  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.
