Superconducting fluctuations in the Hubbard model with an attractive on-site interaction in one dimension

The low-temperature thermodynamics of the one-dimensional Hubbard model with attractive on-site interaction of the electrons is discussed within the framework of the Bethe ansatz for the wave function. At low T the dominant states consist of spin-paired electrons (Cooper-pair-like) and excitations of such pairs (without breaking up the singlet bound states). A critical field H_c (energy required to depair a Cooper pair) is obtained at T=0 (note that the superconducting T_c =0), which disappears for T≠0. There is no magnetic response at T=0 for H<H_c. The one-electron correlation function falls off exponentially with distance, while the singlet pair-singlet pair correlation decreases with a power law. The specific heat is linear in temperature, which is possibly a consequence of the dimension. The elementary excitations of the system are discussed. The excitation spectrum shows some analogies to that of resonant valence bonds in two dimensions.