159 research outputs found
Ultrafast charge carrier and exciton dynamics in an excitonic insulator probed by time-resolved photoemission spectroscopy
No full textAn excitonic insulator phase is expected to arise from the spontaneous formation of electron-hole pairs (excitons) in semiconductors where the exciton binding energy exceeds the size of the electronic band gap. At low temperature, these ground state excitons stabilize a new phase by condensing at lower energy than the electrons at the valence band top, thereby widening the electronic band gap. The envisioned opportunity to explore many-boson phenomena in an exci-tonic insulator system is triggering a very active debate on how ground state excitons can be experimentally evidenced. Here, we employ a nonequilibrium approach to spectrally disentangle the photoinduced dynamics of an exciton condensate from the entwined signature of the valence band electrons. By means of time-and angle-resolved photoemission spectroscopy of the occupied and unoccupied electronic states, we follow the complementary dynamics of conduction and valence band electrons in the photoexcited low-temperature phase of Ta2NiSe5, the hitherto most promising single-crystal candidate to undergo a semiconductor-to-excitonic-insulator phase transition. The photoexcited conduction electrons are found to relax within less than 1 ps. Their relaxation time is inversely proportional to their excess energy, a dependence that we attribute to the reduced screening of Coulomb interaction and the low dimensionality of Ta2NiSe5. Long after (> 10 ps) the conduction band has emptied, the photoemission intensity below the Fermi energy has not fully recovered the equilibrium value. Notably, this seeming carrier imbalance cannot be rationalized simply by the relaxation of photoexcited electrons and holes across the semi-conductor band gap. Rather, a rate equation model involving different photoemission cross -sections of the valence electrons and the condensed excitons is able to reproduce the delayed recovery of the photoemission intensity below the Fermi energy. The model shows that electron quantum tunnelling between the exciton condensate and the valence band top is enabled by an extremely small activation energy of 4 x 10-6 eV and explains the retarded recovery of the exciton condensate. Our findings not only determine the energy gain of ground state exciton formation with exceptional energy resolution, but also demonstrate the use of time-resolved photoemission to unveil the re-formation dynamics of an exciton condensate with femtosecond time resolution
Unveiling the exciton formation in time, energy and momentum domain in layered van der Waals semiconductors
No full textResolving the early-stage dynamics of exciton formation following non-resonant photoexcitation in time, energy, and momentum is quite challenging due to their inherently fast timescales and the proximity of the excitonic state to the bottom of the conduction band. In this study, by combining time- and angle-resolved photoemission spectroscopy with ab initio numerical simulations, we capture the timing of the early-stage exciton dynamics in energy and momentum, starting from the photoexcited population in the conduction band, progressing through the formation of free excitons, and ultimately leading to their trapping in lattice deformations. The chosen material is bismuth tri-iodide (BiI
), a layered semiconductor with a rich landscape of excitons in the electronic structure both in bulk and in monolayer form. The obtained results, providing a full characterization of the exciton formation, elucidate the early stages of the physical phenomena underlying the operation of the ultrafast semiconductor device
Ignorant Cognition. A Philosophical Investigation of the Cognitive Features of Not-Knowing
No full textThis book offers a comprehensive philosophical investigation of ignorance. Using a set of cognitive tools and models, it discusses features that can describe a state of ignorance if linked to a particular type of cognition affecting the agent’s social behavior, belief system, and inferential capacity. The author defines ignorance as a cognitive condition that can be either passively (and unconsciously) borne by an agent or actively nurtured by him or her, and a condition that entails epistemic limitations (which can be any lack of knowledge, belief, information or data) that affect the agent’s behavior, belief system, and inferential capacity.
The author subsequently describes the ephemeral nature of ignorance, its tenacity in the development of human inferential and cognitive performance, and the possibility of sharing ignorance among human agents within the social dimension. By combining previous frameworks such as the naturalization of logic, the eco-cognitive perspective in philosophy and concepts from Peircean epistemology, and adding original ideas derived from the author’s own research and reflections, the book develops a new cognitive framework to help understand the nature of ignorance and its influence on the human condition
Greek Goddness of the Moon in Light of the Orphic Hymn to Selene
No full textHymn to Selene is one of 87 so-called Orphic hymns. Its addressee is goddess of the Moon, who is defined in the hymn both by the name Selene and Mene. Conducting the literary analysis of the above-mentioned hymn, we can notice that it consists of the invocation addressed to the goddess, many epithets revealing the greatness of the divinity and the prayer ending which in its contents resembles the invocation. It is worth mentioning that the basic part of the hymn includes some kind of litany catalogue of epithets by means of which the Hymn’s Author defines and describes the goddess Selene.
