New material in a state over quantum spin liquid

"Observation of a QSL [quantum spin liquid] state is one of the most important goals in condensed-matter physics as well as the development of new spintronic devices."Masayoshi Fujihala, Tokyo University of Science

In addition to the deep understanding of the natural world offered by quantum physics theory, scientists worldwide are working tirelessly to bring forth a technological revolution by leveraging this newfound knowledge in engineering applications. Spintronics is an emerging field that aims to surpass the limits of traditional electronics by using the spin of electrons, which can be roughly seen as their angular rotation, as a means for encoding information.

But the design of devices that can operate using spin is extremely challenging and requires the use of new materials in exotic states, including some that scientists do not fully understand and have not experimentally observed yet. Now, in a paper in Nature Communications, scientists from the Department of Applied Physics at Tokyo University of Science in Japan report a newly synthesized compound with the formula KCu₆AlBiO₄(SO₄)₅Cl that may be key to understanding the elusive quantum spin liquid (QSL) state.

"Observation of a QSL state is one of the most important goals in condensed-matter physics as well as the development of new spintronic devices," said lead scientist Masayoshi Fujihala. "However, the QSL state in two-dimensional (2D) systems has not been clearly observed in real materials owing to the presence of disorder or deviations from ideal models."

What is the QSL state? In antiferromagnetic materials below specific temperatures, the spins of electrons naturally align into large-scale patterns. In materials in a QSL state, however, the spins are disordered, similar to how molecules in liquid water are disordered in comparison to crystalline ice. This disorder arises from a structural phenomenon called frustration, in which there is no possible configuration of spins that is symmetrical and energetically favorable for all electrons.

KCu₆AlBiO₄(SO₄)₅Cl is a newly synthesized compound whose copper atoms are arranged in a particular 2D pattern known as the square kagome lattice (SKL), an arrangement that is expected to produce a QSL state through frustration.

"The lack of a model compound for the SKL system has obstructed a deeper understanding of its spin state," said co-author Setsuo Mitsuda. "Motivated by this, we synthesized KCu₆AlBiO₄(SO₄)₅Cl, the first SKL antiferromagnet, and demonstrated the absence of magnetic ordering at extremely low temperatures – a QSL state."

The scientists could not, however, replicate their experimental findings with theoretical calculations using a standard ‘J1-J2-J3 SKL Heisenberg’ model, which considers the interactions between each copper ion in the crystal network and its nearest neighbors.

"To try to eliminate the discrepancy, we calculated an SKL model considering next-nearest-neighbor interactions using various sets of parameters. Still, we could not reproduce the experimental results. Therefore, to understand the experiment correctly, we need to calculate the model with further interactions," explains co-author Katsuhiro Morita.

This disagreement between experiment and theoretical calculations highlights the need for refining existing theoretical approaches. "While the SKL antiferromagnet we synthesized is a first candidate to investigate SKL magnetism, we may have to consider longer-range interactions to obtain a quantum spin liquid in our models," said co-author Takami Tohyama. "This represents a theoretical challenge to unveil the nature of the QSL state."

This story is adapted from material from Tokyo University of Science, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.