On the Bohr radius relationship to spin-orbit interaction, spin magnitude, and Thomas precession
David C. Lush
2009-03-24
The dynamics of the spin-orbit interaction in atomic hydrogen are studied in a classical electrodynamics-like setting. A Rutherfordian atomic model is used assuming a circular electron orbit, without the quantum principle as imposed arbitrarily in the Bohr model, but with an ad hoc incorporation in the electron of intrinsic spin and associated magnetic dipole moment. Analyzing the motions of the electron spin and orbital angular momenta, it is found that in the presence of Thomas precession, the total angular momentum averaged over the orbit is not generally a constant of the motion. It is noted this differs from the finding of Thomas in a similar assessment of 1927, and the reason for this difference is provided. It is found that although the total orbit-averaged angular momentum is not a constant of the motion, it precesses around a fixed axis similarly to the precession of the total angular momentum vector seen in spin-orbit coupling in quantum theory. The magnitude of the angular velocity of the total orbit-averaged angular momentum is seen to vanish only when the spin and orbital angular momenta are antiparallel and their mutual precession frequencies equate. It is then found, there is a unique radius where the mutual precession frequencies equate. Assuming the electron magnetic moment is the Bohr magneton, and an electron g-factor of two, this radius corresponds to where the orbital angular momentum is the reduced Planck's constant. The orbit radius for stationary total angular momentum for the circular orbit model with nonzero orbital angular momentum is thus the ground-state radius of the Bohr model.
R. Heyrovska; S. Narayan
2005-09-24
Sommerfeld introduced the fine-structure constant into physics, while he was taking into account the relativistic effects in the theory of the hydrogen atom. Ever since, it has puzzled many scientists like Eddington, Dirac, Feynman and others. Here the mysterious fine-structure constant, alpha = (Compton wavelength/de Broglie wavelength) = 1/137.036 = 2.627/360 is interpreted based on the finding that it is close to 2.618/360 = 1/137.508, where the Compton wavelength for hydrogen is a distance equivalent to an arc length on the circumference (given by the de Broglie wavelength) of a circle with the Bohr radius and 2.618 is the square of the Golden ratio, which was recently shown to divide the Bohr radius into two Golden sections at the point of electrical neutrality. From the data for the electron (e) and proton (p) g-factors, it is found that (137.508 - 137.036)= 0.472 = [g(p) - g(e)]/[g(p) + g(e)] (= 2/cube of the Golden ratio), and that (2.627 - 2.618)/360 = (small part of the Compton wavelength corresponding to the intrinsic radii of e and p/de Broglie wavelength) = 0.009/360 = (1- gamma)/gamma, the factor for the advance of perihilion in Sommerfeld's theory of the hydrogen atom, where gamma is the relativity factor.
NASA Astrophysics Data System (ADS)
Saito, H.; Akimoto, R.; Kindo, K.; Takeyama, S.
2011-12-01
CdS/ZnSe multiple quantum wells (MQWs) shows very strong photoluminescence at low temperatures. The strong photoluminescence (PL) arises from the interface localization of the type-II excitons. A systematic study was attempted on the exciton density dependence of magneto-PL behavior up to 53 T by using a pulse magnet. We obtained a strong excitation density dependence of the type-II exciton Bohr radius trapped in interface roughness potentials.
Exciton Mapping at Subwavelength Scales in Two-Dimensional Materials
NASA Astrophysics Data System (ADS)
Tizei, Luiz H. G.; Lin, Yung-Chang; Mukai, Masaki; Sawada, Hidetaka; Lu, Ang-Yu; Li, Lain-Jong; Kimoto, Koji; Suenaga, Kazu
2015-03-01
Spatially resolved electron-energy-loss spectroscopy (EELS) is performed at diffuse interfaces between MoS2 and MoSe2 single layers. With a monochromated electron source (20 meV) we successfully probe excitons near the interface by obtaining the low loss spectra at the nanometer scale. The exciton maps clearly show variations even with a 10 nm separation between measurements; consequently, the optical band gap can be measured with nanometer-scale resolution, which is 50 times smaller than the wavelength of the emitted photons. By performing core-loss EELS at the same regions, we observe that variations in the excitonic signature follow the chemical composition. The exciton peaks are observed to be broader at interfaces and heterogeneous regions, possibly due to interface roughness and alloying effects. Moreover, we do not observe shifts of the exciton peak across the interface, possibly because the interface width is not much larger than the exciton Bohr radius.
ERIC Educational Resources Information Center
Haendler, Blanca L.
