Quantum tunnelling

Quantum tunnelling is the passage of a particle through an energy barrier that classical mechanics forbids it to cross. Its probability depends on the particle’s wavefunction within the barrier.

Schrödinger equation: A fundamental equation describing how a quantum system’s wavefunction changes over time. Its solutions determine how a particle’s wavefunction extends into and beyond a barrier.
Scanning tunneling microscope: A microscope that maps conductive surfaces by measuring electron tunnelling between a sharp tip and a sample. Its image signal comes directly from the distance-sensitive tunnelling current.
Quantum mechanics: The physical theory describing matter and energy at atomic and subatomic scales. Tunnelling is a consequence of quantum mechanics, not a force that pushes particles through barriers.
Thermionic emission: The emission of electrons from a material after thermal energy allows them to overcome a surface barrier. Unlike tunnelling, it crosses the barrier by gaining enough energy to surmount it.
Friedrich Hund: A German physicist whose work included early quantum-mechanical analysis of particles crossing potential barriers. His 1927 treatment helped establish barrier penetration as a consequence of wave mechanics.
Wavefunction: A mathematical description of a quantum system that encodes probabilities for measurement outcomes. A nonzero wavefunction beyond the barrier gives tunnelling a nonzero probability.
Tunnel diode: A semiconductor diode whose heavily doped junction permits electron tunnelling and negative differential resistance. Tunnelling creates its distinctive current peak and high-frequency operation.
Classical mechanics: The framework describing motion using deterministic laws such as Newton’s equations. It forbids a particle from crossing a barrier when its energy is too low.
Over-barrier transport: Transport in which particles possess enough energy to pass over a potential barrier. It is the classical route through a barrier that tunnelling bypasses.
George Gamow: A physicist who explained alpha decay using quantum-mechanical barrier penetration in 1928. His account connected tunnelling to radioactive decay and nuclear structure.

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