Explore the vast range of titles available.

A controllable metal-insulator transition (MIT) of VO2 has been highly desired due to its huge potential applications in memory storage, smart windows or optical switching devices. Recently, interfacial strain engineering has been recognized as an effective approach to tuning the MIT of epitaxial VO2 films. However, the strain-involved structural evolution during the MIT process is still not clear, which prevents comprehensively understanding and utilizing interfacial strain engineering in VO2 films. In this work, we have systematically studied the epitaxial VO2 thick films grown on TiO2 (001) single crystal substrate and the structural transition at the boundary of MIT region. By using in situ temperature-dependent high-resolution x-ray diffractions, a tetragonal-like ('T-like') to 'rutile' structural phase transition is identified during the MIT process. The room-temperature crystal phase of epitaxial VO2 TiO2(001) thick film is clarified to be tetragonal-like, neither strained-rutile phase nor monoclinic phase. The calculated atomic structure of this T-like phase VO2 resembles that of the M1 phase VO2, which has been verified by their similar Raman spectra. More, the crystal lattices of the coexisted phases in the MIT region were revealed in detail. The current findings will not only show some clues on the MIT mechanism study from the structural point of view, but also favor the interface engineering assisted VO2-based devices and applications in the future.

The local structure of the filled tetragonal tungsten bronze (TTB) niobate Ba3Nb5−xTixO15 (x = 0, 0.1, 0.7, 1.0), showing a metal-insulator transition with Ti substitution, has been studied by Nb K-edge extended X-ray absorption fine structure (EXAFS) measurements as a function of temperature. The Ti substitution has been found to have a substantial effect on the local structure, that remains largely temperature independent in the studied temperature range of 80–400 K. The Nb-O bonds distribution shows an increased octahedral distortion induced by Ti substitution, while Nb-Ba distances are marginally affected. The Nb-O bonds are stiffer in the Ti substituted samples, which is revealed by the temperature dependent mean square relative displacements (MSRDs). Furthermore, there is an overall increase in the configurational disorder while the system with Nb 4d electrons turns insulating. The results underline a clear relationship between the local structure and the electronic transport properties suggesting that the metal-insulator transition and possible thermoelectric properties of TTB structured niobates can be tuned by disorder.

We successfully demonstrated a transition from a metallic InOx film into a nondegenerate semiconductor InOx:H film. A hydrogen-doped amorphous InOx:H (a-InOx:H) film, which was deposited by sputtering in Ar, O2, and H2 gases, could be converted into a polycrystalline InOx:H (poly-InOx:H) film by low-temperature (250 °C) solid-phase crystallization (SPC). Hall mobility increased from 49.9 cm2V−1s−1 for an a-InOx:H film to 77.2 cm2V−1s−1 for a poly-InOx:H film. Furthermore, the carrier density of a poly-InOx:H film could be reduced by SPC in air to as low as 2.4 × 1017 cm−3, which was below the metal–insulator transition (MIT) threshold. The thin film transistor (TFT) with a metallic poly-InOx channel did not show any switching properties. In contrast, that with a 50 nm thick nondegenerate poly-InOx:H channel could be fully depleted by a gate electric field. For the InOx:H TFTs with a channel carrier density close to the MIT point, maximum and average field effect mobility (μFE) values of 125.7 and 84.7 cm2V−1s−1 were obtained, respectively. We believe that a nondegenerate poly-InOx:H film has great potential for boosting the μFE of oxide TFTs.

