Disorder-Induced Metal–Insulator Transition in Conducting Polymers

The metal–insulator (M-I) transition in conducting polymers is particularly interesting; critical behavior has been observed over a relatively wide temperature range in a number of systems, including polyacetylene, polypyrrole, poly (p-phenylene vinylene), and polyaniline. In each case, metallic, critical, and insulating regimes have been identified through Zabrodskii plots of the logarithmic derivative of the conductivity. The critical regime (in which the conductivity varies as T, where 1/3) is tunable by varying the extent of disorder and by applying external pressure and/or an external magnetic field. The transitions from metallic to critical behavior and from critical to insulating behavior have been induced with a magnetic field and from insulating to metallic behavior with applied pressure.     

Metal–Insulator Transition in Optical Lattice System with Site-Dependent Interactions

We investigate the half-filled Hubbard model with spatially alternating interactions by means of the two-site dynamical mean-field theory. It is found that a single Mott transition occurs when two kinds of interactions are increased. This implies that the different interactions are essentially irrelevant at the critical point. The nature of the Mott states is also addressed.     

A Thermal Radiation Modulation Platform by Emissivity Engineering with Graded Metal–Insulator Transition

Thermal radiation from a black body increases with the fourth power of absolute temperature (T⁴), an effect known as the Stefan–Boltzmann law. Typical materials radiate heat at a portion of this limit, where the portion, called integrated emissivity (ε_int), is insensitive to temperature (|dε_int/dT| ≈ 10⁻⁴ °C^–1). The resultant radiance bound by the T⁴ law limits the ability to regulate radiative heat. Here, an unusual material platform is shown in which ε_int can be engineered to decrease in an arbitrary manner near room temperature (|dε_int/dT| ≈ 8 × 10⁻³ °C^–1), enabling unprecedented manipulation of infrared radiation. As an example, ε_int is programmed to vary with temperature as the inverse of T⁴, precisely counteracting the T⁴ dependence; hence, thermal radiance from the surface becomes temperature-independent, allowing the fabrication of flexible and power-free infrared camouflage with unique advantage in performance stability. The structure is based on thin films of tungsten-doped vanadium dioxide where the tungsten fraction is judiciously graded across a thickness less than the skin depth of electromagnetic screening.     

Anomalous Domain Wall Magnetoresistance in Ultrathin Manganite Films Near M–I Transition Boundary

Resistance and magnetoresistance in compressively strained epitaxial ultrathin films have been studied. The samples were first demagnetized in different ways so that different magnetic structures were created, such as random domain and single domain states. Very large difference in resistance in zero applied magnetic field was observed between different states. The large change of resistance between states is attributed to spin-dependent scattering at the domain walls. We have shown for the first time that large domain wall resistance can be obtained in strained ultrathin manganite films and the result cannot be explained by the double-exchange model.     

Chemical Modulation of Metal–Insulator Transition toward Multifunctional Applications in Vanadium Dioxide Nanostructures

The metal–insulator transition (MIT) of vanadium dioxide (VO₂) has been of great interest in materials science for both fundamental understanding of strongly correlated physics and a wide range of applications in optics, thermotics, spintronics, and electronics. Due to the merits of chemical interaction with accessibility, versatility, and tunability, chemical modification provides a new perspective to regulate the MIT of VO₂, endowing VO₂ with exciting properties and improved functionalities. In the past few years, plenty of efforts have been devoted to exploring innovative chemical approaches for the synthesis and MIT modulation of VO₂ nanostructures, greatly contributing to the understanding of electronic correlations and development of MIT-driven functionalities. Here, this comprehensive review summarizes the recent achievements in chemical synthesis of VO₂ and its MIT modulation involving hydrogen incorporation, composition engineering, surface modification, and electrochemical gating. The newly appearing phenomena, mechanism of electronic correlation, and structural instability are discussed. Furthermore, progresses related to MIT-driven applications are presented, such as the smart window, optoelectronic detector, thermal microactuator, thermal radiation coating, spintronic device, memristive, and neuromorphic device. Finally, the challenges and prospects in future research of chemical modulation and functional applications of VO₂ MIT are also provided.     

