First-order Metal-insulator Transition Research Articles

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.

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.

Temperature-modulated superradiance near phase transition material

We study the interaction between two quantum emitters connected through vanadium dioxide (VO2) which exhibits a first-order metal–insulator transition (MIT). The experimental data describing the dielectric function of VO2 in this study are collected from previously published works. The radiative states denoted as decay factor of two quantum emitters in the reflection and transmission configurations are investigated in detail. We find that the interconversion of the superradiant and subradiant states can easily be achieved in both configurations by controlling the temperature of VO2. The transition of decay factors from insulator to metal (heating process) and metal to insulator (cooling process) follows different paths. These factors exhibit hysteresis phenomenon and form a hysteresis loop. We also find that the decay factor in either the heating or cooling process increases sharply and reaches a maximum value at the phase transition temperature. The temperature-modulated superradiance obtained in this work offers a new efficient method to control the interaction between quantum emitters.

Glassy electrons at the first-order Mott metal-insulator transition

The Mott metal-insulator transition remains one of the most scrutinized concepts in condensed matter physics. However, the kinetics of the charge carriers at the transition, involving both orbital and spin degrees of freedom, still remains poorly understood. A perfect platform to distinguish between the role of such competing interactions is strongly correlated oxides offering rich phase diagrams, which we use here to address the electron kinetics at the transition. We show a critical slowing down of the electron kinetics at the first-order Mott metal-insulator transition in the Ruddlesden-Popper oxide ${\mathrm{Ca}}_{3}{({\mathrm{Ru}}_{0.9}{\mathrm{Ti}}_{0.1})}_{2}{\mathrm{O}}_{7}$ using low-frequency noise in resistance fluctuations. A critical slowing down of the electron kinetics is manifested as an enhancement of noise by an order of magnitude at the transition with a large shift of the spectral weight to lower frequencies. The second spectrum of noise is frequency dependent, indicating the presence of correlated fluctuations which get suppressed under the application of a magnetic field. Our experiments provide compelling evidence of the formation of a spin-glass phase at the transition in these systems.

Thermodynamics of the metal-insulator transition in the extended Hubbard model from determinantal quantum Monte Carlo

Using finite-temperature determinantal quantum Monte Carlo simulations, we examine the thermodynamic properties of the extended Hubbard model on the half-filled square lattice in the Slater regime at intermediate coupling. We consider both the case of nearest-neighbor interactions and long-range Coulomb interactions, for coupling strengths in which the presence of non-local interactions still allows us to perform sign-problem free quantum Monte Carlo simulations. In particular, we assess a recently proposed scenario from variational calculations in terms of a first-order metal-insulator transition in this interaction regime.

Hysteresis dynamics of rare earth nickelates: unusual scaling exponent and asymmetric spinodal decomposition

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.

Sensitive electric field control of first-order phase transition in epitaxial multiferroic heterostructures

Strongly correlated electron materials that exhibit rich phase transition and multiple phase separation have shown many fascinating properties. Using these properties in the electronic device will require the ability to control their phase transition and phase separation behaviours. In this work, we report a sensitive electric field control of first-order phase transition in epitaxial (011)-Nd0.5Sr0.5MnO3/0.71Pb(Mg1/3Nb2/3)O3–0.29PbTiO3 (PMN-PT) heterostructure. The pristine film shows phase separation character with the penetration of the ferromagnetic metallic phase into the charge and orbital ordered antiferromagnetic insulating phase, and this was revealed by the XMCD and XLD investigation. The (011)-oriented PMN-PT piezoelectric single crystal adopted can exert anisotropic in-plane tensile strains on the epitaxial Nd0.5Sr0.5MnO3 film when applying the electric field. The coaction of the electric field-induced strain and polarization effects of the PMN-PT piezoelectrics enable the efficient manipulation of orbital ordering and the coupled electronic and magnetic phase transitions. Accordingly, a small +1.6 kV cm−1 electric field can recover the first-order metal-insulator transition which was inhibited in the pristine heterostructure, increase the transition temperature by 56 K and the maximum resistance change reaches 7033%. Furthermore, the electric field demonstrated non-volatile control of magnetization, and the magnetoelectric coefficient reaches 1.89 × 10−7 s m−1 at 200 K. This work indicates that choosing the piezoelectric substrate with appropriate electric-field-induced strain opens a route to designing functional electronic devices where the tunable macroscopic properties derive from strongly interacting degrees of freedom present in the manganite.

