Inelastic neutron scattering was employed to investigate the impact of electronic nematic order on the magnetic spectra of LaFeAsO and Ba(Fe$_{0.953}$Co$_{0.047}$)$_{2}$As$_{2}$. These materials are ideal to study the paramagnetic-nematic state, sinc
e the nematic order, signaled by the tetragonal-to-orthorhombic transition at $T_{{rm S}}$, sets in well above the stripe antiferromagnetic ordering at $T_{{rm N}}$. We find that the temperature-dependent dynamic susceptibility displays an anomaly at $T_{{rm S}}$ followed by a sharp enhancement in the spin-spin correlation length, revealing a strong feedback effect of nematic order on the low-energy magnetic spectrum. Our findings can be consistently described by a model that attributes the structural/nematic transition to magnetic fluctuations, and unveils the key role played by nematic order in promoting the long-range stripe antiferromagnetic order in iron pnictides.
The presence of magnetic clusters has been verified in both antiferromagnetic and ferromagnetic quantum critical systems. We review some of the strongest evidence for strongly doped quantum critical systems (Ce(Ru$_{0.24}$Fe$_{0.76}$)$_2$Ge$_2$) and
we discuss the implications for the response of the system when cluster formation is combined with finite size effects. In particular, we discuss the change of universality class that is observed close to the order-disorder transition. We detail the conditions under which clustering effects will play a significant role also in the response of stoichiometric systems and their experimental signature.
In a recent paper, Custers {it et al.} cite{custers} argue for the existence of a new metallic quantum critical phase at 0 K in the Ge-doped heavy-fermion system YbRh$_2$Si$_2$ in the presence of magnetic frustration. In here we discuss the consequen
ces of this identification for the (more standard) field induced quantum critical phase.
