The ground state of the pseudogap in cuprate supercondustors

We present studies of the electronic structure of La_2-xBa _xCuO₄, a system where the superconductivity is strongly suppressed as static spin and charge orders or "stripes" develop near the doping level of x = 1/8. 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 = 1/8. Thus, the nonsuperconducting, striped state at x = 1/8 is consistent with a phase-incoherent d-wave superconductor whose Cooper pairs form spin-charge-ordered structures instead of becoming superconducting.