Unconventional Signatures of a Quantum Griffiths Phase in the Vicinity of a Quenched Nematic Quantum Critical Point*

Upon suppression of a continuous electronic phase transition towards 0K, for example through hydrostatic pressure as an external tuning parameter, a quantum critical point (QCP) occurs in many different material classes. These points constitute a fascinating research area as the diverging quantum fluctuations and electronic correlations could be linked to many unconventional and exciting quantum phases, including the emergence of high-T_c superconductivity. Nevertheless, even more exotic phase may be expected if the symmetry of the electronic order parameter allows for a bilinear coupling to the structural degrees of freedom. The electronic nematic order, ubiquitous in the iron-based superconductors, is one such candidate, where long-range interactions can be mediated through the lattices shear modes.

In this talk, we will first review the unique nature of the isolated nematic QCP in FeSe_0.89S_0.11 under hydrostatic pressure, in the absence of any competing electronic order. At this point, the nematoelastic coupling quenches the quantum critical fluctuations and hence, superconductivity is weakened instead of enhanced. We will then demonstrate that the magnetoresistivity close to the QCP obeys a scaling relation over two decades in temperature with diverging critical exponents at low temperatures. Such divergences are in a stark contrast to the usual ansatz using fixed exponents and the notion of universality classes. We will discuss our findings in the context of an unusual quantum Griffiths phase, emerging from disconnected static and dynamic quantum fluctuations. Moreover, the coupling between the electronic and phononic modes and a possible topological change of the Fermi surface lead to the emergence of an atypical non-zero energy scale at the QCP, consistent with the observation of finite electronic correlations.