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 superconductorsremains 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 YBa ₂ Cu ₃ O _6.56 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 √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.