1982-01-01
Discusses the importance of teaching the Bohr atom at both freshman and advanced levels. Focuses on the development of Bohr's ideas, derivation of the energies of the stationary states, and the Bohr atom in the chemistry curriculum. (SK)
Arne Schirrmacher; A. Shimony
The model of Niels Bohr (1885–1962) for the atom is since long just the one and only conception for atoms of the vast majority\\u000a of educated people. The picture of ? electrons revolving round a nucleus on select avenues has become the icon of the atomic\\u000a age. In stark contrast to this omnipresence, historically, the Bohr atom may be identified
The methodological lesson of complementarity: Bohr’s naturalistic epistemology
NASA Astrophysics Data System (ADS)
Folse, H. J.
2014-12-01
Bohr’s intellectual journey began with the recognition that empirical phenomena implied the breakdown of classical mechanics in the atomic domain; this, in turn, led to his adoption of the ‘quantum postulate’ that justifies the ‘stationary states’ of his atomic model of 1913. His endeavor to develop a wider conceptual framework harmonizing both classical and quantum descriptions led to his proposal of the new methodological goals and standards of complementarity. Bohr’s claim that an empirical discovery can demand methodological revision justifies regarding his epistemological lesson as supporting a naturalistic epistemology.
Exciton-exciton annihilation in organic polariton microcavities
Akselrod, G. M.; Tischler, Jonathan R.; Young, E. R.; Nocera, D. G.; Bulovic, Vladimir
2010-01-01
We investigate the incoherent diffusion of excitons in thin films (5.1±0.1?nm thick) of a highly absorbing J-aggregated cyanine dye material (10{sup 6} ?cm{sup ?1} absorption constant) as the excitonic component of a polariton microcavity. Under high-intensity pulsed laser excitation, the J-aggregated molecular films exhibit significant exciton-exciton annihilation, indicating a large exciton diffusion radius of more than 100 nm. When the material is strongly coupled to a cavity, the polaritonic structure also shows exciton-exciton annihilation, which is a competing process against the establishment of a threshold population of polaritons needed for polariton lasing. This study suggests that exciton-exciton annihilation is a loss process which can significantly increase the lasing threshold in polariton microcavities.
Revisiting Bohr's semiclassical quantum theory.
Ben-Amotz, Dor
2006-10-12
Bohr's atomic theory is widely viewed as remarkable, both for its accuracy in predicting the observed optical transitions of one-electron atoms and for its failure to fully correspond with current electronic structure theory. What is not generally appreciated is that Bohr's original semiclassical conception differed significantly from the Bohr-Sommerfeld theory and offers an alternative semiclassical approximation scheme with remarkable attributes. More specifically, Bohr's original method did not impose action quantization constraints but rather obtained these as predictions by simply matching photon and classical orbital frequencies. In other words, the hydrogen atom was treated entirely classically and orbital quantized emerged directly from the Planck-Einstein photon quantization condition, E = h nu. Here, we revisit this early history of quantum theory and demonstrate the application of Bohr's original strategy to the three quintessential quantum systems: an electron in a box, an electron in a ring, and a dipolar harmonic oscillator. The usual energy-level spectra, and optical selection rules, emerge by solving an algebraic (quadratic) equation, rather than a Bohr-Sommerfeld integral (or Schroedinger) equation. However, the new predictions include a frozen (zero-kinetic-energy) state which in some (but not all) cases lies below the usual zero-point energy. In addition to raising provocative questions concerning the origin of quantum-chemical phenomena, the results may prove to be of pedagogical value in introducing students to quantum mechanics. PMID:17020371
NASA Astrophysics Data System (ADS)
Bagheri, Mehran
2010-07-01
The ground-state characteristics of spatially indirect excitons trapped in radially heteronanostructured type-II band alignment ZnSe/ZnTe nanotubes as functions of the magnetic field for nanotubes with a radial size both smaller and larger than the effective Bohr radius are theoretically investigated. In the former case, dominated by the net kinetic energy of the electron and hole, the magnetic field modifies the exciton spectrum through the well-known Zeeman splitting, intra-orbital-state Aharonov-Bohm oscillations and inter-orbital-state crossovers occurring in very strong magnetic field strengths. However, in the latter case, dominated by the electron-hole Coulomb attraction, the magnetic field adjusts the exciton lines only by means of the Zeeman splitting and inter-orbital-state transitions happening in typical magnetic fields. As a result, the angular momentum transitions occurr at lower magnetic fields when the radial size of the nanotube is increased. Most importantly, another consequence is the substantially unusual exciton oscillator strength in such heteronanostructures. It is shown that when the exciton is optically active, due to the full cylindrical symmetry of the problem, the exciton oscillator strength shows undamped oscillations. This effect is associated with the periodic redistribution of the exciton density as the magnetic field is varied. Also, the magnitude of the magnetically induced excitonic persistent current is decreased with increasing radial size of the nanotube. This study may provide a platform to investigate new photonic quantum interference as well as polarization-sensitive photodetector and photovoltaic devices based on the Aharonov-Bohm effect.
NASA Astrophysics Data System (ADS)
Tyurin, A. N.
