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Measurement of a solid-state triple point at the metal–insulator transition in VO2

2013; Nature Portfolio; Volume: 500; Issue: 7463 Linguagem: Inglês

10.1038/nature12425

1476-4687

Jae Hyung Park, Jim M. Coy, T. Serkan Kasırga, Chunming Huang, Zaiyao Fei, Scott Hunter, David Cobden,

Gas Sensing Nanomaterials and Sensors

The precise location of a solid-state triple point, at which three solid phases coexist in thermal equilibrium, has been determined by controlling the stress and temperature in a nanobeam of vanadium dioxide near its metal–insulator transition. Vanadium dioxide (VO2) is of interest in ultrafast optical and electrical switching applications thanks to the material's unique phase transition between metallic and insulating states involving several competing phases. This study of single-crystal VO2 nanobeams in a system in which the metal–insulator transition is finely controlled by adjusting temperature and strain pinpoints the previously elusive 'triple point' — the transition temperature at which one metallic and two insulating phases can coexist — as 65 °C. Other so-called correlated materials, including manganites and pnictides, also have poorly understood strain-critical phase transitions involving multiple components, and this work demonstrates a method that should be widely applicable in such situations. First-order phase transitions in solids are notoriously challenging to study. The combination of change in unit cell shape, long range of elastic distortion and flow of latent heat leads to large energy barriers resulting in domain structure, hysteresis and cracking. The situation is worse near a triple point, where more than two phases are involved. The well-known metal–insulator transition in vanadium dioxide1, a popular candidate for ultrafast optical and electrical switching applications, is a case in point. Even though VO2 is one of the simplest strongly correlated materials, experimental difficulties posed by the first-order nature of the metal–insulator transition as well as the involvement of at least two competing insulating phases have led to persistent controversy about its nature1,2,3,4. Here we show that studying single-crystal VO2 nanobeams5,6,7,8,9,10,11,12,13,14,15,16 in a purpose-built nanomechanical strain apparatus allows investigation of this prototypical phase transition with unprecedented control and precision. Our results include the striking finding that the triple point of the metallic phase and two insulating phases is at the transition temperature, Ttr = Tc, which we determine to be 65.0 ± 0.1 °C. The findings have profound implications for the mechanism of the metal–insulator transition in VO2, but they also demonstrate the importance of this approach for mastering phase transitions in many other strongly correlated materials, such as manganites17 and iron-based superconductors18.