NMR lineshape and phase soliton effects in incommensurate Rb2ZnCl4

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Abstract
In incommensurate systems the NMR, NQR or EPR resonance frequency varies in space in a way which reflects the spatial variation of the incommensurate modulation wave. An evaluation of the magnetic resonance lineshape for the case of a multi-soliton lattice is presented for different ratios between the soliton width and the inter-soliton spacing as well as for different relations between the resonance frequency and the incommensurate order parameter. The results are compared with the experimental data in Rb2ZnCl4. It is found that in this system the soliton width is large compared with the inter-soliton spacing over most of the incommensurate phase. The temperature variation of the phase soliton density has been determined. The non-classical value of the critical exponent beta =0.35+or-0.03 for the amplitude of the order parameter demonstrates that the whole incommensurate phase is critical in this one-dimensionally modulated system.