Isotopic-mass dependence of the A, B, and C excitonic band gaps in ZnO at low temperatures

Low temperature wavelength-modulated reflectivity measurements of isotopically engineered ZnO samples have yielded the dependence of their A, B, and C excitonic band gaps on the isotopic masses of Zn and O. The observed dependence is analyzed in terms of the band gap renormalization by zero-point vibrations via electron-phonon interaction and the volume dependence on isotopic mass. A simplified, two-oscillator model, employed in the analysis, yields zero-point renormalizations of the band gaps, -154 +/- 14 meV (A), -145 +/- 12 meV (B), and -169 +/- 14 meV (C), for ZnO with natural isotopic composition

Heat Capacity of PbS: Isotope Effects

In recent years, the availability of highly pure stable isotopes has made possible the investigation of the dependence of the physical properties of crystals, in particular semiconductors, on their isotopic composition. Following the investigation of the specific heat ($C_p$, $C_v$) of monatomic crystals such as diamond, silicon, and germanium, similar investigations have been undertaken for the tetrahedral diatomic systems ZnO and GaN (wurtzite structure), for which the effect of the mass of the cation differs from that of the anion. In this article we present measurements for a semiconductor with rock salt structure, namely lead sulfide. Because of the large difference in the atomic mass of both constituents ($M_{\rm Pb}$= 207.21 and ($M_{\rm S}$=32.06 a.m.u., for the natural isotopic abundance) the effects of varying the cation and that of the anion mass are very different for this canonical semiconductor. We compare the measured temperature dependence of $C_p \approx C_v$, and the corresponding derivatives with respect to ($M_{\rm Pb}$ and $M_{\rm S}$), with \textit{\textit{ab initio}} calculations based on the lattice dynamics obtained from the local density approximation (LDA) electronic band structure. Quantitative deviations between theory and experiment are attributed to the absence of spin-orbit interaction in the ABINIT program used for the electronic band structure calculations.Comment: 17 pages including 10 Fig

Admitted Student Publications

In 1999, a mid-sized, public doctoral-intensive university in the Midwest initiated a publication series to help admitted students and their family members better understand the enrollment process and increase the yield of admitted students for the institution. Through focus group research, admitted students and family members provided feedback on the effectiveness of the publications specifically designed for admitted students. This case study presents a model for developing a publications series for admitted students and then using focus group research to refine those publications for greatest effectiveness

Ignition investigations of a novel hypergolic ionic liquid with hydrogen peroxide in drop tests

Conventional hypergolic propellants are highly toxic. Therefore, research is conducted in order to find suitable candidates as a replacement for these kinds of propellants. At DLR a promising propellant combination comprised of highly concentrated hydrogen peroxide and an ionic liquid was identified. The used propellant combination showed an ignition delay time in drop tests of around 30 ms. These drop tests were conducted in a drop test setup keeping the test parameters constant. In the present study different external factors, which could have an impact on the ignition delay time were investigated: drop height, propellant amount and fuel or oxidizer as pool component. For fuel as pool component no impact on the ignition delay time was observed when varying the drop heights or propellant amounts. A significant difference was observed between fuel or oxidizer pools. Here, the ignition delay time for oxidizer pools is twice as long as tests with in the fuel poo

Electronic, vibrational, and thermodynamic properties of ZnS (zincblende and rocksalt structure)

We have measured the specific heat of zincblende ZnS for several isotopic compositions and over a broad temperature range (3 to 1100 K). We have compared these results with calculations based on ab initio electronic band structures, performed using both LDA and GGA exchange- correlation functionals. We have compared the lattice dynamics obtained in this manner with experimental data and have calculated the one-phonon and two-phonon densities of states. We have also calculated mode Grueneisen parameters at a number of high symmetry points of the Brillouin zone. The electronic part of our calculations has been used to investigate the effect of the 3d core electrons of zinc on the spin-orbit splitting of the top valence bands. The effect of these core electrons on the band structure of the rock salt modification of ZnS is also discussed.Comment: 33pages, 16 Figures, submitted to Phys. Rev.

Assessing Student Mindset, Interest, Participation, and Rapport in the Post-Pandemic Public Speaking Classroom: Effects of Modality Change and Communication Growth Mindset

The COVID-19 pandemic created an exigency for educators to reevaluate their approaches to the classroom with one major dimension being course modality. This study uses the Instructional Beliefs Model to examine the impacts of course modality (i.e., hybrid versus face-to-face formats) and students’ communication growth mindset on student engagement in the foundational public speaking course. Consistent with pre-COVID-19 findings, the results indicated that modality does not significantly impact student engagement, with one exception: higher cognitive interest scores were reported among students in the hybrid modality. Communication growth mindset associated positively with all student engagement variables examined: student interest–emotional, student interest–cognitive, participation, and class rapport. The findings offer tentative optimism about the promise of blended public speaking course modalities, and evidence for the necessity of mindset intervention to maximize student success

Discovery of a novel simian pegivirus in common marmosets (Callithrix jacchus) with lymphocytic enterocolitis

From 2010 to 2015, 73 common marmosets