The Ground State of the Pseudogap in Cuprates*

In conventional superconductors, the appearance of an energy gap in the electronic spectrum indicates pairing of electrons into Cooper pairs and a simultaneous transition into a macroscopic superconducting state. In contrast, in the underdoped high temperature superconductors, an energy gap is already present in the normal state. An understanding of this normal state gap or `pseudogap' has proven elusive, because its ground state electronic structure was unknown. Here, we present studies of electronic structure in La$_{2-x}$Ba$_{x}$CuO$_{4}$, a unique system where the superconductivity is strongly suppressed and static spin and charge orders or `stripes' develop near a 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 non-superconducting, `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.