Path integral Monte Carlo simulations for fermion systems: pairing in the electron-hole plasma

We review the path integral method wherein quantum systems are mapped with Feynman's path integrals onto a classical system of 'ring-polymers' and then simulated with the Monte Carlo technique. Bose or Fermi statistics correspond to possible 'cross-linking' of polymers. As proposed by Feynman, superfluidity and Bose condensation result from macroscopic exchange of bosons. To map fermions onto a positive probability distribution, one must restrict the paths to lie in regions where the fermion density matrix is positive. We discuss a recent application to the two-component electron-hole plasma. At low temperature excitons and bi-excitons form. We have used nodal surfaces incorporating paired fermions and see evidence of a Bose condensation in the energy, specific heat and superfluid density. In the restricted path integral picture, pairing appears as intertwined electron-hole paths. Bose condensation occurs when these intertwined paths wind around the periodic boundaries.