An enormous number of electrical systems like any kind of computers, switchboards, telecommunication devices or screens are in use in any city. The more inhabitants, the more energy-consuming appliances. It is known that electrical resistances of every electric device cause power dissipation in the form of heat. The aggregated losses of all devices in a city can be surprisingly high. And they are increasing in consideration of the needed energy for cooling devices like ventilators or air conditioning systems. Zero electrical resistance could save an inconceivable amount of energy but also resources and expenditures for cooling devices.
The electrical resistance of an electric device or material measures the reduction of the electric current when flowing through it. The higher the resistance of a conductor, the higher the reduction of the electric current and the more electrical energy is needed to push current through this resistance. Depending on the quantity of electrical resistances, electrical energy is dissipated and transformed into heat. In some cases, this heating effect is desired, this for example when using an electric kettle. But this power dissipation is often an undesirable effect and causes lost energy, for example a warmed up notebook or TV. Warm server rooms even require additional energy for cooling devices. The reciprocal of the electrical resistance is the electrical conductivity – the capability to conduct an electrical current.
Zero electrical resistance through topological insulators and very thin materials
Scientists at the US-based Massachusetts Institute of Technology (MIT) are also interested in materials and devices with low or better zero electrical resistance. It can reduce energy losses but also extend device capabilities. In recent years, researchers tried to make progress by using very thin materials and topological insulators (TIs). A topological insulator behaves like an insulator in its interior in contrast to its surface which conducts electricity. Electrons are only able to move along the surface of the material.
According to MIT, a breakthrough towards the dissipationless goal has been achieved when the current enters a quantum state without any external magnetic fields at extremely low temperatures. The potential could be enormous if the restriction of the low temperature can be eliminated. MIT postdoc Cui-Zu Chang, then a doctoral student at Tsinghua University in China, and colleagues at Chinese Academy of Sciences-Institute of Physics, Tsinghua, and Stanford University demonstrated a system with remnants of electrical resistance and very close to zero electrical resistance.
Chang and colleagues at MIT were using vanadium instead of chromium for thinner atomic layers for the magnetic topological insulator. With further optimizations, Chang and colleagues at MIT and Penn State University achieved zero electrical resistance at the edge of a sample with extremely low temperatures of 0.025 kelvins. They also stacked sample films on a base of strontium titanate. The results are published in Nature Materials in May 2015.
“A signal entering this system can propagate a long distance without losing any of its energy. While presently it can only be realized at very low temperatures, there are indications that this can be raised,” Chang says.
Chang and colleagues continue their research, for example, by experimenting with different materials. It is called doping if an extra element like chromium or vanadium is added to the material. Compared with chromium, vanadium has three advantages. It is possible to operate at zero resistance at a slightly higher but still very cold temperature. The stability of its intrinsic magnetism can be increased tenfold and it is possible to halve the carrier density.
It is still a long way. However, the practical implementation is a further building block for future green cities.
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