Metal-insulator Phase Transition Research Articles

Ag rearrangement induced metal-insulator phase transition in thermoelectric MgAgSb

In recent years, materials with metal-to-insulator phase transitions have attracted great attention due to their property variations before and after the transition. As nearly all good thermoelectric materials are small band-gap semiconductors, a metal-insulator transition makes a profound difference in thermoelectric performance. Revealing the microscopic reasons for these variations is critical to understanding the structure-property relation in these materials. MgAgSb is an important thermoelectric material around room temperature, due to its high zT value and intrinsic low lattice thermal conductivity in the semiconducting α-phase. However, the thermoelectric properties of mid-temperature β-phase MgAgSb significantly deteriorate because the β-phase has no band gap. In this work, we reveal that the Ag atomic rearrangement between the two phases is the major reason responsible for the metal-insulator transition and deterioration of thermoelectric properties. In MgAgSb, there are strong interactions between Mg and Sb, as well as Ag and Sb, leading to bands of mixed character around the Fermi Level. In α-MgAgSb, the Mg–Sb bands in the conduction band minimum separate well with the bands around the valence band maximum, and form a finite band gap. Due to the Ag rearrangement and the formation of a quasi-two-dimensional structure of β-MgAgSb, the interactions among Mg–Ag–Sb atoms form multiple bands crossing the Fermi level, and realize the “two-dimensional metallization”. In addition, the structural variations in the β-phase results in higher phonon velocities and removal of additional low-frequency optical phonons as only shown in the α-phase, both of which leading to the relatively high κL of the β-phase. Our work provides new perspectives for the performance research and application of materials with metal-insulator phase transitions.

Modulation of the metal-insulator transition in VO2 nanoparticles with nRu+W (n=1–4) codoping: Implications for energy-saving smart windows

Vanadium dioxide (VO2), a remarkably popular thermochromic material, has attracted considerable attention due to its unique reversible metal-insulator phase transition characteristic, which can be used in practical applications such as energy-saving smart windows. However, the advance on enhancing the visible light transmittance while obtaining a useable phase transition temperature remains unresolved issue. Herein, the atomic structures and electronic-optical properties of nRu+W-codoped (n =1–4) VO2 were investigated by the first-principles calculations. The results show that the nRu + W-codoped VO2 is thermally and mechanically stable. The desirable nRu + W co-doped VO2 can be easily prepared under V-rich conditions, and that some V atoms were replaced by Ru and W atoms in the codoping structures, with the [RuO6] and [WO6] octahedra cosharing an edge or oxygen atoms. Furthermore, a lower phase transition temperature and the near-infrared modulation capacity of VO2 could be obtained in theoretical calculations due to the synergistic effect of nRu+W codoping. That is, the introduction of Ru atoms could widen the optical band gap of VO2, increasing the luminous transmittance; Simultaneously, the Fermi levels for nRu+W configurations gradually entered one of the narrow bands, decreasing the electronic band gap and further reducing the phase transition temperature due to the incorporation of the W dopant, which donates extra electrons to the codoping systems due to its higher ion valence. The current work provides an approach for improving the thermochromic performance of VO2 films in energy-saving smart windows, thereby providing new insights into the relationship between overall performance improvement and structural variations induced by elements codoping.

Miniaturized and Actively Tunable Triple-Band Terahertz Metamaterial Absorber Using an Analogy I-Typed Resonator

Triple-band terahertz metamaterial absorber with design of miniaturization and compactness is presented in this work. The unit cell of the terahertz absorber is formed by an analogy I-typed resonator (a rectangular patch with two small notches) deposited on top of dielectric sheet and metallic mirror. The miniaturized structure design exhibits three discrete frequency points with near-perfect absorption at terahertz regime. The three absorption peaks could be ascribed to localized resonances of analogy I-typed resonator, while the response positions of these absorption peaks at the analogy I-typed resonator are different by analyzing the near-field patterns of these resonance peaks. Changes in structure parameters of the analogy I-typed resonator are also investigated. Simulation results revealed that the notch sizes of the rectangular patch are the key factor to form the triple-band near-perfect absorption. Further structure optimization is given to demonstrate triple-band polarization insensitive performance. Moreover, actively tunable absorption properties are realized by inserting or introducing vanadium dioxide with adjustable conductivity into the metamaterial structure. It is revealed that the insulator–metal phase transition of vanadium dioxide is the main reason for the modulation of absorption performance. Compared with previous multiple-band absorbers, the device given here has excellent features of high degrees of simplification, miniaturization, and active modulation, these are important in practical applications.

