Thermomagnetic anomalies in quantum magnon transport caused by tunable junction geometries in cold atomic systems

TL;DR

This paper investigates how tunable junction geometries in cold atomic systems influence magnon-driven spin and heat transport, revealing geometry-dependent criticality and breakdown of classical laws like Wiedemann-Franz.

Contribution

It demonstrates that junction geometry critically affects magnonic transport properties and uncovers quantum-enhanced conductances that break classical transport laws in cold atomic systems.

Findings

Magnonic criticality enhances spin and heat conductances.

Transport properties depend strongly on junction geometry.

Breakdown of the magnonic Wiedemann-Franz law occurs under certain conditions.

Abstract

We study magnon-driven spin and heat transport in a magnetic linear junction (MLJ) formed by two ferromagnets in optical lattices linked via linearly aligned bonds. Using the Schwinger-Keldysh formalism, we uncover that under weak effective Zeeman fields, where Bose-Einstein statistics of magnons dominate, magnonic criticality dramatically enhances spin and thermal conductances. These singular transport properties depend on the junction geometry, and the transport properties qualitatively differ between the linear junction in this study and the point contact in our previous work. The quantum-enhanced conductances result in the breakdown of the magnonic Wiedemann-Franz (WF) law. In the classical regime at temperatures much lower than magnon energy gaps, we find that a magnonic Lorenz number becomes independent of temperature yet dependent on junction geometry, sharply contrasting with the universal WF law for Fermi liquids. We also find that the interface geometry of MLJ decouples spin and heat relaxations between ferromagnets with decay times insensitive to temperature and effective Zeeman fields. These dynamics reveal junction-geometry-sensitive magnon transport distinct from Fermi liquids, paving the way for new avenues in thermomagnetic research leveraging the tunability of cold atomic systems.