Possible Metal/Insulator Transition at B=0 in Two Dimensions

We have studied the zero magnetic field resistivity of unique high- mobility two-dimensional electron system in silicon. At very low electron density (but higher than some sample-dependent critical value, $n_{cr}\sim 10^{11}$ cm$^{-2}$), CONVENTIONAL WEAK LOCALIZATION IS OVERPOWERED BY A SHARP DROP OF RESISTIVITY BY AN ORDER OF MAGNITUDE with decreasing temperature below 1--2 K. No further evidence for electron localization is seen down to at least 20 mK. For $n_s<N_{cr}$, the sample is insulating. The resistivity is empirically found to SCALE WITH TEMPERATURE BOTH BELOW AND ABOVE $n_{cr}$ WITH A SINGLE PARAMETER which approaches zero at $n_s=n_{cr}$ suggesting a metal/ insulator phase transition.Comment: 10 pages; REVTeX v3.0; 3 POSTSCRIPT figures available upon request; to be published in PRB, Rapid Commu

A Composite Fermion Hofstader Problem: Partially Polarized Density Wave States in the 2/5 FQHE

It is well-known that the 2/5 state is unpolarized at zero Zeeman energy, while it is fully polarized at large Zeeman energies. A novel state with charge/spin density wave order for Composite Fermions is proposed to exist at intermediate values of the Zeeman coupling for 2/5. This state has half the maximum possible polarization, and can be extended to other incompressible fractions. A Hartree-Fock calculation based on the new approach for all fractional quantum Hall states developed by R.Shankar and the author is used to demonstrate the stability of this state to single-particle excitations, and compute gaps. We compare our results with a very recent experiment which shows direct evidence for the existence of such a state, and also with more indirect evidence from past experiments.Comment: One reference added, minor clarifying change

Quantum Hall - insulator transitions in lattice models with strong disorder

We report results of numerical studies of the integer quantum Hall effect in a tight binding model on a two-dimensional square lattice with non-interacting electrons, in the presence of a random potential as well as a uniform magnetic field applied perpendicular to the lattice. We consider field magnitudes such that the area per flux quantum is commensurate with the lattice structure. Topological properties of the single electron wave functions are used to identify current carrying states that are responsible for the quantized Hall conductance. We study the interplay between the magnetic field and the disorder, and find a universal pattern with which the current carrying states are destroyed by increasing disorder strength, and the system driven into an insulating state. We also discuss how to interpolate results of lattice models to the continuum limit. The relationship to previous theoretical and experimental studies of quantum Hall-insulator transitions in strongly disordered systems at low magnetic fields is discussed.Comment: 20 pages, 6 figure

Short--range impurity in the vicinity of a saddle point and the levitation of the 2D delocalized states in a magnetic field

The effect of a short--range impurity on the transmission through a saddle--point potential for an electron, moving in a strong magnetic field, is studied. It is demonstrated that for a random position of an impurity and random sign of its potential the impurity--induced mixing of the Landau levels diminishes {\em on average} the transmission coefficient. This results in an upward shift (levitation) of the energy position of the delocalized state in a smooth potential. The magnitude of the shift is estimated. It increases with decreasing magnetic field $B$ as $B^{-4}$.Comment: LaTeX, 20 page

A consideration of the challenges involved in supervising international masters students

This paper explores the challenges facing supervisors of international postgraduate students at the dissertation stage of the masters programme. The central problems of time pressure, language difficulties, a lack of critical analysis and a prevalence of personal problems among international students are discussed. This paper makes recommendations for the improvement of language and critical thinking skills, and questions the future policy of language requirements at HE for international Masters students

Measurements of iodine monoxide at a semi polluted coastal location

Point source measurements of IO by laser induced fluorescence spectroscopy were made at a semi-polluted coastal location during the Reactive Halogens in the Marine Boundary Layer (RHaMBLe) campaign in September 2006. The site, on the NW French coast in Roscoff, was characterised by extensive intertidal macroalgae beds which were exposed at low tide. The closest known iodine active macroalgae beds were at least 300 m from the measurement point. From 20 days of measurements, IO was observed above the instrument limit of detection on 14 days, of which a clear diurnal profile was observed on 11 days. The maximum IO mixing ratio was 30.0 pptv (10 s integration period) during the day, amongst the highest concentrations ever observed in the atmosphere, and 1–2 pptv during the night. IO concentrations were strongly dependent on tidal height, the intensity of solar irradiation and meteorological conditions. An intercomparison of IO measurements made using point source and spatially averaged DOAS instruments confirms the presence of hot-spots of IO caused by an inhomogeneous distribution of macroalgae. The co-incident, point source measurement of IO and ultra fine particles (2.5 nm&amp;ge;&lt;i&gt;d&lt;/i&gt;&amp;ge;10 nm) displayed a strong correlation, providing evidence that IO is involved in the production pathway of ultra fine particles at coastal locations. Finally, a modelling study shows that high IO concentrations which are likely to be produced in a macrolagae rich environment can significantly perturb the concentrations of OH and HO&lt;sub&gt;2&lt;/sub&gt; radicals. The effect of IO on HO&lt;sub&gt;x&lt;/sub&gt; is reduced as NO&lt;sub&gt;x&lt;/sub&gt; concentrations increase

