In the cuprate pseudogap phase, an energy gap of unknown mechanism opens, and both an electronic nematic phase (NE) and a density-wave (DW) phase appear. Perplexingly, the DW, which should generate an energy gap, appears without any new gap opening; and the NE, which should be incapable of opening an energy gap, emerges coincident with the pseudogap opening. Recently, however, it was demonstrated theoretically that a disordered unidirectional DW can generate a vestigial nematic (VN) phase. If the cuprate pseudogap phase were in such a VN state, the energy gap of the NE and DW should be identical to each other and to the pseudogap. We report discovery of such a phenomenology throughout the phase diagram of underdoped Bi₂Sr₂CaCu₂O₈.

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In Cu-based HTS materials, the undoped phase is a robust Mott insulator while, in Fe-based HTS materials, the undoped phase is generally not an insulator at all. Thus, proximity to a Mott insulator appears neither indispensable nor universal to HTS. However, theory predicts that Fe-based materials could still be governed by strong electronic correlations proximate to a Mott insulator if an orbital selective Mott phase (OSMP) exists. A key signature of OSMP would be orbital selective Cooper pairing wherein electrons of a specific orbital character predominantly form the Cooper pairs.
We used Bogoliubov quasiparticle interference imaging to determine the Fermi surface geometry and the corresponding superconducting energy gaps of the famous iron-based superconductor FeSe. We show that both gaps are extremely anisotropic but nodeless, and they exhibit gap maxima oriented orthogonally in momentum space. By introducing a novel technique, we demonstrate that these gaps have opposite sign with respect to each other. This complex gap configuration reveals the existence of orbital-selective Cooper pairing that, in FeSe, is based preferentially on electrons from the dyz orbitals of the iron atoms.

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SI-STM study visualizes the atomic-scale effects of irradiating Fe(Se,Te) superconductor with high-energy heavy ions. Simultaneous imaging of defects, superconducting order parameter and vortex configuration reveals how columnar and point defects pin quantum vortices allowing high critical current density.

By introducing the Dirac-mass 'gapmap' technique, i.e. simultaneously visualizing the mass gap Δ(r) and the ferromagnetic dopant atoms in the atomic-scale, we discover intense nanoscale disorder in the Dirac-mass and demonstrate that this is directly related to fluctuations in the magnetic-dopant atom density n(r). (click the image for expanded version with scale bars)

Quasiparticle interference imaging of heavy fermions reveals the momentum-space structure of the f-electron magnetism in CeCoIn₅. Using this novel information, we then demonstrate directly and quantitatively that the Cooper pairing in this heavy fermion superconductor is mediated by the f-electron magnetic interactions.