Metal-insulator Phase Transition Research Articles

Dimerized Hofstadter model in two-leg ladder quasi-crystals

We theoretically study topological features, band structure, and localization properties of a dimerized two-leg ladder with an oscillating on-site potential. The periodicity of the on-site potential can take either rational or irrational values. We consider two types of dimerized configurations; symmetric and asymmetric models. For rational values of the periodicity as long as inversion symmetry is preserved both symmetric and asymmetric ladders can host topological phases. Additionally, the energy spectrum of the models exhibits a fractal structure known as the Hofstadter butterfly spectrum, dependent on the dimerization of the hopping and the strength of the on-site potential. In the case of irrational values for the periodicity, a metal-insulator phase transition occurs with small values of the critical strength of the on-site potential in the dimerized cases. Our models incorporate the effects of lattice configuration and quasi-periodicity, paving the way for establishing platforms that host both topological and non-topological phase transitions.

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.

Surface doping manipulation of the insulating ground states in Ta2Pd3Te5 and Ta2Ni3Te5

Abstract Manipulating emergent quantum phenomena is a key issue for understanding the underlying physics and contributing to possible applications. Here we study the evolution of insulating ground states of Ta2Pd3Te5 and Ta2Ni3Te5 under in-situ surface potassium deposition via angle-resolved photoemission spectroscopy. Our results confirm the excitonic insulator character of Ta2Pd3Te5.Upon surface doping, the size of its global gap decreases obviously. After a deposition time of more than 7 minutes, the potassium atoms induce a metal-insulator phase transition and make the system recover to a normal state. In contrast, our results show that the isostructural compound Ta2Ni3Te5 is a conventional insulator. The size of its global gap decreases upon surface doping, but persists positive throughout the doping process. Our results not only confirm the excitonic origin of the band gap in Ta2Pd3Te5, but also offer an effective method for designing functional quantum devices in the future.

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.

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.

Dynamic multi-color switching using ultrathin vanadium oxide on aluminum-based asymmetric Fabry–Pérot resonant structure

Vanadium dioxide (VO2) exhibits strong infrared optical switching due to its insulator–metal phase-transition property. However, in the visible wavelengths, its intrinsic optical switching is quite low. Current research explores solutions like multilayering, intricate structural patterning, high thermal budget processes, and costly metals for improved color switching. Nonetheless, the color gamut coverage with these methodologies remains notably limited. This work overcomes these limitations and demonstrates dynamic multi-color switching covering a large color gamut using a simple, unpatterned, ultrathin (∼ λ14, where wavelength λ is taken as 575 nm at the center of the visible spectrum) asymmetric Fabry–Pérot structure of VO2 on aluminum (Al). We use the transfer matrix method to design the VO2/aluminium (Al)/sapphire structure for maximum visible reflectance switching. VO2 films are synthesized using a simple, low thermal budget atmospheric oxidation of vanadium (V). With varying oxidation durations, different colors of the oxidized samples are observed. Consistent and reversible color-switching is observed visibly and in reflectance measurements with the change in temperature from low (RT ∼ 30 °C) to high (HT ∼ 100 °C) or vice versa due to the phase transition property of the VO2 layer in the structure. Compared to the existing studies, this work shows a significant change in chromaticities and covers a large color gamut when plotted on the CIE chromaticity diagram. This work has potential applications in the fields of display, thermochromic structures, and visible camouflage.

Heat accumulation and phase transition induction in a VO2 thin film by a femtosecond pulse-periodic radiation.

The kinetics of optical switching due to the insulator-metal phase transition in a VO2 thin film is studied experimentally at different laser pulse repetition frequencies (PRFs) in the NIR range and compared with temperature kinetics obtained through the thermal conductance calculations. Two switching processes have been found with characteristic times <2 ms and <15 ms depending on the PRF; the former is explained by the accumulation of metallic domains remaining after a single-pulse phase transition, and the latter is referred to the heat accumulation in the film. Consequently, the dynamics of the microscopic domains is leading in the initiation of phase transition under pulse-periodic conditions compared to the macroscopic heat transfer. The reverse transition at the radiation turn-off depends on the PRF with a time coefficient of 17.5 µs/kHz and is determined by the metallic domains' decay in the film. The results are important for understanding the nature of the insulator-metal transition in thin films of VO2 as well as using them in all-optical switches of pulse-periodic laser radiation.

Kirkendall effect induced ultrafine VOOH nanoparticles and their transformation into VO2(M) for energy-efficient smart windows.

