heat transfer

On the lab bench lies a piece of graphite grown from gas, like a deposit on the wall of a reactor. Usually, such material is not suitable for converting heat into electricity: inside it, electrons and holes run in opposite directions and cancel each other out.

A tiny diamond, invisible to the naked eye, sits on a chip and senses heat like no ordinary thermometer can. Hidden inside it are two defects: places where a carbon atom is replaced by a nitrogen atom, with a vacancy nearby.

A frozen orchestra where musicians swap seats when the conductor, magnetism, sets a new score. In a spinel crystal, iron and magnesium atoms sit in two types of seats: some surrounded by four neighbors, others by six. At room temperature, they barely stir, but cool the material to 5 K and a mass migration begins.

If you run a current through two soldered conductors, the junction heats up or cools down: that's the Peltier effect, which powers car refrigerators.

A laser beam passes through a glass cell filled with cesium vapor. The atoms in the vapor dart around chaotically, like a crowd at a train station: each one enters the beam, spends a moment there, and flies back out. Their internal state, something like the orientation of a tiny magnet, changes under the influence of light. If all atoms were identical and stationary, the signal would be steady.

A microchip the size of a fingernail gives off as much heat as a small light bulb. Getting rid of that heat is like trying to cool a cup of hot tea with a teaspoon: the insulating material around transistors is usually a poor heat conductor.

In a hot mug, water sometimes freezes faster than in a warm one. This paradox is over half a century old and is called the Mpemba effect. It is usually attributed to fluid motion or evaporation. Now physicists have shown that a similar trick is possible without water, just through radiation of heat.

The planet WASP-121 b is a gas giant, heated to 2500°C. It always faces its star with one side, like a globe frozen under a lamp. But as it passes in front of the star's disk, its rotation reveals different facets to us — as if that globe were slowly turned, letting us peer into every corner.

Heat a nail in a fire, and it will lengthen — everyone knows that. But heat only the tip and leave the other end cold, and the nail doesn't just get longer: it bends, because hot metal expands more than cold metal. It's this very bending, but at the atomic level, that physicists have turned into electricity.

A metal bar suddenly becomes stiff when heated, and at the same time loses or gains magnetic properties. In alloys based on nickel and manganese, such transformations occur not smoothly but abruptly — atoms rearrange into a new order, and the material changes its behavior.

A gold speck the size of a hundred atoms catches a beam of light, and heats up. In everyday life, that's no big deal, but in the world of nanotechnology, this tiny heat can ruin a biosensor or fry a microchip. You can't just put a fan on it: at these scales, heat gets trapped like in a thick winter coat.

A lithium-ion battery is like a multi-story building where lithium ions live on every floor. When you charge the battery, the ions move from floor to floor, and the whole building warms up a bit. Usually, this heat dissipates evenly, like from underfloor heating. But sometimes one spot gets too hot, and then the whole building can go up in flames.

Hundreds of people on a dance floor stand at equal distances from each other — strict order, like in a crystal lattice. Slow music is thermal vibrations, making dancers sway slightly. If the music gets louder and faster, order collapses: everyone mixes. That's how thermal motion destroys ordered states of electrons in solids.

In a thin semiconductor channel, electrons behave not like lone runners but like a crowd at rush hour. They constantly bump into each other, and these collisions determine how easily heat and electric current pass through the material.

In an insulating crystal, heat usually flows from the hot end to the cold end. But when you switch on a magnetic field, part of the heat flow veers sideways, like water running down a slope that drifts to the side when the wind pushes it. This behavior is called the thermal Hall effect.

If you push heat through a piece of quartz, it usually goes straight, like water through a pipe. But switch on a magnetic field, and the flow veers slightly sideways, as if the pipe suddenly grew a side branch.

Usually, current in a metal acts like boiling water on ice: it heats the material and destroys magnetic order. But in a layered structure of tungsten ditelluride and the ferromagnet Fe3GeTe2, current behaves differently.

Usually, to generate terahertz waves (light between microwaves and infrared), researchers build a sandwich of different metals. One layer acts as the source, another as the receiver of charges. In such designs, platinum was assigned the role of a simple conductor. But it turns out it can work alone.

A thin film of nickel oxide, tens of nanometers thick, is like a sheet of paper folded hundreds of times thinner. At room temperature, it conducts electricity, but cool it slightly and it becomes an insulator.

A liquid that flows without friction is superfluid helium-4, a strange state of matter at temperatures below two kelvins. In a large volume, the transition to superfluidity occurs at the lambda-point temperature, but in narrow channels everything changes.

A handful of sand on a vibrating table is like a crowd on a dance floor that still holds the rhythm of the old song for a second after the DJ has switched to a new one. Scientists made a thin layer of grains bounce on a vertically shaking platform and abruptly changed the shaking intensity.

A diamond speck smaller than a hundredth of a millimeter glows red. Inside it is a crystal defect: two neighboring carbon atoms, where one is missing and a nitrogen atom sits nearby. Such a pair captures light and re-emits it, like a tiny beacon that doesn't fade at room temperature. This speck was brought near a pathway on the chip's surface.

Tiny particles that look like flowers can do two things at once: heat up and tell you how hot they are. This isn't science fiction: it's the result of working with nanoparticles made of cobalt ferrite, a magnetic material that responds to magnetic fields.

Energy as a signature: the sign of the transferred pulse reveals the secret. This metaphor is the key to a new quantum key distribution protocol that uses energy teleportation instead of state measurement.

Thirty nanometers is the distance over which a phonon sprinter hasn't yet had time to accelerate. In thin aluminum nitride films, thermal resistance remains almost unchanged up to this thickness: heat transport proceeds in the ballistic regime, where phonons (quanta of crystal lattice vibrations) fly…

An orchestra without a conductor plays a complex piece. The listener hears the overall music but wants to understand who leads the melody and who merely echoes it in unison.