Papers by Christopher Stanley
2019 IEEE Frontiers in Education Conference (FIE)
This Research-to-Practice Work in Progress (WIP) investigates the format of student assessment qu... more This Research-to-Practice Work in Progress (WIP) investigates the format of student assessment questions. In particular, the focus is on the relationship between student performance on open-ended, constructed-response questions (CRQs) versus close-ended, multiple-choice-response questions (MCQs) in first-year introductory programming courses. We introduce a study to evaluate whether these different response formats return distinct or comparable results. In order to assess this, we compare and correlate student scores on each question type. Our focus is on assessments (exams and tests) in first-year classes. The paper investigates two first-year programming courses with a total of seven sections and approximately 180 combined students. The subject of the sequential set of courses is the procedural C programming language. Based on extant studies comparing student performance on MCQs to their performance on open-ended questions, we investigate whether MCQ scores predict CRQ scores. Preliminary results on the comparison between student performance on these two question formats are presented to assess whether MCQs produce similar results as CRQs, or whether MCQs yield unique contributions. Possible avenues for future work are also discussed.
physica status solidi (a), 2021
Understanding how heat flows across interfaces is vital to energy efficiency and thermal stabilit... more Understanding how heat flows across interfaces is vital to energy efficiency and thermal stability of many electrical devices. However, the thermal resistance caused by the interface between two materials, termed Kapitza resistance, remains poorly understood. To that end, several first‐principles molecular dynamic simulations and a detailed analysis of the phonon processes and associated transfer of heat at the interfaces of both c‐Si|a‐SiO2 and c‐Si|c‐Ge are presented. It is found that in both cases the interface properties are very important. In the case of c‐Si|a‐SiO2, it is found that interface modes cause inelastic phonon interactions and play a significant role in the total energy transferred. In the case of c‐Si|a‐SiO2, one is able to quantify this effect and find that there is a small set of interface modes which carry >10% of the heat, and decrease the ultimate thermal boundary resistance by 26.5%.
Materials Research Express, 2020
Specific heat at constant pressure (Cp) for diamond Si and wurtzite GaN is calculated using a nov... more Specific heat at constant pressure (Cp) for diamond Si and wurtzite GaN is calculated using a novel, first principles method based on density functional theory. This method, termed the Beyond Quasi-Harmonic method, completely takes into account all anharmonic vibrations–both from changes in volume and phonon interactions. Our calculated values for Si show excellent agreement with the generally accepted experimental values. For GaN, however, there is a large discrepancy between two sets of experiments. From an analysis of the available literature, and our own data we develop a set of recommended values.
Journal of Applied Physics, 2020
The electrical and structural properties of two levels (E90 and H180) in diluted n- and p-type Si... more The electrical and structural properties of two levels (E90 and H180) in diluted n- and p-type Si1 − xGex alloys (0 ≤ x ≤ 0.070) are investigated by high-resolution Laplace deep level transient spectroscopy measurements and first-principles calculations. By exploiting the presence of Ge atoms close to a substitutional C atom, we show that E90 and H180 belong to the same C–H pair (labeled CH1BC) with H in a bond-centered configuration (C—HBC—Si). The relative energies of the various configurations of the CH pair are calculated, and the complete vibrational spectra in the lowest-energy structures for each charge state are predicted.The electrical and structural properties of two levels (E90 and H180) in diluted n- and p-type Si1 − xGex alloys (0 ≤ x ≤ 0.070) are investigated by high-resolution Laplace deep level transient spectroscopy measurements and first-principles calculations. By exploiting the presence of Ge atoms close to a substitutional C atom, we show that E90 and H180 belong to the same C–H pair (labeled CH1BC) with H in a bond-centered configuration (C—HBC—Si). The relative energies of the various configurations of the CH pair are calculated, and the complete vibrational spectra in the lowest-energy structures for each charge state are predicted.
