Fermi-liquid behavior in an underdoped high-Tc superconductor

We use magnetic quantum oscillations in the underdoped high T-c superconductor YBa2Cu3O6+x (x approximate to 0.56) measured over a broad range of temperatures 100 mK < T < 18 K to extract the form of the distribution function describing the low-lying quasiparticle excitations in high magnetic fields. Despite the proximity of YBa2Cu3O6.56 to a Mott insulating state, various broken symmetry ground states and/or states with different quasiparticle statistics, we find that our experimental results can be understood in terms of quasiparticle excitations obeying Fermi-Dirac statistics as in the Landau-Fermi liquid theory.