Magneto-transport in high g-factor, low-density two-dimensional electron systems confined in In_0.75Ga_0.25As/In_0.75Al_0.25As quantum wells

We report magneto-transport measurements on high-mobility two-dimensional electron systems (2DESs) confined in In_0.75Ga_0.25As/In_0.75Al_0.25As single quantum wells. Several quantum Hall states are observed in a wide range of temperatures and electron densities, the latter controlled by a gate voltage down to values of 1.10^11 cm^-2. A tilted-field configuration is used to induce Landau level crossings and magnetic transitions between quantum Hall states with different spin polarizations. A large filling factor dependent effective electronic g-factor is determined by the coincidence method and cyclotron resonance measurements. From these measurements the change in exchange-correlation energy at the magnetic transition is deduced. These results demonstrate the impact of many-body effects in tilted-field magneto-transport of high-mobility 2DESs confined in In_0.75Ga_0.25As/In_0.75Al_0.25As quantum wells. The large tunability of electron density and effective g-factor, in addition, make this material system a promising candidate for the observation of a large variety of spin-related phenomena.Comment: 7 pages, 5 figure

Magnetic properties of GaMnAs single layers and GaInMnAs superlattices investigated at low temperature and high magnetic field

Magnetotransport properties of GaMnAs single layers and InGaMnAs/InGaAs superlattice structures were investigated at temperatures from 4 K to 300 K and magnetic fields up to 23 T to study the influence of carriers confinement through different structures. Both single layers and superlattice structures show paramagnetic-to-ferromagnetic phase transition. In GaMnAs/InGaAs superlattice beside the Curie temperature (Tc ~ 40 K), a new phase transition is observed close to 13 K.Comment: 8 pages, 5 figures, Proceedings of the XXXII International School on the Physics of Semiconducting Compounds, Jaszowiec 2003, Polan

Plateau insulator transition in graphene

The quantum Hall effect in a single-layer graphene sample is studied in strong magnetic fields up to 28 T. Our measurements reveal the existence of a metal- insulator transition from filling factor $\nu=-2$ to $\nu=0$. The value of the universal scaling exponent is found to be $\kappa=0.57$ in graphene and therefore in a truly two-dimensional system. This value of $\kappa$ is in agreement with the accepted universal value for the plateau-insulator transitions in standard quasi two-dimensional electron and hole gases.Comment: 10 pages, 5 figure

Quantum Hall states under conditions of vanishing Zeeman energy

We report on magneto-transport measurements of a two-dimensional electron gas confined in a Cd$_{0.997}$Mn$_{0.003}$Te quantum well structure under conditions of vanishing Zeeman energy. The electron Zeeman energy has been tuned via the $s-d$ exchange interaction in order to probe different quantum Hall states associated with metallic and insulating phases. We have observed that reducing Zeeman energy to zero does not necessary imply the disappearing of quantum Hall states, i.e. a closing of the spin gap. The spin gap value under vanishing Zeeman energy conditions is shown to be dependent on the filling factor. Numerical simulations support a qualitative description of the experimental data presented in terms of a crossing or an avoided-crossing of spin split Landau levels with same orbital quantum number $N$

Revealing large-scale homogeneity and trace impurity sensitivity of GaAs nanoscale membranes

III-V nanostructures have the potential to revolutionize optoelectronics and energy harvesting. For this to become a reality, critical issues such as reproducibility and sensitivity to defects should be resolved. By discussing the optical properties of MBE grown GaAs nanomembranes we highlight several features that bring them closer to large scale applications. Uncapped membranes exhibit a very high optical quality, expressed by extremely narrow neutral exciton emission, allowing the resolution of the more complex excitonic structure for the first time. Capping of the membranes with an AlGaAs shell results in a strong increase of emission intensity but also to a shift and broadening of the exciton peak. This is attributed to the existence of impurities in the shell, beyond MBE-grade quality, showing the high sensitivity of these structures to the presence of impurities. Finally, emission properties are identical at the sub-micron and sub-millimeter scale, demonstrating the potential of these structures for large scale applications.Comment: just accepted in Nano Letters, http://pubs.acs.org/doi/abs/10.1021/acs.nanolett.7b0025

Collective character of spin excitations in a system of Mn2+^{2+} spins coupled to a two-dimensional electron gas

We have studied the low energy spin excitations in n-type CdMnTe based dilute magnetic semiconductor quantum wells. For magnetic fields for which the energies for the excitation of free carriers and Mn spins are almost identical an anomalously large Knight shift is observed. Our findings suggests the existence of a magnetic field induced ferromagnetic order in these structures, which is in agreement with recent theoretical predictions [J. K{\"o}nig and A. H. MacDonald, submitted Phys. Rev. Lett. (2002)]Comment: 4 figure

Wastewater quality and unhygienic practices in Minna Abattoir, North Central Nigeria

Wastewater collected from Minna abattoir was analyzed for its microbiological and physicochemical properties using standard methods. The results revealed that the wastewater harboured aerobic heterotrophic bacteria (1.1 x 108 - 4.6 x 108cfu/ml), coliforms (6.3 x 105 – 3.9 x 106cfu/ml), Salmonella species (1.8 x 103 – 2.1 x 104cfu/ml), and fungi (1.4 x 104 – 4.0 x 104cfu/ml). The microorganisms were identified as species of Bacillus, Streptococcus, Escherichia, Klebsiella, Staphylococcus, Pseudomonas, Salmonella, Mucor, Aspergillus, and Penicillium. Escherichia coli was more consistently isolated and constituted 23.23% of the total bacterial isolates while Aspergillus flavus had the highest frequency of occurrence (28.86%) among the fungal isolates. Ova of parasites Taenia saginata, Ascaris sp., and hookworm as well as cysts of Giardia ovis were also detected in the wastewater. The wastewater had a pH of 6.03-7.60, suspended solids (8.76-3960 mg/L), ammonia (760-1140mg/L), COD (81555-8200mg/L), BOD (28-836mg/L) and sulphide (1315.2-2333.6mg/L). These values, with the exception of pH are higher than the permissible limits set by the Federal Ministry of Environment (Nigeria). The sanitary condition of Minna abattoir was poor due to indiscriminate dumping of solid wastes, discharge of untreated wastewater, and poor handling and transportation of meat to sales points. The abattoir lacked necessary gadgets for its operations. Besides, there was no potable water supply in the abattoir and therefore, it depended on commercial water vendors for its water supply. The unhygienic conditions of the abattoir and discharge of untreated wastewater into the environment posed serious threats to the environment and human health. There is the need to provide facilities in the abattoir and enlighten the users on the implications of unhygienic practices in the abattoir

Bostonia. Volume 15

Founded in 1900, Bostonia magazine is Boston University's main alumni publication, which covers alumni and student life, as well as university activities, events, and programs

Enhancement of the spin-gap in fully occupied two-dimensional Landau levels

Polarization-resolved magneto-luminescence, together with simultaneous magneto-transport measurements, have been performed on a two-dimensional electron gas (2DEG) confined in CdTe quantum well in order to determine the spin-splitting of fully occupied electronic Landau levels, as a function of the magnetic field (arbitrary Landau level filling factors) and temperature. The spin splitting, extracted from the energy separation of the \sigma+ and \sigma- transitions, is composed of the ordinary Zeeman term and a many-body contribution which is shown to be driven by the spin-polarization of the 2DEG. It is argued that both these contributions result in a simple, rigid shift of Landau level ladders with opposite spins.Comment: 4 pages, 3 figure