John Jaffe - Academia.edu (original) (raw)

Papers by John Jaffe

Phys Rev B Condens Matter, Jun 1, 1981

By solving the Eliashberg equations for the gap function, the superconducting transition temperat... more By solving the Eliashberg equations for the gap function, the superconducting transition temperature of a proposed low-temperature liquid state of metallic hydrogen is found to be comparable to that obtained, with similar approximations, for the solid. This indicates that metallic hydrogen may be a superconducting liquid in the density range 1.6>=rs>=1.3, where the melting point is expected to be fairly low.

APS Meeting Abstracts, 2005

Phys Rev B, 1994

Periodic Hartree-Fock total-energy calculations on two-dimensional slabs have been used to study ... more Periodic Hartree-Fock total-energy calculations on two-dimensional slabs have been used to study the symmetry-conserving relaxation of the nonpolar (101¯0) surface of ZnO. We find that it is energetically favorable for the Zn-O surface dimers to tilt slightly (by 2.3 °) and move downwards towards the slab, and for the dimer bond to shorten significantly. Our results agree fairly well with those of a recent density-functional calculation, but disagree with empirical tight-binding theory which predicts surface bonds to shorten only slightly while the surface dimers undergo a large tilt (18 °). The available experimental data lies between the ab initio and tight-binding results with large error bars. We have tested the effects of several refinements of our Hartree-Fock calculation, including improvements of the orbital basis set and precision tolerances, the use of thicker slabs in approximating the semi-infinite crystal, and post-self-consistent-field density-functional correlation corrections to the total energy. None of these refinements significantly changed our results. We discuss possible reasons for the disagreement between our results and those of tight-binding theory.

The Journal of Physical Chemistry a, Mar 12, 1999

Phys Rev B, 2005

Low-temperature polymorphs of ZrO 2 and HfO 2 : A density-functional theory study. John E. Jaffe ... more Low-temperature polymorphs of ZrO 2 and HfO 2 : A density-functional theory study. John E. Jaffe 1 , Rafał A. Bachorz 1 , and Maciej Gutowski 1,2 * 1 Chemical Sciences Division, Fundamental Sciences Directorate, Pacific Northwest ...

The Journal of Chemical Physics, 1996

Gaussian basis density functional theory for systems periodic in two or three dimensions: Energy ... more Gaussian basis density functional theory for systems periodic in two or three dimensions: Energy and forces. [The Journal of Chemical Physics 105, 10983 (1996)]. John E. Jaffe, Anthony C. Hess. Abstract. We describe a formulation ...

Ab initio study of a CO monolayer adsorbed on the (1010) surface of ZnO. [The Journal of Chemical... more Ab initio study of a CO monolayer adsorbed on the (1010) surface of ZnO. [The Journal of Chemical Physics 104, 3348 (1996)]. John E. Jaffe, Anthony C. Hess. Abstract. Periodic HartreeFock total energy calculations on two‐dimensionally ...

J Cryst Growth, 2009

The Energy Citations Database (ECD) provides access to historical and current research (1948 to t... more The Energy Citations Database (ECD) provides access to historical and current research (1948 to the present) from the Department of Energy (DOE) and predecessor agencies.

Phys Rev B, 2003

Phys Rev B, 1983

Using a first-principles all-electron band-structure approach, we show that the anomalous (> 50%)... more Using a first-principles all-electron band-structure approach, we show that the anomalous (> 50%) reduction in the band gaps of the AIBIIICVI2 chalcopyrite semiconductors relative to their II-VI isoelectronic analogs results both from a pure structural effect (the anion displacements reflecting the mismatch of classical elemental radii) and from a purely electronic effect (p-d repulsion in the valence band), with a small coupling between the two factors.

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We have investigated the mobility of isolated native point defects in CdTe by first-principles ca... more We have investigated the mobility of isolated native point defects in CdTe by first-principles calculations. Cd vacancies and interstitials, Te interstitials and Te-on-Cd antisites were considered. Diffusion barriers were found by the NEB (nudged- elastic-band) technique within the PAW-LDA method as implemented in the VASP code. Diffusion constants are estimated, and some implications for the growth of radiation detector material are suggested, especially in regard to the formation of Te precipitates. Comparisons to experimental and earlier theoretical studies are also provided.

Aps Meeting Abstracts, Mar 1, 2005

Cobalt-doped titanium dioxide, or CTO, has emerged in the past few years as a semiconducting, tra... more Cobalt-doped titanium dioxide, or CTO, has emerged in the past few years as a semiconducting, transparent, room-temperature ferromagnet. Very recently it has been shown that the magnetism in anatase-structure 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.

