If you adda bit of impurities to certain materials, at low temperatures of a few Kelvin their resistivity actually starts increasing if you go below a certain critical temperature.
The Kondo effect is a phenomenon observed in condensed matter physics, specifically in systems that include magnetic impurities within a metal or semiconductor. Named after Japanese physicist Jun Kondo, who first described the effect in 1964, it relates to the behavior of conduction electrons in the presence of localized magnetic moments, such as those caused by impurity atoms.
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