Abstract
The superconducting state of the 17 K Si-doped Nb-oxynitride superconductor, namely, $({\mathrm{Nb}}_{0.87}$${\mathrm{Si}}_{0.09}{\ensuremath{\square}}_{0.04})$$({\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 $8.7\ifmmode\times\else\texttimes\fi{}{10}^{27}$ ${\mathrm{m}}^{\ensuremath{-}3}$ and $15{m}_{e}$, 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.
Highlights
The search for new superconductors has resulted in surprising new families of materials, such as the pnictides, high temperature superconductors, borocarbides, etc
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
Where A0 is the initial asymmetry, σ is the muon depolarization rate and is related to the field width at the muon site, λ is for any electronic contribution, and C is the background
Summary
The search for new superconductors has resulted in surprising new families of materials, such as the pnictides, high temperature superconductors, borocarbides, etc. Niobium is an elemental superconductor, which shows type II behavior [3], and doping niobium with nitrogen leads to some interesting properties [4,5,6,7,8,9,10], and, depending on the structure formed, the superconducting transition temperature can be enhanced up to 17.7 K for the rocksalt δ-NbN, or suppressed, for example, 4 K for α-NbNx. Recently, oxynitrides of Nb and cation-doped oxynitrides of Nb have been shown to exhibit enhanced values of TC. This relatively small TC reduction under an applied magnetic field suggests a robustness of its superconducting behavior in comparison to that in the parent niobium oxynitride Both Si K-edge x-ray absorption near edge structure (XANES) and 29Si magic angle spinning (MAS)-NMR indicated that the local structure of pinning centers around the silicon atoms close to cationic vacancies was similar to that of Si in amorphous SiO2 in the rocksalt structure of niobium oxynitride Nb(N0.87O0.13) [24]. 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
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