An innovative technique uses ultrafast below-bandgap electric-field pulses to induce and probe an insulator–metal transition in an oxide thin film on which a metamaterial structure has been deposited. The transition from insulating to metallic behaviour and the microscopic interactions that accompany the transition are important phenomena in electronic materials. Until now it has not been possible to observe the transition directly in a time-resolved manner. Here, Richard Averitt and colleagues use ultrafast terahertz pulses to induce a phase transition in a prototypical insulator–metal transition material (vanadium dioxide) on which a metamaterial structure has been deposited. The metamaterial serves to amplify the local terahertz field, as well as to detect macroscopic changes in vanadium dioxide. Through direct, time-resolved observations, the authors establish a detailed microscopic picture of the structural and electronic changes underlying the insulator–metal transition. They conclude that their technique is versatile and could even be used to study phase transitions in superconductors. Electron–electron interactions can render an otherwise conducting material insulating1, with the insulator–metal phase transition in correlated-electron materials being the canonical macroscopic manifestation of the competition between charge-carrier itinerancy and localization. The transition can arise from underlying microscopic interactions among the charge, lattice, orbital and spin degrees of freedom, the complexity of which leads to multiple phase-transition pathways. For example, in many transition metal oxides, the insulator–metal transition has been achieved with external stimuli, including temperature, light, electric field, mechanical strain or magnetic field2,3,4,5,6,7. Vanadium dioxide is particularly intriguing because both the lattice and on-site Coulomb repulsion contribute to the insulator-to-metal transition at 340 K (ref. 8). Thus, although the precise microscopic origin of the phase transition remains elusive, vanadium dioxide serves as a testbed for correlated-electron phase-transition dynamics. Here we report the observation of an insulator–metal transition in vanadium dioxide induced by a terahertz electric field. This is achieved using metamaterial-enhanced picosecond, high-field terahertz pulses to reduce the Coulomb-induced potential barrier for carrier transport9. A nonlinear metamaterial response is observed through the phase transition, demonstrating that high-field terahertz pulses provide alternative pathways to induce collective electronic and structural rearrangements. The metamaterial resonators play a dual role, providing sub-wavelength field enhancement that locally drives the nonlinear response, and global sensitivity to the local changes, thereby enabling macroscopic observation of the dynamics10,11. This methodology provides a powerful platform to investigate low-energy dynamics in condensed matter and, further, demonstrates that integration of metamaterials with complex matter is a viable pathway to realize functional nonlinear electromagnetic composites.

Terahertz-field-induced insulator-to-metal transition in vanadium dioxide metamaterial

Although phase transitions have long been a centerpiece of condensed matter materials science studies, a number of recent efforts focus on potentially exploiting the resulting functional property changes in novel electronics and photonics as well as understanding emergent phenomena. This is quite timely, given a grand challenge in twenty-first-century physical sciences is related to enabling continued advances in information processing and storage beyond conventional CMOS scaling. In this brief review, we discuss synthesis of strongly correlated oxides, mechanisms of metal-insulator transitions, and exploratory electron devices that are being studied. Particular emphasis is placed on vanadium dioxide, which undergoes a sharp metal-insulator transition near room temperature at ultrafast timescales. The article begins with an introduction to metal-insulator transition in oxides, followed by a brief discussion on the mechanisms leading to the phase transition. The role of materials synthesis in influencing functional properties is discussed briefly. Recent efforts on realizing novel devices such as field effect switches, optical detectors, nonlinear circuit components, and solid-state sensors are reviewed. The article concludes with a brief discussion on future research directions that may be worth consideration.

Oxide Electronics Utilizing Ultrafast Metal-Insulator Transitions

In this paper the problem of the Mott metal-insulator transition in vanadium dioxide driven by an external electric field is considered. Delay time (td) measurements have shown that the experimental value of td is almost three orders of magnitude lower than the theoretical value, calculated in a simple electrothermal model. This suggests that under non-equilibrium conditions (in high electric fields) electron correlation effects contribute to the development of the insulator to metal transition. The extra-carrier injection from Si into VO2 was carried out in the structures Si-SiO2-VO2 on p-type silicon with ρ = 0.1 Ω cm and a SiO2 thickness 70 nm. It has been shown that the metal-insulator transition in VO2 can be initiated by injection, i.e. by the increase of the electron density. The value of the critical density was found to be of the order of the electron density in VO2 in the semiconducting phase, approximately 1018-1019 cm-3. This confirms that the metal-insulator transition in VO2 is the purely electronic Mott-Hubbard transition.

https://iopscience.iop.org/article/10.1088/0953-8984/12/41/310/pdf

Electrical switching and Mott transition in VO2

Engineering metamaterials with tunable resonances from mid-infrared to near-infrared wavelengths could have far-reaching consequences for chip based optical devices, active filters, modulators, and sensors. Utilizing the metal-insulator phase transition in vanadium oxide (VO_2), we demonstrate frequency-tunable metamaterials in the near-IR range, from 1.5 - 5 microns. Arrays of Ag split ring resonators (SRRs) are patterned with e-beam lithography onto planar VO_2 and etched via reactive ion etching to yield Ag/VO_2 hybrid SRRs. FTIR reflection data and FDTD simulation results show the resonant peak position red shifts upon heating above the phase transition temperature. We also show that, by including coupling elements in the design of these hybrid Ag/VO_2 bi-layer structures, we can achieve resonant peak position tuning of up to 110 nm.

