Fig. 2: Terahertz spectra of superconducting Nb.

a Normalized transmittance spectra of Nb at various temperatures from 2 K to 8.5 K in the absence of an external magnetic field. The transmittance T_s in the superconducting state is normalized to T_n in the normal state, taken at 11 K. b Normalized transmittance spectra of Nb under in-plane magnetic fields up to 7 T at 1.5 K. The transmittance T_s in the superconducting state is normalized to T_n in the normal state, taken at 7 T. c Real part of the optical conductivity of Nb under the in-plane magnetic field from 0 T to 3.6 T at 1.5 K. d Imaginary part of the optical conductivity as in c. e Real part of the optical conductivity of Nb for several magnetic fields (0–3.5 T) at 1.5 K. The pair-breaking parameter Γ normalized to the pair-broken order parameter Δ(Γ) (see text) corresponding to the magnetic field H is indicated for each curve. The open circles mark experimental data, and the solid lines are fitted to the pair-breaking theory of SBW⁹. The gapped superconducting state (\(0\le \Gamma /\Delta (\Gamma ) \, < \, 1\)) transforms into the gapless superconducting state (\(\Gamma /\Delta (\Gamma )\ge 1\)) and then to the normal state (\(\Gamma /\Delta (\Gamma )=27386\to \infty\)). f Magnetic field dependence of the pair-breaking parameter Γ(H) determined from the experimental optical conductivity in the gapped regime (white) and the gapless regime (yellow). The green area is in a normal state. The error bars represent the uncertainty of the SBW fit with 95% confidence intervals.