Pseudogap and metal-insulator transitions in doped semiconductors

The electronic spectrum of a doped semiconductor described by the Anderson-Holstein impurity model and its conductivity derived from the Kubo linear response theory are calculated. Two characteristic temperatures depending on the doping level x are found in the phase diagram, T _PG and T _λ(x). The pseudogap that opens in the single-particle spectrum at low doping levels and temperatures closes at the lower one, T _PG. The pseudogap state of an insulator is attributed to spin fluctuations in a doped compound. At the higher characteristic temperature T _λ(x),, spin fluctuations vanish and the doped compound becomes a paramagnetic poor metal. Two distinct metal-insulator crossovers between semiconductor-like and metallic temperature dependence of resistivity are found. An insulator-to-poor-metal transition occurs at T ^*(x) ≈ T _λ(x). A poor-metal-to-insulator transition at a lower temperature is attributed to the temperature dependence of density of states in the pseudogap. It is shown that both transitions are observed in La_2?x Sr_xCUO₄.     

Magnetic field induced metal-insulator transitions in p-SiGe

Low density modulation doped p-SiGe, where the holes lie in a strained SiGe quantum well, frequently exhibits anomalous insulating behaviour between filling factors ν=2 and 1. There is also anomalous metallic behavior with a metal-insulator transition between the two. It is shown that in these samples exchange effects are sufficiently large to induce the paramagnetic-ferromagnetic phase transition predicted by Giuliani and Quinn in 1985, also that the metallic and insulating behavior is associated with the coincidence of two Landau levels of opposite spin. A model calculation shows that while a ferromagnetic polarization may occur at integer filling factors screening suppresses it for non-integer filling factors. It is argued the Landau levels then stick-together and allow a spin-density instability to form. Because of the strong spin-orbit coupling in p-SiGe the transport properties of this state differ from those of other systems where a similar quantum Hall ferromagnet probably forms.     

The local field effect and metal-insulator transitions

Possible local field effects which accompany the metal-insulator transition have been investigated. It is shown that the intensity of light absorption by an impurity molecule may increase several times in dielectric phase due to local field corrections. A simple model of Peierls transition is explored.     

Aspects of simulating metal-insulator transitions in CrS and CoS

The electron-energy structure of CrS and CoS monosulfides at a temperature of 0 K is investigated theoretically using the augmented plane wave approach in combination with the WIEN2k program code. Computations of the total and partial densities of electron states are performed in the LDA + U approximation with allowance for the antiferromagnetic ordering of magnetic moments of 3d-metal atoms. In our models, the optical forbidden band gap E _g in the CrS compound emerges in the monoclinic crystal structure but is missing from the NiAs structure. For the CoS compound in the NiAs structure, computations of the electron-energy structure are performed for both the highand low-spin configurations of the Co atom’s 3d ⁷ shell. The optically forbidden E _g band gap appears in the high-spin configuration with strong correlations of d-electrons, while CoS remains a conductor in the low-spin configuration.     

Dielectric theory of metal-insulator transitions in doped systems

Model dielectric functions for doped systems show how the dopant excitations change from single-particle-like to collective in character as the concentration is increased from a dilute insulating limit to a more concentrated metallic regime.     

Glassy behavior of electrons near metal-insulator transitions

The emergence of glassy behavior of electrons is investigated for systems close to disorder-driven and interaction-driven metal-insulator transitions. Our results indicate that Anderson localization effects strongly stabilize such glassy behavior, while Mott localization tends to suppress it. We predict the emergence of an intermediate metallic glassy phase separating the insulator from the normal metal. This effect is expected to be most pronounced for sufficiently disordered systems, in agreement with recent experimental observations.     

Dielectric breakdown and avalanches at nonequilibrium metal-insulator transitions

Motivated by recent experiments on the finite temperature Mott transition in VO(2) films, we propose a classical coarse-grained dielectric breakdown model where each degree of freedom represents a nanograin which transitions from insulator to metal with increasing temperature and voltage at random thresholds due to quenched disorder. We describe the properties of the resulting nonequilibrium metal-insulator transition and explain the universal characteristics of the resistance jump distribution. We predict that by tuning voltage, another critical point is approached, which separates a phase of boltlike avalanches from percolationlike ones.     

Reemergent metal-insulator transitions in manganites exposed with spatial confinement

The metal-insulator transition is characterized as a single peak in the temperature-dependent resistivity measurements; exceptions to this have never been seen in any single crystal material system. We show that by reducing a single crystal manganite thin film to a wire with a width comparable to the mesoscopic phase-separated domains inherent in the material, a second and robust metal-insulator transition peak appears in the resistivity versus temperature measurement. This new observation suggests that spatial confinement is a promising route for the discovery of emergent physical phenomena in complex oxides.     

Kinetics of phase transitions under high pressure

Abstract

Phase transitions under high pressure have attracted increasing attention i n connection with high pressure synthesis of new materials (e.g. superhard materials, ceramics, semiconductors, high temperature superconductors) and the exploration of geological processes like the formation of rocks and minerals. For t h e investigation of thermodynamic and electronic equilibrium properties a broad spectrum of methods for pressure generation and physical measurements have been developed ^1,2,3,4. Tha measurement of equilibrium properties, however, gives only poor insight into the detailed mechanisnts of phase changes. The accurate determination of phase equilibria and phase diagrams becomes inore and more difficult a t low temperatures due to increasing hysteresis effects end t h e extrapolation of experimental data to zerc, teiiipwature a nd the compar ison with quantumiiiechanical ah in.itio calculations are questionable.     

Nonmetal-metal transitions in liquid substances under high pressure

Abstract

In the melts of Te, Se, S, I₂ and Mg₃Bi₂ the nonmetal-metal transitions were found under pressure. The transitions are accompanied by a decrease of the volume. The transitions seem to terminate at high temperature by “critical regions”. For S and Se the kinetics of the transitions and the pressure influence on the solidification of the melts were investigated.

The existence of the transitions of this kind gives an explanation of anomalies of melting curves of some substances.     

The hydrogen bond under pressure

The hydrogen bonds are quite pervasive in several classes of materials. Its parameters are known to show systematic variations with hydrogen bond length, and pressure variable is thus a natural way for studying hydrogen bonded substances. In this article, we review the unifying features as obtained through several experimental and theoretical investigations. Amongst other things, it is examined whether the observed pressure-induced variations in parameters of hydrogen bonds are consistent with the co-relations known on different chemical substances at normal pressure. In particular, the controversies on variations of O–H and H- - -O pairs with pressure and symmetrization of hydrogen bond have been resolved. The effects of close packing promoted by pressure such as formation of muli-centered hydrogen bonds and steric repulsions and the way the hydrogen bonds counter these in different ways are also examined.     

Second-order phase transitions under pressure in polycrystalline compounds and rocks

Bulletin of the Russian Academy of Sciences: Physics - The dependence of thermal conductivity in mono-and polycrystalline tellurium and rocks at hydrostatic pressures of up to 400 MPa in the...     

Structural phase transitions of tellurium nanoplates under pressure

In situ high-pressure angle dispersive x-ray diffraction experiments using synchrotron radiation on Te nanoplates were carried out with a diamond anvil cell at room temperature. The results show that Te-I with a trigonal structure transforms to triclinic Te-II at about 4.9 GPa, Te-II transforms to monoclinic Te-III at about 8.0 GPa, Te-III turns to rhombohedral Te-IV at about 23.8 GPa, and Te-IV changes to body centered cubic Te-V at 27.6 GPa. The bulk moduli B0 of Te nanoplates are higher than those of Te bulk materials.