The challenge

Magnetism has been studied for thousands of years, from natural lodestones to modern technologies. Today, it remains central to physics, especially at the quantum level, where atomic spins interact in unexpected ways. In certain geometries, competing interactions lead to “frustration”, which prevents spins from settling into an ordered pattern, even at the lowest temperatures. This can give rise to a spin-liquid quantum state, hosting emergent excitations and long-range quantum entanglement. Identifying such states remains challenging, as disorder can obscure their signatures. Y-kapellasite offers a rare, clean platform where pressure can continuously tune magnetic interactions.

A key challenge in quantum magnetism research is to disentangle the roles of disorder and intrinsic geometric frustration, which can produce similar signatures of a quantum spin liquid. Geometric frustration suppresses magnetic order and, together with quantum fluctuations, can stabilise such a state. The central question is whether a clean kagome system [1], which provides an ideal platform to isolate the role of geometric frustration, can be tuned in a controlled manner. Pressure allows continuous tuning of the balance of magnetic interactions. High-brilliance X-ray diffraction under pressure follows subtle structural changes that directly modify these interactions.

 The experiment

High-pressure X-ray diffraction at beamline ID27 was combined with muon spin spectroscopy measurements at the Paul Scherrer Institute (PSI) to investigate how pressure modifies Y-kapellasite.

Under hydrostatic pressure, X-ray diffraction measurements reveal subtle structural changes that reduce lattice anisotropy without any structural transition, thereby continuously enhancing geometric frustration (Figure 1).
 

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Fig. 1: Schematic of a single kagome layer of Y-kapellasite inside a diamond anvil cell. Under pressure, the Y spacer atom is driven closer to the kagome plane, modifying the relative positions of the Cu1 (orange) and Cu2 (blue) sites and thereby tuning the magnetic interactions.

Complementary muon spin spectroscopy results from PSI show that static magnetic order is fully suppressed and replaced by a dynamic ground state (Figure 2). The absence of spin freezing is a key signature of a spin-liquid-like phase. Together, these results demonstrate that pressure tunes the underlying interactions and stabilises a fluctuating quantum state in a clean system, enabling the role of geometric frustration to be isolated without the influence of disorder, providing a clear example of “quantum disorder by design.

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Fig. 2: Zero-field asymmetries measurements at 0.28 K under increasing pressures: ambient pressure, 1.14 GPa, 1.8 GPa, and 2.3 GPa. The black dotted line indicates the contribution from the pressure cell.

The impact

These results show that spin-liquid behaviour can emerge from intrinsic geometric frustration alone, without the need for disorder. By using pressure as a non-thermal tuning parameter, the two effects can be effectively disentangled, addressing a long-standing challenge in quantum magnetism. Y-kapellasite is thus established as a model system to study frustration-driven quantum states in a controlled way. More broadly, this work demonstrates how pressure can be used to engineer and explore exotic quantum phases, advancing our ability to understand and control quantum spin liquids.