Vanadium dioxide (VO2) has received widespread attention for application in energy-efficient smart windows because of its distinct thermochromic property in the near-infrared region during the reversible metal-insulator phase transition. In this study, lepidocrocite VOOH ultrafine nanoparticles (NPs) with a diameter less than 30 nm were prepared by a mild and efficient hydrothermal method, and the Kirkendall effect played a vital role in the growth of the VOOH NPs. It was found that VOOH could be transformed into VO2via a subsequent annealing treatment during which the size and morphology of VOOH are well preserved even though the annealing temperature is up to 500 °C. The ultrafine VO2 NPs are crucial for achieving excellent nanothermochromic performance with a luminous transmittance (Tlum) up to 56.45% and solar modulation ability (ΔTsol) up to 14.95%. The environmental durability is well improved by coating VO2 NPs with an SiO2 shell as confirmed via progressive oxidation and acid corrosion experiments. Meanwhile, the Tlum of the VO2@SiO2 film is further increased from 56.45% to 62.29% while the ΔTsol remained unchanged. This integrated thermochromic performance presents great potential for the development of VO2-based smart windows.

First-principles predictions of room-temperature ferromagnetism in orthorhombic MnX2 (X = O, S) monolayers.

Two-dimensional ferromagnets with high spin-polarization at ambient temperature are of considerable interest because they might be useful for making nanoscale spintronic devices. We report that even though bulk phases of MnO2 are generally antiferromagnetic with low ordering temperatures, the corresponding MnO2 and MnS2 monolayers are ferromagnetic, and MnS2 is a high temperature half metallic ferromagnet. Based on first-principles calculations, we find that the MnO2 monolayer is an intrinsic ferromagnetic semiconductor with a Curie temperature TC of ∼300 K, while the half-metallic MnS2 monolayer has a remarkably high TC of ∼1150 K. Both compounds have substantial magnetocrystalline anisotropy, out of plane in the case of MnO2 monolayers, and in plane along the b-axis of orthorhombic MnS2 monolayer. Interestingly, a metal-insulator phase transition occurs in the MnS2 monolayer when the applied biaxial strain is beyond -2%. Tuning near this metal-insulator transition offers additional possibilities for devices. The present work shows that MnX2 (X = O, S) monolayers have the properties required for ultrathin nano-spintronic devices.

Influence of Mo/V coupled multi-electron reactions and the crystalline phase transition of VO2 on high specific capacity of lithium-ion batteries

To further improve the specific capacity and stability of amorphous vanadium-based lithium-ion battery cathode materials, MoS2 was introduced for the first time into the 70V2O5–30Li3PO4 (VP) glass cathode system. Both Mo and S participated in the redox reaction, achieving for the first time an increase in the V4+ content from 21.8% to 50.4%. This improved the electrical conductivity. Moreover, the realization of a Mo/V multi-electron coupling reaction promoted the capacity. The precipitation of LiV2O5 and MoP2O8 nanocrystals during electrocycling enhanced the cycling stability and reaction kinetics. Compared to VP, the capacity of 67V2O5–30Li3PO4–3MoS2 (VPMo3) was enhanced by more than 50% at a current density of 100 mA g-1, and the capacity retention after 50 cycles was improved by about 20%. Notably, the VO2 nanocrystals precipitated during the melt preparation underwent a metal-insulator phase transition (MIT), and their structural transformation affected conductivity.

A thermally reconfigurable metamaterial with switchable wideband absorption and sensing at THz band based on VO2 thin film

The VO2 thin film has the advantage of thermally controlled insulator-metal phase transition. Based on this, we presented a thermally reconfigurable metamaterial with switchable wideband absorption and sensing at THz band in this paper. At low temperature ([Formula: see text][Formula: see text]S/m), the metamaterial can realize nearly perfect absorption at the range of 6.88–9[Formula: see text]THz. When the temperature rises to a certain extent ([Formula: see text][Formula: see text]S/m), an absorption peak which can be used to sensing appears at 4.08[Formula: see text]THz with the permittivity sensitivity of 0.5[Formula: see text]THz/PU. The metamaterial has the advantages of simple structure and switchable wideband absorption/sensing functions with potential application value on terahertz stealth, detection, sensing, and so on.

Boosting Polysulfide Redox Kinetics by Temperature-Induced Metal-Insulator Transition Effect of Tungsten-Doped Vanadium Dioxide for High-Temperature Lithium-Sulfur Batteries.