The literary analysis of the respective epithets which are present in the Hymn to Selene, allows to state that the picture of the goddess of the Moon included in it possesses very classic, known to us from mythology, features and also clearly seen orphic features. Undoubtedly, dual sexuality that the goddess Selene was attributed with in verse 4 is such a feature. The phrase ϑῆλυς τε καὶ ἄρσην is to be found in this verse. In this context we have the direct reference to the primeval orphic godhead, Fanes, who was a two-gender god. Thus, Selene is perceived as one of hypostasis of one and the only one divine creature which can be defined as “One”.
Basing on the results of the literary analysis, we can notice that the goddess Selene, who was not particularly popular with the ancient Greeks, was somehow re-discovered by Orphics, who saw her beauty through the person of the Hymn’s Author. Due to the fact that Selene was the goddess of the Moon by the light of which the orphic usually practiced their own rituals, certainly occupied much space in the orphic cult
Educación y salud reproductiva en la biosfera de Calakmul, Campeche. Antropología. Boletín Oficial del Instituto Nacional de Antropología e Historia: Grupos sociales y diversidad cultural. Num. 66 Nueva Época (2002) abril-junio
No full textÁlvarez-Larrauri, Selene, “Un experimento educativo-comunicativo para fomentar el uso de la terapia de hidratación oral. Promotion and Education”, 1994, I(4), pp. 22-6; S.Álvarez-Larrauri, Selene, “Cultura de prevención de enfermedades sexuales transmisibles y embarazo en adolescentes”, en Dimensión Antropológica, 1998, vol. 12, pp. 77-98.Álvarez-Larrauri, Selene et al., “Aprendiendo a prevenir la deshidratación en comunidades alejadas y mercados mexicanos”, Soc. Sci. & Med., 1994, 38(11), pp. 1499-1508.Berbaum. J., Aprendizaje y formación. Una pedagogía por objetivos, México, FCE, 1988.Bourdieu P. y L. Wacquant. Responses, Paris, Editions Seuil,1992.Bryant I., Usher R., La educación de adultos como teoría, práctica e investigación. El triángulo cautivo, Barcelona, Morata, 1992.Díaz, B. F., “El aprendizaje significativo desde una perspectiva constructivista”, en Educar, 1993, núm. 1(4), pp. 23-36.Ehrenfeld-Lenkiewicz, N, “Educación para la salud reproductiva y sexual de la adolescente embarazada”, en Salud Pública de México, 1994, núm. 36, pp. 154-60.Green, L.W., M.W. Kreuter, S.G. Deeds, K.B. Partidge, Health Education Planning. A Diagnostic Approach, USA Mayfield Publishing Co., 1980
Instituto Guttmacher, Aborto Clandestino: una realidad latinoamericana, Nueva York, 1994.Instituto Nacional de Perinatología, Anuario Estadístico 1990, México, 1990.
