Effect of Aerogel Anisotropy in Superfluid $^3$He-A

Two theories have been advanced to describe the effects of anisotropic impurity introduced by stretched silica aerogel on the orientation of the orbital angular momentum $\hat{l}$ in superfluid $^3$He-A. These theories disagree on whether the anisotropy will orient $\hat{l}$ perpendicular\footnote{G. E. Volovik, J. Low Temp. Phys. \textbf{150}, 453 (2008).} or parallel\footnote{J. A. Sauls, arXiv:1307.7656.} to the strain axis. In order to examine this question we have produced and characterized a homogeneous aerogel sample with uniaxial anisotropy introduced during growth, corresponding to stretching of the aerogel. These samples have been shown to stabilize two new chiral states;\footnote{J. Pollanen et al. Nature Physics, 8, 317-320 (2012).} the higher temperature state being the subject of the present study. Using pulsed NMR we have performed experiments on $^3$He-A imbibed in this sample in two orientations: strain parallel and perpendicular to the applied magnetic field. From the NMR frequency shifts as a function of tip angle and temperature, we find that the angular momentum $\hat{l}$ is oriented along the strain axis, providing evidence for the theory advanced by Sauls.