Charge-Entropy-Stabilized Superconductor (Ag,Sn)Se
Mohamed Oudah
Research Associate, Stewart Blusson Quantum Matter Instiute
University of British Columbia
Thursday, January 15, 2026
2 p.m.
In-person: C2-361
Abstract: The study of entropy in materials has two overlapping areas of research: exploring the role of stabilizing phases above a critical entropy threshold and enhancing material properties through configurational disorder [1,2]. In both cases, the focus has been on the contribution of atomic species to the total entropy. Here, we extend this framework by introducing the concept of charge-entropy stabilization in the rocksalt superconductor (Ag,Sn)Se.
In (Ag,Sn)Se, we expect Sn to exist in a mixed-valent state, fluctuating between Sn²⁺/Sn⁴⁺, for the experimentally realized compositions [3]. Charge fluctuations, particularly in systems with valence-skipping elements like Sn, have been proposed as a mechanism for unconventional superconductivity [4]. We present spectroscopic evidence from HEXPES and Mössbauer measurements confirming the persistence of charge fluctuations down to 4 K, below the superconducting transition temperature [5]. We propose that these fluctuating states contribute to the entropy and stabilize the metastable rocksalt structure, an entropy-stabilized phase where the entropy term dominates the Gibbs free energy landscape [5].
We develop a model based on the mixed valence of Sn, considering the ionic species Ag¹⁺/Sn²⁺/Sn⁴⁺, that captures the observed phase stability region. This proposed model may allow for the synthesis of other rocksalt compounds containing valence-skipping elements and finding new superconductors where charge fluctuations are important.
[1] S. Schweidler, et al., Nat. Rev. Mater. 9: 266-281 (2024).
[2] S. S. Aamlid, M. Oudah, et al., JACS 145.11: 5991-6006 (2023).
[3] A. Wold, and R. Brec., Mater. Res. Bull. 11, 761 (1976).
[4] C. M. Varma, Rep. Prog. Phys. 75: 052501 (2012).
[5] M. Oudah, et al., Commun. Mater. 6.1: 58 (2025).
Mohamed Oudah is a Research Associate at the Quantum Matter Institute (QMI), University of British Columbia, Canada, where he works on the synthesis and discovery of novel quantum materials, including high-entropy materials and correlated systems. Dr. Oudah earned his Ph.D. in Physics from Kyoto University for his work on superconductivity in antiperovskite oxides. He also holds an M.Sc. in Chemistry–Nanotechnology from the University of Waterloo, where he worked on thermoelectric chalcogenides, and a B.Eng. in Chemical Engineering from the University of Ottawa. His research background includes positions at Princeton University, the Max Planck Institute for Solid State Research in Stuttgart, and Kyoto University. He has received the MEXT Research Scholarship. He has more than 30 publications, including Nature Materials, Physical Review X, Nature Communications, JACS, npj Quantum Materials, Inorganic Chemistry etc.