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Research Highlights

Quantum phase transition near organic quantum spin liquid unveiled by chemical tuning

writerShigeki Fujiyama

Vol.35 (Aug) 2025 | Article no.26 2025

RIKEN, Hirosawa 2–1, Wako, 351–0198, Japan.

Quantum spin liquids (QSLs)—exotic magnetic states where electron spins remain disordered even at absolute zero—have long fascinated physicists for their rich entanglement and potential to host exotic quasiparticles [27,28,29]. Yet, the challenge has always been in stabilizing and probing such states in real materials. Molecular solids offer a clean platform with two-dimensional triangular lattices, which can be an ideal system to host QSLs. Two candidates for gapless QSLs, κ-(BEDT-TTF)2Cu2(CN)3 (BEDT-TTF = bisethylenedithiotetrathiafulvalene) and X [Pd(dmit)2]2 (X = EtMe3Sb, dmit = 1,3-dithiole-2-thione-4,5-dithiolate) have been well examined [30].

Geometrical frustration in triangular antiferromagnetic networks is accepted as a potential source to prevent classical magnetic ordering. Experimentally, however, the anisotropy of the transfer integral (t′/t), (t’s are often approximated as isosceles triangles) of molecular-based gapless QSLs is not exactly 1. Therefore, the mechanism of the QSL phenomenon is still an open issue. A systematic material control with precise tuning of t′/t from the antiferromagnetically ordered to the QSL states across the quantum phase transition (QPT) has long been awaited.

It has recently been shown that mixed salts of multiple cations can be systematically synthesized as X [Pd(dmit)2]2, allowing precise control of the antiferromagnetic order temperature (TN) as well as t′/t [31] as shown in Fig. 2a. It is important to note that the charge gaps for these mixed salts remain open. This is different from the mixed salts of κ-(BEDT-TTF)2Y (Y: mixed anion) where the mixture of Y’s easily causes an insulator–metal transition.

Fig. 2
figure 2

a The antiferromagnetic order temperatures (TN) of X [Pd(dmit)2]2 as a function of t′/t. The TN’s for X = (Me4As)1−y (Me4Sb)y, (Me4Sb)1−y (EtMe3Sb)y, and (Et2Me2As)1−y (Et2Me2Sb)y are shown as triangles, diamonds, and squares, respectively. The QSL samples are shown as open symbols. The transfer integrals, t and t′ are defined in the inset. b Magnitude of the isolated magnetic moment (nonlinear component of the magnetization to the field) as a function of t′/t. In the inset, we plot the susceptibility (linear component of the magnetization) at the lowest temperature in the QSL phase


The development of a system covering the antiferromagnetic to the QSL states has made it possible to experimentally explore the spin state near the QPT. Contrary to the common notion that a gapless QSL would be realized only at a specific point of physical parameters [32], no magnetic ordering is observed over a finite range of t′/t as shown in Fig. 2a. No sample with 2 K TN < 6 K was found. We cannot claim that the QPT from the antiferromagnetic to the QSL states is a second-order transition. This suggests that significant quantum fluctuations near the QPT may lead to lattice instabilities as well as a damping of magnetic correlations. The amount of uncorrelated isolated moments evaluated from field-dependent magnetization measurements remains nearly constant across all samples exhibiting the QSL behavior. This forms a plateau with respect to t′/t as shown in Fig. 2b, a finding that has not been noted so far. Molecular solids are recognized as homogeneous systems with fewer impurities than inorganic compounds, so the discussions on disorder effects to QSLs are limited. However, theoretical work on weakly disordered dimer magnets considering inorganic.

QSLs appears to share features with the data, such as nonlinear magnetization scaling with an external field [33]. This could help illustrate the physics of QSLs regardless of the materials’ chemical nature.

The uniform magnetic susceptibility (χ(q = 0)), which is the linear magnetization component in response to a magnetic field at the lowest temperature in the QSL phase, has been found to be suppressed as the system approaches the QPT boundary, as shown in the inset of Fig. 2b [31]. The magnetic susceptibility of the antiferromagnetic mode, χ(q = Q), is widely accepted to be enhanced near the quantum critical point. However, the effect of quantum critical fluctuations on χ(q = 0) remains unclear. Future detailed experiments on the spin state at dilution refrigeration temperatures, using spectroscopic methods such as nuclear magnetic resonance (NMR), are expected to reveal nontrivial quantum fluctuations near the quantum phase transition.

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[Source: https://link.springer.com/article/10.1007/s43673-025-00165-7]