C3 symmetry breaking metal-insulator transitions in a flat band in the half-filled Hubbard model on the decorated honeycomb lattice

We study the single-orbital Hubbard model on the half-filled decorated honeycomb lattice. In the noninteracting theory at half filling the Fermi energy lies within a flat band where strong correlations are enhanced. The lattice is highly frustrated. We find a correlation driven first-order metal-insulator transition to two different insulating ground states---a dimer valence bond solid Mott insulator when intertriangle correlations dominate, and a broken ${C}_{3}$-symmetry antiferromagnet that arises from frustration when intratriangle correlations dominate. The metal-insulator transitions into these two phases have very different characters. The metal-broken ${C}_{3}$ antiferromagnetic transition is driven by spontaneous ${C}_{3}$ symmetry breaking that lifts the topologically required degeneracy at the Fermi energy and opens an energy gap in the quasiparticle spectrum. The metal-dimer valence bond solid transition breaks no symmetries of the Hamiltonian. It is caused by strong correlations renormalizing the electronic structure into a phase that is adiabatically connected to both the trivial band insulator and the ground state of the spin-1/2 Heisenberg model in the relevant parameter regime. Therefore, neither of these metal-insulator transitions can be understood in either the Brinkmann-Rice or Slater paradigms.

Effects of Ga doping on the phase transitions of V2O3

${\mathrm{V}}_{2}{\mathrm{O}}_{3}$ undergoes a first-order metal-insulator transition accompanied by a magnetostructural transition at ${T}_{\mathrm{MI}}\ensuremath{\sim}160\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Here, we report a comprehensive study on gallium-doped (4%) polycrystalline ${\mathrm{V}}_{2}{\mathrm{O}}_{3}$ by employing various experimental techniques such as synchrotron x-ray diffraction, thermal expansion, Raman spectroscopy, DC magnetization, time-of-flight neutron diffraction, heat capacity, and electrical transport. Our studies show several interesting features related to the structural, magnetic, and electronic phase transitions in Ga-doped sample. Intriguingly, the nonmagnetic doping in ${\mathrm{V}}_{2}{\mathrm{O}}_{3}$ enhances the N\'eel temperature by 25 K $({T}_{\mathrm{N}}=185\phantom{\rule{0.16em}{0ex}}\mathrm{K})$. Further, we find the decoupling of the structural transition from the magnetic and insulator-insulator transition $({T}_{\mathrm{II}}\ensuremath{\sim}185\phantom{\rule{0.16em}{0ex}}\mathrm{K})$. Raman spectroscopic studies reveal that the structural phase transition begins at ${T}^{*} (=195\phantom{\rule{0.16em}{0ex}}\mathrm{K})$, which is slightly above ${T}_{\mathrm{N}}$ or ${T}_{\mathrm{II}}$, as evidenced from the softening of ${E}_{g}$ mode and the splitting of ${A}_{1g}$ mode. Additionally, the rhombohedral and monoclinic phases coexist in a broad temperature range, and a complete phase transformation to monoclinic phase occurs at 127 K.

