## Bose Gas Sum

To explicitly include indistinguishability of bosons we use the grad canonical ensemble and treat each single particle state as a system in equilibrium with a reservoir of all the other single particle states at a temperature $T$ and chemical potential $\mu$. The number of bosons and energy are given by $$N = \sum_{\epsilon_{\bf n}} \overline{n}(\epsilon_{\bf n}),$$ $$E = \sum_{\epsilon_{\bf n}} \epsilon_{\bf n}\overline{n}(\epsilon_{\bf n}),$$ $$\overline{n}(\epsilon_{\bf n}) = \frac{1}{e^{(\epsilon_{{\bf n}} -\mu)/kT} - 1}$$ where $\epsilon_{\bf n}$ is a single particle energy, and $\overline{n}(\epsilon_{\bf n})$ is the mean number of bosons in a single particle microstate state of energy $\epsilon_{\bf n} = A(n_x^2 + n_y^2+n_z^2)$. The integers $n_x$, $n_y$, and $n_z$ can take on any positive nonzero value and $A$ is a constant that depends on the mass of the boson and the volume of the system.

For a given value of $N$ and each value of $T$ the program numerically determines the value of $\mu$ that satisfies Eq. (1), and then uses Eq. (2) to find the energy and the specific heat $c = (1/N)\Delta E/\Delta T$ as a function of $T$.

Problem: The ideal Bose gas for a finite number of particles

Our discussion of Bose condensation has involved some subtle issues associated with the chemical potential and the limit of an infinite system. It is instructive to consider the ideal Bose gas for a finite number of particles and see how Bose condensation arises as the number of particles is increased. (This problem is based on a paper by Price and Swendsen.) The idea is to start with the expressions for a finite system \begin{align} N(T, \mu) & = \sum_{\vec n} \frac{1}{e^{\beta (\epsilon_{\vec n}-\mu)} - 1} \label{eq:6/Nsum}\\ E(T, \mu) & = \sum_{\vec n} \epsilon_{\vec n} \frac{1}{e^{\beta (\epsilon_{\vec n}-\mu)} - 1}, \end{align} and do the sums over $\vec n = (n_x, n_y, n_z)$. We write $\epsilon_{\vec n} = A (n_x^2 + n_y^2 + n_z^2)$, where $A = \hbar^2 \pi^2 /2mL^2$. For simplicity, choose units such that $A=1$.

1. Explain why the value of $T_c$ depends on $A$ and $N$.
2. Write a program to do the sums in \eqref{eq:6/Nsum} as three nested loops over $n_x$, $n_y$, and $n_z$. Because $\overline{n}( \epsilon)$ goes to zero rapidly for large $\epsilon$, restrict the sums to values of $\vec n$ for which $\overline{n}(\epsilon) > \delta$ with $\delta = 10^{-8}$. Take $n_x^2 + n_y^2 + n_z^2 < M$, where \begin{equation} \frac{1}{e^{\beta (AM - \mu)} - 1} = \delta, \end{equation} or \begin{equation} M = \ln(\delta ^{-1} + 1)/\beta A + \mu/A. \label{eq:priceM} \end{equation} Determine $M$ from \eqref{eq:priceM} and calculate the upper limits on the nested sums successively as $n_x^2 \lt M$, $n_y^2 < M - n_x^2$, and $n_z^2 < M - n_x^2 - n_y^2$. However, your program will run reasonably quickly if you choose $M$ as the upper limit for all three loops.
3. Compute $N(T, \mu)$ for values of $\mu$ between 1.0 and 7.0 for $T=1.0$. Note the divergence of $N$ at $\mu= 3 A$ (the lowest single-particle energy state) and the negative values of $N$ that can occur for $\mu \geq 3A$. Are there other values of $\mu$ where $N$ diverges? Why does this nonphysical behavior of $N(T, \mu)$ occur?
4. Use your results for $N(T,\mu)$ to determine $\mu(N,T)$ by successively halving the interval containing the correct value of $\mu$ until $N_{\rm trial}$ is close to the desired value. Choose the initial upper and lower limits of the interval to be $\mu_+ =\epsilon_0 = 3A$ and $\mu_- = -10\epsilon_0$. A summary of the algorithm is given by the following:
1. Set $\mu_{\rm trial} = \frac12(\mu_+ + \mu_-)$.
2. Compute $N_{\rm trial} = N(\mu_{\rm trial},T)$
3. If $N_{\rm trial} > N$, set $\mu_+ = \mu_{\rm trial}$; otherwise, set $\mu_- = \mu_{\rm trial}$.
4. If $\mu_+ - \mu_- > \delta \mu$, go to step 1.
First choose $N=5$ to check that your program is working correctly and then consider $N = 500$.
5. Use the results from part(d) to plot $\mu$ as a function of $T/T_c$ for $N=500$ and describe its qualitative behavior. (In your program include code to find $T_c$ using the answer to part(a).) What is the upper limit of $\mu$?
6. Plot the fraction of particles in the two lowest energy states as a function of $T$. Discuss their behavior as a function of temperature.
7. Although it is instructive to write your own program, Program BoseGasSum implements the sum over $\vec n$ and computes $\mu$ by varying its value until the computed $N$ equals the desired input value. The program plots $\overline{N}_0$, $\overline{N}_{\epsilon} = N-\overline{N}_0$ and the specific heat $C = (1/N)\Delta E/\Delta T$ by using two neighboring values of the temperature. Use the default values and explain why there appears to be a phase transition. How does the value of the transition temperature depend on the difference between the ground state and the first excited state? Does the transition temperature become better defined as $N$ is increased?

## Resources

1. Problem 6.67 in Statistical and Thermal Physics: With Computer Applications, 2nd ed., Harvey Gould and Jan Tobochnik, Princeton University Press (2021).
2. T. Price and R. H. Swendsen, “ Numerical computation for teaching quantum statistics,” Am. J. Phys. 81, 866–872 (2013).

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