Metal-insulator Transition Research Articles

Tuning the metal-insulator transition temperature by the controlled generation of oxygen vacancies on La0.7Ca0.3MnO3-x epitaxial thin films

This report presents a study on the effect of the controlled generation of oxygen vacancies on the electronic transport properties of La0.7Ca0.3MnO3-x epitaxial thin films. Oxygen defects cause significant changes in the crystalline structure of mixed-valence manganites and lower the Mn valence, resulting in a shift of the metal-insulator transition to lower temperatures due to modifications in the angles and lengths of Mn-O bonds. Experiments using Polarized Extended X-Ray Absorption Fine Structure Spectroscopy have provided strong evidence that oxygen vacancies are mainly formed in the basal plane of the MnO6 octahedra. The accurate control of oxygen vacancies generation opens possibilities for managing and enhancing the magneto-electronic properties of epitaxial manganite thin films with potential in industrial applications.

Selective modulation of electronic transport in VO2 induced by 10 keV helium ion irradiation

Vanadium dioxide (VO2) manifests an abrupt metal–insulator transition (MIT) from monoclinic to rutile phases, with potential use for tunable electronic and optical properties and spiking neuromorphic devices. Understanding pathways to modulate electronic transport in VO2, as well as its response to irradiation (e.g., for space applications), is critical to better enable these applications. In this work, we investigate the selective modulation of electronic transport in VO2 films subject to different 10 keV helium ion (He+) fluences. Under these conditions, the resistivity in the individual monoclinic and rutile phases varied by 50%–200%, while the MIT transformation temperature remains constant within 4 °C independent of irradiation fluence. Importantly, different trends in the resistivity of the monoclinic and rutile phases were observed both as a function of total He fluence as well as in films grown on different substrates (amorphous SiO2/Si vs single crystal Al2O3). Through a combination of measurements including majority carrier sign via Seebeck, low frequency noise, and TEM, our investigation supports the presence of different kinds of point defects (V in; O in), which may arise due to grain boundary defect interactions. Our work suggests the utility of He irradiation for the selective modulation of VO2 transport properties for neuromorphic, in contrast to other established but non-selective methods, like doping.

Terahertz beam reconfigurable phase gradient metasurface of VO2 based on different metal–insulator transition temperatures

Terahertz beam reconfigurable phase gradient metasurface of VO2 based on different metal–insulator transition temperatures

Формирование наноструктур из поликристаллических пленок диоксида ванадия с помощью сканирующей зондовой литографии

Vanadium dioxide (VO2) undergoes a reversible first-order metal-insulator phase transition near room temperature, accompanied by a structural phase transition. This causes a significant change in its electrical and optical properties, making it useful for practical applications. VO2-based nanostructures exhibit notable mechanical resistance to structural transition caused by their small size and demonstrate vivid properties during the phase transition. Obtaining VO2 nanostructures is an extremely demanded objective. This research study presents the use of scanning probe lithography techniques to fabricate nanostructures on polycrystalline VO2 films. The present study focuses on the modification of VO2 films when a positive bias is applied to the sample. The effect of the value and duration of the applied voltage, relative humidity on the quality of the formed nanolithographic pattern was analyzed. The oxidation mechanism was determined. It was found that as a result of local anodic oxidation the formed oxide structures consisting of vanadium penta-oxide (V2O5) completely dissolve in water. This process leads to the separation of the continuous polycrystalline VO2 film into individual nanostructures with precise dimensions. The presented method of forming nanostructures from crystalline VO2 films is promising for nanophotonics and nanoelectronics.

Spacing-driven collapse of band gap and magnetic order in Sr3Ir2O7

Using first-principles methods, we investigated the effect of structural deformation on the electronic and magnetic properties of Sr3Ir2O7 by two tuning ways: reducing only the interlayer spacing and the uniaxial-like compression. In both cases, an insulator-metal transition is successfully achieved with significantly different Fermi surface features. The former consists of a hole pocket and an electronic pocket, while for the latter, a pseudo-gap is captured near the center of the crystallographic Brillouin zone. The anti-ferromagnetism collapses completely and is not fully synchronized with the closing of the Mott gap in two kinds of compression cases, indicating that the conventional correlation between magnetic order and Mott insulating state does not exist here. Our research is conducive to understand the nature of lattice distortion-induced electronic and magnetic transitions, and will be expected to provide a new theoretical reference for the experimental regulation of novel metallic properties in iridium.

