Magnetic Systems Called Spin Ice Offer Peek into Quantum World

Although the question ‘What is light?’ sounds like a basic question, it is one that has interested many of the brilliant scientific minds for over thousands of years.

Now, a collaborative research with scientists at the Okinawa Institute of Science and Technology Graduate University (OIST) has incorporated another twist to the theory, converting an abstract concept about the quantum properties of magnets into a testable hypothesis about a new variety of light.

From the time when Isaac Newton refracted light through prisms in 1672, researchers have been divided over whether light is composed of waves or particles. Light appears to travel in straight lines, as would be anticipated of a particle, but Newton’s experiments have revealed it also has wavelength and frequency, similar to sound waves.

Nearly two centuries years later, the Scottish physicist James Clerk Maxwell provided one portion of the answer, when he understood that light was composed of fluctuating electric and magnetic fields. It was only in the 20^th century through Einstein’s work, that light was finally understood to be composed of fundamental particles known as photons, which act like both waves and particles.

This finding helped stimulate the new science of quantum mechanics, which defines the behavior of energy and matter on the atomic and subatomic level.

More recently, in the late 20^th century, physicists started exploring an occurrence termed as emergence. Just as the actions of large groups of people can vary from that of any single member of the group, emergence describes how particles in large groups can act in unanticipated ways, exposing new laws of physics or providing a new perspective for old ones. One question being probed was, “Could there be such a thing as emergent light?”

The recent work of OIST Professor Nic Shannon, Han Yan, a PhD student in his Theory of Quantum Matter Unit, and their colleagues in Switzerland and in the US centers on a strange group of magnetic systems called spin ice, which escape all conservative forms of magnetic order and instead offers a peek into the quantum world.

In conventional magnets such as the ones on a fridge, magnetic atoms create a minute magnetic field and work together to produce the much larger magnetic fields which allow them to “stick” to metal objects. This is possible because the minute magnetic fields related to each different atom in the magnet arrange themselves such that they point in the same direction.

In spin ice, nonetheless, atoms do not order magnetically, but still function together to create a magnetic field which wavers on the atomic scale.

In recent times, scientists have comprehended that quantum effects at low temperatures can add an emergent electric field in spin ice, with an astonishing consequence: Emergent electric and magnetic fields integrate to yield magnetic excitations that act precisely like photons of light.

“It behaves like light, but you can’t see it with your eyes,” said Prof Shannon “Imagine the crystal of spin ice is a tiny universe with its own laws of nature, and you are on the outside looking in. How could you figure out what is going on inside.”

In 2012 Prof. Shannon and his then PhD student Owen Benton suggested a method to detect the light within a quantum spin ice by bouncing neutrons off the magnetic atoms within the crystal. They predicted a typical signature in how the crystal absorbs the energy of the neutrons, which indicates the existence of the emergent electrodynamics of a quantum spin ice.

Now, in a paper reported in Nature Physics, the authors state that they have detected this signature in a material known as praseodymium hafnate (Pr₂Hf₂O₇).

Locating the signatures of emergent light in a real material turned out to be very difficult, as it required working at temperatures as low as 50 mK—less than a tenth of a degree above absolute zero—with crystals free of any dirt and flaws.

A study team led by Dr. Romain Sibille from the Paul Scherrer Institut (PSI) in Switzerland, in partnership with colleagues at the University of Warwick in the UK, managed to produce a flawless crystal of a quantum spin ice material with which they could at last test the hypothesis.

“It’s very beautiful, like a precious stone,” said Prof Shannon, “and it’s amazing to think it’s all one big crystal with no imperfections.”

This crystal was taken Sibille to the European Institut Laue-Langevin (ILL) in Grenoble, France, as well as the Oak Ridge National Laboratory (ORNL) in Tennessee, USA, to use these facilities’ specially-developed neutron spectrometers.

In a very challenging experiment, Sibille’s team used an array of 960 supermirrors coated with cobalt, iron, and vanadium alloys that could selectively reflect various types of neutrons— something that his home institution PSI has formed, and used the HYSPEC instrument (ORNL) to get a 3D analysis of their reflection patterns.

Along with a detailed mapping of the scattered neutrons using the IN5 instrument (ILL), this permitted them to measure the polarization of the scattered particles and map the energy signatures those particles generated.

Dr Benton and Prof Shannon’s theory bore a strange similarity to the experimental energy maps. The graphical representation of neutron reflection showed so-called pinch points, which are typical features of a quantum spin ice. When the spin ice was scanned at low temperatures, the pinch points vanished in a way that strongly indicated emergent light.

Yan worked on the theory and examined the experimental data to establish the speed of the emergent light—a modest 3.6 m per second, about as fast as a person running a marathon in four hours. The photons of normal light—the kind one might sunbathe under—could cover the same distance within a thousandth of a second.

“To me it’s very cool that this material behaves like a mini-universe with its own light and charged particles” said Han.

“At present, we don’t know any way of explaining these results without invoking quantum mechanics,” said Prof Shannon, “so it really does look like we have seen emergent light.”