Kirscht, J.P., “Preventive health behavior: A review of research issues”, en Health Psychol, 1983, núm. 2, pp. 277-301.“Mortalidad materna”, en Sistema Nacional de Salud. La salud de la mujer en México. Cifras concentradas, México, Programa Nacional, Mujer, Salud y Desarrollo, 1990.Shoham-Yakubovish, I., J.S. Pliskin y D. Carr. “The impact of a health education course on maternal knowledge: a comparative study in a low socioeconomic rural region”, en Health Educ Q, 1991,núm. 18, pp. 145-50.Stevenson, H.C., K.M. Gay y L. Josar, “Culturally Sensitive AIDS. Education and Perceived AIDS Risk Knowledge. Reaching the Know-It-All Teenager”, en Aids Education and Prevention, 1995, núm. 7 (2), pp. 134-44
Inhibition of the photoinduced structural phase transition in the excitonic insulator Ta2NiSe5
No full textFemtosecond time-resolved midinfrared reflectivity is used to investigate the electron and phonon dynamics occurring at the direct band gap of the excitonic insulator Ta2NiSe5 below the critical temperature of its structural phase transition. We find that the phonon dynamics show a strong coupling to the excitation of free carriers at the Γ point of the Brillouin zone. The optical response saturates at a critical excitation fluence FC=0.30±0.08 mJ/cm2 due to optical absorption saturation. This limits the optical excitation density in Ta2NiSe5 so that the system cannot be pumped sufficiently strongly to undergo the structural change to the high-temperature phase. We thereby demonstrate that Ta2NiSe5 exhibits a blocking mechanism when pumped in the near-infrared regime, preventing a nonthermal structural phase transition
Toxicological profile for dinitrotoluenes
No full textA Toxicological Profile for Dinitrotoluenes, Draft for Public Comment was released in April 2013. This edition supersedes any previously released draft or final profile.Chemical manager(s)/author(s): Carolyn Harper, Nickolette Roney, Mike Fay, Selene Chou, Division of Toxicology and Human Health Sciences, Atlanta, GA;Heather Carlson-Lynch, Julie M. Klotzbach, Kelly Salinas, H. Danielle Johnson, Mario Citra, SRC, Inc., North Syracuse, NY
Mott resistive switching initiated by topological defects
No full textResistive switching is the fundamental process that triggers the sudden
change of the electrical properties in solid-state devices under the action of
intense electric fields. Despite its relevance for information processing,
ultrafast electronics, neuromorphic devices, resistive memories and
brain-inspired computation, the nature of the local stochastic fluctuations
that drive the formation of metallic nuclei out of the insulating state has
remained hidden. Here, using operando X-ray nano-imaging, we have captured the
early-stages of resistive switching in a V2O3-based device under working
conditions. V2O3 is a paradigmatic Mott material, which undergoes a first-order
metal-to-insulator transition coupled to a lattice transformation that breaks
the threefold rotational symmetry of the rhombohedral metal phase. We reveal a
new class of volatile electronic switching triggered by nanoscale topological
defects of the lattice order parameter of the insulating phase. Our results
pave the way to the use of strain engineering approaches to manipulate
topological defects and achieve the full control of the electronic Mott
switching. The concept of topology-driven reversible electronic transition is
of interest for a broad class of quantum materials, comprising transition metal
oxides, chalcogenides and kagome metals, that exhibit first-order electronic
transitions coupled to a symmetry-breaking order
Toxicological profile for hydrogen sulfide (update)
No full textprepared by Syracuse Research Corporation ; prepared for U.S. Dept. of Health and Human Services, Public Health Service, Agency for Toxic Substances and Disease Registry."Chemical manager(s)/author(s): Selene Chou"--P. ix."A Toxicological Profile for Hydrogen Sulfide, Draft for Public Comment was released in September, 2004. This edition supersedes any previously released draft or final profile."--p. iii"July 2006""Under Contract No. 200-2004-09793"Also available via the World Wide Web.Includes bibliographical references (p. 159-199) and index
Effect of Photoinduced Screening on the Spectroscopic Signature of Exciton-Phonon Coupling
No full textThe light-mediated interaction of fermionic and bosonic excitations governs the optoelectronic properties of condensed matter systems. In photoexcited semiconductors, the coupling of electron-hole pairs (excitons) to coherent optical phonons enables a modulation of the excitonic resonance that is phase-locked to the frequency of the coupled vibrational mode. Moreover, due to the Coulombic nature of excitons, their dynamics are sensitive to transient changes in the screening by the photoexcited carriers. Interestingly, the effect of photoinduced screening on the transient optical signal originating from the exciton dynamics coupled to phonons is not yet established. By means of broadband transient reflectance spectroscopy, we disclose how exciton-phonon coupling manifests in either the presence or absence of dynamical screening in a layered semiconductor. Furthermore, we unveil the promoting role of photoinduced screening on these exciton-phonon coupled dynamics as opposed to the case in which the unscreened exciton-exciton repulsion likely dominates the nonequilibrium optical response. These findings set a protocol to look at an excitonic resonance and its fundamental many-body interactions on the ultrafast timescale and provide new perspectives on the access to nonequilibrium coupled dynamics
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