Intrinsic and Extrinsic Factors Influencing the Dynamics of VO2 Mott Oscillators

Oscillatory devices have recently attracted significant interest as key components of computing systems based on biomimetic neuronal spiking. An understanding of the time scales underlying the spiking is essential for engineering fast, controllable, low-energy devices. However, we find that the intrinsic dynamics of these devices is difficult to properly characterize, as they can be heavily influenced by the external circuitry used to measure them. Here we demonstrate these challenges using a VO2 Mott oscillator with a sub-100-nm effective size, achieved using a nanogap cut in a metallic carbon nanotube electrode. Given the nanoscale thermal volume of this device, it would be expected to exhibit rapid oscillations. However, due to external parasitics present within commonly used current sources, we see orders-of-magnitude slower dynamics. We outline methods for determining when measurements are dominated by extrinsic factors and discuss the operating conditions under which intrinsic oscillation frequencies may be observed.</p

Nonlinear Magnetization Dynamics Driven by Strong Terahertz Fields

We present a comprehensive experimental and numerical study of magnetization dynamics triggered in a thin metallic film by single-cycle terahertz pulses of $\sim20$ MV/m electric field amplitude and $\sim1$ ps duration. The experimental dynamics is probed using the femtosecond magneto-optical Kerr effect (MOKE), and it is reproduced numerically using macrospin simulations. The magnetization dynamics can be decomposed in three distinct processes: a coherent precession of the magnetization around the terahertz magnetic field, an ultrafast demagnetization that suddenly changes the anisotropy of the film, and a uniform precession around the equilibrium effective field that is relaxed on the nanosecond time scale, consistent with a Gilbert damping process. Macrospin simulations quantitatively reproduce the observed dynamics, and allow us to predict that novel nonlinear magnetization dynamics regimes can be attained with existing table-top terahertz sources.Comment: 6 pages, 4 figure

Metallization of Epitaxial VO₂ Films by Ionic Liquid Gating through Initially Insulating TiO₂ Layers

Ionic liquid gating has been shown to metallize initially insulating layers formed from several different oxide materials. Of these vanadium dioxide (VO₂) is of especial interest because it itself is metallic at temperatures above its metal–insulator transition. Recent studies have shown that the mechanism of ionic liquid gated induced metallization is entirely distinct from that of the thermally driven metal–insulator transition and is derived from oxygen migration through volume channels along the (001) direction of the rutile structure of VO₂. Here we show that it is possible to metallize the entire volume of 10 nm thick layers of VO₂ buried under layers of rutile titanium dioxide (TiO₂) up to 10 nm thick. Key to this process is the alignment of volume channels in the respective oxide layers, which have the same rutile structure with clamped in-plane lattice constants. The metallization of the VO₂ layers is accompanied by large structural expansions of up to ∼6.5% in the out-of-plane direction, but the structure of the TiO₂ layer is hardly affected by gating. The TiO₂ layers become weakly conducting during the gating process, but in contrast to the VO₂ layers, the conductivity disappears on exposure to air. Indeed, even after air exposure, X-ray photoelectron spectroscopy studies show that the VO₂ films have a reduced oxygen content after metallization. Ionic liquid gating of the VO₂ films through initially insulating TiO₂ layers is not consistent with conventional models that have assumed the gate induced carriers are of electrostatic origin