Imaging the two gaps of the high-temperature superconductor Bi2Sr2CuO6+x

The nature and behaviour of electronic states in high-temperature superconductors are the centre of much debate. The pseudogap state, observed above the superconducting transition temperature, T-c, is seen by some as a precursor to the superconducting state. Others view it as a competing phase. Recently, this discussion has focused on the number of energy gaps in the system. Some experiments indicate a single energy gap, implying that the pseudogap is a precursor state. Others indicate two, suggesting that it is a competing or coexisting phase. Here, we use temperature-dependent scanning tunnelling spectroscopy of (Bi1-yPby)(2)Sr2CuO6+x to clarify the situation. We find a previously unobserved narrow and homogeneous gap that vanishes near T-c, superimposed on the typically observed inhomogeneous and broad gap, which is only weakly temperature dependent. These results not only support the two-gap picture, but also explain previously troubling differences between scanning tunnelling microscopy and other experimental measurements.