Band-Resolved Superconducting Gaps via the Thermal Hall Effect in CeRu2

summary

Video file (mp4)

The gist

The gist: Thermal Hall transport resolves ambiguity in multiband superconductors by assigning superconducting gaps to specific Fermi surface sheets through a low-field sign reversal in thermal Hall

In short

Standard probes cannot assign superconducting gaps to specific Fermi surface sheets because they average over all carriers. This study uses thermal Hall transport ($\kappa_{xy}$) in CeRu2 to resolve this ambiguity. By fixing gap scales using specific heat and mobility ratios, a low-field sign reversal in $\kappa_{xy}$ proves the larger gap is on the heavy Ce 4f electron sheets, while the smaller gap is on hole sheets.

Key concepts

Thermal Hall Conductivity ($\kappa_{xy}$)
This measures how heat flows perpendicular to an applied magnetic field. It is sensitive to entropy transport from thermally excited Bogoliubov quasiparticles (BQPs) without interference from phonon backgrounds, making it a tool to probe superconducting properties.
Multiband Superconductor
CeRu2 is a superconductor with multiple electronic bands (electron and hole sheets). Standard probes fail because they measure a weighted sum of all these gaps, obscuring which specific band has which gap magnitude.
Sign Reversal in $\kappa_{xy}$
A change in the sign of the transverse thermal Hall conductivity as the magnetic field is varied. In this study, a low-field reversal below 1 T indicates that the larger superconducting gap is associated with one specific carrier band (the electron sheets) and the smaller gap with another (the hole sheets).
Zero-Field Specific Heat
This thermodynamic measurement was used to fix two distinct temperature-equivalent gap scales ($\Delta_e/k_B \approx 13$ K and $\Delta_h/k_B \approx 8$ K). This provided the necessary initial constraints on the magnitude of the superconducting gaps before transport measurements were used.

Terminology used across episodes

This episode discusses

The paper

Band-Resolved Superconducting Gaps via the Thermal Hall Effect in CeRu2 · Read on arXiv

Darian Hall, Luke Pritchard Cairns, Nico Rowland, Yuanqi Lyu, Kohtaro Yamakawa, Josue Rodriguez, Kirin Chanteloup, Didier Perrodin, Joel E. Moore, §James G. Analytis

Department of Physics, University of California, Berkeley · Kavli Energy NanoScience Institute, Berkeley · Materials Sciences Division, Lawrence Berkeley National Laboratory · CIFAR Quantum Materials

Transcript

Introduction to the show: ident: Quantum Radio. Generated commentary on the latest quantum physics and condensed matter papers.

Kai: Today's paper: "Band-Resolved Superconducting Gaps via the Thermal Hall Effect in CeRu2".

Mira: The gist: Thermal Hall transport resolves ambiguity in multiband superconductors by assigning superconducting gaps to specific Fermi surface sheets through a low-field sign reversal in thermal Hall conductivity.

Kai: First, who's behind it and why it matters.

Paper summary: Kai: So to recap, this paper is showing that bulk probes can't assign gaps because they average over everything; this work uses thermal Hall transport kappa xy constrained by specific heat and normal-state magnetotransport measurements to resolve that ambiguity in CeRu2.

Mira: The thesis is that fixing the gap magnitudes using specific heat—finding e/k B about thirteen K and h/k B about eight K—and band mobilities completely determines the normal-state electronic degrees of freedom <ref:2610.22361#pg1,8$ K—and band mobilities completely determines the normal-state electronic degrees>.

Lev: They then use a low-field sign reversal in kappa xy to place the larger gap on the heavy Ce 4f electron sheets and the smaller one on the hole sheets, which is what makes it useful for understanding these materials <ref:2610.22361#pg1,the larger gap on the heavy Ce 4f electron sheets and the>.

Kai: Why does this matter for us right now? Because kappa xy remains finite in the mixed state and selectively tracks entropy transport of thermally excited Bogoliubov quasiparticles without a competing phonon background.

Mira: Developing a robust kappa xy methodology is essential for disentangling multiple superconducting gaps and assigning them to specific Fermi surface sheets, which is what this paper demonstrates is possible.

Lev: For error correction researchers like me, this means we need to know if the quasiparticle excitations we create are dominated by one gap or the other because that dictates how much noise we expect in our system.

Kai: It’s about getting a handle on these different bands, which is crucial when you start thinking about materials for superconducting qubits or anything involving correlated electrons.

Conclusion: Kai: So looking at "Band-Resolved Superconducting Gaps via the Thermal Hall Effect in CeRu2", this paper shows that thermal hall transport, combined with specific heat and magnetotransport, is a powerful way to assign superconducting gaps to individual Fermi surface sheets.

Mira: In simple terms, it means you can take a bulk measurement and use it to figure out exactly which part of the material—the electron sheet or the hole sheet—is responsible for which energy gap.

Lev: It’s about getting specific information about the quasiparticle dynamics, not just a single averaged response across all carriers.

Kai: The implication is that we have a general blueprint here for assigning carrier bands to superconducting gaps and untangling multiband quasiparticle dynamics in various heavy-fermion superconductors.

Mira: They confirm that the larger gap resides on the electron band while BQP excitations across the smaller hole gap govern that low-field sign reversal at low temperatures.

Lev: It sets a precedent for how we can use these transport measurements to isolate signed quasiparticle currents, which is exactly what we need when designing systems where these different channels interact.

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