Quantum Materials Expertise at Georgia Tech

Quantum Materials Expertise at Georgia Tech

Researchers in Germany and Japan report preparing a compound that could realize a quantum spin liquid, a rarely observed and delicate state of magnetic matter. Published in the journal Nature, the paper is cause for excitement among researchers in condensed-matter physics. Among them is Martin Mourigal, who wrote an accompanying News & Views piece in Nature to put the research in context and convey the excitement of the field of quantum materials.

Researchers in Germany and Japan report preparing a compound that could realize a quantum spin liquid, a rarely observed and delicate state of magnetic matter. Published in the journal Nature, the paper – “A spin-orbital-entangled quantum liquid on a honeycomb lattice” – is cause for excitement among researchers in condensed-matter physics. Among them is Martin Mourigal, who wrote an accompanying News & Views piece in Nature – “The two faces of a magnetic honeycomb” – to put the research in context and convey the excitement of the field of quantum materials.

Mourigal is an assistant professor in the School of Physics. His research group studies magnetic phenomena in quantum materials. In these materials, the impact of quantum mechanics transcends the atomic scale to produce new effects up to the human scale.

“This field of research touches on the deep and profound organizing principles of the universe,” Mourigal says. “At the same time, it is an area where we can explore and test abstract theoretical ideas from a simple piece of ceramic grown in the lab.”

In the following Q&A, Mourigal explains quantum spin liquids, why the recent findings of the research team led by Hidenori Takagi are noteworthy, and the role Georgia Tech can play in quantum materials research.

What is exciting about quantum spin liquids?
They are new forms of matter, predicted to exist in some quantum materials at temperatures close to zero degree Kelvin.

Spins are atomic-scale magnetic moments that make up any magnetic material. In ferromagnets – think refrigerator magnets – the spins are frozen in a periodic pattern.

In quantum spin liquids, spins do not freeze and keep dancing even at close to absolute zero temperature. As spins fluctuate, they share the same quantum state, and their collective behavior cannot be represented by the sum of individual behaviors. This phenomenon is called quantum entanglement.

Entanglement leads to physical properties that make quantum spin liquids appealing for both fundamental inquiries and practical purposes. Entanglement is required for quantum computers to work; it may also be used to create new electronic devices and technologies for the post-Moore’s law era.

Most importantly, quantum spin liquids quench our thirst for fundamental discoveries about matter. In particular, understanding the relationship between atomic-scale properties and macroscopic behavior is a central challenge. Quantum mechanics complicates the picture, primarily because of entanglement.

What is the breakthrough reported by the researchers in Nature?
Quantum spin liquids are notoriously difficult to realize in real materials, and their confirmation involves many steps.

The group led by Takagi started from a magnetically ordered material – a well-researched iridium oxide with a honeycomb structure – known to host an important ingredient of the quantum spin liquid recipe.

That ingredient is called Kitaev interactions. Usually, spins interact with neighbors in a simple way: they align in the same or the opposite direction. Kitaev interactions are different. Each spin is getting contradictory information from its neighbors and cannot decide what to do. Because spins cannot pick a preferred direction, they remain fluctuating all the way to absolute zero temperature.

The researchers applied a soft chemistry approach to modify the original iridium oxide, heating it to 120 degrees Celsius. In solid-state synthesis, reagents are usually heated to around 1,000 degrees Celsius to make a new compound.

Despite the mild conditions, the researchers got a dramatic change in the material’s physical properties, sufficient to produce a strong contender for a quantum spin liquid. More generally, it means it is possible to realize exotic states of matter through gentle modifications of existing materials.

How did the researchers show that the new material could be a quantum spin liquid?
It is not fully confirmed that the new material is a quantum spin liquid, but it passed a first battery of tests, which included nuclear magnetic resonance and heat capacity measurements. These tests showed that the spins remain fluctuating at 1 degree Kelvin and that no magnetic order is present at 0.05 degree Kelvin, which is very exciting.

Nevertheless, more investigations are required. The challenge for the researchers is to further characterize the spin dynamics in the new material. Progress in materials research usually comes from the confluence of different experimental techniques and expertise, and I expect this new material will generate a lot of interest in that direction.

What is Georgia Tech doing about quantum materials?
Quantum materials research is on the rise at Georgia Tech. Understanding and controlling quantum states of matter is a priority for the U.S. We have a lot of talent on campus to make an impact in that burgeoning area.

First, there is strong intellectual overlap between quantum materials research and the fields of atomic physics, quantum optics, and quantum information sciences. In all these fields, a profound understanding of quantum mechanics and entanglement is central.

Furthermore, quantum materials research cuts across all material-centric disciplines on campus: from the chemistry, synthesis, and characterization of new materials to their integration in devices for new electronics.

Finally, and close to my own research, Georgia Tech is ideally located to utilize the world-class neutron-scattering facilities at Oak Ridge National Laboratory. Because neutrons themselves carry a spin, they can scatter off the excitations of magnetic materials and are thus an ideal probe of quantum spin liquids and other magnetic quantum materials.

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