90 research outputs found
Doping a semiconductor to create an unconventional metal
Landau Fermi liquid theory, with its pivotal assertion that electrons in
metals can be simply understood as independent particles with effective masses
replacing the free electron mass, has been astonishingly successful. This is
true despite the Coulomb interactions an electron experiences from the host
crystal lattice, its defects, and the other ~1022/cm3 electrons. An important
extension to the theory accounts for the behaviour of doped semiconductors1,2.
Because little in the vast literature on materials contradicts Fermi liquid
theory and its extensions, exceptions have attracted great attention, and they
include the high temperature superconductors3, silicon-based field effect
transistors which host two-dimensional metals4, and certain rare earth
compounds at the threshold of magnetism5-8. The origin of the non-Fermi liquid
behaviour in all of these systems remains controversial. Here we report that an
entirely different and exceedingly simple class of materials - doped small gap
semiconductors near a metal-insulator transition - can also display a non-Fermi
liquid state. Remarkably, a modest magnetic field functions as a switch which
restores the ordinary disordered Fermi liquid. Our data suggest that we have
finally found a physical realization of the only mathematically rigourous route
to a non-Fermi liquid, namely the 'undercompensated Kondo effect', where there
are too few mobile electrons to compensate for the spins of unpaired electrons
localized on impurity atoms9-12.Comment: 17 pages 4 figures supplemental information included with 2 figure
Pressure-induced quantum phase transition in Fe₁−ᵪCoᵪSi (x = 0.1,0.2)
Please refer to full text to view abstrac
Linear magnetoresistance in commercial n-type silicon due to inhomogeneous doping
Free electron theory tells us that resistivity is independent of magnetic
field. In fact, most observations match the semiclassical prediction of a
magnetoresistance that is quadratic at low fields before saturating. However, a
non-saturating linear magnetoresistance has been observed in exotic
semiconductors such as silver chalcogenides, lightly-doped InSb, N-doped InAs,
MnAs-GaAs composites, PrFeAsO, and epitaxial graphene. Here we report the
observation of a large linear magnetoresistance in the ohmic regime in
commonplace commercial n-type silicon wafer. It is well-described by a
classical model of spatially fluctuating donor densities, and may be amplified
by altering the aspect ratio of the sample to enhance current-jetting:
increasing the width tenfold increased the magnetoresistance at 8 T from 445 %
to 4707 % at 35 K. This physical picture may well offer insights into the large
magnetoresistances recently observed in n-type and p-type Si in the non-ohmic
regime.Comment: submitted to Nature Material
Resistivity of MnFeSi single crystals: Evidence for quantum critical behavior
Resistivity measurements have been made on MnFeSi single crystals
between 2 and 300K for = 0, 0.05, 0.08, 0.12 and 0.15. Fe doping is found
to depress the magnetic ordering temperature from 30K for = 0 to below 2K
for = 0.15. Although Fe doping results in a large increase of the
low-temperature residual resistivity, the temperature dependence of the
resistivity above the magnetic transition remains practically unaffected by
increasing Fe content. An analysis of the temperature derivative of the
resistivity provides strong evidence for the existence of a non-Fermi-liquid
ground state near = 0.15 and thus for a quantum critical point tuned by Fe
content.Comment: 9 pages, 4 figures, Proceedings of the NATO Advanced Research
Workshop on Properties and Application of Thermoelectric Materials, Hvar,
Croatia, 21-26 September 200
Large Anomalous Hall effect in a silicon-based magnetic semiconductor
Magnetic semiconductors are attracting high interest because of their
potential use for spintronics, a new technology which merges electronics and
manipulation of conduction electron spins. (GaMn)As and (GaMn)N have recently
emerged as the most popular materials for this new technology. While Curie
temperatures are rising towards room temperature, these materials can only be
fabricated in thin film form, are heavily defective, and are not obviously
compatible with Si. We show here that it is productive to consider transition
