Studies of underdoped cuprate compounds such as YBa 2 Cu 3 O 6+ x through probes such as ARPES have indicated that these phases show characteristics of non-Fermi liquids, and in particular, the absence of well-defined Landau quasiparticles. However, quantum oscillations have been observed in these materials at low temperatures, if their superconductivity is suppressed by a sufficiently high magnetic field, which is evidence for the presence of well-defined quasiparticles with fermionic statistics. These experimental results thus disagree with those from ARPES and other probes.