1,720,956 research outputs found
Bogolon-mediated light absorption in atomic condensates of different dimensionality
No full textIn the case of structureless bosons, cooled down to low temperatures, the absorption of electromagnetic waves by their Bose-Einstein condensate is usually forbidden due to the momentum and energy conservation laws: the phase velocity of the collective modes of the condensate called bogolons is sufficiently lower than the speed of light. Thus, only the light scattering processes persist. However, the situation might be different in the case of composite bosons or the bosons with an internal structure. Here, we develop a microscopic theory of electromagnetic power absorption by a Bose–Einstein condensates of cold atoms in various dimensions, utilizing the Bogoliubov model of a weakly-interacting Bose gas. Thus, we address the transitions between a collective coherent state of bosons and the discrete energy levels corresponding to excited internal degrees of freedom of non-condensed individual bosons. It is shown, that such transitions are mediated by one and two-bogolon excitations above the condensate, which demonstrate different efficiency at different frequencies and strongly depend on the condensate density, which influence varies depending on the dimensionality of the system. © 2023, The Author(s).11Nsciescopu
Optical valleytronics of impurity states in two-dimensional Dirac materials
No full text© 2021 American Physical Society.We analyze the valley selection rules for optical transitions from impurity states to the conduction band in two-dimensional Dirac materials, taking a monolayer of MoS2 as an example. We employ the analytical model of a shallow impurity potential which localizes electrons described by a spinor wave function, and, first, find the system eigenstates taking into account the presence of two valleys in the Brillouin zone. Then, we find the spectrum of the absorbance and calculate the photon-drag electric current due to the impurity-band transitions, drawing the general conclusions regarding the valley optical selection rules for the impurity-band optical transitions in gapped Dirac materials.11Nsciescopu
Partial quantum revivals of localized condensates in distorted lattices
No full textWe report on a peculiar propagation of bosons loaded by a short Laguerre–Gaussian pulse in a nearly flat band of a lattice potential. Taking a system of exciton polaritons in a kagome lattice as an example, we show that an initially local- ized condensate propagates in a specific direction in space, if anisotropy is taken into account. This propagation consists of quantum jumps, collapses, and revivals of the whole com- pact states, and it persists given any direction of anisotropy. This property reveals its signatures in the tight-binding model, and, surprisingly, it is much more pronounced in a continuous model. Quantum revivals are robust to the repulsive interaction and finite lifetime of the particles. Since no magnetic field or spin–orbit interaction is required, this system provides a new kind of easily implementable optical logic. ©2020 Optical Society of Americ
Observation of a single quantized vortex vanishment in exciton-polariton superfluids
No full text© 2022 American Physical Society. We report the direct observation of a single quantized vortex vanishing from a microcavity exciton-polariton superfluid. Exciton-polariton vortices generated by a nonresonant Laguerre-Gaussian optical pumping beam reveal themselves in the energy-integrated emission image, representing a multimode entity consisting of the ground- and excited states. From the time-resolved spectroscopy measurements utilizing various Laguerre-Gaussian beam sizes, we find that the two lowest-energy states get populated and compete with each other, manifested by the change in their mutual population with the beam diameter. Furthermore, we study the transition from the excited state characterized by the finite orbital angular momentum (and a vortex in the direct space) to the ground state under pulsed excitation conditions. Our experimental findings are in excellent agreement with the numerical calculations employing the driven-dissipative Gross-Pitaevskii equation coupled with pumping reservoirs. Thus, our study provides an experimental and theoretical platform to investigate nonequilibrium vortex dynamics and manipulate multistate polariton condensates in semiconductor microcavities.11Nsciescopu
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Get PDFWe 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
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe 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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