Scattering rate

The interaction picture

Define the unperturbed Hamiltonian by $H_{0}$, the time dependent perturbing Hamiltonian by $H_{1}$ and total Hamiltonian by $H$.

The eigenstates of the unperturbed Hamiltonian are assumed to be

$H=H_{0}+H_{1}\$

$H_{0}|k\rangle =E(k)|k\rangle$

In the interaction picture, the state ket is defined by

$|k(t)\rangle _{I}=e^{{iH_{0}t/\hbar }}|k(t)\rangle _{S}=\sum _{{k'}}c_{{k'}}(t)|k'\rangle$

By a Schrödinger equation, we see

$i\hbar {\frac {\partial }{\partial t}}|k(t)\rangle _{I}=H_{{1I}}|k(t)\rangle _{I}$

which is a Schrödinger-like equation with the total $H$ replaced by $H_{{1I}}$.

Solving the differential equation, we can find the coefficient of n-state.

$c_{{k'}}(t)=\delta _{{k,k'}}-{\frac {i}{\hbar }}\int _{0}^{t}dt'\;\langle k'|H_{1}(t')|k\rangle \,e^{{-i(E_{k}-E_{{k'}})t'/\hbar }}$

where, the zeroth-order term and first-order term are

$c_{{k'}}^{{(0)}}=\delta _{{k,k'}}$

$c_{{k'}}^{{(1)}}=-{\frac {i}{\hbar }}\int _{0}^{t}dt'\;\langle k'|H_{1}(t')|k\rangle \,e^{{-i(E_{k}-E_{{k'}})t'/\hbar }}$

The transition rate

The probability of finding $|k'\rangle$ is found by evaluating $|c_{{k'}}(t)|^{2}$.

In case of constant perturbation,$c_{{k'}}^{{(1)}}$ is calculated by

$c_{{k'}}^{{(1)}}={\frac {\langle \ k'|H_{1}|k\rangle }{E_{{k'}}-E_{k}}}(1-e^{{i(E_{{k'}}-E_{k})t/\hbar }})$

$|c_{{k'}}(t)|^{2}=|\langle \ k'|H_{1}|k\rangle |^{2}{\frac {sin^{2}({\frac {E_{{k'}}-E_{k}}{2\hbar }}t)}{({\frac {E_{{k'}}-E_{k}}{2\hbar }})^{2}}}{\frac {1}{\hbar ^{2}}}$

Using the equation which is

$\lim _{{\alpha \rightarrow \infty }}{\frac {1}{\pi }}{\frac {sin^{2}(\alpha x)}{\alpha x^{2}}}=\delta (x)$

The transition rate of an electron from the initial state $k$ to final state $k'$ is given by

$P(k,k')={\frac {2\pi }{\hbar }}|\langle \ k'|H_{1}|k\rangle |^{2}\delta (E_{{k'}}-E_{k})$

where $E_{k}$ and $E_{{k'}}$ are the energies of the initial and final states including the perturbation state and ensures the $\delta$-function indicate energy conservation.

The scattering rate

The scattering rate w(k) is determined by summing all the possible finite states k' of electron scattering from an initial state k to a final state k', and is defined by

$w(k)=\sum _{{k'}}P(k,k')={\frac {2\pi }{\hbar }}\sum _{{k'}}|\langle \ k'|H_{1}|k\rangle |^{2}\delta (E_{{k'}}-E_{k})$

The integral form is

$w(k)={\frac {2\pi }{\hbar }}{\frac {L^{3}}{(2\pi )^{3}}}\int d^{3}k'|\langle \ k'|H_{1}|k\rangle |^{2}\delta (E_{{k'}}-E_{k})$

References

• C. Hamaguchi (2001). Basic Semiconductor Physics. Springer. pp. 196–253.
• J.J. Sakurai. Modern Quantum Mechanics. Addison Wesley Longman. pp. 316–319.