Critical charge fluctuations at the Kondo breakdown of heavy-fermions*

Recent experiments on quantum critical materials CeRuIn5, YbRh2Si2 and YbAlB4 have observed a natural coincidence of critical charge and spin fluctuations, a phenomenon that goes beyond the Landau-Ginzburg paradigm. We study these effects in the framework of local quantum criticality and argue that soft charge fluctuations are natural consequences of an abrupt change in Fermi surface volume and the entanglement pattern that accompanies Kondo breakdown in heavy-fermion systems.
In a simplified Kondo lattice model, in which each moment is connected to a separate conduction bath [1], we show that a Kondo breakdown develops between a heavy Fermi liquid and a gapped spin liquid via a QCP with ω/T scaling, which features a critical charge mode associated with the break-up of Kondo singlets. We discuss the experimental implications of this effect [2].
Another development is the recent observation of quantum critical point and strange metal behavior in ferromagnetic stoichiometric heavy-fermion CeRh6Ge4 [3]. This is surprising because the abrupt change in the entanglement entropy that is required for singular charge fluctuations are absent in a pure ferromagnet. We argue that the easy-plane magnetic anisotropy produces triplet resonating valence bonds, which produce a highly entangled ordered state, similar to a magnetically frustrated anti-ferromagnetic system. We discuss the possibility that the RVB state can escape to conduction band producing triplet superconductors [4].

References:
[1] Y. Komijani, P. Coleman, PRL 120, 157206 (2018); PRL 122, 217001 (2019).
[2] L. Prochaska, et al., Science 367, 285 (2020); H. Kobayashi, et al., in review (2020).
[3] B. Shen, Y. Zhang, Y. Komijani, M. Nicklas, R. Borth, A. Wang, Y. Chen, Z. Nie, R. Li, X. Lu, H. Lee, M. Smidman, F. Steglich, P. Coleman, H. Yuan, Nature 579, 51 (2020).
[4] P. Coleman, Y. Komijani, E. Konig, Phys. Rev. Lett. 125, 077001 (2020).