Unravelling the temperature dependence of superconductivity
High-temperature superconductors are those that transition to superconductivity at temperatures closer to ambient temperature, making attainment easier and less expensive. Despite decades of research and advances, there is no widely accepted theory to explain the properties of high-temperature superconductivity. Understanding the mechanisms could speed development and exploitation for widespread application. EU-funded scientists on the project 'Iron-based superconductivity: Fermi surface and superconducting gap anisotropy' (FESUME) used iron-based high-temperature superconductors discovered in 2008 to test certain hypotheses. Most scientists believe that the unique topology of a mathematically defined boundary related to thermal, electrical, magnetic and optical properties of semiconductors (the Fermi surface topology) plays an important role. A common theory states that the Fermi surface becomes gapped to avoid instabilities, driving the system into the superconducting state. FESUME scientists set out to explore the details of the Fermi surface and its propensity for instabilities in an effort to shed light on the mechanisms of high-temperature superconductivity. Investigators exploited three versatile, powerful and established techniques to measure the Fermi surface and the superconducting gap structure (de Haas van Alphen, London penetration depth and specific heat measurements) in 111 iron pnictide (layered iron–arsenic) superconductors. Results not only pointed to important conditions for superconductivity but also ruled out others that seem not to play a role. Outcomes provided the first thermodynamic evidence for the existence of a quantum phenomenon related to superconductivity phase transitions (quantum critical point) that affects the majority of the Fermi surface. It does so by increasing the quasiparticle mass. Overall, results emphasise the role of mass renormalisation in increasing the superconducting critical temperature in iron-based superconductors. This phenomenon is accompanied by quantum critical instabilities in some systems. FESUME made groundbreaking progress in addressing the long-unanswered yet critical question of the mechanisms of high-temperature superconductivity. Better understanding will provide a fresh viewpoint on the search for new materials for future superconducting applications, speeding innovation and development.
Keywords
Superconductivity, high temperature superconductor, iron-based, Fermi surface, gap structure, quantum critical instabilities
