Chiral Superconductivity and Charge Density Waves Compete in Twisted Molybdenum Ditelluride.

The pursuit of superconductivity arising from highly unusual electron interactions continues to drive materials science, and a new study sheds light on how this might occur near exotic quantum states of matter. Taige Wang and Michael P. Zaletel, both from the University of California, Berkeley, alongside their colleagues, investigate a pathway to achieving chiral superconductivity – a state where electrons pair in a way that breaks symmetry and allows for unusual properties – by exploring the behaviour of electrons in materials exhibiting a ‘fractional Chern insulator’ state. Their work demonstrates that as this insulating state weakens, a chiral superconductor emerges alongside a competing charge-density wave, differing in energy by a remarkably small amount, and mirroring observations in twisted materials like molybdenum ditelluride. This finding is significant because it proposes a new mechanism for creating spin-polarized chiral superconductivity and re-entrant integer quantum Hall states, potentially paving the way for novel electronic devices and a deeper understanding of quantum materials.

Superconductivity holds immense promise for both scientific advancement and technological innovation. Researchers are now investigating whether superconductivity can arise in systems where electrons repel each other and are fully spin-polarized – a scenario that challenges conventional understanding. Two-dimensional materials exhibiting moiré patterns are proving to be ideal platforms for this exploration.

Recent investigations into carefully constructed layered materials, such as twisted MoTe₂ and rhombohedral graphene, reveal the emergence of superconductivity from a unique insulating state called a fractional Chern insulator. Slight alterations to these materials, like introducing impurities, can disrupt this insulating state and induce superconductivity, even without the usual mechanisms that drive this phenomenon. Theoretical modelling and large-scale computational simulations demonstrate that this transition isn’t a simple shift to a metallic state, but rather a branching into two distinct phases: a region of chiral superconductivity and a re-entrant integer quantum Hall (RIQH) phase.

The research suggests a unified microscopic explanation for both spin-polarized chiral superconductivity and the RIQH state, originating from the disruption of the parent insulating state. The superconducting state is identified as a Luther-Emery (LE) liquid, a unique phase where single electrons are gapped, but Cooper pairs remain gapless, allowing for long-range correlations. This is confirmed by examining the decay of electron and Cooper pair correlations and calculating the system’s central charge.

The superconductivity exhibits a chiral f-wave pairing symmetry, crucial for understanding the material’s properties and potential applications. The research also demonstrates the existence of magnetic Bloch states, fundamental to understanding the material’s behaviour. Within the superconducting region, the phase of the superconducting order parameter indicates chiral f + if pairing.

The system exhibits five co-propagating Majorana edge modes – exotic particles that are their own antiparticles – in a specific topological state. The superconducting region represents a single chiral phase, presenting a clear observable for phase-sensitive probes. The superconducting order parameter evolves smoothly from strong to weak coupling, transitioning from tightly bound bosonic pairs to weakly connected Cooper pairs.

However, superconductivity in these materials faces competition from other phases. Calculations reveal a delicate balance between superconductivity and charge ordering, with a superconducting dome separating insulating and metallic phases. A quasi-one-dimensional obstruction to superconductivity arises from the finite circumference of the simulated wires, but this effect diminishes in larger systems.

A more significant competitor is a commensurate √3 × √3 charge-density wave (CDW), which emerges under specific conditions and exhibits a quantized Hall response, mirroring observations in other materials. Despite these competing phases, the research establishes that the observed chiral superconductivity is remarkably resilient. It persists across realistic screening lengths, carrier densities, and lattice geometries, and is even enhanced by modest electron doping.

These findings suggest the underlying mechanism is linked to the melting of the insulating state rather than specific Fermi surface details, opening avenues for exploring new quantum phenomena and developing advanced electronic devices.