Observation of ordered vortices with Andreev bound states in Ba0.6K0.4Fe2As2

For a type-II superconductor in an applied magnetic field greater than the lower critical field H-c1, the magnetic flux will penetrate into the superconductor and form quantized vortices with Andreev bound states in the vortex cores. The characteristics of the bound states are related to the pairing symmetry and band structure of the superconductor. Recently, a new family of high temperature (high-T-c) superconductors, the iron pnictides, has been discovered. Surprisingly, in electron-doped Ba(Fe1-xCox)(2)As-2, no bound state was found in the vortices. Here, we use a low-temperature scanning tunnelling microscope to study the electronic structure of hole-doped Ba0.6K0.4Fe2As2. Two superconducting gaps (with gap values 2 Delta/k(B) T-c approximate to 2.2 and 5.1, where k(B) is Boltzmann's constant) were observed in the superconducting state. By applying magnetic fields, we observed ordered vortices with Andreev bound states in the vortex cores. The bound states and their spatial evolution can be qualitatively explained by our numerical calculations for multiband s-wave superconductivity.