Metal-insulator Transition Properties Research Articles

Effect of O 2 concentration on metal–insulator transition properties of vanadium oxide thin films prepared by radio frequency magnetron sputtering

The influence of O 2 concentration on metal–insulator transition (MIT) of vanadium oxide (VO x ) thin films was studied in terms of structural and electrical properties. The VO x films were prepared by varying O 2 concentration using reactive radio frequency magnetron sputtering with a vanadium target. As the O 2 concentration in O 2/Ar gas increased from 1% to 7%, the deposition rate of VO x films abruptly decreased and the crystalline phases of the films were transformed from V 2O 3 to V 2O 5. The VO x films deposited at 2% O 2 mainly consisted of the VO 2 phase and showed better crystallinity than those deposited at other O 2 concentrations. From the current–voltage measurements of Pt/VO x /Pt capacitors, the VO x films deposited at 2% O 2 showed excellent MIT properties which can be applied to memory devices.

METAL-INSULATOR TRANSITIONS IN POLYCRYSTALLINE VOx THIN FILMS

ABSTRACT Metal-insulator transition properties in polycrystalline VOx thin films, which were deposited using rf magnetron sputtering with a vanadium target on Pt/Ti/SiO2/Si substrates, were studied in terms of structural and electrical characteristics. The phase change of VOx thin films by annealing was confirmed from amorphous VOx films to mixed crystalline phases containing V2O3,VO2,V6O13, and V2O5. The metal-insulator transition of VOx thin films was observed by current-voltage measurement as a function of top-electrode area and the temperature. It was demonstrated that the VOx thin films can be applied to memory devices employing the transition from a high-resistance (off) insulating state to a low-resistance (on) metallic state.

Metal-insulator transition, magnetoresistance, and magnetic properties of 3d-sulfides (Review)

The results of a study of the transport and magnetic properties of some sulfides of 3d elements are reported. The concentration transitions with a change of conductivity type and a change of magnetic order are considered, and the features of the colossal magnetoresistance in FexMn1−xS and CuVxCr1−xS2 solid solutions are discussed.

Hidden scaling in the quantum Hall metal-insulator transition

Scaling properties of the quantum Hall metal-insulator transition are severely affected by finite-size effects in small systems. Surprisingly, despite the narrow spatial range where probability structure functions exhibit multifractal scaling, we clearly verify the existence of extended self-similarity---a hidden infrared scaling phenomenon related to the peculiar form of the crossover at the onset of nonmultifractal behavior. As finite-size effects get stronger for structure functions with negative orders, the parabolic approximation for the multifractal spectrum loses accuracy. However, by means of an extended self-similarity analysis, an improved evaluation of the multifractal exponents is attained for negative orders too, rendering them consistent with previous results, which rely on computations performed for considerably larger systems.

Filling-Control Metal-Insulator Transition in the Hubbard Model Studied by the Operator Projection Method

Thermodynamic and dynamical properties of filling-control metal-insulator transition (MIT) in the Hubbard model are studied by the operator projection method, especially in two dimensions. This is a non-perturbative analytic approach to many-body systems. The present theory incorporates the Mott-Hubbard, Brinkman-Rice and Slater pictures of the MIT into a unified framework, together with reproducing low-energy narrow band arising from spin-charge fluctuations. At half filling, single-particle spectra $A(\omega, k)$ show formation of two Hubbard bands and their antiferromagnetic shadows separated by a Mott gap in the plane of energy $\omega$ and momentum $k$ with lowering temperatures. These four bands produce splitting to two low-energy narrow bands and two SDW-like bands in the dispersion. Near half filling, the low-energy narrow band persists at low temperatures. This narrow band has a particularly weak dispersion and large weights around $(\pi, 0)$ and $(0, \pi)$ momenta. The velocity of these low-energy excitations is shown to vanish towards the MIT, indicating the mass divergence as in the Brinkman-Rice picture, but most prominently around $(\pi,0)$ and $(0,\pi)$ with strong momentum dependence. This reflects the suppression of the coherence near the MIT. Main structures in $A(\omega, k)$ show remarkable similarities to quantum Monte-Carlo results in two dimensions as well as in the one-dimensional Hubbard model. The charge compressibility appears to diverge with decreasing doping concentration in both one and two dimensions in agreement with the exact and quantum Monte-Carlo results. We also discuss implications of the flat dispersion formed near the Fermi level to the observations in high-$\Tc$ cuprate superconductors.

Metal-insulator transition and magnetic properties in disordered systems of solid solutions MexMn1−xS

Investigations of the structure, thermal, electric and magnetic properties of disordered systems of MexMn1−xS (MeCr, Fe, V) solid solutions with fcc α-MnS lattice are reported. Metal-insulator transitions (MITs) of the Anderson type and magnetic transitions have been found, and the correlation between them has been established. Magnetic phase diagrams have been obtained.

A Unified Theory of Magnetism in Narrow Band Electron Systems

A unified description is given for magnetism of narrow band electron systems on the basis of a functional integral formalism within the static approximation. A method is developed to calculate the free energy functional from a given band structure and its closed form expression is given for arbitrary amplitudes of the spin and charge density fluctuations, correponding to arbitrary strength of the electron-electron interaction. This expression is shown to lead to essentially correct results in both the weak and strong coupling limits. The metal-insulator (Mott) transition and related magnetic properties for the case of half-filled band and the magnetism of metals in the case of non-integral occupation number per atom are discussed with a numerical example for a tight-binding band in a simple cubic lattice.

Magnetism, Metal-Insulator Transition, and Optical Properties in Sm- and Some Other Divalent Rare-Earth Monochalcogenides

In Sm2+X compounds (X = S, Se, Te) an unusualy large dependence of the low-temperature Van Vleck susceptibility on the ligand has been observed. SmSe, SmTe, and also TmTe undergo a continuous insulator-metal transition under pressure. In contrast, SmS shows a discontinuous transition to the metal at 6.5 kbar. In all cases the transition is accompanied by a large volume change of around 20%, which is due to the promotion of a 4f electron into the 5d-6s conduction band. Optical spectroscopy yields three distinct absorption peaks. They may be interpreted as transitions from 4f6↔4f55dt2g and 4f6↔4f55deg and probably an excitonic peak. The activation energy of a 4f electron is found to be 0.63, 0.46, 0.20, and 0.22 eV for SmTe, SmSe, SmS, and TmTe, respectively, from both the pressure-dependent electrical resistivity at room temperature and the observed absorption edge.

Electrical conductivity and thermoelectric power of niobium dioxide

VO2 thin films were prepared on quartz glass substrates by using high power impulse magnetron sputtering with different pulsed bias voltages. X-ray diffraction patterns revealed that all as-deposited films were polycrystalline monoclinic VO2 and the preferred crystalline orientation of the as-deposited films varied with the magnitude of the bias voltage. Scanning electron microscopy images exhibited that the crystalline size of the VO2 thin films reduced with the magnitude of the bias voltage. X-ray photoelectron spectroscopy results revealed that oxygen vacancies were formed in all films and their quantities were similar. UV–visible-near IR spectra of the VO2 thin films at different temperatures confirmed that all films possessed typical metal–insulator transition properties. Four-point probes resistivity results exhibited that the phase transition temperature was reduced from 54 to 31.5°C when the magnitude of the bias voltage was increased from − 50 to − 250 V.