Search

Micro-focused X-ray beams produced by third generation synchrotron sources offer new perspective of studying strains and processes at nanoscale. Atomic force microscope setup combined with a micro-focused synchrotron beam allows precise positioning and nanomanipulation of nanostructures under illumination. In this paper, we report on integration of a portable commercial atomic force microscope setup into a hard X-ray synchrotron beamline. Details of design, sample alignment procedure and performance of the setup are presented., Comment: To be submitted to Review of Scientific Instruments

Taking into account the available experimental results, we model the electronic properties and current-voltage characteristics of a ferromagnet-semiconductor junction. The Fe/GaAs interface is considered as a Fe/(i-GaAs)/n+-GaAs/n-GaAs multilayer structure with the Schottky barrier. We also calculate numerically the current-voltage characteristics of a double-Schottky-barrier structure Fe/GaAs/Fe, which are in agreement with available experimental data. For this structure, we have estimated the spin current in the GaAs layer, which characterizes spin injection from the ferromagnet to the semiconductor., Comment: 3 pages, 5 figures, presented on The European Conference Physics of Magnetism 2014 (PM'14), June 23-27, 2014 Pozna\'n, POLAND

We study the low-temperature thermopower of micron sized, free-standing membranes containing a two-dimensional electron system. Suspended membranes of 320 nm thickness including a high electron mobility structure in Hall bar geometry of 34 {\mu}m length are prepared from GaAs/AlGaAs heterostructures grown by molecular beam epitaxy. Joule heating on the central region of the membrane generates a thermal gradient with respect to the suspension points where the membrane is attached to cold reservoirs. Temperature measurements on the membrane reveal strong thermal gradients due to the low thermal conductivity. We measure the zero-field thermopower and find that the phonon-drag contribution is suppressed at low temperatures up to 7 K., Comment: 5 pages

We present thermopower measurements on a high electron mobility two-dimensional electron gas (2DEG) in a thin suspended membrane.We show that the small dimension of the membrane substantially reduces the thermal conductivity compared to bulk material so that it is possible to establish a strong thermal gradient along the 2DEG even at a distance of few micrometers. We find that the zero-field thermopower is significantly affected by the micro patterning. In contrast to 2DEGs incorporated in a bulk material, the diffusion contribution to the thermopower stays dominant up to a temperature of 7 K until the phonon-drag becomes strong and governs the run of the thermopower. We also find that the coupling between electrons and phonons in the phonon-drag regime is due to screened deformation potentials, in contrast to piezoelectric coupling found with bulk phonons., Comment: 7 pages

We study the diffraction of Damon-Eshbach-type spin waves incident on a one-dimensional grating realized by micro slits in a thin permalloy film. By means of time-resolved scanning Kerr microscopy we observe unique diffraction patterns behind the grating which exhibit replications of the spin-wave field at the slits. We show that these spin-wave images, with details finer than the wavelength of the incident Damon-Eshbach spin wavelength, arise from the strongly anisotropic spin wave dispersion., Comment: 5 pages, 3 figures

Low temperature magnetotransport measurements and far infrared transmission spectroscopy are reported in molecular beam epitaxial grown two-dimensional hole systems confined in strained InAs quantum wells with magnetic impurities in the channel. The interactions of the free holes spin with the magnetic moment of 5/2 provided by manganese features intriguing localization phenomena and anomalies in the Hall and the quantum Hall resistance. In magnetic field dependent far infrared spectroscopy measurements well pronounced cyclotron resonance and an additional resonance are found that indicates an anticrossing with the cyclotron resonance.