The practical application of Li-S batteries is still severely restricted by poor cyclic performance caused by the intrinsic polysulfides shuttle effect, which is even more severe under the high-temperature condition owing to the inevitable increase of polysulfides' solubility and diffusion rate. Herein, tungsten-doped vanadium dioxide (W-VO2) micro-flowers are employed with first-order metal-insulator phase transition (MIT) property as a robust and multifunctional modification layer to hamper the shuttle effect and simultaneously improve the thermotolerance of the common separator. Tungsten doping significantly reduces the transition temperature from 68 to 35 °C of vanadium dioxide, which renders the W-VO2 easier to turn from the insulating monoclinic phase into the metallic rutile phase. The systematic experiments and theoretical analysis demonstrate that the temperature-induced in-suit MIT property endows the W-VO2 catalyst with strong chemisorption against polysulfides, low energy barrier for liquid-to-solid conversion, and outstanding diffusion kinetics of Li-ion under high temperatures. Benefiting from these advantages, the Li-S batteries with W-VO2 modified separator exhibit significantly improved rate and long-term cyclic performance under 50°C. Remarkably, even at an elevated temperature (80°C), they still exhibit superior electrochemical performance. This work opens a rewarding avenue to use phase-changing materials for high-temperature Li-S batteries.

Thermally Reconfigurable, 3D Chiral Optical Metamaterials: Building with Colloidal Nanoparticle Assemblies.

The three-dimensional, geometric handedness of chiral optical metamaterials allows for the rotation of linearly polarized light and creates a differential interaction with right and left circularly polarized light, known as circular dichroism. These three-dimensional metamaterials enable polarization control of optical and spin excitation and detection, and their stimuli-responsive, dynamic switching widens applications in chiral molecular sensing and imaging and spintronics; however, there are few reconfigurable solid-state implementations. Here, we report all-solid-state, thermally reconfigurable chiroptical metamaterials composed of arrays of three-dimensional nanoparticle/metal bilayer heterostructures fabricated from coassemblies of phase change VO2 and metallic Au colloidal nanoparticles and thin films of Ni. These metamaterials show dynamic switching in the mid-infrared as VO2 is thermally cycled through an insulator-metal phase transition. The spectral range of operation is tailored in breadth by controlling the periodicity of the arrays and thus the hybridization of optical modes and in position through the mixing of VO2 and Au nanoparticles.

Terahertz Metasurfaces Exploiting the Phase Transition of Vanadium Dioxide.

Artificially designed modulators that enable a wealth of freedom in manipulating the terahertz (THz) waves at will are an essential component in THz sources and their widespread applications. Dynamically controlled metasurfaces, being multifunctional, ultrafast, integrable, broadband, high contrasting, and scalable on the operating wavelength, are critical in developing state-of-the-art THz modulators. Recently, external stimuli-triggered THz metasurfaces integrated with functional media have been extensively explored. The vanadium dioxide (VO2)-based hybrid metasurfaces, as a unique path toward active meta-devices, feature an insulator-metal phase transition under the excitation of heat, electricity, and light, etc. During the phase transition, the optical and electrical properties of the VO2 film undergo a massive modification with either a boosted or dropped conductivity by more than four orders of magnitude. Being benefited from the phase transition effect, the electromagnetic response of the VO2-based metasufaces can be actively controlled by applying external excitation. In this review, we present recent advances in dynamically controlled THz metasurfaces exploiting the VO2 phase transition categorized according to the external stimuli. THz time-domain spectroscopy is introduced as an indispensable platform in the studies of functional VO2 films. In each type of external excitation, four design strategies are employed to realize external stimuli-triggered VO2-based THz metasurfaces, including switching the transreflective operation mode, controlling the dielectric environment of metallic microstructures, tailoring the equivalent resonant microstructures, and modifying the electromagnetic properties of the VO2 unit cells. The microstructures' design and electromagnetic responses of the resulting active metasurfaces have been systematically demonstrated, with a particular focus on the critical role of the VO2 films in the dynamic modulation processes.

Universal Scaling near Band-Tuned Metal-Insulator Phase Transitions.

We present a theory for band-tuned metal-insulator transitions based on the Kubo formalism. Such a transition exhibits scaling of the resistivity curves in the regime where Tτ>1 or μτ>1, where τ is the scattering time and μ the chemical potential. At the critical value of the chemical potential, the resistivity diverges as a power law, R_{c}∼1/T. Consequently, on the metallic side there is a regime with negative dR/dT, which is often misinterpreted as insulating. We show that scaling and this "fake insulator" regime are observed in a wide range of experimental systems. In particular, we show that Mooij correlations in high-temperature metals with negative dR/dT can be quantitatively understood with our scaling theory in the presence of T-linear scattering.

Magnetic Stress-Driven Metal-Insulator Transition in Strongly Correlated Antiferromagnetic CrN.

