The Kondo Effect

 by Sarah Laubacher

KONDO Q & A

Some people see science as a game of question and answer. If you wonder about how the world works, it’s easy to develop the questions. If there were only one way of studying the world, it would be easy to find answers. But the world of science isn’t that simple. Sometimes, scientists examine the same problem over and over and develop more questions than answers.  “The Kondo Effect” is one topic that researchers have been mulling over for years – it was observed in the 30s, named in the 60s, and is currently being examined by the tools of nanotechnology. 

So what is the Kondo effect? It’s a phenomenon observed when an electrical current is passed through a metal alloy. An alloy is any material whose crystalline structure is infiltrated by another type of atom (or atoms.) In Kondo’s case, the alloy involved is a “diluted alloy,” which means the extraneous atoms are few and far between on the crystalline structure. In order for Kondo effect to be observed, these extraneous atoms can’t be any old atoms – they must be magnetic. Scientists have a pretty good grasp on how a current flows through a normal piece of metal, but when this magnetic impurity is present, whether it is iron, cobalt, or chromium, an electric current behaves in a mysterious manner.

Temperature is another variable in this experiment, and that’s where things get complicated. Scientific lingo says: When passing a current through a nonmagnetic metal, resistivity decreases when temperature decreases. When a magnetic impurity is present, resistivity decreases when temperature decreases, until a certain point, and then resistivity increases. This description of the Kondo effect might not make sense to you if you don’t know what “resistivity” means, or if you don’t understand the effects of a magnetic impurity.

PRETEND

Pretend you are a scientist who does understand these terms. You just finished some equations and graphed your findings, and now your colleagues want an explanation. “Why did the pattern of resistivity change its tune?” they ask. A good nano-scientist would have pondered and visualized until a nice narrative was prepared. Since you aren’t a good nano-scientist yet, I will offer another type of narrative – one that uses intuitive pictures to explain a foggy concept. If we personify the elements contributing to Kondo, the phenomenon is clarified. Some scientific precision is lost in translation, but once you understand the intuitive picture, you will be on your way to understanding the more correct, quantum version of the story.

Maybe you’ve never met an electron, but I can assure that they are charismatic little characters. (Link to: “Electrons: The Life of the Party.”) They mingle with the other subatomic particles (protons and neutrons), but electrons are the ones who really know how to party. Sometimes they throw dance parties, and one style of dance they really enjoy is the “Kondo” dance.

PARTY

Let’s say a group of electrons are going to a party. The dance room (the metal) is in between the front door and the kitchen, but once they enter the party they attempt a beeline to the kitchen for refreshments. Before the Kondo dancer arrives, this trip goes more smoothly. If the temperature of the dance room is down, the electrons make it through quickly because there aren’t a lot of party guests (atoms) dancing yet. When things heat up and electrons start to bounce around and party, the arriving electrons experience more trouble (resistivity) as they buzz through the crowd.

The story changes when the Kondo dancer arrives, (when the magnetic impurity is plopped onto the metal.) The Kondo dancer positions herself in the middle of the dance floor and starts spinning – she can start off spinning either up, or down. (Link to Spin article.) Her manner of dancing is very interesting and strangely magnetic to the subatomic partiers. When an electron (current) enters the room, it definitely notices this temptress, (magnetic impurity.) The first electron is most attracted to her spin-up style, and approaches her to dance. He had been spinning down before, but when he gets close to the Kondo dancer, he flips his spin from down to up. The urge to flip is mutual, and the Kondo dancer flips her spin from up to down so she can lure in another electron. Spin patterns at the Kondo party can seem complicated, but to put it simply: all of the electron partiers possess spin – some up, some down – and they change their spin according to who’s dancing nearby.

Before long, all of the subatomic spinners are crowded around the dancer in a “Kondo cloud,” which is a very important part of these parties. The cloud-crowd takes shape when the room temperature is lowered. Their networking quiets down, and they linger there temporarily. Eventually the crowd escapes the Kondo dancer’s allure, only to be replaced by another group of hovering electrons. This “lingering” and “hovering” of the curious particles explains why resistivity increases as the temperature goes down – the curious crowd gets in the way of newcomers (the current) who are trying to cruise through to the kitchen.

REVERSE PARTY

Examining the concept in reverse might clarify the party pattern. When the temperature is increased, the Kondo effect isn’t evident, even when the Kondo dancer is spinning center stage. Before she arrived, a hot dance floor made it difficult for electrons to cruise through, but now, the heat gives them the energy they need to surpass this magnetic diva and her surrounding crowd. To review: an electrical current passing through metal usually prefers cold and gets held up by hot, but when a magnetic impurity is present, heat actually helps the current make it through, while lack of heat leads to a Kondo cloud, which obstructs the electrical current.

SO WHAT?

Why is this important? Because it’s a natural phenomenon that is slowly being understood. Nanoscience is a branch of science that unites chemistry, physics and biology with its tools of subatomic exploration. Even though the Kondo Effect interests scientists in these three fields, it is nanotechnology in particular whose powerful lens unveils answers, while developing new questions at the same time. Questions such as: What would happen if several impurities were placed close together on the metal? One exciting possibility is that arranging impurities a certain way could eliminate the Kondo effect. This means scientists could develop a material that transports a current efficiently, with magnetic order, leading to computer chips that carry more information and less heat.

Other interests include, how is the Kondo effect observed in semiconductors?  What happens when the relationship between the impurity and the lattice of atoms is altered? These questions could lead to interesting answers that shed light on the elusive natural phenomenon. The unfinished story of Kondo is one example of how fascinating nanoscience can be – it facilitates a whole new game of question and answer.