Imaging the two gaps of the high-temperature superconductor Bi ₂ Sr ₂ CuO _6+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, Tc, 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 (Bi(1y)Pb(y))"2Sr"2CuO(6+x) to clarify the situation. We find a previously unobserved narrow and homogeneous gap that vanishes near Tc, 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 experimentalmeasurements.