Image courtesy of David H Cobden
Measured phase diagram for the three solid phases of vanadium oxide (VO2): metallic (golden), insulating M1 (grey), and insulating M2 (purple). To locate the triple point, the material is stretched under a microscope, and the phases are determined by polarized optical microscopy.
Materials typically have more than one possible structure, or phase, depending upon the temperature, pressure, or variation in the material’s elemental composition. Phase transitions in materials are notoriously challenging to study. The situation is worse near a triple point, where more than two phases are involved. The well-known metal–insulator transition in vanadium dioxide, a popular candidate for ultrafast optical and electrical switching applications, is a case in point. In a new approach to studying this important phenomena, stretching single crystal “nanobeams” of vanadium dioxide (VO2) under a microscope revealed when the material switches between conducting and insulating states – and showed that this transition occurs when three solid phases have equal energy, the . first ever determination of a solid-state “triple point.”
Such insights are key to mastering the optical and electrical switching properties of the material. The techniques developed may also allow control of magnetic and superconducting properties for an array of energy-relevant applications.
The metal-insulator transition, the point where the material goes from conducting to insulating, in VO2 involves a rapid and drastic change in its optical and electrical properties, making it a classic candidate material for applications in switching and sensing. The detailed behavior near the transition was not known because of the complexity of bulk samples and because two insulating phases (known as M1 and M2) compete with the metallic phase near the transition. By working with nanobeams of VO2, which are tiny single-crystal wires of the material, in a purpose-built apparatus for applying axial stress under a microscope, researchers from the University of Washington were able to observe the convergence of the three phases with unprecedented control. They could map the stress and temperature at which each pair of phases coexisted and also deduce the precise triple point, at which all three coexist. It was found to be at 65.0 °C and at zero applied stress, meaning that it coincides with the metal-insulator transition in a free-standing crystal. This insight has important theoretical implications, and should help in efforts to exploit the transition in developing new technologies. It also demonstrates the value of such a controlled approach to complex materials in general.
David H. Cobden
University of Washington
Department of Energy Office of Science, Basic Energy Sciences program.
Jae H. Park, Jim M. Coy, T. Serkan Kasirga, Chunming Huang, Zaiyao Fei, Scott Hunter, and David H. Cobden, “Measurement of a solid-state triple point at the metal-insulator transition in VO2", Nature 500, 431 (2013). [DOI: 10.1038/nature12425]
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