A strange new quantum state appears when atoms get “frustrated”

In research published in Nature Materials, Wilson's team describes a new way to use a phenomenon known as frustration of long range order in a material to create unconventional magnetic states. These states may eventually have relevance for quantum technologies. Wilson emphasized that the work focuses on basic science rather than immediate applications. "This is fundamental science aimed at addressing a basic question. It's meant to probe what physics may be possible for future devices."

Their study, titled "Interleaved bond frustration in a triangular lattice antiferromagnet," examines how multiple forms of frustration can arise in these systems. One important type is geometric frustration. This occurs when magnetic moments in a material cannot settle into a single stable pattern and instead remain in a fluctuating configuration.

Tiny Atomic Magnets and Frustrated Geometry

Wilson explained magnetism using a simple analogy. "You can think of magnetism as being derived from tiny bar magnets sitting at the atomic sites in a crystal lattice," he said. These tiny magnets are called magnetic dipole moments. Depending on the structure of a material, they interact with one another and arrange themselves in ways that minimize their energy or, in other words, reach their ground state. The ground state represents the lowest possible energy configuration of a system, and at absolute zero temperature every system exists in this state.

Wilson continued, "If those magnetic moments interact in a way that wants them to point antiparallel to one another, we call that antiferromagnetism." In a square arrangement of atoms, this interaction works easily. Each magnetic moment can point opposite to its neighbors, producing a stable configuration.

However, things change when the atoms form a triangular arrangement. In that geometry, it becomes impossible for every magnetic moment to point opposite to all of its neighbors at the same time. As Wilson described it, the moments begin competing with each other. They are effectively frustrated because the geometry of the lattice prevents them from achieving the lowest energy arrangement. The system tries to reach equilibrium but cannot fully do so because of the structure it occupies.

Bond Frustration and Electron Sharing

A similar type of frustration can occur in another aspect of electrons. Instead of involving magnetism, it can arise from electron charge. When two nearby ions attempt to share an electron across a bond, they may form what scientists call an atomic dimer.

Just as magnetic interactions can be frustrated in certain lattice structures, these dimers can also face restrictions in geometries such as triangular lattices or honeycomb networks. The result can be a network of bonds that is itself frustrated. Such a network is often very sensitive to strain, and applying strain can partially relieve the frustration within the bonding pattern.

Wilson's study focuses on an extremely rare class of materials where both types of frustration exist at the same time. Magnetic frustration and bond frustration appear together in the same structure.

Coupling Two Frustrated Systems

Wilson described the finding as "exciting" because it opens a possible route to controlling one frustrated system by influencing the other. Over the past six or seven years, scientists have learned how to create frustrated magnetic states using materials built from triangular networks of lanthanides, a group of elements found along the bottom row of the periodic table.

"In principle, this triangular lattice network of properly chosen lanthanide moments can cause a special kind of intrinsically quantum disordered state to arise," Wilson said. The team's goal was to build on that idea. "One thing we tried to do in this project was to functionalize that exotic state by embedding it in a crystal lattice that has an additional degree of bond frustration."

Researchers know that quantum disordered magnetism can take several forms. Some of these states may support long range entanglement among spins, which is a key concept in quantum information science. Wilson explained, "Some states can host long-range entanglement of spins, which is of interest in the realm of quantum information. Gaining control over those states via applying a strain in the frustrated bond network would be exciting."

Toward Controlling Quantum States

When two frustrated systems exist together and are both highly sensitive to disturbances such as strain or magnetic fields, an important question emerges. Scientists want to know whether the two systems can influence each other. If one layer becomes ordered under certain conditions, it could potentially affect the other layer as well.

"It's a way of imparting in things a functionality or response to other things to which it would otherwise not respond," Wilson explained. "So, in principle, one can engineer large ferroic responses.You can apply a bit of strain, which induces magnetic order, or you can apply a bit of magnetic field and induce changes to the structure.

"Again, in principle, if you can find a quantum disordered ground state that hosts long-range entanglement, the question then becomes whether you can access that entanglement by, for instance, coupling to another layer, such as bond frustration."

Wilson is also interested in whether this approach could lead to multiple types of order emerging together. "Basically, you could have different types of order that get nucleated because of the proximity of these two frustrated lattices," he said. "That's the big-picture idea."