Cell cycle times of short-term cultures of brain cancers as predictors of survival

Tumour cytokinetics estimated in vivo as potential doubling times (Tpot values) have been found to range in a variety of human cancers from 2 days to several weeks and are often related to clinical outcome. We have previously developed a method to estimate culture cycle times of short-term cultures of surgical material for several tumour types and found, surprisingly, that their range was similar to that reported for Tpot values. As Tpot is recognised as important prognostic variable in cancer, we wished to determine whether culture cycle times had clinical significance. Brain tumour material obtained at surgery from 70 patients with glioblastoma, medulloblastoma, astrocytoma, oligodendroglioma and metastatic melanoma was cultured for 7 days on 96-well plates, coated with agarose to prevent proliferation of fibroblasts. Culture cycle times were estimated from relative 3H-thymidine incorporation in the presence and absence of cell division. Patients were divided into two groups on the basis of culture cycle times of ⩽10 days and >10 days and patient survival was compared. For patients with brain cancers of all types, median survival for the ⩽10-day and >10-day groups were 5.1 and 12.5 months, respectively (P=0.0009). For 42 patients with glioblastoma, the corresponding values were 6.5 and 9.0 months, respectively (P=0.03). Lower grade gliomas had longer median culture cycle times (16 days) than those of medulloblastomas (9.9 days), glioblastomas (9.8 days) or melanomas (6.7 days). We conclude that culture cycle times determined using short-term cultures of surgical material from brain tumours correlate with patient survival. Tumour cells thus appear to preserve important cytokinetic characteristics when transferred to culture

Proceedings IMWA 2010

Abstract A research-grade passive treatment system was constructed to receive 1000 L/minute of mine water from abandoned boreholes (pH 5.95, net alkalinity 29 mg/L CaCO₃, Fe 192 mg/L, Zn 11 mg/L, Cd 17 μg/L, Pb 60 μg/L and As 64 μg/L). The 2-ha system includes an oxidation pond followed by parallel treatment trains of aerobic wetlands, vertical flow bioreactors, re-aeration ponds, and horizontal-flow limestone beds and a final polishing wetland. Final effluent waters had pH &gt;7 and contained &lt; 1 mg/L total Fe and &lt; 0.1 mg/L total Zn, with concentrations of Cd, Pb and As below detectable limits. Key Words hard rock mining, metal mining, acid mine drainage, natural treatment systems Introduction This paper describes the initial evaluation of an innovative, ecologically engineered passive system designed to treat abandoned ferruginous Pb-Zn mine waters at the Tar Creek Superfund Site, part of the historic Tri-State Mining District (TSMD) of Oklahoma, Kansas and Missouri, USA. Significant quantities of Pb and Zn were produced from the TSMD from the 1890s through the 1960s. By the early 1970s when mining ceased, two and nine million tons of Pb and Zn, respectively, had been produced During mining, large capacity dewatering operations pumped approximately 50,000 m³ d⁻¹ of water from the mines (Reed et al. 1955). Upon decline and cessation of mining, groundwater began to accumulate in the mine voids. By late 1979, metal-rich waters began to discharge via artesian pressure into Tar Creek and its tributaries. The first documented discharges of mine drainage were at a location near southeast Commerce, OK (Oklahoma Water Resources Board 1983) and were subsequently identified for passive treatment implementation Methods For this study, periodic water quality and quantity data collection efforts for the subject discharges began in 1998, with regular monthly sampling beginning in 2004 and continuing to the present. The targeted discharges have circum-neutral pH (5.96 ± 0.06), total alkalinity of 405 ± 13 mg/L as CaCO₃ and combined flow rates of up to 1000 L/minute. Metals and sulfate concentrations are elevated above expected levels and degrade the receiving waters ). Design and construction details for the passive treatment system are summarized in Sydney, NS IMWA 2010 &quot;Mine Water and Innovative Thinking&quot; Wolkersdorfer &amp; Freund Results and Discussion In the year of operation, the passive treatment system performed as designed from a water quality perspective ( Other metals of specific interest in these waters were Cd, Pb, and As. All three were removed to below detection limits (0.64, 19.5 and 22 µg/L, respectively) before the outflow of the second process units, presumably through sorptive processes. Although the vertical-flow bioreactors were designed to remove Cd and Pb as well as Zn, Cd and Pb rarely remained in measureable concentrations at this stage of the treatment system. The other trace metal found in significant concentrations in these waters was Ni. A small percentage (&lt;10%) of Ni was removed through co-precipitation and sorption in Cell 1. However, the majority of Ni (≈ 95%) was removed via re- IMWA 2010 Sydney, NS &quot;Mine Water and Innovative Thinking&quot