Physicists solve low-temperature magnetic mystery: Mechanism of the Kondo Effect unlocked

March 24, 2015, University of Connecticut
From Left to Right: Jason Hancock, Diego Casa, and Jung-ho Kim, shown with one of the instruments used in the experiment. Credit: Argonne National Laboratory

Researchers have made an experimental breakthrough in explaining a rare property of an exotic magnetic material, potentially opening a path to a host of new technologies. From information storage to magnetic refrigeration, many of tomorrow's most promising innovations rely on sophisticated magnetic materials, and this discovery opens the door to harnessing the physics that governs those materials.

The work, led by Brookhaven National Laboratory physicist Ignace Jarrige, and University of Connecticut professor Jason Hancock, together with collaborators at the Argonne National Laboratory and in Japan, marks a major advance in the search for practical materials that will enable several types of next-generation technology. A paper describing the team's results was published this week in the journal Physical Review Letters.

The work is related to the Kondo Effect, a physical phenomenon that explains how magnetic impurities affect the electrical resistance of materials. The researchers were looking at a material called ytterbium-indium-copper-four (usually written using its chemical formula: YbInCu4).

YbInCu4 has long been known to undergo a unique transition as a result of changing temperature. Below a certain temperature, the material's magnetism disappears, while above that temperature, it is strongly magnetic. This transition, which has puzzled physicists for decades, has recently revealed its secret. "We detected a gap in the electronic spectrum, similar to that found in semiconductors like silicon, whose energy shift at the transition causes the Kondo Effect to strengthen sharply," said Jarrige.

Electronic energy gaps define how electrons move (or don't move) within the material, and are the critical component in understanding the electrical and magnetic properties of materials. "Our discovery goes to show that tailored semiconductor gaps can be used as a convenient knob to finely control the Kondo Effect and hence magnetism in technological materials," said Jarrige.

Ignace Jarrige, shown with the sample used in the experiment. Credit: Brookhaven National Laboratory

To uncover the energy gap, the team used a process called Resonant Inelastic X-Ray Scattering (RIXS), a new experimental technique that is made possible by an intense X-ray beam produced at a synchrotron operated by the Department of Energy and located at Argonne National Laboratory outside of Chicago. By placing materials in the focused X-ray beam and sensitively measuring and analyzing how the X-rays are scattered, the team was able to uncover elusive properties such as the energy gap and connect them to the enigmatic magnetic behavior.

The new physics identified through this work suggest a roadmap to the development of materials with strong "magnetocaloric" properties, the tendency of a material to change temperature in the presence of a magnetic field. "The Kondo Effect in YbInCu4 turns on at a very low temperature of 42 Kelvin (-384F)," said Hancock, "but we now understand why it happens, which suggests that it could happen in other materials near room temperature." If that material is discovered, according to Hancock, it would revolutionize cooling technology.

Household use of air conditioners in the US accounts for over $11 billion in energy costs and releases 100 million tons of carbon dioxide annually. Use of the magnetocaloric effect for as an alternative to the mechanical fans and pumps in widespread use today could significantly reduce those numbers.

In addition to its potential applications to technology, the work has advanced the state of the art in research. "The RIXS technique we have developed can be applied in other areas of basic energy science," said Hancock, noting that the development is very timely, and that it may be useful in the search for "topological Kondo insulators," which have been predicted in theory, but have yet to be discovered.

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9 comments

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Danknstein
5 / 5 (1) Mar 25, 2015
Yahoo needs to have more pieces like this
Dethe
3 / 5 (2) Mar 25, 2015
So, which is the mechanism behind Kondo effect?
diracolyte
not rated yet Mar 26, 2015
Hi Dethe - The Kondo effect can be defined either as the screening of a localized magnetic moment by delocalized electrons from the metallic host (seen in the magnetic susceptibility), or alternatively as the scattering of delocalized by localized electrons seen in resistance (electrical susceptibility) for the limiting case of isolated impurities supporting the localized electrons. Briefly, the Kondo effect in this limit was discovered in the 30s, the mechanism has been well established since the 50s, but the theoretical solution awaited Nobel-winning theoretical progress in the 70s wit the advent of renormalization group theory by the late Ken Wilson. The current work regards a lattice of localized electrons, so is more difficult to solve, but the single impurity concepts are useful.
diracolyte
not rated yet Mar 26, 2015
A point about the single impurity case is that each system has a single energy scale TK associated with the Kondo effect, called the Kondo temperature TK. YbInCu4 is amazing and unusual in that it changes Kondo effect in a 1st order phase transition (like evaporating water), where TK is tiny above the valence temperature of 42 K and huge below it. This is very exotic behavior, even if you understand the single impurity case, which is a rather advanced case of many-body quantum physics. In the single-impurity case, the density of states at the Fermi level is important, and the point of the paper is that there is a rather drastic change of this quantity with a tiny shift in the energy scales due to a peculiarity in this material. The underpinnings of this sensitivity is the presence of an energy gap in the electronic spectrum, which sets up a competition between Kondo screening influences and Fermi pressure which drives the phase transition in this material.
diracolyte
not rated yet Mar 26, 2015
The authors had to develop a new experimental technique to see this gap, since direct optical absorption didn't work in this case. The technique of RIXS is a new one could be useful for making discoveries of similar importance in other materials.
In summary, this work contributes significantly to understanding of extensions of the Kondo effect to the case of a lattice of localized moments and could impact other areas significantly.
KBK
not rated yet Mar 27, 2015
.......edit......
diracolyte
not rated yet Mar 27, 2015
KBK, really? Tell me more?
KBK
not rated yet Mar 27, 2015
sorry, edited. You'll have to look up the lectures of david hudson, and data on his experiments. The new science is the place where people are not looking, not where they've been.

It's not in the textbooks and is probably not indicated by them either. This is the history of science, and it's reality, no matter what others may insist, be they fool or gatekeeper.

It is part of a transition mode of an assembled nano material. Built in a very near room temperature situation. Over months, it is tickled into existence. check on the book by robert e. cox as well.
KBK
not rated yet Mar 27, 2015
check youtube, there is a nice 131 minute video (you'll know it when you see it) that will give you verifiable data, with scientific data, solid data on methodology, etc. the works.

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