2000-10-01
This paper combines algebraic and Lagrangian geometry to construct a special basis in every space of conformal blocks, the Bohr-Sommerfeld (BS) basis. We use the method of Borthwick-Paul-Uribe [3], in which every vector of a BS basis is determined by some half-weight Legendrian distribution coming from a Bohr-Sommerfeld fibre of a real polarization of the underlying symplectic manifold. The advantage of BS bases (compared to the bases of theta functions in [23]) is that we can use the powerful methods of asymptotic analysis of quantum states. This shows that Bohr-Sommerfeld bases are quasiclassically unitary. Thus we can apply these bases to compare the Hitchin connection [11] and the KZ connection defined by the monodromy of the Knizhnik-Zamolodchikov equation in the combinatorial theory (see, for example, [14] and [15]).
ERIC Educational Resources Information Center
Brunori, Maurizio
2012-01-01
Before the outbreak of World War II, Jeffries Wyman postulated that the "Bohr effect" in hemoglobin demanded the oxygen linked dissociation of the imidazole of two histidines of the polypeptide. This proposal emerged from a rigorous analysis of the acid-base titration curves of oxy- and deoxy-hemoglobin, at a time when the information on the…
A Simple Relativistic Bohr Atom
ERIC Educational Resources Information Center
Terzis, Andreas F.
2008-01-01
A simple concise relativistic modification of the standard Bohr model for hydrogen-like atoms with circular orbits is presented. As the derivation requires basic knowledge of classical and relativistic mechanics, it can be taught in standard courses in modern physics and introductory quantum mechanics. In addition, it can be shown in a class that…
Atempts to link Quanta & Atoms before the Bohr Atom model
NASA Astrophysics Data System (ADS)
Venkatesan, A.; Lieber, M.
2005-03-01
Attempts to quantize atomic phenomena before Bohr are hardly ever mentioned in elementary textbooks.This presentation will elucidate the contributions of A.Haas around 1910. Haas tried to quantize the Thomson atom model as an optical resonator made of positive and negative charges. The inherent ambiguity of charge distribution in the model made him choose a positive spherical distribution around which the electrons were distributed.He obtained expressions for the Rydberg constant and what is known today as the Bohr radius by balancing centrifugal energy with Coulomb energy and quantizing it with Planck's relation E=h?. We point out that Haas would have arrived at better estimates of these constants had he used the virial theorem apart from the fact that the fundamental constants were not well known. The crux of Haas's physical picture was to derive Planck's constant h from charge quantum e , mass of electron m and atomic radius. Haas faced severe criticism for applying thermodynamic concepts like Planck distribution to microscopic phenomena. We will try to give a flavor for how quantum phenomena were viewed at that time. It is of interest to note that the driving force behind Haas's work was to present a paper that would secure him a position as a Privatdozent in History of Physics. We end with comments by Bohr and Sommerfeld on Haas's work and with some brief biographical remarks.
Ehrenfest's adiabatic hypothesis in Bohr's quantum theory
Pérez, Enric
2015-01-01
It is widely known that Paul Ehrenfest formulated and applied his adiabatic hypothesis in the early 1910s. Niels Bohr, in his first attempt to construct a quantum theory in 1916, used it for fundamental purposes in a paper which eventually did not reach the press. He decided not to publish it after having received the new results by Sommerfeld in Munich. Two years later, Bohr published "On the quantum theory of line-spectra." There, the adiabatic hypothesis played an important role, although it appeared with another name: the principle of mechanical transformability. In the subsequent variations of his theory, Bohr never suppressed this principle completely. We discuss the role of Ehrenfest's principle in the works of Bohr, paying special attention to its relation to the correspondence principle. We will also consider how Ehrenfest faced Bohr's uses of his more celebrated contribution to quantum theory, as well as his own participation in the spreading of Bohr's ideas.
Corda, Christian
2015-01-01
The idea that black holes (BHs) result in highly excited states representing both the "hydrogen atom" and the "quasi-thermal emission" in quantum gravity is today an intuitive but general conviction. In this paper it will be shown that such an intuitive picture is more than a picture. In fact, we will discuss a model of quantum BH somewhat similar to the historical semi-classical model of the structure of a hydrogen atom introduced by Bohr in 1913. The model is completely consistent with existing results in the literature, starting from the celebrated result of Bekenstein on the area quantization.
Christian Corda
2015-03-11
The idea that black holes (BHs) result in highly excited states representing both the "hydrogen atom" and the "quasi-thermal emission" in quantum gravity is today an intuitive but general conviction. In this paper it will be shown that such an intuitive picture is more than a picture. In fact, we will discuss a model of quantum BH somewhat similar to the historical semi-classical model of the structure of a hydrogen atom introduced by Bohr in 1913. The model is completely consistent with existing results in the literature, starting from the celebrated result of Bekenstein on the area quantization.
