diff --git a/chapters/fundamentals/examples.pdf b/chapters/fundamentals/examples.pdf deleted file mode 100644 index f2f4eb8..0000000 Binary files a/chapters/fundamentals/examples.pdf and /dev/null differ diff --git a/chapters/fundamentals/examples.tex b/chapters/fundamentals/examples.tex index 6def9f0..0c54a4c 100644 --- a/chapters/fundamentals/examples.tex +++ b/chapters/fundamentals/examples.tex @@ -1,4 +1,4 @@ -% !TeX root = ../../script.tex +% !TeX root = ../script.tex \documentclass[../../script.tex]{subfiles} \begin{document} @@ -136,7 +136,7 @@ Thus the wave function for $x < 0$ becomes \psi_{\text{I}}(x) = A \left[ e^{ikx} + \frac{ik + \alpha}{ik - \alpha}e^{-ikx} \right] \end{equation} The fraction of reflected particles is calculated as -\begin{equation} +\begin{equation}\label{eq:reflectioncoefffirst} R = \frac{\abs{Be^{-ikx}}^2}{\abs{Ae^{ikx}}^2} = \frac{\abs{B}^2}{\abs{A}^2} = \abs{\frac{ik + \alpha}{ik - \alpha}}^2 = 1 \end{equation} which means that \textit{all} particles are being reflected if $E < \energy{0}$. This corresponds to the expected classical behaviour or particles. @@ -157,4 +157,57 @@ of the penetrating wave sinks to $1/e$ of its initial value after a distance $x Particles with energy $E$ can penetrate into potential areas $\energy{0} > E$ with a certain probability, even if they shouldn't be able to according to classic particle physics. \end{tcolorbox} Once we accept that particles are described by waves, we can come to the conclusion that particles are allowed to exist in \textit{classically forbidden} locations. + +\subsubsection{(b) $E > \energy{0}$} +In this case, the kinetic energy $ekin = E$ of the incidental particle is greater than the potential jump $\energy{0}$, and in the classical model all particles would enter the area $x > 0$, +losing speed in the process, because their kinetic energy decreases to $\ekin = E - \energy{0}$. What does this look like in the wave model? The quantity $\alpha$ is now purely imaginary, so we introduce the real quantity +\begin{equation} + k' = \frac{\sqrt{2m(E - \energy{0})}}{\hbar} = i\alpha +\end{equation} +The solution~\eqref{eq:potentialstepsolution} then becomes +\begin{equation} + \psi_{\text{II}} = Ce^{-ik'x} + De^{ik'x} +\end{equation} +Since there are no particles moving in the $-x$ direction, we know that $C = 0$, and we are left with $\psi_{\text{II}} = De^{ik'x}$. +From the boundary conditions~\eqref{eq:boundaryconditions} we can deduce +\begin{align} + B = \frac{k - k'}{k + k'}A && \text{and} && D = \frac{2k}{k + k'}A +\end{align} +thus leaving us with the wave functions +\begin{equation} + \begin{split} + \psi_{\text{I}}(x) &= A \cdot \left(e^{ikx} + \frac{k - k'}{k + k'} \cdot e^{-ikx} \right) \\ + \psi_{\text{II}}(x) &= \frac{2k}{k + k'}Ae^{ik'x} + \end{split} +\end{equation} +The \textbf{\textit{reflection coefficient}} $R$, i.e.\ the fraction of reflected particles is then analogous to the result from optics +\begin{equation}\label{eq:reflectioncoeff} + R = \frac{\abs{B}^2}{\abs{A}^2} = \abs{\frac{k - k'}{k + k'}}^2 +\end{equation} +\textbf{Remark.} Since the wave number $k$ is proportional to the refractive index in optics ($k = n \cdot k_0$), the result~\eqref{eq:reflectioncoeff} can easily be transformed into the reflection coefficient +\[ + R = \abs{\frac{n_1 - n_2}{n_1 + n_2}}^2 +\] +of a lightwave encountering a boundary surface between two mediums with refractive indexes $n_1$ and $n_2$. + +To compute the amount of particles being transmitted per unit of time (i.e.\ the number of particles passing through the area $x = x_0 > 0$ divided by the amount of particles passing through an area $x < 0$ per unit of time) +one has to consider that the velocities in both areas are different. +The ratio $v' / v = k' / k = \lambda / \lambda'$ is given by the ratio of the wave numbers. That is why the \textbf{\textit{transmission coefficient}} +\begin{equation}\label{eq:transmissioncoeff} + T = \frac{v' \abs{D}^2}{v \abs{A}^2} = \frac{4k \cdot k'}{(k + k')^2} +\end{equation} +From comparing~\eqref{eq:reflectioncoeff} and~\eqref{eq:transmissioncoeff} we can see that +\[ + T + R = 1 +\] +which makes sense because the amount of particles overall has to be conserved. + +\textbf{\textit{Remarks.}} +\begin{itemize} + \item Total reflection also occurs for $E = \energy{0}$. In this case $\alpha = 0$ and $k' = 0$, and thus according to~\eqref{eq:reflectioncoefffirst} or~\eqref{eq:reflectioncoeff} we see that $R = 1$ + \item Instead of a positive potential barrier one can also consider a negative one with $\energy{0} < 0$, where reflection and transmission occurs as well. + One has to let the wave incide from the right in order to yield the same equations. The optical equivalent would be the transition from a medium with higher optical density into one with a lower optical density. +\end{itemize} + +\subsection{Quantum Tunnelling} \end{document} \ No newline at end of file diff --git a/chapters/fundamentals/schroedinger.tex b/chapters/fundamentals/schroedinger.tex index 14e3e39..ceb47cb 100644 --- a/chapters/fundamentals/schroedinger.tex +++ b/chapters/fundamentals/schroedinger.tex @@ -1,4 +1,4 @@ -% !TeX root = ../../script.tex +% !TeX root = ../script.tex \documentclass[../../script.tex]{subfiles} \begin{document} diff --git a/script.pdf b/script.pdf index ece4377..6b64465 100644 Binary files a/script.pdf and b/script.pdf differ