Emergence of exotic superconducting states in multiorbital systems under extreme conditions

• Applicants: Professorin Dr. Anna E. Böhmer; Professor Dr. Ilya Eremin
• Subject Area: Experimental Condensed Matter Physics ; Theoretical Condensed Matter Physics
• Term: since 2026
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 572794210

The GOSSAMER project is a joint German (Ruhr-University Bochum)-Japanese (University of Tokyo) initiative in condensed matter physics that combines experimental and theoretical approaches to investigate the emergence of exotic superconducting states in multiorbital systems under extreme conditions—low temperatures, high magnetic fields, applied pressure and strain. The central objective is to understand the formation of multicomponent superconducting phases in unconventional materials where spin-orbit coupling, band topology, and potentially structural disorder play crucial roles. Our primary focus lies on the unconventional nematic superconductor FeSe and its chemically substituted alloys FeSe₁₋ₓSₓ and FeSe₁₋ᵧTeᵧ. These systems exhibit a rich substitution-temperature phase diagram, potentially hosting multiple superconducting states. Despite active investigations, the evolution of the superconducting gap structure upon suppression of nematic order by the different chemical substitutions, and the consequences of the structural inhomogeneity resulting from those substitutions, remain unresolved. We will address the following research questions: (i) how does the nematic phase influence the superconducting order parameter, and what mechanisms account for the large residual density of states upon its suppression? (ii)What is the complete phase diagram of FeSe₁₋ₓSₓ at high sulfur concentrations, including potential magnetic phases? (iii) How are electronic correlations, gap anisotropy, nodal structure, and topological superconductivity interrelated in FeSe₁₋ᵧTeᵧ? To tackle these questions, our three groups in Germany and Japan, who made pioneering contributions in the field of unconventional multiorbital superconductors, will join forces. The originality of our proposal lies in combining two single-crystal growth approaches, high-resolution thermodynamic probes under extreme conditions, and theoretical modelling that embraces the real structure of the substituted compounds. Employing methods of the highest reliability and exchanging our complementary experimental expertise will guarantee the most conclusive results. The project is divided into three work packages and seven tasks. Each of the PIs is in charge of two or three tasks. Four PhD students will work in this project, two from the Japanese and two from the German side. The project requires several types of expertise, and the skills of the partners are well complemented to meet the requirements. Furthermore, all three PIs have proven strong record of joint research, ensuring seamless cooperation. Through our comprehensive approach, GOSSAMER will identify the origin of exotic superconducting properties in the platform materials FeSe₁₋ₓSₓ and FeSe₁₋yTey, and advance our understanding of the complex landscape of unconventional superconductivity in multiorbital systems.

• DFG Programme: Research Grants
• International Connection: Japan
• Partner Organisation: Japan Society for the Promotion of Science (JSPS)
• Cooperation Partner: Professor Dr. Takasada Shibauchi