Pressure Unlocks a Quantum Spin Liquid State in Kagome Material Y-Kapellasite, Ruling Out Disorder as Its Origin
A European team used muon spin spectroscopy under hydrostatic pressure to show that squeezing Y-kapellasite suppresses its magnetic order without any structural change, producing a clean quantum spin liquid state driven purely by geometric frustration.
Overview
A team of physicists from a dozen European institutions has demonstrated that a crystalline material called Y-kapellasite can be coaxed into a quantum spin liquid state by applying pressure — and that the transition happens without any structural rearrangement of its atoms. The result, published in Physical Review Letters on April 21, 2026, addresses a question that has dogged the field for decades: whether the exotic spin liquid behavior seen in candidate materials is a genuine quantum phenomenon or merely an artifact of chemical impurities and disorder.
The answer, in Y-kapellasite at least, is that it is genuine. As first author Dipranjan Chatterjee noted, “pressure serves as a control parameter that distinguishes disorder-driven states from genuine quantum fluctuations,” according to Phys.org.
What We Know
Y-kapellasite — formula Y₃Cu₉(OH)₁₉Cl₈ — belongs to a broader family of kagome materials. In a kagome lattice, magnetic atoms are arranged in a repeating pattern of corner-sharing triangles that resembles a Japanese woven bamboo design, according to Phys.org. The geometry forces neighboring magnetic spins into competition with one another — a state physicists call geometric frustration — because no single arrangement can simultaneously satisfy all pairwise interactions. Under the right conditions, that competition prevents the spins from settling into any ordered pattern even at temperatures approaching absolute zero, producing a quantum spin liquid: a state in which spins remain perpetually fluctuating and quantum-entangled.
At ambient pressure, Y-kapellasite does not reach that ideal. Its kagome lattice is slightly distorted by displaced yttrium atoms, which creates an imbalance among the magnetic exchange pathways and allows a conventional long-range magnetic order to emerge below 2.2 K, according to the primary paper. The ordered state follows the theoretically predicted in-plane (1/3 1/3) pattern. The key insight is that this distortion is small — the material sits close to the spin liquid regime — and compression might nudge it across the boundary.
To test that hypothesis, the team, led by senior author Pascal Puphal of the Max-Planck Institute for Solid State Research, applied hydrostatic pressure using a piston cylinder pressure cell. Senior author Puphal explained the technique: “In a µSR experiment, spin-polarized muons are implanted into the sample and act as extremely sensitive local magnetic probes,” as reported by Phys.org.
As pressure increased, two things happened in parallel. First, high-pressure X-ray diffraction showed that the crystal compressed isotropically, with no structural phase transitions detected up to 10 GPa and a bulk modulus of 80 GPa, according to the primary paper. The superstructure reflections that confirm the underlying kagome arrangement persisted at all measured pressures. Second, optical phonon measurements revealed that the kagome lattice distortion was gradually relieved — the exchange pathways became more symmetric — again without any abrupt structural change.
The magnetic response, tracked by muon spin spectroscopy, was the decisive measurement. At ambient pressure the muons detected a static internal magnetic field from the ordered spins. Under approximately 2.3 GPa, that field vanished and was replaced by dynamic spin fluctuations that persisted down to the lowest accessible temperatures. The muon relaxation rate — which measures whether spins are frozen or fluctuating — approached zero at that pressure, indicating the complete absence of magnetic freezing, according to the primary paper. The authors conclude: “the ZF and LF measurements confirm the absence of magnetic freezing and indicate that only dynamic fluctuations are present as pressure increases, clearly suggesting the presence of a SL ground state.”
Critically, because neither structural transitions nor chemical disorder introduced by pressure were present, the paper argues that the spin liquid-like state arises solely from enhanced magnetic frustration. The paper states: “Suppressing long-range magnetic order via frustration tuning with pressure, in an ultraclean material, establishes a major step towards realizing a well-controlled spin liquid ground state.”
Why This Matters
The race to confirm a quantum spin liquid in a real material has been running for decades. Most leading candidates — herbertsmithite, α-RuCl₃, and several organic compounds — suffer from a fundamental ambiguity: their spin liquid-like signals could in principle be produced by quenched disorder, local defects, or other non-quantum mechanisms. As the primary paper notes, “most experimental spin liquid candidates inevitably deviate from perfectly frustrated models due to chemical and structural disorder, blurring the nature and origin of their ground states.”
The Y-kapellasite experiment sidesteps that objection in an unusual way. Rather than searching for a material that happens to be near the spin liquid phase and hoping disorder is not responsible, the team used a material that is known to order magnetically — ruling out disorder by construction — and then applied a clean external control (pressure) to move it toward the spin liquid regime. According to Phys.org, the result represents “the first fingerprint for the realization of a quantum spin liquid without strong disorder.”
Separate work in late 2025 corroborated the broader field’s momentum. A Stanford-led team reported in Nature Physics that two distinct kagome materials — herbertsmithite and Zn-barlowite — both exhibited spinon excitations, fractionalized quasiparticles that only emerge in strongly entangled quantum systems. As Professor Young Lee of Stanford told Interesting Engineering: “our measurements revealed that the fundamental excitations of the kagome spins appear in the form of ‘spinons’.” The convergence of evidence from multiple independent groups and materials is strengthening the case that quantum spin liquids are not merely theoretical constructs.
What We Don’t Know
The Y-kapellasite result is described as spin liquid-like behavior, and the paper’s own language is measured: it speaks of “clearly suggesting” a spin liquid ground state rather than definitively confirming one. Fully conclusive identification of a quantum spin liquid requires additional probes — in particular, measurements of fractionalized excitations like spinons — that the current muon spectroscopy and X-ray experiments do not directly access.
The transition under pressure also occurs at cryogenic temperatures (below 2.2 K at ambient pressure, with the ordered phase suppressed progressively at higher pressures). Operating a material at such temperatures and at gigapascal pressures simultaneously is an experimental tour de force, but it also means the spin liquid state in Y-kapellasite cannot yet be considered a practical platform for any device application.
Finally, the team has not yet mapped the complete pressure-temperature phase diagram, and it remains unclear whether the system reaches a fully quantum-critical spin liquid at some higher pressure or gradually crossovers into a different disordered phase.
Analysis
What makes the Y-kapellasite result methodologically distinctive is its use of pressure as a clean control parameter. Chemical substitution — the more common route to tuning frustrated magnets — invariably introduces some lattice disorder. Pressure does not. The authors’ demonstration that the kagome distortion is gradually relieved by compression, without any structural transition and without chemical doping, creates a uniquely controlled experimental trajectory from an ordered magnet to a putative spin liquid.
In the broader context of condensed matter physics, this puts Y-kapellasite in a rare category: a system where the ordered and spin liquid phases can both be studied in the same single-crystal sample, simply by changing the applied pressure. That tunability is precisely what theorists have requested for decades as a means to test competing models of frustrated magnetism and to characterize the quantum phase transition that separates conventional magnetic order from a fully entangled spin liquid ground state.