Quantum critical point lying beneath the superconducting dome in iron-pnictides

Whether a quantum critical point (QCP) lies beneath the superconducting dome has been a long-standing issue that remains unresolved in many classes of unconventional superconductors, notably cuprates, heavy fermion compounds and most recently iron-pnictides. The existence of a QCP may offer a route to understand: the origin of their anomalous non-Fermi liquid properties, the microscopic coexistence between unconventional superconductivity and magnetic or some exotic order, and ultimately the mechanism of superconductivity itself. The isovalent substituted iron-pnictide BaFe$_2$(As$_{1-x}$P$_x$)$_2$ offers a new platform for the study of quantum criticality, providing a unique opportunity to study the evolution of the electronic properties in a wide range of the phase diagram. Recent experiments in BaFe$_2$(As$_{1-x}$P$_x$)$_2$ have provided the first clear and unambiguous evidence of a second order quantum phase transition lying beneath the superconducting dome.