Mar 12 2020
For many years, physicists have been attempting to view the Kondo cloud, a quantum phenomenon.
This is a schematic illustration of the Kondo cloud detection. Image Credit: Jeongmin Shim.
An international team of researchers, which also included a researcher from City University of Hong Kong (CityU), has now designed an innovative device that effectively measures the length of the Kondo cloud and also helps in managing the Kondo cloud.
This latest discovery can be considered as a turning point in the field of condensed matter physics, and can offer insights for interpreting the numerous impurity systems, like high-temperature superconductors.
Assistant Professor, Dr Ivan Valerievich Borzenets, from the Department of Physics at CityU achieved this breakthrough in association with researchers from Korea, Japan, and Germany. The team’s study findings were published in the new issue of Nature, a highly prominent scientific journal.
What is the Kondo Cloud?
The physical phenomenon called the Kondo effect was identified in the 1930s. Metals’ electrical resistance often decreases with decreasing temperatures. But if the metal contains some magnetic impurities, the result will be the exact opposite. Resistance will reduce initially; however, when it is below a certain threshold temperature, the resistance will rise as the temperature reduces further.
Japanese theoretical physicist Jun Kondo successfully solved this puzzle that existed for more than five decades ago; hence the effect was named after him. According to him, when a magnetic atom, or an impurity, is located within a metal, it exhibits a spin. However, instead of simply binding with a single electron to create two spin-down and spin-up, it binds collectively with the entire electrons present inside certain areas around it and forms a cloud of electrons that surround the impurity. This phenomenon is called the Kondo cloud.
Hence, when a voltage is applied over the Kondo cloud, the electrons are screened off by the Kondo cloud or cannot move, and this results in increased resistance.
How Big is the Cloud?
Certain rudimentary properties of the Kondo effect have been experimentally demonstrated and were found to be associated with the Kondo temperature (that is, the threshold temperature where the resistance begins to increase at low temperatures). But the measurement of the length of the Kondo cloud has not been achieved yet. At the theoretical level, the Kondo cloud can spread out across several micrometers from the impurity present in semiconductors.
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.
Dr Ivan Valerievich Borzenets, Assistant Professor, Department of Physics, City University of Hong Kong
Dr Borzenets carried out the experimental measurement in this study. He is a scientist who enjoys performing difficult experiments and hence decided to take up this challenge.
Isolating a Single Kondo Cloud in the Device
The improvements in nanotechnology allowed the researchers to develop a device that can restrict an unpaired electron spin, or magnetic impurity, in a quantum dot, similar to a tiny conducting island that has a diameter of just a few hundred nanometers.
“Since the quantum dot is very small, you can know exactly where the impurity is,” added Dr Borzenets.
Linking to the quantum dot is a long and one-dimensional (1D) channel. The unpaired electron is forced to bind with the electrons present in this channel and create 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.
Dr Ivan Valerievich Borzenets, Assistant Professor, Department of Physics, City University of Hong Kong
The system’s novelty is that when a voltage is applied at different points within the channel with numerous distances away from the quantum dot, they would induce “weak barriers” along the channel. Scientists later visualized the ensuing change in electron flow as well as the Kondo effect with different barrier position and strength.
The Secret Lies in the Oscillation Amplitude
When the voltages were changed, it was observed that the conductance increased and decreased, regardless of where the barriers are placed. Oscillations in Kondo temperature were also observed when there were oscillations in conductance.
When the oscillation amplitude of Kondo temperature was plotted against the barrier distance from the impurity split by the theoretical length of the Kondo cloud, the scientists discovered that all their data points fall onto one curve, as theoretically predicted.
We have experimentally confirmed the original theoretical result of the Kondo cloud length which is in micrometre scale. 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.
Dr Ivan Valerievich Borzenets, Assistant Professor, Department of Physics, City University of Hong Kong
Provide Insights into Multiple Impurity Systems
The scientists spent nearly three years in this study. Their subsequent step is to analyze different methods to manage the Kondo state.
“Many other manipulations on the device can be done. 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,” concluded Dr Borzenets.
The corresponding authors of the study are Dr Borzenets, Professor Sim Heung-Sun from Korea Advanced Institute of Science and Technology (KAIST), and Dr Michihisa Yamamoto of RIKEN Center for Emergent Matter Science (CEMS) in Japan. The co-first authors of the study are Dr Borzenets and Dr Shim Jeongmin from KAIST.
Other co-authors of the study included Jason Chen C. H. from the University of Tokyo, Professor Dr Andreas D. Wieck, and Dr Arne Ludwig from Rurh-University Bochum, and also Professor Seigo Tarucha from RIKEN CEMS.
The research was supported by Hong Kong Research Grants Council, Grants-in-Aid for Scientific Research (KAKENHI), CityU, the National Research Foundation of Korea, Japan Science and Technology Agency, the Federal Ministry of Education and Research (BMBF), and Deutsche Forschungsgemeinschaft.