/pdf/frequency-tunable-near-infrared-metamaterials-based-on-vo2-12x9a4j9o0.pdf

Frequency tunable near-infrared metamaterials based on VO2 phase transition.

When holes of about 0.018% are induced into a conduction band (breakdown of critical on-site Coulomb energy), an abrupt first-order Mott metal–insulator transition (MIT) rather than a continuous Hubbard MIT near a critical on-site Coulomb energy U/Uc=1, where U is on-site Coulomb energy between electrons, is observed on an inhomogeneous VO2 film, a strongly correlated Mott insulator. As a result, discontinuous jumps of the density of states on the Fermi surface are observed and inhomogeneity inevitably occurs. The off-current and temperature dependences of the abrupt MIT in a two-terminal device and the gate effect in a three-terminal device are clear evidence that the abrupt Mott MIT was induced by the excitation of holes. Raman spectra measured by a micro-Raman system show an MIT without the structural phase transition. Moreover, the magnitude of the observed jumps ΔJobserved at the abrupt MIT is an average over an inhomogeneous measurement region of the maximum true jump, ΔJtrue, deduced from the Brinkman–Rice picture. A brief discussion of whether VO2 is a Mott insulator or a Peierls insulator is presented.

https://iopscience.iop.org/article/10.1088/1367-2630/6/1/052/pdf

Mechanism and observation of Mott transition in VO2-based two- and three-terminal devices

Metal-oxide-semiconductor field-effect transistors (MOSFET) have been for a long time the key elements of modern electronics industry. For the purpose of a permanent integration enhancement, the size of MOSFET has been decreasing exponentially for over decades in compliance with Moore's Law, but nowadays, owing to the intrinsic restrictions, the further scaling of MOSFET devices either encounters fundamental limits or demands for more and more sophisticated and expensive engineering solutions. Alternative approaches and device concepts are currently designed both in order to sustain an increase of the integration degree, and to improve the functionality and performance of electronic devices. Oxide electronics is one of such promising approaches which could enable and accelerate the development of information and computing technology. The behavior of delectrons in transition metal oxides is responsible for the unique properties of these materials, causing strong electronelectron correlations, which play an important role in the mechanism of metal-insulator transition. The Mott transition in vanadium dioxide is specifically the phenomenon that researchers consider as a corner stone of oxide electronics, particularly, in its special direction known as a Motttransition field-effect transistor (MTFET). This review focuses on current research, latest results, urgent problems and nearterm outlook of oxide electronics with special emphasis on the state of the art and recent progress in the field of VO2-based MTFETs. Index Terms — Oxide electronics, Mott transition FET, metal-insulator transition, transition metal oxides, vanadium dioxide. 1 Manuscript received November 25, 2013; accepted December 10, 2013. Date of online publication: December 15, 2013. This work was supported by the Strategic Development Program of Petrozavodsk State University (2012 – 2016), Ministry of Education and Science of Russian Federation “Scientific and Educational Community of Innovation Russia (2009-2013)” Program through contracts no. 14.740.11.0895, no. 14.740.11.0137, no. 14.740.11.1157, no. 16.740.11. 0562, and no. 14.B37.21.0755, as well as “Development of Scientific Potential of High School” Program, contracts no. 2.3282.2011 and no. 2.2774.2011. A. L. Pergament is with the Department of Information Measuring Systems and Physical Electronics, Faculty of Physical Engineering of Petrozavodsk State University, 185910 Petrozavodsk, Russia (e-mail: aperg@psu.karelia.ru). G. B. Stefanovich is with the Department of Information Measuring Systems and Physical Electronics, Faculty of Physical Engineering of Petrozavodsk State University, 185910 Petrozavodsk, Russia (e-mail: gstef@yandex.ru). A. A. Velichko is with the Department of Electronics and Electrical Power Engineering, Faculty of Physical Engineering of Petrozavodsk State University, 185910 Petrozavodsk, Russia (e-mail: velichko@ psu.karelia.ru).

/pdf/oxide-electronics-and-vanadium-dioxide-perspective-a-review-42h5seia31.pdf

Oxide Electronics and Vanadium Dioxide Perspective: A Review

We have proposed a method to probe metal to insulator transition in VO2 measuring photoluminescence response of colloidal quantum dots deposited on the VO2 film. In addition to linear luminescence intensity decrease with temperature that is well known for quantum dots, temperature ranges with enhanced photoluminescence changes have been found during phase transition in the oxide. Corresponding temperature derived from luminescence dependence on temperature closely correlates with that from resistance measurement during heating. The supporting reflectance data point out that photoluminescence response mimics a reflectance change in VO2 across metal to insulator transition. Time-resolved photoluminescence study did not reveal any significant change of luminescence lifetime of deposited quantum dots under metal to insulator transition. It is a strong argument in favor of the proposed explanation based on the reflectance data.

/pdf/photoluminescence-response-of-colloidal-quantum-dots-on-vo2-4fzsfsjx7s.pdf

G. B. Stefanovich

Papers

Electrical switching and Mott transition in VO2

Oxide Electronics and Vanadium Dioxide Perspective: A Review

Photoluminescence response of colloidal quantum dots on VO2 film across metal to insulator transition.

Phase composition of anodic oxide films on transition metals: a thermodynamic approach

Vanadium oxide thin films and fibers obtained by acetylacetonate sol–gel method