Posted in | Quantum Physics

Mysterious Activities in YbMgGaO4 Crystal Probably Relate to Quantum Spin Liquids

The tiny YbMgGaO4 crystal, here perched on a stand for testing, appears to be the next extremely rare material to produce an equally as rare observable quantum spin liquid. Credit: Georgia Tech / Martin Mourigal

Physicist Martin Mourigal has discovered intense signs of “spooky” action, and a great deal of it, inside a new exotic crystal. If validated over time, the outcomes of his research would mean that this crystal type is a rare new material with the ability to produce a quantum spin liquid.

At present, very few materials are known to possibly possess such properties. Only a year ago was the new crystal synthesized for the maiden time. Validation of experimental data, recently produced by Mourigal, by other physicists can even take 10 years or more.

Confused? Meet quantum physics

Many people may be puzzled by the concept of a “liquid” observed inside a solid object.

This concept is regarding quantum materials, a portion of the twilight zone known as quantum physics, which researchers have been striving hard to grasp, more than 100 years, only a nanometer at a time. While many concepts related to quantum physics are still undiscovered, it outlines the unrevealed reality of matter.

Quantum physics is employed to engineer computers, superconductors, smart phones, and MRI machines. However, the laws of quantum physics in relation to the atomic realm go against human understanding of what is real, and certain laws sound so irrational that they have turned into popular science brain teasers.

‘Spooky’ entanglement bonanza

Talking about quantum entanglement—the center point of Mourigal’s analysis of the obvious spin liquid in the crystal—when two particles, e.g. electrons, get entangled, they can be closely linked to each other even if they are physically separated by many miles. Hence actions applied to one of the particles instantaneously affect the other.

Initially, this theory was felt to be too strange even by Albert Einstein, the father of relativity, who mocked it as “spooky action at a distance.”

Although entanglement has ever since been demonstrated in various experiments, researchers like Mourigal, who is an experimental physicist at the Georgia Institute of Technology, and his group have developed the concept ever more. The synthetic crystal he has investigated—an ytterbium (Yb) compound bearing the formula YbMgGaO4—is probably overflowing with detectable ‘spooky’ connections.

Imagine a state of matter where this entanglement doesn’t involve two electrons but involves, three, five, 10 or 10 billion particles all in the same system. You can create a very, very exotic state of matter based on the fact that all these particles are entangled with each other. There are no individual particles anymore, but one huge electron ensemble acting collectively.

Martin Mourigal, experimental physicist at the Georgia Institute of Technology

Mourigal, Joseph Paddison (a former postdoctoral fellow), and Marcus Daum (a graduate student) reported their findings in the Nature Physics journal on 5 December 2016. They worked along with collaborators from Oak Ridge National Laboratory and the University of Tennessee. The U.S. Department of Energy and the National Science Foundation funded the research.

Quantum computing dreams

The immense “spooky” entanglement transforms a system of electrons into quantum spin “liquid.” The term must not to be taken in the everyday sense, such as in water. In contrast, it outlines the collective nature of spins of the electrons in the crystal.

In a spin ‘liquid,’ the directions of the spins are not tidily aligned, but frenzied, although the spins are interconnected, whereas in a spin ‘solid’ the spin directions have a neat organization.

Martin Mourigal, experimental physicist at the Georgia Institute of Technology

In case the findings of the research are proven to be valid, it could lead to unearthing of quantum spin liquid materials which are still unknown and which physicists propose to exist according to mathematical equations and theory. In the long run, with respect to current standards, new quantum materials can become veritable sorcerer’s stones in the hands of quantum computing engineers.

Beijing’s ytterbium crystal success?

Scientists from China were the first to synthesize the Yb crystal a year ago. The Beijing government has made large investments with the anticipation of producing synthetic quantum materials possessing novel properties. Mourigal, an assistant professor at Georgia Tech’s School of Physics, stated that they may have now succeeded.

The only evident quantum spin liquids detected earlier occurred in a natural crystal known as herbertsmithite, which was an emerald green stone unearthed from a mine in Chile in the year 1972. It gets its name from the mineralogist Herbert Smith, who passed away nearly 20 years before to the discovery.

The evident spin liquid nature of the crystal was detected by scientists from the Massachusetts Institute of Technology, who were triumphant in reproducing a purified piece of the crystal in 2012 in their lab.

Encyclopedia of spin liquids

That maiden discovery was only the start of an Odyssey. The chemical composition of herbertsmithite leads to only a single entanglement scheme. However, according to physics math, there might be innumerable schemes.

“Finding herbertsmithite was like saying, ‘animals exist.’ But there are so many different species of animals, or mammals, or fish, reptiles and birds,” Mourigal stated. “Now that we have found one, we are looking for different kinds of spin liquids.”

The more spin liquids confirmed by experimental physicists, the more theoretical physicists can use them to unearth mysteries related to quantum physics. “It’s important to create the encyclopedia of them,” Mourigal explained. “This new crystal may be only our second or third entry.”

What neutron scattering revealed

Physicists at the University of Tennessee victoriously replicated the original Yb crystal. Mourigal investigated it at the Oak Ridge National Laboratory (ORNL), by cooling it down to a temperature of 273.09 C, or 0.06 K.

Due to the cooling, the natural motion of the atoms was slowed down to a near stop, thus enabling the physicists to detect the dance of the electron spins around the Yb atoms in the YbMgGaO4 crystal. By using a powerful superconducting magnet they lined the spins up in an orderly manner to form a starting point for their analysis.

“Then we removed the magnetic field, and let them go back to their special kind of wiggling,” Mourigal explained. His group performed the analysis at the ORNL Spallation Neutron Source (SNS)—a Department of Energy Office of Science User Facility. With the power and size of a particle supercollider, the SNS enabled the researchers to observe the dance of the electron spins by bombarding them using neutrons.

In general, if one electron flips its spin, physicists will anticipate it to form a neat chain reaction that makes a wave to pass through the crystal. The wave caused by the sequential flipping of electron spins may be similar to fans at a football game standing and sitting back down to make a wave go around the stadium.

However, there was a strange occurrence. “This jumbly kind of spin wave broke down into many other waves, because everything is collective, everything is entangled,” explained Mourigal. “It was a continuum of excitations, but breaking down across many electrons at once.”

The resulting phenomenon was qualitatively similar to what was detected by applying the same technique to herbertsmithite.

Nobel Prize topology donut

Theoretical physicists working towards the authentication of the findings of Mourigal and his team might have to crunch the data using techniques that are partly dependent on topology, which was the center of interest of the 2016 Nobel Prize in Physics. Mourigal hopes that the results will be satisfactory. “At first glance, this material is screaming, ‘I’m a quantum spin liquid’,” he stated.

However, it must go through a years-long array of rigorous mathematical tests. The theoretical physicists are expected to wrap the data around a mathematical “donut” to make sure whether or not it is a quantum spin liquid.

That’s meant seriously. As a mathematical mental exercise, they virtually spread the spin liquid around a donut shape, and the way it responds to being on a donut tells you something about the nature of that spin liquid.

Martin Mourigal, experimental physicist at the Georgia Institute of Technology

Despite the fact that entangled particles appear to defy space and time, the shape of space occupied by them has an impact on the nature of the entanglement pattern.

The probability of a quantum spin liquid was first indicated in the 1930s, though only involving atoms positioned in a straight line. From then, physicists have been looking for materials containing such quantum spin liquids.

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