Section: Research Program

From the Schrödinger equation to Boltzmann-like equations

Participant : François Castella.

Schrödinger equation, asymptotic model, Boltzmann equation.

The Schrödinger equation is the appropriate way to describe transport phenomena at the scale of electrons. However, for real devices, it is important to derive models valid at a larger scale.

In semi-conductors, the Schrödinger equation is the ultimate model that allows to obtain quantitative information about electronic transport in crystals. It reads, in convenient adimensional units,

i t ψ ( t , x ) = - 1 2 Δ x ψ + V ( x ) ψ , (11)

where V(x) is the potential and ψ(t,x) is the time- and space-dependent wave function. However, the size of real devices makes it important to derive simplified models that are valid at a larger scale. Typically, one wishes to have kinetic transport equations. As is well-known, this requirement needs one to be able to describe “collisions” between electrons in these devices, a concept that makes sense at the macroscopic level, while it does not at the microscopic (electronic) level. Quantitatively, the question is the following: can one obtain the Boltzmann equation (an equation that describes collisional phenomena) as an asymptotic model for the Schrödinger equation, along the physically relevant micro-macro asymptotics? From the point of view of modelling, one wishes here to understand what are the “good objects”, or, in more technical words, what are the relevant “cross-sections”, that describe the elementary collisional phenomena. Quantitatively, the Boltzmann equation reads, in a simplified, linearized, form :

t f ( t , x , v ) = 𝐑 3 σ ( v , v ' ) [ f ( t , x , v ' ) - f ( t , x , v ) ] d v ' . (12)

Here, the unknown is f(x,v,t), the probability that a particle sits at position x, with a velocity v, at time t. Also, σ(v,v') is called the cross-section, and it describes the probability that a particle “jumps” from velocity v to velocity v' (or the converse) after a collision process.