  By directly measuring electrical hysteresis loops using the Positive-Up
Negative-Down (PUND) method, we accurately determined the remanent
ferroelectric polarization Pr of orthorhombic RMnO3 (R = Ho, Tm, Yb, and Lu)
compounds below their E-type spin ordering temperatures. We found that LuMnO3
has the largest Pr of 0.17 uC/cm^2 at 6 K in the series, indicating that its
single-crystal form can produce a Pr of at least 0.6 \muuC/cm^2 at 0 K.
Furthermore, at a fixed temperature, Pr decreases systematically with
increasing rare earth ion radius from R = Lu to Ho, exhibiting a strong
correlation with the variations in the in-plane Mn-O-Mn bond angle and Mn-O
distances. Our experimental results suggest that the contribution of the Mn t2g
orbitals dominates the ferroelectric polarization.
