Mechanism Of Metal-insulator Transition Research Articles

Cell Size Effects and Distinct Current Conduction Behaviors for Hafnium‐Oxide‐Based Selectors with Vanadium as Top Electrode

Selector devices have been incorporated in the resistance random access memory to realize one‐selector‐one‐resistor device for the achievement of high‐density storage applications. Among many possible materials for selector devices, the metal–insulator transition mechanism of the vanadium oxide (VOx) is interesting due to its formation under the influence of temperature. Previous studies have also indicated that VOx is formed after high‐temperature annealing. Herein, the V/HfO2/TiN with different deice cell sizes is investigated. After the forming process, devices with small cell sizes can exhibit threshold switching (TS) characteristics, while devices with large cell sizes can show resistive switching property. Electrical measurements of DC and AC operations are also conducted to verify stable TS characteristics for selector applications. The current fitting method and the computer‐aid simulation are also carried out to compare different current conduction behaviors of devices with large and small cell sizes, which indicates that heat plays an important role in the formations of VOx layer of the V/HfO2/TiN device.

Tight-binding based modeling to demonstrate metal-insulator transition in VO2 within Mott-Hubbard picture

The mechanism of metal-insulator transition (MIT) in vanadium dioxide (VO2) is still under debate whether it is due to the structural phase transition or electron correlations. A previous study, indicating that MIT appears far below the critical temperature of Tc ≈ 340 K when a small amount of oxygen is taken out from the system, suggested that oxygen screening and electronic correlations play a dominant role in MIT of VO2. To demonstrate how the Mott-Hubbard picture works in this mechanism, we construct a Hamiltonian with a kinetic part constructed within tight-binding scheme and an interaction part treated within the mean-field approximation for the system mimicking the situation of VO2 without and with oxygen deficiencies. Keeping the Hubbard repulsion parameter, U, unchanged, we propose that the MIT is dominantly controlled by the changes of hopping parameters and the on-site energies due to possible orbital reorientations induced by correlation effects. We calculate the projected density of states (PDOS) using Green’s function technique and demonstrate the opening/closing of the insulating gap within the Mott-Hubbard picture. Our results qualitatively reproduce the band renormalization picture inferred from the experimental data suggested by literature. In addition, we find that ferromagnetic electron spin configuration is necessary for the overall MIT scenario to work.

Metal‐Insulator Transition: Spectroscopic Studies on the Metal–Insulator Transition Mechanism in Correlated Materials (Adv. Mater. 42/2018)

Metal‐Insulator Transition: Spectroscopic Studies on the Metal–Insulator Transition Mechanism in Correlated Materials (Adv. Mater. 42/2018)

Investigation of the vanadium L23 -edge x-ray absorption spectrum of SrVO3 using configuration interaction calculations: Multiplet, valence, and crystal-field effects

Configuration interaction cluster calculations have been performed to investigate the electronic structure of orthorhombic distorted ${\mathrm{SrVO}}_{3}$ compound, where a considerable tetragonal crystal-field splitting of larger than $180\phantom{\rule{4.pt}{0ex}}\text{meV}$ for ${t}_{2g}$ orbital levels is suggested for the good consistence between the simulated and the experimental spectral line shape. This supports the crystal-field splitting scenario for the metal-insulator transition mechanism. We further calculate V ${L}_{23}$-edge absorption spectra of vanadium oxides with similar ${\mathrm{VO}}_{6}$ octahedral environment for formal $d$ electrons varying from $3{d}^{0}$ to $3{d}^{3}$, where we obtain energy parameters representing the local ground-state properties and provide the theoretical guidance for identifying the chemical valence and the crystal-field strength of a particular vanadium oxide. Our simulated polarization-dependent spectra at different tetragonal crystal-field values reveal a nonlinear orbital-lattice interaction. The orbital polarization $P$ reflecting the electron occupations in different ${d}_{xy}$ and ${d}_{xz(yz)}$ orbital symmetries shows strong sensitivity to small tetragonal crystal field, indicating the strongly correlated electron effect and further addressing the efficiency of strain engineering in functional SVO-based thin films and heterostructures.

Spectroscopic Studies on the Metal-Insulator Transition Mechanism in Correlated Materials.

