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In second-order quantum phase transitions from magnetically ordered to paramagnetic states at T = 0, tuned by pressure or chemical substitution, a quantum critical point is expected to appear with critical behavior manifesting in the slowing down of spin fluctuations in the paramagnetic state and a continuous development of the order parameter in the ordered state. Quantum criticality is discussed widely as a possible driving force for unconventional superconductivity and other exotic phenomena in correlated electron systems. In the real world, however, quantum critical points and quantum criticality are often masked by a preceding first-order transition and/or the development of competing states. Pressure tuning of the itinerant-electron helical magnet MnSi is a well-known example of the suppression of a quantum critical point due to a first-order phase transition and resulting destruction of the ordered state. Utilizing muon spin relaxation experiments, here we report that 15% Fe-substituted (Mn,Fe)Si exhibits completely different behavior with pressure tuning, including the restoration of second-order quantum critical behavior and a quantum critical point at pQPC ~ 21–23 kbar, which coincides with the T = 0 crossing point of the extrapolated phase boundary line of pure MnSi. This result is quantitatively consistent with the recent theory of itinerant-electron ferromagnets by Sang, Belitz, and Kirkpatrick, who argued that disorder would restore a quantum critical point which is otherwise hidden by a first-order transition.Quantum criticality: Iron aids restorationAn exotic quantum state of matter is identified in a new material by researchers in the USA, Switzerland and Germany. Yasutomo Uemura from Columbia University and co-workers observe quantum criticality in a material in which it was previously unseen just by adding iron. As pressure is increasingly applied to pure manganese silicon at absolute zero temperature, it can suddenly change from an ordered magnetic state to an unordered one. Uemura et al. now show that a slower continuous, or second-order, transition is observed instead when fifteen percent of the manganese atoms are replaced with iron atoms. This is indicative that a quantum critical point exists at a pressure of between 21 and 23 kilobar. They believe that criticality is restored because the iron atoms introduce disorder into the system.

Restoration of quantum critical behavior by disorder in pressure-tuned (Mn,Fe)Si