Simultaneous Structural and Electronic Transitions in Epitaxial VO_{2}/TiO_{2}(001)

Modulation-doping a correlated electron insulator

Correlated electron materials (CEMs) host a rich variety of condensed matter phases. Vanadium dioxide (VO2) is a prototypical CEM with a temperature-dependent metal-to-insulator (MIT) transition with a concomitant crystal symmetry change. External control of MIT in VO2-especially without inducing structural changes-has been a long-standing challenge. In this work, we design and synthesize modulation-doped VO2-based thin film heterostructures that closely emulate a textbook example of filling control in a correlated electron insulator. Using a combination of charge transport, hard X-ray photoelectron spectroscopy, and structural characterization, we show that the insulating state can be doped to achieve carrier densities greater than 5 × 1021 cm-3 without inducing any measurable structural changes. We find that the MIT temperature (TMIT) continuously decreases with increasing carrier concentration. Remarkably, the insulating state is robust even at doping concentrations as high as ~0.2 e-/vanadium. Finally, our work reveals modulation-doping as a viable method for electronic control of phase transitions in correlated electron oxides with the potential for use in future devices based on electric-field controlled phase transitions.

High-Strain-Induced Local Modification of the Electronic Properties of VO2 Thin Films

Vanadium dioxide (VO₂) is a popular candidate for electronic and optical switching applications due to its well-known semiconductor-metal transition. Its study is notoriously challenging due to the interplay of long- and short-range elastic distortions, as well as the symmetry change and the electronic structure changes. The inherent coupling of lattice and electronic degrees of freedom opens the avenue toward mechanical actuation of single domains. In this work, we show that we can manipulate and monitor the reversible semiconductor-to-metal transition of VO₂ while applying a controlled amount of mechanical pressure by a nanosized metallic probe using an atomic force microscope. At a critical pressure, we can reversibly actuate the phase transition with a large modulation of the conductivity. Direct tunneling through the VO₂-metal contact is observed as the main charge carrier injection mechanism before and after the phase transition of VO₂. The tunneling barrier is formed by a very thin but persistently insulating surface layer of the VO₂. The necessary pressure to induce the transition decreases with temperature. In addition, we measured the phase coexistence line in a hitherto unexplored regime. Our study provides valuable information on pressure-induced electronic modifications of the VO₂ properties, as well as on nanoscale metal-oxide contacts, which can help in the future design of oxide electronics.

Fabrication and Optical Characterization of VO2-Based Thin Films Deposited on Practical Float Glass by Magnetron Sputtering and Professional Annealing

In this paper, VO2 thin films with good optical properties are fabricated on practical float glass by magnetron sputtering and a professional annealing method. The near-infrared switching efficiency (NIRSE) of the prepared film reaches 39% (@2000 nm), and its near-infrared energy modulation ability (ΔTir) reaches 10.9% (780-2500 nm). Further, the highest integral visible transmittance Tlum is 63%. The proposed method exhibits good reproducibility and does not cause any heat damage to the magnetron sputtering machine. The crystalline structure of the VO2 film is characterized by X-ray diffraction (XRD). The lattice planes (011) and (-211) grow preferentially (JCPDS 65-2358), and a large number of NaV2O5 crystals are detected simultaneously. The microstructures are characterized by scanning electron microscopy (SEM), and a large number of long sheet crystals are identified. The phase transition temperature is significantly reduced by an appropriate W doping concentration (Tc = 29 °C), whereas excessive W doping causes distortion of the thermal hysteresis loop and a reduction in the NIRSE. Oxygen vacancies are created by low pressure annealing, due to which the phase transition temperature of VO2 film decreases by 8 °C. The addition of an intermediate SiO2 layer can prevent the diffusion of Na+ ions and affect the preparation process of the VO2 thin film.

Non-universal current flow near the metal-insulator transition in an oxide interface

In systems near phase transitions, macroscopic properties often follow algebraic scaling laws, determined by the dimensionality and the underlying symmetries of the system. The emergence of such universal scaling implies that microscopic details are irrelevant. Here, we locally investigate the scaling properties of the metal-insulator transition at the LaAlO₃/SrTiO₃ interface. We show that, by changing the dimensionality and the symmetries of the electronic system, coupling between structural and electronic properties prevents the universal behavior near the transition. By imaging the current flow in the system, we reveal that structural domain boundaries modify the filamentary flow close to the transition point, preventing a fractal with the expected universal dimension from forming.

Macroscopic properties usually follow algebraic scaling laws near phase transitions. Here, the authors investigate the scaling properties of the metal‐insulator transition at the LaAlO3/SrTiO3 interface, finding that coupling between structural and electronic properties prevents the universal behavior.

Decoupling between metal-insulator transition and structural phase transition in an interface-engineered VO2

The coupling between the metal-insulator transition (MIT) and the structural phase transition (SPT) in VO₂ has been at the center of discussion for several decades, while the underlying mechanisms of electron-lattice or electron-electron interactions remain an open question. Until recently, the equilibrium state VO₂ is believed to be a non-standard Mott-Hubbard system, i.e., both of the two interactions cooperatively work on MIT, indicating the association between MIT and SPT. However, due to the pronounced contribution of strain in strongly correlated systems, it is desirable to explore the correspondence in an interface-engineered VO₂. Herein, we investigate the carrier dynamics in the VO₂ films with anomalous MIT on the basis of time-resolved transient differential reflectivity measurements. Unexpectedly, MIT is decoupled from SPT, in sharp contrast with the case of strain-free VO₂ films: MIT is triggered by bandgap recombination below 75 °C during heating, while intense SPT-induced signal appears separately between 70 °C and 100 °C. The decoupling between MIT and SPT provides insights into the interfacial interactions in VO₂ thin films.

Detection of Spin Polarized Band in VO2/TiO2(001) Strained Films via Orbital Selective Constant Initial State Spectroscopy

The VO2 is a 3d1 electron system that undergoes a reversible metal–insulator transition (MIT) triggered by temperature and characterized by an interplay between orbital, charge and lattice degrees of freedom. The characterization of the MIT features are therefore extremely challenging and powerful investigation tools are required. In this work, we demonstrate how a combination of resonant photoemission and constant initial state (CIS) spectroscopy can be used as an orbital selective probe of the MIT studying three different VO2/TiO2(001) strained films. The CIS spectra of the V 3d and V 3p photo-electrons shows sensitivity to different orbital contribution and the presence of a spin polarized band close to the Fermi level.

Directly measuring the structural transition pathways of strain-engineered VO2 thin films

Epitaxial films of vanadium dioxide (VO₂) on rutile TiO₂ substrates provide a means of strain-engineering the transition pathways and stabilizing of the intermediate phases between monoclinic (insulating) M1 and rutile (metal) R end phases. In this work, we investigate structural behavior of epitaxial VO₂ thin films deposited on isostructural MgF₂ (001) and (110) substrates via temperature-dependent Raman microscopy analysis. The choice of MgF₂ substrate clearly reveals how elongation of V-V dimers accompanied by the shortening of V-O bonds triggers the intermediate M2 phase in the temperature range between 70-80 °C upon the heating-cooling cycles. Consistent with earlier claims of strain-induced electron correlation enhancement destabilizing the M2 phase our temperature-dependent Raman study supports a small temperature window for this phase. The similarity of the hysteretic behavior of structural and electronic transitions suggests that the structural transitions play key roles in the switching properties of epitaxial VO₂ thin films.

Advances in soft X-ray RIXS for studying redox reaction states in batteries

High-efficiency mapping of resonant inelastic X-ray scattering (mRIXS) for detecting and quantifying both cationic and anionic redox states in batteries.