The Bohr effect before Perutz.
Brunori, Maurizio
2012-01-01
Before the outbreak of World War II, Jeffries Wyman postulated that the Bohr effect in hemoglobin demanded the oxygen linked dissociation of the imidazole of two histidines of the polypeptide. This proposal emerged from a rigorous analysis of the acid-base titration curves of oxy- and deoxy-hemoglobin, at a time when the information on the chemistry and structure of the protein was essentially nil. The magnetochemical properties of hemoglobin led Linus Pauling to hypothesize that the (so called) Bohr histidines were coordinated to the heme iron in the fifth and sixth positions; and Wyman shared this opinion. However, this structural hypothesis was abandoned in 1951 when J. Wyman and D. W. Allen proposed the pK shift of the oxygen linked histidines to be the result of "...a change of configuration of the hemoglobin molecule as a whole accompanying oxygenation." This shift in paradigm, that was published well before the 3D structure of hemoglobin was solved by M.F. Perutz, paved the way to the concept of allostery. After 1960 the availability of the crystallographic structure opened new horizons to the interpretation of the allosteric properties of hemoglobin. PMID:22987550
Rutherford and Bohr describe atomic structure 1913
NSDL National Science Digital Library
1998-01-01
This page is from PBS A Science Odyssey: People and Discoveries. It describes Bohr's contribution to theories of the atom and its relationship to others' such as Rutherford's. Links to relevant pages are provided. An image of Bohr's conception drawn in his notebook is included.
NSDL National Science Digital Library
This is a set of animations that demonstrates properties of the spherical holes formed when uniform spheres are packed. Cubic, octahedral and tetrahedral packing arrangements may be examined without anything in the holes,and with the repective holes filled. The sizes of the various holes relative to the spheres being packed are shown, which can lead students into an exploration of the radius ratio concept. An example is given of computing the relative size of an octahedral hole.
NASA Astrophysics Data System (ADS)
Robinson, L. M.; Smith, L. M.; Sun, T.-B.; Jonker, B. T.
1998-03-01
Exciton magnetic polarons (EMP) are excitons dressed by the local magnetization of the magnetic impurities. The EMP lower their energy by spontaneously aligning the magnetic impurities within the Bohr radius. In 14% ZnMnSe/ZnSe multiple quantum wells, the hole becomes confined to the ZnMnSe ``barriers'' while the electron remains in the ZnSe ``wells.'' This weak electron-hole overlap results in a 1000-fold increase in the exiton lifetime (10 ns). We use time-resolved photoluminescence spectroscopy to study the magnetization of spatially-indirect EMP in small magnetic fields. We find that the EMP polarization approaches 100% in fields as small as 200 mT, consistent with their extremely long recombination lifetime. This is in contrast with previous measurements which show that the polarization for short-lived spatially direct EMP never completely saturate.(I.A. Merkulov, D.R. Yakovlev, K.V. Kavokin, G. Mackh, W. Ossau, A. Waag, and G. Landwehr. JETP Letters 62 (4), 335 (1995).)
Excitons and Peierls Distortion in Conjugated Carbon Nanotubes
Tretiak, Sergei
Excitons and Peierls Distortion in Conjugated Carbon Nanotubes Sergei Tretiak,*, Svetlana Kilina investigate coupled excitonic and vibrational effects in carbon nanotubes using a time-dependent Hartree the entire nanotube, particularly in large-radius CNTs. However, we demonstrate that vibrational relaxation
Dow, J. D.; Hjalmarson, H. P.; Sankey, O. F.; Allen, R. E.; Buettner, H.
1980-01-01
The observation of core excitons with binding energies much larger than those of the valence excitons in the same material has posed a long-standing theoretical problem. A proposed solution to this problem is presented, and Frenkel excitons and Wannier excitons are shown to coexist naturally in a single material. (GHT)
Epistemological Dimensions in Niels Bohr's Conceptualization of Complementarity
NASA Astrophysics Data System (ADS)
Derry, Gregory
2008-03-01
Contemporary explications of quantum theory are uniformly ahistorical in their accounts of complementarity. Such accounts typically present complementarity as a physical principle that prohibits simultaneous measurements of certain dynamical quantities or behaviors, attributing this principle to Niels Bohr. This conceptualization of complementarity, however, is virtually devoid of content and is only marginally related to Bohr's actual writing on the topic. Instead, what Bohr presented was a subtle and complex epistemological argument in which complementarity is a shorthand way to refer to an inclusive framework for the logical analysis of ideas. The important point to notice, historically, is that Bohr's work involving complementarity is not intended to be an improvement or addition to a particular physical theory (quantum mechanics), which Bohr regarded as already complete. Bohr's work involving complementarity is actually an argument related to the goals, meaning, and limitations of physical theory itself, grounded in deep epistemological considerations stemming from the fundamental discontinuity of nature on a microscopic scale.
