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pkirwin 2021-04-09 16:10:05 -06:00
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@ -360,65 +360,65 @@ S_u = [1, s; s, 1];
\item %3b
To find the E-\textbf{k} relationship in terms of cosine functions, we must first
To find the \(E\)-\(\vec{k}\) relationship in terms of cosine functions, we must first
find \( h_0 \) in terms of trigonometric functions.
We will require the following trigonometric identities to do so:
\begin{align}
sin(\alpha \pm \beta) = sin(\alpha)cos(\beta) \pm sin(\beta)cos(\alpha)
\sin(\alpha \pm \beta) = \sin(\alpha)\cos(\beta) \pm \sin(\beta)\cos(\alpha)
\label{eq:sin_sum}
\end{align}
\begin{align}
cos(\alpha \pm \beta) = cos(\alpha)cos(\beta) \mp sin(\alpha)sin(\alpha)
\cos(\alpha \pm \beta) = \cos(\alpha)\cos(\beta) \mp \sin(\alpha)\sin(\alpha)
\label{eq:cos_sum}
\end{align}
\begin{align}
1 = sin^2(\theta) cos^2(\theta)
1 = \sin^2(\theta) \cos^2(\theta),
\label{eq:sum_squares}
\end{align}
As well as the following general sine/cosine properties:
as well as the following general sine/cosine properties:
\begin{align}
cos(\theta) &= cos(-\theta)
sin(\theta) &= -sin(-\theta)
\begin{split}
\cos(\theta) &= \cos(-\theta) \\
\sin(\theta) &= -\sin(-\theta).
\label{eq:neg_sin_cos}
\end{split}
\end{align}
First we can rewrite the exponentials in \( h_0 \) using Euler's identity:
\begin{align*}
h_0 = -t_c \left( 1 + cos(-k_x a - k_y b) + i sin(-k_x a - k_y b) + cos(-k_x a + k_y b) + i sin(-k_x a + k_y b) \right)
h_0 = -t_c \left( 1 + \cos(-k_x a - k_y b) + i \sin(-k_x a - k_y b) + \cos(-k_x a + k_y b) + i \sin(-k_x a + k_y b) \right)
\end{align*}
Using identities (\ref{eq:sin_sum}) and (\ref{eq:cos_sum}), we can expand this relationship further.
% TODO no idea how to format this...
\begin{align*}
h_0 = -t+c ( 1 &+ cos(-k_x a)cos(-k_y b) - sin(-k_x a)sin(-k_y b) + i sin(-k_x a)cos(-k_y b) + i sin(-k_y b)cos(-k_x a) \\
&+ cos(k_y b)cos(k_x a) + sin(k_y b)sin(k_x a) + i sin(k_y b)cos(k_x a) - i sin(k_x a)cos(k_y b) )
h_0 = -t+c \big( 1 &+ \cos(-k_x a)\cos(-k_y b) - \sin(-k_x a)\sin(-k_y b) + i \sin(-k_x a)\cos(-k_y b) + i \sin(-k_y b)\cos(-k_x a) \big. \\
\big. &+ \cos(k_y b)\cos(k_x a) + \sin(k_y b)\sin(k_x a) + i \sin(k_y b)\cos(k_x a) - i \sin(k_x a)\cos(k_y b) \big)
\end{align*}
Here we can use the properties from (\ref{eq:neg_sin_cos}) to reduce this equation. Since \( \left| h_0 \right| ^2 = h_0 h^*_0 \),
we also obtain a simple expression for \( h^*_0 \).
\begin{align*}
h_0 &= -t_c ( 1 + 2cos(k_x a)cos(k_y b) - 2 i sin(k_x a)cos(k_y b)) \\
h^*_0 &= -t_c ( 1 + 2cos(k_x a)cos(k_y b) + 2 i sin(k_x a)cos(k_y b))
h_0 &= -t_c \left( 1 + 2\cos(k_x a)\cos(k_y b) - 2 i \sin(k_x a)\cos(k_y b)\right) \\
h^*_0 &= -t_c \left( 1 + 2\cos(k_x a)\cos(k_y b) + 2 i \sin(k_x a)\cos(k_y b)\right)
\end{align*}
We can now find \( \left| h_0 \right| ^2 = h_0 h^*_0 \):
\begin{align*}
h_0 h^*_0 = 1 + 4cos(k_x a)cos(k_y b) + 4cos^2(k_y b)
h_0 h^*_0 = 1 + 4\cos(k_x a)\cos(k_y b) + 4\cos^2(k_y b)
\end{align*}
From this, we can finally obtain an expression for \( E(k) \):
From this, we can finally obtain an expression for \( E(\vec{k}) \):
\begin{align}
E(k) = E_0 \pm t_c \sqrt{1 + 4cos(k_x a)cos(k_y b) 4 cos^2(k_y b)}
E(\vec{k}) = E_0 \pm t_c \sqrt{1 + 4\cos(k_x a)\cos(k_y b) 4 \cos^2(k_y b)}
\end{align}
\end{enumerate}
@ -491,7 +491,11 @@ S_u = [1, s; s, 1];
\item %4e
To generate the points, we used a simple MATLAB script, found in Appendix (A).
\end{enumerate}
\app
\end{document}

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