Electronic structure of the cuprate superconducting and pseudogap phases from spectroscopic imaging STM

A. R. Schmidt, K. Fujita, E. A. Kim, M. J. Lawler, H. Eisaki, S. Uchida, D. H. Lee, J. C. Davis

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We survey the use of spectroscopic imaging scanning tunneling microscopy (SI-STM) to probe the electronic structure of underdoped cuprates. Two distinct classes of electronic states are observed in both the d-wave superconducting (dSC) and the pseudogap (PG) phases. The first class consists of the dispersive Bogoliubov quasiparticle excitations of a homogeneous d-wave superconductor, existing below a lower energy scale E = Δ0. We find that the Bogoliubov quasiparticle interference (QPI) signatures of delocalized Cooper pairing are restricted to a k-space arc, which terminates near the lines connecting k = ±(π/a0, 0) to k = ±(0, π/a 0). This arc shrinks continuously with decreasing hole density such that Luttinger's theorem could be satisfied if it represents the front side of a hole-pocket that is bounded behind by the lines between k = ±(π/a0, 0) and k = ±(0, π/a0). In both phases, the only broken symmetries detected for the \E\ < Δ0 states are those of a d-wave superconductor. The second class of states occurs proximate to the PG energy scale E = Δ1. Here the non-dispersive electronic structure breaks the expected 90°-rotational symmetry of electronic structure within each unit cell, at least down to 180°-rotational symmetry. This electronic symmetry breaking was first detected as an electronic inequivalence at the two oxygen sites within each unit cell by using a measure of nematic (C2) symmetry. Incommensurate non-dispersive conductance modulations, locally breaking both rotational and translational symmetries, coexist with this intra-unit-cell electronic symmetry breaking at E = Δ1. Their characteristic wavevector Q is determined by the k-space points where Bogoliubov QPI terminates and therefore changes continuously with doping. The distinct broken electronic symmetry states (intraunit-cell and finite Q) coexisting at E ∼ Δ1 are found to be indistinguishable in the dSC and PG phases. The next challenge for SI-STM studies is to determine the relationship of the E ∼ Δ1 broken symmetry electronic states with the PG phase, and with the E < Δ0 states associated with Cooper pairing.

Original languageEnglish
Article number065014
JournalNew Journal of Physics
StatePublished - Jun 2011


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