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87
main.tex
87
main.tex
@@ -197,7 +197,7 @@ We can keep the same expressions as earlier with the same reduced dimensionless
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\end{align}
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And then our expression, with the changes becomes:
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\begin{equation}
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\sigma(\kappa, \xi) = -i \frac{3 \sigma_0}{4} \frac{1}{\xi}\left[\int_{1 - \xi}^{1}\dd{\xi} \tanh(\frac{\xi + \xi' - \mu}{2 t}) I_1 + \int_{1}^{\infty} \dd{\xi'} \left( \tanh(\frac{\xi + \xi' - \mu}{2t}) I_1 - \tanh(\frac{\xi' - \mu}{2t})I_2 \right) \right]
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\sigma(\kappa, \xi) = -i \frac{3 \sigma_0}{4} \frac{1}{\xi}\left[\int_{1 - \xi}^{1}\dd{\xi} \tanh(\frac{\xi + \xi' - \mu}{2 t}) I_1 + \int_{1}^{\infty} \dd{\xi'} \left( \tanh(\frac{\xi + \xi' - \mu}{2t}) I_1 - \tanh(\frac{\xi' - \mu}{2t})I_2 \right) \right] \label{eq:nam}
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\end{equation}
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with
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\begin{align}
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@@ -210,6 +210,39 @@ with
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g &= \frac{\xi' \left( \xi + \xi'\right) + 1}{\sqrt{\xi'^2 - 1}\sqrt{(\xi + \xi')^2 - 1}}
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\end{align}
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\section{Mathematica implementation}
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We want to dimensionally reduce these when we implement the code, by writing $\corr$, $\Delta$, $\omega_D$ and $T$ in units of $\Delta_0$:
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\begin{align}
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o_D &= \frac{\omega_D}{\Delta_0} \\
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m &= \frac{\corr}{\Delta_0} \\
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d &= \frac{\Delta}{\Delta_0} \\
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t &= \frac{T}{\Delta_0}
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\end{align}
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giving us
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\begin{equation}
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\left[ N(0) V \right]^{-1} = \int_{- o_D}^{o_D} \frac{\dd{\varepsilon}}{\sqrt{d^2 + \varepsilon^2}} \tanh{\frac{\sqrt{d^2 + \varepsilon^2} - m}{2 t}}
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\end{equation}
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\begin{equation}
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n = \int_0^\infty \dd{\epsilon} \left( \frac{1}{1 + \exp(\frac{\sqrt{d + \varepsilon^2} - m}{t})} - \frac{1}{1 + \exp(\frac{\sqrt{d^2 + \varepsilon^2}}{t})} \right)
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\end{equation}
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We can look at the Nam conductivity now:
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\begin{figure}[htp]
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\centering
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\includegraphics[width=14cm]{realpartofconductivity}
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\caption{$\frac{\Re[\sigma_{SC}]}{\sigma_N}$} \label{fig:real}
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\end{figure}
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\begin{figure}[htp]
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\centering
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\includegraphics[width=14cm]{imagpartofconductivity}
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\caption{$\frac{\Im[\sigma_{SC}]}{\sigma_N}$} \label{fig:imag}
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\end{figure}
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As there is as yet no filtering based on the free energy, this is messy, so the comparison to \cref{fig:osRepro1b} is necessary to see which combinations of $T$ and $n$ are valid.
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However, this is still quite numerically unstable, even with the dimensional reduction procedure described above for $n$ and $\corr$.
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The information that higher $n$ leads to lower $\sigma$ is expected, and does suggest that the implementation is correct, although still very noisy.
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\section{Noise calculation}
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For our noise calculation, we assume a material parameterised by a Debye frequency $\debye$ and the interaction parameter $N(0) V$.
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@@ -217,6 +250,8 @@ Because these aren't necessarily experimentally accessible, I tried to keep thei
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In order to keep $N(0) V$ small and $\debye$ bigger than the other parameters, I chose $N(0)V = 0.25$ and $\debye = \SI{1e13}{\per\s}$.
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This leads to $T_{c0} = T_c(\mu = 0) = \SI{1.44e11}{\per\s}$, similar to the values used for the equilibrium Nam case.
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Because that particular choice of $N(0) V$ and $\debye$ lead to some additional numerical noise for $T$ close to $T_c$, I changed the values slightly to $N(0) V = 0.2$ and $\debye = \SI{1e14}{\per\s}$, which leads to $T_{c0} = \SI{7.64e11}{\per\s}$, and ensures the graphs below look good for $T > 0.8 T_c$.
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\begin{enumerate}
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\item Use the Owen Scalapino coupled integral equations \cref{eq:gap,eq:n}, find $\mu$ and $\Delta$ for fixed $n$.
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\item Find the expected gap from the approximation in OS, $T_c(n) \approx (1 - 4n) T_{c0}$.
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@@ -226,6 +261,56 @@ This leads to $T_{c0} = T_c(\mu = 0) = \SI{1.44e11}{\per\s}$, similar to the val
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\item Calculate the noise as usual.
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\end{enumerate}
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\subsection{Figures}
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Some examples of the noise are potrayed below, from \crefrange{fig:smallomeganoise}{fig:largerTnoise}.
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There are some interesting features in the graphs:
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\begin{itemize}
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\item In the curves for constant omega, \crefrange{fig:smallomeganoise}{fig:bigomeganoise}, there seems to be a consistent dip, where initially as $T$ increases, the noise decreases.
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That seems like it could be an artifact of the method used to calculate $\Delta$, but it is very odd (because nothing besides $\Delta$) should affect any other step.
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\item In the constant omega graphs, for $n = .02$ and $n = .04$ the noise is greater than for $n = 0$ even in the region where it goes to the `normal state'.
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This is confusing.
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\item Most values of $n$ should not be superconducting for most of the range of $T$, as expected.
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Near the critical temperature for a particular $n$, the noise increases as expected, showing a little gap.
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See \cref{fig:mediumTnoise}.
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\end{itemize}
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\begin{figure}[htp]
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\centering
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\includegraphics[width=14cm]{constOmega1/1}
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\caption{$T_1$ vs temperature, for very small omega} \label{fig:smallomeganoise}
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\end{figure}
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\begin{figure}[htp]
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\centering
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\includegraphics[width=14cm]{constOmega1/10}
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\caption{$T_1$ vs temperature, for medium omega} \label{fig:mediumomeganoise}
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\end{figure}
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\begin{figure}[htp]
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\centering
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\includegraphics[width=14cm]{constOmega1/13}
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\caption{$T_1$ vs temperature, for big omega} \label{fig:bigomeganoise}
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\end{figure}
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\begin{figure}[htp]
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\centering
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\includegraphics[width=14cm]{constT1/43}
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\caption{$T_1$ vs frequency, for a temperature where all the chosen $n$ values are allowed, showing the gap between $n = 0$ and $n > 0$, which seems potentially spurious.} \label{fig:smallTnoise}
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\end{figure}
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\begin{figure}[htp]
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\centering
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\includegraphics[width=14cm]{constT1/35}
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\caption{$T_1$ vs frequency, for a temperature where the $n = .04$ state is almost, but not quite surpressed.} \label{fig:mediumTnoise}
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\end{figure}
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\begin{figure}[htp]
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\centering
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\includegraphics[width=14cm]{constT1/15}
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\caption{$T_1$ vs frequency, for temperature so large that only the $n = 0$ state is energetically allowed to be superconducting} \label{fig:largerTnoise}
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\end{figure}
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\printbibliography
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\end{document}
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