Y. Gallais, R. M. Fernandes, I. Paul, L. Chauviere, Y. -X. Yang, M. -A. Measson, M. Cazayous, A. Sacuto, D. Colson, A. Forget
Electronic analogues of nematic states, in which rotational symmetry is broken but translational invariance is preserved, have been proposed in a variety of correlated materials, such as quantum Hall systems, cuprates, ruthenates, heavy fermions and, more recently, iron pnictide superconductors. In the latter, several experiments on strained samples have collected strong but indirect evidence that the tetragonal-to-orthorhombic structural transition is driven not by the lattice, but by electronic nematicity. However, these measurements cannot disentangled the roles of the spin, charge and orbital degrees of freedom in the nematic instability. Here, using electronic Raman spectroscopy, we report the first direct measurements of incipient charge nematicity in the tetragonal phase of strain-free Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ single crystals. The diverging Raman response at low temperatures unveils an underlying charge nematic state that has hitherto remained unnoticed. Its fluctuations persist above the superconducting dome, suggesting they may play a role in the pairing mechanism. Comparison with the behavior of the elastic modulus allows us to disentangle the charge contribution to the nematic instability, imposing strong constraints on theoretical models for this intricate correlated state.
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http://arxiv.org/abs/1302.6255
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