metal monosilicides as potential alternatives. In particular, we report the
discovery that the bulk metallic magnets derived from doping the narrow gap
insulator FeSi with Co share the very high anomalous Hall conductance of
(GaMn)As, while displaying Curie temperatures as high as 53 K. Our work opens
up a new arena for spintronics, involving a bulk material based only on
transition metals and Si, and which we have proven to display a variety of
large magnetic field effects on easily measured electrical properties.Comment: 19 pages with 5 figure
Microwave assisted synthesis of MnO2 on nickel foam-graphene for electrochemical capacitor
A green chemistry approach (hydrothermal microwave irradiation) has been used to deposit manganese
oxide on nickel foam-graphene. The 3D graphene was synthesized using nickel foam template by chemical
vapor deposition (CVD) technique. Raman spectroscopy, X-ray diffraction (XRD), scanning electron
and transmission electron microscopies (SEM and TEM) have been used to characterize structure and
surface morphology of the composite, respectively. The Raman spectroscopy measurements on the samples
reveal that 3D graphene consists of mostly few layers with low defect density. The composite was
tested in a three electrode configuration for electrochemical capacitor, and exhibited a specific capacitance
of 305 F g−1 at a current density of 1Ag−1 and showed excellent cycling stability. The obtained
results demonstrate that microwave irradiation technique could be a promising approach to synthesis
graphene based functional materials for electrochemical applications.The South African Research Chairs Initiative of the Department of Science and Technology (SARCHi-DST) and the National Research Foundation (NRF). A. Bello, M. Fabiane, and O.O. Fashedemi acknowledge financial support from University of Pretoria and NRF for PhD bursaries.http://www.elsevier.com/locate/electactahb2016ChemistryPhysic
Distribution of Potentially toxic elements in Water, Sediment and Soils in the Riparian Zones around a Kraft Pulp and Paper Mill in Western Kenya
This chapter discusses the impact of effluent discharge on the distribution of potentially toxic elements (Pb, Cd, Cu and Zn) in water, sediments and soils near a Kraft mill in Western Kenya. Potentially Toxic Elements (PTEs) concentrations were determined at three sites: Water Intake Point, Effluent Discharge Point and Downstream Point. The mill liquid effluent parameters and gaseous emissions were also characterized. One-way ANOVA was used to analyze the spatial differences in PTEs concentrations. Principal component analysis determined the correlations between the proximity to the Kraft mill and the PTEs in soils, water and sediments. In riverine soils, Cd was 0.78 ± 0.01 mg/kg, while Pb was 94.38 ± 9.65 mg/kg. In sediments, the concentration was 16.81 ± 2.46 mg/kg for Zn, 6.16 ± 0.72 mg/kg for Cd and 75.28 ± 5.97 mg/kg for Pb. In water, Zn was 0.26 ± 0.038 mg/L, Cu was 0.75 ± 0.11 mg/L, Cd was 0.05 ± 0.004 mg/L and Pb was 1.26 ± 9.65 mg/L. The spatial distributions of PTEs in soils near the factory and across the river may have resulted from the factory’s effluent discharge and gaseous emissions. These findings should help formulate more stringent industrial effluent management programs in Western Kenya
Interesting magnetic properties of FeCoSi alloys
Solid solution between nonmagnetic narrow gap semiconductor FeSi and
diamagnetic semi-metal CoSi gives rise to interesting metallic alloys with
long-range helical magnetic ordering, for a wide range of intermediate
concentration. We report various interesting magnetic properties of these
alloys, including low temperature re-entrant spin-glass like behaviour and a
novel inverted magnetic hysteresis loop. Role of Dzyaloshinski-Moriya
interaction in the magnetic response of these non-centrosymmetric alloys is
discussed.Comment: 11 pages and 3 figure
Nickel-copper graphene foam prepared by atmospheric pressure chemical vapour deposition for supercapacitor applications
Please read abstract in the article.The National Research Foundation (NRF) of South Africa via iThemba LABS Materials Research Department (MRD) and the South African Research Chairs Initiative (SARChI) of the Department of Science and Technology and the NRF.http://elsevier.com/locate/surfcoathj2021Physic
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