Key equations from quantum statistical tools

Equilibrium

Expectation values

$A=\mathrm{T}\mathrm{r}[{\hat{\rho }}_{\mathrm{e}\mathrm{q}}\hat{A}]$

Density matrix of fermions in equilibrium

${\hat{\rho }}_{\mathrm{e}\mathrm{q}}=\sum _{\alpha }f(E_{\alpha })|E_{\alpha }⟩⟨E_{\alpha }|=f(\hat{H}-\mu \hat{I})$

• Fermi-Dirac distribution function: $f(E)=[\exp ((E-\mu )/k_{B}T)+1{]}^{-1}$
• Hamiltonian and its spectral decomposition: $\hat{H}=\sum _{\alpha }{E}_{\alpha }|E_{\alpha }⟩⟨E_{\alpha }|$
• function of Hamiltonian: $F(\hat{H})=\sum _{\alpha }F(E_{\alpha })|E_{\alpha }⟩⟨E_{\alpha }|$

Charge density

• charge density operator: $\hat{n}(\mathbf{𝐫})=|\mathbf{𝐫}⟩⟨\mathbf{𝐫}|$

• expectation value: $n(\mathbf{𝐫})=\mathrm{T}\mathrm{r}[{\hat{\rho }}_{\mathrm{e}\mathrm{q}}|\mathbf{𝐫}⟩⟨\mathbf{𝐫}|]=⟨\mathbf{𝐫}|{\hat{\rho }}_{\mathrm{e}\mathrm{q}}|\mathbf{𝐫}⟩$ (in some discrete representation these is just diagonal matrix element)

Density of states

• local density of states: $n(\mathbf{𝐫})=\mathrm{T}\mathrm{r}[{\hat{\rho }}_{\mathrm{e}\mathrm{q}}|\mathbf{𝐫}⟩⟨\mathbf{𝐫}|]=\sum _{\alpha }|{\mathrm{\Psi }}_{\alpha }(\mathbf{𝐫}){|}^{2}f(E_{\alpha })=\int dE[]f(E-E_{\alpha })\int dEg(r,E)f(E)$

Nonequilibrium

• Expectation values:

• Current operator:

• Spin torque operator: