Kondo effect contributes to the abnormal scattering mechanism of conduction electrons in a metal due to magnetic defects. The electrical resistivity is low, increasing with temperature as the logarithmic temperature T is lowered log (T). It is sometimes more commonly used to describe multi-body scattering processes from impurities or ions, with low energy quantum mechanical degrees of freedom. In a more general sense, it has become a significant concept in condensed matter physics in understanding the behavior of metal systems with strongly interacting electrons.

History:

In 1934, the minimum resistance in gold was observed as a function of temperature (de Boer, de Haas, and van den Berg 1934), indicating that there must be some additional scattering mechanisms that make a significant contribution to the resistance — strength increases as a temperature decreases.

In depth Understanding Of Kondo Effect

The Kondo effect affects the electrical resistance of metals at low temperatures and generates complex magnetic and electronic commands. Novel concepts for data processing and storage, such as using quantum dots, are based on this. In 1998, US researchers published a spectroscopic study on the Kondo effect using scanning tunneling microscopy, which is considered ground-breaking and has triggered countless others. Many of these studies may now have to re-examine whether Julich researchers have shown that this method cannot prove the Kondo Effect beyond doubt.  Instead, another phenomenon is creating accurate spectroscopic “fingerprints” attributed to the Kondo effect.

The resistance of metals to temperature drops is usually reduced. The Kondo effect causes it to increase again below a threshold value for the material in question, the so-called Kondo temperature. This phenomenon occurs when foreign magnetic atoms, such as iron, form non-magnetic host metals, such as copper. Simply put, when a current flows, atomic nuclei are enclosed by electrons. Iron atoms have a quantum mechanical, magnetic moment. This causes the surrounding electrons to align their spin counterpart to the moment of the atom at low temperature and to revolve around the cobalt atom like a cloud on a mountaintop. This obstructs the flow of electrons – the electrical resistance again increases. In physics, this is known as entanglement, the strong coupling of the impurity moment with nearby electrons’ spin. This effect can be exploited, for example, in the form of quantum dots: Nanocrystals that may one day serve as minuscule information storage or processor elements.

The Kondo effect was already seen in 1934 and was fundamentally explained by the 1964 Jun Kondo. In 1998, experimental physicists made a methodological breakthrough in the study of effects. Through scanning tunneling microscopy, it was possible to position and detect individual atoms on surfaces and record energy spectra exclusively at these points. A characteristic dip in the measurement curve was found in cobalt atoms on a gold surface, which has since been considered a marker for the Kondo effect. Earlier, the Kondo effect could only be detected indirectly through resistance measurements. As a result of further investigation of other material combinations and atomic arrangements using this technique, and a separate field of research was created, which is devoted to the investigation of many-body phenomena with atomic resolution.

However, physicists at the Peter Gruenberg Institute and the Institute for Advanced Simulation Forschungszentrum Jülich have now found an alternative reason for the dip in the energy spectrum: the so-called magnetic anisotropy. Below a specific temperature, it causes the foreign atom’s magnetic moment in pairs to the crystal lattice of the host metal so that the orientation of the moment virtually freezes. Above this temperature, the magnetic moment excitation occurs due to the spin properties of the tunneling electrons of the microscope. In 1998, scientists were not able to measure this type of spin stimulation.

Researchers have been working for years to improve the theoretical model for spin stimulation. Initially, they found evidence of markers similar to those of a Kondo. Initially, however, they could not still incorporate consistently significant, so-called relativistic effects in their calculations. Once they succeeded in doing so, they took another look at the cobalt and gold system. They were now able to perform their calculations effectively with data from scanning tunneling spectroscopy studies. Both measured and calculated spectra are almost in agreement.

“This means what we thought and learned about the Kondo effect over the last two decades, and they have already found their way into textbooks, need to be analyzed again,” Prof. Samir Lounis. Head of the group. Functional Nanoscale Structure Probe and Simulation Laboratory (Funsilab). Scientists are already proposing new experiments based on their predictions.

References:

https://www.eurekalert.org/pub_releases/2021-01/fj-rqf010721.php

Bouaziz, J., Mendes Guimarães, F.S. & Lounis, S. A new view on the origin of zero-bias anomalies of Co atoms atop noble metal surfaces. Nat Commun 11, 6112 (2020). https://doi.org/10.1038/s41467-020-19746-1

Peter Grünberg Institute – Quantum Theory of Materials (PGI-1/IAS-1): https://www.fz-juelich.de/pgi/pgi-1/EN/Home/home_node.html