Effective models for excitons in carbon nanotubes
Horia D. Cornean; Pierre Duclos; Benjamin Ricaud
2006-05-04
We analyse the low lying spectrum of a model of excitons in carbon nanotubes. Consider two particles with an attractive Coulomb self-interaction, placed on an infinitely long cylinder. If the cylinder radius becomes small, the low lying spectrum is well described by a one-dimensional effective Hamiltonian which is exactly solvable.
Ultrafast Optical Studies of Multiple Exciton Generation in Lead Chalcogenide Quantum Dots
NASA Astrophysics Data System (ADS)
Midgett, Aaron G.
2011-12-01
Providing affordable, clean energy is one of the major challenges facing society today, and one of the promising solutions is third generation solar energy conversion. Present day, first and second-generation solar cells can at most convert each absorbed photon into a single electron hole pair, thereby establishing a theoretical limit to the power conversion efficiency. The process of multiple exciton generation (MEG) in semiconductor quantum dots increases that theoretical efficiency from 33% to 42% by utilizing the excess energy of high energy photons that is otherwise wasted as heat to excite a second electron-hole pair, thereby boosting the potential photocurrent. This thesis explores the benefits of MEG in quantum confined systems and shows that quantum dots are more efficient at generating multiple excitons from a single photon than bulk semiconductors. The variations in optical measurements of MEG have raised skepticism and brought into question the validity of these experiments. The two important questions that this thesis attempts to address are (1) what are the enhanced QYs in isolated PbSe QDs and (2) does quantum confinement enhance MEG over bulk semiconductors. Experimental variations in the enhanced QYs are partially explained by the production of a long-lived photocharged state that increases the apparent photon-to-exciton QYs. A procedure is detailed that decreases the possibility of producing this charged state. By studying the production of these states, conditions are found that minimize their effect and produce less variation in the reported QYs. Variations in the MEG efficiency were studied in films of chemically treated PbSe quantum dots where a different mechanism was responsible for an apparent decrease of the measured QYs. Finally, for the first time, a quantum dot size-dependence in the MEG efficiency was found in colloidal PbSe, PbS, and PbSxSe1-x quantum dot solutions and is attributed to the increased Coulomb interaction in materials with a larger Bohr exciton radius. These results will allow a better understanding of MEG and how this important process may be used to enhance solar energy conversion.
The operation : The interview of Niels Bohr
NASA Astrophysics Data System (ADS)
Terletskij, Yakov P.
The article represents a stenogramme of the recollections of professor Yakov Petrovich Terletskij (1912, St. Petersburg-1993, Moscow) concerning his meeting with professor Niels Bohr in Kopenhagen from 24 september 1945 till 22 november 1945 under the task of Soviet KGB and personally Lavrentii Berija. Some additional informations concerning previous preparations of the trip, the final stage of the mission, as well as general considerations about the development of Physics, persons of physicists and nuclear technologies has been given by the author of recollections throughout the text
Bohr's Creation of his Quantum Atom
NASA Astrophysics Data System (ADS)
Heilbron, John
2013-04-01
Fresh letters throw new light on the content and state of Bohr's mind before and during his creation of the quantum atom. His mental furniture then included the atomic models of the English school, the quantum puzzles of Continental theorists, and the results of his own studies of the electron theory of metals. It also included the poetry of Goethe, plays of Ibsen and Shakespeare, novels of Dickens, and rhapsodies of Kierkegaard and Carlyle. The mind that held these diverse ingredients together oscillated between enthusiasm and dejection during the year in which Bohr took up the problem of atomic structure. He spent most of that year in England, which separated him for extended periods from his close-knit family and friends. Correspondence with his fianc'ee, Margrethe Nørlund, soon to be published, reports his ups and downs as he adjusted to J.J. Thomson, Ernest Rutherford, the English language, and the uneven course of his work. In helping to smooth out his moods, Margrethe played an important and perhaps an enabling role in his creative process.
Bohr's Principle of Complementarity and Beyond
NASA Astrophysics Data System (ADS)
Jones, R.
2004-05-01
All knowledge is of an approximate character and always will be (Russell, Human Knowledge, 1948, pg 497,507). The laws of nature are not unique (Smolin, Three Roads to Quantum Gravity, 2001, pg 195). There may be a number of different sets of equations which describe our data just as well as the present known laws do (Mitchell, Machine Learning, 1997, pg 65-66 and Cooper, Machine Learning, Vol. 9, 1992, pg 319) In the future every field of intellectual study will possess multiple theories of its domain and scientific work and engineering will be performed based on the ensemble predictions of ALL of these. In some cases the theories may be quite divergent, differing greatly one from the other. The idea can be considered an extension of Bohr's notions of complementarity, "...different experimental arrangements.. described by different physical concepts...together and only together exhaust the definable information we can obtain about the object" (Folse, The Philosophy of Niels Bohr, 1985, pg 238). This idea is not postmodernism. Witchdoctor's theories will not form a part of medical science. Objective data, not human opinion, will decide which theories we use and how we weight their predictions.
