Probing the superconducting properties of the Si-doped Nb-oxynitride superconductor $({\mathrm{Nb}}_{0.87}$${\mathrm{Si}}_{0.09}{\square }_{0.04})$$({\mathrm{N}}_{0.87}$${\mathrm{O}}_{0.13})$

The superconducting state of the 17 K Si-doped Nb-oxynitride superconductor, namely, $({\mathrm{Nb}}_{0.87}$${\mathrm{Si}}_{0.09}{\square }_{0.04})$$({\mathrm{N}}_{0.87}$${\mathrm{O}}_{0.13})$, has been investigated using muon spin rotation and relaxation. Our results show that there is no evidence of time-reversal symmetry breaking and that the temperature dependence of the magnetic penetration depth is consistent with an isotropic singlet “$s$-wave” ground state. The magnetic penetration depth has been determined to be 218(5) nm and the superconducting gap was found to be 3.4(1) meV, giving a BCS ratio of 4.6. The superelectron density and effective mass were found to be $8.7\times {10}^{27}$ ${\mathrm{m}}^{-3}$ and $15m_e$, respectively. Also, using the Uemura plot, it is suggested that the mechanisms for superconductivity in this material may not be entirely conventional and could be similar to the cuprates, organics, and heavy fermion systems.

Figure 1

The zero-field $\mu$SR spectra for $({\mathrm{Nb}}_{0.87}$${\mathrm{Si}}_{0.09}{\square }_{0.04})$$({\mathrm{N}}_{0.87}$${\mathrm{O}}_{0.13})$. The red symbols are the data collected at 1.5 K and the blue symbols are the data collected at 20 K. The line is a fit to the data using the model described in the text.

Figure 2

Typical muon asymmetry spectra in $({\mathrm{Nb}}_{0.87}$${\mathrm{Si}}_{0.09}{\square }_{0.04})$$({\mathrm{N}}_{0.87}$${\mathrm{O}}_{0.13})$ taken in a transverse field of 40 mT at 2.0 (upper), 10.0 (middle), and 20.0 K (lower). The line is a fit to the data (see text). For clarity, only one of the two virtual detectors has been shown.

Figure 3

The temperature dependence of ${\sigma }_1$ in a field of 40 mT.

Figure 4

The temperature dependence of the ${\sigma }_{\text{sc}}\left(T\right)$. The line is a fit to the data using an isotropic $s$-wave model (see text for details).

Figure 5

The “Uemura plot,” which shows a classification scheme for conventional and “unconventional” superconductors. The green star shows the position of $({\mathrm{Nb}}_{0.87}$${\mathrm{Si}}_{0.09}{\square }_{0.04})$$({\mathrm{N}}_{0.87}$${\mathrm{O}}_{0.13})$ within this scheme and suggests that the mechanisms for superconductivity may not be entirely conventional.