Phonon and electronic properties of semiconducting silicon nitride bilayers

The two-dimensional (2D) IV-V semiconductors have attracted much attention due to their fascinating electronic and optical properties. In this work, we predicted three phases of silicon nitrides, denoted α-Si2N2, β-Si2N2, and γ-Si4N4, respectively. Both α-Si2N2 and β-Si2N2 consist of two buckled SiN sheets, and γ-Si4N4 consists of two puckered SiN sheets. It is challenging to transform between α-Si2N2 and β-Si2N2 because of the high energy barrier. The three dynamically stable bilayers are semiconductors with fundamental indirect band gaps from 0.25 eV to 2.92 eV. As expected, only the s and p orbitals contribute to the electronic states, and the pz orbitals dominate near the Fermi level. Furthermore, insulator-metal transitions occur in α-Si2N2 and β-Si2N2 under the biaxial strain of 16 %. These materials perhaps have potential applications in microelectronics and spintronics.

Heat activated VO2 (R) cathodes for high-performance aqueous zinc ion batteries

We introduce a novel approach for design and production of vanadium dioxide (VO2) cathodes activated by heat, specifically tailored for use in aqueous zinc-ion batteries (ZIBs). Our cathodes exhibit distinctive metal-insulator transition (MIT) characterized by the shift between monoclinic (M) and tetragonal rutile (R) phases. The employment of an aqueous electrolyte in ZIBs enables successful operation of VO2 cathodes, showcasing phase transition and enhanced electrochemical performance above a readily attainable temperature of approximately 68 °C. The MIT feature enhances the charge transfer properties at electrode-electrolyte interfaces, resulting in notable improvements in specific capacity and energy densities. Fabricated VO2(M) cathodes underwent heat-triggered transformation to VO2(R), resulting in a remarkable increase (400 %) in capacity. At 0.1 A g−1, VO2(R) cathodes show specific capacity of 354.6 mAh g−1, yielding an impressive energy density of 461.0 Wh kg−1 for aqueous ZIBs. This exceptional performance can be attributed to abrupt change in electrical conductivity upon thermal activation of MIT. This work underscores pivotal role of thermally activated MIT in finely tuning electrochemical performance of energy storage systems, prospecting a promising avenue for further advancements.

Competitive coexistence of ferromagnetism and metal–insulator transition of VO2 nanoparticles

We investigated the magnetic and electric properties of nanometer-sized vanadium dioxide (VO2) particles. VO2 nanoparticles were formed by milling VO2 powder. We measured the magnetic field dependence of the magnetization of the VO2 powder and nanoparticles. The VO2 powder did not exhibit ferromagnetism, whereas the VO2 nanoparticles exhibited ferromagnetism. In addition, we fabricated samples by bridging between electrodes with the VO2 nanoparticles, and the temperature dependence of their resistance was measured. Metal-insulator transitions (MITs) were observed, and the temperature range where the MIT occurred was wider than that in a typical bulk VO2. The VO2 nanoparticles exhibited these properties of ferromagnetism and MIT possibly because of the surface and size effects of the VO2 nanoparticles. These results indicate the first observation of the competitive coexistence of ferromagnetism and MIT of VO2 nanoparticles.

Electronic Structure Evolution in the Temperature Range of Metal–Insulator Transitions on Sn/Ge(111)

One‐third of monolayer of Sn adatoms on a Ge(111) substrate forms a 2D triangular lattice with one unpaired electron per site. The system presents a metal–insulator transition when decreasing the temperature and it is known to exhibit strong electron–phonon coupling at 120–150 K. Herein, a study of the electronic band structure for α‐Sn/Ge(111) between 150 and 5 K is reported. Both the experimental Fermi surfaces and the energy dispersions along high symmetry directions as a function of the temperature are presented. At 5 K it is observed a weakly or low‐dispersing spectral feature, exhibiting an extended gap in the reciprocal space. This feature is derived from the topmost occupied band, which is metallic at high temperature and which develops a kink associated with the strong electron–phonon coupling. The spectral evolution is partially explained with an increase of the electron–phonon coupling when decreasing the temperature. The increase of the electron–phonon coupling at low temperatures gives light into the new physics of this 2D system. The bandwidth is progressively reduced when reducing the temperature, enhancing the electronic correlation effects, and triggering the Mott transition.

