Previously I

posted about some fascinating experimental results on anisotropic thermal expansion and elastic softening near superconducting and magnetic transitions in organic charge transfer salts.

Subsequently, I became aware that the new iron pnictide superconductors do exhibit somewhat similar phenomena. A combined theory-experimental

PRL (10 co-authors!) describes shear acoustic mode spectroscopy in terms of

**nematic spin fluctuations**.

They find in undoped BaFe2As2 that the shear modulus (C66) softens significantly as one approaches the magnetically ordered phase (which is a associated with a tetragonal-orthorhombic lattice distortion). For the optimally doped material there is a hardening of the lattice as one enters the superconducting phase.

The figure above explains the nematic order parameter and how it couples to shear lattice distortions. A key is the that the magnetic phase consists of Neel antiferromagnetic order on two separate sublattices . They are weakly coupled together and the

**nematic order parameter phi** equals the dot product of m1 and m2, the antiferromagnetic order parameters on the two separate lattices. phi then couples directly to the shear strain.

The softening into the superconducting (SC) phase is explained by a coupling of the SC and AFM order parameters. This leads to a change in the static spin susceptibility upon entering the SC phase. This in turn effects the fluctuations in the nematic order parameter.

A couple of comments:

a. Experimental data is presented just for C66. It would be helpful to see it for longitudinal sound and for other transverse modes besides epsilon_s=epsilon_xy. These modes should not have significant coupling to superconductivity and magnetism if the nematic mode is where all the action is.

b. In other antiferromagnets lattice anomalies at magnetic transitions are explained in terms of the spin anisotropy (e.g. due to the

Dzyaloshinsky-Moriya interaction) coupling to the different components of the stain tensor. This is reviewed

here by Lines. Can that be ruled out in the pnictides?