Neither solid nor liquid : Neutrons help reveal an exotic state of matter
A team of researchers recently used the thermal-neutron two-axis diffractometer D23 at the ILL to investigate Na2 BaCo(PO4 )2 (NBCP), a material that surprisingly behaves as a ‘spin supersolid’ -a state combining properties of both a solid and a liquid . Neutrons, acting like tiny magnets themselves, were the ideal probes to reveal the hidden magnetic order and dynamics within this material. This discovery, which is also relevant for energy-efficient cooling, provides the first real-life evidence of a supersolid state in a quantum magnet.
Supersolids are a peculiar quantum state of matter and are actively sought after in material science. Intriguingly, they combine properties of both an incompressible solid and a superfluid with zero viscosity. Potential application of these materials include sub-Kelvin refrigeration, that is, cooling to extremely low temperatures. This is of particular interest in areas such as space technology where very low temperatures are required for optimal functioning of certain devices.
The recently synthesised substance Na2BaCo(PO4)2 (NBCP for short, consisting of Na = sodium, Ba = barium, Co = cobalt, P = phosphorous and O = oxygen), is a potential candidate for the quantum analogue of a supersolid, known as a spin supersolid.
NBCP is a so-called anti-ferromagnet, meaning that its magnetic moments align in an anti-parallel fashion and cancel each other out. Therefore, although it does have classical magnetic properties, it is overall effectively non-magnetic. Below a certain temperature, known as the Néel temperature, such anti-ferromagnetic materials become paramagnetic: they can then be magnetised if a magnetic field is applied.
Measured and calculated adiabatic cooling curves of NBCP from an initial temperature T0 = 2 K and various initial fields B0 = 3, 3.5 and 4 T.
These properties of NBCP make it a very promising material for diverse applications. In a recent study, a team of researchers explored it in great detail and published their results in the prestigious journal Nature. “We used high-quality single NBCP crystals to perform neutron diffraction on ILL's D23”, explains Wentao Jin from Beihang University, member of the research team. The sample environment of D23 is optimised for studying magnetic materials and their properties as a function of externally applied magnetic fields. In addition, neutrons - who themselves behave like tiny magnets - are ideal probe particles for an in-depth characterisation of materials such as NBCP.
“Our experiments at the ILL indeed provided evidence that NBCP features elements of both solid and superfluid phases”, says Jin. Importantly, this is the first time that a supersolid state has been observed in a real-life quantum magnet.
Furthermore, the scientists discovered a so-called giant magnetocaloric effect (MCE) of the NBCP system. MCE describes the heating up of the material upon application of a magnetic field and is, notably, the scientific basis for energy-efficient, sustainable refrigeration technologies. In view of current concerns about global helium shortages, this discovery is of particular interest.
The experimental data were in excellent agreement with theoretical simulations of supersolid systems, underlining the importance of combining both theory and experimental observations particularly when searching for novel states of matter.
Reference: Xiang, J., Zhang, C., Gao, Y. et al. Giant magnetocaloric effect in spin supersolid candidate Na2BaCo(PO4)2. Nature625, 270–275 (2024). https://doi.org/10.1038/s41586-023-06885-w
ILL instrument:D23
ILL contacts: Karin Schmalzl, Wolfgang Schmidt