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Phases of matter are often described by an order parameter. For example the spins in a ferromagnet are randomly oriented at high temperature, but as the temperature falls below the Curie temperature, the spins begin to line up. A local order parameter given by the magnetization (the average spin density), distinguishes the normal and ferromagnetic phases. In contrast, topological insulators represent a broad class of band insulators distinguished not by local order but by global topological properties. In analogy with order parameters in the usual study of phases and phase transitions, topological phases are distinguished by topological invariants, but the lack of local order makes it difficult to experimentally distinguish between different topological phases. Now, a theoretical paper appearing in Physical Review Letters from Emilio Alba at the Instituto de Física Fundamental, Spain, and his colleagues describes how the usually hidden topological order that can occur in certain cold atom lattices could be measured directly

Preprint: http://arxiv.org/abs/1105.2775
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