VO2 as a Highly Efficient Electrocatalyst for the Oxygen Evolution Reaction.

Herein, we report high electrocatalytic activity of monoclinic VO2 (M1 phase) for the oxygen evolution reaction (OER) for the first time. The single-phase VO2 (M1) nanoparticles are prepared in the form of uniformly covering the surface of individual carbon fibers constituting a carbon fiber paper (CFP). The VO2 nanoparticles reveal the metal-insulator phase transition at ca. 65 °C (heating) and 62 °C (cooling) with low thermal hysteresis, indicating a high concentration of structural defect which is considered a grain boundary among VO2 nanoparticles with some particle coalescence. Consequently, the VO2/CFP shows a high electrocatalytic OER activity with the lowest η10 (350 mV) and Tafel slope (46 mV/dec) values in a 1 M aqueous solution of KOH as compared to those of the vacuum annealed V2O5 and the hydrothermally grown VO2 (M1), α-V2O5, and γ′-V2O5. The catalytically active site is considered V4+ components and V4+/5+ redox couples in VO2. The oxidation state of V4+ is revealed to be more favorable to the OER catalysis compared to that of V5+ in vanadium oxide through comparative studies. Furthermore, the amount of V5+ component is found to be increased on the surface of VO2 catalyst during the OER, giving rise to the performance degradation. This work suggests V4+ and its redox couple as a novel active component for the OER in metal-oxide electrocatalysts.

Reconfigurable broadband polarisation conversion metasurface based on VO2

Polarisation is an important characteristic parameter of electromagnetic waves. The controllable adjustment of the polarisation of electromagnetic waves is important for applications in fields such as antenna radiation, satellite communication, and radar stealth. In this study, a reconfigurable metasurface polarisation converter with a square ring structure and an opening was developed based on the reversible metal–insulator phase transition (MIT) of vanadium dioxide (VO2). VO2 is in the insulating high-resistance state (M phase) when its temperature is lower than the MIT temperature (68 °C). Finite element simulations showed that the metasurface could convert linearly polarised waves into cross-polarised reflected waves at frequencies of 8–18 GHz, corresponding to the X and Ku bands. The polarisation conversion rate (PCR) was over 90% and the relative bandwidth was up to 79%. VO2 is in the metallic low-resistance state (R phase) when its temperature is higher than the MIT temperature. At frequencies > 9.4 GHz, the PCR of the metasurface was less than 20%. Under these conditions, the polarisation of most of the reflected waves was the same as that of the incident wave. The polarisation converter was designed to work over a wide frequency range, and thus multiple resonances could be produced based on multiple resonance points in the working frequency range. Moreover, the electromagnetic wave component in the v-direction was rotated along 180°; hence, the polarisation of the incident electromagnetic wave rotated along 90°. When the resistance was low, the phases in the u- and v-directions were basically the same and the electromagnetic waves were reflected in the same direction. A smaller VO2 resistance denotes a higher co-polarisation reflection coefficient (Rxx). Rxx > 80% when the resistance of VO2 < 10 Ω, and Rxx > 90% when the resistance was 1 Ω. The maximum VO2 power loss was achieved when the resistance of VO2 was 188 Ω, i.e., the impedance matched with that of air. The experimental and simulation results were generally consistent. The proposed polarisation converter could have widespread applications in the dynamic control of electromagnetic waves, with uses in antenna radiation and radar cross-section reduction.

Phase transition behavior in nanostructured VO2 with M1, M2, and R phases observed via temperature-dependent XRD measurements

Vanadium dioxide (VO2) exhibits a metal-insulator phase transition at approximately 70 °C. Two different crystal structures, M1 and M2 phases, in the insulating state have been reported. The M2 phase is expected to be a Mott insulator, whereas the M1 phase is expected to act as a band insulator. It is important to clarify the origin and transition properties of the M2 phase. In this study, we fabricated VO2 nanostructures via sputtering deposition and subsequent lamp annealing at various O2 pressures. Temperature-dependent XRD measurements revealed that the nanostructured VO2 exhibits the M2 phase in temperatures just before the phase transition from M1 to R (metal state) during the heating process. The ratio of V5+/(V4+ + V5+) in nanostructured VO2 was estimated from XPS results; an increase in this ratio resulted in the broadening of the temperature range of the M2 phase. Furthermore, in the cooling process, direct phase transitions from R to M1 were observed. Structural changes are likely to occur in different pathways during heating and cooling. Our findings should contribute to the identification of the phase transition mechanism and to the development of a Mott field-effect transistor that utilizes nanostructured VO2 in the M2 phase.

