Interpreting quantum oscillation experiments on underdoped YBa2Cu3O6+x

On the basis of negative transport coefficients, it has been argued that the quantum oscillations observed in underdoped YBa2Cu3O6+x in high magnetic fields must be due to antinodal electron pockets. We point out a counterexample in which electronlike transport in a hole-doped cuprate is associated with Fermi-arc states. We also present evidence that the antinodal gap in YBa2Cu3O6.67 is robust to modest applied magnetic fields. We suggest that these observations should be taken into account when interpreting the results of the quantum oscillation experiments.