Capitalizing on the Mott transition in vanadium dioxide for ultrafast modulators (Conference Presentation)

Metal-insulator transition in vanadium dioxide Metal-insulator transition in vanadium dioxide

The physics of the metal-insulator transition (MIT) in vanadium dioxide remains a subject of intense interest. Because of the complicating effects of elastic strain on the phase transition, there is interest in comparatively strain-free means of examining VO2 material properties. We report contact-free, low-strain studies of the MIT through an inductive bridge approach sensitive to the magnetic response of VO2 powder. Rather than observing the expected step-like change in susceptibility at the transition, we argue that the measured response is dominated by an analog of the Barkhausen effect, due to the extremely sharp jump in the magnetic response of each grain as a function of time as the material is cycled across the phase boundary. This effect suggests that future measurements could access the dynamics of this and similar phase transitions.

Vanadium dioxide (VO2) is a paradigmatic example of a strongly correlated system that undergoes a metal-insulator transition at a structural phase transition. To date, this transition has necessitated significant post hoc adjustments to theory in order to be described properly. Here we report standard state-of-the-art first principles quantum Monte Carlo (QMC) calculations of the structural dependence of the properties of VO2. Using this technique, we simulate the interactions between electrons explicitly, which allows for the metal-insulator transition to naturally emerge, importantly without ad hoc adjustments. The QMC calculations show that the structural transition directly causes the metal-insulator transition and a change in the coupling of vanadium spins. This change in the spin coupling results in a prediction of a momentum-independent magnetic excitation in the insulating state. While two-body correlations are important to set the stage for this transition, they do not change significantly when VO2 becomes an insulator. These results show that it is now possible to account for electron correlations in a quantitatively accurate way that is also specific to materials.

On the metal–insulator transition in vanadium dioxide

: Vanadium oxides are very interesting compounds which exhibit exotic transport phenomena. In particular vanadium dioxide (VO2) undergoes a first-order transition from a high-temperature metallic phase to a low-temperature insulating phase at almost the room temperature (T = 340 K). The resistivity jumps by several orders of magnitude through this transition, and the crystal structure changes from rutile (R-phase) at high-temperature to monoclinic (so-called M1-phase) at low-temperature. The latter is characterized by a dimerization of the vanadium atoms into pairs, as well as a tilting of these pairs with respect to the c-axis. VO2 has also attracted a great deal of attention for its ultrafast optical response, switching between the R and the M1 phase. Despite the large number of experimental studies focusing on this material the physics driving this phase transition and the resulting optical properties is still mysterious. There are intensive reports around the world to make devices such as switches, transistors, detectors, varistors, phase change memory, exploiting the unique properties of VO2. Two physical effects, Peierls, i.e. dimerization, and the Mott mechanism due to strong Coulomb repulsion are important in the metal-insulator transition (MIT) of VO2. Understanding the detailed interplay and the relative importance of both Peierls and Mott mechanism is important for controlling this material with an eye towards applications. For example, whether the driving force of this transition is electronic (i.e. occurring on femtosecond timescales) or structural (occurring on the picosecond timescale) is important to understand the speed of the switching from the M1 to the rutile phase. The insights obtained in this study together with the computational machinery developed, will serve as a basis for rational material design of VO2 based applications.

With a vanadium dioxide thin film as the Q-switcher, a pulsed laser can be realized. The pulsed laser performance shows a bifurcation due to the insulator–metal transition of the vanadium dioxide with unchanged patterns. By analyzing the pulsed laser performance, the kinetics of the nonlinear optical response during insulator–metal transition of the vanadium dioxide can be investigated.

Electrically driven metal-insulator transition in vanadium dioxide (VO2) is of interest in emerging memory devices, neural computation, and high speed electronics. We report on the fabrication of out-of-plane VO2 metal-insulator-metal (MIM) structures and reproducible high-speed switching measurements in these two-terminal devices. We have observed a clear correlation between electrically-driven ON/OFF current ratio and thermally-induced resistance change during metal-insulator transition. It is also found that sharp metal-insulator transition could be triggered by external voltage pulses within 2 ns at room temperature and the achieved ON/OFF ratio is greater than two orders of magnitude with good endurance.

Metal-to-insulator transition (MIT) behaviors accompanied by a rapid reversible phase transition in vanadium dioxide (VO2) have gained substantial attention for investigations into various potential applications and obtaining good materials to study strongly correlated electronic behaviors in transition metal oxides (TMOs). Although its phase-transition mechanism is still controversial, during the past few decades, people have made great efforts in understanding the MIT mechanism, which could also benefit the investigation of MIT modulation. This review summarizes the recent progress in the phase-transition mechanism and modulation of VO2 materials. A representative understanding on the phase-transition mechanism, such as the lattice distortion and electron correlations, are discussed. Based on the research of the phase-transition mechanism, modulation methods, such as element doping, electric field (current and gating), and tensile/compression strain, as well as employing lasers, are summarized for comparison. Finally, discussions on future trends and perspectives are also provided. This review gives a comprehensive understanding of the mechanism of MIT behaviors and the phase-transition modulations.