100th anniversary of Bohr's model of the atom.
Schwarz, W H Eugen
2013-11-18
In the fall of 1913 Niels Bohr formulated his atomic models at the age of 27. This Essay traces Bohr's fundamental reasoning regarding atomic structure and spectra, the periodic table of the elements, and chemical bonding. His enduring insights and superseded suppositions are also discussed. PMID:24123759
Zimmermann, Roland
Magneto-optical studies of excitons in Zn1 xCdxSe/ZnSe quantum wells J. Puls, V. V. Rossin,* and F at room temperature.2 Up to now, a number of studies for ZnxCd1 xSe/ZnSe multiple quantum wells MQW's have a systematic study of magnetoexcitons in Zn1 xCdxSe/ZnSe MQW's with well widths close to the bulk exciton Bohr
Wiseman, Howard M., E-mail: H.Wiseman@Griffith.edu.au
2013-11-15
By rigorously formalizing the Einstein–Podolsky–Rosen (EPR) argument, and Bohr’s reply, one can appreciate that both arguments were technically correct. Their opposed conclusions about the completeness of quantum mechanics hinged upon an explicit difference in their criteria for when a measurement on Alice’s system can be regarded as not disturbing Bob’s system. The EPR criteria allow their conclusion–incompleteness–to be reached by establishing the physical reality of just a single observable q (not of both q and its conjugate observable p), but I show that Bohr’s definition of disturbance prevents the EPR chain of reasoning from establishing even this. Moreover, I show that Bohr’s definition is intimately related to the asymmetric concept of quantum discord from quantum information theory: if and only if the joint state has no Alice-discord, she can measure any observable without disturbing (in Bohr’s sense) Bob’s system. Discord can be present even when systems are unentangled, and this has implications for our understanding of the historical development of notions of quantum nonlocality. -- Highlights: •Both the EPR argument, and Bohr’s reply, were technically correct. •Their opposed conclusions came from different criteria for disturbance. •Bohr’s criterion works against even the simplified (one-variable) EPR argument. •Bohr’s criterion for disturbance is intimately related to quantum discord. •This illuminates the historical development of notions of quantum nonlocality.
Paul Ehrenfest, Niels Bohr, and Albert Einstein: Colleagues and Friends
NASA Astrophysics Data System (ADS)
Klein, Martin J.
2010-09-01
In May 1918 Paul Ehrenfest received a monograph from Niels Bohr in which Bohr had used Ehrenfest's adiabatic principle as an essential assumption for understanding atomic structure. Ehrenfest responded by inviting Bohr, whom he had never met, to give a talk at a meeting in Leiden in late April 1919, which Bohr accepted; he lived with Ehrenfest, his mathematician wife Tatyana, and their young family for two weeks. Albert Einstein was unable to attend this meeting, but in October 1919 he visited his old friend Ehrenfest and his family in Leiden, where Ehrenfest told him how much he had enjoyed and profited from Bohr's visit. Einstein first met Bohr when Bohr gave a lecture in Berlin at the end of April 1920, and the two immediately proclaimed unbounded admiration for each other as physicists and as human beings. Ehrenfest hoped that he and they would meet at the Third Solvay Conference in Brussels in early April 1921, but his hope was unfulfilled. Einstein, the only physicist from Germany who was invited to it in this bitter postwar atmosphere, decided instead to accompany Chaim Weizmann on a trip to the United States to help raise money for the new Hebrew University in Jerusalem. Bohr became so overworked with the planning and construction of his new Institute for Theoretical Physics in Copenhagen that he could only draft the first part of his Solvay report and ask Ehrenfest to present it, which Ehrenfest agreed to do following the presentation of his own report. After recovering his strength, Bohr invited Ehrenfest to give a lecture in Copenhagen that fall, and Ehrenfest, battling his deep-seated self-doubts, spent three weeks in Copenhagen in December 1921 accompanied by his daughter Tanya and her future husband, the two Ehrenfests staying with the Bohrs in their apartment in Bohr's new Institute for Theoretical Physics. Immediately after leaving Copenhagen, Ehrenfest wrote to Einstein, telling him once again that Bohr was a prodigious physicist, and again expressing the hope that he soon would see both of them in Leiden.
