Evidence for insulator–metal transition in amorphous chalcogenide Se–Ge–Te films

The temperature dependence of the dc conductivity and thermoelectric power was determined for five different amorphous chalcogenide Se–Ge–Te films, with Ge?=?3.0–22?at.%, Se?=?0–97?at.% and Te?=?0–97?at.%. The films were prepared by thermal evaporation of GeSe₄, GeTe₄ and GeSe₂Te₂ quenched bulk materials. Values of the activation energy calculated from the temperature dependence of both electrical conductivity and thermoelectric power showed a decrease with increasing Ge content in the Se–Ge films as well as with replacement of Te for Se in the Se–Ge–Te films. The results showed an Anderson transition, with the conductivity showing insulating behaviour on the Ge–Se side to metallic behaviour at the binary composition Ge–Te. The radius of localization was obtained for the different compositions investigated. The wave function associated with the charge carriers at the composition Ge_3.3Te_96.7 is non-localized. A minimum metallic conductivity of 237?±?5?(Ω?cm)^?1 was found.     

Metal–insulator transition in Si–MOS structures

A survey is presented on recent investigations of the metal-to-insulator transition in two-dimensional systems with special emphasis on n-Si–MOS structures. Experimental facts are presented and the currently open questions on the nature of this transition are addressed.     

Unusual metal–insulator transition in disordered ferromagnetic films

We present a theoretical interpretation of recent data on the conductance near and farther away from the metal–insulator transition in thin ferromagnetic Gd films of thickness b≈2$b\approx 2$–10 nm. For increasing sheet resistances a dimensional crossover takes place from d=2 to d  =3 dimensions, since the large phase relaxation rate caused by scattering of quasiparticles off spin wave excitations renders the dephasing length L_??b$L_{?}?b$ at strong disorder. The conductivity data in the various regimes obey fractional power-law or logarithmic temperature dependence. One observes weak localization and interaction induced corrections at weaker disorder. At strong disorder, near the metal–insulator transition, the data show scaling and collapse onto two scaling curves for the metallic and insulating regimes. We interpret this unusual behavior as proof of two distinctly different correlation length exponents on both sides of the transition.     

Metal–insulator phase transition and topology in a three-component system

Due to the topology, insulators become non-trivial, particularly those with large Chern numbers which support multiple edge channels, catching our attention. In the framework of the tight binding approximation, we study a non-interacting Chern insulator model on the three-component dice lattice with real nearest-neighbor and complex next-nearest-neighbor hopping subjected to Λ-or V-type sublattice potentials. By analyzing the dispersions of corresponding energy bands, we find that the system undergoes a metal–insulator transition which can be modulated not only by the Fermi energy but also the tunable extra parameters. Furthermore, rich topological phases, including the ones with high Hall plateau, are uncovered by calculating the associated band’s Chern number. Besides, we also analyze the edge-state spectra and discuss the correspondence between Chern numbers and the edge states by the principle of bulk-edge correspondence. In general, our results suggest that there are large Chern number phases with C = ±3 and the work enriches the research about large Chern numbers in multiband systems.     

On the metal–insulator transition in vanadium dioxide

The conductivity σ of vanadium dioxide (VO₂) drops at a metal–insulator transition by four orders of magnitude due to the structural change between tetragonal and monoclinic crystals. In order to elucidate this conductivity drop, we introduce the semiclassical equation of motion to describe the dynamics of the conduction electron (wave packet), where the existence of a k-vector k is prerequisite for the conduction. We showed that the periodicity using the non-orthogonal bases does not legitimize the electron dynamics in solids. The theory suggests that the decrease in the dimensionality of the k-vectors due to the structural change is the cause of the conductivity drop.     

Superconductivity in amorphous thin films of tin–copper

The properties of the superconducting state in the amorphous Sn_1?xCu_x thin films were characterized. The concentration of copper changes in the range from 0.08 to 0.41. The calculations were conducted in the framework of the strong-coupling formalism, wherein the Eliashberg functions determined in the tunnel experiment were used. The value of the Coulomb pseudopotential equal to 0.1 was adopted. It will be shown that the critical temperature (T_C) decreases from 7 to 3.9 K. The ratio, R_Δ = 2Δ(0)/k_BT_C, differs from the BCS value: R_Δ ∈ <4.4, 3.95>, where Δ(0) represents the order parameter. Similarly behave the ratios: R_C = ΔC(T_C)/C^N(T_C) ∈ <2.2, 1.75> and R_H = T_CC^N(T_C)/H²_C(0) ∈ <0.141, 0.154>. The parameter ΔC(T_C) is the specific heat jump, C^N(T_C) denotes the specific heat of the normal state and H_C(0) is the thermodynamic critical field.     

