Nonequilibrium phase transitions in a driven sandpile model

We construct a driven sandpile slope model and study it by numerical simulations in one dimension. The model is specified by a threshold slope sigma(c), a parameter alpha, governing the local current-slope relation (beyond threshold), and j(in), the mean input current of sand. A non-equilibrium phase diagram is obtained in the alpha-j(in) plane. We find an infinity of phases, characterized by different mean slopes and separated by continuous or first-order boundaries, some of which we obtain analytically. Extensions to two dimensions are discussed.z

Probing the structural changes in the phase transitions of a Bi2MoO6 catalyst: the nature of the intermediate-temperature phase

The nature of the phase transitions of Bi2MoO6 has been investigated by the combined use of X-ray diffraction and Xray absorption spectroscopy. The distorted MoO6 octahedra in the low-temperature form are shown to undergo further distortion in the intermediate-temperature form before transforming to MoO4 tetrahedra in the high-temperature phase. (C) 1995 Academic Press, Inc.

Liquid alloy semiconductors revisited - Discussion

In 1990 Enderby and Barnes reviewed the electrical properties of liquid alloys which show features associated with semiconducting behaviour. They proposed an empirical classification scheme based on the notion that some liquid semiconductors are characterized by a finite gap in σ (E), the energy-dependent conductivity (narrow definition), whereas in others σ (E) is continuous (broad definition). Interesting behaviour occurs for systems at the narrow/broad boundary and further analysis of these liquid alloys will form the subject matter of this paper. Particular attention will be focused on liquid silver chalcogenides as these offer a severe test of current theories.

Magnetic field-induced metal-insulator transitions in graphite and diluted magnetic semiconductors - Discussion

Two topical subjects related with the effect of magnetic field on electrical conduction and the metal-insulator transition are discussed. The first topic is an electronic phase transition in graphite, which is interpreted as a manifestation of a nestingtype instability inherent to a one-dimensional narrow Landau sub-band. The second topic is spin-dependent tranport in III-V based diluted magnetic semiconductors; in particular, a large negative magnetoresistance observed in the vicinity of metal-nonmetal transition.

Mott insulator high T-c bipolaronic superconductor transition in cuprates - Discussion

All ‘undoped’ cuprates are antiferromagnetic Mott insulators. We argue that with doping they remain to be insulators including the ‘overdoped’ samples. Hence, there is no clear dividing line between non–metallic cuprates and high–temperature superconductors. Based on the generic Hamiltonian including the electron–phonon interaction and the direct Coulomb repulsion the ground state of doped cuprates is shown to be a charged 2e Bose liquid of small bipolarons. A theory of the normal state transport of copper oxides is developed. The temperature dependence of the resistivity and of the Hall effect agrees remarkably well with the experimental data in La2–xSrxCuO4 for the entire temperature regime including unusual ‘logarithmic’ low–temperature region. The violation of Kohler's rule in magnetoresistivity is explained. The resistive and thermodynamic superconducting transitions in a magnetic field are quantitatively described.

Magnetism, charge ordering, and metal-insulator transition in the lanthanum manganites

We examine the magnetic and structural properties of the lanthanum manganite-based double-exchange magnets exhibiting colossal magnetoresistance. A model Hamiltonian containing the double-exchange, superexchange, and the Hubbard terms, with parameters obtained from density–functional calculations (Ref. 1), is studied within a mean-field approximation both at temperature T=0 and T>0 and with the effects of the magnetic field included. The phase diagrams we obtain with magnetic and charge-ordered phases enable us to examine the competition between the double- and superexchange terms as functions of doping and temperature. Our theoretical study provides a qualitative understanding of the phase diagram observed in the experiments. © 1997 American Institute of Physics.