Elusive Kondo Cloud Seen For The First Time

An international team of scientists has observed Kondo clouds, a physical phenomenon first predicted nearly 90 years ago.

AsianScientist (Mar. 17, 2020) – Kondo clouds, a physical phenomenon first proposed in the 1930s, have been detected in the lab for the first time. The result of a decades-long search, these findings by an international team of scientists have been published in Nature.

How well a metal can conduct or resist electricity depends at least in part on the temperature. In pure metals, electrical resistance decreases as temperature decreases; however, magnetic impurities in a metal behave differently, showing reduced electrical resistance at first until the temperature drops below a certain threshold, after which the electrical resistance begins to increase.

This unusual behavior was explained by Jun Kondo, a Japanese theoretical physicist after whom the effect is named. He explained that when a magnetic atom (an impurity) is placed inside a metal, it has a spin. But instead of just coupling with one electron to form a pair like normal electrons, it couples collectively with all the electrons within some area around it, forming a cloud of electrons surrounding the impurity. When this so-called Kondo cloud has a voltage applied over it, the electrons are not free to move or are screened off by the Kondo cloud, resulting in resistance increase.

Although basic properties of the Kondo effect have been proved experimentally and found to be related to the threshold or Kondo temperature, scientists have not directly observed the Kondo cloud until now.

“The difficulty in detecting the Kondo cloud lies in the fact that measuring spin correlation in the Kondo effect requires the fast detection of tens of gigahertz. And you cannot freeze time to observe and measure each of the individual electrons,” explained Dr. Ivan Valerievich Borzenets, an assistant professor at the City University of Hong Kong who performed the experimental components of the study.

The researchers fabricated a device that can confine an unpaired electron spin (magnetic impurity) in a quantum dot, like a small conducting island with a diameter of only a few hundred nanometers. Connected to the quantum dot is a long, one-dimensional channel. The unpaired electron is thus made to couple to the electrons in this channel and form a Kondo cloud there.

“In this way, we isolate a single Kondo cloud around a single impurity, and we can control the size of the cloud as well,” said Borzenets.

By applying a voltage at different points inside the channel, the team was able to induce weak barriers along the channel. They then varied the barrier strength and position to observe how the electrons would flow in response.

The researchers observed that the the electrical conductance went up and down, regardless of where they placed the barriers. Furthermore, these oscillations in conductance matched oscillations in the measured Kondo temperature. When the researchers plotted the oscillation amplitude of Kondo temperature versus the barrier distance from the impurity divided by the theoretical cloud length, they found that all their data points fell onto a single curve, as theoretically expected.

“We have experimentally confirmed the original theoretical result of the Kondo cloud length which is in micrometer scale,” said Borzenets. “For the first time, we have proved the existence of the cloud by directly measuring the Kondo cloud length. And we found out the proportionality factor connecting the size of the Kondo cloud and Kondo temperature.”

Their next step is to investigate different ways to control the Kondo state, Borzenets said.

“Many other manipulations on the device can be done,” he said. “For example, we can use two impurities at the same time and see how they will react when the clouds overlap. We hope the findings can provide insights into the understanding of multiple impurity systems such as Kondo lattices, spin glasses and high transition-temperature superconductors.”

“It is very satisfying to have been able to obtain real space image of the Kondo cloud, as it is a real breakthrough for understanding various systems containing multiple magnetic impurities,” added Dr. Michihisa Yamamoto of the RIKEN Center for Emergent Matter Science, who led the international collaboration.

The article can be found at: Borzenets et al. (2020) Observation of the Kondo Screening Cloud.


Source: City University of Hong Kong; Photo: Shutterstock.
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