▎ 摘　　要

Understanding the material parameters that control the superconducting transition temperature T-c is a problem of fundamental importance. In many novel superconductors phase fluctuations determine T-c, rather than the collapse of the pairing amplitude. We derive rigorous upper bounds on the superfluid phase stiffness for multiband systems, valid in any dimension. This in turn leads to an upper bound on T-c in two dimensions, which holds irrespective of pairing mechanism, interaction strength, or order-parameter symmetry. Our bound is particularly useful for the strongly correlated regime of low-density and narrow-band systems, where mean-field theory fails. For a simple parabolic band in 2D with Fermi energy E-F, we find that k(B)T(c)