In materials scientist Steven Wilson’s lab at the University of California, Santa Barbara, researchers study the physics behind unusual states of matter while designing materials that can support properties useful for future quantum technologies.
In a study published in natural materialsWilson’s team describes a new way to create unconventional magnetic states by exploiting a phenomenon known as frustration of long-range order in materials. These states may eventually be relevant to quantum technology. Wilson emphasized that the research focuses on basic science rather than direct applications. “This is fundamental science that aims to address fundamental questions. It aims to investigate what kind of physics will be possible in future devices.”
Their study, entitled “Interleaved coupling frustration in triangular lattice antiferromagnets,” investigates how multiple forms of frustration occur in these systems. One important type is geometric frustration. This occurs when the magnetic moments within the material do not settle into a single stable pattern but remain in a fluctuating configuration.
Tiny atomic magnets and frustrated geometry
Wilson explained magnetism with a simple analogy. “Magnetism can be thought of as coming from little bar magnets at the atomic sites of the crystal lattice,” he said. These small magnets are called magnetic dipole moments. Depending on the structure of the materials, the materials interact and are arranged to minimize energy, or reach the ground state. The ground state represents the lowest possible energy configuration of a system, and at absolute zero all systems exist in this state.
Wilson continued, “When these magnetic moments interact in such a way that they are oriented antiparallel to each other, we call it antiferromagnetism.” This interaction works easily if the atoms are arranged in a square. Each magnetic moment can point in the opposite direction to its neighboring magnetic moments, producing a stable configuration.
However, the situation changes when the atoms form a triangular arrangement. Its geometry makes it impossible for all magnetic moments to point opposite all neighboring magnetic moments at the same time. As Wilson explained, the moments begin to compete with each other. They are effectively frustrating because the geometry of the lattice does not allow achieving the lowest energy configuration. The system tries to reach equilibrium, but due to its structure, it cannot reach equilibrium completely.
Bond frustration and electron sharing
A similar kind of frustration can occur on other aspects of electrons. Instead of involving magnetism, it can be caused by the charge of electrons. When two nearby ions try to share electrons through a bond, they can form what scientists call an atomic dimer.
Just as magnetic interactions can be hindered in certain lattice structures, these dimers can also face limitations in geometries such as triangular lattices and honeycomb networks. As a result, the network of bonds itself may become frustrated. Such networks are often very sensitive to strain, and adding strain can partially alleviate frustration within the bonding pattern.
Wilson’s research focuses on the very rare class of substances in which both types of frustration exist simultaneously. Magnetic frustration and bond frustration appear simultaneously in the same structure.
Combine two stressed systems
Wilson said the discovery was “exciting” because it opens the door to potentially 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 a triangular network of lanthanides, a group of elements found along the bottom row of the periodic table.
“In principle, this triangular lattice network of appropriately chosen lanthanide moments could give rise to a special kind of intrinsic quantum disorder state,” Wilson said. The team’s goal was to build on that idea. “One of the things we tried to do in this project was to functionalize that exotic state by embedding it in a crystal lattice with additional bonding frustration.”
Researchers know that quantum disordered magnetism can take several forms. Some of these states may support long-range entanglement between spins, an important concept in quantum information science. “Long-range entanglement of spins can occur in some states, which is interesting in the field of quantum information. It would be exciting to be able to control these states by straining the frustrated coupling network,” Wilson explained.
Towards control of quantum states
An important problem arises when two stressed systems exist together, both highly sensitive to disturbances such as strain or magnetic fields. Scientists want to know whether the two systems can influence each other. Ordering one layer under certain conditions can also affect other layers.
“This is a way to give things functions and reactions that they wouldn’t otherwise have,” Wilson explained. “So, in principle, you can manipulate large ferroic responses. You can induce magnetic order by applying a little strain, or you can induce changes in the structure by applying a little magnetic field.
“Again, in principle, if we could find a quantum disordered ground state that hosts long-range entanglement, the question becomes whether we can access that entanglement by coupling it with another layer, for example through bond frustration.”
Wilson is also interested in whether this approach might allow multiple types of order to emerge together. “Essentially, because of the proximity of these two frustrated lattices, there can be different types of core order,” he said. “That’s the big idea.”
