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UB researchers identify a mixed-order phase transition

An article by a team of the University of Barcelona, published in the journal Proceedings of the National Academy of Sciences (PNAS), shows the theoretical prediction and experimental observation that a mixed-order transition in a magnetic colloidal crystal.

An article by a team of the University of Barcelona, published in the journal Proceedings of the National Academy of Sciences (PNAS), shows the theoretical prediction and experimental observation that a mixed-order transition in a magnetic colloidal crystal.

20/12/2017

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UB researchers identify a mixed-order phase transition Phase transitions are phenomena of matter transformation, such as when water boils or it freezes, which is quite important in our daily life. The current classification describes three types: first and second order transitions, described since the thirties, and the mixed-order transitions, predicted in 1969 and which gathered features from the other two. Now, an article by a team of the University of Barcelona, published in the journal Proceedings of the National Academy of Sciences (PNAS), shows the theoretical prediction and experimental observation that a mixed-order transition in a magnetic colloidal crystal, that is, a solid formed by a suspension of magnetic microparticles.

“This result constitutes the first experimental creation of a mixed-order transition. Also, the colloidal system we developed could become a paradigmatic experimental system to study the features of mixed-order transition, which so far were only described in theory” says Ricard Alert, researcher from the Institute of Complex Systems (UBICS) and first signer of the article.  Researchers from the same institute, and Professor Jaume Casademunt and Pietro Tierno –member of the Institute of Nanoscience and Nanotechnology  (IN2UB), also took part in the research.

A new milestone in the field of phase transitions

First order transitions are known for their discontinuity in the phase transition. For example, water undergoes a sudden reduction of density when freezing and boiling, when liquid water becomes solid and gaseous, respectively. Regarding second order transitions, the system develops in a continuous way from one phase to the other. Again with the case of water, at a temperature of 374 ºC and a pressure of 218 atmospheres gets to the critical point, in which density of liquid and solid phases become the same. Therefore, transition in water between these phases through the critical point is continuous, and therefore, a second order transition. In addition, a system in the critical point obtains extraordinary features, becoming highly sensitive to small changes. With fluids, for instance, this leads to large density fluctuations that make the fluid to appear cloudy –known as critical opalescence.

In 1969, David J. Thouless –awarded the Nobel Prize on Physics in 2016- predicted a phase transition that despite showing second order transition features, was discontinuous –like first order transitions. This discovery broadened the classification of phase transitions, adding mixed-order transitions to this list. Although so far, these transitions had been predicted theoretically in a wide range of phenomena, it had not been observed in an equilibrium experimental system. The system the UB team designed is made of a suspension of paramagnetic particles forming a crystal pattern that can be controlled with external magnetic fields.

Article reference:

R. Alert, P. Tierno, and J. Casademunt. Mixed-order phase transition in a colloidal crystal. Proc. Natl. Acad. Sci. USA 114, 12906 (2017). Doi: 10.1073/pnas.1712584114

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