Pairing fluctuations and pseudogaps in the attractive Hubbard model - art.no. 075116

The two-dimensional attractive Hubbard model is studied in the weak-to-inte rmediate-coupling regime by employing a nonperturbative approach. It is sho wn that this approach is in quantitative agreement with Monte Carlo calcula tions for both single-particle and two-particle quantities. Both the densit y of states and the single-particle spectral weight show a pseudogap at the Fermi energy below some characteristic temperature T*, also in good agreem ent with quantum Monte Carlo calculations. The pseudogap is caused by criti cal pairing fluctuations in the low-temperature renormalized classical regi me ((h) over bar omega <k(B)T) of the two-dimensional system. With increasi ng temperature the spectral weight fills in the pseudogap instead of closin g it, and the pseudogap appears earlier in the density of states than in th e spectral function. Small temperature changes around T* can modify the spe ctral weight over frequency scales much larger than temperature. Several qu alitative results for the s-wave case should remain true for d-wave superco nductors.