Phase diagrams and vortex pinning studies of multiband superconductors
WAI-KWONG KWOK, ULRICH WELP, CARLOS CHAPARRO, LEI FANG, ALEXEI
KOSHELEV, 1Materials Science Division, Argonne National Laboratory
We report specific heat measurements on a series of multi-band superconductors from MgB2
to the iron pnictide single crystals including BaFe2(As1−xPx)2 with phosphorous contents
ranging from optimal doping (x0.3, Tc = 29.5 K) to highly overdoped (x0.6, Tc = 11K),
SmFeAsO0.85F0.15, (Ba1-xKx}As2Fe2 and Ba(Fe1-x,Cox)2As2. For BaFe2(As1−xPx)2 we find a
sharp superconducting transition at Tc for all doping levels, a suppression of the ∆C-step at
Tc with increasing doping and enhanced magnetic field dependence at higher doping. The
phase diagrams determined from specific heat data show a decrease of dHc2/dT with
increasing doping and a nearly constant superconducting anisotropy of Γ2.5. For
SmFeAsO0.85F0.15 we find pronounced c-axis superconducting fluctuations inducing strong
broadening and suppression of the specific heat anomaly, an upper critical field slope of -3.5
T/K and anisotropy of  = 8. We compare our results with the proposed ‘universal’ scaling of
∆Cp/Tc and dHc2/dT due to quantum criticality and non Fermi liquid behavior and due to
strong pair-breaking and non-magnetic interband scattering, respectively. In addition, we
report on the vortex pinning behavior of iron pnictide crystals with defects induced by proton
and heavy ion irradiation.
This work was supported by DOE-BES under Contract No. DE-AC02- 06CH11357
1