The ceramic that fooled physicists into thinking it was a quantum spin liquid

Quantum spin liquids are among the most sought-after states of matter in condensed matter physics. They refuse to settle into magnetic order even at temperatures near absolute zero, and their exotic quantum properties have potential applications in quantum computing. So when a material called cerium magnesium hexalluminate (CeMgAl11O19) displayed two signature characteristics of a quantum spin liquid -- a continuum of energy states and a lack of magnetic ordering -- researchers naturally classified it as one.

They were wrong. A study published in Science Advances, co-led by Rice University physicist Pengcheng Dai, has shown that CeMgAl11O19 is not a quantum spin liquid at all. It is something new: a non-quantum state of matter that mimics quantum behavior through an entirely different mechanism.

What quantum spin liquids actually do

In a conventional magnetic material cooled to near absolute zero, the magnetic ions (the atoms carrying magnetic charge) align into one of two ordered states: ferromagnetic (all pointing the same direction) or antiferromagnetic (alternating directions). The material settles into a single low-energy configuration and stays there.

In a quantum spin liquid, things are different. The material transitions between multiple low-energy states through quantum mechanical processes, never locking into a single arrangement. This produces a continuum of observable states and an absence of magnetic ordering -- the two features that led researchers to classify CeMgAl11O19 as a quantum spin liquid.

A weaker boundary, a richer set of choices

By bombarding the material with neutrons and conducting additional measurements, Dai's team discovered what was actually happening. In CeMgAl11O19, the boundary between the ferromagnetic and antiferromagnetic states is weaker than in most materials. The magnetic ions have more flexibility to adopt either state, so within the same crystal structure, some ions go ferromagnetic and others go antiferromagnetic. The result is a lack of magnetic ordering -- but not for quantum reasons.

This disordered arrangement also opens up a wider array of possible low-energy configurations. When the material is cooled toward absolute zero, it can select from multiple states, producing what looks like the energy continuum found in quantum spin liquids. But there is a critical difference: once CeMgAl11O19 enters a low-energy state, it stays there. It cannot hop between states the way a true quantum spin liquid does.

"The material's unique ability to 'choose' between different low energy states produced observational data very similar to a quantum spin liquid state," said Dai. "This is a new state of matter that, to our knowledge, we are the first to describe."

The case for careful observation

Co-first author Tong Chen, a research scientist at Rice, noted the challenge: "It was not a quantum spin liquid, yet we were observing what we thought were quantum spin liquid-associated behaviors."

The finding underscores a methodological concern in the search for quantum spin liquids. The two most commonly used diagnostic criteria -- a continuum of states and absence of magnetic ordering -- can apparently be produced by non-quantum mechanisms. Researchers hunting for genuine quantum spin liquids will need to go beyond these surface signatures to confirm that quantum mechanical processes are actually responsible.

"It underscores the importance of careful observation and thorough investigation of your data," Dai said.

Limitations and open questions

The study describes the phenomenon in one material. Whether similar non-quantum mimicry occurs in other candidate spin liquids remains to be determined. The precise conditions under which the weak ferromagnetic-antiferromagnetic boundary arises, and whether this new state of matter has practical applications, are open questions. The work also does not address whether other materials previously classified as quantum spin liquids might be misidentified for similar reasons.

The neutron scattering and magnetic susceptibility work at Rice was supported by the U.S. Department of Energy's Basic Energy Sciences program. Crystal growth was supported by the Robert A. Welch Foundation and the Gordon and Betty Moore Foundation. The study also involved researchers at Rutgers University, the Tata Institute of Fundamental Research in India, and other institutions.