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Spectroscopic Imaging STM Studies of Electronic Structure in the Superconducting and Pseudogap Phases of Cuprate High-Tc Superconductors

One of the key motivations for the development of atomically resolved spectroscopic imaging STM (SI-STM) has been to probe the electronic structure of cuprate high temperature superconductors. In both the d-wave superconducting (dSC) and the pseudogap (PG) phases of underdoped cuprates, two distinct classes of electronic states are observed using SI-STM. The first class consists of the dispersive Bogoliubov quasiparticles of a homogeneous d-wave superconductor. These are detected below a lower energy scale |E|=Δ0 and only upon a momentum space (k-space) arc which terminates near the lines connecting k=\pm(π/a0,0) to k=\pm(0, π/a0). In both the dSC and PG phases, the only broken symmetries detected in the |E|\leq Δ0 states are those of a d-wave superconductor. The second class of states occurs at energies near the pseudogap energy scale |E| Δ1 which is associated conventionally with the 'antinodal' states near k=\pm(π/a0,0) and k=\pm(0, π/a0). We find that these states break the 90o-rotational (C4) symmetry of electronic structure within CuO2 unit cells, at least down to 180o rotational (C2) symmetry (nematic) but in a spatially disordered fashion. This intra-unit-cell C4 symmetry breaking coexists at |E| Δ1 with incommensurate conductance modulations locally breaking both rotational and translational symmetries (smectic). The properties of these two classes of |E| Δ1 states are indistinguishable in the dSC and PG phases. To explain this segregation of k-space into the two regimes distinguished by the symmetries of their electronic states and their energy scales |E| Δ1 and |E|\leqΔ0, and to understand how this impacts the electronic phase diagram and the mechanism of high-Tc superconductivity, represents one of a key challenges for cuprate studies.