EMSL: John E Jaffe's Publications

Jordan DV, AS Renholds, JE Jaffe, KK Anderson, LR Corrales, and AJ Peurrung. 2008. "Simple classical model for Fano statistics in radiation detectors." Nuclear Instruments and Methods in Physics Research. Section A, Accelerators, Spectrometers, Detectors and Associated Equipment 585(3):146-154. doi:10.1016/j.nima.2007.009 Abstract A simple classical model that captures the essential statistics of energy partitioning processes involved in the creation of information carriers (ICs) in radiation detectors is presented. The model pictures IC formation from a fixed amount of deposited energy in terms of the statistically analogous process of successively sampling water from a large, finite-volume container (“bathtub”) with a small dipping implement (“shot glass”). The model exhibits sub-Poisson variance in the distribution of the number of ICs generated (the “Fano e_ect”). Elementary statistical analysis of the model clarifies the role of energy conservation in producing the Fano e_ect and yields Fano’s prescription for relating the IC number distribution to the mean and variance of the underlying IC energy distribution. The connection between the model and energy partitioning in semiconductor radiation detectors is illustrated, and the implications of this simple picture for guiding or constraining more detailed, “microscopic” physical models of detector material response to ionizing radiation are discussed.

Jaffe JE, RA Bachorz, and MS Gutowski. 2007. "BAnd offset and magnetic property engineering for epitaxial interfaces: a Monolayer of M2O3 (M=Al, Ga, Sc, Ti, Ni) at the alpha-Fe203/alpha-Cr203 (0001) Interface." Physical Review. B, Condensed Matter and Materials Physics 75(20):205323. doi:10.1103/PhysRevB.75.205323 Abstract We have used density functional theory with the gradient corrected exchange-correlation functional PW91 to study the effect of an interfactant layer, where Fe and Cr are replaced by a different metal, on electronic and magnetic properties of an epitaxial interface between -Fe2O3 and -Cr2O3 in the hexagonal (0001) basal plane. We studied a monolayer of M2O3 (M=Al, Ga, Sc, Ti, Ni) sandwiched with 5 layers of chromia and five layers of hematite through epitaxial interfaces of two types, termed “oxygen divided” or “split metal.” We found that both the magnetic and electronic properties of the superlattice are modified by the interfactant monolayer. For the split metal interface, which is favored through the growth pattern of chromia and hematite, the band offset can be changed from 0.62 eV (no interfactant) up to 0.90 eV with the Sc2O3 interfactant, and down to –0.51 eV (i.e. the a-Fe2O3/a-Cr2O3 heterojunction changes from Type II to Type I) with the Ti2O3 interfactant, due to a massive interfacial charge transfer. The band gap of the system as a whole remains open for the interfactant monolayers based on Al, Ga, and Sc, but it closes for Ti. For Ni, the split-metal interface has a negative band offset and a small band gap. Thus, nanoscale engineering through layer-by-layer growth will strongly affect the macroscopic properties of this system.

Jaffe JE. 2007. "Energy and Length Scales in Scintillator Nonproportionality." Nuclear Instruments and Methods in Physics Research. Section A, Accelerators, Spectrometers, Detectors and Associated Equipment 580(3):1378-1382. doi:10.1016/j.nima.2007.07.059 Abstract The causes of energy nonlinearity (nonproportionality) in gamma-ray spectroscopy of scintillators have not been quantitatively explained to date, but are believed to involve the spatial density of thermalized electrons and holes, their transport and interactions, and competition between radiative and nonradiative recombination. Here, we relate electron-hole density to the initial ray’s energy through expressions for the stopping power and distance, and then attempt to fit some “plateau-type” (e.g. BaF2, CdWO4 etc.) light curves via competing radiative and nonradiative processes obeying different energy power laws. We find that reasonable power laws and the assumption of a uniform energy density in the excited region lead to a fairly good agreement with many experimental curves. We then show that a simple approximation for averaging over regions of different excitation density leads to improved agreement with experiment, at the expense of making it more difficult to uncover the underlying kinetics from the experimental data. These considerations should carry over to more sophisticated models or simulations of scintillation processes.

