User:Aalopes/SandboxRPA



The random phase approximation (RPA) is an approximation method in condensed matter physics and in nuclear physics. It was first introduced by David Bohm and David Pines as an important result in a series of seminal papers. For decades physicists had been trying to incorporate the effect of microscopic quantum mechanical interactions between electrons in the theory of matter. Bohm and Pines RPA approximation accounts for the weak screened Coulomb interaction, and RPA is commonly used for describing the dynamic linear electronic response of electron systems.

In the RPA, electrons are assumed to respond only to the total electric potential V(r) which is the sum of the external perturbing potential Vext(r) and a screening potential Vsc(r). The external perturbing potential is assumed to oscillate at a single frequency ω, so that the model yields via a self-consistent field (SCF) method a dynamic dielectric function


 * εRPA(k, ω).

The contribution to the dielectric function from the total electric potential is assumed to average out, so that only the potential at wave vector k contributes. This is what is meant by the random phase approximation. The resulting dielectric function, also called the Lindhard dielectric function, correctly predicts a number of properties of the electron gas, including plasmons.

The RPA was criticized in the late 50's for overcounting the degrees of freedom and the call for justification lead to intense work among theoretical physicists. In a seminal paper Murray Gell-Mann and Keith Brueckner showed that the RPA can be derived from a summation of leading-order chain Feynman diagrams in a dense electron gas.

The consistency in these results became an important justification and motivated a very strong growth in theoretical physics in the late 50's and 60's.

RPA Ground State
The RPA vacuum $$\left|\mathbf{RPA}\right\rangle$$ for a bosonic system can be expressed in terms of non-correlated bosonic vacuum $$\left|\mathbf{MFT}\right\rangle$$ and original boson excitations $$\mathbf{a}_{i}^{\dagger}$$

$$\left|\mathrm{RPA}\right\rangle=\mathcal{N}\mathbf{e}^{Z_{ij}\mathbf{a}_{i}^{\dagger}\mathbf{a}_{j}^{\dagger}/2}\left|\mathrm{MFT}\right\rangle$$

where Z is a symmetric matrix with  $$|Z|\leq 1$$ and

$$\mathcal{N}= \frac{\left\langle \mathrm{MFT}\right\|\left.\mathrm{RPA}\right\rangle}{\left\langle  \mathrm{MFT}\right\|\left.\mathrm{MFT}\right\rangle}$$

The normalization can be calculated by

$$\langle \mathrm{RPA}|\mathrm{RPA}\rangle= \mathcal{N}^2 \langle \mathrm{MFT}| \mathbf{e}^{z_{i}(\tilde{\mathbf{q}}_{i})^2/2} \mathbf{e}^{z_{j}(\tilde{\mathbf{q}}^{\dagger}_{j})^2/2} $$
 * \mathrm{MFT}\rangle=1

where $$Z_{ij}=(X^{\mathrm{t}})_{i}^{k} z_{k} X^{k}_{j}$$ is the singular value decomposition of $$Z_{ij}$$. $$\tilde{\mathbf{q}}^{i}=(X^{\dagger})^{i}_{j}\mathbf{a}^{j}$$

$$\mathcal{N}^{-2}= \sum_{m_{i}}\sum_{n_{j}} \frac{(z_{i}/2)^{m_{i}}(z_{j}/2)^{n_{j}}}{m!n!} \langle \mathrm{MFT}| \prod_{i\,j} (\tilde{\mathbf{q}}_{i})^{2 m_{i}} (\tilde{\mathbf{q}}^{\dagger}_{j})^{2 n_{j}} $$
 * \mathrm{MFT}\rangle

$$=\prod_{i} \sum_{m_{i}} (z_{i}/2)^{2 m_{i}} \frac{(2 m_{i})!}{m_{i}!^2}= $$

$$ \prod_{i}\sum_{m_{i}} (z_{i})^{2 m_{i}} {1/2 \choose m_{i}}=\sqrt{\det(1-|Z|^2)} $$

the connection between new and old excitations is given by

$$\tilde{\mathbf{a}}_{i}=\left(\frac{1}{\sqrt{1-Z^2}}\right)_{ij}\mathbf{a}_{j}+ \left(\frac{1}{\sqrt{1-Z^2}}Z\right)_{ij}\mathbf{a}^{\dagger}_{j}$$