Critical charge fluctuations and $\omega$/T scaling at the Kondo breakdown of Heavy-fermion systems

A number of recent experiments on quantum critical materials CeRuIn$_5$, YbRh$_2$Si$_2$, CeCu$_2$Si$_2$ and YbAlB$_4$ have observed critical charge fluctuations coinciding with magnetic transition, a phenomena that goes beyond the Landau-Ginzburg paradigm. We will argue that such soft charge fluctuations are natural consequences of an abrupt change in the Fermi surface volume that accompanies Kondo breakdown (KBD) in heavy-fermion systems.

Using a model 1D Kondo lattice in which each moment is connected to a separate conduction bath, we show that a KBD transition develops between a heavy Fermi liquid and a gapped spin liquid via a QCP with $\omega/T$ scaling, which features residual entropy and a critical charge mode directly associated with the break-up of Kondo singlets.

Furthermore, We contrast the cases of ferromagnetic (FM) and antiferromagnetic (AFM) couplings. While with a purely Ising magnetic coupling, both models map to the dissipative transverse-field Ising model, the Heisenberg limit is quite different: The charge mode is incoherently broadened in the FM system, while it remains sharp, protected by the spin liquid gap, in frustrated AFM system. We discuss the implications of these effects on the experiments.

Ref.: Phys. Rev. Lett. 120, 157206 (2018); arXiv:1810.08148.