The surface tension at the metal-insulator phase transition in the magnetic field

Any metal-insulator transition in a crystalline matter must be the transition from a situation in which the electronic bands overlap to that when they do not [1]. On the basis of the phenomenological theory [2], various singularities are considered in the magnetic field for the surface tension at the metal-insulator contact. The surface tension displays also the quantum magnetic oscillations at low temperatures [3]. The consideration is applied to the Mott insulator in the magnetic field.

On the Conductivity of the 2D Rayleigh Model in the Region of the Metal–Insulator Phase Transition

On the Conductivity of the 2D Rayleigh Model in the Region of the Metal–Insulator Phase Transition

Role of spin-phonon and electron-phonon interactions in the phonon renormalization of (Eu1−xBix)2Ir2O7 across the metal-insulator phase transition: Temperature-dependent Raman and x-ray studies

We report temperature-dependent Raman scattering and X-ray diffraction studies of pyrochlore iridates, (Eu$_{1-x}$Bi$_x$)$_2$Ir$_2$O$_7$, for x=0, 0.02, 0.035, 0.05 and 0.1. The temperature variation in Raman experiments spans from 4 K to 300 K, covering the metal-insulator phase transition accompanied by paramagnetic to all-in/all-out (AIAO) spin ordering (T$_N$). These systems also show a Weyl semi-metal (WSM) phase at low temperatures (below ~50 K). We show that the Ir-O-Ir bond bending mode, A$_{1g}$ (510 cm$^{-1}$), shows anomalous softening in the magnetically ordered AIAO state, arising primarily from the spin-phonon interaction due to the phonon-modulation of the Dzyaloshinskii-Moriya (DM) spin-exchange interaction. The two stretching modes, T$_{2g}^1$ (307 cm$^{-1}$) and T$_{2g}^2$ (382 cm$^{-1}$) harden significantly in the magnetic insulating phase. The T$_{2g}$ phonons also show anomalous temperature dependence of their mode frequencies, hitherto unreported, due to strong electron-phonon coupling. The signatures of the WSM state are observed in phonon renormalization below 50 K due to strong electron-phonon interaction. Our experimental results establish strong magneto-elastic coupling below T$_N$ and significant electron-phonon interactions in the metallic phase above T$_N$ as well as in the low-temperature WSM state.

Raman Spectroscopy of V4O7 Films

A thin film of vanadium oxide Magnéli phase V4O7 was produced using cathodic arc sputtering. X-ray diffraction, Rutherford backscattering spectrometry and Raman investigations confirmed the formation of this phase. The Raman spectrum of V4O7 differs considerably from the spectrum of another Magnéli oxide, V3O5, showing that Raman spectroscopy is an excellent tool for distinguishing between these two phases. Temperature-dependent Raman measurements revealed a significant change of the spectra near the V4O7 metal–insulator phase transition.

Ar-implanted vanadium dioxide thin film with the reduced phase transition temperature

VO2 layers with a low metal–insulator phase transition (MIT) temperature TMIT below 40 °C were prepared using magnetron sputtering, rapid thermal annealing, and Ar2+ ion implantation. The implantation-induced strains and point defects effectively lower the metal–insulator transition temperature, while the subsequent thermal cycling (cooling-heating cycles) improves the temperature coefficient of resistance (TCR) up to the values >10%/°C within the temperature range 30–40 °C. These approaches can be used to extend the range of applications of VO2 thin films, especially for uncooled microbolometers. A methodology for studying the influence of defects and strains on electrical parameters of MIT in VO2 films has been proposed.

