Singular Charge Response of Strange Metals

Quantum electronic matter has long been understood in terms of two limiting behaviors of electrons: one of delocalized metallic states, and the other of localized magnetic states. Understanding the strange metallic behavior which develops at the brink of the localization demands new probes of the underlying electronic charge dynamics.

I will highlight two recent experimental studies on strange metals in heavy-fermions:

1. A synchrotron-radiation-based Mössbauer spectroscopy study reveals temperature- and pressure-dependent charge fluctuations. The usual single absorption peak in the Fermi-liquid regime splits into two peaks upon entering the critical regime. This phenomenon is naturally interpreted as a single nuclear transition modulated by nearby electronic valence fluctuations [1], whose long time scales are further enhanced due to the formation of charged polarons

2. Measurements of the thermodynamic Grüneisen ratio [2] in the ferromagnetic quantum critical material CeRh₆Ge₄ [3] reveal a divergent behavior, indicating a dynamical critical exponent of z = 3. This serves as a fingerprint of quantum criticality with conserved dynamics, despite the fact that the magnetization order parameter itself is not conserved in this easy-plane ferromagnet.

I will discuss the significance of these findings and their connection to earlier experiments. I will argue that the the conserved quantity underlying the Grüneisen measurement is charge, and that the Mössbauer experiment provides a direct measurement of this charge mode. Lastly, I will present ongoing theoretical efforts aimed at understanding these phenomena.

[1] H. Kobayashi, Y. Sakaguchi, H. Kitagawa, M. O. S. Ikeda, S. Suzuki, S. Nakatsuji, R. Masuda, Y. Kobayashi, M. Seto, Y. Yoda, K. Tamasaku, Y. Komijani, P. Chandra, P. Coleman, Observation of a Critical Charge Mode in a Strange Metal, Science 379, 908 (2023).

[2] J. Zhan, Y. Zhang, J. Zhang, Z. Nie, Y. Chen, L. Jiao, Y. Komijani, M. Smidman, F. Steglich, P. Coleman, H. Yuan, Critical fluctuations and conserved dynamics in a strange ferromagnetic metal, submitted (2025).

[3] B. Shen, Y. Zhang, Y. Komijani, M. Nicklas, R. Borth, A. Wang, Y. Chen, Z. Nie, Z. Wang, G. Cao, H. Lee, M. Smidman, F. Steglich, P. Coleman, H. Yuan, Strange metal behavior in a pure ferromagnetic Kondo lattice, Nature 579, 51 (2020).