强磁场在ZnCr2Se4中诱导的各向异性太赫兹共振吸收

Anisotropic resonance absorptions induced by high magnetic field in ZnCr2Se4

作为典型的具有螺旋磁结构的材料,ZnCr2Se4承载着诸如磁电耦合、磁致伸缩和负热膨胀等有趣特性,并可能具备多种不同的量子基态.本文利用太赫兹时域光谱技术研究了ZnCr2Se4在低温强磁场(T=4-60 K,H=0-10 T)下的自旋动力学行为.当外加磁场高于4 T时,可以观察到亚太赫兹频率范围的磁共振吸收,并呈现出随磁场增加蓝移特征.当磁场(H)方向垂直于太赫兹波矢(k)方向时,仅观察到单个共振吸收,且其磁场行为符合线性拉莫尔进动关系.这种磁场依赖性对应传统的铁磁共振,意味着螺旋自旋态在高磁场下演化为线性铁磁态.然而,在H和k同时平行于样品的<111>晶向配置下,当磁场强度高于7 T时,其太赫兹共振明显劈裂为高频和低频两个吸收峰,并且其高频吸收表现出非线性磁场依赖关系.这种奈尔温度以下特有的各向异性太赫兹自旋动力学效应可能与最近发现的量子临界区域有关.

As a typical helimagnet,ZnCr2Se4 possesses fascinating effects including magnetoelectric coupling,magnetostriction,negative thermal expansion,as well as possible diversity in quantum ground states.Here in this work,we investigate magnetic excitation arising from spiral spin structure in ZnCr2Se4 single crystal by using terahertz(THz)time domain spectroscopy(THz-TDS)under magnetic fields up to 10 T and at low temperatures.The magnetic resonance absorption is observed in a sub-THz region as the applied magnetic field is above 4 T,featuring the blue shift with magnetic field increasing.As the THz wave vector(k)is vertical to the external magnetic field(H),the single resonance frequency conforms well with the linear Larmor relation,corresponding to a spin structure transformation from helical to ferromagnetic state with magnetic field increasing in ZnCr2Se4.However,in the geometry in which both k and H are along the(111) direction of crystal,a well-defined resonance splitting emerges when H>7 T.Especially,the high-frequency absorption shows pronouncedly nonlinear magnetic field dependence.It is suggested that such anisotropic spin dynamics below Néel temperature be linked with the field-driven quantum criticality unveiled in recent work.