Mobility edges and localization characteristics in one-dimensional quasiperiodic quantum walk

Abstract We construct a one-dimensional quasiperiodic quantum walk to investigate the localization–delocalization transition. The inverse participation ratio and Lyapunov exponent are employed as two indexes to determine the mobility edge, a critical energy to distinguish the energy regions of extended and localized states. The analytical solution of mobility edge is obtained by the Lyapunov exponents in global theory, and the consistency of the two indexes is confirmed. We further study the dynamic characteristics of the quantum walk and show that the probabilities are localized to some specific lattice sites with time evolution. This phenomenon is explained by the effective potential of the Hamiltonian which corresponds to the phase in the coin operator of the quantum walk.

Tuning the Conductivity by Disorder and Electric Field Near the Metal–Insulator Transition

Tuning the Conductivity by Disorder and Electric Field Near the Metal–Insulator Transition

Study of fractality nature in VO2 films and its influence on metal-insulator phase transition

The mechanisms underlying the origin of fractal shape of inclusions of a new phase in VO2 films during metal-insulator phase transition are discussed. The obtained results show that hysteresis of the temperature dependence of resistance R(T) significantly depends on the film morphology and texture. Moreover, some fractal features are observed. To determine the fractal dimension D of the structural elements of the studied films from their images, different fractal analysis approaches were preliminary compared and discussed. As a result of the film image treatments, the boundaries of the structural elements were found to have fractal dimensions of 1.3 to 1.5 or higher and to correlate with the shape of R(T). The fractal boundaries indicate the dominant role of elastic stress on the phase transition of films, which is confirmed by numerical modeling. Based on these results, an analytical model is proposed that relates the free energy of a film to the fractal dimension of its constituents. Depending on the ratio of the elastic and interface specific energies, the position of the free energy minimum F corresponds to a certain fractal dimensionality D. A small interface energy leads to a higher fractal dimension making the initial phase more stable. This conclusion explains well all the effects observed experimentally in VO2. The obtained results provide a better understanding of the influence of structure and morphology on other properties of the studied films.

Electric current driven insulator–metal transition and large electroresistance in single crystal Sr2Ir1–xRuxO4

A series of single crystal Sr2Ir1–xRuxO4 (0 ≤ x ≤ 0.55) have been synthesized, and their physical properties have been investigated. Ru-substitution at Ir-site can drastically reduce the resistivity, which is accompanied by serious suppression of the long range antiferromagnetic order. With the application of electric current, remarkable electroresistance exceeding 90% is obtained at x ∼ 0.2, which may contain significant contribution from the strong magnetic competitions. Interestingly, insulator–metal transition driven by electric current is observed (e.g., below 100 K), which can be ascribed to the lattice modulation. The findings indicate that the electric current can be an efficient route to tune the physical properties of the Jeff = 1/2 Mott insulator.

Nanoelectrical monitoring the nonvolatile behavior of VO2 under multi-field stimulate by conductive atomic force microscopy

The ability to control the metal–insulator phase transition is crucial for future neuromorphic and memristive technologies. The key goal of this implementation is to understand and control the nanoscale mechanisms that support these two fundamental switching modalities (volatile and non-volatile states). Here, by adjusting the temperature (60 °C) close to the metal–insulator transition temperature, external application of low voltage (1 V) and laser excitation (18.6 mw · cm−2) can complete the metal–insulator transition of vanadium dioxide (VO2). The nonvolatile behavior of the VO2 devices was nanoelectrically monitored using conductive atomic force microscopy (C-AFM). Using this technique, we were able to reversibly transform the local switching response from volatile to nonvolatile, providing a unique approach for nanoelectrical monitoring in phase transition materials.

