Ordinary electric current heats up wires — anyone who has touched a phone charger knows that. But there's another kind of current: instead of electrons rushing in a crowd in one direction, they spin in sync, like figure skaters in a pairs routine. This coordinated rotation is called a spin current, and it produces almost no heat.
A group of physicists set out to see what happens when you send a pure spin current through a thin plate, a spin Hall bridge, and connect it to an external load, a regular resistor. They applied the principle of least energy dissipation (a variational principle) and calculated how currents, charges, and heat would distribute. The result surprised them: when the load was moved a distance significantly exceeding the spin relaxation length, the path over which electron spin rotation decays, the heating in the resistor dropped to zero. Current flowed, but no heat was generated.
This is like water flowing through a hose without pressing on the walls. All the energy goes not into heat but into sustaining the current itself — pure, without any charge contamination. This regime, a global steady state, turned out to be the hallmark of spin current. Ordinary current, where electrons also carry charge, doesn't behave this way.
The absence of heat isn't just a curiosity. It means spin circuits could be made far more efficient, without bulky cooling. The first link in such a circuit, a plate carrying a pure spin current, already doesn't heat up the resistor that follows it, as long as that resistor is placed far enough away.
In ordinary copper, the spin relaxation length reaches hundreds of nanometers — thousands of times longer than in many semiconductors, which is why copper is often used in spin current experiments.