Evidence mounts for quantum criticality theory

January 30, 2015
A 'Fermi surface' is kind of three-dimensional map representing the collective energy states of electrons in a material. These computer-generated illustrations show how the Fermi surface for CeRhIn5 changes, depending upon whether the electrons are strongly interacting (left) or weakly interacting (right). Credit: Q. Si/Rice University and J.X. Zhu/Los Alamos National Laboratory

A new study by a team of physicists at Rice University, Zhejiang University, Los Alamos National Laboratory, Florida State University and the Max Planck Institute adds to the growing body of evidence supporting a theory that strange electronic behaviors—including high-temperature superconductivity and heavy fermion physics—arise from quantum fluctuations of strongly correlated electrons.

The study, which appeared in the Jan. 20 issue of Proceedings of the National Academy of Sciences, describes results from a series of experiments on a layered composite of cerium, rhodium and indium. The experiments tested, for the first time, a prediction from a theory about the origins of quantum criticality that was published by Rice physicist Qimiao Si and colleagues in 2001.

"Our theory was a surprise at the time because it broke with the textbook framework and suggested that a broad range of phenomena—including high-temperature superconductivity—can only be explained in terms of the collective behavior of strongly correlated electrons rather than by the more familiar theory based on essentially decoupled electrons," said Si, a co-corresponding author on the new study and Rice's Harry C. and Olga K. Wiess Professor of Physics and Astronomy.

Experimental evidence in support of the theory has mounted over the past decade, and the PNAS study fills yet another gap. In the experiments, researchers probed high-quality samples of a heavy-fermion material known as CeRhIn5.

Heavy fermion materials like CeRhIn5 are prototype systems for quantum criticality. In these materials, electrons tend to act in unison, and even one electron moving through the system causes widespread effects. This "correlated electron" behavior is very different from the electron interactions in a common metal like copper, and physicists have become increasingly convinced that correlated electron behavior plays an important role in phenomena like superconductivity and quantum criticality.

Quantum critical points, near which these strange correlated effects are particularly pronounced, mark a smooth phase change, or transition from one state of matter to another. Just as the melting of ice involves a transition from a solid to a liquid state, the electronic state of quantum materials changes when the material is cooled to a quantum critical point.

The critical temperature of a material can be raised or lowered if the material is chemically altered, placed under high pressure or put into a strong magnet. In the new experiments, which were carried out using the high magnetic field facilities at Los Alamos National Laboratory in New Mexico and at Florida State University, researchers observed a magnetically induced quantum critical point at ambient pressure and compared it to the previously studied case of a pressure-induced quantum critical point.

Qimiao Si. Credit: Jeff Fitlow/Rice University

The nature of the quantum critical point was probed by something called the "Fermi surface," a sort of three-dimensional map that represents the collective energy states of all electrons in the material. When physicists have previously attempted to describe quantum phase transitions using traditional theories, equations dictate that the Fermi surface must change smoothly and gradually as the material passes through the critical point. In that case, most of the electrons on the Fermi surface are still weakly coupled to each other.

In contrast, Si's theory predicts that the Fermi surface undergoes a radical and instantaneous shift at the critical point. The electrons on the entire Fermi surface become strongly coupled, thereby giving rise to the strange-metal properties that allow unusual electronic states, including superconductivity.

"We observed exactly the sort of a sharp Fermi surface reconstruction predicted by theory of unconventional quantum criticality," said study co-author Frank Steglich, director of the Max Planck Institute for Chemical Physics of Solids in Dresden, Germany, and also of the Center for Correlated Matter at Zhejiang University in Hangzhou, China.

Zhejiang physicist Huiqiu Yuan, co-corresponding author on the study, said, "Our experiments demonstrate that direct measurements of a Fermi surface can distinguish theoretically proposed models of and point to a universal description of ."

Heavy-fermion metals and high-temperature superconductors are examples of quantum matter, and the new research is an example of the pathbreaking, collaborative research that Rice hopes to foster with the new Rice Center for Quantum Materials.

Si, who also directs the new center, said, "Our study exemplifies the kind of progress in that can be made through collaborations among theory, materials synthesis and spectroscopic measurements. At the Rice Center for Quantum Materials, we seek to foster this type of synergy, both internally at Rice University and through collaborations with our domestic and international partner institutions."

Explore further: Quantum criticality observed in new class of materials

More information: Proceedings of the National Academy of Sciences, www.pnas.org/content/112/3/673.full

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3.3 / 5 (10) Jan 30, 2015
Hmmmm...Quantum materials. Notice they are 'rare earth' elements in combinations. Now 'rare earths' are indeed rare on our planet. Price tag for getting them is subservience to either the Chinese Communists or the Taliban or the Islamic State.

There IS, however, a MUCH larger source for these.....the asteroid belt. Many if not most asteroids and other system bodies are rich in Iridium, for example. Other needed elements will plausibly be found with them. Where we find a lot of Iridium today is from a meteorstrike at Chixculub in Yucatan that crashed and exploded, distributing and iridium layer in rock strata all over the world.

The future of space exploration and large energy distribution carriers here is superconductivity, and this requires the high Tc materials made from rare earths. So we will have to go to space now to mine them. The quicker the better before we are all forced to either bow down to the red Chinese or bow and sssscrape to the radical moslems.
Jan 30, 2015
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4.7 / 5 (3) Jan 30, 2015
I agree: strongly correlated electrons give rise to the strange electronic behaviors, including high temp superconductivity. I have many Physorg posts on this topic. They draw on Art Winfree's theory of coupled periodic oscillators.
Jan 31, 2015
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1 / 5 (3) Jan 31, 2015
"QUANTUM": I am growing tired of that word it means everything, and yet it means nothing. Just use the word, and everyone will believe you are a genius.
Whydening Gyre
4.3 / 5 (6) Jan 31, 2015
Ol' "movementis.." is startin' off pretty quick in this thread.
My only advice to him is -
slowdown on the caffeine or whatever other amphetamine you might be on...
He's a perfect posterboy for a "This is your brain on drugs" commercial..
Whydening Gyre
4.2 / 5 (5) Jan 31, 2015
"QUANTUM": I am growing tired of that word it means everything, and yet it means nothing. Just use the word, and everyone will believe you are a genius.

I use it all the time. I don't think anybody makes that mistake with me...
Jan 31, 2015
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Jan 31, 2015
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1 / 5 (1) Feb 02, 2015
Without paying 10 bucks for access to the paper it is hard to comment on this article which is very sparse on what they did and filled with a whole lot of silly jargon about how great their theory is. It would be nice if you said what they did and how they did it rather than essentially run an ad campaign saying how great the researchers were. Apparently they simply cooled the material down to the critical point and than used a high strength magnet to vary the critical point. To measure the change in state they must have measured in some way the change as they changed the magnetic field. It boils down to the change is pretty rapid and they assign it to a phase change with a lot of jargon. That tells you naught of how to make a room temperature superconductor but there is always another grant.
Feb 02, 2015
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