反铁磁体Cs_2CuCl_4锥态的弹性常数和超声衰减
Elastic constants and ultrasonic attenuation in the cone state of the frustrated antiferromagnet Cs_2CuCl_4
摘要：
在垂直于层平面的外磁场中，准二维阻挫反铁磁体Cs_2CuCl_4在低温下呈现出磁有序的“锥态”.在这种状态下，磁矩在磁场方向上的分量是有限的，而它们在层平面上的投影形成螺旋。我们提出的理论和实验结果的磁场依赖的弹性常数和超声衰减率在锥形状态。我们的理论分析是基于通常的自旋波展开周围的经典基态的海森堡模型的各向异性三角晶格与Dzyaloshinskiii-Moriya相互作用。磁振子-声子相互作用的模型，通过扩大交换相互作用的声子坐标的幂的二阶。只要外部磁场不太接近临界场，锥态变得不稳定，我们得到合理的理论和实验之间的协议，这表明至少在这个制度磁振子行为定义良好的准粒子。我们还表明，良好定义的磁振子的假设意味着在长波长下，在Cs_2CuCl_4的锥态的超声衰减率是成比例的声子动量的四次方。
英文摘要：
In an external magnetic field perpendicular to the plane of the layers, the quasi two-dimensional frustrated antiferromagnet Cs_2CuCl_4 exhibits a magnetically ordered "cone state" at low temperatures. In this state the component of the magnetic moments in field direction is finite, while their projection onto the plane of the layers forms a spiral. We present both theoretical and experimental results for the magnetic field dependence of the elastic constants and the ultrasonic attenuation rate in the cone state. Our theoretical analysis is based on the usual spin-wave expansion around the classical ground state of a Heisenberg model on an anisotropic triangular lattice with Dzyaloshinskii-Moriya interactions. Magnon-phonon interactions are modeled by expanding the exchange interactions up to second order in powers of the phonon coordinates. As long as the external magnetic field is not too close to the critical field where the cone state becomes unstable, we obtain reasonable agreement between theory and experiment, suggesting that at least in this regime magnons behave as well-defined quasiparticles. We also show that the assumption of well-defined magnons implies that at long wavelengths the ultrasonic attenuation rate in the cone state of Cs_2CuCl_4 is proportional to the fourth power of the phonon momentum.
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