Weak-coupling Bardeen-Cooper-Schrieffer superconductivity in the electron-doped cuprate superconductors

We use in-plane tunneling spectroscopy to study the temperature dependence of the local superconducting gap $\Delta \left(T\right)$ in electron-doped copper oxides with various $T_c$’s and Ce-doping concentrations. We show that the temperature dependence of $\Delta \left(T\right)$ follows the expectation of the Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity, where $\Delta \left(0\right)∕k_B{T}_c\approx 1.72\pm 0.15$ and $\Delta \left(0\right)$ is the average superconducting gap across the Fermi surface, for all the doping levels investigated. These results suggest that the electron-doped superconducting copper oxides are weak-coupling BCS superconductors.

Figure 1

Point-contact spectra measured at $2\mathrm{K}$ and well above $T_c$ for various doping levels. The conductance drop has been subtracted from the spectra above $T_c$ as explained in the text. Those two vertical gray lines are added as references to identify the positions of the coherence peaks.

Figure 2

Comparison of experimental data and theoretical calculations with the isotropic BTK model (black lines) and the anisotropic one (gray lines) for two samples: (a) plcco-un21 and (b) ncco-op25. (c) The schematic diagram of the anisotropic gap (or nonmonotonic $d$-wave gap), in which the black fragments indicate the antinodal region and the gray fragments indicate the nodal region. (d) $\Delta \left(\text{averaged}\text{gap}\right)\sim T$ relations determined with two different models; solid lines are guides for the eyes.

Figure 3

(Color online) Temperature dependence of the normalized spectra (balck dots) and theoretical calculations with the isotropic BTK model (solid lines) for (a) plcco-un21, (b) ncco-op25, and (c) plcco-ov17; all curves except the lowest one are shifted upward for clarity. (d)–(f) Fitting parameter of $Z$ corresponding to the data shown in (a)–(c), respectively. (g)–(i) Fitting parameter of $\Gamma$ corresponding to the data shown in (a)–(c), respectively. (j) The temperature dependence of the superconducting gap in a reduced scale for various doping levels.

Figure 4

Relationship between the zero-temperature averaged gap and transition temperature. The gray symbols indicate previous reports from ARPES (Ref. 13), Raman (Ref. 12), point contact (Ref. 16, 17), and grain boundary (Ref. 14), respectively. The open circles denote the results of fitting to the nonmonotonic $d$-wave model. The crosses indicate the data of Nb measured by the same experimental apparatus used in this work. The data of some conventional superconductors are also presented as open stars for comparison (Ref. 29).