Exact diagonalization and cluster mean-field study of triangular-lattice XXZ antiferromagnets near saturation

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説明

Quantum magnetic phases near the magnetic saturation of triangular-lattice antiferromagnets with XXZ anisotropy have been attracting renewed interest since it has been suggested that a nontrivial coplanar phase, called the π-coplanar or Ψ phase, could be stabilized by quantum effects in a certain range of anisotropy parameter J/Jz besides the well-known 0-coplanar (known also as V) and umbrella phases. Recently, Sellmann et al. [Phys. Rev. B 91, 081104(R) (2015)] claimed that the π-coplanar phase is absent for S=1/2 from an exact-diagonalization analysis in the sector of the Hilbert space with only three down-spins (three magnons). We first reconsider and improve this analysis by taking into account several low-lying eigenvalues and the associated eigenstates as a function of J/Jz and by sensibly increasing the system sizes (up to 1296 spins). A careful identification analysis shows that the lowest eigenstate is a chirally antisymmetric combination of finite-size umbrella states for J/Jz≳2.218 while it corresponds to a coplanar phase for J/Jz≲2.218. However, we demonstrate that the distinction between 0-coplanar and π-coplanar phases in the latter region is fundamentally impossible from the symmetry-preserving finite-size calculations with fixed magnon number. Therefore, we also perform a cluster mean-field plus scaling analysis for small spins S≤3/2. The obtained results, together with the previous large-S analysis, indicate that the π-coplanar phase exists for any S except for the classical limit (S→∞) and the existence range in J/Jz is largest in the most quantum case of S=1/2.(Some mathematical expressions are not properly displayed. Please use the attachment.)

source:https://journals.aps.org/prb/abstract/10.1103/PhysRevB.96.014431#fulltext

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  • Physical Review B

    Physical Review B 96 (1), 014431-, 2017-07-25

    American Physical Society

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