Broken symmetry and critical transport properties of random metals

Recent experimental data on the conductivity σ+(T), T → 0, on the metallic side of the metal–insulator transition in ideally random (neutron transmutation-doped) 70Ge:Ga have shown that σ+(0) ∝ (N − Nc)μ with μ = ½, confirming earlier ultra-low-temperature results for Si:P. This value is inconsistent with theoretical predictions based on diffusive classical scaling models, but it can be understood by a quantum-directed percolative filamentary amplitude model in which electronic basis states exist which have a well-defined momentum parallel but not normal to the applied electric field. The model, which is based on a new kind of broken symmetry, also explains the anomalous sign reversal of the derivative of the temperature dependence in the critical regime.

Critical transport properties of random metals in large magnetic fields

The threshold behavior of the transport properties of a random metal in the critical region near a metal–insulator transition is strongly affected by the measuring electromagnetic fields. In spite of the randomness, the electrical conductivity exhibits striking phase-coherent effects due to broken symmetry, which greatly sharpen the transition compared with the predictions of effective medium theories, as previously explained for electrical conductivities. Here broken symmetry explains the sign reversal of the T → 0 magnetoconductance of the metal–insulator transition in Si(B,P), also previously not understood by effective medium theories. Finally, the symmetry-breaking features of quantum percolation theory explain the unexpectedly very small electrical conductivity temperature exponent α = 0.22(2) recently observed in Ni(S,Se)2 alloys at the antiferromagnetic metal–insulator transition below T = 0.8 K.

Overexpression of the large subunit of the protein Ku suppresses metallothionein-I induction by heavy metals

Metallothioneins (MT) are involved in the scavenging of the toxic heavy metals and protection of cells from reactive oxygen intermediates. To investigate the potential role of the protein Ku in the expression of MT, we measured the level of MT-I mRNA in the parental rat fibroblast cell line (Rat 1) and the cell lines that stably and constitutively overexpress the small subunit, the large subunit, and the heterodimer of Ku. Treatment with CdS04 or ZnS04 elevated the MT-I mRNA level 20- to 30-fold in the parental cells and the cells (Ku-70) that overproduce the small subunit or those (Ku-7080) overexpressing the heterodimer. By contrast, the cells (Ku-80) overexpressing the large subunit of Ku failed to induce MT-I. In vitro transcription assay showed that the MT-I promoter activity was suppressed selectively in the nuclear extracts from Ku-80 cells. The specificity of the repressor function was shown by the induction of hsp 70, another Cd-inducible gene, in Ku-80 cells. Addition of the nuclear extract from Ku-80 cells at the start of the transcription reaction abolished the MT-l promoter activity in the Rat 1 cell extract. The transcript once formed in Rat 1 nuclear extract was not degraded by further incubation with the extract from Ku-80 cells. The repressor was sensitive to heat. The DNA-binding activities of at least four transcription factors that control the MT-I promoter activity were not affected in Ku-80 cells. These observations have set the stage for further exploration of the mechanisms by which the Ku subunit mediates suppression of MT induction.

Mineral surfaces and bioavailability of heavy metals: A molecular-scale perspective

There is a continual influx of heavy metal contaminants and pollutants into the biosphere from both natural and anthropogenic sources. A complex variety of abiotic and biotic processes affects their speciation and distribution, including adsorption onto and desorption from mineral surfaces, incorporation in precipitates or coprecipitates, release through the dissolution of minerals, and interactions with plants and microbes. Some of these processes can effectively isolate heavy metals from the biosphere, whereas others cause their release or transformation to different species that may be more (or less) bioavailable and/or toxic to organisms. Here we focus on abiotic adsorption and precipitation or coprecipitation processes involving the common heavy metal contaminant lead and the metalloids arsenic and selenium in mine tailings and contaminated soils. We have used extremely intense x-rays from synchrotron sources and a structure-sensitive method known as x-ray absorption fine structure (XAFS) spectroscopy to determine the molecular-level speciation of these elements at concentrations of 50 to several thousand ppm in the contaminated environmental samples as well as in synthetic sorption samples. Our XAFS studies of As and Pb in the mine tailings show that up to 50% of these contaminants in the samples studied may be present as adsorbed species on mineral surfaces, which makes them potentially more bioavailable than when present in sparingly soluble solid phases. Our XAFS studies of Se(VI) sorption on Fe2+-containing sulfates show that this element undergoes redox reactions that transform it into less bioavailable and less toxic species. This type of information on molecular-level speciation of heavy metal and metalloid contaminants in various environmental settings is needed to prioritize remediation efforts and to assess their potential hazard to humans and other organisms.

New physics of metals: fermi surfaces without Fermi liquids.

I relate the historic successes, and present difficulties, of the renormalized quasiparticle theory of metals ("AGD" or Fermi liquid theory). I then describe the best-understood example of a non-Fermi liquid, the normal metallic state of the cuprate superconductors.