Detector counting rate nonlinearity, though a known problem, is commonly ignored in the analysis of angle resolved photoemission spectroscopy where modern multichannel electron detection schemes using analog intensity scales are used. We focus on a n
early ubiquitous inverse saturation nonlinearity that makes the spectra falsely sharp and beautiful. These artificially enhanced spectra limit accurate quantitative analysis of the data, leading to mistaken spectral weights, Fermi energies, and peak widths. We present a method to rapidly detect and correct for this nonlinearity. This algorithm could be applicable for a wide range of nonlinear systems, beyond photoemission spectroscopy.
A Fermi arc is a disconnected segment of a Fermi surface observed in the pseudogap phase of cuprate superconductors. This simple description belies the fundamental inconsistency in the physics of Fermi arcs, specifically that such segments violate th
e topological integrity of the band. 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 success. Here we show the Fermi arc, while composed of real spectral weight, lacks the quasiparticles to be a true Fermi surface. 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 processes. While real, this weight does not form a true Fermi surface, because the quasiparticles, though significantly broadened, remain at the gap edge. This non-quasiparticle weight may account for much of the unexplained behavior of the pseudogap phase of the cuprates.
Ultrahigh resolution angle-resolved photoemission spectroscopy with low-energy photons is used to study the detailed momentum dependence of the well-known nodal kink dispersion anomaly of Bi2Sr2CaCu2O8+{delta}. We find that the kinks location transit
ions smoothly from a maximum binding energy of about 65 meV at the node of the d-wave superconducting gap to 55 meV roughly one-third of the way to the antinode. Meanwhile, the self-energy spectrum corresponding to the kink dramatically sharpens and intensifies beyond a critical point in momentum space. We discuss the possible bosonic spectrum in energy and momentum space that can couple to the k-space dispersion of the electronic kinks.
Conventional superconductivity is robust against the addition of impurities unless the impurities are magnetic in which case superconductivity is quickly suppressed. Here we present a study of the cuprate superconductor Bi$_2$Sr$_2$Ca$_1$Cu$_2$O$_{8+
delta}$ that is intentionally doped with the magnetic impurity, Fe. Through the use of our Tomographic Density of States (TDoS) technique, we find that while the superconducting gap magnitude is essentially unaffected by the inclusion of iron, the onset of superconductivity, T$_{C}$, and the pair-breaking rate are strongly dependent and correlated. These findings suggest that, in the cuprates, the pair-breaking rate is critical to the determination of T$_{C}$ and that magnetic impurities do not disrupt the strength of pairing but rather the lifetime of the pairs.
Using low-photon energy angle-resolved photoemission (ARPES), we study the low-energy dispersion along the nodal (pi, pi) direction in Bi(2)Sr(2)CaCu(2)O(8+delta) (Bi2212) as a function of temperature. Less than 10 meV below the Fermi energy, the hig
h-resolution data reveals a novel kink-like feature in the real part of the electron self-energy that is distinct from the larger well-known kink roughly 70 meV below E_F. This new kink is strongest below the superconducting critical temperature and weakens substantially as the temperature is raised. A corollary of this finding is that the Fermi velocity, as measured over this energy range, varies rapidly with temperature - increasing by almost 30% from 70 to 110 K.