What are the ground states of an interacting, low-density electron system? In the absence of disorder, it has long been expected that as the electron density is lowered, the exchange energy gained by aligning the electron spins should exceed the enhancement in the kinetic (Fermi) energy, leading to a (Bloch) ferromagnetic transition. At even lower densities, another transition to a (Wigner) solid, an ordered array of electrons, should occur. Experimental access to these regimes, however, has been limited because of the absence of a material platform that supports an electron system with very high quality (low disorder) and low density simultaneously. Here we explore the ground states of interacting electrons in an exceptionally clean, two-dimensional electron system confined to a modulation-doped AlAs quantum well. The large electron effective mass in this system allows us to reach very large values of the interaction parameter rs , defined as the ratio of the Coulomb to Fermi energies. As we lower the electron density via gate bias, we find a sequence of phases, qualitatively consistent with the above scenario: a paramagnetic phase at large densities, a spontaneous transition to a ferromagnetic state when rs surpasses 35, and then a phase with strongly nonlinear current-voltage characteristics, suggestive of a pinned Wigner solid, when rs exceeds ≃38. However, our sample makes a transition to an insulating state at rs ≃27, preceding the onset of the spontaneous ferromagnetism, implying that besides interaction, the role of disorder must also be taken into account in understanding the different phases of a realistic dilute electron system.

SignificanceOur combined theoretical and experimental study of bulk and heterostructured forms of a correlated electron material leads to insights into the metal–insulator transition. Comparison of single-layer, bilayer, and very thick samples validates a combined ab-initio/many-body theoretical approach and enables a clear disentangling of electronic and lattice contributions to the transition by independently changing each. Analysis of the lattice relaxations associated with the metal–insulator transition highlights the importance of the elastic properties of and propagation of distortions into the electronically inert counterlayer, defining a control parameter for tuning electronic properties. Counterlayer-induced bond-angle changes and electronic confinement provide separate tuning parameters, with bond-angle changes found to be a much less effective tuning parameter. In complex oxide materials, changes in electronic properties are often associated with changes in crystal structure, raising the question of the relative roles of the electronic and lattice effects in driving the metal–insulator transition. This paper presents a combined theoretical and experimental analysis of the dependence of the metal–insulator transition of NdNiO3 on crystal structure, specifically comparing properties of bulk materials to 1- and 2-layer samples of NdNiO3 grown between multiple electronically inert NdAlO3 counterlayers in a superlattice. The comparison amplifies and validates a theoretical approach developed in previous papers and disentangles the electronic and lattice contributions, through an independent variation of each. In bulk NdNiO3, the correlations are not strong enough to drive a metal–insulator transition by themselves: A lattice distortion is required. Ultrathin films exhibit 2 additional electronic effects and 1 lattice-related effect. The electronic effects are quantum confinement, leading to dimensional reduction of the electronic Hamiltonian and an increase in electronic bandwidth due to counterlayer-induced bond-angle changes. We find that the confinement effect is much more important. The lattice effect is an increase in stiffness due to the cost of propagation of the lattice disproportionation into the confining material.

Controlling the electronic properties of oxides that feature a metal– insulator transition (MIT) is a key requirement for developing a new class of electronics often referred to as “Mottronics.” A simple, controllable method to switch the MIT properties in real time is needed for practical applications. Here we report a giant, nonvolatile resistive switching (ΔR/R > 1,000%) and strong modulation of the MIT temperature (ΔTc > 30 K) in a voltage-actuated V₂O₃/PMN-PT [Pb(Mg,Nb)O₃-PbTiO₃] heterostructure. This resistive switching is an order of magnitude larger than ever encountered in any other similar systems. The control of the V₂O₃ electronic properties is achieved using the transfer of switchable ferroelastic strain from the PMN-PT substrate into the epitaxially grown V₂O₃ film. Strain can reversibly promote/hinder the structural phase transition in the V₂O₃, thus advancing/suppressing the associated MIT. The giant resistive switching and strong Tc modulation could enable practical implementations of voltage-controlled Mott devices and provide a platform for exploring fundamental electronic properties of V₂O₃.

