1] Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA [2] Ames Laboratory-USDOE, Ames, Iowa 50011, USA [3].
1] Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA [2] Ames Laboratory-USDOE, Ames, Iowa 50011, USA.
Nat Commun. 2014;5:3229. doi: 10.1038/ncomms4229.
Many of the iron pnictides have strongly anisotropic normal-state characteristics, important for the exotic magnetic and superconducting behaviour these materials exhibit. Yet, the origin of the observed anisotropy is unclear. Electronically driven nematicity has been suggested, but distinguishing this as an independent degree of freedom from magnetic and structural orders is difficult, as these couple together to break the same tetragonal symmetry. Here we use time-resolved polarimetry to reveal critical nematic fluctuations in unstrained Ba(Fe1-xCox)2As2. The femtosecond anisotropic response, which arises from the two-fold in-plane anisotropy of the complex refractive index, displays a characteristic two-step recovery absent in the isotropic response. The fast recovery appears only in the magnetically ordered state, whereas the slow one persists in the paramagnetic phase with a critical divergence approaching the structural transition temperature. The dynamics also reveal a gigantic magnetoelastic coupling that far exceeds electron-spin and electron-phonon couplings, opposite to conventional magnetic metals.
许多铁基磷化物具有强烈各向异性的正常态特性，这对这些材料表现出的奇异磁性和超导行为很重要。然而，观察到的各向异性的起源尚不清楚。有人提出电子驱动的向列性，但要将其与磁序和结构序区分开来，作为一个独立的自由度是很困难的，因为它们结合在一起会破坏相同的四方对称性。在这里，我们使用时间分辨极化法来揭示未受应变的 Ba(Fe1-xCox)2As2 中的临界向列性波动。飞秒各向异性响应源于复折射率的两倍面内各向异性，显示出特征的两步恢复，而各向同性响应中则没有。快速恢复仅出现在磁有序状态，而缓慢恢复则在顺磁相中持续存在，其临界发散接近结构相变温度。动力学还揭示了一种巨大的磁弹耦合，远远超过电子自旋和电子声子耦合，与传统的磁性金属相反。
