kondo effect

In a pot of boiling water, steam bubbles appear and disappear — a similar struggle takes place in two-dimensional materials. Here, electrons both create magnetism and try to extinguish it.

Two tiny magnets sit in a piece of metal. Around them is a sea of conduction electrons. Usually, electrons try to "shield" the magnetism of the impurities: they line up around them and quench it. This phenomenon is called the Kondo effect.

Quantum matter rarely treats us to the spectacle of self-motion. Where the classical world teems with starling murmurations, fish schools, and bustling bacteria, quantum thermodynamics long remained serene.

A tiny crystal of charged atoms, trapped in a trap, trembles with heat. Each ion oscillates like a ball on a spring, and these vibrations hinder using the crystal for precise quantum operations. Usually, excess energy is removed with a laser — the beam hits the ions and slows their motion, like a stream of air damping a pendulum.

A pebble falls into a pond, and ripples spread across the water. In the quantum world, the pond is not made of water but of many identical particles, and the pebble is also a particle, just a foreign one. Such a foreign particle is called a Fermi polaron.

Ordinary water freezes as expected. But sometimes hot water turns to ice faster than cold — the Mpemba effect. Recently, a similar rule-breaking was found in quantum entanglement — the invisible link between particles discovered by Erwin Schrödinger.

A superconductor is like a perfect electron dance: electrons move in pairs without losing energy, like dancers who never miss a beat. But add a few magnetic impurities, and strangers crash into the dance, disrupting the synchronized motion. The temperature at which this dance begins drops.

The magnetic field increases, but the magnetization stays put. It's like a scale where you add weight, yet the needle doesn't move until enough has accumulated to jump to the next mark. That's exactly how rare-earth tetraboride crystals behave, and physicists have been puzzled for decades.

Take a long thread, close it into a ring, and tie a knot in it. The knot slides along the thread until it hits something. Now imagine the thread is made of two different segments: one stiff and slippery, the other soft and sticky. When heated, the sticky segment softens, and the knot can slide off it, like a sled sliding off a melting hill.

The dance floor is packed with people, each moving on their own. Suddenly, some of them break into pairs (spinning around, barely noticing their neighbors) while the rest line up in chains. That's roughly how atoms behave in the metal CsCr₃Sb₅, a member of the kagome metals, whose crystal lattice resembles a woven basket.

A tiny magnet, made from just two molecules, was placed on a metal surface, and it behaved like a temperamental conductor. Normally, electrons in a metal move in a coordinated way, like a crowd on an escalator. But when cooled significantly, this order breaks down, and the motion becomes unpredictable.

A thin film of soap on your hand conforms to the shape of your palm, but clench your fingers and the tension changes unevenly. A monolayer of molybdenum disulfide on a graphite substrate behaves in a similar way.

A crystal of calcium ions in a trap, illuminated by a laser, is a tiny model of the matter inside a white dwarf. There, in the depths of a dying star, atomic nuclei arrange into a rigid lattice as the temperature drops.

The surface of the uranium compound UTe2 crystal looks like a frozen sea: in some places, electrons move freely; in others, they get stuck near atoms. A scanning tunneling microscope, a device that probes the surface with an atomically sharp tip and senses individual electrons, revealed that this landscape is not random.

A semiconductor diode normally conducts current in only one direction, like a spring-loaded door: it opens easily one way but pushes back the other. But if you dope the crystal heavily, the door starts behaving strangely: under reverse bias, the current doesn't just flow: it shoots up.

An ordinary conductor carries current equally in both directions, like a door that swings both inward and outward. A diode, by contrast, is a door with a self-closing hinge: it opens easily one way but pushes back the other.

In the macroscopic world we know, any motion inevitably decays: friction, a dissipative process that scatters energy, intervenes. But quantum reality has surprises in store.

Metals are full of free electrons: they dart between atoms like a swarm of bees around a hive. If you poke such a metal with a thin magnetic filament, the electrons start to feel its field. A new calculation shows they don't just go around the field; they trap it inside, like a cork sealing water in a bottle.

At 78 K, a crystal of Mn₃Si₂Te₆ turns into an inductor that needs neither copper wire nor a superconductor. The electrons inside it spontaneously organize into closed vortices with a definite sense of rotation: chiral orbital currents.

The temperature in a room drops by a few degrees while the refrigerator runs without a single moving compressor. Inside, there is no Freon, but a piece of metal that is alternately magnetized and demagnetized.

The dance floor where magnetic moments and conduction electrons take turns leading and following: that is the Kondo lattice in the crystal V5S8.

In a chain of eight spins, a defect lurks — a tiny magnetic moment. The Kondo effect, known since the 1930s, makes electrons around it align into a collective screen. The material's resistance abnormally increases. But detecting such an impurity hidden deep in the structure is like finding a stone at the bottom of a dark pond.

A stone falls into water, and ripples spread outward. In the quantum world, a magnetic impurity (that pebble) in a metal perturbs the surrounding electrons, and they form a special cloud around it. This is called the Kondo cloud. Like ripples on water in the dark, it long remained elusive.

A drop of ink in a glass of water quickly spreads, tinting the whole liquid. Similarly, electrons in a metal move freely, like molecules in a gas. But when a magnetic impurity, an atom with its own magnetic moment, enters this medium, something strange happens.

A crowd in a square. At first, everyone spins to their own rhythm, but suddenly the music changes, and they all start moving in sync, as one. That's roughly how electrons behave in some metals: they either cling to magnetic impurities or break away and form a collective order.