Smooth-barrier semiclassical tunnelling
For a smooth opaque barrier between two simple turning points, the leading transmitted-to-incident probability-flux fraction is T≈exp[−2∫_(x₁)^(x₂)κ(x)dx], κ=√[2m(V−E)]/ħ.
Key knowledge
For a smooth opaque barrier between two simple turning points, the leading transmitted-to-incident probability-flux fraction is T≈exp[−2∫_(x₁)^(x₂)κ(x)dx], κ=√[2m(V−E)]/ħ. This exponential depends on the full forbidden-region action rather than the barrier height alone.
Values and equations
T≈e^(−2Θ); Θ=∫_(x₁)^(x₂)√[2m(V(x)−E)]dx/ħ≫1
- T,Θ
- Dimensionless flux fraction and forbidden action
- x,x₁,x₂
- Position and turning points m
- m
- Mass kg
- V,E
- Potential and incident energy J
- ħ
- Reduced Planck constant J s
Conditions and scope
- One-dimensional real elastic potential, constant nonrelativistic mass, separated simple turning points, valid Airy/WKB overlaps and forbidden action much larger than one.
- The expression is the leading opaque single-barrier result; near barrier top, resonant multiple barriers, abrupt steps and uncontrolled WKB errors need different methods. Transmission is a flux ratio.
Sources · 1
References
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Evidence and review
Quantum Physics III: chapter 3, semiclassical approximation: §3.5, printed pages 67–69, equations 3.5.1–3.5.9: incident/transmitted WKB current, forbidden action θ and T=e^(−2θ); wide smooth barrier and small transmission assumption stated.
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