Universality and nonmonotonic finite-size effects above the upper critical dimension

We analyze universal and nonuniversal finite-size effects of lattice systems in a $L^d$ geometry above the upper critical dimension d = 4 within the O(n) symmetric $\phi^4$ lattice theory. On the basis of exact results for $n \to\infty$ and one-loop results for n = 1 we identify significant lattice effects that cannot be explained by the $\phi^4$ continuum theory. Our analysis resolves longstanding discrepancies between earlier asymptotic theories and Monte Carlo (MC) data for the five-dimensional Ising model of small size. We predict a {\it nonmonotonic} L dependence of the scaled susceptibility $\chi L^{-d/2}$ at $T_c$ with a weak maximum that has not yet been detected by MC data.