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| Figure 1: (a) An example of the classical phase transition in a ferromagnet. See more at APS link below. |
Theoretically predicted quantum critical behavior in a model magnetic material has been experimentally confirmed at a quantitative level.
Every physicist knows how a ferromagnet like iron behaves as the temperature is increased [Fig. 1(a)]. At low temperatures, the constituent spins are spontaneously aligned as a result of the local magnetic fields from neighboring spins. Thermal fluctuations act against such local fields, inducing random reorientation of the spins. As the temperature increases, thermal fluctuations grow and the net magnetization in the ordered state continuously decreases. The magnetization drops to zero at a critical temperature Tc (1043 kelvin in the case of iron). In a narrow temperature range around Tc, thermal fluctuations of the spins extend over all length scales of the material—scale invariance is a key feature of critical points. This is an example of a continuous classical phase transition driven by thermal fluctuations.
Fluctuations driving quantum phase transitions are of a different nature, however. For example, a continuous quantum phase transition [1, 2, 3] is driven by quantum fluctuations resulting from Heisenberg’s uncertainty principle. The transition takes place when the material is at zero temperature but as a function of a nonthermal control parameter, such as applied pressure, an external magnetic field, or the density of electrons manipulated by the chemical composition. Theory has predicted that, surprisingly, an additional presence of thermal fluctuations at finite temperatures does not eliminate the critical fluctuations present around the quantum critical point [4]. Instead, the region of quantum criticality becomes progressively broader with increasing temperature and extends to temperatures significantly above zero [Fig. 1(b)]. This has now been experimentally demonstrated by Alison Kinross and her colleagues at McMaster University, Canada, in cooperation with theorist Subir Sachdev from Harvard University. In a paper in Physical Review X [5], they report the phase diagram of CoNb2O6, a model magnetic material for quantum criticality [6]. Their results correspond perfectly to the general phase diagram outlined in Fig. 1(b). Furthermore, they provide the first quantitative confirmation of any theory—although there are not many—aiming to predict the temperature evolution of the quantum critical behavior. Although the work by Kinross et al. is concerned with a particular compound, the results are important in a broader sense. Namely, the quantum critical behavior observed in such diverse systems as metals, magnets, superconductors, gases of cold atoms, and black holes, shares many fundamental characteristics—universality is another key feature of critical points.
Quantum criticality is related to high temperature superconductors, which would make our power-consuming lives a lot easier (and in a geopolitical sense, maybe more peaceful). "Wars and rumors of wars" are essentially serial struggles for scarce resources, usually pilfered by the country with the biggest weapons from those with little or none.
Related links follow:
American Physical Society: Viewpoint: A Critical Test of Quantum Criticality
Martin Klanjšek, Jožef Stefan Institute, Jamova cesta 39, 1000 Ljubljana, Slovenia
Physics arXiv: Quantum Criticality
Subir Sachdev, Department of Physics, Harvard University, Cambridge MA 02138
Bernhard Keimer, Max-Planck Institute for Solid State Research, Stuttgart, Germany
Rutgers: Quantum Criticality
Science Daily: Quantum criticality observed in a new class of materials, Rice University
