Type-1.5 Superconductivity

We demonstrate the existence of a novel superconducting state in high quality two-component MgB₂ single crystalline superconductors where a unique combination of both type-1 (λ₁/ξ₁<1/√2) and type-2 (λ₂/ξ₂>1/√2) superconductor conditions is realized for the two components of the order parameter. This condition leads to a vortex-vortex interaction attractive at long distances and repulsive at short distances, which stabilizes unconventional stripe- and gossamerlike vortex patterns that we have visualized in this type-1.5 superconductor using Bitter decoration and also reproduced in numerical simulations.