Charge-Entropy-Stabilized Superconductor (Ag,Sn)Se

Synopsis

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 evidence from hard x-ray photoelectron spectroscopy (HAXPES) and Mössbauer spectroscopy 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).