Spin doctors summon coppers

Imagine three toddlers, but only two toys to share between them. The result? Three frustrated toddlers.

Using this basic principle, in the form of magnets, physicists recently discovered a new state of matter – called quantum spin liquid. And now, reporting in the Journal of the American Chemical Society, researchers at Boston College have created the most perfect quantum spin liquid yet.

The new material could solve some of the problems hampering development of a large-scale quantum computer.

Although people have known about natural magnets since ancient times, magnetism was only explained by the new science of quantum theory in the early twentieth century. Physicists including Niels Bohr and Paul Dirac realised that magnetism was all down to the alignment of the ‘quantum spin’ of electrons in the materials.

In ‘ferromagnetic’ materials, electron spins tend to align with one another, and the material can be magnetised. While in antiferromagnetic materials adjacent electrons force one another to have opposite spin — so no magnetism.

In 1973, the physicist and Nobel laureate Philip Anderson theorised a new situation, where you have a repeated triangular pattern of three antiferromagnetic electrons. In this case, the electron spins want to be opposite to their neighbours, but because of the triangular structure they can’t.

Just like the three toddlers constantly grabbing the two toys from one another’s fingers, the electrons constantly force one another to flip their spin direction.  This is what’s called a ‘frustrated magnet’. A material made up of this repeated structure is known as a quantum spin liquid.

The ‘liquid’ part comes from the ability of the spins to move around effortlessly through the material—a property that may allow the flow of quantum information, a vital element in the quest to build large scale quantum computers.

But quantum spin liquids are rare. It was 40 years after Anderson’s proposal that the first, a greenish mineral called herbertsmithite, was found.

In 2006 Alexiei Kitaev at CalTech in the US predicted that materials with a particular crystal structure, where the atoms are arranged in a kind of honeycomb, could make for ideal quantum spin liquids.

Since then, only two materials have been found that fit the model.

These are lithium (Li₂IrO₃) and a sodium iridate (Na₂IrO₃). Both have the right kind of honeycomb arrangement, with the lithium or sodium atoms sitting inside each honeycomb cell, like larvae in a beehive, but neither is a perfect fit.

For example, sodium iridate can only hold its quantum spin liquid state down to about 15 degrees Kelvin (minus 258 degrees Celsius), whereas an ideal Kitaev material should hold the state right down to absolute zero.

When Fazel Tafti at Boston college and his team studied these materials, they found imperfections which might explain their middling properties. For example, the bond angles of the triangle were not the ideal 120 degrees as defined by Kitaev. The lithium atom was too small, the sodium one too big.

To overcome this issue Tafti’s team turned to copper, because it’s between lithium and sodium in size.

They found that they could take sodium iridate and replace all the sodium atoms with copper atoms. Their process was pretty simple, simply mixing the sodium iridate with copper chloride salt and heating them.

The resulting material, a copper iridium binary metal oxide described as Cu₂IrO₃,  exhibits almost perfect geometry for the Kitaev model, with bond angles closer to the ideal 120°. The scientists describe it as having “the nearly ideal honeycomb structure … closer to a Kitaev spin liquid than its predecessors”.

“Copper is ideally suited to the honeycomb structure,” comments Tafti. “There is almost no distortion.”

Cu₂IrO₃ can hold quantum spin liquid behaviour right down to just a couple of degrees above absolute zero.

Variations on this theme could lead to the first ideal quantum spin liquid. After more than 40 years of trying, scientists are finally getting the hang of this weird new state of matter.