Session S50: Vanadium Oxides Metal Insulator Transitions

Unraveling the Mott-Peierls intrigue in Vanadium dioxide

Vanadium dioxide is one of the most studied strongly correlated materials.
The nature of the rutile metal to monoclinic insulator transition in this system triggered a longstanding debate about which comes first, the Mott localisation or the Peierls distortion. Here, we show that the electronic correlations and the lattice vibrations conspire to stabilise the monoclinic insulator, and so they must be treated on equal footing. We design a minimal model for VO2 that includes all the important physical ingredients: the electronic correlations, the multi-orbital character, and the two components antiferrodistortive mode that condenses in the monoclinic insulator. We solve this model by dynamical mean-field theory within the adiabatic Born-Oppenheimer (BO) approximation. Consistently with the first-order character of the metal-insulator transition, the BO potential has a rich landscape describing the distorted and undistorted phases, and which translates into an equally rich thermodynamics that we uncover by the Monte Carlo method. Remarkably, we find that a distorted metal phase intrudes between the low-temperature distorted insulator and high-temperature undistorted metal, which sheds new light on the debated experimental evidence of a monoclinic metallic phase.   

Correlation induced emergent charge order in metallic vanadium dioxide thin films

Understanding the metal-insulator transition in vanadium dioxide is complicated by symmetry breaking structural transitions accompanying correlated electronic transitions. Resolving this problem though is at the heart of many scientific and technological advancements. To that end, we develop a cross-phase, symmetry-consistent approach to treat structural distortions and electronic correlations in epitaxial VO₂ films from first principles. By adopting a uniform Bravais symmetry across several epitaxial growth configurations, we demonstrate the exquisite sensitivity of correlation physics to apical bond lengths in the presence of lattice symmetry breaking, and discover emergent charge order preceding the dimerized insulating phase even in the metallic rutile phase. We discuss the deep physical implications for the phase diagram of VO₂, and argue for the unparalleled significance of correlation physics and sample quality in light of this result.
  

Probing the Mott transition in V2O3 with Pair Distribution Function Analysis

V₂O₃ is a popular system for studying Mott insulators, which are materials that are driven into an insulating state by strong electron correlations. Despite decades of research, a complete understanding of the metal-insulator transition in V₂O₃ has not been conclusively established. Here, we present comprehensive atomic and magnetic pair distribution function (PDF) analyses of V₂O₃ using both x-ray and neutron total scattering measurements, shedding new light on the mechanism of the transition from the point of view of short-range structural and magnetic correlations on both sides of the transition. The results allow us to thoroughly examine the transition from the metallic state to the insulating state with complementary sensitivity provided by the x-rays (sensitive to the vanadium displacements) and neutrons (sensitive to the oxygen displacements and magnetic correlations). We discuss the implications of these results in the context of other recent experimental and theoretical studies of V₂O₃.
  

Nanosecond fluctuators in V2O3 film within the metal-insulator-transition

Vanadium sesquioxide (V₂O₃) is an archetypal Mott insulator, and exhibits a first-order transition at 160 K between a low temperature monoclinic, antiferromagnetic insulator and a high temperature, rhombohedral, paramagnetic, metallic phase. Resistive noise as measured through electronic transport can be used as a probe of the fluctuating dynamics of the two-phase domain structure. We measure noise spectra at both low frequencies (up to 300 kHz) and radio frequencies (between 200 MHz and 1 GHz). At low current densities the noise power is quadratic in bias current, as expected for resistive fluctuations probed nonperturbatively by the current. The low frequency noise generally resembles flicker-type 1/f^a noise, often taking on the form of Lorentzian noise dominated by a small number of fluctuators as the insulating phase is approached. The presence of radio frequency noise power that is quadratic in the bias current allow identification of domain fluctuations with lifetimes below 1 ns, comparable to timescales seen in non-equilibrium pump-probe studies of the transition.   

Low-temperature metallic phase in V2O3 films due to structural confinement

We show that high purity V₂O₃ films grown on certain substrates exhibit a suppressed metal insulator transition (MIT) and in some cases even a reentrant metallic phase at low temperature. This occurs only in films which have the c-axis in the out-of-plane direction. For this lattice orientation the MIT entails a structural expansion which is entirely in the film plane. As the MIT progresses, some expansion into the low temperature monoclinic phase occurs but in-plane confinement prohibits a full expansion of the lattice. Instead, a coexisting metallic phase is stabilized down to the lowest temperatures measured. This low temperature metallic phase exhibits structural and electrical properties which are distinct from the high temperature metallic phase.   