Chang, Yuan-Ming; Shieh, Jiann; Chu, Pei-Yuan; Lee, Hsin-Yi; Lin, Chih-Ming; Juang, Jenh-Yih
2011-11-01
Room-temperature ultraviolet (UV) luminescence was investigated for the atomic layer deposited ZnO films grown on silicon nanopillars (Si-NPs) fabricated by self-masking dry etching in hydrogen-containing plasma. For films deposited at 200 °C, an intensive UV emission corresponding to free-exciton recombination (~3.31 eV) was observed with a nearly complete suppression of the defect-associated broad visible range emission peak. On the other hand, for ZnO films grown at 25 °C, albeit the appearance of the defect-associated visible emission, the UV emission peak was observed to shift by ~60 meV to near the direct band edge (3.37 eV) recombination emission. The high-resolution transmission electron microscopy (HRTEM) showed that the ZnO films obtained at 25 °C were consisting of ZnO nanocrystals with a mean radius of 2 nm embedded in a largely amorphous matrix. Because the Bohr radius of free-exictons in bulk ZnO is ~2.3 nm, the size confinement effect may have occurred and resulted in the observed direct band edge electron-hole recombination. Additionally, the results also demonstrate order of magnitude enhancement in emission efficiency for the ZnO/Si-NP structure, as compared to that of ZnO directly deposited on Si substrate under the same conditions. PMID:21967063
Exciton states in quasi-zero-dimensional semiconductor nanosystems
Pokutnyi, S. I., E-mail: Pokutnyi_Sergey@inbox.ru [National Academy of Sciences of Ukraine, Kurdyumov Institute for Metal Physics (Ukraine)
2012-02-15
The variational method in the context of the modified effective mass approximation is used to calculate the dependence of exciton ground-state energy for a quantum dot embedded in a borosilicate glassy matrix on the quantum dot radius. It is shown that the peaks in the absorption and low-temperature luminescence spectra of such a nanosystem are shifted to shorter wavelengths due to size quantization of the exciton ground-state energy in the quantum dot.
Relativistic Corrections to the Bohr Model of the Atom
ERIC Educational Resources Information Center
Kraft, David W.
1974-01-01
Presents a simple means for extending the Bohr model to include relativistic corrections using a derivation similar to that for the non-relativistic case, except that the relativistic expressions for mass and kinetic energy are employed. (Author/GS)
Exciton-exciton annihilation in organic polariton microcavities
Bulovic, Vladimir
Sublinear intensity dependence of photoluminescence from organic exciton-polariton microcavities under non-resonant excitation in two power regimes is shown. The sublinearity is attributed to exciton-exciton annihilation, ...
Stacking in colloidal nanoplatelets: tuning excitonic properties.
Guzelturk, Burak; Erdem, Onur; Olutas, Murat; Kelestemur, Yusuf; Demir, Hilmi Volkan
2014-12-23
Colloidal semiconductor quantum wells, also commonly known as nanoplatelets (NPLs), have arisen among the most promising materials for light generation and harvesting applications. Recently, NPLs have been found to assemble in stacks. However, their emerging characteristics essential to these applications have not been previously controlled or understood. In this report, we systematically investigate and present excitonic properties of controlled column-like NPL assemblies. Here, by a controlled gradual process, we show that stacking in colloidal quantum wells substantially increases exciton transfer and trapping. As NPLs form into stacks, surprisingly we find an order of magnitude decrease in their photoluminescence quantum yield, while the transient fluorescence decay is considerably accelerated. These observations are corroborated by ultraefficient Förster resonance energy transfer (FRET) in the stacked NPLs, in which exciton migration is estimated to be in the ultralong range (>100 nm). Homo-FRET (i.e., FRET among the same emitters) is found to be ultraefficient, reaching levels as high as 99.9% at room temperature owing to the close-packed collinear orientation of the NPLs along with their large extinction coefficient and small Stokes shift, resulting in a large Förster radius of ?13.5 nm. Consequently, the strong and long-range homo-FRET boosts exciton trapping in nonemissive NPLs, acting as exciton sink centers, quenching photoluminescence from the stacked NPLs due to rapid nonradiative recombination of the trapped excitons. The rate-equation-based model, which considers the exciton transfer and the radiative and nonradiative recombination within the stacks, shows an excellent match with the experimental data. These results show the critical significance of stacking control in NPL solids, which exhibit completely different signatures of homo-FRET as compared to that in colloidal nanocrystals due to the absence of inhomogeneous broadening. PMID:25469555
Intermediately bound exciton in a quantum well doped with transition metal impurities
P. Dahan
2007-01-01
The possibility of an intermediately bound exciton in a quantum well, doped with transition metal impurities, is considered. Such bound excitons, in which both carriers are captured in an intermediate-radius orbital, can appear in a quantum well due to strong hybridization of the two-dimensional band states and the impurity d states that are strongly suppressed in the bulk case due
Howard, Don
in the history of twentieth-century physics, and rightly so, Albert Einstein and Niels Bohr being the figuresRevisiting the Einstein-Bohr Dialogue Don Howard Einstein and Bohr Â No names loom larger identified complementarity as the chief novelty in the quantum description of nature, Einstein for having
