Quantum critical points—zero-temperature phase transitions driven by quantum fluctuations—are among the most exotic states in condensed matter. Reaching one usually requires precision tuning of pressure, composition, or magnetic field. Disorder is the enemy: it smears transitions, broadens critical features, and generally degrades quantum coherence.
Krenkel, Tanatar, Grasset, Konczykowski, Chen, Petrovic, Levchenko, and Prozorov reach a quantum critical point by adding disorder. They irradiate single crystals of (Ca,Sr)₃Rh₄Sn₁₃ with electrons, introducing non-magnetic point defects. The defects progressively suppress the charge-density wave order. As the CDW is destroyed, electrical resistivity becomes nearly perfectly linear in temperature—the hallmark of quantum criticality.
The physics: the CDW was a competing order that masked the quantum critical point. The system was always close to criticality, but the CDW state formed first and hid it. Disorder, by destroying the CDW, doesn't create the criticality—it reveals it. The disorder is a tuning knob that navigates the system to the most interesting point by removing what was in the way.
The transition from Fermi liquid (T² resistivity, well-defined quasiparticles) to non-Fermi liquid (linear T, no quasiparticles) is sharp and controlled. Each dose of irradiation moves the system closer to the QCP.
Removing structure can expose a deeper organizing principle that the original order was hiding. The disorder doesn't add complexity—it subtracts a complication.