Description
We investigated superfluid density (penetration depth) as a
function of temperature and flux flow resistivity as a function
of magnetic field of various kinds of Fe based superconductor
systematically by microwave conductivity measurement
techniques[1-5]. Reflecting the multiply gapped nature of
these materials, large variety of the phenomena was
observed both in the temperature dependence of the
penetration depth and in the magnetic field dependence of
flux flow resistivity. We developed a model that describes the
superfluid density and the flux flow resistivity for a two gap
superconductor, which take the Fermi surface structure
explicitly into account[5]. With available data of the Fermi
surface measured by ARPES experiments, we succeeded in
explaining the observed behaviors of these two independent
quantities QUANTITATIVELY very well in terms of the two
band model. Depending on the magnitude of the obtained
anisotropy parameters, we confirmed the presence/the
absence of the nodes on each Fermi surface. Thus, we can
determine the superconducting gap structure investigating
these two quantities in detail. Therefore, our method can be
called as a novel method to discuss the structure of the
superconducting order parameter.
In Fe(Se,Te), the dissipation by the flux flow was found to be
exceptionally small, which turn out to be the result of the
backflow of supercurrent by the disorder specific to this
system[4].
Another interesting aspect is, in all materials investigated,
the quasiparticle scattering time in the vortex core is rather
short so that the mean free path of the quasiparticle in the
vortex core is limited by the core radius. Indeed, we already
obtained essentially the same features in many other
superconductors, which cannot be explained by any existing
theories, and may suggest the presence of a novel
mechanism of dissipation by quasiparticles in the vortex core.
[1] T. Okada et al., Phys. Rev. B86 (2012) 064516.
[2] H. Takahashi et al., Phys. Rev. B86 (2012) 144525.
[3] T. Okada et al., Physica C484 (2013) 27, ibid C494 (2013)
109, ibid C504 (2014) 24.
[4] T. Okada et al., Phys. Rev. B91 (2015) 054510.
[5] A. Maeda et al., Quantum Matt. 4 (2015) 308, and T.
Okada, in preparation.
