Origin of spiral-spin order:back to origin DE+SE+NNN AFM+JT term 0.5 (c) 0.4 J2=J2b2 J2=0 (3)energy gap 0.3 0.2 0.1 f5 120 JAF(0.001)30 40 60 80 100 00 20 J2(0.001) (d) J2=J26,J2a-0 A 3 2 (1)proper wave-number -3 -2 0 2 E-Er  DE+SE+NNN AFM+JT term (1) proper wave-number (3) energy gap J2=J2b, J2a=0 J2=J2b, J2a=0 Origin of spiral-spin order: back to origin