Metal–insulator transition in NiS2−x Se x : chemical vs external pressure effects

The Se alloying (x)- and the pressure (P)-induced metal–insulator transitions on the strongly correlated NiS_2?x Se _x system have been investigated through Raman and infrared (IR) spectroscopies. Raman and IR responses of NiS₂ to lattice compression are correlated to a metallization transition, occurring at ～4 GPa. This result suggests a strong interaction between lattice and electronic degrees of freedom. In particular, IR measurements carried out by applying P on NiS₂ (i.e. lattice contraction) and on Se alloying (i.e. lattice expansion) reveal that in both cases a metallic state is obtained. Our optical spectroscopy results deviate from the idea of a simple scaling factor between P and x previously claimed by transport measurements, but, on the contrary, point out the substantially different microscopic origin of the two transitions.     

Universal role of quantum uncertainty in Anderson metal–insulator transition

We explore quantum uncertainty, based on Wigner–Yanase skew information, in various one-dimensional single-electron wave functions. For the power-law function and eigenfunctions in the Aubry–André model, the electronic localization properties are well-defined. For them, we find that quantum uncertainty is relatively small and large for delocalized and localized states, respectively. And around the transition points, the first-order derivative of the quantum uncertainty exhibits singular behavior. All these characters can be used as signatures of the transition from a delocalized phase to a localized one. With this criterion, we also study the quantum uncertainty in one-dimensional disorder system with long-range correlated potential. The results show that the first-order derivative of spectrum-averaged quantum uncertainty is minimal at a certain correlation exponent α_m${\alpha }_m$ for a finite system, and has perfect finite-size scaling behaviors around α_m${\alpha }_m$. By extrapolating α_m${\alpha }_m$, the threshold value α_c?1.56±0.02${\alpha }_c?1.56\pm 0.02$ is obtained for the infinite system. Thus we give another perspective and propose a consistent interpretation for the discrepancies about localization property in the long-range correlated potential model. These results suggest that the quantum uncertainty can provide us with a new physical intuition to the localization transition in these models.     

Metal–insulator transition and colossal magnetoresistance: relevance of electron–lattice coupling and electronic phase separation

In this article, we review the insulator–metal transition and the colossal magnetoresistance effect in manganites. The relevance of electron–lattice coupling and the resulting Jahn–Teller polaron is elaborated. The general features of electronic phase separation, which results from disorder and strain effects, are discussed along with electron–lattice coupling effects. Although a comprehensive theory is still lacking that can account for all the intricate features of manganite physics, electronic-phase separation and electron–lattice coupling appear to capture the essence of the colossal magnetoresistance effect in manganites.     

Metal–insulator phase transition in hydrogenated thin films of V<Subscript>2</Subscript>O<Subscript>3</Subscript>

Temperature dependences of the electrical conductivity of thin vanadium sesquioxide V2O3 films obtained by using the laser sputtering technique have been studied. A significant decrease (by four–five orders of magnitude) in the electrical conductivity has been observed below 150 K as a result of a metal–insulator phase transition. It is shown that hydrogenation of films lowers the temperature of this phase transition.     

Controllable fabrication of self-organized nano-multilayers in copper–carbon films

In order to clarify the influence of methane concentration and deposition time on self-organized nano-multilayers,three serial copper-carbon films have been prepared at various methane concentrations with different deposition times using a facile magnetron sputtering deposition system. The ratios of methane concentration(CH4/Ar+CH4) used in the experiments are 20%, 40%, and 60%, and the deposition times are 5 minutes, 20 minutes, and 40 minutes, respectively.Despite the difference in the growth conditions, self-organizing multilayered copper-carbon films are prepared at different deposition times by changing methane concentration. The film composition and microstructure are investigated by x-ray photoelectron spectroscopy(XPS), x-ray diffraction(XRD), field emission scanning electron microscopy(FESEM), and high-resolution transmission electron microscopy(HRTEM). By comparing the composition and microstructure of three serial films, the optimal growth conditions and compositions for self-organizing nano-multilayers in copper-carbon film are acquired. The results demonstrate that the self-organized nano-multilayered structure prefers to form in two conditions during the deposition process. One is that the methane should be curbed at low concentration for long deposition time,and the other condition is that the methane should be controlled at high concentration for short deposition time. In particular, nano-multilayered structure is self-organized in the copper-carbon film with copper concentration of 10-25 at.%.Furthermore, an interesting microstructure transition phenomenon is observed in copper-carbon films, that is, the nanomultilayered structure is gradually replaced by a nano-composite structure with deposition time and finally covered by amorphous carbon.     

Electron emission degradation of nano-structured sp^2-bonded amorphous carbon films

The initial field electron emission degradation behaviour of original nano-structured sp^2-bonded amorphous carbon films has been observed, which can be attributed to the increase of the work function of the film in the field emission process analysed using a Fowler-Nordheim plot. The possible reason for the change of work function is suggested to be the desorption of hydrogen from the original hydrogen termination film surface due to field emission current-induced local heating. For the explanation of the emission degradation behaviour of the nano-structured sp2-bonded amorphous carbon film, a cluster model with a series of graphite （0001） basal surfaces has been presented, and the theoretical calculations have been performed to investigate work functions of graphite （0001） surfaces with different hydrogen atom and ion chemisorption sites by using first principles method based on density functional theory-local density approximation.     