We study the transport behavior of anti-dot graphene both theoretically and experimentally, where the term ‘anti-dot’ denotes the graphene layer to be nanostructured with a periodic array of holes. It has been shown that the electronic band structure of the anti-dot graphene can be described by a 4 by 4 effective Hamiltonian (Pan J et al 2017 Phys. Rev. X. 7 031043) with a gap around the Dirac point, attendant with a 0 to π variation of the Berry phase as a function of energy, measured from the band edge. Based on the diagrammatic method analysis and experiments, we identify an energy-dependent metal-to-insulator transition (MIT) in this two-dimensional (2D) system at a critical Fermi energy ɛ c , characterized by the divergence of the localization length in the Anderson localization phase to a de-localized metallic phase with diffusive transport. By measuring the conductance of square samples with varying dimension and at different Fermi energies, experiments were carried out to verify the theory predictions. While both theory and experiment indicate the existence of a 2D MIT with similar localization length divergence exponent, the values of the critical energy ɛ c and that of the localization length do not show quantitative agreement. Given the robust agreement in the appearance of a 2D MIT, we attribute the lack of quantitative agreement to the shortcomings in the theoretical model. The difficulties in addressing such shortcomings are discussed.

Understanding the dynamics of phase-transitions, interpretations of their experimental observations and their agreement with theoretical predictions continue to be a long-standing research interest. Here, we present detailed phase-transition dynamics of rare earth nickelates associated with its first-order metal–insulator transition. The thermal hysteresis shows absence of training effect and defies the Preisach model. A large phase-coexistence in insulating state during cooling suggests kinetically arrested glassy dynamics of the phase-transition. Experimentally derived hysteresis scaling exponent is much larger than the mean-field predicted universal value of 2/3. In the phase-coexistence region, the quench and hold measurement depicts higher stability of the metallic state compare to that of the insulating one; highlighting the manifestation of phase-coexistence via asymmetric spinodal decomposition. All these observations for nickelates are in stark contrast to the phase-transition dynamics of canonically similar vanadates but are closer to those of glasses, alloys. A substantial disagreement between the experiment and theory emphasizes the necessity to incorporate system-dependent details for the accurate interpretation of the experimental results.

Terahertz (THz) metamaterials based on phase-change materials (PCMs) offer promising approaches to the dynamic modulation of electromagnetic responses. In this study, we design and experimentally demonstrate a tunable THz metamaterial composed of a symmetric split-ring resonator (SRR) pair, with the left halves covered by a 35 nm thick epitaxial vanadium dioxide (VO2) film, enabling the simultaneous exploitation of both permittivity- and conductivity-induced modulation mechanisms. During the metal–insulator transition (MIT) of VO2, cooperative changes in permittivity and conductivity lead to the excitation, redshift, and eventual disappearance of a quasi-bound state in the continuum (QBIC) resonance. Finite element simulations, using optical parameters of VO2 film defined by the Drude–Smith model, predict the evolution of the transmission spectra well. These results indicate that the permittivity change originating from mesoscopic carrier confinement is a non-negligible factor in THz metamaterials hybridized with VO2 film and also reveal the potential for developing reconfigurable THz metamaterials based on the dielectric modulation effects of VO2 film.

Methane (CH 4 ) sensors based on metal oxide semiconductors are widely used for both safety and environmental applications. The need of further improvements in performance provides great motivation for the search of novel sensing materials and strategies. Herein, CH 4 sensor based on samarium nickelate (SmNiO 3 , SNO), a complex perovskite oxide, was developed. Under exposure to CH 4 , SNO exhibited a large sensitivity arising from metal–insulator transition induced by electron doping, which was a fundamentally different strategy from the carrier depletion or accumulation on the surface of the conventional metal oxide semiconductors. The electronic phase transition was confirmed by both X-ray characterization and electrochemical impedance spectroscopy measurements. In addition, the SNO exhibited a mixed ionic and electronic conductivity in the CH 4 environment. The kinetics of phase transformation was modeled with the Johnson–Mehl–Avrami equation. The performance of SNO-based CH 4 sensors were also carefully evaluated. Overall, the electron doping-induced metal–insulator transition was demonstrated as a promising alternative strategy for CH 4 sensing. Graphical abstract