title: 'Inverse' melting of a vortex lattice.
abstract: Inverse melting is the process in which a crystal reversibly transforms into a liquid or amorphous phase when its temperature is decreased. Such a process is considered to be very rare, and the search for it is often hampered by the formation of non-equilibrium states or intermediate phases. Here we report the discovery of first-order inverse melting of the lattice formed by magnetic flux lines in a high-temperature superconductor. At low temperatures, disorder in the material pins the vortices, preventing the observation of their equilibrium properties and therefore the determination of whether a phase transition occurs. But by using a technique to 'dither' the vortices, we were able to equilibrate the lattice, which enabled us to obtain direct thermodynamic evidence of inverse melting of the ordered lattice into a disordered vortex phase as the temperature is decreased. The ordered lattice has larger entropy than the low-temperature disordered phase. The mechanism of the first-order phase transition changes gradually from thermally induced melting at high temperatures to a disorder-induced transition at low temperatures.
authors.affiliation: Department of Condensed Matter Physics, The Weizmann Institute of Science, Rehovot 76100, Israel. nurit.avraham@weizmann.ac.il
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authors.lastname: Avraham
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authors.lastname: Khaykovich
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authors.lastname: Myasoedov
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authors.lastname: Rappaport
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authors.lastname: Shtrikman
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authors.lastname: Feldman
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authors.lastname: Tamegai
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authors.lastname: Li
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authors.lastname: Konczykowski
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authors.lastname: van der Beek
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authors.lastname: Zeldov