Electric-field induced ion-leveraged metal–insulator transition in conducting polymer-based field effect devices

The field effect devices prepared completely from conducting polymers, especially poly(3,4-ethylenedioxythiophene)/poly(styrene sulfonic acid) (PEDOT/PSS), were studied. Normally in a conductive “on” state, the transistor-like device has a transition to a substantially less conductive “off” state at an applied positive gate voltage, typically ∼15–25 V. The current ratio I_off/I_on can exceed 10⁻⁴ at room temperature. We have found that the field effect is strongly temperature dependent and is substantially reduced upon decreasing the temperature by only a 10 °C. This loss of current reduction upon application of a gate voltage is not due to the temperature dependence of the electrical conductivity of polymers of which the devices are made. The temperature dependence of the dc conductivity of the PEDOT/PSS follows the variable range hopping law both before and after application of the gate voltage, though with an increased activation energy, T₀. We suggest that the conducting polymer is near the metal–insulator transition and that the field effect in the device is related to the electric field modulating this transition in the region underneath the gate electrode. The transition is controlled and leveraged by ion motion. The time dynamics of the current with the gate modulation strongly supports our conjecture. We demonstrate the generality of the phenomena by presenting similar results for devices fabricated from the conducting polypyrrole doped with Cl.     

Excitonic metal–insulator phase transition of the Mott type in compressed calcium

It has been experimentally found that, under the static compression of a calcium crystal at room temperature, it undergoes a series of structural phase transitions: face-centered cubic lattice → body-centered cubic lattice → simple cubic lattice. It has been decided to investigate precisely the simple cubic lattice (because it is an alternative lattice) with the aim of elucidating the possibility of the existence of other (nonstructural) phase transitions in it by using for this purpose the Hubbard model for electrons with half-filled ns-bands and preliminarily transforming the initial electronic system into an electron–hole system by means of the known Shiba operators (applicable only to alternative lattices). This transformation leads to the fact that, in the new system of fermions, instead of the former repulsion, there is an attraction between electrons and holes. Elementary excitations of this new system are bound boson pairs—excitons. This system of fermions has been quantitatively analyzed by jointly using the equation-of-motion method and the direct algebraic method. The numerical integration of the analytically exact transcendental equations derived from the first principles for alternative (one-, two-, and three-dimensional) lattices has demonstrated that, in systems of two-species (electrons + hole) fermions, temperature-induced metal–insulator phase transitions of the Mott type are actually possible. Moreover, all these crystals are in fact excitonic insulators. This conclusion is in complete agreement with the analytically exact calculations of the ground state of a one-dimensional crystal (with half-filled bands), which were performed by Lieb and Wu with the aim to find out the Mott insulator–metal transition of another type.     

Detection of the metal–insulator transition in disordered systems of magnetic nanoislands

We have studied the conductivity and permittivity of a series of nanoisland-type FeNi films with an effective thickness of up to 3.2 nm on different substrates. It has been observed that the quantity Re ε changes its sign at effective thickness d ^* ≈ 1.5–1.8 nm, because of the metal–insulator transition. Analysis of the temperature dependences of the conductivity has confirmed the existence of the metal–insulator transition at the same thickness d ^*. It has been concluded that the introduced effective permittivity can serve as a characteristic of island metal systems.     

Metal–insulator transition induced by non-stoichiometry of surface layer and molecular reactions on single crystal KTaO3

In the study we present results on topography, morphology, chemical composition, electronic structure and electrical properties of the (100) surface layer of KTaO₃ single crystal caused by sputtering with Ar⁺ ion beam with energy of 1 keV. Several surface sensitive techniques, i.e. X-ray photoelectron spectroscopy (XPS), local conductivity of atomic force microscopy (LC-AFM), and Kelvin Probe Force Microscopy (KPFM) were used. The observed changes in the electronic structure were explained as a result of the chemical decomposition of the surface layer. A correlation between the electronic states which appeared in the energy gap and the changes in charge state of Ta ions was found. The activation energy related to averaged local conductivity temperature dependence was estimated from Arrhenius plot. It was also found, that variations in the local contact potential difference (LCPD) indicated changes in the chemical composition in nano-scale. The chemical reconstruction of the KTaO₃ surface modified by Ar⁺ ion beam was deduced. This non-homogeneity corresponded to 2-D non-homogeneity of the local electric conduction (LC-AFM), which occurred within nano-areas after sputtering. Chemical reactivity of the modified surface with CO₂ and O₂ was observed. The reversibility of the Ar⁺ induced loss of oxygen non-stoichiometry was observed after the sample was exposed to various doses of O₂. The successful reversibility occurred after oxidation process at high temperature, i.e. 300 °C.