New Critical Density in Metal-Insulator Transition, obtained in n(p)- Type Degenerate [ InAs1-xPx(Sbx), GaTe1-xAsx(Sbx,Px), CdTe1-xSx(Sex)]-Crystalline Alloys, 0≤x≤1, being just That of Carriers Localized in Exponential

By basing on the same physical model and treatment method, as used in our recent works [1, 2], for GaAs1-xTex(Sbx,Px) -crystalline alloys, 0≤x≤1 , we will investigate the critical impurity density in the metal-insulator transition (MIT), obtained now in n(p)-type degenerate X(x)=[InAs1-xPx(Sbx), GaTe1-xAsx(Sbx,Px), CdTe1-xSx(Sex)]- crystalline alloys, being due to the effects of the size of donor (acceptor) d(a)-radius, rd(a), and the x- concentration, assuming that all the impurities are ionized even at T=0 K. In such n(p)-type degenerate X(x)= -crystalline alloys, we will determine:
 (i)-the critical impurity density NCDn(CDp)(rd(a), x) in the MIT, as given in Eq. (8), by using an empirical Mott parameter Mn(p)=0.25, and
 (ii)-the density of electrons (holes) localized in the exponential conduction (valence)-band tails (EBT), NEBTCDn(CDp)(rd(a),x), as given in Eq. (26), by using our empirical Heisenberg parameter, Hn(p)=0.47137, as given in Eq. (15), suggesting that: for given rd(a) and x, NEBTCDn(CDp)(rd(a),x)=NCDn(CDp)(rd(a),x) obtained with a precision of the order of 2.91x10-7, as observed in Tables 2-8.
 In other words, such the critical d(a)-density NCDn(CDp)(rd(a),x) is just the density of electrons (holes) localized in the EBT, NEBTCDn(CDp)(rd(a),x).
 So, if denoting the total impurity density by N, the effective density of free electrons (holes), N* given in the parabolic conduction (valence) band of the n(p)-type degenerate - crystalline alloy, can thus be defined by: N*(N,rd(a),x)=N-NCDn(NDp)=N-NEBTCDn(CDp) , as that given in compensated crystals, needing to determine various optical, electrical, and thermoelectric properties in such n(p)-type degenerate X(x)-crystalline alloys, as those studied in n(p)-type degenerate crystals [3-7].

New Critical Density in Metal-Insulator Transition, obtained in n(p)- Type Degenerate InP1-x Asx (Sbx), GaAs1-xTex(Sbx,Px), CdS1-xTex(Sex)-Crystalline Alloys. (II)

By basing on the same physical model and treatment method, as used in our recent works [1, 2], we will investigate the critical impurity densities in the metal-insulator transition (MIT), obtained in n(p)-type degenerate [〖InP_(1-x) As_x (Sb_x),GaAs〗_(1-x) Te_x 〖(Sb_x,P_x),CdS〗_(1-x) Te_x (Se_x)]- crystalline alloys, 0≤x≤1, being due to the effects of the size of donor (acceptor) d(a)-radius, r_(d(a)), the x- concentration, and finally the high d(a)-density, N, assuming that all the impurities are ionized even at T=0 K. In such n(p)-type degenerate crystalline alloys, we will determine:(i)-the critical impurity density N_(CDn(CDp)) (r_(d(a)),x) in the MIT, as that given in Eq. (8), by using an empirical Mott parameter M_(n(p))=0.25, and(ii)-the density of electrons (holes) localized in the exponential conduction (valence)-band tails (EBT), N_CDn(CDp)^EBT (〖 r〗_d(a) ,x), as that given in Eq. (26), by using our empirical Heisenberg parameter, H_(n(p))=0.47137, as given in Eq. (15), according to: for given〖 r〗_d(a) and x, N_CDn(CDp)^EBT (〖 r〗_d(a) ,x)≅N_(CDn(CDp)) (r_(d(a)),x), with a precision of the order of 2.92×10^(-7) , as observed in Tables 2-8 in Appendix 1.In other words, such the critical d(a)-density N_CDn(NDp) (r_(d(a))),x), is just the density of electrons (holes) localized in the EBT, N_CDn(CDp)^EBT (〖 r〗_(d(a)),x), respectively.

