There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

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 the classic transistor, the number of electric charge carriers--and thus the electrical conductivity--is precisely controlled by external voltage, providing electrical switching capability. This simple but powerful feature is essential for information processing technology, and also provides a platform for fundamental physics research. As the number of charges essentially determines the electronic phase of a condensed-matter system, transistor operation enables reversible and isothermal changes in the system's state, as successfully demonstrated in electric-field-induced ferromagnetism and superconductivity. However, this effect of the electric field is limited to a channel thickness of nanometres or less, owing to the presence of Thomas-Fermi screening. Here we show that this conventional picture does not apply to a class of materials characterized by inherent collective interactions between electrons and the crystal lattice. We prepared metal-insulator-semiconductor field-effect transistors based on vanadium dioxide--a strongly correlated material with a thermally driven, first-order metal-insulator transition well above room temperature--and found that electrostatic charging at a surface drives all the previously localized charge carriers in the bulk material into motion, leading to the emergence of a three-dimensional metallic ground state. This non-local switching of the electronic state is achieved by applying a voltage of only about one volt. In a voltage-sweep measurement, the first-order nature of the metal-insulator transition provides a non-volatile memory effect, which is operable at room temperature. Our results demonstrate a conceptually new field-effect device, extending the concept of electric-field control to macroscopic phase control.

Collective bulk carrier delocalization driven by electrostatic surface charge accumulation

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

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).

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Oxide Electronics and Vanadium Dioxide Perspective: A Review

The effect of a strong electric field on the metal-insulator phase transition in vanadium dioxide was studied. It was found that the field application to a silicon-silicon oxide-silicon nitride-vanadium dioxide (Si-SiO2-Si3N4-VO2) heterostructure shifts the critical temperature of this transition toward lower values under conditions when the thermal effects are minimized. Numerical modeling of the current-voltage characteristics of the vanadium dioxide-based sandwich-type switches measured at various temperatures in the range from 15 to 350 K showed that the applied electric field influences the critical concentration and temperature of the phase transition.

The effect of electric field on metal-insulator phase transition in vanadium dioxide

On the basis of the Mott criterion for metal–insulator transition (MIT), an expression for the correlation length, identical to that for the coherence length in the theory of superconductivity, is obtained. This correlation length characterizes the size of an electron–hole pair (in an excitonic insulator) or the effective Bohr radius (as, e.g., in doped semiconductors). The relation obtained is used for calculation of the coherence length in vanadium dioxide. The presence of two characteristic coherence lengths (ξ1 ~ 20 A and ξ2 ~ 2 A) is found. This is associated with the specific features of the transition mechanism in VO2: this mechanism represents a combination of the purely electronic Mott–Hubbard contribution and the structural (Peierls-like) one. It is shown, however, that the driving force of the MIT in VO2 is the electron-correlation Mott–Hubbard transition.

https://iopscience.iop.org/article/10.1088/0953-8984/15/19/322/pdf

Metal–insulator transition: the Mott criterion and coherence length

Thin films of amorphous vanadium oxide have been prepared by electrochemical anodic oxidation. The phase composition of anodic films on vanadium has been shown to depend on the oxidation conditions (electrolyte composition, oxidation current, and time), and the stoichiometry can be controlled from V O2 to V2O5. Physical properties of the oxide films, including the metal–insulator transition in amorphous V O2, are studied. In addition, it is shown that non-equilibrium electrochemical oxidation leads to the formation of metastable vanadium oxides with extremely high sensitivity to laser () and electron-beam () irradiation. Such films are of considerable technical interest, particularly because of potential applications as an efficient resist material for both photonanolithography and electron-beam nanolithography.

Alexander Pergament

Papers

Electrical switching and Mott transition in VO2

Oxide Electronics and Vanadium Dioxide Perspective: A Review

The effect of electric field on metal-insulator phase transition in vanadium dioxide

Metal–insulator transition: the Mott criterion and coherence length

Anodic oxidation of vanadium and properties of vanadium oxide films