Homework Set 1

Problem 2: Autocorrelation function of harmonic oscillator at finite temperature

For one-dimensional harmonic oscillator, the expectation value of the number operator at finite temperature $T$ is given by

$⟨{\hat{a}}^{†}\hat{a}⟩=\frac{1}{e^{\beta \hslash \omega }-1}$

where \beta=1/(k_B T).

(a) Compute the autocorrelation function of the position operator

$\frac{1}{2}⟨\{\hat{x}(t),\hat{x}(0)\}⟩$

and give physical interpretation of its first and second term. Here $\{\hat{A},\hat{B}\}$ is antucommutator of two operators.

(b) What happens to the autocorrelation function in the high temperature limit?

Problem 4: Phonons in one-dimensional chain with two different atoms per unit cell

Consider an infinite one-dimensional chain described by classical Hamiltonian:

$H=\sum _j[\frac{p_j^2}{2m_j}+\frac{k}{2}(x_j-x_{j-1}{)}^2]$

whose even sites host atoms of mass $m_{2j}=m$ and odd sites host atoms of mass $m_{2j+1}=M$.

(a) Find classical normal mode frequencies and sketch their dispersion curves $\omega (k)$.

(b) What is the gap in the excitation spectrum?

(c) Write the diagonalized Hamiltonian in second quantized form and discuss how you might arrive at your final answer. You will now need two types of creation operators.