Coherence peak; a universal feature in high-Tc superconductors
The BCS (Bardeen-Cooper-Schriefer) theory tells us that two electrons are condensed to make a pair (Cooper pair) in the superconducting state, producing a superconducting coherence peak (SCP) just below the Fermi level. However, it has been a long-standing puzzle why single-layered high-Tc superconductors (HTSCs) such as La2-xSrxCuO4 (LSCO) do not exhibit the SCP in contract to multi-layered HTSCs such as Bi2Sr2CaCa2O8. This inconsistency has provoked a lot of debates in modeling the high-Tc mechanism, for example, an essential difference between single- and multi-layered HTSCs. This PRL paper reports the first direct observation of a SCP in LSCO, together with the precise momentum dependence by angle-resolved photoemission spectroscopy (ARPES). They found that the SCP is confined in a surprisingly narrow momentum region between the node and the antinode in sharp contrast to multi-layered HTSCs. The experimental results suggest that a pseudogap dominant around the antinode affects this anomalous momentum dependence of SCP in LSCO. Thus, the present high-resolution ARPES study has established the universality of a coherence peak in HTSCs and at the same time provided a key for understanding the difference in the superconducting transition temperature between single- and multi-layered HTSCs.
