  We have studied the ground state properties of Hubbard model on long six-leg
square cylinders with doped hole concentration per site $5.55\% \leq \delta\leq
12.5\%$ using density-matrix renormalization group. By keeping a large number
of states for long system sizes, we find that the nature of the ground state is
remarkably sensitive to the presence of next-nearest-neighbor electron hopping
$t'$. In the positive $t'$ side, we find a robust $d$-wave superconducting (SC)
phase characterized by coexisting quasi-long-range SC and charge density wave
(CDW) correlations. Without $t'$ the ground state forms an insulating stripe
phase with long-range CDW order but short-range spin-spin and SC correlations.
In stark contrast to four-leg cylinders, our results show that the lightly
doped Hubbard model on six-leg cylinders remains insulating in the negative
$t'$ side where the SC correlations decay exponentially with short correlation
lengths. In the larger negative $t'$ side, the doped holes form a novel holon
Wigner crystal with one doped hole per emergent unit cell and short-range
spin-spin correlations.
