V NMR evidence for coexistence of cubic and tetragonal phases

 The Martensitic transformation (MT) in A15 binary-alloy superconductor V₃Si is a second-order, displacive structural transition from cubic to tetragonal symmetry, at temperature T_m a few K above the superconducting transition temperature T_c = 17 K. Though studied extensively, the MT has not yet been conclusively linked with a transition to superconductivity, and remains relevant due to renewed interest in soft phonon modes, whileV₃Si continues to be of interest, e.g. due to similarities with Fe-As superconductors.  Previous NMR studies on the MT in V₃Si have been on powder samples, and with little emphasis on temperature dependence during the transformation. Here we study a high-quality single crystal, where quadrupolar splitting and Knight shift of NMR spectra for ⁵¹V allowed us to distinguish between spectra from transverse chains of V as a function of temperature. This revealed evidence of the coexistence of untransformed cubic phase and transformed tetragonal phase over a few K below and above T_m, and that the Martensitic lengthening of one axis occurs in a plane perpendicular to the crystal growth axis, as twinned domains. The data below T_m suggest that superconductivity may only be fully realized upon complete transformation to the tetragonal phase. More details on the effects on relaxation time T₁, the electric field gradient, and the hyperfine field due to spin/orbital susceptibility of electrons are also discussed.