Jaffe JE, DV Jordan, and AJ Peurrung. 2007. "Energy Nonlinearity in Radiation Detection Materials: Causes and Consequences." Nuclear Instruments and Methods in Physics Research. Section A, Accelerators, Spectrometers, Detectors and Associated Equipment 570(1):72-83. doi:10.1016/j.nima.2006.09.097 Abstract The phenomenology and present theoretical understanding of energy nonlinearity (nonproportionality) in radiation detection materials is reviewed, with emphasis on gamma-ray spectroscopy. Semiconductor detectors are extremely linear, while scintillators display varying degrees and patterns of nonlinearity, and gas detectors show a characteristic form of nonproportionality associated with core levels. The relation between nonlinear response (to both primary particles and secondary electrons) and spectrometer resolution is also discussed. We review the qualitative ideas about the origin of nonlinearity in scintillators that have been proposed to date, with emphasis on transport and recombination of information carriers. Recent computational and experimental work on the basic physics of scintillators is leading towards a better understanding of energy nonlinearity and should result in new, more linear scintillator materials in the near future.

Jaffe JE. 2006. "Computational Study of Ge and Sn Doping of CdTe." Journal of Applied Physics 99(3):033704. doi:10.1063/1.2168237 Abstract The formation and ionization energies of substitutional Ge and Sn dopants in CdTe are calculated in supercell model within the local density approximation (LDA) to density functional theory. Doping on both the Cd and Te sublattices are considered, but the formation energy of both defects is predicted to be much lower on the Cd site under most growth conditions. Ge and Sn on the Cd sites are predicted to be deep donors and hole traps with defect ionization levels near the midgap, with Ge slightly lower than Sn, in good agreement with experiments. Ge and Sn on the Te sites are predicted to be shallow acceptors.

Azad S, OA Marina, CM Wang, LV Saraf, V Shutthanandan, DE McCready, A El-Azab, JE Jaffe, MH Engelhard, CHF Peden, and S Thevuthasan. 2005. "Nanoscale Effects on Ion Conductance of Layer-by-Layer Structures of Gadolinia-doped Ceria and Zirconia." Applied Physics Letters 86(13):131906-131909. Abstract Layer-by-layer structures of gadolinia-doped ceria and zirconia have been synthesized on Al2O3(0001) using oxygen plasma-assisted molecular beam epitaxy. Oxygen ion conductivity greatly increased with an increasing number of layers compared to bulk polycrystalline yttria-stabilized zirconia and gadolinia doped ceria electrolytes. The conductivity enhancement in this layered electrolyte is interesting, yet the exact cause for the enhancement remains unknown. For example, the space charge effects that are responsible for analogous conductivity increases in undoped layered halides are suppressed by the much shorter Debye screening length in layered oxides. Therefore, it appears that a combination of lattice strain and extended defects due to lattice mismatch between the heterogeneous structures may contribute to the enhancement of oxygen ionic conductivity in this layered oxide system.

Jaffe JE, RA Bachorz, and MS Gutowski. 2005. "Low-temperature Polymorphs of ZrO2 and HfO2. A Density Functional Theory Study ." Physical Review. B, Condensed Matter and Materials Physics 72(14):144107. Abstract We present density functional calculations of the total energies and equations of state of the monoclinic, tetragonal, cubic, orthorhombic-I (Pbca) and orthorhombic-II (cotunnite)-structure phases of zirconia and hafnia in the local density (LDA) and generalized-gradient (GGA) approximations. The accuracy of the LDA approximation is not sufficient and GGA corrections are critical to obtain low-temperature phase transitions under pressure that are consistent with experiment, i.e., (monoclinic‡ orthorhombic-I ‡ cotunnite). The GGA values of the bulk modulus of the cotunnite phase were found to be 251 and 259 GPa for ZrO2 and HfO2, respectively. We developed a new population analysis scheme in which atomic radii are adapted to the actual charge distribution in the material. The results indicate that the effective atomic radius of Hf is smaller than that of Zr, which is a drastic manifestation of the relativistic lanthanide contraction. The population analysis results demonstrate that ionicity: (i) increases from the monoclinic to the cotunnite phase, and (ii) is larger for HfO2 than for ZrO2. This variable ionicity may be the reason why LDA fails to describe the relative stability of different polymorphs. The bandgap and heat of formation are also larger for monoclinic HfO2 than for ZrO2 by 0.6 eV and 0.60 eV/formula unit, respectively. The tetragonal phase, which often exists as a metastable phase at ambient conditions, has a bandgap larger than the monoclinic phase by 0.35 and 0.65 eV for ZrO2 and HfO2, respectively.

