NMR and order parameter symmetry of unconventional superconductors: Sr₂RuO₄ to UTe₂

A consistent experimental interpretation of order parameter symmetry in unconventional superconductors can be elusive. A case in point is Sr₂RuO₄, long held to exhibit a triplet pairing state that breaks time reversal symmetry. Significant progress on that problem resulted from the incorporation of variable stress into a range of experimental techniques such as NMR. The question arises, can similar systematics occur in systems like UTe₂ or others? For example, Knight shift measurements on UTe₂ have been interpreted as evidence for a B₃u ground state—although the inferred relative decreases in spin susceptiblity are extraordinarily small for all directions of the applied field. We show that UTe₂, more so than layered materials like Sr₂RuO₄ with much greater conductivity anisotropy, presents significant challenges for accurate single-crystal NMR in low temperature measurements. Indeed, results on small-grain powders in the normal state contrast significantly (and quantifiably) from similarly conducted experiments on single crystals and allow for superconducting state measurements for fields aligned along all three principal axes to considerably lower temperatures than previously reported. In addition, the outcome of these experiments charts a path for examining the high-field phases of UTe₂.