  At ambient pressure CrAs undergoes a first-order transition into a
double-helical magnetic state at TN = 265 K, which is accompanied by a
structural transition. The recent discovery of pressure-induced
superconductivity in CrAs makes it important to clarify the nature of quantum
phase transitions out of its coupled structural/helimagnetic order. Here we
show, via neutron diffraction on the single-crystal CrAs under hydrostatic
pressure (P), that the combined order is suppressed at Pc ~ 10 kbar, near which
bulk superconductivity develops with a maximal transition temperature Tc ~ 2 K.
We further show that the coupled order is also completely suppressed by
phosphorus doping in CrAs1-xPx at a critical xc ~ 0.05, above which inelastic
neutron scattering evidenced persistent antiferromagnetic correlations,
providing a possible link between magnetism and superconductivity. In line with
the presence of antiferromagnetic fluctuations near Pc (xc), the A coefficient
of the quadratic temperature dependence of resistivity exhibits a dramatic
enhancement as P (x) approaches Pc (xc), around which Res(T) has a
non-Fermi-liquid form. Accordingly, the electronic specific-heat coefficient of
CrAs1-xPx peaks out around xc. These properties provide clear evidences for
quantum criticality, which we interpret as originating from a nearly
second-order helimagnetic quantum phase transition that is concomitant with a
first-order structural transition. Our findings in CrAs highlight the distinct
characteristics of quantum criticality in bad metals, thereby bringing out new
insights into the physics of unconventional superconductivity such as occurring
in the high-Tc iron pnictides.
