Spin supersolid appears in a quantum antiferromagnet – Physics World

Spin supersolid appears in a quantum antiferromagnet – Physics World

Time Stamp: January 26, 2024 7:00 AM

Figure illustrating the adiabatic cooling process of a spin supersolid, as compared to paramagnetic cooling
The figure illustrates the adiabatic cooling process of a spin supersolid, as compared to paramagnetic cooling, highlighting the unique features of spin supersolid cooling. The triangular lattice structure and experimental devices (with NBCP compounds) for adiabatic magnetocaloric measurements are also included in the insets. (Courtesy: W Li)

Researchers in China, France and Australia have found new evidence for an exotic quantum state of matter called a spin supersolid. The discovery, made in an antiferromagnetic material with a triangular atomic lattice structure, represents a breakthrough in fundamental physics and might also aid the development of new cooling techniques that do not require liquid helium, since the material also shows a giant magnetocaloric effect.

As their name implies, supersolids are materials that flow without friction (like a superfluid) even though their component particles are arranged in a crystalline lattice (like a solid). As such, these materials break two continuous symmetries: translational invariance, due to the crystalline order; and gauge symmetry, due to the material’s frictionless flow.

Theorists predicted in the 1960s that supersolids should exist in quantum solids with so-called mobile bosonic vacancies – that is, gaps left behind as atoms with integer spin values move through the crystalline lattice. Beginning in the 1980s, experimental research focused on hints that supersolidity might occur in superfluid helium-4. In 2004, physicists at Pennsylvania State University in the US reported evidence for supersolidity in this material. However, further investigation by the same researchers revealed that they were mistaken, and their observations could be explained in other ways.

More recent experiments have shown that dipolar quantum gases elongated in one direction can undergo a phase transition from a regular Bose-Einstein condensate (BEC) to a state with supersolid properties. Atoms in dipolar gases have large magnetic moments and it is the interactions between them that give rise to supersolidity in these systems.

Layers of evidence

Researchers led by Gang Su at the University of Chinese Academy of Sciences (CAS) in Beijing now say they have found the quantum magnetic analogue of a supersolid in a recently synthesized antiferromagnet with the chemical formula Na2BaCo(PO4)2. This compound, known as NBCP, also displays a giant magnetocaloric effect, meaning that it heats up and cools down dramatically when an external magnetic field is applied and removed.

Su and colleagues Wei Li of the Institute of Theoretical Physics, CAS; Junsen Xiang and Peijie Sun from the Institute of Physics, CAS; and Wentao Jin at Beihang University carried out their magnetocaloric measurements at temperatures below 1 K. The excellent agreement between their experimental data and theoretical calculations of supersolid quantum phase transitions helped convince them that they were observing a new spin supersolid.

Further confirmation came from microscopic evidence they gained by conducting neutron diffraction experiments on high-quality samples of NBCP at the Institut Laue-Langevin in France and the Australian Nuclear Science and Technology Organisation. “The diffraction peaks revealed in-plane three-sublattice order, solid order and incommensurability in the out-of-plane direction,” says Su. “The latter can be related to the existence of gapless Goldstone modes (a form of symmetry breaking in bosons) and therefore supports the existence of spin superfluidity in the compound.”

A new quantum state of matter and a new cooling mechanism

The CAS team chose to study NBCP because it exhibits strong low-energy spin fluctuations, indicating a possible quantum spin liquid state. It is also an antiferromagnet, meaning that unlike conventional ferromagnets, which have parallel electron spins, its electron spins tend to align antiparallel to each other. This anti-alignment leads to strong interactions among the spins.

After one of the team’s members suggested a spin supersolid might exist in NBCP, Li and Gang asked their experimentalist colleagues Xiang, Jin and Sun if it was possible to look for new quantum spin states in the compound. “They did and observed the new quantum state of matter, the spin supersolid,” Li recalls.

As well as revealing a new quantum state of matter, the discovery could also lead to new helium-free sub-Kelvin cooling methods. These are highly sought after for materials science,  quantum technology and space applications, among others, Li tells Physics World.

Li explains that there are currently two main ways to cool materials to few-Kelvin temperatures. The first is to use helium, which becomes a liquid at temperatures below 4.15 K. The second is to exploit the magnetocaloric effect, in which certain materials change temperature under the influence of an applied magnetic field.  Both these techniques have their drawbacks: helium is scarce and therefore expensive, while the special class of compounds used for magnetocaloric cooling (known as hydrated paramagnetic salts) have low magnetic entropy density, poor chemical stability and low thermal conductivity. However, Li claims that the giant magnetocaloric effect in the newly-discovered spin supersolid could “effectively overcome these drawbacks” by exploiting collective spin excitations at low energies.

Looking for other spin supersolids

The researchers are now trying to obtain additional dynamical evidence for spin supersolidity in NBCP. To this end, Jin says they are performing inelastic neutron scattering measurements to investigate the Goldstone modes associated with the spin superfluid order. They also plan to conduct polarized neutron diffraction experiments to further strengthen their findings.

Finally, the team is investigating other triangular lattice compounds in an effort to identify additional spin supersolid states or other exotic spin states. “By doing so, we hope to better understand the underlying physical phenomena that give rise to these intriguing quantum phases of matter,” Su says.

Their present study is detailed in Nature.

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