Evolution of Magnetic Double Helix and Quantum Criticality near a Dome of Superconductivity in CrAs

At ambient pressure, CrAs undergoes a first-order transition into a double-helical magnetic state atTN=265K, 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 the coupled structural/helimagnetic order in this system. Here, we show, via neutron diffraction on the single-crystal CrAs under hydrostatic pressure (P), that the combined order is suppressed atPc≈10kbar, near which bulk superconductivity develops with a maximal transition temperatureTc≈2K. We further show that the coupled order is also completely suppressed by phosphorus doping inCrAs1−xPxat a criticalxc≈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 nearPc(xc), theAcoefficient of the quadratic temperature dependence of resistivity exhibits a dramatic enhancement asP(x) approachesPc(xc), around whichρ(T)has a non-Fermi-liquid form. Accordingly, the electronic specific-heat coefficient ofCrAs1−xPxpeaks aroundxc. These properties provide clear evidence 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 those occurring in the high-Tciron pnictides.