Discovery of orbital-selective Cooper pairing in FeSe

Abstract

FeSe is the focus of intense research interest because of its unusual non-magnetic nematic state and because it forms the basis for achieving the highest critical temperatures of any iron-based superconductor. However, its Cooper pairing mechanism has not been determined because an accurate knowledge of the momentum-space structure of superconducting energy gaps ∆ i ( k) on the different electron-bands E i ( k) does not exist. Here we use Bogoliubov quasiparticle interference (BQPI) imaging to determine the coherent Fermi surface geometry of the α-and ε-bands surrounding the Γ = (0, 0) and X = (π/a Fe , 0) points of FeSe, and to measure their superconducting energy gaps ∆ α ( k) and ∆ ε ( k).We show directly that both gaps are extremely anisotropic but nodeless, and are aligned along orthogonal crystal axes. Moreover, by implementing a novel technique we demonstrate the sign change between ∆ α ( k) and ∆ ε ( k). This complex configuration of ∆ α ( k) and ∆ ε ( k), which was unanticipated within pairing theories for FeSe, reveals a unique form of superconductivity based on orbital selective Cooper pairing of electrons from the d yz orbitals of iron atoms. This new paradigm of orbital selectivity may be pivotal to understanding the microscopic interplay of quantum paramagnetism, nematicity and high temperature superconductivity. BIOGRAPHICAL SKETCHPeter Oliver Sprau was born on June 13th 1986 in the small town of Kirchheimbolanden, Germany, where he completed both his primary and secondary education. Long before he was a physicist, Peter was an active member of the track and field team in his school and a local club, even going on to compete in the dash and relay event on the state and federal youth level. Upon finishing school, he fulfilled his civic duty and carried out his alternative civilian service in the hospital in Kirchheimbolanden. While Peter's academic interests were diverse, including not just science but also Latin and history, his natural curiosity about the world finally urged him to pursue a higher education in physics. Mistakes. Make glorious, amazing mistakes. Make mistakes nobody's ever made before. Don't freeze, don't stop, don't worry that it isn't good enough, or it isn't perfect, whatever it is: art, or love, or work or family or life.Whatever it is you're scared of doing, Do it. Make your mistakes, next year and forever." I also want to acknowledge in no specific order the following people for useful discussions throughout my PhD:

s+is superconductivity with incipient bands: Doping dependence and STM signatures

“…The QPI intensity integrated over the wave vectors corresponding to scattering between two bands has been shown to have a dependence on energy very different for s +− and s ++ scenarios, leading to a strong enhancement of the integrated response for the s +− but not the s ++ case. This method has been recently successfully applied to confirm the signchanging nature of the order parameter in FeSe 73 , where the superconducting gaps are also extremely anisotropic. More recently, the method has been also applied to (Li 1−x Fe x )OHFe 1−y Zn y Se with only electron Fermi surface pockets 74 .…”

Gap structure of FeSe determined by angle-resolved specific heat measurements in applied rotating magnetic field

“…Even if the multi-gap structure is established [17,18], gap nodes or minima must still be located in the corresponding bands. Unfortunately, there have been few reports on this issue except for the recent Bogoliubov quasiparticle interference (BQPI) experiments, which found gap minima in both the α and ε bands [19]. However, there is still no bulk evidence of the locations of nodes or gap minima, and also no information about the gap from the δ band.…”

“…2(g) by simply assuming an equal ratio, which shows four-fold symmetry similar to the experimental observation of ∆C(φ)/T . On the other hand, ∆ α manifests a waveform of cos 2φ [19], whose oscillation differs from the four-fold symmetry, when the effects of TBs are considered (Supplementary S4 [24]). Hence, the ∆ 1 with nodes (gap minima) is assigned to the electron-type ε band.…”

Orbital superconductivity, defects, and pinned nematic fluctuations in the doped iron chalcogenide FeSe0.45Te0.55

“…This pinning effectively increases the spatial extent of a defect's scattering potential along the x-or y-axis. Alternatively, the observed broken C 4 -symmetry of the defect states could reflect the presence of orbital selective Mottness [11][12][13][14] .…”

“…There exists strong evidence for a s ± -wave symmetry of the superconducting order parameter 1,2 , arising from the pairing between electronlike and hole-like Fermi surface pockets that is mediated by magnetic fluctuations [3][4][5][6] . However, the existence of nematicity in the iron chalcogenide superconductor FeSe 7 , and the observation of high temperature superconductivity in iron chalcogenides without hole pockets [8][9][10] , has recently cast doubt on such a simple picture, giving rise to the proposal of orbital selective superconducting pairing [11][12][13][14] . Moreover, recent scanning tunneling spectroscopy experiments 15 have reported that even outside the nematic phase in doped FeSe 0.…”