Quantum mysteries in 3D: A new spin on magnetic materials

In the world of quantum physics, certain materials behave in ways that defy our classical understanding of magnetism. An example are quantum spin liquids (QSLs), exotic states of matter where magnetic moments – or “spins” – remain in a fluid-like state even at absolute zero temperature, unlike conventional magnets that freeze into a rigid structure. QSLs also maintain a high degree of quantum entanglement, leading to fascinating phenomena such as fractional excitations and long-range quantum entanglement.

While QSLs have been extensively studied in two-dimensional systems, doing this in three dimensions is a significant challenge, as the higher dimensionality tends to suppress the quantum fluctuations necessary for maintaining a liquid-like state. Despite this, scientists have been on the hunt for 3D materials that could host QSLs, as they could unlock new frontiers in quantum computing and other advanced technologies.

A prominent candidate is the material potassium nickel sulfate (K₂Ni₂(SO₄)₃), a crystalline material in the langbeinite family of crystals, known for their complex crystal structures that endow them with exotic magnetic properties.

What makes K₂Ni₂(SO₄)₃ stand out is its complex structure, which is composed of two interconnected trillium lattices, a type of 3D crystal structure of interlocking triangular units that create a network of corner-sharing tetrahedra. Although the material eventually develops a weakly magnetically ordered state at very low temperatures, its dynamic magnetic behavior suggests it is on the brink of a QSL phase, making it a valuable subject of study for understanding the delicate balance between order and quantum chaos.

Now, researchers at EPFL, the Helmholtz-Zentrum Berlin and Freie Universität Berlin combined experimental and theoretical techniques to delve deeper into the unique properties of K₂Ni₂(SO₄)₃. Their findings reveal that the material is governed by a complex network of magnetic interactions that place it tantalizingly close to a QSL phase.

The research was led by Ivica Živković from the Laboratory for Quantum Magnetism at EPFL.

The researchers employed a multifaceted approach to study K₂Ni₂(SO₄)₃. They started with inelastic neutron scattering experiments, a powerful technique that allows scientists to probe the dynamic behavior of spins within a material. By bombarding the material with neutrons and analyzing how they scatter, the researchers could infer the underlying magnetic structure and dynamics.

They then compared their experimental data to theoretical models derived from density functional theory (DFT) and advanced computational methods like pseudo-fermion functional renormalization group (PFFRG) and classical Monte Carlo simulations. These techniques allowed the team to simulate the magnetic interactions within K₂Ni₂(SO₄)₃ and explore how close it is to a spin-liquid state.

The study revealed that K₂Ni₂(SO₄)₃ exhibits a highly dynamic magnetic state, even at temperatures where one would expect conventional magnetic order to dominate. This state is characterized by a “freezing” of only a small fraction of the available magnetic entropy, which is a hallmark of spin liquid behavior – as if the quantum spin liquid is covered in a frozen layer. Moreover, applying a magnetic field enhances this dynamic state, pushing the material further toward a full QSL phase.

The researchers traced the origin of these dynamics to the novel arrangement of magnetic moments, characterized by a trillium lattice of tetrahedra. They found that it closely resembles a theoretical model known to support spin liquid behavior, suggesting that the material’s properties are governed by its proximity to this quantum critical point.

The findings have significant implications for the field of quantum materials. By demonstrating that K₂Ni₂(SO₄)₃ is near a QSL phase, the study opens up new avenues for exploring how quantum fluctuations can stabilize exotic states of matter in three dimensions. This could lead to the discovery of new materials with similar properties, potentially advancing technologies that rely on quantum information processing.

Other contributors

• EPFL Crystal Growth Facility
• STFC Rutherford Appleton Laboratory
• Okayama University
• Indian Institute of Technology Madras