Physics – Condensed Matter – Superconductivity
Scientific paper
2004-06-17
Europhys. Lett. 71, 359 (2005)
Physics
Condensed Matter
Superconductivity
4 pages, 1 figure
Scientific paper
10.1209/epl/i2005-10097-y
A long-standing problem with the many-body approximations for interacting condensed bosons has been the dichotomy between the ``conserving'' and ``gapless'' approximations, which either obey the conservations laws or satisfy the Hugenholtz-Pines condition for a gapless excitation spectrum, in the order. It is here shown that such a dichotomy does not exist for a system of composite bosons, which form as bound-fermion pairs in the strong-coupling limit of the fermionic attraction. By starting from the constituent fermions, for which conserving approximations can be constructed for any value of the mutual attraction according to the Baym-Kadanoff prescriptions, it is shown that these approximations also result in a gapless excitation spectrum for the boson-like propagators in the broken-symmetry phase. This holds provided the corresponding equations for the fermionic single- and two-particle Green's functions are solved self-consistently.
Pieri Pierbiagio
Strinati Giancarlo Calvanese
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