Ultrafast observation of critical nematic fluctuations and giant magnetoelastic coupling in iron pnictides.
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.
1] Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA [2] Ames Laboratory-USDOE, Ames, Iowa 50011, USA [3].
Aaron
A
Patz
1] Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA [2] Ames Laboratory-USDOE, Ames, Iowa 50011, USA [3].
Tianqi
T
Li
1] Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA [2] Ames Laboratory-USDOE, Ames, Iowa 50011, USA.
Sheng
S
Ran
School of Physics and Astronomy, University of Minnesota, Minneapolis, Minnesota 55455, USA.
Rafael M
RM
Fernandes
Institute for Theory of Condensed Matter and Institute for Solid State Physics, Karlsruhe Institute of Technology, 76131 Karlsruhe, Germany.
Joerg
J
Schmalian
1] Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA [2] Ames Laboratory-USDOE, Ames, Iowa 50011, USA.
Sergey L
SL
Bud'ko
1] Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA [2] Ames Laboratory-USDOE, Ames, Iowa 50011, USA.
Paul C
PC
Canfield
1] Department of Physics, University of Crete, Heraklion 71003, Greece [2] Institute of Electronic Structure and Laser, Foundation for Research and Technology-Hellas, Heraklion 71110, Greece.
Ilias E
IE
Perakis
1] Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA [2] Ames Laboratory-USDOE, Ames, Iowa 50011, USA.
Jigang
J
Wang