Planes, Chains, and Orbits: Quantum Oscillations and High Magnetic Field Heat Capacity in Underdoped YBCO

TL;DR

This study measures the heat capacity of underdoped YBCO under high magnetic fields, revealing quantum oscillations consistent with Fermi Liquid behavior while maintaining a d-wave superconducting gap, challenging existing theories.

Contribution

It provides thermodynamic evidence that both Fermi Liquid-like quantum oscillations and a persistent d-wave gap coexist in high magnetic fields in underdoped cuprates.

Findings

Quantum oscillations observed in heat capacity indicate Fermi Liquid behavior.

The quasiparticle density of states follows a H dependence.

The d-wave superconducting gap persists across the entire magnetic field range.

Abstract

The underlying physics of the magnetic-field-induced resistive state in high temperature cuprate superconductors remains a mystery. One interpretation is that the application of magnetic field destroys the d-wave superconducting gap to uncover a Fermi surface that behaves like a conventional (i.e.Fermi Liquid) metal (1). Another view is that an applied magnetic field destroys long range superconducting phase coherence, but the superconducting gap amplitude survives (2, 3). By measuring the specific heat of ultra-clean YBa2Cu3O6.56 (YBCO 6.56), we obtain a measure of the quasi-particle density of states from the superconducting state well into the magnetic-field-induced resistive state. We have found that 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 quasi-particle density of states follows a \sqrt{H} behavior that persists right through the zero-resistance transition, evidencing the fully developed d-wave superconducting gap over the entire magnetic field range measured. The coexistence of these two phenomena pose a rigorous thermodynamic constraint on theories of high-magnetic-field resistive state in the cuprates.