The mechanism for superconductivity, where a current flows through a medium without generating resistance or heat, does not follow Ohm’s law of current, resistance and voltage and may be caused by quantum tunnelling and electron substitution, says materials scientist and theoretical physicist Dr Johan Prins.
“Measurements of superconductivity show that the voltage across the medium drops to an immeasurably low value. However, Ohm’s law dictates that a current through a medium must cause resistance, owing to the acceleration of charge carriers and their subsequent scattering, thus releasing heat. This would result in a potential difference, or voltage, because the charges accelerate and move to counter the electrical field applied across the medium.
“However, in superconductivity, there is no resistance and, thus, no acceleration and scattering. The physics underpinning Ohm’s law does not apply because it cannot explain the reason for the drop in voltage across a superconductor. “The current flowing through a superconductor must, therefore, be caused by a different charge-carrying mechanism,” he says.
The fact that zero voltage can never be measured means the mechanism for superconductivity cannot be experimentally verified by direct measurement. Current theories of superconductivity may, therefore, be inaccurate, especially as they run counter to several experimentally determined behaviours.
“The conundrum is that a moving charge carrier has kinetic energy, which is dissipated as heat. Charge carriers within a superconductor also move when a current flows, but this motion cannot be caused by acceleration (increase in kinetic energy) of the charge carriers caused by the electric field.
“Since the charge carriers in a superconductor must, thus, be stationary entities when no current flows, the regions between them are insulating barriers and superconductivity occurs across these insulating barriers,” he says.
Prins deduces his results from his work on diodes and transistors. Positive charges accumulate on the cathode and negative charges on the anode of a transistor interface (called a pn-junction, or Schottky diode, if one of the materials is a metal and the other a semicon- ductor), causing the electric field across the interface to become weaker. An insulating layer, which is called a depletion layer, is formed on the nonmetallic side of a Schottky junction.
This layer is free of electron charges and should, therefore, prevent a current from flowing through it. However, owing to the wave nature of electrons, a charge carrier can tunnel through the depletion layer without gaining or losing any energy. This enables charge carriers to flow without an increase in kinetic energy, which would result in heat dissipation through scattering.
Prins proposes that this quantum tunnelling, where an electron wave can affect another electron wave on the other side of an insulating barrier, causes superconductivity. Quantum tunnelling preserves the energy of both affected charge carriers and does not violate the law of the conservation of energy as was proposed by Sir Isaac Newton.
“By making the depletion zones (insulating layers) between the charge carriers sufficiently small, which is achieved by increasing the density of insulated charge carriers, electron waves can tunnel through, affect and displace the next insulated electron wave. The charge of this electron wave then displaces the series of subsequent charge carriers along the superconducting medium, enabling a current to flow without resistance.
“This quantum tunnelling theory of superconductivity can be experimentally tested and does not cause resistance or conflict with estab- lished, experimentally verified laws of physics, such as Ohm’s law and Coulomb’s law,” says Prins.
Since the mechanism is known, materials can be optimised to superconduct at room and higher temperatures. Potential applications of room temperature superconductivity include a magnetic battery that stores charge mechanically and heat-free transistors that will enable further miniatur- isation of computing hardware, he concludes.