Heat capacity through the magnetic-field-induced resistive transition in an underdoped high-temperature superconductor

The underlying physics of the magnetic-field induced resistive state in lightly doped high-temperature cuprate superconductors remains a mystery. One interpretation is that the application of magnetic field destroys the d-wave superconducting gap, uncovering a Fermi surface that behaves as a Fermi liquid. Another view is that an applied magnetic field destroys long-range superconducting phase coherence, but the superconducting gap amplitude survives. By measuring the specific heat of YBa2Cu3O6.56 we determine the quasiparticle density of states from the superconducting state well into the magnetic-field induced resistive state. At very high magnetic fields the specific heat exhibits both the conventional temperature dependence and quantum oscillations expected for a Fermi liquid. On the other hand, the magnetic-field dependence of the quasiparticle density of states follows root H behaviour that persists smoothly through the zero-resistance transition, giving evidence of a developed d-wave superconducting gap over the entire magnetic field range measured.