The origin and non-quasiparticle nature of Fermi arcs in Bi2Sr2CaCu2O8+delta

A Fermi arc1,2 is a disconnected segment of a Fermi surface observed in the pseudogap phase3,4 of cuprate superconductors. This simple description belies the fundamental inconsistency in the physics of Fermi arcs, specifically that such segments violate the topological integrity of the band5. Efforts to resolve this contradiction of experiment and theory have focused on connecting the ends of the Fermi arc back on itself to form a pocket, with limited and controversial success6–9. Here we show the Fermi arc, although composed of real spectral weight, lacks the quasiparticles to be a true Fermi surface5. To reach this conclusion we developed a new photoemission-based technique that directly probes the interplay of pair-forming and pair-breaking processes with unprecedented precision. We find the spectral weight composing the Fermi arc is shifted from the gap edge to the Fermi energy by pair-breaking processes10. Although real, this weight does not form a true Fermi surface, because the quasiparticles, although significantly broadened, remain at the gap edge. This non-quasiparticle weight may account for much of the unexplained behaviour of the pseudogap phase of the cuprates. In a solid the behaviour of the electrons is most fully described in terms of the electron Green’s function, the poles of which map the energy versus momentum dependence of the electronic quasiparticles (the dressed electronic states)5. The locus of poles at the Fermi energy, EF, defines the Fermi surface of the material, from which almost all of the electronic properties of a material emanate. For a single continuous band, the Fermi surface should form a continuous loop. Consequently, the broken segments of Fermi surface known as Fermi arcs apparently require a major rethinking of someof the basic tenets of condensedmatter physics. One approach to resolve this problem is completely discarding the notion of electron quasiparticles, and with it almost all of the understanding of solids built up from generations of condensed matter physicists. Much support for this line of reasoning came from angle-resolved photoemission spectroscopy (ARPES), which is unique in its ability to directly probe the electronic excitations as a function of energy and momentum, that is the quasiparticles. Earlier ARPES studies found that the ARPES peaks were either anomalously broad or vanishingly weak11–13 especially in the underdoped ‘pseudogap’ regime of the cuprates—aspects that were widely taken as evidence for the lack of electron-quasiparticles. The recent introduction of laser and low-energy14 ARPES made tremendous advances in the peak sharpness and spectral weight at EF, handicapping this line of argument. We, for example, now see sharp nodal quasiparticle-like peaks for all doping levels of Bi2Sr2CaCu2O8+δ (Bi2212) studied (down to moderately underdoped x = 0.10 samples), in contrast to recent studies