Quantum oscillation and topology change of the uncondensed Landau Fermi surface in superconducting CeCoIn5

TL;DR

This study detects uncondensed electrons in the superconducting state of CeCoIn5 using quantum oscillations, revealing a Fermi surface reconstruction linked to spin-density wave order, thus clarifying a longstanding debate.

Contribution

First direct observation of uncondensed electrons in a superconductor through quantum oscillations, demonstrating Fermi surface changes associated with SDW order in CeCoIn5.

Findings

Quantum oscillations from uncondensed electrons detected via thermal conductivity.

Phase shift in oscillations linked to Fermi surface reconstruction.

Uncondensed electrons confirmed in the superconducting state of CeCoIn5.

Abstract

Metals typically have multiple Fermi surface sheets, and when they enter the superconducting state, some electrons on these sheets may remain uncondensed, or their superconducting pairs can be rapidly destroyed by a magnetic field. Detecting uncondensed electrons within the superconducting state provides key information about the underlying electronic structure; however, this task remains a significant experimental challenge. Here we demonstrate quantum oscillations from the uncondensed electrons in the heavy-fermion superconductor CeCoIn5, observed through thermal conductivity measurements with a magnetic field rotating within the tetragonal a-b plane. We detect a fine structure in thermal conductivity, characterized by multiple small resonances (oscillations) in a rotating magnetic field. Remarkably, the phase of these resonances shifted by as much as {\pi} for a field above 9.7 T where spin-density wave (SDW) order emerges and coexists with superconductivity. This phase shift is naturally explained by a change in the Berry phase of the uncondensed Fermi surface, driven by the Fermi surface reconstruction associated with the onset of SDW order. Our work unambiguously shows the existence of uncondensed electrons in the superconducting state of CeCoIn5, thus resolving a longstanding debate on this issue.