1,721,192 research outputs found
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
First Principles Study of Oxides : Bulk, Interfaces and Defects
No full textIn the recent years, oxides have been the focus of numerous theoretical and experimental
studies. This is because of a wide variety of exotic physical phenomenon,
such as multiferroicity, charge ordering, metal-insulator transitions, high-Tc superconductivity
etc that have been observed in these materials. Moreover, most
oxides are earth-abundant, stable, non-toxic and easy to produce in a wide range
of environmental conditions. As a result, they have also been used in a variety of
technological applications. In this thesis, we study bulk oxides, interfaces between
different oxides, and defects in bulk oxides. We use first-principles methods to
calculate different properties of these systems as discussed below. These state-of-the-
art methods based on density functional theory (for ground-state properties)
and many-body perturbation theory (for excited-state properties) have been shown
to predict properties that are in excellent agreement with experiments.
Our study of bulk properties of oxides is motivated by the possibility of constructing
an efficient all-oxide solar cell. We explore two ferroelectric transition metal
oxides, YMnO3 and Zn2Mo3O8, as potential candidates for photoabsorbers. We
calculate the electronic structure and optical properties of these materials and
compare our results with available experiments.
A technologically and fundamentally interesting phenomenon at oxide interfaces
is the formation of a two-dimensional electron gas (2DEG). We propose a novel
oxide heterostructure system, consisting of two materials with chemical formula
A2Mo3O8 (A = Zn, Mg, Cd), which has the potential to host a 2DEG. Our calculations
predict the formation of 2DEG at this interface with electron densities
and localization comparable to that of other well-known 2DEG systems.
In the last part of the thesis, we investigate the electronic structure and optical
properties of the oxygen vacancies (F-centers) in -alumina. -Alumina or sapphire
is a widely used and well-studied material. We propose a modi fication of the
existing method for calculation of defect charge transition levels (CTLs) in solids.
Using this modi fication we calculate CTLs for F-centers in -alumina. We show
that our modi fication improves the accuracy of the results signifi cantly. Furthermore,
we calculate excited state properties of these F-centers to understand and
explain photoluminescence experiments performed on these system
Manipulating phonons and electrons in two-dimensional materials
No full textTwo-dimensional (2D) materials are one or few atoms thick layered materials. The
interaction between the layers in their parent three-dimensional material is weak. Therefore,
one can stack different 2D materials on top of each other like “Lego”, or one can rotate
one of the layers on top of another layer of 2D materials. The ability to controllably
“stack” and “twist” is unique to these materials and provides a great platform to manipulate
the electronic, vibrational, and optical properties. Experimental evidence of correlated
insulating states, superconductivity, ferromagnetism in the case of twisted bilayer graphene
at a certain rotation angle has led to a flurry of research activity in understanding the
behavior of electrons in these materials. However, two important facets attracted very
little attention: effects of twisting on the collective vibration of atoms (i.e. phonons), and
structural reconstruction of rigidly twisted moiré lattice.
In this thesis, we explore the layer and twist angle dependence of the phonon modes in
several 2D materials. We combine membrane theory and molecular dynamics simulations
to show that layer breathing modes can be mapped consistently to vibrations of a simple
linear chain model. Our study provides a simple and efficient way to probe the interlayer
interaction in few layers of 2D materials. The introduction of twist between two layers
gives rise to a large scale moiré lattice. We find that the Raman active phonon modes,
especially low-frequency shear and layer breathing modes, are quite sensitive to the twist
angle. We discover the existence of phason modes (with frequency 1 cm −1 , comparable
to acoustic modes) for any nonzero twist, corresponding to an effective translation of the
moiré lattice by relative displacement of the constituent layers in a nontrivial way. Our
calculations shed new insights into the origin of friction at the nanoscale. An important
step in understanding the exotic electronic and optical properties of the moiré lattices is
the inclusion of the effects of structural relaxation of the un-relaxed moiré lattices. All
the studies conducted on moiré materials to date presume that the moiré lattice constant
of the un-relaxed twisted structure remains intact even after relaxation. We explore if
novel lattice reconstructions of the moiré lattices are possible and the consequences of
such lattice reconstructions on the electronic properties. In the last part of the thesis,
in collaboration with experimentalists, we investigate the softening and broadening of the
high-frequency phonon modes due to temperature, doping, and twist angle in MoS 2 , a
prototypical transition metal dichalcogenide.
This thesis has been organized as follows:
• In Chapter 1, we describe the motivations behind studying properties of 2D materi-
als, focusing on moiré materials. We point out some key experimental and theoretical
1challenges in the field of moiré materials. In the end, we also highlight the issues
addressed in this thesis and summarize the key results.
• In Chapter 2, we describe the methods adopted in this thesis. We use multiscale
simulations to efficiently compute the structural, vibrational, and electronic proper-
ties presented in this thesis. All the electronic structure calculations are performed
with first-principles density-functional theory (DFT) based calculations. We briefly
summarize key concepts behind DFT. We also outline some of the technical aspects
of our DFT calculations. All the structure predictions and vibrational properties
calculations are performed using molecular dynamics (MD) simulations. We briefly
summarize some key concepts of MD simulations.
• In Chapter 3, we develop an efficient strategy to compute breathing modes of 2D ma-
terials, including the finite temperature anharmonic effects. Relative out-of-plane dis-
placements of the constituent layers of 2D materials give rise to unique low-frequency
breathing modes. The breathing modes can be used as a direct probe to determine
layer thickness using Raman spectroscopy. We compare our calculations with exper-
iments and first-principles calculations and find that they are in excellent agreement
with each other.