Static and Dynamic Properties of the Insulator–Metal Transition in Magnetic Semiconductors, Including the Perovskites

All magnetic semiconductors experience an insulator–metal transition with decreasing temperature near and below the magnetic ordering temperature. T_c, as long as the carrier concentration, whether electrons or holes, is low enough. For somewhat higher carrier concentrations, a resistivity anomaly ordinarily occurs near T_c, which cannot be explained in terms of scattering from the magnetic fluctuations alone. The thrust of this paper is to review the magnetotransport properties of various magnetic semiconductors and to present arguments that magnetic polarons, a many body state involving charge carriers and the localized magnetic spins in their immediate neighborhoods, are involved in the physical processes leading to the insulator–metal transition. Magnetic polarons have been observed directly by a variety of physical techniques including, for example, magnetic measurements, neutron diffraction, scanning tunneling microscopy, and, most recently, by noise measurements. This article will review these physical manifestations of the existence of magnetic polarons and relate them to the transition.     

Electrical Conductivity of Composite Materials Based on Fine-Particle Silicon near the Metal–Insulator Transition

Si/SiO₂ composites containing 17, 19, and 21 wt % crystalline B-doped Si particles (near the insulator–metal transition) are studied by impedance spectroscopy. The results indicate that, at low frequencies, the nonlinear variation of the real part of electrical conductivity, Re, with applied dc voltage V for the composite containing 17 wt % Si (dielectric properties) is due to the current being limited by the space charge buildup at traps in the silica layers between Si particles. At high frequencies, the nonlinear Re(V) behavior is due to tunneling through the contacts between the particles.     

Semiconductor–Metal Phase Transition and “Tristable” Electrical Switching in Nanocrystalline Vanadium Oxide Films on Silicon

Technical Physics Letters - Temperature dependences of the ac conductivity of nanocrystalline mixed vanadium oxide films on silicon revealed a multistep shape of the hysteresis loop observed during...     

Dynamic Electronic Structure in Transition Metal Oxides

The isomer shift of La1-xCaxFeO3 decreases gradually from the value for trivalent iron to that for tetravalent iron, as the Ca content x increases from 0 to 1. This indicates that iron in La1.x-CaxFeO3 has an intermediate valence state. The intermediate valence state of iron increases gradually from trivalence to tetravalence. This can be interpreted as being due to electron delocalization among iron ions on identical octahedral sites. The Mossbauer spectra at various temperatures of La0.5Ca0.5FeO3 and their theoretical treatment show that electron delocalization slows down with decreasing temperature.     

Surface Modification of α-Fe Metal Particles by Chemical Surface Coating

The structure of α-Fe metal magnetic recording particles coated with silane coupling agents have been studied by TEM, FT-IR, EXAFS, Mossbauer. The results show that a close, uniform, firm and ultra thin layer, which is beneficial to the magnetic and chemical stability, has been formed by the cross-linked chemical bond Si-O-Si. And the organic molecule has chemically bonded to the particle surface, which has greatly affected the surface Fe atom electronic structure. Furthermore, the covalent bond between metal particle surface and organic molecule has obvious effect on the near edge structure of the surface Fe atoms.     

Dynamic Scattering of Light in Liquid-Crystal Cells with Metal–Insulator–Semiconductor Microstructures Containing Gold Nanoparticles

Technical Physics Letters - The possibility of reducing the threshold of the effect of dynamic light scattering in liquid-crystal cells has been demonstrated. The threshold lowering is favored by...     

Electrical and Thermal Characteristics of the Insulator–Metal Transition in Crystalline $$\hbox {V}_{2}\hbox {O}_{5}$$ Films

