Structural and Magnetic Phase Transitions in Iron-Based Superconductors La(O_1-xF_x)FeAs

(Above)  Graphical representation of the splitting of the {220} diffraction peak as the structural phase transition proceeds.  The horizontal axis is diffraction angle, and the vertical axis is temperature.  (Below)  Quantitative data.  If you follow the peaks from low temperature where they are well separated, the peaks move towards each other with increasing temperature, suggesting a second order type transition, but then the peaks rapidly collapse into a single peak, which could indicate that the transition is actually weakly first order.  This is an aspect that needs further detailed measurements in the vicinity of the transition.

The structural transition was a surprise as it was thought that the strong anomalies in the bulk properties were due to the formation of a spin-density-wave magnetic state.  But even more of a surprise was the observation that the magnetic transition occurred at a temperature that was substantially below the structural transition.

Magnetic structure and order parameter (solid blue points) obtained at the NCNR.  Green squares-- order parameter measurements at Oak Ridge National Laboratory.

Publication

Magnetic Order Close to Superconductivity in the Iron-based Layered La(O_1-xF_x)FeAs systems, C. de la Cruz, Q. Huang, J. W. Lynn, J. Li, W. Ratcliff II, J. L. Zarestky, H. A. Mook, G. F. Chen, J. L. Luo, N. L. Wang, and P. Dai, Published online 5/28/2008 in Nature (article).  Supplementary Material

Last modified 22-July-2008