Figure 1

Schematic diagram of the formation process of a Bogoliubov-quasiparticle (BQP) band. In the normal state above $T_c$ (a), the electron band has an equal weight at any momentum, while in the superconducting state below $T_c$ (b), particle-hole mixing (mixing of electron and hole bands) takes place due to the pairing, leading to opening of a superconducting gap as well as a transfer of weight between the electron and hole bands.Reuse & Permissions

Figure 2

(a) ARPES spectra of Bi2223 in the normal state (140 K) measured along a yellow line in the Brillouin zone shown in the inset in (e). Spectrum at $k_F$ is indicated by a dark red line. (b) ARPES spectra taken under the same condition as in (a) but at the superconducting state (60 K). (c) Same as (b) in an expanded intensity scale above $E_F$. (d) ARPES spectra in (b) divided by the FD function at 60 K convoluted with a Gaussian reflecting the instrumental resolution. Spectrum at $k_F$ is indicated by a dark green line. Red lines are fitting curves for unoccupied states using a Lorentzian with energy-dependent broadening factor. Fittings are restricted to the spectra of which peak positions are located within $5k_{B}T$ from $E_F$. (e) Intensity plot of normalized ARPES spectra in (d) as a function of binding energy and wave vector. Momentum region is the same in (a)–(d).Reuse & Permissions

Figure 3

(a) Comparison of the ARPES intensity plot [same as Fig. 2e] with the calculated BQP band dispersions based on BCS theory (red and blue dashed lines). Calculation has been performed using the BCS formula $E_k=[{ϵ}_k^2+|\Delta (k){|}^2{]}^{1/2}$, where ${ϵ}_k$ (white solid line) has been determined by fitting the spectral peak positions at 140 K (white open circles) using parabolic function. The corresponding hole band ($-{ϵ}_k$) is also shown by a white dashed line. Peak positions of Lorentzian to fit the normalized ARPES spectra in the unoccupied states [Fig. 2d] are shown by white filled squares for comparison. (b) ARPES spectra (dots) measured at three representative points in the Brillouin zone shown in (a) (points A, B, and C), together with the fitting curves (solid lines) to obtain the coherence factors ($|u_k{|}^2$ and $|v_k{|}^2$). The inset shows expansion near $E_F$. (c) Coherence factors as a function of ${ϵ}_k$ determined from ARPES intensity (red and blue solid circles), compared with those derived from ARPES dispersion using the BCS formula (solid lines). The sum of two coherence factors ($|u_k{|}^2+|v_k{|}^2$) obtained by ARPES intensity is shown by gray solid circles.Reuse & Permissions