Secondary phase limited metal-insulator phase transition in chromium nitride thin films

Chromium nitride (CrN) is a well-known hard coating material that has found applications in abrasion and wear-resistant cutting tools, bearings, and tribology applications due to its high hardness, high-temperature stability, and corrosion-resistant properties. In recent years, CrN has also attracted significant interest due to its high thermoelectric power factor, and for its unique and intriguing metal-insulator phase transition. While CrN bulk single-crystals exhibit the characteristic metal-insulator transition accompanied with structural (orthorhombic-to-rocksalt) and magnetic (antiferromagnetic-to-paramagnetic) transition at ∼260–280 K, observation of such phase transition in thin-film CrN has been scarce, and the exact cause of the absence of such transitions in several studies is not well-understood. In this work, the formation of the secondary metallic Cr2N phase during the growth is demonstrated to inhibit the observation of metal-insulator phase transition in CrN thin films. When the Cr-flux during deposition is reduced below a critical limit, epitaxial and stoichiometric CrN thin film is obtained that reproducibly exhibits the phase transition. Annealing of the mixed-phase film inside reducing NH3 environment converts the Cr2N into CrN, and a discontinuity in the electrical resistivity at ∼277 K appears, which supports the underlying hypothesis. Demonstration of the inhibited metal-semiconductor phase transition in CrN due to the presence of secondary Cr2N phase is similar to the previous finding of the substantial change in its mechanical hardness and reduction in thermoelectric properties. A clear demonstration of the origin behind the controversy of the metal-insulator transition in CrN thin films marks significant progress and would enable its nanoscale device realization.

Pressure-induced concomitant topological and metal-insulator quantum phase transitions in Ce3Pd3Bi4

The electronic property and magnetic susceptibility of Ce3Pd3Bi4 were systemically investigated from 18 to 290 K for varying values of cell volume using dynamic mean-field theory coupled with density functional theory. By extrapolating to zero temperature, the ground state of Ce3Pd3Bi4 at ambient pressure is found to be a correlated semimetal due to insufficient hybridization. Upon applying pressure, the hybridization strength increases and a crossover to the Kondo insulator is observed at finite temperatures. The characteristic temperature signaling the formation of Kondo singlet, as well as the characteristic temperature associated with f-electron delocalization–localization change, simultaneously vanishes around a critical volume of 0.992 V0, suggesting that such metal–insulator transition is possibly associated with a quantum critical point. Finally, Wilson’s loop calculations indicate that the Kondo insulating side is topologically trivial, thus a topological transition also occurs across the quantum critical point.

Independent regulation of electrical properties of VO2 for low threshold voltage electro-optic switch applications

Vanadium dioxide has a unique insulator-metal phase transition characteristic near room temperature, meeting optical switches' requirements. However, due to its high resistance in the insulator state, the working voltage threshold of the VO2-based optical switch is also high. It is one of the obstacles for VO2-based optical switch applications. In this work, high-quality VO2 films were prepared by DC pulsed magnetron sputtering. By inserting a conductive ITO buffer layer, VO2 sheet resistance reduced about 2 orders, and the electrical phase change characteristics have almost disappeared. Still, the high optical modulation ability is hardly affected. The interesting results meet the requirements of a low threshold working voltage electro-optic switch. In the case of a 40 µm electrode gap size, the threshold voltage for VO2/ITO is only 1.4 V, which is much lower than 20 V of pure VO2 in the same condition, and the optical switch modulation depth at the wavelength of 1550 nm reaches 35%. For the switch with 5 µm electrode gap size, the threshold voltage is only 1.3 V, and the response times of "ON" and "OFF" are 0.24 ms and 2.1 ms, respectively. This result is valuable for the practical application of VO2-based optical switches.

Orbital-selective Mott phase and non-Fermi liquid in FePS3

The layered metal phosphorous trisulfide FePS$_3$ is reported to be a Mott insulator at ambient conditions and to undergo structural and insulator-metal phase transitions under pressure. However, the character of the resulting metallic states has not been understood clearly so far. Here, we theoretically study the phase transitions of FePS$_3$ using first-principles methods based on density functional theory and embedded dynamical mean field theory. We find that the Mott transition in FePS$_3$ can be orbital-selective, with $t_{2g}$ states undergoing a correlation-induced insulator-to-metal transition while $e_g$ states remain gapped. We show that this orbital-selective Mott phase, which occurs only when non-hydrostatic pressure is used, is a bad metal (or non-Fermi liquid) with large fluctuating moments due to Hund's coupling. Further application of pressure increases the crystal-field splitting and converts the system to a conventional Fermi liquid with low-spin configurations dominant. Our results show that FePS$_3$ is a novel example of a system that realizes an orbital-selective Mott phase, allowing tuning between correlated and uncorrelated metallic properties in an accessible pressure range ($\leq$ 18 GPa).