The metal-insulator transition (MIT) in correlated materials is a novel phenomenon that accompanies a large change in resistivity, often many orders of magnitude. It is important in its own right but its switching behavior in resistivity can be useful for device applications. From the material physics point of view, the starting point of the research on the MIT should be to understand the microscopic mechanism. Here, an overview of recent efforts to unravel the microscopic mechanisms for various types of MITs in correlated materials is provided. Research has focused on transition metal oxides (TMOs), but transition metal chalcogenides have also been studied. Along the way, a new class of MIT materials is discovered, the so-called relativistic Mott insulators in 5d TMOs. Distortions in the MO6 (M = transition metal) octahedron are found to have a large and peculiar effect on the band structure in an orbital dependent way, possibly paving a way to the orbital selective Mott transition. In the final section, the character of the materials suitable for applications is summarized, followed by a brief discussion of some of the efforts to control MITs in correlated materials, including a dynamical approach using light.

Evolution of Structural and Electrical Properties of Oxygen-Deficient VO2 under Low Temperature Heating Process.

Structural stability and functional performances of vanadium dioxide (VO2) are strongly influenced by oxygen vacancies. However, the mechanism of metal-insulator transition (MIT) influenced by defects is still under debate. Here, we study the evolution of structure and electrical property of oxygen-deficient VO2 by a low temperature annealing process (LTP) based on a truss-structured VO2 nanonet. The oxygenation process of the oxygen-deficient VO2 is greatly prolonged, which enables us to probe the gradual change of properties of the oxygen-deficient VO2. A continuous lattice reduction is observed during LTP. No recrystallization and structural collapse of the VO2 nanonet can be found after LTP. The valence-band X-ray photoelectron spectroscopy (XPS) measurements indicate that the oxygen deficiency strongly affects the energy level of the valence band edge. Correspondingly, the resistance changes of the VO2 films from 1 to 4.5 orders of magnitude are achieved by LTP. The effect of oxygen vacancy on the electric field driven MIT is investigated. The threshold value of voltage triggering the MIT decreases with increasing the oxygen vacancy concentration. This work demonstrates a novel and effective way to control the content of oxygen vacancies in VO2 and the obvious impact of oxygen vacancy on MIT, facilitating further research on the role of oxygen vacancy in structure and MIT of VO2, which is important for the deep understanding of MIT and exploiting innovative functional application of VO2.

Metal-insulator transition in epitaxial NdNiO3 thin film: A structural, electrical and optical study

NdNiO3 thin film has been prepared by pulsed laser deposition on LaAlO3 (001) single crystalline substrate. Temperature-dependent resistivity measurement shows a sharp metal-insulator transition in such thin film. The phase transition temperature can be tuned from 90K to 121K by changing the thickness of thin film. The structure evolution during phase transition is studied by Raman spectroscopy. Optical conductivity reveals that the variation carrier density in the process of phase transition. The results of structural, electrical and optical studies provide useful insights to understand the mechanism of metal-insulator transition of NdNiO3 thin film.

First-principles investigation on solar radiation shielding performance of rutile VO2 filters for smart windows

Vanadium dioxide (VO2) undergoing reversible metal-insulator phase transition could allow for the formation of an efficient thermochromic material for smart windows. However, solar radiation shielding performance is determined by transparent rutile VO2 filters, and the puzzling metal-insulator transition mechanism makes it challenging to explain the origin of the coexistence of strong near infrared absorption with high optical transparency. The band structure, the density of states, and the optical properties of rutile VO2 were calculated using the first-principles calculations. The calculated results of the structural and optical properties are in good agreement with the previously reported experimental findings. The calculated dielectric functions, electron energy-loss function and solar radiation shielding performance of the rutile VO2 filters indicate that rutile VO2 is a promising near-infrared absorption/reflectance material with the near-infrared radiation insulating abilities and a visible light transmittance. These properties arise from plasma oscillation and a collective oscillation (volume plasmons) of carrier electrons.

Thermal kinetic analysis of metal–insulator transition mechanism in W-doped VO2

Tungsten (W) doping can decrease the phase transition temperature of VO2, but underlining reasons are not clear. In this study, differential scanning calorimetry was employed to investigate the kinetics of the solid–solid transition in hydrothermal-synthesized W (X = 0.50, 1.09, 2.58 and 3.81 %)-doped VO2 nanoparticles. We firstly revealed the W-doping mechanism by combining the classical nucleation kinetics model with isoconversional kinetic analysis and applied them on the solid–solid transition taking place in doping. The experimentally observed large lag in the cooling stage and asymmetry effects of the decreasing temperature on insulator–metal transition and metal–insulator transition can be rationally explained. In the heating stage, W doping decreases free energy barrier (ΔG*) for homogenous nucleation and reduces geometrical factor (f(Θ)) and both factors promote the transition and thus lower the phase transition temperature quickly. However, in cooling stage, the free energy barrier (ΔG het * ) for heterogeneous nucleation was much larger than that of heating stage due to lacking of proper nucleation sites. The effect of decreasing geometrical factor was accompanied with the effect of increasing free energy barrier for homogenous nucleation by doping W. Such a competition mechanism slows down the trend of reducing temperature. It is important to unravel interaction mechanisms of doped W on different VO2 phases, which is helpful to further tailor kinetic properties of VO2 phase transition.

