Strong electronic correlations, emerging from the parent Mott insulator phase, are key to copper-based high-temperature superconductivity. By contrast, the parent phase of an iron-based high-temperature superconductor is never a correlated insulator. However, this distinction may be deceptive because Fe has five actived d orbitals while Cu has only one. In theory, such orbital multiplicity can generate a Hund's metal state, in which alignment of the Fe spins suppresses inter-orbital fluctuations, producing orbitally selective strong correlations. The spectral weights Z of quasiparticles associated with different Fe orbitals m should then be radically different. Here we use quasiparticle scattering interference resolved by orbital content to explore these predictions in FeSe. Signatures of strong, orbitally selective differences of quasiparticle Z appear on all detectable bands over a wide energy range. Further, the quasiparticle interference amplitudes reveal that [Formula: see text], consistent with earlier orbital-selective Cooper pairing studies. Thus, orbital-selective strong correlations dominate the parent state of iron-based high-temperature superconductivity in FeSe.

Pair density wave (PDW) states are defined by a spatially modulating superconductive order parameter. To search for such states in transition-metal dichalcogenides (TMDs), we used high-speed atomic-resolution scanned Josephson-tunneling microscopy. We detected a PDW state whose electron-pair density and energy gap modulate spatially at the wave vectors of the preexisting charge density wave (CDW) state. The PDW couples linearly to both the s-wave superconductor and the CDW and exhibits commensurate domains with discommensuration phase slips at the boundaries, conforming those of the lattice-locked commensurate CDW. Nevertheless, we found a global δΦ≅±2π/3 phase difference between the PDW and CDW states, possibly owing to the Cooper-pair wave function orbital content. Our findings presage pervasive PDW physics in the many other TMDs that sustain both CDW and superconducting states.

The elementary CuO 2 plane sustaining cuprate high-temperature superconductivity occurs typically at the base of a periodic array of edge-sharing CuO 5 pyramids. Virtual transitions of electrons between adjacent planar Cu and O atoms, occurring at a rate t/ℏ and across the charge-transfer energy gap E , generate “superexchange” spin–spin interactions of energy J ≈ 4 t 4 / E 3 in an antiferromagnetic correlated-insulator state. However, hole doping this CuO 2 plane converts this into a very-high-temperature superconducting state whose electron pairing is exceptional. A leading proposal for the mechanism of this intense electron pairing is that, while hole doping destroys magnetic order, it preserves pair-forming superexchange interactions governed by the charge-transfer energy scale E . To explore this hypothesis directly at atomic scale, we combine single-electron and electron-pair (Josephson) scanning tunneling microscopy to visualize the interplay of E and the electron-pair density n P in Bi 2 Sr 2 CaCu 2 O 8+x . The responses of both E and n P to alterations in the distance δ between planar Cu and apical O atoms are then determined. These data reveal the empirical crux of strongly correlated superconductivity in CuO 2 , the response of the electron-pair condensate to varying the charge-transfer energy. Concurrence of predictions from strong-correlation theory for hole-doped charge-transfer insulators with these observations indicates that charge-transfer superexchange is the electron-pairing mechanism of superconductive Bi 2 Sr 2 CaCu 2 O 8+x .

The most essential characteristic of any fluid is the velocity field !(#) and this is particularly true for macroscopic quantum fluids 1 . Although rapid advances 2 -7 have occurred in quantum fluid !(#) imaging 8 , the velocity field of a charged superfluid -a superconductor -has never been visualized. Here we use superconductive-tip scanning tunneling microscopy 9,10,11 to image the electron-pair density % ! (#) and velocity ! ! (#) fields of the flowing electron-pair fluid in superconducting NbSe2. Imaging ! ! (#) surrounding a quantized vortex 12 , 13 finds speeds reaching 10,000 km/hr . Together with independent imaging of % ! (#) via Josephson tunneling, we visualize the supercurrent density / ! (#) ≡ % ! (#)! ! (#), which peaks above 3 × 10 " A/cm 2 . The spatial patterns in electronic fluid flow and magneto-hydrodynamics reveal hexagonal structures co-aligned to the crystal lattice and quasiparticle bound states 14 , as long anticipated [15][16][17][18] . These novel techniques pave the way for electronic fluid flow visualization in many other quantum fluids.