Nonequilibrium Field Theories of Driven Materials

• Applicant: Dr. Carl Philipp Zelle
• Subject Area: Theoretical Condensed Matter Physics
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 570906600

The concept of thermal equilibrium forms the foundation of our understanding of phases and phase transitions in many-body systems. In recent years, systems that break the conditions of thermal equilibrium through drives and dissipation have fueled a stream of research activity both experimentally as well as theoretically. This raises fundamental questions: What phases and transitions can arise in materials far from equilibrium? What novel phenomena emerge that have no equilibrium counterparts? In this project I will identify and analyze universal structures in nonequilibrium phases and transitions that are relevant for current experimental platforms of optically pumped materials. Numerous recent experiments show that pumping cuprate compounds with a laser pulse can induce a transient superconducting phase. Such pump induced nonequilibrium phase transitions can differ starkly from their equilibrium counterparts. As demonstrated in my doctoral work they can realize new universality whose corresponding scaling forms defy thermal fluctuation-dissipation relations. To understand the nature of the photo-induced superconducting transition in high T_c materials, I will devise nonequilibrium gauge theories. Electromagnetic gauge fields are a necessary ingredient to understand superconductivity. Further, nonabelian gauge structures emerge in effective descriptions of the pseudogap phase of the material compounds in question and need to be considered, as well. These ingredients allow for the construction of a theory of driven-dissipative phase transitions in the presence of gauge fields. It promises the identification of novel nonequilibrium universality classes relevant to current experimental platforms. Another key platform is nonequilibrium condensates of exciton-polaritons in cavity-embedded materials. The long-distance correlation functions of these systems are governed by U(1) Goldstone that mode realizes the Kardar-Parisi-Zhang (KPZ) universality class, which originally was identified in surface growth processes. In materials that have an additional long-range order, the polariton mode couples to the soft modes of this order parameter, as was demonstrated experimentally in Van-der-Waals materials immersed in an optically pumped cavity. I will analyze how this impacts scaling behaviors of observables in these systems using the respective nonequilibrium nonlinear sigma models. This leads to a generalization of the KPZ universality class to larger symmetry groups and points to novel scaling laws in nonthermal phases of quantum matter.

• DFG Programme: WBP Fellowship
• International Connection: USA
• Host: Professor Dr. Subir Sachdev