Laser Based Angle-Resolved Photoemission, the Sudden Approximation, and Quasiparticle-Like Spectral Peaks in ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$

A new low photon energy regime of angle-resolved photoemission spectroscopy is accessed with lasers and used to study the high $T_c$ superconductor ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$. The low energy increases bulk sensitivity, reduces background, and improves resolution. With this we observe spectral peaks which are sharp on the scale of their binding energy—the clearest evidence yet for quasiparticles in the normal state. Crucial aspects of the data such as the dispersion, superconducting gaps, and the bosonic coupling kink are found to be robust to a possible breakdown of the sudden approximation.

Figure 1

Comparison of ARPES along the node in near-optimally doped Bi2212 using (a) 6 eV laser photons at $T=25\mathrm{K}$, (b) 28 eV photons at $T=26\mathrm{K}$, and (c) 52 eV photons at $T=16\mathrm{K}$. The images are scaled identically in $E$ and $\mathbf{k}$, and all three contain MDC derived dispersion for the laser data (red circles). Additionally, the dispersions for the data of panels (b) and (c) are shown as blue squares and black triangles, respectively.

Figure 2

(a) The MDC at the Fermi energy (red circles) is shown along with a Lorentzian fit (blue line). (b) Lorentzian MDC half-widths from the 25 K laser ARPES data of Fig. 1a. (c) Comparison of nodal (solid red line) and off-nodal (dotted blue line) laser ARPES in the superconducting state. The location of the cuts in the first Brillouin zone are shown in the inset. (d) Comparison of the off-nodal cut from (c) in the normal (solid red line) and superconducting (dotted blue line) states.

Figure 3

(a) EDCs (triangles) and Lorentzian fits (blue lines) at different temperatures (offset for clarity) for three emission angles each. (b) Summary of EDC fitting results showing full-width $2\mathrm{Im}\Sigma$ versus peak position. The shaded region indicates where peak full widths are sharper than their energy, which should be considered quasiparticle-like. (c) Raw EDCs from the laser (red circles) and 52 eV synchrotron source (black triangles) measured at the same $\mathbf{k}$ value.