1,721,015 research outputs found
Small polarons and point defects in barium cerate
No full textPublisher's PDFBarium cerate (BaCeO3) is a well-known ionic conductor of both hydrogen and oxygen. In applications, it
is frequently doped (for instance with Y) to increase stability and promote diffusion. However, the effects of
doping and native defects are not fully understood. Computational studies have been stymied by the nature
of the conduction band, which is made up of cerium 4f states. These states present a challenge to ab initio
techniques based on density functional theory within the standard approximations for exchange and correlation.
Using a hybrid functional, we investigate the effects of hydrogen impurities and native defects on the electrical
and optical properties of BaCeO3. We discuss the tendency of excess electrons or holes to localize in the form
of small polarons. We also explore the interactions of polarons with hydrogen impurities and oxygen vacancies,
and their impact on luminescence properties.University of Delaware. Department of Materials Science and Engineering
Defects as qubits in 3C- and 4H-SiC
No full textPublisher's PDFWe employ hybrid density functional calculations to search for defects in different polytypes of SiC that
may serve as qubits for quantum computing. We explore the divacancy in 4H- and 3C-SiC, consisting of a
carbon vacancy adjacent to a silicon vacancy, and the nitrogen-vacancy (NV) center in 3C-SiC, in which the
substitutional NC sits next to a Si vacancy (NC-VSi). The calculated excitation and emission energies of the
divacancy in 4H-SiC are in excellent agreement with experimental data, and aid in identifying the four unique
configurations of the divacancy with the four distinct zero-phonon lines observed experimentally. For 3C-SiC,
we identify the paramagnetic defect that was recently shown to maintain a coherent quantum state up to room
temperature as the spin-1 neutral divacancy. Finally, we show that the (NC-VSi)− center in 3C-SiC is highly
promising for quantum information science, and we provide guidance for identifying this defect.University of Delaware. Department of Materials Science and Engineering
Band alignment and p-type doping of ZnSnN2
No full textPublisher's PDFComposed of earth-abundant elements, ZnSnN2 is a promising semiconductor for photovoltaic and
photoelectrochemical applications. However, basic properties such as the precise value of the band gap and
the band alignment to other semiconductors are still unresolved. For instance, reported values for the band gap
vary from 1.4 to 2.0 eV. In addition, doping in ZnSnN2 remains largely unexplored. Using density functional
theory with the Heyd-Scuseria-Ernzerhof hybrid functional, we investigate the electronic structure of ZnSnN2,
its band alignment to GaN and ZnO, and the possibility of p-type doping. We find that the position of the
valence-band maximum of ZnSnN2 is 0.39 eV higher than that in GaN, yet the conduction-band minimum is
close to that in ZnO, which suggests that achieving p-type conductivity is likely as in GaN, yet it may be difficult
to control unintentional n-type conductivity as in ZnO. Among possible p-type dopants, we explore Li, Na, and
K substituting on the Zn site. We show that while LiZn is a shallow acceptor, NaZn and KZn are deep acceptors,
which we trace back to large local relaxations around the Na and K impurities due to the atomic size mismatches.University of Delaware. Department of Materials Science and Engineering
First-principles study of surface charging in LaAlO3/SrTiO3 heterostructures
No full textPublsiher's PDFThe two-dimensional electron gas (2DEG) observed at the interface between LaAlO3 (LAO) and SrTiO3 (STO)
is known to be very sensitive to the proximity of the LaAlO3 surface and the conditions to which the surface is
exposed. We use first-principles calculations to study surface reconstructions on LAO films, taking into account
that the LAO surface can be charged. The results for the charged surfaces and for the coupling between the
surface and the 2DEG enable us to account not only for the behavior of the 2DEG as a function of thickness of the
LAO layer, but simultaneously determine the stable terminations and reconstructions on the LAO surface under
a variety of conditions. Our studies of charged surfaces are based on an extension of the methodology of A. Y.
Lozovoi et al. [J. Chem. Phys. 115, 1661 (2001)]. From the calculated electronic structure of the unreconstructed
(but relaxed) AlO2 and LaO surface terminations of LAO, we find surface states having excess holes (AlO2
termination) or excess electrons (LaO termination). This result is central to understanding the mechanism of
2DEG formation, and is consistent with a 2DEG of density 3.3 × 1014 cm−2 being intrinsic to the LaO-TiO2
interface in the LAO/STO system. We explore the effects of the Al-adatom, O-vacancy, and H-adatom surface
reconstructions on the 2DEG density, and find that the stability of different reconstructions is tied to the thickness
of the LAO layer as well as the surface exposure conditions. We find that including the effects of charging of
the surface significantly stabilizes the AlO2 termination versus the LaO termination. Overall, our methodology
has the advantage of decoupling first-principles calculations for the interface from those for the charged surface,
and constitutes a general approach that can be applied to the commonly occurring problem of charge exchange
between the surface and the interface of a thin film with a substrate, or between the surface and defects/impurities
in the bulk of a material.
DOI: 10.1103/PhysRevB.University of Delaware. Department of Materials Science and Engineering
(InxGa1−x)2O3 alloys for transparent electronics
No full textPublisher's PDF(InxGa1−x )2O3 alloys show promise as transparent conducting oxides. Using hybrid density functional
calculations, band gaps, formation enthalpies, and structural parameters are determined for monoclinic and
bixbyite crystal structures. In the monoclinic phase the band gap exhibits a linear dependence on alloy
concentration, whereas in the bixbyite phase a large band-gap bowing occurs. The calculated formation enthalpies
showthat the monoclinic structure is favorable for In compositions up to 50% and bixbyite for larger compositions.
This is caused by In strongly preferring sixfold oxygen coordination. The formation enthalpy of the 50:50
monoclinic alloy is much lower than the formation enthalpy of the 50:50 bixbyite alloy and also lower than most
monoclinic alloys with lower In concentration; these trends are explained in terms of local strain. Consequences
for experiment and applications are discussed.University of Delaware. Department of Materials Science and Engineering
Correct Implementation of Polarization Constants in Wurtzite Materials and Impact on III-Nitrides
No full textPublisher's PDFAccurate values for polarization discontinuities between pyroelectric materials are critical for understanding and designing the electronic properties of heterostructures. For wurtzite materials, the zincblende structure has been used in the literature as a reference to determine the effective spontaneous polarization constants. We show that, because the zincblende structure has a nonzero formal polarization, this method results in a spurious contribution to the spontaneous polarization differences between materials. In addition, we address the correct choice of “improper” versus “proper” piezoelectric constants. For the technologically important III-nitride materials GaN, AlN, and InN, we determine polarization discontinuities using a consistent reference based on the layered hexagonal structure and the correct choice of piezoelectric constants, and discuss the results in light of available experimental data.University of Delaware. Department of Materials Science and Engineering
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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