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Register a new userCompeting Phases in Epitaxial Vanadium Dioxide at Nanoscale
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Phase competition in correlated oxides offers tantalizing opportunities as many intriguing physical phenomena occur near the phase transitions. Owing to a sharp metal-insulator transition (MIT) near room temperature, correlated vanadium dioxide (VO2) exhibits a strong competition between insulating and metallic phases that is important for practical applications. However, the phase boundary undergoes strong modification when strain is involved, yielding complex phase transitions. Here, we report the emergence of the nanoscale M2 phase domains in VO2 epitaxial films under anisotropic strain relaxation. The phase states of the films are imaged by multi-length-scale probes, detecting the structural and electrical properties in individual local domains. Competing evolution of the M1 and M2 phases indicates a critical role of lattice-strain on both the stability of the M2 Mott phase and the energetics of the MIT in VO2 films. This study demonstrates how strain engineering can be utilized to design phase states, which allow deliberate control of MIT behavior at the nanoscale in epitaxial VO2 films.
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The metal-insulator transition (MIT) in vanadium dioxide (VO2) has the potential to lead to a number of disruptive technologies, including ultra-fast data storage, optical switches, and transistors which move beyond the limitations of silicon. For applications, VO2 films are deposited on crystalline substrates to prevent cracking observed in bulk VO2 crystals across the thermally driven MIT. Near the MIT, VO2 films exhibit nanoscale coexistence between metallic and insulating phases, which opens up further potential applications such as memristors, tunable capacitors, and optically engineered devices such as perfect absorbers. It is generally believed that the formation of phase domains must be affected to some extent by random processes which lead to unreliable performance in nanoscale MIT based devices. Here we show that nanoscale randomness is suppressed in the thermally driven MIT in sputtered VO2 films; the observed domain patterns of metallic and insulating phases in the vicinity of the MIT in these films behave in a strikingly reproducible way. This result opens the door for realizing reliable nanoscale VO2 devices.
We investigate the electronic and structural changes at the nanoscale in vanadium dioxide (VO2) in the vicinity of its thermally driven phase transition. Both electronic and structural changes exhibit phase coexistence leading to percolation. In addition, we observe a dichotomy between the local electronic and structural transitions. Nanoscale x-ray diffraction reveals local, non-monotonic switching of the lattice structure, a phenomenon that is not seen in the electronic insulator-to-metal transition mapped by near-field infrared microscopy.
We present the first comprehensive broadband optical spectroscopy data on two insulating phases of vanadium dioxide (VO2): monoclinic M2 and triclinic. The main result of our work is that the energy gap and the electronic structure are essentially unaltered by the first-order structural phase transition between the M2 and triclinic phases. Moreover, the optical interband features in the M2 and triclinic phases are remarkably similar to those observed in the well-studied monoclinic M1 insulating phase of VO2. As the energy gap is insensitive to the different lattice structures of the three insulating phases, we rule out Peierls effects as the dominant contributor to the opening of the gap. Rather, the energy gap arises from intra-atomic Coulomb correlations.
The metal-insulator transition and unconventional metallic transport in vanadium dioxide (VO$_2$) are investigated with a combination of spectroscopic ellipsometry and reflectance measurements. The data indicates that electronic correlations, not electron-phonon interactions, govern charge dynamics in the metallic state of VO$_2$. This study focuses on the frequency and temperature dependence of the conductivity in the regime of extremely short mean free path violating the Ioffe-Regel-Mott limit of metallic transport. The standard quasiparticle picture of charge conduction is found to be untenable in metallic VO$_2$.
We use apertureless scattering near-field optical microscopy (SNOM) to investigate the nanoscale optical response of vanadium dioxide (VO2) thin films through a temperature-induced insulator-to-metal transition (IMT). We compare images of the transition at both mid-infrared (MIR) and terahertz (THz) frequencies, using a custom-built broadband THz-SNOM compatible with both cryogenic and elevated temperatures. We observe that the character of spatial inhomogeneities in the VO2 film strongly depends on the probing frequency. In addition, we find that individual insulating (or metallic) domains have a temperature-dependent optical response, in contrast to the assumptions of a classical first-order phase transition. We discuss these results in light of dynamical mean-field theory calculations of the dimer Hubbard model recently applied to VO2.
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