Bulletin of the American Physical Society, Mar 13, 2006

Native point defects in reduced TiO2, namely the oxygen vacancy and Ti interstitial, were investi... more Native point defects in reduced TiO2, namely the oxygen vacancy and Ti interstitial, were investigated computationally for both the rutile and anatase structures. The generalized gradient approximation to density functional theory was used along with a plane-wave expansion and ultrasoft pseudopotentials. Defect formation energies were calculated after geometry relaxation from O vacancies created in the bulk rutile and anatase lattices, from the experimental Ti interstitial position in rutile, and from several trial initial geometries for the Ti interstitial in anatase. Contrary to traditional assumptions but consistent with much recent evidence, the Ti interstitial was found to be predominant over the O vacancy in rutile under most conditions. Donor ionization energies in rutile were consistent with experiment. Surprisingly, the calculations also indicate a dominant role for the Ti interstitial in anatase (lower formation energy than the O vacancy, though not by as much as in rutile, and much shallower ionization levels.) We evaluate these findings against experimental data on pure n-type TiO2, and discuss possible implications for transition metal cation doping as well as anion doping of TiO2.

Surface Sci, 2009

We have used high-energy resolution X-ray photoelectron spectroscopy to measure valence band offs... more We have used high-energy resolution X-ray photoelectron spectroscopy to measure valence band offsets at the epitaxial anatase TiO 2(0 0 1)/ n-SrTiO 3(0 0 1) heterojunction prepared by molecular beam epitaxy. The valence band offsets range between -0.06 ± 0.05 and +0.16 ± 0.05 eV for anatase thicknesses between 1 and 8 monolayers and three different methods of substrate surface preparation, with no systematic dependence on film thickness. The conduction band offset (CBO) varies over a comparable range by virtue of the fact that anatase and SrTiO 3 exhibit the same bandgap (˜3.2 eV). In contrast, density functional theory predicts the VBO to be +0.55 eV. The lack of agreement between theory and experiment suggests that either some unknown factor in the interface structure or composition excluded from the modeling is influencing the band offset, or that density functional theory cannot accurately calculate band offsets in these oxide materials. The small experimental band offsets have important implications for the use of this interface for fundamental investigations of surface photocatalysis. Neither electrons nor holes are likely to become trapped in the substrate and thus be unable to participate in surface photocatalytic processes.

The Journal of Physics and Chemistry of Solids, 2003

In contrast to zinc-blende semiconductors, where the nonpolar (110) surface has the lowest energy... more In contrast to zinc-blende semiconductors, where the nonpolar (110) surface has the lowest energy, our first-principles calculations on the chalcopyrite semiconductor CuInSe2 reveal that facets terminated by the (112)-cation and (1̄1̄2̄)-Se polar surfaces are lower in energy than the unfaceted (110) plane, despite the resulting increased surface area. This explains the hitherto puzzling existence of polar microfacets on nominally nonpolar (110) chalcopyrite surfaces. The extraordinary stability of these polar facets originates from the effective neutralization of surface charge by low-energy ordered CuIn antisite or Cu vacancy surface defects, while the relaxed but defect-free (112) surface is metallic and much higher in energy. We explain the low carrier density of the observed faceted surface in terms of autocompensation between opposite-polarity facets.

$Research paper thumbnail of Trapping of Holes and Excitons in Scintillators: CsI and LaX_{3} ( X=Cl, Br)$

Ieee Transactions on Nuclear Science, Aug 1, 2010

Aps March Meeting Abstracts, Mar 1, 2005

TiO2 in both the rutile and anatase structures has been proposed as an absorber of sunlight for p... more TiO2 in both the rutile and anatase structures has been proposed as an absorber of sunlight for photocatalytic processes, including water splitting to produce hydrogen from solar energy. However, the large band gap of TiO2, about 3.0 eV, permits the use of only a small part of the solar spectrum. Alloying with other elements, and especially anion doping with nitrogen, sulfur or other anions in place of oxygen, has shown promise for reducing the band gap of the material, permitting more of the solar spectrum to be used so that photocatalysis with TiO2 can be made more efficient. However, little is known about the concentration dependence of the bandgap reduction, or about autocompensation of nonisovalent dopants by native defects such as the oxygen vacancy. We have calculated defect formation energies and changes in the DFT bandgap for rutile TiNxO2-x for x = 0.042, 0.083, 0.167 and 0.25. The band gap initially decreases with increasing x, but the reduction largely saturates at about 0.3 eV for x >= 0.083. We also report preliminary results on the formation energy and electronic structure of O vacancies in TiNxO2-x-δ, and on interactions and complexes of these defects.