Condensed Matter Seminar

Phase Diagram and Vortex Pinning of Iron-Arsenide Superconductors

Dr. Ulrich Welp
Materials Science Division
Argonne National Laboratory

Thursday, October 29, 2009 - 4:00 P.M., NSH 184

 

The discovery of superconductivity at temperatures up to 55 K and the emergence of unconventional superconducting order-parameters in layered iron-arsenide compounds have stimulated widespread research into the properties of these new materials.  We present specific heat and magnetization measurements on single crystals of the nearly optimally doped pnictide superconductors NdFeAsO_1-xF_x and Ba_1-xK_xFe₂As₂. We determine the anisotropic phase diagrams from the specific heat measurements using an entropy conserving construction, which does not require the choice of a resistivity criterion to define the transition temperature.  Both materials are characterized by a low superconducting anisotropy near T_c of G ~ 4 for NdFeAsO_1-xF_x and 2.6 for Ba_1-xK_xFe₂As₂, which is promising for potential applications. The upper critical field slopes are extraordinarily high, m₀¶H^c_c2/¶T = -6.5 T/K and m₀¶H^ab_c2/¶T= -17.4 T/K for Ba_1-xKxFe₂As₂, which points to the emergence of paramagnetic limiting effects at low temperatures.  A thermodynamic analysis reveals that this material is extreme type-II with k_c ~ 100 and k_ab ~ 260.

Measurements of the magnetization hysteresis reveal strong vortex pinning and critical current densities that for Ba_1-xK_xFe₂As₂ at low temperatures reach 1 MA/cm².  At temperatures close to T_c the critical current is characterized by a pronounced peak effect.  Upon irradiation with energetic protons and heavy ions the critical currents can be increased by up to a factor of ten. The suppression of T_c and changes of the phase diagram caused by scattering due to the irradiation-induced defects will be discussed in terms of the s_± - symmetry of the superconducting order-parameter.