Controlled growth of VO2 Nanowires by direct conversion of V2O5 solution film

Vanadium dioxide is known to exhibit fascinating properties due to its phase transition from insulating to metallic phase at 340 K. Effectively growing single-crystal VO₂ nanowires has been difficult to realize in a controlled, repeatable way. Through the use of a chemical vapor deposition using solutions of V₂O₅, we realized a consistent growth of single crystals with their size ranging from nanowires to microbeams. A comprehensive understanding of the growth mechanism has also been obtained. With combined probing techniques, including an Enhanced Darkfield Optical Microscopy, Atomic Force Microscopy, and Raman Microspectroscopy, we captured the dynamics of growth, along with the stoichiometry and crystal structure for nanocrystals at different growth stages. We revealed that the growth starts with the vaporization of V₂O₅, followed by dense seed nucleation on the substrate, and then the growing of large crystals as a result of consuming neighboring seeds.
The new method of VO₂ nanowires growth based on V₂O₅ film provides a simpler and more economical solution for their application in industry.   

Observation of the voltage-triggered insulator to metal transition in a VO2 thin film as a function of temperature.

VO₂ undergoes an insulator to metal transition with a resistivity change up to five orders of magnitude upon heating through 340 K. The transition has also been observed via current-voltage (IV) measurements as a sharp jump in the current when sweeping the applied voltage. We have previously used the tip of an atomic force microscope (AFM) to map the local current response of voltage-biased polycrystalline VO₂ thin films[1]. We have fit the IV curve immediately preceding the transition to the temperature-dependent Poole-Frenkel conduction mechanism and calculated the local temperature of the film. While these measurements were nominally performed at room temperature, we calculated an increased local temperature of 335 K immediately preceding the transition, confirming the role of Joule heating in our voltage-biased, tip-sample geometry. Here, we extend these measurements to a wider temperature range to further understand the effects of Joule heating on the voltage-triggered insulator to metal transition in VO₂ thin films. ([1] Spitzig et al. arXiv:1903.03062v1)   

Selective phonon-mode excitation revealed by tracing the pathway of photo-induced structural-phase-transition in single-crystal VO2

The Metal-Insulator (M-I) transition in VO₂ that is associated with a monoclinic-rutile (M-R) structure phase transition has been extensively studied due to the potential application of its fast electrical/optical switch properties. It remains a big challenge to reveal the transition pathway to fully understand the underlying mechanism. In this study, we use MeV ultrafast electron diffraction to monitor the structure evolution in single-crystal VO₂ under femtosecond laser excitation. The M-R phase transition was observed at a high sample temperature (T = 314 K) under a specific pump fluence of 16 mJ/cm². Crystallographic structural analysis reveals that the atomic motions have a two-step dynamics. During the first a few ps, both stretching and rotation A_g phonon modes are involved in the collective vanadium motions. Then only rotation A_g phonon mode is continuously excited in the following hundreds of ps. Moreover, such the M-R phase transition cannot be observed at a much lower sample base temperature (T = 77 K), indicating a certain role of thermal fluctuation in this photo-induced phenomenon.   

Probing the insulator-to-metal transition in VO2 using non-contact atomic force microscopy

Vanadium dioxide (VO₂) undergoes an insulator-to-metal transition (IMT) when heated above a critical temperature of 340 K. The role of Joule heating and applied electric field in inducing this transition is an active area of research. In this study, the IMT of VO₂ thin films grown using oxygen plasma molecular beam epitaxy are examined using non-contact atomic force microscopy (AFM) in ultrahigh vacuum. By measuring the dissipated power through a qPlus AFM tuning fork, the competing effects of Joule heating and applied electric field are assessed. With this non-invasive and highly sensitive technique, we can measure the IMT as a function of tip-sample distance and bias voltage.   

Resistive switching and filament formation in VO2 vertical devices

Vanadium oxides exhibit electrically-triggered, volatile, resistive switching and provide a promising opportunity to mimic spiking neurons for neuromorphic computing. The control of the physical properties of these oxides is an important requirement to develop oxide electronics for future technology. However, a detailed understanding of the non-volatile filamentary formation in these Mott materials is still lacking. In this work, we investigated the mechanisms behind volatile and non-volatile resistive switching in electrically-driven VO₂ vertical devices. By using electrical transport measurement and in-situ transmission electron microscopy, the metal-insulator transition properties of the conductive filament and its nanoscale lattice structure are studied. Our works address the important issues in resistive switching based neuromorphic technologies.   

