Strain-tuning nematic order and signatures of nematic quantum criticality*

Quantum criticality associated with electronic nematic order has been suggested as a possible avenue for a range of exotic electronic effects, from non-Fermi liquid behavior to superconductivity. In order to study the behavior of metals proximate to such a quantum critical point, it is useful to establish effective tuning parameters that can drive the critical temperature of an electronic nematic phase to zero. I will describe how both symmetric and orthogonal antisymmetric strains can play this role, and demonstrate these effects in an archetypal Fe-based superconductor. For compositions progressively closer to the putative nematic quantum critical point, these tuning parameters become increasingly more effective, the precise variation of which provides evidence for a wide range of composition and temperature over which quantum critical fluctuations play a key role in shaping the properties of this family of materials. Additional evidence can be found in the temperature and doping dependence of the nematic suceptibility, which, by applying large magnetic fields, can be measured in the absence of superconductivity down to low temperatures. Further insights can be obtained by consideration of other model material systems, in particular materials that undergo ferroquadrupolar order of local 4f atomic orbitals.