We study the Coulomb pseudogap for tunneling into the two-dimensional electron system of high-mobility (Al,Ga)As/GaAs heterojunctions subjected to a quantizing magnetic field at filling factor $\nu \leq 1$. Tunnel current-voltage characteristics show that for the double maximum observed in the tunnel resistance at $\nu \approx 1$ the pseudogap is linear in energy with a slope that depends on filling factor, magnetic field, and temperature. We give a qualitative account of the filling factor dependence of the pseudogap slope and we confirm the recently reported appearance of another relaxation time for tunneling at $\nu \approx 1$. For the tunnel resistance peaks at $\nu=1/3$ and 2/3 a completely different behaviour of the current-voltage curves is found and interpreted as manifestation of the fractional gap., Comment: 7 pages, 9 figures included. Accepted to PRB

We demonstrate the localisation of quantum well excitons in a periodic array of linear traps using photoluminescence experiments. The excitonic traps are induced by applying spatially alternating external voltages via interdigitated metal gates. The localisation originates from the periodical modulation of the strength of the quantum-confined Stark effect in the plane of the quantum well. In our experiments, the trap depth is easily tuned by the voltages applied to the interdigitated gates. Furthermore, we find that a perpendicular magnetic field reduces the exciton diffusion length. In short-period lateral superlattices, we observe a magnetic-field-induced stabilisation of excitons in the presence of strong in-plane electric fields., Comment: including 11 figures

The Basic Opportunity Grants symbolize one approach to the financing of higher education. In this paper, the formula of the grant is tested against the theory and practice of negative income taxation. A key concept in both Basic Opportunity Grants and in other types of income taxation is that of the family unit whose income must be counted in calculating benefits. This concept is critical to the success or failure of the Basic Opportunity Grant in achieving the aims that have been set for it. The conclusion is that college students cannot be held in dependent status in many cases and that this means that some of the arguments for the Basic Opportunity Grants are questionable. (Author)

The extent to which student financial aid has changed the composition of young people attending and planning to attend college is considered. The income redistribution policies of the 1960s and 1970s included rapid expansion of federal financial aid programs and reflected a shift from the goal of promoting economic efficiency and faster economic growth to the goal of providing greater equity. Data from the annual Current Population Surveys show that little or no change occurred in the ratio of college enrollment rates for youth from families with income below the median relative to youth from families with above-median incomes. Additionally, data from the National Longitudinal Study of the High School Class of 1972 show no real changes in college attendance plans of high school seniors, after controlling for socioeconomic status and ability. It is concluded that student financial aid programs substituted public for private funds, thus reducing the burden of educational costs to college students and their parents. They did relatively little, however, to induce more youth from lower income families to attend college and they impeded economic growth by sacrificing economic efficiency. The major student financial aid programs, their rationale, and expected effects are reviewed. (SW)

Reasons for the increasing numbers of independent college students are considered, with attention to the provision for independent students in the Basic Educational Opportunity Grant (Pell) program. The availability of grants to independent students may have two consequences: college students under age 22 with access to parental resources might take steps to gain independent status; and individuals over 25 who have not attended college might enter higher education. In classifying independent students, California examines the students' situation in the current and 3 preceding years, in contrast to information on only the current and prior year that is used by the federal government to determine eligibility for Pell grants. The extent that enrolled students shift from dependent to independent status is assessed, based on data from the California Survey of Student Expenditures and Resources for 1979-1983, and longitudinal data for freshmen entering the University of Wisconsin System in fall 1979. Based on the limited available evidence, it is concluded that the incentives for students under age 25 to change their status are not that substantial. Reasons for the growth of students over age 25 need to be explored. (SW)

Definitions of manageable college student debts are discussed and one definition is applied to California data, using the Student Expenses and Resources Survey. Definitions proposed by Daniere, Hartman, and Horch define manageable debt levels in terms of future income. A comparison of the three proposals shows the number of years of repayment required for accumulated debts of different amounts. For the analysis of California data, a manageable debt limit is defined as the maximum debt that can be repaid within the standard 10-year repayment period. Any debt in excess of $14,000 is designated as excessive, although the present maximum borrowing allowed under the Guaranteed Student Loan (GSL) program is $12,500. The analysis, which is restricted to seniors at four-year public and private colleges, considers average debt size, the distribution of debt size, borrowing patterns, and graduate and professional education debt. The data indicate that at most 3% of seniors with debt might on average experience repayment problems. The situation is most serious for independent students attending private colleges; perhaps as many as 9% of this group have unmanageable debts. However, slightly over half of students borrow and the average accumulated debt is well under half the GSL maximum. (SW)