# Correlation function of single annihilation/creation operator vanishes

I could not find anything on that on google, or here on physics stack exchange, which surprises me. My problem is, that I do not see, why exactly

$$\left = \left = 0,\tag{1}$$

where $$<...>$$ is defined via the path integral, with $$A$$, $$B$$ being operators, $$a$$,$$b$$ their coherent states and S the action of the physical system:

$$\left = \frac{\int\mathcal{D}(a,b) ~a, b ~\mathrm{exp}(-S)}{\int\mathcal{D}(a,b)~ \mathrm{exp}(-S)},\tag{2}$$

e.g. used to calculate the correlation function / Green's function, e.g. in imaginary time:

$$G(\tau)=\left.\tag{3}$$

Now I know, that $$\left$$ and $$\left$$ have to vanish, as long as the Hamiltonian in S conserves the particle number. But I can't see, why it is zero on mathematical grounds.

Note: Usually I compute the correlation function as follows. $$\mathcal{Z}$$ is the partition function, $$\bar \phi, \phi$$ are coherent states, defined via $$a \left|\phi\right> = \phi \left|\phi\right>$$ and $$H$$ is the Hamiltonian.

$$S=\int\mathrm{d}\tau ~ \bar\phi ~ (\partial_\tau-H) ~\phi.\tag{4}$$

Lets assume a quadratic Hamiltonian and $$\phi$$, $$\bar \phi$$, $$u$$ and $$\bar u$$ all are functions of $$\tau$$.

$$\left = \frac{1}{\mathcal{Z_0}} \int\mathcal{D}(\bar \phi, \phi) ~\phi \bar \phi ~\mathrm{exp}(-S)$$

$$= \frac{1}{\mathcal{Z_0}} \frac{\delta^2}{\delta u \delta \bar u}\Big|_{u=\bar u=0} \int\mathcal{D}(\bar \phi, \phi) ~\mathrm{exp}\Big[-\int\mathrm{d}\tau ~ \big\{\bar\phi ~ (\partial_\tau-H) ~\phi + \bar\phi u + \bar u \phi\big\}\Big]$$

$$= \frac{\delta^2}{\delta u \delta \bar u}\Big|_{u=\bar u=0} ~ ~\mathrm{exp}\Big[\int\mathrm{d}\tau ~ \bar u (\partial_\tau - H)^{-1} u\Big]= \mathrm{det}\big(\partial_\tau-H\big)^{-1} .\tag{5}$$

Now I cannot see, where this goes wrong, if I take $$\left$$ or $$\left$$. Especially (!) because for a superconductor, $$\left$$ can be non-zero, so it depends on the Hamiltonian, which correlators vanish and which do not, right?

• More generally, one may prove that

1. if $$G~=~U(1)^m\times \mathbb{Z}_{n_1}\times\ldots\times \mathbb{Z}_{n_r}\tag{A}$$ is an abelian group$$^1$$;

2. if the $$k$$th field of the theory $$\phi^k~\longrightarrow~ \phi^{\prime k}~=~g^{q_k}\phi^k, \qquad g~\in~G,\tag{B}$$ transforms in an 1-dimensional representation$$^1$$ characterized by an integer charge tuple $$q_k\in\mathbb{Z}^{m+r}$$;

3. if the action $$S[\phi]$$ and the path integral measure $${\cal D}\phi$$ have a global $$G$$-symmetry;

4. and if the quantity $$F[\phi]~\longrightarrow~ F[\phi^{\prime}]~=~g^QF[\phi], \qquad g~\in~G,\tag{C}$$ has a non-trivial charge $$Q\in\mathbb{Z}^{m+r}$$: $$\exists g\in G:~~ g^Q~\neq~1~\in~\mathbb{C}; \tag{D}$$

then by changing the integration variables in the path integral one may show that the corresponding correlator transforms as \begin{align} \langle F \rangle_J ~:=~~& \frac{1}{Z[J]}\int \! {\cal D}\phi~\exp\left(\frac{i}{\hbar}\left(S[\phi] +J_k\phi^k \right)\right)F[\phi] \cr ~=~~&\frac{1}{Z[J]}\int \! {\cal D}\phi^{\prime}~\exp\left(\frac{i}{\hbar}\left(S[\phi^{\prime}] +J_k\phi^{\prime k} \right)\right)F[\phi^{\prime}] \cr ~\stackrel{(B)+(C)}{=}& \frac{1}{Z[Jg^q]}\int \! {\cal D}\phi~\exp\left(\frac{i}{\hbar}\left(S[\phi] +J_kg^{q_k}\phi^k \right)\right)g^QF[\phi] \cr ~=~~&g^Q\langle F \rangle_{Jg^q}. \end{align} \tag{E}

• In particular then

$$\langle F \rangle_{J=0} ~\stackrel{(D)+(E)}{=}~0.\tag{F}$$

• The $$G$$ symmetry could e.g. be global gauge symmetry, flavor symmetry, particle number symmetry, ghost number symmetry, parity symmetry, etc. OP's scenario (1) can be viewed as a special case of eq. (F).