physica status solidi (a), 2018
Electronic devices generate unwanted heat. The removal of this heat often involves a layer of a h... more Electronic devices generate unwanted heat. The removal of this heat often involves a layer of a high-thermal-conductivity material X deposited at a strategic location on the Si chip. The layer is then cooled using a mechanical fan or more sophisticated means. The authors present here the results of abinitio molecular-dynamics simulations which examine the thermal interactions between heat generated in Si and three materials X: Ge, C, and Si itself. The geometry is a very thin wire of atoms X located within a Si slab. Heat is periodically removed from the wire as if it were connected to a heat sink. The wire then absorbs heat from the warmer Si slab, thus cooling it. The rate at which the Si temperature drops depends on how efficiently heat crosses the Si|X boundary and then is absorbed by the wire. The interactions involve the coupling of Si-Si vibrational modes with SiX (interface) and then X-X (layer) modes. The worst performance occurs for X5 5C as lower-frequency Si-Si modes must decay into higher-frequency Si-C then C─C modes. This involves slow two-(or more) phonon processes. The fastest cooling rate occurs when the wire is made of Si itself: there is no Si|Si interface and the vibrational interactions involve only Si bulk modes which couple to each other resonantly. The Ge wire performs quite well, as the Si-Si modes easily decay into lower-frequency Si-Ge and Ge-Ge modes.
physica status solidi (a), 2018
The interactions between heat flow and an oxide layer in Si are studied within two temperature wi... more The interactions between heat flow and an oxide layer in Si are studied within two temperature windows using non-equilibrium ab initio moleculardynamics (MD). The model system is a H-saturated Si nanowire containing an amorphous SiO x layer. The nanowire is in a large 1-D periodic box which prevents thermal contamination between image nanowires. The results show that the oxide acts as barrier to heat flow and substantially increases the time required for the system to reach thermal equilibrium. This effect is caused by the higher-frequency vibrational modes in the oxide relative to Si, and is unrelated to the low thermal conductivity of SiO x. A new firstprinciples method to calculate the Kapitza resistance of the interface directly from the MD data is proposed.
physica status solidi (a), 2017
Oxide layers are ubiquitous in Si technology including nanostructures. How such layers interact w... more Oxide layers are ubiquitous in Si technology including nanostructures. How such layers interact with heat flow is not well understood. In this contribution, we present the preliminary results of ab initio molecular-dynamic simulations of such interactions. We show that oxide layers reflect (part of) the incoming heat, which results in the accumulation of energy on the warmer side of the layer for longer times than without the presence of the oxide. The results are consistent with earlier predictions that phonon-defect interactions are determined by the vibrational properties of the defect (here, the oxide layer).
Biophysical Journal, 2015
Cardiac cells express more than one isoform of the Na, K-ATPase (NKA), the heteromeric enzyme tha... more Cardiac cells express more than one isoform of the Na, K-ATPase (NKA), the heteromeric enzyme that creates the Na þ and K þ gradients across the plasmalemma. Cardiac isozymes contain one catalytic a-subunit isoform (a1, a2, or a3) associated with an auxiliary b-subunit isoform (b1 or b2). Past studies using biochemical approaches have revealed minor kinetic differences between isozymes formed by different a-b isoform combinations; these results make it difficult to understand the physiological requirement for multiple isoforms. In intact cells, however, NKA enzymes operate in a more complex environment, which includes a substantial transmembrane potential. We evaluated the voltage dependence of human cardiac NKA isozymes expressed in Xenopus oocytes, and of native NKA isozymes in rat ventricular myocytes, using normal mammalian physiological concentrations of Na þ o and K þ o. We demonstrate that although a1 and a3 pumps are functional at all physiologically relevant voltages, a2b1 pumps and a2b2 pumps are inhibited by~75% and~95%, respectively, at resting membrane potentials, and only activate appreciably upon depolarization. Furthermore, phospholemman (FXYD1) inhibits pump function without significantly altering the pump's voltage dependence. Our observations provide a simple explanation for the physiological relevance of the a2 subunit (~20% of total a subunits in rat ventricle): they act as a reserve and are recruited into action for extra pumping during the long-lasting cardiac action potential, where most of the Na þ entry occurs. This strong voltage dependence of a2 pumps also helps explain how cardiotonic steroids, which block NKA pumps, can be a beneficial treatment for heart failure: by only inhibiting the a2 pumps, they selectively reduce NKA activity during the cardiac action potential, leading to an increase in systolic Ca 2þ , due to reduced extrusion through the Na/Ca exchanger, without affecting resting Na þ and Ca 2þ concentrations.
Journal of Applied Physics, 2015
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Papers by Christopher Stanley