第1回 物理工学科教室談話会 （講師：Dr. Kees van der Beek）
|講演者：||Dr. Kees van der Beek（Ecole Polytechnique, France）|
|題 目：||Vortex pinning as a probe of superconductivity in iron pnictide compounds|
The recently discovered iron-based superconductors are the second family of high temperature superconductors after the cuprates. Contrary to cuprates though, superconductivity in the iron-based compounds involves Cooper pairing on different electron and hole bands. Proposed mechanisms of superconductivity include the anti-ferromagnetic-fluctuation mediated s± scenario, in which the superconducting order parameter changes sign between the hole-like and electron-like Fermi-surface sheets, and orbital-fluctuation mediated s++ superconductivity, in which no order parameter sign change occurs. In the first case, superconductivity is expected to be exquisitely sensitive to disorder, while in the second case it should be robust.
An unexpected tool to take a look at both the fundamental superconducting properties and the effects of disorder is the pinning of vortex lines in the superconducting mixed state by material defects, and the resulting critical current. Vortex pinning in the iron-based superconductors is due to extrinsic material inhomogeneity on the one hand, and to atomic-sized point defects on the other hand. I will discuss how vortex pinning by material inhomogeneity on the nm scale can be characterized, and how it can be exploited to examine the temperature- and field evolution of the material anisotropy. The latter returns information on the relative weight of electron-like and hole-like Fermi surface sheets in defining superconductivity.
On the other hand, atomic-sized point defects pin vortex lines because they scatter the quasi-particles inside, and around the vortex core. This quasi-particle scattering is the same mechanism that is liable to be prejudicial to s± superconductivity. I shall discuss how scattering rates can be extracted from critical current density measurements.