Magnetic imaging of interlayer Josephson vortices

We have magnetically imaged interlayer Josephson vortices emerging parallel to the planes of single crystals of the organic superconductor κ-(BEDT-TTF)2 Cu(NCS)2, and the single layer cuprate high-Tc superconductors Tl2Ba2CuO6+δ (Tl-2201) and (Hg,Cu)Ba2CuO4+δ) (Hg-1201), using a scanning Superconducting Quantum Interference Device (SQUID) microscope. These images provide a direct measurement of the interlayer penetration depth, which is approximately 63μm for κ-(BEDT-TTF)2 Cu(NCS)2, 18μm for Tl-2201 and 8μm for Hg-1201. The lengths for the cuprates are about a factor of 10 larger than originally predicted by the interlayer tunneling model for the mechanism of superconductivity in layered compounds, indicating that this mechanism alone cannot account for the high critical temperatures in these materials.