Theoretical physicists have stumbled upon a counterintuitive finding.
BY DEBDUTTA PAUL
ಈ ಲೇಖನವನ್ನು ಕನ್ನಡದಲ್ಲಿ ಓದಲು ೨ನೇ ಪುಟಕ್ಕೆ ಹೋಗಿ.
We are surrounded by wires conducting electricity. From the cables that charge our gadgets to those coiled inside the myriad electrical devices that power our homes, all transport electricity. Metals, which make the wires, conduct electricity efficiently, while other materials, like plastic, do not.
The physical law that explains how electricity is conducted via the wires is called Ohm’s law. The conducted current is proportional to the voltage difference applied, it states. The resistance relates the two and is proportional to the length of the conducting material.
Researchers at ICTS-TIFR and their colleagues from IISER-Pune and IIT-Hyderabad have found that in special scenarios, the resistance can decrease with length. Although scientists have observed deviations from Ohm’s law earlier, this is the first time researchers have found such a strikingly opposite behaviour.
Electricity, electrons, and quantum mechanics
Electricity conduction is the transport of electrons, elementary particles constituting all atoms and molecules. The physical laws that govern their transport are quantum mechanical. According to quantum mechanics, electrons behave both like particles and waves, which is crucial to studying their movement inside wires.
Madhumita Saha from ICTS, a co-author, explained that they found resistance decreasing with length in a special kind of wire. Here, electrons can travel only in one direction, the length of the wire, unlike in metals, where they zoom around in three dimensions.
Electron transport is like highway traffic
Archak Purkayastha from IIT-Hyderabad, another co-author, explained that the electrons’ transport inside the wires is akin to travelling on a highway in a car. “If there is traffic from all lanes, causing traffic jams, the larger the car has to cover, the slower it becomes,” he said. This scenario is akin to the usual behaviour of wires’ resistances.
If there is no traffic, the time the car takes does not depend on the distance between its start and end points. However, if it has leaking fuel, then the more the distance between them, the more time it takes. “In this case, despite having no traffic, you would slow down the longer you drive, simply because you are running out of your energy source,” he said.
The team’s research looked at a scenario when the distance between the two points is not too large. In the absence of traffic, despite fuel leakage, “our research suggests that electrons take less time to cover larger distances!”
The team considered the effect of impurities in the crystals or the presence of other atoms, which mimic the impact of traffic. Quantum mechanics limits that there is no way to distinguish different electrons, and all of them behave as waves till a specific length. However, wires around us are typically much longer than the lengths at which the wave-like effects of individual electrons are significant.
Archak explained that the exciting effects they discovered kicked in as they studied small lengths of the special wires. They are, in fact, so small that the wave-like nature of the electrons persists over the entire length of the wires.
Even for such small lengths of wires, the electrons can possess energies only for specific values that researchers call ‘energy bands’. Electrons collectively display a unique behaviour — and accordingly, the wire’s resistance decreases with the wire’s length — only when the electrons occupy energies very close to the minimum or maximum of the allowed energy.
The team’s paper was published in the journal Physical Review B Letters. They also found that the strange behaviour disappeared if they completely isolated the wire from its environment.
A mystery and future scope
“Clearly, more research is required to unveil the full understanding of this regime of electronic transport,” said Archak. “Nevertheless, from a fundamental physics perspective, finding such a new and counterintuitive effect is exciting.”
The team’s finding can have potential implications in “nanoscale systems and can be envisioned to find novel device applications,” Archak signed off.
The author thanks Madhumita Saha and Archak Purkayastha for insightful discussions.
Quite an interesting phenomenon indeed!