Magnetic memory and spontaneous vortices in a van der Waals superconductor

TL;DR

This paper investigates the magnetic properties of a van der Waals superconductor, 4Hb-TaS2, revealing a hidden magnetic phase that coexists with superconductivity and exhibits spontaneous vortices without detectable magnetic signals.

Contribution

It uncovers a novel hidden magnetic phase in 4Hb-TaS2 that breaks time reversal symmetry without ferromagnetic order, advancing understanding of magnetism in correlated superconducting systems.

Findings

Discovery of a spontaneous vortex phase with trainable vortex density.

Identification of a magnetic phase breaking time reversal symmetry.

Absence of detectable magnetic signals despite magnetic ordering.

Abstract

Doped Mott insulators exhibit some of the most intriguing quantum phases of matter, including quantum spin-liquids, unconventional superconductors, and non-Fermi liquid metals. Such phases often arise when itinerant electrons are close to a Mott insulating state, and thus experience strong spatial correlations. Proximity between different layers of van der Waals heterostructures naturally realizes a platform for experimentally studying the relationship between localized, correlated electrons and itinerant electrons. Here, we explore this relationship by studying the magnetic landscape of 4Hb-TaS2, which realizes an alternate stack of a candidate spin liquid and a superconductor. We report on a spontaneous vortex phase whose vortex density can be trained in the normal state. We show that time reversal symmetry is broken above Tc, indicating the presence of a magnetic phase independent of the superconductor. Strikingly, this phase does not generate detectable magnetic signals. We use scanning superconducting quantum interference device (SQUID) microscopy to show that it is incompatible with ferromagnetic ordering. The discovery of this new form of hidden magnetism illustrates how combining superconductivity with a strongly correlated system can lead to new, unexpected physics.