At first glance, liquid water and solid inorganic substances do not have much in common. However, a detailed scientific look reveals that inorganic compounds, such as some minerals, can assume states known as spin liquids. This brings them closer to the liquids we know from our everyday life than we may imagine.
We are all familiar with the phenomenon of freezing: water self-organises into ordered ice structures when cooled. Similarly, the degree of order in spin liquids is temperature-dependent. In some spin liquids, residual disorder (molecular movement) persists even at very low temperatures. This gives rise to a characteristic diffuse magnetic signal, which is hard to detect with standard experimental methods. However, neutrons - which can be imagined as tiny magnets - can easily interact with magnetic materials and are the ideal probe for detecting this signal.
This crucial advantage of neutrons was exploited in earlier studies of the mineral Tb2Ti2O7, a quantum analogue of classical spin ices. The results revealed not only the diffuse magnetic scattering mentioned above, but also an additional exotic signal hinting at the presence of novel molecular interactions.
These interesting results inspired an interdisciplinary team of scientists from French research institutions, the Paul Scherrer Institut (Switzerland) and the ILL to go one step further. "We synthesised the molecule Tb2(Ti,Zr,Hf,Sn,Ge)2O7, aso-called high entropy (high disorder) counterpart compound of Tb2Ti2O7. This allowed us to probe the robustness of these newly discovered interactions towards disorder", explains Florianne Vayer (ICCMO, Université Paris-Saclay) the first author of the study, "In particular, we used neutron diffraction and inelastic neutron scattering at both ILL and the PSI to reveal the magnetic response of Tb2(Ti,Zr,Hf,Sn,Ge)2O7", adds Claudia Decorse, research team member from the same institution.
At the ILL, neutron diffraction measurements were carried out on the D1B neutron diffractometer, while inelastic neutron scattering experiments were carried out on the thermal time-of-light instrument Panther. Stephane Rolls, ILL researcher and local contact point for this study, emphasises that “the developments made to the Panther spectrometer during the recent Endurance upgrade programme have made it possible to see clearly the spectroscopic signatures specific to these systems.”
Unexpectedly, the exotic signal initially discovered in Tb2Ti2O7 was found to be even stronger in the high-entropy compound. A detailed analysis of these data, combined with further experiments and modelling by Sylvain Petit, allowed the team to show that random movements of oxygen (O) atoms surrounding the central terbium (Tb) atoms profoundly alter the interactions between the Tb ions. This destabilises the spin liquid state, which implies that disorder is an important factor for the emergence of peculiar properties of such compounds. Moreover, theoretical considerations revealed that changing the degree of disorder can make the material go back into its liquid state.
Although this study is primarily rooted in fundamental research, its implications go way beyond exotic physics. "Notably, the quantum liquid spin state is characterised by a phenomenon known as 'large-scale entanglement', the very same quantum mechanical property the scientific community would like to use for quantum computation", says Florianne Vayer. This next-level technology is expected to solve computational challenges that are too complex for our current state-of-the-art computers.
Text: Olga Matsarskaia
Reference: Vayer F, Petit S, Damay F, Embs J, Rols S, Colin C, Lhotel E, Bounoua D, Dragoe N, Bérardan D, Decorse C. Entropy-stabilized materials as a platform to explore terbium-based pyrochlore frustrated magnets. Communications Materials. 2024 Aug 21;5(1):162.
https://doi.org/10.1038/s43246-024-00589-y
ILL contact:Stéphane Rols