"Fluctuations and Multi-Component Effects in Two-Dimensional Superconductors"

Phase transitions in low-dimensional systems offer a unique perspective on the role of fluctuations in condensed matter physics. In two-dimensional superconductors, enhanced fluctuations prevent true long-range order at finite temperature, and the superconducting transition becomes topological in nature, as described by the Berezinskii–Kosterlitz– Thouless (BKT) mechanism driven by vortex–antivortex unbinding. We present a qualitative theoretical framework to describe fluctuation effects in 2D superconductors, based on an extended Ginzburg–Landau approach that incorporates the full effects of the order-parameter fluctuations up to the Gaussian level. Non-topological fluctuations of the amplitude and phase renormalize the mean-field critical temperature and thermodynamic properties, while topological phase fluctuations treated within a renormalization-group approach determine the true transition temperature of the system. We then discuss a quantitative microscopic approach for the evaluation of the BKT transition temperature across the BCS–BEC crossover, with direct comparison to experiments in novel layered LixZrNCl superconductor, highlighting the increasing role of phase fluctuations at low carrier density. Finally, we discuss how multi-band superconductivity in artificial high-Tc superlattices (AHTS) leads to distinctive signatures in the temperature dependence of the upper critical magnetic field, exhibiting an upward curvature that differs from the behavior expected in single-component superconductors.