Understanding Unconventional Superconductors: On The Origin Of Cooper Pairing In Sr2Ruo4 And The Broken Symmetries In The Pseudogap Regime Of The Cuprates

Abstract

Unconventional superconductors provide, today, some of the most fundamental challenges in condensed-matter physics, besides their unimaginable potential for applications. Here, we will discuss two of these materials. Firstly, we will focus on Sr2 RuO4 , one of the most promising candidates to exhibit chiral pwave superconductivity, analogously to a two-dimensional (2D) film of 3 He-A. Some of the outstanding challenges regarding this multi-band superconductor remain, including the knowledge of which band(s) are primarily responsible for the appearance of superconductivity. Solving this issue will help us understand the symmetry of the pairing and, eventually, the pairing mechanism. We report scanning tunneling microscopy (STM) studies confirming a nodal superconducting gap structure and indicating that the quasi-one-dimensional (1D) bands are most important for superconductivity in Sr2 RuO4 . We suggest an experimental avenue to confirm this observation and determine the pairing symmetry beyond doubt. We then turn to a discussion of the hole-doped cuprate high-temperature superconductors, with a focus on their mysterious pseudogap regime and its Q = 0 and Q 0 electronic broken symmetries. Development of novel spectroscopic imaging STM (SI-STM) techniques allowed the quantitative measurement of such broken symmetries (also seen by other probes) and their inter-relations. We report on these novel techniques in detail, in partic- ular Fourier phase determination of STM data, and study extensively the hole doping, p, dependence of the broken symmetries as well as the Fermi surface topology for Bi2 Sr2 CaCu2 O8+[delta] samples spanning the phase diagram between 0.06 [LESS-THAN OR EQUAL TO] p [LESS-THAN OR EQUAL TO] 0.23. We show that the electronic symmetry breaking tendencies weaken with increasing p and disappear close to pc = 0.19. Concomitantly, the coherent k-space topology undergoes an abrupt transition, from arcs to closed contours, at the same pc . These data reveal that the k-space topology transformation in the cuprates is linked intimately with the disappearance of the electronic symmetry breaking at a concealed critical point.