• In Chapter 4, we computationally explore the engineering of phonons with the twist
angle in TMD bilayers. We establish that the phonons and related properties can
be controlled by twisting, and we refer to this engineering as “twistnonics”. The
sensitiveness of low-frequency phonon modes with twist angle can be used to monitor
structural reconstruction. Moreover, we show that the velocities of the phason modes
are quite sensitive to the twist angle, unlike the acoustic modes. Our study reveals
the possibility of an intriguing θ-dependent superlubric to pinning behavior and the
existence of phason modes in all two-dimensional materials.
• In Chapter 5, we demonstrate a dramatic reconstruction of moiré lattices in twisted
transition metal dichalcogenides for θ > 58.4 ◦ . Our calculations suggest that the
presumption that the moiré lattice constant of the rigidly twisted structure continues
to characterize the relaxed structure is not always valid. We also find multiple flat
bands both near the valence and conduction band edges in the reconstructed lattice,
which can lead to the realization of exotic correlated electronic states.
• In Chapter 6, we use several techniques to investigate the temperature, doping, and
twist angle dependence of the high-frequency Raman modes in MoS 2 and compare our
results directly to the experiments. We compute the temperature dependence of the
phonon modes using DFT based calculations incorporating three-phonon processes,
and the doping dependence of the modes by explicitly computing electron-phonon
coupling matrix elements with DFT. On the other hand, the twist angle dependence
of the modes is computed with classical simulations.
• In Chapter 7, we summarize and provide some future directions based on the work
presented in this thesis
Taming Electrons in 2D Materials: Influence of Substrates, Defects and Moire Superlattices
No full textTwo-dimensional materials have been extensively studied in the last decade owing
to the rich physics at reduced dimensions and their applications in electronics and
optoelectronics. Controlled modifi cation of electronic structure is often desirable
to engineer 2D materials for specifi c applications. In this thesis we use ab initio
density functional theory (DFT) and GW calculations to study various intrinsic
and extrinsic factors that modify the electronic properties of 2D materials. Intrinsic
factors include the dependence of band structure on the number of layers and
point defects in the material. Extrinsic factors include substrate screening effects
and the introduction of a twist between the two layers of a bilayer materia
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
Electronic structure of two-dimensional materials: Effect of point defects and moiré patterns
No full textIn the last two decades, research on two-dimensional (2D) materials has gained momentum due to their capability to host a wide range of novel phases of matter. These materials have also found applications in electronic, optoelectronic and sensor devices. The electronic structure of material forms a basic building block to understand its electronic phases. It can be altered by several external factors, for example, strain, defects, moiré patterns or electric field. In this thesis, we focus on two such external factors: point defects and moiré patterns. Using first-principle calculations, we study the influence of these factors on the electronic structure of phosphorene and transition metal dichalcogenides.
Defects appear spontaneously in a material at a finite temperature. We investigate native point defects: vacancy and self-interstitial in mono- and bilayer phosphorene. We have studied formation energies, quasiparticle energies of defect states, and charge transition levels of these defects using ab initio density functional theory and GW approximation to the electron self-energy. These defects have low formation energies of 0.9–1.6 eV in the neutral state. Furthermore, the vacancy in phosphorene behaves as an acceptor-like defect which can explain the p-type conductivity in phosphorene. On the other hand, interstitial can show both acceptor- and donor-like behaviour.
In homobilayers of transition metal dichalcogenides, a small twist can be introduced to form a moiré superlattice (MSL). Recently, there is experimental evidence of flat bands in such twisted bilayer structures. We study the flat bands in twisted WSe2 where strong spin-orbit interaction gives rise to novel and interesting phenomenon. Flat bands emerge at both the band edges in twisted bilayer WSe2. The flat bands at the valence band edge originate from the K point of the unit cell Brillouin zone, unlike other twisted transition metal dichalcogenide structures. For twist angle (θ) close to 0◦, the bands at the valence band edge also possess a non-trivial topology. Quantum spin Hall insulating state, which is a consequence of this non-trivial topology should be experimentally accessible for θ < 3.5◦. For θ close to 60◦, the flattening of the bands arising from the K point is a consequence of the atomic reconstructions of the individual layer. Our findings are in excellent agreement with spectroscopic measurements on this material.
Another way to generate a moiré pattern from the homobilayer is by applying strain to one of the layers. This induces a lattice constant mismatch between the two layers and causes the formation of an MSL. This strained MSL shows very different structural and electronic properties than the twisted ones. We show that the strained MSLs provide a platform to study the ionic Hubbard model on a honeycomb lattice and the Hubbard model on a triangular lattice.
Two different TMD layers can be rotated and stacked on top of each other to form a twisted heterostructure. We study the electronic structure and optical properties of a twisted MoS_2/MoSe_2 heterostructure. The electronic band structure of this system and its origin in the individual layer’s unit cell has been explored. Experimentally, it has been found that interlayer excitons in heterostructure are long-lived compared to the intralayer ones. We have studied both kinds of excitons in this system. Our calculations show that optically-dark-finite-centre-of-mass-momentum intralayer excitons in bilayers of MoS2 and MoSe2 become optically allowed in the heterostructure. This is due to the presence of the moiré potential. Furthermore, we clarify the nature of the interlayer excitons. Our calculations show that this heterostructure has a type II band alignment. This results in the lowest energy interlayer excitons as those arising from transitions between the valence band of MoSe2 to the conduction band of MoS2
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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