New Non-Quantum State of Matter

A study published in Science Advances and co-led by Rice University’s Pengcheng Dai discovered that the material cerium magnesium hexalluminate (CeMgAl₁₁O₁₉) is not in a quantum spin liquid phase.

“The material has been classified as a quantum spin liquid due to two properties: observation of a continuum of states and lack of magnetic ordering. But closer observation of the material showed that the underlying cause of these observations wasn’t a quantum spin liquid phase,” states Bin Gao, co-first author and research scientist at Rice.

Behavior of Typical Magnetic States

Insulating materials can have magnetic ions that take on one of two magnetic states: ferromagnetic or antiferromagnetic. Once an ion is in a ferromagnetic state, it will bring other ions nearby into that state; therefore, all the ions in a structure align in a ferromagnetic state. The same is true for an antiferromagnetic state.

This magnetic alignment can be seen when researchers bring the material down to near-absolute zero temperatures. At low temperatures, non-quantum materials will settle into a low-energy configuration, and because all the ions will be either in a ferromagnetic or antiferromagnetic state, observers will only see one low-energy configuration.

Quantum Liquid Spin Material

The behavior of a quantum liquid spin material is different at near-absolute zero temperatures. These materials transition to and from different low-energy states via quantum mechanics. Therefore, observers will witness a continuum of different states, not just one.

Additionally, both ferromagnetic and antiferromagnetic states can be observed rather than just one, like in conventional magnetically ordered materials.

How Researchers Were Fooled

CeMgAl₁₁O₁₉ was observed with both a lack of magnetic ordering and a continuum of different states. After careful analysis of the continuum of states, it was discovered that it does not arise from the state originally believed, but a degeneration of states from the competition of ferromagnetic and antiferromagnetic interactions.

“We were interested in this material, which has a collection of characteristics we hadn’t seen before. It was not a quantum spin liquid, yet we were observing what we thought were quantum spin liquid-associated behaviors,” explains Tong Chen, co-first author and research scientist at Rice.

The team bombarded the material with neutrons and took careful measurements to conclude in the case of CeMgAl₁₁O₁₉, the boundary between the ferromagnetic and antiferromagnetic states was weaker than in most materials.

The magnetic ions did not align into a single-ordered state. Instead, in the same structure, some were ferromagnetic, and some were antiferromagnetic, producing the lack of magnetic ordering, opening up a wider array of possible low-energy states.

When the material was brought to near-absolute zero, it could choose from a variety of different low-energy states. The result is a mix of observable states that resemble the continuum of different states found in quantum spin liquids. However, the material was not in a quantum spin liquid state; once it entered a low-energy state, it could not shift to another state.

“The material’s unique ability to ‘choose’ between different low-energy states produced observational data very similar to a quantum spin liquid state. This is a new state of matter that, to ur knowledge, we are the first to describe,” explains Dai, who adds, “It underscores the importance of careful observation and thorough investigation of your data.”

Sources:

Phys.org: Material previously thought to be is actually a new, state of matter
ScienceBlog: Material Mimic Reveals a New Nonquantum State of Matter
Bioengineer.org: New Discovery Reveals  a new, Nonquantum State of Matter

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