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Destabilization effect of exchange dipole-dipole interaction on the spectrum of electric dipolar ultracold Fermi gas

The self-consistent field approach for the electric dipolar ultracold spin-1/2 fermions is discussed. Contribution of the exchange part of the electric dipole interaction is found. Hence we obtain a model of dipolar fermions beyond the self-consistent field approximation. It is shown that the exchange interaction of electric dipolar fermions depends on the spin-polarisation of the system. For instance the electric dipole exchange interaction equals to zero for spin-unpolarised systems, namely all low laying quantum states occupied by two-particles with opposite spins. In opposite limit of the full spin polarisation of the degenerate fermions, then we have one particle in each quantum states, the exchange interaction has maximum value, which is comparable with the self-consistent field part of the dipole-dipole interaction. The self-consistent part of the electric dipole-dipole interaction gives a positive contribution into the spectrum of collective excitations, while the exchange part of the dipole-dipole interaction leads to a negative term in the spectrum. At angles between the equilibrium polarisation and the direction of wave propagation close to $π/2$ the full dipolar part of the spectrum becomes negative. At the electric dipole moment of fermions of order of several Debay the dipolar part is large enough to exceed the Fermi pressure, that reveals in an instability.