Recently many interesting magnetic nanostructures have been fabricated and much attention is arising on the rich magnetic properties that originate in the quantum effects eminent in the nanoscale world. One of the peculiar aspects of the quantum effects is the spin excitation gap. In the spin-1/2 low-dimensional systems, the spin gap often appears when the lattice dimerization or the frustration in the spin-spin interaction are introduced. In the present study, we investigate the ground-state property of the spin-1/2 antiferromagnetic spin chiral nanotubes with the spatial modulation in the spin-spin interaction. The ground-state phase diagrams of them are determined by observing the behavior of the expectation value of the Lieb-SchultzMattis slow-twist operator calculated by the quantum Monte Carlo method with the continuous-time loop algorithm. We discuss the relation between the characteristic of the topology of the phase diagram and the chiral vector of the nanotubes.Key words: chiral nanotubes; spin gap; quantum Monte Carlo method PACS: 75.10. Jm, 75.40.Mg, 75.50.Ee, 75.75.+a
MotivationThese days the experimental techniques have made a lot of progress and good experimentalists have synthesized the nanoscale magnets which realize the models that had been discussed only theoretically before. Among them are the BIP-TENO as the spin-1 twoleg ladder [1], the compound [(CuCl2tachH)3Cl]Cl2 (tach=cis,trans-1,3,5-triamino-cyclohexane) as the triangular spin nanotube [2], and the oxygen molecules adsorbed on the inner surface of the porous material as a many-leg spin tube [3]. The spin ladders [4] and tubes [5] have attracted attention in the course of the extensive studies on the low-dimensional magnets, part of which started from the discussions on the Haldane gap [6] of the one-dimensional spin chains. The appearance of the spin excitation gap is one of the interesting macroscopic quantum phenomena. In spin-1/2 systems, it occurs when the frustration in the spin-spin interaction or the lattice dimerization is introduced. A good example of the former case is the triangular spin nanotube [7] whose spontanous dimerized and gapped ground state had been theoretically investigated [8] and now got a strong possibility of an experimental realization in the near future [2]. During the course of studies on the low-dimensional systems, the huge amount of studies on the carbon nanotubes