Metal-insulator transition switching in VOx−VSe2 heterojunctions

First-order metal-insulator transition (MIT) observed in strongly correlated systems such as vanadium dioxide $({\mathrm{VO}}_{2})$ holds potential in electronics, energy, to optical applications. Starting from a vanadium diselenide $({\mathrm{VSe}}_{2})$ bulk crystal, we demonstrated a direct surface conversion from ${\mathrm{VSe}}_{2}$ to ${\mathrm{VO}}_{2}$ via laser exposure in ambient condition. The process generates defects, and the heat from the laser promotes oxidation forming ${\mathrm{VO}}_{x}$. Raman spectra at room temperature suggest the resulting oxide formed is monoclinic (M1) ${\mathrm{VO}}_{2}$. Above the transition temperature $({T}_{C})$, all the phonon modes are damped indicating formation of the rutile phase (metallic). Photoluminescence (PL) intensity enhancement and peak shifts observed at ${T}_{C}$ suggest correlation to the band structure transformation. In addition, we observed electrically induced MIT in our lateral ${\mathrm{VSe}}_{2}\text{\ensuremath{-}}{\mathrm{VO}}_{x}$ heterojunction device.

Unusually thick metal-insulator domain walls around the Mott point

Many Mott systems feature a first-order metal-insulator transition at finite temperatures, with an associated phase coexistence region displaying inhomogeneities and local phase separation. Here one typically finds "bubbles" or domains of the respective phases, which are separated by surprisingly thick, or fat, domain walls, as revealed both by imaging experiments and recent theoretical modeling. To gain insight into this unexpected behavior, we perform a systematic model study of the structure of such metal-insulator domain walls around the Mott point, within the dynamical mean-field theory framework. Our study reveals that a mechanism producing such "fat" domain walls can be traced to strong magnetic frustration, which is expected to be a robust feature of "spin-liquid" Mott systems.

Phase diagram of the Hubbard model on a square lattice: A cluster slave-spin study

The cluster slave-spin method is employed to investigate systematically the ground state properties of the Hubbard model on a square lattice with doping $\ensuremath{\delta}$ and coupling strength $U$ being its parameters. At half-filling, a relation between the staggered magnetization $M$ and the antiferromagnetic (AFM) gap ${\mathrm{\ensuremath{\Delta}}}_{\text{AFM}}$ is established in the small $U$ limit to compare with that from the Hartree-Fock theory, and a first-order metal-insulator Mott transition in the paramagnetic state is substantiated, which is characterized by discontinuities and hystereses at ${U}_{\text{Mott}}=10t$. The interaction ${U}_{c}$ for the crossover in the AFM state, separating the weak- and strong-coupling regimes, is found to remain almost unchanged with large dopings, and smaller than ${U}_{\text{Mott}}$ at half-filling because of long range AFM correlations. Finally, an overall phase diagram in the $U\ensuremath{-}\ensuremath{\delta}$ plane is presented, which is composed of four regimes: the AFM insulator at half-filling, the AFM metal with the compressibility $\ensuremath{\kappa}>0$ or $\ensuremath{\kappa}<0$, and the paramagnetic metal, as well as three phase transitions: (i) From the AFM metal to the paramagnetic metal, (ii) between the AFM metal phases with positive and negative $\ensuremath{\kappa}$, and (iii) separating the AFM insulating phase at $\ensuremath{\delta}=0$ from the AFM metal phase for $\ensuremath{\delta}>0$.

Doubling of the Phase Transition Temperature of VO2 by Fe Doping.

Vanadium dioxide (VO2) undergoes a fully reversible first-order metal-insulator transition from the M1 monoclinic phase (P21/c) to a high-temperature tetragonal phase (P42/mnm) at around 68 °C. Modulation of the phase transition of VO2 by chemical doping is of fundamental and technological interest. Here, we report the synthesis of highly crystallized Fe-doped VO2 powders by a carbo-thermal reduction process. The impact of Fe doping on the structural and phase transition of VO2 is studied. The as-prepared Fe-doped VO2 samples crystallize in the M2 monoclinic form (C2/m), which is linked to segregation of the doping ions in the V2 zigzag chains. A large increase in the transition temperature to 134 °C is observed, which does correspond to a breakthrough in VO2-type thermochromic materials.