Composition-driven Mott transition within SrTi 1−x V x O3

The last few decades has seen the rapid growth of interest in the bulk perovskite-type transition metal oxides SrVO3 and SrTiO3. The electronic configuration of these perovskites differs by one electron associated to the transition metal species which gives rise to the drastically different electronic properties. Therefore, it is natural to look into how the electronic structure transitions between these bulk structures by using doping. Measurements of the substitutional doped SrTi 1−x V x O3 shows an metal–insulator transition (MIT) as a function of doping. By using supercell density functional theory with dynamical mean field theory (DFT+DMFT), we show that the MIT is indeed the result of the combination of local electron correlation effects (Mott physics) within the t2g orbitals and the atomic site configuration of the transition metals which may indicate dependence on site disorder. SrTi 1−x V x O3 may be an ideal candidate for benchmarking cutting-edge Mott–Anderson models of real systems. We show that applying an effective external perturbation on SrTi 1−x V x O3 can switch the system between the insulating and metallic phase, meaning this is a bulk system with the potential use in Mott electronic devices.

Effect of Pr3+ Substitution at the A-Site on the Structural and Electrical Properties of Hole-Doped La-Based Manganites

In this study, the effect of praseodymium substitution at La-site on structural and electrical transport properties of La0.5Ba0.5MnO3 was investigated. Polycrystalline La0.5-xPr0.5x Ba0.5MnO3 (x = 0, 0.50, 1.00) were synthesized using a conventional solid-state method. The powder X-ray diffraction patterns show a single-phase orthorhombic distorted perovskite structure with space group Pnma. The Rietveld refinement analysis showed that the unit cell volume decreased as Pr3+ substitution increased which may be attributed to the different ionic radii of ions. Electrical resistivity measurements by using standard four-point probe resistivity measurement in a temperature range of 30 K to 300 K. As the Pr3+ concentration increases, metal-insulator transition, TMI decreases from 264K (x=0) to 157K (x=1.00) while resistivity increases from 1.16 Ω.cm (x = 0) to 20.3 Ω.cm (x =1.00). The decreased TMI are attributed to the decrease in tolerance factor which indicates enhancement in MnO6 octahedral distortion consequently reduce double exchange interaction. The electrical resistivity in the metallic region for all samples was fitted with the combination of domain/ grain boundary, electron-electron, electron-magnon and electron-phonon scattering processes. The resistivity behaviour in the insulating region for all samples was attributed to small polaron hopping model which revealed that the activation energies increased as Pr3+ content increased due to the enhancement in the distortion of MnO6 octahedral.

Critical Impurity Density in the Mott Metal-Insulator Transition, obtained in the n(p)-Type Degenerate

By basing on the same physical model and treatment method, as used in our recent works (Van Cong, 2024; 2023; 2023), we investigate the critical impurity density in the metal-insulator transition (MIT), obtained in the n(p)-type degenerate Si1−xGex- crystalline alloy, 0≤x≤1, and also applied to determine the optical band gap, being due to the effects of the size of donor (acceptor) d(a)-radius, rd(a), the x-Ge concentration, the temperature T, and finally the high d(a)-density, N, assuming that all the impurities are ionized even at T=0 K. In such the n(p)-type degenerate Si1−xGex- crystalline alloy, we will determine: (i)-the critical impurity density

Carrier-doping-driven insulator-metal transition in disordered materials for memristive switching with high uniformity

Attaining highly uniform operations in a disordered system presents a persistent challenge. The utilization of ion migration in amorphous materials to trigger the resistive switching process of the material usually results in inferior uniformity of the memristive device. Here, we demonstrate that the resistive switching behavior can be activated through carrier doping in the disorder system, and highly ordered resistance modulation is achieved in Ag-doped albumen. By manipulating the doping level of the carrier, the localization of the free electron wavefunction can be tuned, leading to multi-level variations in resistance. This memristive switching behavior is in all electronic and displays excellent switching uniformity, holding great potential for applications in high-density memories and neuromorphic computing chips.