Effective Hamiltonians for correlated narrow energy band systems and magnetic insulators: Role of spin-orbit interactions in metal-insulator transitions and magnetic phase transitions.

Using a second-quantized many-electron Hamiltonian, we obtain (a) an effective Hamiltonian suitable for materials whose electronic properties are governed by a set of strongly correlated bands in a narrow energy range and (b) an effective spin-only Hamiltonian for magnetic materials. The present Hamiltonians faithfully include phonon and spin-related interactions as well as the external fields to study the electromagnetic response properties of complex materials and they, in appropriate limits, reduce to the model Hamiltonians due to Hubbard and Heisenberg. With the Hamiltonian for narrow-band strongly correlated materials, we show that the spin-orbit interaction provides a mechanism for metal-insulator transition, which is distinct from the Mott-Hubbard (driven by the electron correlation) and the Anderson mechanism (driven by the disorder). Next, with the spin-only Hamiltonian, we demonstrate the spin-orbit interaction to be a reason for the existence of antiferromagnetic phase in materials which are characterized by a positive isotropic spin-exchange energy. This is distinct from the Néel-VanVleck-Anderson paradigm which posits a negative spin-exchange for the existence of antiferromagnetism. We also find that the Néel temperature increases as the absolute value of the spin-orbit coupling increases.

Tailoring LaAlO3/SrTiO3 Interface Metallicity by Oxygen Surface Adsorbates.

We report an oxygen surface adsorbates induced metal-insulator transition at the LaAlO3/SrTiO3 interfaces. The observed effects were attributed to the terminations of surface Al sites and the resultant electron-accepting surface states. By controlling the local oxygen adsorptions, we successfully demonstrated the nondestructive patterning of the interface two-dimensional electron gas (2DEG). The obtained 2DEG structures are stable in air and also robust against general solvent treatments. This study provides new insights into the metal-insulator transition mechanism at the complex oxide interfaces and also a highly efficient technique for tailoring the interface properties.

Research progress of metal-insulator phase transition mechanism in VO2

VO2 is a metal oxide that has a thermally-induced phase-transition. In the vicinity of 341 K, VO2 undergoes a reversible transition from the high-temperature metal phase to the low-temperature insulator phase. Associated with the metal-insulator transition (MIT), there are drastic changes in its optical, electrical and magnetic characteristics. These make VO2 an attractive material for various applications, such as optical and/or electrical switches, smart glass, storage media, etc. Thus, the reversible metal-insulator phase transition in VO2 has long been a research hotspot. However, the metal-insulator transition mechanism in VO2 has been a subject of debate for several decades, and yet there is no unified explanation. This paper first describes changes of the crystal structure and the energy band structure during VO2 phase transition. With regard to the crystal structure, VO2 transforms from the low-temperature monoclinic phase VO2(M) into the high-temperature stable rutile phase VO2(R), and in some special cases, this phase transition process may also involve a metastable monoclinic VO2(B) phase and a tetragonal VO2(A) phase. In respect of the energy band structure, VO2 undergoes a transition from the low-temperature insulator phase into a high-temperature metal phase. In the band structure of low-temperature monoclinic phase, there is a band gap of about 0.7 eV between d// and * bands, and the Fermi level falls exactly into the band gap, which makes VO2 electronically insulating. In the band structure of high-temperature rutile phase, the Fermi level falls into the overlapping portion of the * and d// bands, which makes VO2 electronically metallic. Next, this paper summarizes the current research status of the physical mechanism underlying the VO2 MIT. Three kinds of theoretical perspectives, supported by corresponding experimental results, have been proposed so far, which includes electron-correlation-driven MIT, Peierls-like structure-driven MIT, and MIT driven by the interplay of both electron-correlation and Peierls-like structural phase transition. It is noted that recent reports mostly focus on the controversywhether VO2 is a Mott insulator, and whether the structural phase transition and the MIT accurately occur simultaneously in VO2. Finally, the paper points out the near-future development direction of the VO2 research.

