, H., Sow, C., Maeno, Y., Uchoa, B., Chiang, T. C., Batson, P. E., Phillips, P. W., Abbamonte, P. Nature 621, 66-70 (2023).