Fundamental origin of the large impact of strain on superconducting Nb₃Sn

Abstract

Superconductors can carry very high current densities without resistive loss. This makes them highly suitable for the construction of compact high field magnets of several tens of tesla. However, with increasing current density and magnetic field come high Lorentz loads and high strain levels, and it is empirically known that superconducting properties are affected by strain. The superconducting properties of the most commonly used high field material, Nb 3 Sn, are particularly badly affected by strain. Here we demonstrate that strain causes significant sub-lattice distortion in the A15 lattice structure of Nb 3 Sn, and show how this leads to the relatively large reduction of its superconducting properties. The changes are found to be primarily due to changes in the electron density of states, with a lesser contribution due to changes in the phonon spectrum. The amount of sub-lattice distortion further depends on crystal orientation. These findings suggest that it is possible to mitigate the reduction of the current carrying capacity by strain through crystal alignment and sub-lattice stabilization. This would enable superconducting magnets to reach a significantly higher magnetic field, and ease their construction by simplifying react-and-wind coil fabrication.

Effects of the initial axial strain state on the response to transverse stress of high-performance RRP Nb3Sn wires

“…An equally important aspect of Nb 3 Sn superconductors is, however, their reduced mechanical strain tolerance. The critical current density of Nb 3 Sn wires strongly depends on the mechanical strain applied to the superconducting phase [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27], this last being, in addition, a brittle intermetallic compound with a noteworthy propensity to fracture. For the most relevant cases in magnet design, the action of mechanical loads translates consequently into a reduction of the conductor performance, i.e.…”

Multi-metallic conduction cooled superconducting radio-frequency cavity with high thermal stability

“…One possibility is the contamination of the Nb 3 Sn film by the plating solution. Another possibility is related to strain of the Nb 3 Sn film given by the differential ther- mal expansion coefficient between the Cu layer and the Nb layer, since it is well known that the superconducting properties of Nb 3 Sn are very sensitive to strain [23]. A finite element mechanical analysis with ANSYS showed that stresses as high as ∼ 275 MPa at the irises and ∼ 185 MPa elsewhere on the cell may be applied to the Nb 3 Sn-coated Nb, due to the larger thermal contraction of Cu.…”

Ultrathin epitaxial NbN superconducting films with high upper critical field grown at low temperature

“…A systematic increase of T C with epitaxial tensile strain in MgB 2 films has also been reported, resulted from the softening of the bondstretching phonon mode [42]. On the other hand, it has been reported that strain can cause significant sub-lattice distortion in the lattice structure of Nb 3 Sn, and lead to reduction of its superconducting properties, primarily due to changes in the density of states at Fermi surface with a lesser contribution due to change in the phonon spectrum [43]. For superconducting NbN, the situation can be more complex since the lattice parameter can increase continuously with an increase in N concentration due to an expansion and distortion in NbN lattice caused by the interstitial incorporation of N atoms [33].…”