Singular angular magnetoresistance and sharp resonant features in a high-mobility metal with open orbits

We report high-resolution angular magnetoresistance (AMR) experiments performed on crystals of with high mobility  s at 2 K) and extremely low residual resistivity . The Fermi surface, comprised of intersecting cylinders, supports open orbits. The resistivity in a magnetic field  T displays a singular pattern of behavior. With and initially , tilting in the longitudinal plane leads to a steep decrease in by a factor of 40. However, if is tilted in the transverse plane, increases steeply by a factor of 8. Using the Shockley-Chambers tube integral approach, we show that, in , the singular behavior results from the rapid conversion of closed to open orbits, resulting in opposite signs for AMR in orthogonal planes. The floor values of in both AMR scans are identified with specific sets of open and closed orbits. Also, the “completion angle” detected in the AMR is shown to be an intrinsic geometric feature that provides a new way to measure the Fermi radius . However, additional sharp resonant features that appear at very small tilt angles in the longitudinal AMR scans are not explained by the tube integral approach.