Acoustoelectric effect in strongly correlated oxides

The acoustoelectric (AE) effect, which manifests itself as the generation of a DC current by a propagating acoustic wave, may convey crucial information on the interdependence between the structural and electronic properties of strongly correlated oxides. We observed that acoustic waves may be coupled not only to an electronic but also to a magnonic state of these materials. In this work, we studied the AE effect in non-magnetic (VO₂, V₂O₃) and ferromagnetic La_0.7Sr_0.3MnO₃ wires grown on top of piezoelectric LiNbO₃ substrates. It was observed that the resistance change across a metal-insulator transition results in a significant enhancement of the AE current in the vanadates wires. Additionally, we show that the sign of the AE current differs for excitations produced by bulk and surface acoustic waves. Importantly, it was observed that the AE current in the La_0.7Sr_0.3MnO₃ wire can be significantly tuned by applying an external magnetic field. Our work provides a new platform for surface acoustic wave devices based on Mott transitions.   

Fragile 3D Order in V(1-x)MoxO2

VO₂ displays a first-order metal-insulator phase transition near 340 K. Accompanying this electronic transition is a structural transition. The connection between these two transitions is unclear, with electronically-driven and structurally-driven models providing conflicting results. Electron doping with molybdenum enhances metallicity and reduces the structural transition temperature; between 17% and 19% Mo, we find that the long-range structural transition is suppressed entirely while the electronic transitions remain similar.
Diffuse x-ray scattering measurements were performed on a single crystal of the V_0.81Mo_0.19O₂. In the low-temperature insulating phase, sharp rods are observed, indicating two-dimensional ordering of atomic displacements. The slight oscillation of these rods about the [110] direction in reciprocal space can be explained by weak, inherently frustrated coupling between the ordered planes. Such fragile embedded order is predicted by an Ising-like ferrodistortive model proposed by Lovorn and Sarker. 3D-ΔPDF analysis of the diffuse scattering and structural simulations provide a clear picture of the local order in this system.   

Nonequilibrium-induced quantum fluctuations in resistive phase transition

We investigate the quantum mechanical origin of resistive phase transitions in solids driven by a constant electric field in the vicinity of a metal-insulator transition. The mean-field theory [1] showed that the self-consistent Landau-Zener mechanism reproduced main experimental features, and we show that it is equivalent to the empirical resistor network theories based on electronic scenarios applicable to systems like VO₂. Despite the qualitative agreement, reliable quantitative predictions for the switching electric-field has remained elusive. Theoretical estimates based on the Landau-Zener mechanism are typically orders of magnitude larger than experimentally observed threshold electric-fields of kV/cm for the insulator-to-metal transition in transition metal oxides and chalcogenides. We investigate the similarities between the resistive switching and the charge-density-wave (CDW) system. Motivated by the CDW theories, we construct a Lagrangian for the gap parameter and investigate the role of the quantum fluctuations to the gap parameter as a quantum variable. We discuss how quantum corrections affect the nonequilibrium resistive transition.

[1] J. E. Han, J. Li, C. Aron, and G. Kotliar, Phys. Rev. B 98, 035145 (2018).   

Emergent properties in films of transition metal oxides

Transition metal oxides exhibit diverse emergent phenomena such as strongly correlated Mott insulating states, magnetic and structural phase transitions, and metal-insulator transitions. Many of these emergent states occur at nanometer length scales which cannot be accessed with traditional infrared focusing methods due to the Abbe diffraction limit. However, scattering-type scanning near-field infrared microscopy (S-SNIM) can circumvent the Abbe diffraction limit and probe optical properties with spatial resolution of tens of nanometers. We have coupled table-top plasma light sources developed in-house to a commercial S-SNIM apparatus from Neaspec GmbH to demonstrate broadband infrared nano-spectroscopy. This is used to study the novel nanoscale properties of ultrathin vanadium dioxide (VO₂) film on rutile titanium dioxide (TiO₂) substrate. Due to strain from the substrate, the metal-insulator transition in the VO₂ film occurs at a temperature of 305 K which is much lower than in bulk VO₂. We also study the surface metallic layer induced by vacuum-annealing of insulating strontium titanate (SrTiO₃) crystals and extract the dynamical properties of the free charge carriers.