An Orderly Transition

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Science  13 Aug 2010:
Vol. 329, Issue 5993, pp. 729-731
DOI: 10.1126/science.329.5993.729-d

Classical phase transitions, such as boiling or freezing, are usually driven by heat exchange, which introduces the order or disorder necessary for rearrangement. In contrast, quantum phase transitions occur at zero temperature as a result of quantum (rather than thermal) fluctuations and are controlled by parameters such as pressure or chemical composition. Most quantum phase transitions have been observed in compounds with carefully tuned chemical compositions, which are inevitably subject to disorder. Jaramillo et al. now induce such a transition in elemental chromium, which presents an unusually clean system. Chromium is antiferromagnetic at ambient pressure with a transition (Néel) temperature TN close to room temperature; as the authors increase the pressure, TN decreases and eventually disappears at a quantum critical point close to 10 GPa. Along the way, the antiferromagnetic phase changes character from mean-field–like to quantum critical. This change is reflected in the value of a critical exponent for resistivity, which approaches 0.25 at very low temperatures, easily distinguishable from the mean-field value of 0.5. Comparison with a related nonstoichiometric system indicates that pressure and chemical doping lead to distinct phase transitions.

Proc. Natl. Acad. Sci. U.S.A. 107, 13631 (2010).

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