New Antiferromagnet May Help Develop and Monitor Key Materials

Superconductors are materials that conduct electricity without resistance at very low temperatures. From the moment when superconductors were discovered by Physicists, they in fact wondered whether it could be possible to produce materials that are capable of exhibiting the same properties at warmer temperatures.

Physics Professor Markus Grainer (in back) and his team: Anton Mazurenko (from left), senior grad student, Daniel Greif, postdoc fellow, and Geoffrey Ji, graduate student, have created an antiferromagnet, an exotic form of matter that can give important insights into how room temperature superconductors might be created. Rose Lincoln/Harvard Staff Photographer

According to a group of Harvard Scientists, the key to doing so could exist in another exotic material called antiferromagnet.

A Team of Physicists, headed by Physics Professor Markus Greiner, have taken a vital step toward understanding those materials by developing a quantum antiferromagnet from an ultracold gas of hundreds of lithium atoms. The research has been explained in a paper published on May 25th in the journal Nature.

We have created a model system for real materials … and now, for the first time, we can study this model system in a regime where classical computers get to their limit. Now, we can poke and prod our antiferromagnet. It’s a beautifully tunable system, and we can even freeze time to take a snapshot of where the atoms are. That’s something you won’t be able to do with an actual solid.

Professor Markus Greiner, Harvard University

But what, exactly, is an antiferromagnet?

Standard magnets, the ones that can be stuck to a refrigerator, work because of the fact that the electron spins in the material are aligned, permitting them to work in unison. However, those spins are arranged in a checkerboard pattern in an antiferromagnet. One spin could be pointing south, while the next pointing north, and so on.

Greiner and Physics Professor Eugene Demler emphasized the need to understand antiferromagnets because experimental work has suggested that the unusual state may be a precursor to high-temperature superconductivity in extremely promising high-temperature superconductors, which refer to a class of copper-containing compounds called cuprates.

Demler said that presently the best cuprates display superconductivity at almost minus 160 degrees Fahrenheit, which is cold based on everyday standards, but considered to be far higher than any other type of superconductor. That temperature has adequate warmness such that it can allow practical applications of cuprate superconductors in transportation, telecommunications and in the production and transmission of electric power.

This antiferromagnet stage is a crucial stepping-stone for understanding superconductors. Understanding the physics of these doped antiferromagnets may be the key to high-temperature superconductivity.

Professor Eugene Demler, Harvard University

Greiner and his team built one by trapping a cloud of lithium atoms in a vacuum and then using a technique they dubbed, “Entropy Redistribution” in order to cool them to just 10 billionths of a degree above absolute zero, which indeed permitted them to observe the strange physics of antiferromagnets.

“We have full control over every atom in our experiment,” said Daniel Greif, the Postdoctoral Fellow working in Greiner’s lab. “We use this control to implement a new cooling scheme, which allows us to reach the lowest temperatures so far in such systems.”

Greiner and his team were able to photograph the system with sufficient detail in order to identify and extract information about individual atoms with the help of that degree of control. It is also possible now for the team to alter the atomic density of the antiferromagnet in order to search for a superconducting state.

The system is not just a model, but is also a special-purpose quantum computer capable of simulating the complex physics of antiferromagnets and how their change into superconductors can work.

Even though it is possible for Scientists to simulate the quantum properties of simple atoms and also relatively simple materials, compounds that are much more exotic like cuprates are simply too complicated to be accurately modeled by standard computers, and many in the field consider that quantum computers could be the answer.

Many people expect that the first field where quantum computers will make a major impact is in quantum simulation. If Scientists want to test the airflow and other flight characteristics of an airplane, they would build a wind tunnel to test that. This is, essentially, a quantum wind tunnel for real materials.So what we have done in the past is to come up with what we think are simple models. The truth is we still cannot solve those models. The end result is that our predictions disagree with experimental results, but we don’t know if our model was incorrect or if we didn’t compute it correctly. With this system, we know exactly which model describes it. And now … if we make a prediction, they can tell us if it is accurate.

Professor Eugene Demler, Harvard University

Demler stated that, even though the system may one day play a role in designing a new series of superconductors, its ultimate significance may depend on helping Researchers develop a foundation of knowledge for Materials Science.

“The problem in trying to come up with better superconductors is that if you take a material and change one parameter … lots of things are changing,” Demler said. “With this simulation, we have full control of parameters. So we can actually understand what helps and what suppresses superconductivity, and then we can become wiser in terms of choosing elements” to investigate.

The Air Force Office of Scientific Research, Army Research Office, the Gordon and Betty Moore Foundation EPiQS Initiative, the Harvard Quantum Optics Center, the National Science Foundation and the Swiss National Science Foundation supported the research.

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