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arXiv 0901.4803cond-mat.str-el
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三角晶格反铁磁体中的自旋波:衰变、谱重整化和奇异性

Spin-waves in triangular lattice antiferromagnet: decays, spectrum renormalization, and singularities

A. L. Chernyshev, M. E. Zhitomirsky

摘要:

本文在自旋波理论的框架下,对三角晶格上量子海森堡反铁磁体的动力学性质进行了全面的研究。三角晶格反铁磁体中自旋波激发的显著特征是:(i)由于自发的两个磁振子衰变,在零温度下的有限寿命;(ii)磁振子能量相对于谐波结果的强重整化;(iii)衰变率γ_k的对数奇异性。利用最低阶磁振子自能的Dyson方程的在壳层和离壳层解,得到了磁振子谱的量子修正。在低能时,磁振子激发仍然是明确的,尽管在k->0和γ_k ~ 2处有反常的衰变率γ_k ~ k^2。|k-Q_AF|在高能量时,磁振子的衰减很大,当S=1/2时,衰减率达到(2 Gamma_k/E_k)~ 0.3。在壳层上的解在Gamma_k中表现出对数奇异性,同时在E_k中沿动量空间中的某些轮廓出现跳跃状间断。这种奇异性在磁振子谱函数A(k,w)中甚至更为突出。虽然离壳解消除了这种对数奇异性,但衰变率仍然大大提高。我们还讨论了高阶校正的作用,并表明,这种奇异性可能会导致完全消失的频谱在附近的某些k点。分析了三角晶格反铁磁体的各种推广以及可果美晶格上的XXZ模型的双磁振子衰变的运动学条件。我们的结果表明,衰变和奇异性的自旋波谱必须是普遍存在的所有这些系统。

英文摘要:

We present a comprehensive study of the dynamical properties of the quantum Heisenberg antiferromagnet on a triangular lattice within the framework of spin-wave theory. The distinct features of spin-wave excitations in the triangular-lattice antiferromagnet are (i) finite lifetime at zero temperature due to spontaneous two-magnon decays, (ii) strong renormalization of magnon energies with respect to the harmonic result, and (iii) logarithmic singularities in the decay rate Gamma_k. Quantum corrections to the magnon spectrum are obtained using both the on-shell and off-shell solutions of the Dyson equation with the lowest-order magnon self-energy. At low-energies magnon excitations remain well-defined albeit with the anomalous decay rate Gamma_k ~ k^2 at k->0 and Gamma_k ~ |k-Q_AF|^7/2 at k->Q_AF. At high energies, magnons are heavily damped with the decay rate reaching (2 Gamma_k/E_k) ~ 0.3 for the case S=1/2. The on-shell solution shows logarithmic singularities in Gamma_k with the concomitant jump-like discontinuities in E_k along certain contours in the momentum space. Such singularities are even more prominent in the magnon spectral function A(k,w). Although the off-shell solution removes such log-singularities, the decay rates remain strongly enhanced. We also discuss the role of higher-order corrections and show that such singularities may lead to complete disappearance of the spectrum in the vicinity of certain k-points. The kinematic conditions for two-magnon decays are analyzed for various generalizations of the triangular-lattice antiferromagnet as well as for the XXZ model on a kagome lattice. Our results suggest that decays and singularities in the spin-wave spectra must be ubiquitous in all these systems.

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