We report a large transition temperature (TC) decrease in a VO2 thin film device under photo-irradiation. With the increasing photo-irradiation intensity (PIntensity) with a wavelength of 405 nm, the TC of the VO2 device decreased at a rate of ∼3.2×10−2°CW/cm2 and reached as low as 40.0 °C at a PIntensity of 8.4×102 W/cm2. While the change in TC is primarily due to the photothermal effect when the PIntensity is below 3.6×102 W/cm2, both the photothermal and photo-induced carrier density effects contribute to the change in TC when the PIntensity is above 6.4×102 W/cm2.

The burgeoning interest in sub-wavelength metal optics is fueled by applications ranging from cancer therapy and proteomics to opto-electronics and novel photovoltaic materials. The optical properties of nanoscale metallic structures are determined not only by the electromagnetics of localized and propagating surface plasmons, but also by the details of metal nanoparticle size, shape and spatial arrangement, and dielectric environment. This paper presents several ways in which interesting plasmonic structures and applications are made possible by nanoscale materials synthesis and structuring techniques based on pulsed laser and electron-beam deposition, electron-beam lithography and focused ion-beam nanomachining. Examples include (1) using a focused ion beam to create arrays of subwavelength holes in which the extraordinary optical transmission effect can be switched; (2) fabricating gold::VO2 composite nanoparticles to measure the size dependence of the metal-insulator transition in vanadium dioxide using surface-enhanced Raman scattering; and (3) the construction of heterostructures in which excitons can be coupled to surface plasmons.

We report the first application of critical cluster techniques to the Mott metal-insulator transition in vanadium dioxide. We show that the geometric universal properties of the metallic and insulating puddles observed by scanning near-field infrared microscopy are consistent with the system passing near criticality of the random field Ising model as temperature is varied. The resulting large barriers to equilibrium may be the source of the unusually robust hysteresis phenomena associated with the metal-insulator transition in this system.

Vanadium dioxide (VO 2 ) undergoes a metal-insulator transition (MIT) at 340 K with the structural change between tetragonal and monoclinic crystals as the temperature is lowered.The conductivity σ drops at MIT by four orders of magnitude.The low-temperature monoclinic phase is known to have a lower ground-state energy.The existence of a k-vector k is prerequisite for the conduction since the k appears in the semiclassical equation of motion for the conduction electron (wave packet).Each wave packet is, by assumption, composed of the plane waves proceeding in the k direction perpendicular to the plane.The tetragonal (VO 2 ) 3 unit cells are periodic along the crystal's x-, y-, and z-axes, and hence there are three-dimensional k-vectors.The periodicity using the non-orthogonal bases does not legitimize the electron dynamics in solids.There are one-dimensional k along the c-axis for a monoclinic crystal.We believe this decrease in the dimensionality of the k-vectors is the cause of the conductivity drop.Triclinic and trigonal (rhombohedral) crystals have no k-vectors, and hence they must be insulators.The majority carriers in graphite are "electrons", which is shown by using an orthogonal unit cell for the hexagonal lattice.

We show that the main mechanism for the dc voltage or dc current induced insulator-metal transition in vanadium dioxide VO(2) is due to local Joule heating and not a purely electronic effect. This "tour de force" experiment was accomplished by using the fluorescence spectra of rare-earth doped micron sized particles as local temperature sensors. As the insulator-metal transition is induced by a dc voltage or dc current, the local temperature reaches the transition temperature indicating that Joule heating plays a predominant role. This has critical implications for the understanding of the dc voltage or dc current induced insulator-metal transition and has a direct impact on applications which use dc voltage or dc current to externally drive the transition.

The microwave radiation reflection coefficient of pure vanadium dioxide and its compositions with dielectrics exhibits a jumplike change upon a sample transition from dielectric into metallic state. The reflection coefficient may either increase or decrease, the sign and magnitude of the effect depending on the electromagnetic field frequency and the sample thickness. The phenomenon is explained by changes in the wave interference conditions upon the phase transition.

Nanostructured thin-film composites vanadium oxide/porous anodic aluminum oxide (VO/PAO) were investigated. Thin layers of vanadium oxide were formed by sol-gel deposition from the solution of vanadium isopropoxide in isopropanol with subsequent annealing in Ar atmosphere at 500°C. As a substrate, porous anodic aluminum oxide and Si/SiO2 with PAO and without it were used. The orienting effect of PAO on the formation of the vanadium oxide layer has been established. Using the techniques of X-ray diffraction and electron microscopy, the formation of the crystalline vanadium dioxide VO2 phase, the orientation of the grains, which correlates with the morphology of the PAO substrate, has been established. The influence of electrolyte and stresses during the filling of the pores of aluminum oxide on the formation of grains, nanoclusters, and agglomerates of vanadium oxide is analyzed. Electrical properties of the nanostructured vanadium oxide layer were investigated. The characteristic of electrical resistance indicate the reversible insulator-metal phase transition in vanadium dioxide. The capacitances-voltage characteristics of the Al/VO/PAO/Al sandwich structures had an asymmetric character with an increase in capacitance upon increasing the negative bias voltage on vanadium oxide. This effect is determined by the phase transition insulator-metal under the influence of electric field. In addition, injection of electrons from the Al electrode or their extraction from vanadium oxide as well as recharging of the VO/PAO interface states can exert the influence. The mechanism for vanadium dioxide formation on the orienting substrate made of porous anodic aluminum oxide is proposed.