The metal–insulator phase change in vanadium dioxide and its applications

Vanadium dioxide is an unusual material that undergoes a first-order Metal–Insulator Transition (MIT) at 340 K, attracting considerable interest for its intrinsic properties and its potential applications. However, the nature of MIT has not been fully determined. Variants of density functional theory (DFT) have been widely used to study the MIT in pure and doped VO2. A full description of MIT is complicated by several related factors such as V–V dimerization, magnetic properties, and spin correlations. Each of these requires careful attention. In this Perspective, we explain why DFT fails, introduce a spin-pairing model of MIT, and propose a new way to estimate the transition temperature. We then use the method to study the doping and alloying process. Finally, we give an overview of some applications of MIT. This work aims to provide insight into and stimulate more research studies in this promising field.

Hubbard model on the Bethe lattice via variational uniform tree states: Metal-insulator transition and a Fermi liquid

We numerically solve the Hubbard model on the Bethe lattice with finite coordination number $z=3$, and determine its zero-temperature phase diagram. For this purpose, we introduce and develop the `variational uniform tree state' (VUTS) algorithm, a tensor network algorithm which generalizes the variational uniform matrix product state algorithm to tree tensor networks. Our results reveal an antiferromagnetic insulating phase and a paramagnetic metallic phase, separated by a first-order doping-driven metal-insulator transition. We show that the metallic state is a Fermi liquid with coherent quasiparticle excitations for all values of the interaction strength $U$, and we obtain the finite quasiparticle weight $Z$ from the single-particle occupation function of a generalized "momentum" variable. We find that $Z$ decreases with increasing $U$, ultimately saturating to a non-zero, doping-dependent value. Our work demonstrates that tensor-network calculations on tree lattices, and the VUTS algorithm in particular, are a platform for obtaining controlled results for phenomena absent in one dimension, such as Fermi liquids, while avoiding computational difficulties associated with tensor networks in two dimensions. We envision that future studies could observe non-Fermi liquids, interaction-driven metal-insulator transitions, and doped spin liquids using this platform.

Magneto-optical and thermo-optical modulations of Goos-Hänchen effect in one-dimensional photonic crystal with graphene-VO2

We have theoretically studied the magneto-optical and thermo-optical modulations of Goos-Hänchen (GH) effect in a one-dimensional photonic crystal with graphene-vanadium dioxide (VO2) periodic structure. Due to the characteristics of graphene and VO2 in the terahertz region, this structure can be used for magnetic field and temperature modulation. Magneto-Optical effect of graphene in the terahertz region and the first-order metal–insulator transition (MIT) of VO2 near 68 degreesC will have a significant effect on the GH shift modulation. We obtain their relationship between the amplified GH shift with the VO2 temperature under the magnetic field environment, and the relationship between the magnetic field and VO2 is in the insulation (298 K) and the metallic state (358 K), respectively. In the case of a constant magnetic field, the increase of temperature will decrease the GH shift before and after amplification, and the increase of magnetic field will significantly enhance the GH shift, including VO2 in a fully insulated state and a metallic state, respectively. This provides a new idea and method for the design of terahertz equipment which modulates by temperature and magnetic.

Local moments versus itinerant antiferromagnetism: Magnetic phase diagram and spectral properties of the anisotropic square lattice Hubbard model