Oxide Electronics and Vanadium Dioxide Perspective: A Review

Metal-oxide-semiconductor field-effect transistors (MOSFET) have been for a long time the key elements of modern electronics industry. For the purpose of a permanent integration enhancement, the size of MOSFET has been decreasing exponentially for over decades in compliance with Moore's Law, but nowadays, owing to the intrinsic restrictions, the further scaling of MOSFET devices either encounters fundamental limits or demands for more and more sophisticated and expensive engineering solutions. Alternative approaches and device concepts are currently designed both in order to sustain an increase of the integration degree, and to improve the functionality and performance of electronic devices. Oxide electronics is one of such promising approaches which could enable and accelerate the development of information and computing technology. The behavior of delectrons in transition metal oxides is responsible for the unique properties of these materials, causing strong electronelectron correlations, which play an important role in the mechanism of metal-insulator transition. The Mott transition in vanadium dioxide is specifically the phenomenon that researchers consider as a corner stone of oxide electronics, particularly, in its special direction known as a Motttransition field-effect transistor (MTFET). This review focuses on current research, latest results, urgent problems and nearterm outlook of oxide electronics with special emphasis on the state of the art and recent progress in the field of VO2-based MTFETs. Index Terms — Oxide electronics, Mott transition FET, metal-insulator transition, transition metal oxides, vanadium dioxide. 1 Manuscript received November 25, 2013; accepted December 10, 2013. Date of online publication: December 15, 2013. This work was supported by the Strategic Development Program of Petrozavodsk State University (2012 – 2016), Ministry of Education and Science of Russian Federation “Scientific and Educational Community of Innovation Russia (2009-2013)” Program through contracts no. 14.740.11.0895, no. 14.740.11.0137, no. 14.740.11.1157, no. 16.740.11. 0562, and no. 14.B37.21.0755, as well as “Development of Scientific Potential of High School” Program, contracts no. 2.3282.2011 and no. 2.2774.2011. A. L. Pergament is with the Department of Information Measuring Systems and Physical Electronics, Faculty of Physical Engineering of Petrozavodsk State University, 185910 Petrozavodsk, Russia (e-mail: aperg@psu.karelia.ru). G. B. Stefanovich is with the Department of Information Measuring Systems and Physical Electronics, Faculty of Physical Engineering of Petrozavodsk State University, 185910 Petrozavodsk, Russia (e-mail: gstef@yandex.ru). A. A. Velichko is with the Department of Electronics and Electrical Power Engineering, Faculty of Physical Engineering of Petrozavodsk State University, 185910 Petrozavodsk, Russia (e-mail: velichko@ psu.karelia.ru).

Role of epitaxial microstructure, stress and twin boundaries in the metal–insulator transition mechanism in VO2/Al2O3 heterostructures

The microstructures of epitaxial polycrystalline VO2 thin films grown on (001) sapphire were investigated by means of X-ray diffraction, Cs-corrected scanning transmission electron microscopy (STEM), in both plane and transverse geometry, in relation to its metal–insulator transition (MIT) properties. It is shown that the epitaxial relationship between the thin film and the substrate can be defined as out-of-plane twofold twinning symmetry {020}M//(006)S (where subscripts “M” and “S” denote the monoclinic phase of VO2 and the sapphire α-Al2O3 substrate, respectively) with in-plane threefold twinning structure (200)M//{110}S. The origin of these twinning structures is discussed: the in-plane threefold twinning structure comes from the threefold symmetry of the Al2O3 (001) plane, and the twofold twinning symmetry is induced by the MIT phase transition. The STEM planar view observations of the thin film demonstrate the presence of elongated grains down to nanoscale, with a high density of twin boundaries (TB). These TB are highly orientated into two sets of families. STEM low-angle annular dark-field imaging and STEM dark-field atomic displacement measurements evidence very different strain behaviors for these two TB families. Most of the TB and some of the smaller grains with typical dimensions of only a couple of nanometers exhibit locally an enhanced tetragonality. They are proposed to act as nucleation centers during the MIT process and then to influence the dynamics of the transition.

