Density-matrix renormalization-group study of the spin gap in a one-dimensional Hubbard model: Effect of the distant transfer and exchange coupling

The spin gap of a one-dimensional repulsive Hubbard model is numerically calculated with the density-matrix renormalization group, with a special emphasis on the effect of a next-nearest-neighbsr hopping (t') and the nearest-neighbor ferromagnetic exchange (J) interact ion. At half filling, a significant spin gap opens if \t'\similar or equal to\t\ and J=0, in agreement with the weak-coupling theory, while the gap is strongly suppressed by the introduction of J. On the other hand, the quarter-filled system has very small spin gaps regardless of the values of t' and J. Implications for the CuO2 chain in Sr14Cu24O41 and related materials are discussed. [S0163-1829(98)07617-6].