• We can say more. Such symmetry implies $$G$$-invariance of

1. the path integral/partition function $$Z[J]~=~Z[Jg^q],\tag{G}$$ i.e. the generator of all Feynman diagrams;

2. the generator $$W_c[J]~=~W_c[Jg^q]\tag{H}$$ of connected Feynman diagrams;

3. and the effective/proper action $$\Gamma[\phi_{\rm cl}]~=~\Gamma[g^q\phi_{\rm cl}],\tag{I}$$ i.e. the generator of 1PI Feynman diagrams.$$^2$$

• In particular, the following quantities must respect the $$G$$-symmetry, i.e. be $$G$$-invariant or vanish:

• The connected $$n$$-point correlation functions $$\langle \phi^{k_1}\ldots \phi^{k_n}\rangle^c_{J=0}~=~\left(\frac{\hbar}{i}\right)^{n-1}W_{c,n}^{k_1\ldots k_n}.\tag{J}$$

• The $$n$$-point 1PI-correlation functions $$\Gamma_{n,k_1\ldots k_n}~=~\left.\frac{\delta^n \Gamma[\phi_{\rm cl}]}{\delta\phi^{k_1}_{\rm cl}\ldots \delta\phi^{k_n}_{\rm cl}} \right|_{\phi_{\rm cl}=0}.\tag{K}$$

• Feynman diagrams. This e.g. explains why a fermion line cannot break in a Feynman diagram.

$$^1$$ Recall that an irreducible representation $$\rho$$ of the abelian group (A) is 1-dimensional and classified by an integer charge tuple $$q=(q_1,\ldots,q_{m+r})\in\mathbb{Z}^{m+r}$$: $$G~\ni~g~=~(g_1,\ldots,g_{m+r})\quad\stackrel{\rho}{\mapsto}\quad \rho(g)~=~g^q~:=~g_1^{q_1}\ldots g_{m+r}^{q_{m+r}}~\in~\mathbb{C}. \tag{L}$$

$$^2$$ Eq. (I) can be seen from the fact that if we introduce a counter-transformation of the sources
$$J~\longrightarrow~ J^{\prime}_k~=~J_kg^{-{q_k}},\tag{M}$$ then the classical fields $$\phi_{\rm cl}=\frac{\delta W_c[J]}{\delta J}$$ also transform as $$\phi_{\rm cl}~\longrightarrow~ \phi^{\prime k}_{\rm cl}~=~g^{q_k}\phi^k_{\rm cl}.\tag{N}$$

Yes, it depends on the Hamiltonian. Specifically, if we add a linear term $$H\to H+\lambda (a+a^{\dagger})$$ this will not vanish. A simple mathematical way the expectation value $$\langle a \rangle$$ must vanish for a standard Hamiltonian quadratic in $$a$$ is that while the Hamiltonian is even under $$a\to -a$$ this expectation value is odd, therefore it must vanish (unless we take into account a spontaneous symmetry breaking. Then it can get a non-zero VEV)

• Ah that's how you can show it, thank you!! What about $<aa>$ though? It is also even under $a \rightarrow -a$, right? Commented Mar 13, 2020 at 20:02
• Oh wait, I think, I misunderstood you. With "quadratic in a" you mean quadratic in the annihilation operator, not like $a a^\dagger$, right? Then I see your argument! Thank you. But then still, I have no idea how to see, that $<a>$ is zero for example for a tight binding Hamiltonian $H=-t \sum_{<ij>}a_i^\dagger a_j$. Commented Mar 13, 2020 at 21:12
• No, I mean a standard Hamiltonian like the tight-binding one you wrote. Note that $a$ and $a^{\dagger}$ are not independent. If you take $a\to -a$ you must do the same for its hermitian conjugate, in order for it to remain the hermitian conjugate. In fact, for tight-binding Hamiltonians the symmetry is more general $a\to \exp(i\theta)a$ and this $U(1)$ symmetry reflects the conservation of number of particles. For a SC Hamiltonian this $U(1)$ symmetry breaks and only the $Z_2$ remains (multiplication by $-1$) as the number of particles if not conserved, but the parity of it is.
– user245141
Commented Mar 14, 2020 at 8:11

The average is taken over the ground state or it is a thermodynamic average (not average over the coherent states), i.e. $$$$\langle A \rangle = \sum_n e^{-\beta E_n} \langle n | A| n\rangle .$$$$ The quantum numbers implied by $$n$$ include the number of particles. If operator $$A$$ changes this number, the matrix elements are zero, because the wave functions corresponding to different quantum numbers are orthogonal.

• Thank you! Your right, maybe I should have made it more clear, that it's a thermodynamic average! But still, this average (at least in QM) depends on the Hamiltonian, as can be seen for conventional superconductors, where $<a^\dagger a^\dagger>$ does not vanish, because the ground state is a superposition of the vacuum and $a_k^\dagger a_{-k}^\dagger |0>$. So there must be another way to argue, that it vanishes, depending on the Hamiltonian, right? Commented Mar 13, 2020 at 21:06
• I don't think that it vanishes in the case of a superconductor... but this could be checked by direct calculation. Commented Mar 13, 2020 at 21:11
• In fact, for superconductor the particle number should change by 2 - a Cooper pair. Commented Mar 13, 2020 at 21:13
• Oh yeah you are right. I was thinking way too hard in the path integral direction! Thank you! Commented Mar 13, 2020 at 21:19