Low-temperature thermodynamics of Heisenberg helimagnets

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Abstract
The authors present a new variational approach suitable for studying the temperature-dependent properties of Heisenberg helimagnets. This problem brings intriguing difficulties; indeed a direct extension to helimagnets of the techniques which are successful for collinear systems, provides unsatisfactory results. Recent calculations concerning quantum contributions to the elementary excitation spectrum at zero temperature have pointed out that magnon-magnon cubic interactions, which are neglected in standard variational methods, are essential to prevent an obvious violation of the Goldstone theorem. Thus the authors modify the variational approach in order to take such interactions into account and we obtain the temperature dependences of the helix wave-vector and the spin wave spectrum which at vanishing temperature substantiate previous perturbative calculations. The phase diagram of the ANNNH model is obtained on the basis of the authors' theory.