Observation of the J-sheet of the Fermi surface of YbRh₂Si₂

We report a previously unobserved quantum oscillation frequency in YbRh₂Si₂. This is the first quantum oscillation that can definitely be assigned to the second of the two major sheets of the Fermi surface predicted by band structure calculations. These calculations have been done for two scenarios: the "large" Fermi surface, which includes the Yb 4f-hole in the Fermi volume, and the "small" Fermi surface, which does not. Previously observed frequencies were interpreted in terms of a "large" Fermi surface that has been so strongly spin-split that it closely resembles the "small" Fermi surface. The new frequency can also be incorporated into this picture, however there are some indications that the situation is more complicated than was assumed, suggesting a need for more advanced band structure calculations.