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

Abstract

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 dioxide, 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 nature. Here we show that studying single-crystal VO2 nanobeams 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 manganites and iron-based superconductors.

Inhomogeneity of the ultrafast insulator-to-metal transition dynamics of VO2

“…With their well-defined, controllable strain state and ability to withstand a high degree of strain, microcrystal samples have been studied to investigate the strain-temperature phase diagrams for VO 2 (see Fig. 2b) [27][28][29][30][31] . The very low-luminescence background and narrow linewidths in Raman spectroscopy indicate a low defect density and confirm a high structural quality of the VO 2 microcrystals.…”

Modification of electronic structure in compressively strained vanadium dioxide films

“…Thus, for experimental measurements of these systems to provide meaningful insight, the external parameters, the intrinsic interacting degrees of freedom, and the resulting properties must all be well characterized. Hence, measurements on samples subject to external perturbations, for example pressure and strain [5][6][7][8][9][10] , can provide additional insight into the underlying physics of these systems.…”

Visualization of one-dimensional diffusion and spontaneous segregation of hydrogen in single crystals of VO2

“…They are usually elongated along the rutile c-axis, and when released from the substrate they are optically uniform and exhibit sharp MITs at 65 °C implying a lack of impurities or twinning 15,20,21 . To counter strain caused by substrate adhesion 22 , the crystals are either transferred to a soft polymer, polydimethylsiloxane (PDMS), or cantilevered from the edge of an oxidized silicon chip.…”

“…Diffusion perpendicular to these channels requires the protons to move between densely spaced metal ions, and has a much higher energy barrier 12,13 . Pure, unstrained rutile-structured (R) VO2 has a well-known first-order metalinsulator transition (MIT) to a monoclinic insulating phase (M1) on cooling through = 65 °C 14,15 . Interestingly, it has recently been found 16,17 that hydrogen in VO2 reduces the gap in the insulating phase while slightly reducing , and the material becomes fully metallic above some concentration.…”