The electrical and thermal properties with respect to the crystallization in \(\hbox {V}_{2}\hbox {O}_{5}\) thin films were investigated by measuring the resistance at different temperatures and applied voltages. The changes in the crystal structure of the films at different temperatures were also explored using Raman measurements. The thermal diffusivity of the crystalline \(\hbox {V}_{2}\hbox {O}_{5}\) film was measured by the nanosecond thermoreflectance method. The microstructures of amorphous and crystalline \(\hbox {V}_{2}\hbox {O}_{5}\) were observed by SEM and XRD measurements. The temperature-dependent Raman spectra revealed that a structural phase transition does not occur in the crystalline film. The resistance measurements of an amorphous film indicated semiconducting behavior, whereas the resistance of the crystalline film revealed a substantial change near \(250\,{^{\circ }}\hbox {C}\), and Ohmic behavior was observed above \(380\,{^{\circ }}\hbox {C}\). This result was due to the metal–insulator transition induced by lattice distortion in the crystalline film, for which \(T_{\mathrm{c}}\) was \(260\,{^{\circ }}\hbox {C}\). \(T_{\mathrm{c}}\) of the film decreased from 260 \({^{\circ }}\hbox {C}\) to \(230\,{^{\circ }}\hbox {C}\) with increasing applied voltage from 0 V to 10 V. Furthermore, the thermal diffusivity of the crystalline film was \(1.67\times 10^{-7}\,\hbox {m}^{2}\cdot \hbox {s}^{-1}\) according to the nanosecond thermoreflectance measurements.     

Magnetocaloric Effect in Perovskite Manganite La0.65Nd0.05Ba0.3MnO3 with Double Metal–Insulator Peaks

With Nd³⁺ ionic doping, series of perovskite manganites La_0.7−x Nd _x Ba_0.3MnO₃ (x=0, 0.05, 0.1) were prepared by the sol-gel technique. The magnetocaloric correlative measurement of La_0.65Nd_0.05Ba_0.3MnO₃ was representatively carried out. Double metal–insulator peaks being found on the resistivity–temperature curves of samples, the maximum of magnetic entropy change was found to be ΔS _H=−1.3 J/kg K and its refrigerant capacity 52 J/kg, obtained in the vicinity of room temperature under the variation of 1 T magnetic field. The comparative faint Nd³⁺ magnetic inhomogeneity submerged in the double-exchange strong magnetic signal along with the differential coefficient of the magnetic moment with temperature, in the physical definition of the magnetic entropy change, resulting in a single peak on the magnetic entropy change curve.     

Metal–Nonmetal Transition in Rubidium Tungsten Bronze RbxWOy

An oxygen concentration dependent metal–nonmetal (MN) transition was observed for Rb_0.23WO _y with 2.80 < y < 3.08. As 2.80 < y < 3.0, the room temperature resistivity (_RT) of the Rb_0.23WO _y is about 5 × 10^–4 cm. While in the case of y > 3.04, the _RT of the Rb_0.23WO _y exhibits a four orders of magnitude increase with a value of 5 cm. Correspondingly, the lattice constant along c-direction slightly shortens as oxygen concentration increases from 2.80 to 3.08. The observed results suggest that the hybridization between W 5d (t_2g) and O 2p orbitals might be responsible for the MN transition. In addition, similar measurements were performed for Rb _x WO_3.04 and Rb _x WO_2.85 with 0.19 < x < 0.27. No rubidium concentration dependent MN transition was observed, indicating the electronic structure of the host WO _y is not modified significantly by varying the soluble rubidium concentration.     

Recent Progress in Thin 3He–4He Films

We review recent work on the properties of ³He–⁴He thin film mixtures. The subject is introduced by reviewing the contributions made by Gasparini and co-workers. In the first part of this survey, we examine a microscopic calculation of the effects that an adsorbed ³He component has on the collective excitations associated with a ⁴He film, third sound. The work in question computes the change in the third sound velocity due to adding ³He, and we can compare this with existing experiment. For small ³He coverage, the change in the third sound speed depends on the derivatives of the species chemical potentials with respect to the areal density. The microscopic calculation shows oscillations in the third sound speed that reflect the layered structure of the film. In the second part of this survey, we concentrate on the ³He component. An important impact of the ⁴He environment is to change the bare mass in the limit of zero ³He concentration to a hydrodynamic mass. Quasiparticle interactions are introduced through the Landau parameters. Our Landau parameters are determined by fitting s-wave and p-wave T-matrix components using experimental results for the effective mass and the spin susceptibility. We can then predict the thermodynamic behavior of the mixture films at finite polarization, and in addition we can predict the spectrum of zero-sound excitations at all polarizations.