Unraveling Metal-insulator Transition Mechanism of VO2Triggered by Tungsten Doping

Understanding the mechanism of W-doping induced reduction of critical temperature (TC) for VO2 metal-insulator transition (MIT) is crucial for both fundamental study and technological application. Here, using synchrotron radiation X-ray absorption spectroscopy combined with first-principles calculations, we unveil the atomic structure evolutions of W dopant and its role in tailoring the TC of VO2 MIT. We find that the local structure around W atom is intrinsically symmetric with a tetragonal-like structure, exhibiting a concentration-dependent evolution involving the initial distortion, further repulsion, and final stabilization due to the strong interaction between doped W atoms and VO2 lattices across the MIT. These results directly give the experimental evidence that the symmetric W core drives the detwisting of the nearby asymmetric monoclinic VO2 lattice to form rutile-like VO2 nuclei, and the propagations of these W-encampassed nuclei through the matrix lower the thermal energy barrier for phase transition.

Inverse Photoemission Spectroscopy

Inverse photoemission spectroscopy (IPES) is the powerful technique to directly investigate the unoccupied electronic states of materials. In this technique, electrons are injected to samples, and then emitted photons from the samples are detected. In this article, we describe the outline of IPES including its principle, and apparatuses we have developed and constructed. As examples, we present experimental results of Ti 3p-3d resonant IPES of the Mott insulator YTiO3, which is an important material for understanding of the mechanism of metal-insulator transition due to electron correlation, and angle-resolved IPES of LaB6(001), which is used as an electron source. Finally, we report a spin polarized electron gun in progress.

Percolative metal-insulator transition inLa0.9Sr0.1MnO3ultrathin films by resistive relaxation

We report an experimental study of the time dependence of resistivity of a ${\mathrm{La}}_{0.9}{\mathrm{Sr}}_{0.1}{\mathrm{MnO}}_{3}$ ultrathin film in order to elucidate the underlying mechanism for metal-insulator transition and colossal magnetoresistance (CMR) effect. There is a clear change of sign in the resistive relaxation rate across the metal-insulator transition driven by temperature or magnetic field. When measuring in increasing temperature or decreasing magnetic field, the resistivity increases with time in the metallic state but decreases with time in the insulating state. These relaxation processes indicate that the metal-insulator transition and the associated CMR are a direct result of phase separation and of percolation of the metallic phase.

Investigations of physical mechanisms of metal-insulator transition in highly strained n-Si and n-Ge crystals

Investigations of physical mechanisms of metal-insulator transition in highly strained n-Si and n-Ge crystals

Structural Effects in Magnetoresistive Manganites and Mechanism of Metal-Insulator Transition

Colossal magnetoresistivity (CMR) is caused by a magnetic field-induced insulator-to-metal transition. It is arguably one of the most dramatic phenomena that occur in solids due to competing forces in a complex system. In the CMR oxides the electron-phonon coupling and spin correlations that favor charge localization are competing against the electron kinetic energy and lattice elasticity that prefer charge delocalization. The CMR phenomenon occurs at the crossover point of charge-localized (insulating) and charge-delocalized (metallic) states, where the system is particularly susceptible to external stimuli such as a magnetic field. The competitions among these forces are usually considered globally as volume averages. However, since the system is complex and the phenomenon is non-linear, local fluctuations dominate the behavior of the system near the critical point. Therefore the system would appear spatially inhomogeneous if the measurement is made with certain time and length scales. To characterize and understand such a system it is required to deploy local approaches in which the competing interactions are evaluated locally. Experimentally, the pulsed neutron pair-density function (PDF) analysis is one of the methods of local structural study. Using the PDF technique it is suggested that the phase transition occurs via local and percolative processes. Also through the analysis of the local structure it is shown that the ionic size effect on the CMR phenomenon is not caused by a change in the band width as is usually assumed, but is due to local structural changes that affect polaron stability. The critical ionic size determined by this approach is in excellent agreement with experimental observations. The PDF results also indicate that the local structure of layered manganites is close to that of perovskite, suggesting that they share common elements of the CMR mechanisms.

Variational Monte Carlo Studies of the Hubbard Model in One- and Two-Dimensions –Off-Diagonal Intersite Correlation Effects–

We investigate the ground state properties of the one- and the two-dimensional Hubbard models using a new variational Monte Carlo method. By generalizing the Gutzwiller variational wave function (GF) to take into account an off-diagonal intersite correlation, we examine the metal-insulator transition mechanism at the half filling. As Kaplan et al. first pointed out, the binding mechanism between doubly occupied sites and empty sites plays a crucial role for this transition; we confirm this by examining the real space correlation function between these two kinds of sites. Results for the spin and the charge structure factors are presented as well as the momentum distribution function and are discussed in connection with the pairing effect. We also find that in less-than half filling, the improvements of the structure factors at 2 k F are realized as compared with GF results.