A spin-Peierls system (DMe-DCNQI) 2 Li has been studied with electron paramagnetic resonance ͑EPR͒ under hydrostatic pressure. A definite frequency dependence of the EPR linewidth is observed for the Curie spins that appear below T SP , demonstrating a clear one-dimensional diffusive character similar to t- (CH) x . This is strong evidence for Curie spins to arise from spin solitons as domain walls caused by structural defects. The estimated extent of the spin soliton is less than several

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... lar units. A relation with the antiferromagnetic ordering provoked by impurities as found in CuGeO 3 is discussed. Lithium salt of dimethyl-dicyanoquinone-diimine ͓(DMe-DCNQI) 2 Li͔ is a one-dimensional ͑1D͒ -band system composed of 1D stacks of DMe-DCNQI molecules, where one electron transferred from each Li counter ion to each couple of DMe-DCNQI molecules. At room temperature this system is an insulator 1 with a probable 4k F dimerization gap at the Fermi energy, suggested from quite similar physical properties to (DMe-DCNQI) 2 Ag. 2 A relatively higher electrical conductivity is believed to arise from a collective mode of domain walls excited thermally. 1 In an iodized system (DI-DCNQI) 2 Ag with a narrower bandwidth, the Wigner crystallization of electrons occurs below 220 K, demonstrating the crucial role of the long-range Coulomb interaction between DCNQI molecules in these systems. 3 Magnetic properties suggest (DMe-DCNQI) 2 Li has a nonmagnetic ground state below 60 K accompanied by a 2k F superstructure, corresponding to tetramerization of DCNQI molecules, 4 ascribable to a spin-Peierls transition like the Ag-salt. 5 If the number of DCNQI molecules in a chain is 4Nϩn, where N is an integer and n is 1, 2, or 3, the spin soliton could be present in the spin-Peierls state. Such a spin soliton might be mobile with the Goldstone mode of kink solitons, as actually observed in trans-polyacetylene ͓t-(CH) x ͔. 6,7 The spin solitons induced by impurity doping would be concerned with the appearance of a long-range antiferromagnetic ͑AF͒ ordering below T SP , as in the inorganic spin-Peierls system CuGeO 3 doped with impurities such as Zn or Si. [8] [9] [10] [11] To induce AF long-range ordering in the spin-Peierls state, the spatial extent of the spin solitons becomes crucial. In this meaning, it is interesting to investigate the nature of spin solitons in spin-Peierls systems. Since the magnitude of the Curie tail in (DMe-DCNQI) 2 Li depends on sample quality, the Curie tail has been assigned to localized impurity spins. From such a reason, the dynamics of the Curie spins, that is, the spin solitons, has remained unclear. 12 In this report, we demonstrate evidence for spin solitons and their dynamics in the spin-Peierls system (DMe-DCNQI) 2 Li with electron paramagnetic resonance ͑EPR͒ in a wide frequency range from 10 to 20 000 MHz. Strong evidence for the presence of spin solitons is obtained from a definite quasi-one-dimensional diffusive motion of the Curie spins coming out below T SP , which is deduced from the frequency dependence of the EPR linewidth. 7 The observed behavior of the diffusion rate similar to t-(CH) x reminds us that the present system has the same degeneracy in the ground-state energy as t-(CH) x . In addition, the frequency spectrum at the lowest temperature 4.2 K tells us that the spin soliton extent should be less than several molecular units. A weak intra-and interchain interaction between the solitons arising from such a small extent elucidates why AF long-range ordering was not observed in (DMe-DCNQI) 2 Li, although it takes place in CuGeO 3 systems doped with impurity concentrations less than 1 at. % which is comparable to that for the present spin solitons. The small extent of the spin soliton would be consistent with the crucial role of the long-range Coulomb interaction in systems such as the Li and Ag salts of DCNQI. 4,13