Following up on earlier theoretical predictions, MIT researchers have now demonstrated experimentally the existence of a fundamentally new kind of magnetic behavior, adding to the two previously known states of magnetism.
Ferromagnetism – the simple magnetism of a bar magnet or compass needle – has been known for centuries. In a second type of magnetism, antiferromagnetism, the magnetic fields of the ions within a metal or alloy cancel each other out. In both cases, the materials become magnetic only when cooled below a certain critical temperature. The prediction and discovery of antiferromagnetism – the basis for the read heads in today’s computer hard disks – won Nobel Prizes in physics for Louis Neel in 1970 and for MIT professor emeritus Clifford Shull in 1994.
“We are showing that there is a third fundamental state for magnetism,” said MIT professor of physics Young Lee.
This new state, called a quantum spin liquid (QSL), is a solid crystal, but its magnetic state is described as liquid: Unlike the other two kinds of magnetism, the magnetic orientations of the individual particles within it fluctuate constantly, resembling the constant motion of molecules within a true liquid.
Finding the Evidence
“There is no static order to the magnetic orientations, known as magnetic moments, within the material. But there is a strong interaction between them, and due to quantum effects, they don’t lock in place,” said Mr. Lee.
Although it is extremely difficult to measure, or prove the existence, of this exotic state, this is claimed to be one of the strongest experimental data sets out there that [does] this. What used to just be in theorists’ models is a real physical system now, according to the researcher.
Philip Anderson, a leading theorist, first proposed the concept in 1987, saying that this state could be relevant to high-temperature superconductors.
“Ever since then, physicists have wanted to make such a state. It is only in the past few years that we have made progress,” said Mr. Lee.
The material itself is a crystal of a mineral called herbertsmithite. Mr. Lee and his colleagues first succeeded in making a large, pure crystal of this material last year – a process that took 10 months – and have since been studying its properties in detail.
“This was a multidisciplinary collaboration, with physicists and chemists. You need both … to synthesize the material and study it with advanced physics techniques. Theorists were also crucial to this,” explained the researcher.
Through its experiments, the team made a significant discovery. They found a state with fractionalized excitations, which had been predicted by some theorists but was a highly controversial idea. While most matter has discrete quantum states whose changes are expressed as whole numbers, this QSL material exhibits fractional quantum states. In fact, the researchers found that these excited states, called spinons, form a continuum.
To measure this state, the team used a technique called neutron scattering, which is Mr. Lee’s specialty. To actually carry out the measurements, they used a neutron spectrometer at the National Institute of Standards and Technology (NIST) in Gaithersburg, Maryland.
“The results are really strong evidence of this fractionalization of the spin states. That is a fundamental theoretical prediction for spin liquids that we are seeing in a clear and detailed way for the first time. We have to get a more comprehensive understanding of the big picture. There is no theory that describes everything that we are seeing,” said the researcher.
It may take a long time to translate this “very fundamental research” into practical applications. The work could possibly lead to advances in data storage or communications, perhaps using an exotic quantum phenomenon called long-range entanglement, in which two widely separated particles can instantaneously influence each other’s states. The findings could also bear on research into high-temperature superconductors, and could ultimately lead to new developments in that field, he says.
“These findings, which have been anticipated for decades, are very significant and open a new chapter in the study of quantum entanglement in many-body systems. The detection of such states was an exceptionally difficult task. Young Lee and his group brilliantly overcame these challenges in their beautiful experiment,” said Subir Sachdev, a professor of physics at Harvard University.