Jaffe JE, TC Droubay, and SA Chambers. 2005. "Oxygen Vacancies and Ferromagnetism in CoxTi₁–xO₂–x–y." Journal of Applied Physics 97(7):073908 (6 p.). Abstract Abstract: Cobalt-doped titanium dioxide, or CTO, has emerged in the past two years as a semiconducting, transparent, room-temperature ferromagnet. Very recently it has been shown that the magnetism in CTO often originates in surface nanoparticles or Co-rich regions that have a much-enhanced substitutional Co content up to 40% of Ti sites, so that magnetic CTO is not a true dilute magnetic semiconductor (DMS), but rather a fairly high-density spin system. In this work we describe a computational study of Co-rich CTO using the Generalized Gradient Approximation (GGA) to density functional theory (DFT) within the supercell model. Our total energy calculations show a strong tendency for Co-atom clustering or segregation on Ti sites. There is also a strong tendency for the oxygen vacancies to form complexes with the Co atoms. In addition, we find that the oxygen stoichiometry plays an essential role in determining the system’s magnetic order. The largest ordered moments require at least enough oxygen vacancies to put all of the Co atoms in the +2 charge state, as they indeed appear to be experimentally, so that the conventional DMS mechanism could only apply via n-type carriers. We find a small but not negligible spin density associated with Ti atoms near the vacancy sites, suggesting an F-center-mediated interaction between the much larger Co moments. We also present experimental data showing that the ferromagnetic remanence and coercive field increase with the n-type conductivity.

Jaffe JE, M Dupuis, and MS Gutowski. 2004. "First-principles study of noncommutative band offsets at [alpha]-Cr2O3/[alpha]-Fe2O3(0001) interfaces." Physical Review. B, Condensed Matter and Materials Physics 69(20):205106. Abstract Using first-principles density functional theory, we have modeled the atomic, electronic and magnetic structure of epitaxial interfaces between alpha-hematite and alpha-chromia (corundum structure) in the hexagonal (0001) basal plane. Our model was a superlattice with a period of about 27.5Å, corresponding to the shortest-period superlattice considered in a recent series of experiments (Chambers et al., Phys. Rev. B 61, 13223 (2000)). Two different epitaxial interface structures were studied: (i) an oxygen plane separating an Fe double layer from a Cr double layer, or (ii) a metal double layer split between Fe and Cr. We found that these two structures are close in total energy but have distinct spin structure and different valence band offsets (chromia above hematite by 0.4 and 0.6 eV for (i) and (ii) respectively), possibly explaining the experimental non-commutative band offset seen in this system (0.3±0.1 eV for hematite grown atop chromia, and 0.7±0.1 eV for the reverse).

Alfonso DR, JE Jaffe, AC Hess, and MS Gutowski. 2003. "Formation of the c(1X1) Cu Monolayer on CaO(100): A Theoretical Study." Physical Review. B, Condensed Matter 6815(15):, doi:10.1103/PhysRevB.68.155411 Abstract The Cu overlayer adsorbed on CaO(100) was studied at the density functional level of theory. Following an experimental suggestion, we examined a complete c(1X1) structure with copper atoms adsorbed on every hollow site of CaO(100). The binding energy/atom for this structure is 3.19 eV. However, another c(1X1) structure proposed by us, with copper atoms on every surface Ca and O atoms, displayed a larger binding energy of 3.37 eV. When the copper coverage is reduced by half, the preferred adsorption site fo rhte Cu overlayer is above the O site and the dominant mechanism for the metal adhesion is rehybridization of the metal 3d and surface O 2p bands. Our results suggest a delicate balance between the formation of a monolayer film and 3D particles on the substrate.