131 research outputs found
Theory of optical and THz transitions in carbon nanotubes, graphene nanoribbons and flat nanoclusters
This thesis is devoted to the optical properties of low-dimensional structures based
on such two-dimensional materials as graphene, silicene and phosphorene.
We investigate optical properties of a variety of quasi-one dimensional and quasi-zero-dimensional structures, which are promising for future optoelectronics. Primarily we focus on their low-energy optical properties and how these properties
are influenced by the structures’ geometry, external fields, intrinsic strain and edge
disorder.
As a consequence of this endeavor, we find several interesting effects such as
correlation between the optical properties of tubes and ribbons whose periodic
and ‘hard wall’ boundary conditions are matched and a universal value of matrix
element in narrow-gap tubes and ribbons characterizing probability of transitions
across the band gap opened up by intrinsic strain originating from the tube’s surface
curvature or ribbon’s edge relaxation. The analytical study of the gapped 2D
Dirac materials such as silicene and germanene, which have some similarity to
the aforementioned quasi-one-dimensional systems in terms of physical description,
reveals a valley- and polarization-dependent selection rules. It was also found that
absorption coefficient should change in gapped materials with increasing frequency
and become a half of its value for gap edge transitions when the spectrum is linear.
Our analysis of the electronic properties of flat clusters of silicene and phosphorene relates the emergence and the number of the peculiar edge states localized
at zero energy, so-called zero-energy states, which are know to be of topological
origin, to the cluster’s structural characteristics such as shape and size. This allows to predict the presence and the number of such states avoiding complicated topological arguments and provides a recipes for design of metallic and dielectric
clusters. We show that zero-energy states are optically active and can be efficiently
manipulated by external electric field. However, the edge disorder is important to
take into account. We present a new fractal-based methodology to study the effects
of the edge disorder which can be applied also to modeling of composite materials.
These finding should be useful in design of optoelectronic devices such as tunable
emitters and detectors in a wide region of electromagnetic spectrum ranging form
the mid-infrared and THz to the optical frequencies.EU FP7 ITN NOTEDEVEU H2020 RISE project CoExA
Momentum alignment and the optical valley Hall effect in low-dimensional Dirac materials
We study the momentum alignment phenomenon and the optical control of valley
population in gapless and gapped graphene-like materials. We show that the
trigonal warping effect allows for the spatial separation of carriers belonging
to different valleys via the application of linearly polarized light. Valley
separation in gapped materials can be detected by measuring the degree of
circular polarization of band-edge photoluminescence at different sides of the
sample or light spot (optical valley Hall effect). We also show that the
momentum alignment phenomenon leads to the giant enhancement of near-band-edge
interband optical transitions in narrow-gap carbon nanotubes and graphene
nanoribbons independent of the mechanism of the gap formation. A detection
scheme to observe these giant interband transitions is proposed which opens a
route for creating novel terahertz radiation emitters.Comment: 28 pages, 9 figure
Tuning terahertz transitions in a double-gated quantum ring
We theoretically investigate the optical functionality of a semiconducting
quantum ring manipulated by two electrostatic lateral gates used to induce a
double quantum well along the ring. The well parameters and corresponding
inter-level spacings, which lie in the THz range, are highly sensitive to the
gate voltages. Our analysis shows that selection rules for inter-level dipole
transitions, caused by linearly polarized excitations, depend on the
polarization angle with respect to the gates. In striking difference from the
conventional symmetric double well potential, the ring geometry permits
polarization-dependent transitions between the ground and second excited
states, allowing the use of this structure in a three-level lasing scheme.Comment: 7 pages, 6 figure
Zigzag-Shaped Superlattices on the Basis of Graphene Nanoribbons: Structure and Electronic Properties
This is the author accepted manuscript. The final version is available from Springer Verlag via the DOI in this record.The paper focuses on superlattices consisting of two coplanar fragments of one-layer graphene nanoribbons that have different width and are connected at an angle. Classification of such superlattices was carried out; their electronic properties were studied using the tight-binding method. It was demonstrated that in superlattices consisting of two fragments of graphene nanoribbons with armchair edges connected at an angle of 60°, the band gap can be regulated by the number of dimeric carbon atom chains of one of the fragments. In that case one can observe a periodic dependence of the band gap on the number of chains with a characteristic period equal to three dimeric chains. The number of dimeric chains of the second superlattice fragment regulates the average band gap value near which the periodic oscillations occur, as well as the amplitude of those oscillations. Therefore, one can accomplish a sufficiently precise band gap tuning for such structures. Such tuning can find its wide application in the booming carbon nanoelectronics industry when creating generators, amplifiers and sensors in the nanochains.This research was supported by projects FP7 ITN NOTEDEV(FP7-607521), CACOMEL(FP7-247007), FAEMCAR (FP7-318617) and CANTOR (FP7-612285); project H2020-MSCA-RISE-2014 CoExAN (SEP-210156718); European Graphene Flagship project (604391), as well as by the Belarusian Ministry of Education (grant 20140773), Belarusian State University (grant 11, 2014), and the international grant AFOSR “Nanosized CherenkovType Terahertz Light Emitter Based on Double-Walled Carbon Nanotubes and Bi-graphene Nanoribbons”
Optical selection rules of zigzag graphene nanoribbons
