Samuel Graham

Bio: Samuel Graham is an academic researcher from Georgia Institute of Technology. The author has contributed to research in topics: Thermal conductivity & Thermal resistance. The author has an hindex of 48, co-authored 347 publications receiving 9774 citations. Previous affiliations of Samuel Graham include Merck & Co. & United States Military Academy.

Hiv protease inhibitors having polyether substituents

Abstract: Polyether derivatives of the form, A-G-B-B-J wherein G is a dipeptide isostere substituted with a polyether, B an amino acid or analog thereof, and J a small terminal group are described. These compounds are useful in the inhibition of DIV protease, the prevention or treatment of infection by HIV and the treatment of AIDS, either as compounds, pharmaceutically acceptable salts, pharmaceutical composition ingredients, whether or not in combination with other antivirals, immunomodulators, antibiotics or vaccines. Methods of treating AIDS and methods of preventing or treating infection by HIV are also described.

Optimizing Crack Onset Strain for Silicon Nitride/Fluoropolymer Nanolaminate Barrier Films

Abstract: Nanolaminates using alternating inorganic and organic layers have the potential to provide ultrabarrier with high resistance to gas permeation while also changing the crack onset strain (COS) to im...

Temperature- and Doping-Dependent Anisotropic Stationary Electron Velocity in Wurtzite GaN

Abstract: The influences of temperature, dopant density, free-carrier density, and field orientation on the electron drift mobility and velocity-field relationship in wurtzite c -axis GaN are quantified by means of theoretical investigation. Electron velocity perpendicular to the growth plane is uniformly lower than that parallel to the growth plane for field strengths below 500 kV/cm, although anisotropy within the basal plane itself is found to be insignificant. The calculated low-field electron mobility is demonstrated to be consistent with recent Hall measurements over a range of dopant densities. Low-field mobility is enhanced under the influence of free-carrier densities above the background doping due to both increased screening of ionized impurities and a reduction in A1-LO phonon lifetime through the plasmon-phonon interaction.

Optical studies of MOCVD-grown GaN-based ferromagnetic semiconductor epilayers and devices

Abstract: Ga1–xMnxN epilayers and p-i-n device structures have been grown by metalorganic chemical vapor deposition. Optical studies were found to investigate the role of Mn concentration and the role of different co-dopants to elucidate the origin of the room temperature ferromagnetism in Ga1–xMnxN epilayers. Increasing Mn concentration was found to significantly affect long-range lattice ordering. This observation was supported by the existence of a disorder-induced Raman mode at 300 cm–1 and local vibrational modes (LVMs) at 669 cm–1 that has been attributed to the formation of self-compensating nitrogen. The E1(TO) phonon frequency was found to linearly increase with Mn composition, which is expressed by (558 + 2.7x) cm–1. The intensity and the linewidth of an absorption band near 1.5 eV was found to increase with Mn concentration, and is assigned to the e to t2 transition in the split 5T2 band of Mn3+. No absorption was detected in Si co-doped Ga1–xMnxN. In this case, a significantly increased EPR signal of Mn2+ ions confirmed the trapping of electrons provided by the shallow Si donor and reduced the available states for ferromagnetic exchange. A change in the measured magnetization is observed under ultraviolet illumination, though the nature of this phenomenon is still not understood. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Pseudomorphic growth of InAs on misoriented GaAs for extending quantum cascade laser wavelength

Abstract: The authors have studied the impact of epilayer strain on the deposition of InAs/GaAs on (100) and (111)B with 2° offset toward⟨2-1-1⟩ surfaces. Consequences of a 7% lattice mismatch between these orientations in the form of three-dimensional growth are less apparent for (111)B with 2° offset toward⟨2-1-1⟩ surfaces compared to (100). By exploring a range of molecular beam epitaxy process parameters for InAs/GaAs growth and utilizing scanning electron microscopy, atomic force microscopy, and Raman spectroscopy to evaluate the quality of these strained layers, the authors develop empirical models that describe the influence of the process conditions in regards to surface roughness with >92% accuracy. The smoothest InAs/GaAs samples demonstrated average surface roughness of 0.08 nm for 10 μm2 areas, albeit at very low deposition rates. The authors have found the most important process conditions to be substrate temperature and deposition rate, leading us to believe that controlling diffusion length may be the key to reducing defects in severely strained structures. InGaAs/AlGaAs quantum cascade laser structures were also produced on (111)B with 2° offset toward⟨2-1-1⟩ to take advantage of the piezoelectric effect, and the modified laser transitions due to these effects were observed.

I and i

Abstract: There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality. Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

Fast parallel algorithms for short-range molecular dynamics

Abstract: Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Interface engineering of highly efficient perovskite solar cells

Abstract: Advancing perovskite solar cell technologies toward their theoretical power conversion efficiency (PCE) requires delicate control over the carrier dynamics throughout the entire device. By controlling the formation of the perovskite layer and careful choices of other materials, we suppressed carrier recombination in the absorber, facilitated carrier injection into the carrier transport layers, and maintained good carrier extraction at the electrodes. When measured via reverse bias scan, cell PCE is typically boosted to 16.6% on average, with the highest efficiency of ~19.3% in a planar geometry without antireflective coating. The fabrication of our perovskite solar cells was conducted in air and from solution at low temperatures, which should simplify manufacturing of large-area perovskite devices that are inexpensive and perform at high levels.

An electron acceptor challenging fullerenes for efficient polymer solar cells.

Abstract: A novel non-fullerene electron acceptor (ITIC) that overcomes some of the shortcomings of fullerene acceptors, for example, weak absorption in the visible spectral region and limited energy-level variability, is designed and synthesized. Fullerene-free polymer solar cells (PSCs) based on the ITIC acceptor are demonstrated to exhibit power conversion effi ciencies of up to 6.8%, a record for fullerene-free PSCs.