Using a cluster extension of the dynamical mean-field theory (CDMFT) we map out the magnetic phase diagram of the anisotropic square lattice Hubbard model with nearest-neighbor intrachain $t$ and interchain $t_{\perp}$ hopping amplitudes at half-filling. A fixed value of the next-nearest-neighbor hopping $t'=-t_{\perp}/2$ removes the nesting property of the Fermi surface and stabilizes a paramagnetic metal phase in the weak-coupling regime. In the isotropic and moderately anisotropic regions, a growing spin entropy in the metal phase is quenched out at a critical interaction strength by the onset of long-range antiferromagnetic (AF) order of preformed local moments. It gives rise to a first-order metal-insulator transition consistent with the Mott-Heisenberg picture. In contrast, a strongly anisotropic regime $t_{\perp}/t\lesssim 0.3$ displays a quantum critical behavior related to the continuous transition between an AF metal phase and the AF insulator. Hence, within the present framework of CDMFT, the opening of the charge gap is magnetically driven as advocated in the Slater picture. We also discuss how the lattice-anisotropy-induced evolution of the electronic structure on a metallic side of the phase diagram is tied to the emergence of quantum criticality.

Influence of temperature on the electro-induced MIT of VO2 prepared by the magnetron sputtering method

Vanadium dioxide (VO2) is a typical representative of the strongly correlated electronic system. It presents a reversible first-order metal-insulator transition once stimulated by temperature, voltage, light, or pressure. The insulation-metal phase transition (MIT) makes it a new type of functional material with excellent potential applications in many fields. In this paper, magnetron sputtering technology and the "two-step method" process are used to prepare the VO2 films with phase transition properties. The results show that after the classic V film is processed for more than 100 minutes in air, and annealed in N2 atmosphere for 150 minutes, the VO2 (M) can be obtained with a resistance change rate more than three orders of magnitude. The study of the resistance change under the action of temperature and voltage found that the temperature of the heating and cooling phase transition are 67.4ºC and 62.8ºC respectively, and the hysteresis width is 4.6ºC. The relationship between phase transition voltage and ambient temperature is presented based on the heat dissipation model. It is found that Joule heating plays a significant role in the electro-induced phase transition of VO2. Furthermore, the ambient temperature has an obvious regulation effect on the phase transition voltage of VO2 film. The research results can provide efficient guidance to the preparation of VO2 film and the application and regulation of electro-induced phase transition.

Unveiling the role of Fe3O4 in polymer spin valve near Verwey transition

The spinterface formed between ferromagnetic (FM) electrode and organic materials is vital for performance optimization in organic spin valve (OSV). Half-metallic Fe3O4 with drastic change in structure, conductivity and magnetic property near Verwey transition can serve as an intrinsic spinterface regulator. However, such modulating effect of Fe3O4 in OSV has not been comprehensively investigated, especially below the Verwey transition temperature (Tv). Here, we highlight the important role of Fe3O4 electrode in reliable-working and controllable Fe3O4/P3HT/Co polymer spin valves by investigating the magnetoresistance (MR) above and below Tv. In order to distinguish between different contributions to charge transport and related MR responses, the systematic electronic and magnetic characterizations were carried out in full temperature range. Particularly, the first-order metal-insulator transition in Fe3O4 has a dramatic effect on the MR enhancement of polymer spin valves at Tv. Moreover, both the conducting mode transformation and MR line shape modulation could be accomplished across Tv. This research renders unique scenario to multimodal storage by external thermodynamic parameters, and further reveals the importance of spin-dependent interfacial modification in polymer spin valves.

Magnetic-field-induced insulator\u2013metal transition in W-doped VO2 at 500 T

Metal–insulator (MI) transitions in correlated electron systems have long been a central and controversial issue in material science. Vanadium dioxide (VO2) exhibits a first-order MI transition at 340 K. For more than half a century, it has been debated whether electron correlation or the structural instability due to dimerised V ions is the more essential driving force behind this MI transition. Here, we show that an ultrahigh magnetic field of 500 T renders the insulator phase of tungsten (W)-doped VO2 metallic. The spin Zeeman effect on the d electrons of the V ions dissociates the dimers in the insulating phase, resulting in the delocalisation of electrons. As the Mott–Hubbard gap essentially does not depend on the spin degree of freedom, the structural instability is likely to be the more essential driving force behind the MI transition.