Active beam manipulation and convolution operation in VO2-integrated coding terahertz metasurfaces.

Coding metasurfaces have received tremendous interest due to their unprecedented control of beams through the flexible design of coding sequences. However, realizing tunable coding metasurfaces with scattering-pattern shifts in the terahertz range is still challenging. Here, we propose a VO2-integrated coding metasurface to realize a thermally controlled scattering-pattern shift by convolution operation. The required phase profiles and high amplitudes of 1-bit and 2-bit coding metasurfaces are easily obtained only by changing the length of the VO2 cut-wires. The insulator-metal phase transition of the VO2 cut-wires leads to an ultrafast switching effect between multiple deflected scattering beams and one normally reflected beam. In particular, the VO2 phase transition contributes to dynamical convolution operations of the 2-bit coding metasurface. The proposed VO2-integrated coding metasurfaces are important for realizing tunable terahertz beam manipulation as well as arbitrary required scattering beams.

First-principles study on the heterostructure of twisted graphene/hexagonal boron nitride/graphene sandwich structure

In recent years, the discovery of ‘magic angle’ graphene has given new inspiration to the formation of heterojunctions. Similarly, the use of hexagonal boron nitride, known as white graphene, as a substrate for graphene devices has more aroused great interest in the graphene/hexagonal boron nitride heterostructure system. Based on the first principles method of density functional theory, the band structure, density of states, Mulliken population, and differential charge density of a tightly packed model of twisted graphene/hexagonal boron nitride/graphene sandwich structure have been studied. Through the establishment of heterostructure models twisted bilayer-graphene inserting hBN with different twisted angles, it was found that the band gap, Mulliken population, and charge density, exhibited specific evolution regulars with the rotation angle of the upper graphene, showing novel electronic properties and realizing metal–insulator phase transition. We find that the particular value of the twist angle at which the metal–insulator phase transition occurs and propose a rotational regulation mechanism with angular periodicity. Our results have guiding significance for the practical application of heterojunction electronic devices.

Integrated Hybrid VO2–Silicon Optical Memory

Vanadium dioxide (VO2) is an interesting material for hybrid photonic integrated devices due to its insulator–metal phase transition. Utilizing the hysteresis of the phase transition in voltage-biased VO2, we demonstrate a compact hybrid VO2–silicon optical memory element integrated into a silicon waveguide. An optical pulse writes the VO2 memory, leading to an optical attenuation that can be read out by the optical transmission in a silicon waveguide. Our on-chip memory cell can be optically written with energy as low as 23.5 pJ per pulse and with a 10–90% rise time of ∼100 ns. This approach is promising for optical data storage in silicon photonic integrated circuits.

Ellipsometric characterization of VO-=SUB=-2-=/SUB=-, VO-=SUB=-2-=/SUB=- : Mg, VO-=SUB=-2-=/SUB=- : Ge nanocrystalline films

The spectra of the refractive index n(λ) and the extinction coefficient k(λ) of thin VO2, VO2 : Mg, VO2 : Ge films were measured using the ellipsometric method. For an undoped VO2 film at a wavelength λ=632.8 nm, near the insulator-metal phase transition, the n(T) and k(T) thermal hysteresis loops were studied. An interpretation of the results is given on the base of the Moss relation, the idea of a change in n(T) and k(T) with an impurity variation of the material density, and also on the base of the ideology of the Coulomb transformation of the density of states function in strongly correlated materials. Keywords: ellipsometry, vanadium dioxide, insulator-metal phase transition, strongly correlated materials.

Electrically and optically erasable non-volatile two-dimensional electron gas memory.

The high-mobility two-dimensional electron gas (2DEG) generated at the interface between two wide-band insulators, LaAlO3 (LAO) and SrTiO3 (STO), is an extensively researched topic. In this study, we have successfully realized reversible switching between metallic and insulating states of the 2DEG system via the application of optical illumination and positive pulse voltage induced by the introduction of oxygen vacancies as reservoirs for electrons. The positive pulse voltage irreversibly drives the electron to the defect energy level formed by the oxygen vacancies, which leads to the formation of the insulating state. Subsequently, the metallic state can be achieved via optical illumination, which excites the trapped electron back to the 2DEG potential well. The ON/OFF state is observed to be robust with a ratio exceeding 106; therefore, the interface can be used as an electrically and optically erasable non-volatile 2DEG memory.