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## Homework Statement

A system is subject to Hamiltonian ##\hat{H}=-\gamma B_z \hat{S}_z##. Write down the density matrix.[/B]

## Homework Equations

For canonical ensemble

##\hat{\rho}=\frac{1}{Tr(e^{-\beta \hat{H}})}e^{-\beta \hat{H}}##

In general ##\rho=\sum_m |\psi_m\rangle \langle \psi_m|##

## The Attempt at a Solution

How to know which definition of density matrix to use? Thanks.