PublishedThis is the final version of the article. Available from American Physical Society via the DOI in this record.We present an analytical tight-binding theory of the optical properties of graphene nanoribbons with zigzag edges. Applying the transfer matrix technique to the nearest-neighbor tight-binding Hamiltonian, we derive analytical expressions for electron wave functions and optical transition matrix elements for incident light polarized along the structure axis. It follows from the obtained results that optical selection rules result from the wave function parity factor (−1)J, where J is the band number. These selection rules are that ΔJ is odd for transitions between valence and conduction subbands and that ΔJ is even for transitions between only valence (conduction) subbands. Although these selection rules are different from those in armchair carbon nanotubes, there is a hidden correlation between absorption spectra of the two structures that should allow one to use them interchangeably in some applications. The correlation originates from the fact that van Hove singularities in the tubes are centered between those in the ribbons if the ribbon width is about a half of the tubes circumference. The analysis of the matrix elements dependence on the electron wave vector for narrow ribbons shows a smooth nonsingular behavior at the Dirac points and the points where the bulk states meet the edge states.This work was supported by the EU FP7 ITN NOTEDEV
(Grant No. FP7-607521), EU H2020 RISE project CoExAN
(Grant No. H2020-644076), FP7 IRSES projects CANTOR
(Grant No. FP7-612285), QOCaN (Grant No. FP7-316432),
InterNoM (Grant No. FP7-612624), and Graphene Flagship
(Grant No. 604391). The authors are very thankful to R. Keens
and C. A. Downing for a careful reading of the manuscript and
to A. Shytov and K. G. Batrakov for useful advice and fruitful
discussions
Electro-absorption of silicene and bilayer graphene quantum dots
We study numerically the optical properties of low-buckled silicene and
AB-stacked bilayer graphene quantum dots subjected to an external electric
field, which is normal to their surface. Within the tight-binding model, the
optical absorption is calculated for quantum dots, of triangular and hexagonal
shapes, with zigzag and armchair edge terminations. We show that in triangular
silicene clusters with zigzag edges a rich and widely tunable infrared
absorption peak structure originates from transitions involving zero energy
states. The edge of absorption in silicene quantum dots undergoes red shift in
the external electric field for triangular clusters, whereas blue shift takes
place for hexagonal ones. In small clusters of bilayer graphene with zigzag
edges the edge of absorption undergoes blue/red shift for triangular/hexagonal
geometry. In armchair clusters of silicene blue shift of the absorption edge
takes place for both cluster shapes, while red shift is inherent for both
shapes of the bilayer graphene quantum dots.Comment: 7 pages, 7 figure
Are economists rational or just different?
Economics students are more likely than others to act self-interestedly and less likely to behave cooperatively, behaviour which is rational from the viewpoint of many economic theories. Students in other disciplines may have another conception of wha t is rational. The latter may be more likely to behave cooperatively and less likely to behave self-interestedly. We have been comparing the behaviour of students from different disciplines in simple ultimatum bargaining and prisoner\u27s dilemma games. Our paper discusses some of the ways in which different academic disciplines both reinforce and elaborate upon student\u27s conceptions of rationality
Development of control system for automated manipulator
The paper is devoted to investigation of manipulators for plating lines and their modes of operation, as well as the development of a control system for gantry autooperator (LLC KTM-2000). The manipulator motion cyclograms for a given technological regulations were developed and presented.
Article in English.
Automatinio manipuliatoriaus valdymo sistema
Santrauka
Analizuojama galvaninio padengimo linijos automatinio manipuliatoriaus valdymo sistema, apibendrinamos išvados. Pateiktos pagal nustatytą technologinio proceso aprašą sudarytos judėjimo ciklogramos bei loginės funkcijos.
Reikšminiai žodžiai: valdymo sistema, automatinis manipuliatorius, ciklograma
The Moderating Effects of the Need for Multinational Investment on State Repression
What are the effects of resource endowments on state repression? This paper theorizes that states are more likely to engage in repression to secure resource-rich areas to maximize the state’s profits, with repression intensity varying by whether the resource requires outside investment to extract. Should resource extraction require outside investment, states must restrain their repression and share profits with an external multinational corporation. This theory yields predictions that there will be higher levels of state repression closer to sites of natural resources, and that overall levels of repression will be lower in areas with resources that require multinational investment than in areas with resources that the state can extract with its own capabilities. This is tested using a logit model and PRIO-GRID cell-years and repression data from the UCDP's Georeferenced Event Dataset.Master of Art
Strong Light-Matter Coupling in Carbon Nanotubes as a Route to Exciton Brightening
We show that strong light-matter coupling can be used to overcome a long
standing problem that has prevented efficient optical emission from carbon
nanotubes. The luminescence from the nominally bright exciton states of carbon
nanotubes is quenched due to the fast nonradiative scattering to the dark
exciton state having a lower energy. We present a theoretical analysis to show
that by placing carbon nanotubes in an optical microcavity the bright exctonic
state may be split into two hybrid exciton-polariton states, while the dark
state remains unaltered. For sufficiently strong coupling between the bright
exciton and the cavity, we show that the energy of the lower polariton may be
pushed below that of the dark exciton. This overturning of the relative
energies of the bright and dark excitons prevents the dark exciton from
quenching the emission. Our resutls pave the way for a new approach to
band-engineering the properties of the nanoscale optoelectronic devices.Comment: 35 pages, 5 figures, 6 pages of supplementary materials, 1
supplementary figur
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