The Ground State of the Pseudogap in Cuprate Superconductors

Abstract

We present studies of the electronic structure of La _2–x Ba _x CuO ₄ , a system where the superconductivity is strongly suppressed as static spin and charge orders or “stripes” develop near the doping level of x = ⅛. Using angle-resolved photoemission and scanning tunneling microscopy, we detect an energy gap at the Fermi surface with magnitude consistent with d -wave symmetry and with linear density of states, vanishing only at four nodal points, even when superconductivity disappears at x = ⅛. Thus, the nonsuperconducting, striped state at x = ⅛ is consistent with a phase-incoherent d -wave superconductor whose Cooper pairs form spin-charge–ordered structures instead of becoming superconducting.