The Thermodynamics of Quantum Critical Points

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Science  11 Sep 2009:
Vol. 325, Issue 5946, pp. 1348-1349
DOI: 10.1126/science.1179046

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A thermodynamic state, such as a gas or liquid, is usually characterized by well-defined properties such as density, but at high temperatures and pressures, a critical point can be reached suddenly where properties such as density fluctuate wildly. Quantum fluctuations that arise through the Heisenberg uncertainty principle can also lead to critical behavior but do so in the limit of low temperatures. Quantum critical points are often seen as fluctuations in electronic ordering driven by an external magnetic field. Because a quantum critical point can affect the properties of a material well above absolute zero, the search for unusual electronic phases of matter can be aided by their presence. However, it has proven difficult to see the changes in thermodynamic properties that must occur near quantum critical points. On page 1360 of this issue, Rost et al. (1) characterize the entropy changes of an unusual electronic phase that was observed in highly pure Sr3Ru2O7 single crystals (2). These results show that the spin nematic state, an analog of the molecular ordering that occurs in nematic liquid crystals, is a true thermodynamic phase.