Quantum critical point and BCS-BEC crossover in iron-based high-Tc superconductors
松田 祐司 教授
The discovery of iron-pnictide high-Tc superconductivity has been one of the most exciting recent developments in condensed matter physics. The most important aspect of the iron pnictides may be that they open a new landscape in which to study mechanisms of unconventional pairing that lead to high-Tc superconductivity. Here we discuss two interesting aspects of iron-based superconductors.
1)Quantum critical point (QCP): A QCP is a special class of second-order phase transition that takes place at absolute zero temperature, typically in a material in which the phase transition temperature has been driven to zero by nonthermal parameters, such as doping. We show the evidence for a QCP hidden beneath the superconducting dome, which we believe to be crucially important for understanding the anomalous normal-state properties and the high-Tc superconductivity .
2) BCS-BEC crossover: Amongst the iron-based superconductors, iron-selenide FeSe (Tc=9 K) with very simple crystal structure is particularly intriguing. The superconducting transition temperature increases up to ~40 K by applying pressure and up to ~60 K in monolayer films. What is remarkable is that all the Fermi surface have small effective Fermi energies comparable to the superconducting gap energies, which implies that the system is situated deep in the crossover regime between the weak-coupling BCS and strong-coupling BEC limit . We address the issue of preformed pairs and highly spin polarized superconducting phase.
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