Traditionally, the Coulomb repulsion or Peierls instability causes the metal-insulator phase transitions in strongly correlated quantum materials. In comparison, magnetic stress is predicted to drive the metal-insulator transition in materials exhibiting strong spin-lattice coupling. However, this mechanism lacks experimental validation and an in-depth understanding. Here we demonstrate the existence of the magnetic stress-driven metal-insulator transition in an archetypal material, chromium nitride. Structural, magnetic, electronic transport characterization, and first-principles modeling analysis show that the phase transition temperature in CrN is directly proportional to the strain-controlled anisotropic magnetic stress. The compressive strain increases the magnetic stress, leading to the much-coveted room-temperature transition. In contrast, tensile strain and the inclusion of nonmagnetic cations weaken the magnetic stress and reduce the transition temperature. This discovery of a new physical origin of metal-insulator phase transition that unifies spin, charge, and lattice degrees of freedom in correlated materials marks a new paradigm and could lead to novel device functionalities.

Preparation of VO2/graphene/SiC film by water vapor oxidation

Abstract Vanadium dioxide (VO2) has attracted extensive attention due to the specific metal-insulator phase transition as well as the wide device applications. The practical performance of VO2-based device strongly depends on the quality of VO2, since the higher quality of VO2 film always shows much more pronounced phase transition behavior. Thus, the preparation of high quality VO2 film is essential and highly desirable. In this work, we have prepared high-quality VO2 film on SiC substrate by water vapor oxidation with graphene (G) buffer layer, which showed excellent phase transformation properties. Compared with the VO2/SiC sample without G buffer layer, the VO2/G/SiC films show the resistance changes up to four-orders of magnitude across the phase transition boundary and superior optoelectronic properties, which indicates the significant role of G layer in the film growth process. The current study not only provides an economical and feasible method for VO2/G/SiC thin film preparation with high quality, but also supply some clues for the application of G-based VO2 devices in the future.

The ground state of the Kondo insulator

The Kondo insulator (KI) is the state of an electron liquid in the Kondo lattice at half filling, studied within the mean field approach. We demonstrate, that the {Z}_2-field, which is formed by interaction between electrons and local moments, leads to an insulator state in a lattice with a double cell lattice. In the ground state, electrons and local moments form singlets; in this case, no spin or charge density waves are realized in a lattice with a double cell. The Majorana-type gap spectrum of the quasi-particle excitations is realized in the { Z}_2 -field. The gap in the spectrum decreases with increasing external magnetic field; it closes at a critical value at the insulator-metal phase transition point. In metal phase the lattice remains with a double cell. Thus, the introduction of the { Z}_2 -field allows us to answer the key question what is the ground state of KI.

Terahertz metamaterials for broadband, high modulation depth modulator, and tunable dual-band absorber based on metal-vanadium dioxide hybrid structure

Terahertz metamaterials for broadband, high modulation depth modulating, and tunable dual-band absorbing are designed based on the similar composite structure of metal and vanadium dioxide film arrays. By using external excitation to induce the insulator-metal phase transition of the vanadium dioxide layer, the transmission characteristics of the metamaterial can be manipulated. High modulation depths of more than 80% are achieved in the range of 0.2–0.8 THz, and the bandwidth width with modulation depths exceeding 60% is up to 140%. By increasing the dielectric thickness and adding a metal ground, the initial broadband modulator can be switched to a dual-band absorber when the vanadium dioxide is in the metal phase. Furthermore, the modulation effect and the absorption performance exhibit insensitive characteristics to the polarization angle of incident waves. This work provides potential applications in broadband modulation of terahertz communication as well as dual-band absorption for terahertz detection.

Nanostructured Vanadium Dioxide Materials for Optical Sensing Applications.

Vanadium dioxide (VO2) is one of the strongly correlated materials exhibiting a reversible insulator-metal phase transition accompanied by a structural transition from a low-temperature monoclinic phase to high-temperature rutile phase near room temperature. Due to the dramatic change in electrical resistance and optical transmittance of VO2, it has attracted considerable attention towards the electronic and optical device applications, such as switching devices, memory devices, memristors, smart windows, sensors, actuators, etc. The present review provides an overview of several methods for the synthesis of nanostructured VO2, such as solution-based chemical approaches (sol-gel process and hydrothermal synthesis) and gas or vapor phase synthesis techniques (pulsed laser deposition, sputtering method, and chemical vapor deposition). This review also presents stoichiometry, strain, and doping engineering as modulation strategies of physical properties for nanostructured VO2. In particular, this review describes ultraviolet-visible-near infrared photodetectors, optical switches, and color modulators as optical sensing applications associated with nanostructured VO2 materials. Finally, current research trends and perspectives are also discussed.