Randomness rules

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Science  30 Oct 2015:
Vol. 350, Issue 6260, pp. 509
DOI: 10.1126/science.aad4136

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Phase transitions are perfect examples of physical phenomena for which statistical physics offers powerful predictions. Different types of phase transitions, ranging from liquid-vapor to ferromagnetic transitions, can be treated within the same theoretical framework. This yields useful expressions for characteristic physical properties of the system, such as resistivity, heat capacity, or free energy near the phase transition. The theoretical predictions can be strongly affected by random disorder, such as impurities or vacancies that are inevitably present in all real physical systems. In some systems, rare but large spatial regions are present in which there are no impurities. Such rare regions may be in a phase different from that of the bulk of the system and can dramatically alter the nature of the transition, causing certain physical properties of the system to diverge to infinity in the vicinity of the transition. These infinities are called Griffiths singularities (1) and can be expected to occur in a variety of systems, but they are not easily observed experimentally (2). On page 542 of this issue, Xing et al. (3) report the first experimental evidence of a Griffiths singularity near a quantum phase transition in a two-dimensional (2D) superconducting system.