Origin of the magnetization enhancement and spontaneous vortices in 4Hb‑TaS2

Determine the microscopic mechanism responsible for the pronounced increase in total magnetization in the van der Waals superconductor 4Hb‑TaS2 upon entering the superconducting state—manifested by spontaneous vortices after an out-of-plane training field is applied only above the superconducting transition temperature and removed before cooling—despite scanning SQUID magnetometry indicating normal-state magnetization at least three orders of magnitude too small to account for the observed vortex density.

Background

The paper studies a new magnetoelectric mechanism in spin-orbit coupled superconductors, where charged magnetic inclusions and Rashba spin-orbit coupling generate anomalous supercurrents that can enhance magnetization and nucleate vortices without applied orbital magnetic fields.

This mechanism qualitatively reproduces features of the ‘magnetic memory’ observed in 4Hb‑TaS2, where spontaneous vortices appear even when the training field is applied only above Tc and removed before cooling. However, the authors estimate that in their model the superconductivity-induced magnetization enhancement is at most comparable to the normal-state Pauli contribution, which appears insufficient to explain the experimental vortex densities.

Consequently, despite several proposed scenarios in the literature (e.g., proximity to spin liquids, unconventional interlayer pairing, or heavy Fermi liquid behavior), there is no consensus or experimentally validated explanation for the observed magnetization amplification and spontaneous vortices in 4Hb‑TaS2.