Control of Exciton Fluxes in an Excitonic Integrated Circuit
Alex A. High; Ekaterina E. Novitskaya; Leonid V. Butov; Micah Hanson; Arthur C. Gossard
2008-01-01
Efficient signal communication uses photons. Signal processing, however, uses an optically inactive medium, electrons. Therefore, an interconnection between electronic signal processing and optical communication is required at the integrated circuit level. We demonstrated control of exciton fluxes in an excitonic integrated circuit. The circuit consists of three exciton optoelectronic transistors and performs operations with exciton fluxes, such as directional switching
Optically programmable excitonic traps
Alloing, Mathieu; Lemaître, Aristide; Galopin, Elisabeth; Dubin, François
2013-01-01
With atomic systems, optically programmed trapping potentials have led to remarkable progress in quantum optics and quantum information science. Programmable trapping potentials could have a similar impact on studies of semiconductor quasi-particles, particularly excitons. However, engineering such potentials inside a semiconductor heterostructure remains an outstanding challenge and optical techniques have not yet achieved a high degree of control. Here, we synthesize optically programmable trapping potentials for indirect excitons of bilayer heterostructures. Our approach relies on the injection and spatial patterning of charges trapped in a field-effect device. We thereby imprint in-situ and on-demand electrostatic traps into which we optically inject cold and dense ensembles of excitons. This technique creates new opportunities to improve state-of-the-art technologies for the study of collective quantum behavior of excitons and also for the functionalisation of emerging exciton-based opto-electronic circuits. PMID:23546532
Anomalous magnetization of a carbon nanotube as an excitonic insulator
NASA Astrophysics Data System (ADS)
Rontani, Massimo
2014-11-01
We show theoretically that an undoped carbon nanotube might be an excitonic insulator—the long-sought phase of matter proposed by Keldysh, Kohn, and others fifty years ago. We predict that the condensation of triplet excitons, driven by intervalley exchange interaction, spontaneously occurs at equilibrium if the tube radius is sufficiently small. The signatures of exciton condensation are its sizable contributions to both the energy gap and the magnetic moment per electron. The increase of the gap might have already been measured, albeit with a different explanation [V. V. Deshpande, B. Chandra, R. Caldwell, D. S. Novikov, J. Hone, and M. Bockrath, Science 323, 106 (2009), 10.1126/science.1165799]. The enhancement of the quasiparticle magnetic moment is a pair-breaking effect that counteracts the weak paramagnetism of the ground-state condensate of excitons. This property could rationalize the anomalous magnitude of magnetic moments recently observed in different devices close to charge neutrality.
Jocular Physics: A Tribute to Bohr in Humor
NASA Astrophysics Data System (ADS)
Halpern, Paul
2011-04-01
Copenhagen, starting in the 1920s and 1930s and continuing after the Second World War, was not just a center for extraordinary developments in quantum and nuclear physics; it also provided a perfect stage for physicists' abundant humor. We will examine the Journal of Jocular Physics, a humorous tribute to Bohr published on the occasions of his 50^th, 60^th and 70^th birthdays. We will discuss how the articles in the journal reflected attempts by the contributors, such as L'eon Rosenfeld, Victor Weisskopf and others, to interpret and explain aspects of Bohr's philosophy, such as complementarity and the abandonment of pure Laplacian determinism.
NASA Technical Reports Server (NTRS)
Harris, Michael R.
1987-01-01
Resonator configurations permitting operation with large mode radius while maintaining good transverse mode discrimination are considered. Stable resonators incorporating an intracavity telescope and unstable resonator geometries utilizing an output coupler with a Gaussian reflectivity profile are shown to enable large radius single mode laser operation. Results of heterodyne studies of pulsed CO2 lasers with large (11mm e sup-2 radius) fundamental mode sizes are presented demonstrating minimal frequency sweeping in accordance with the theory of laser-induced medium perturbations.
Control of Exciton Fluxes in an Excitonic Integrated Circuit
NASA Astrophysics Data System (ADS)
High, Alex A.; Novitskaya, Ekaterina E.; Butov, Leonid V.; Hanson, Micah; Gossard, Arthur C.
2008-07-01
Efficient signal communication uses photons. Signal processing, however, uses an optically inactive medium, electrons. Therefore, an interconnection between electronic signal processing and optical communication is required at the integrated circuit level. We demonstrated control of exciton fluxes in an excitonic integrated circuit. The circuit consists of three exciton optoelectronic transistors and performs operations with exciton fluxes, such as directional switching and merging. Photons transform into excitons at the circuit input, and the excitons transform into photons at the circuit output. The exciton flux from the input to the output is controlled by a pattern of the electrode voltages. The direct coupling of photons, used in communication, to excitons, used as the device-operation medium, may lead to the development of efficient exciton-based optoelectronic devices.
Coherently coupled exciton lasing
Bulovic, Vladimir
Lowest reported threshold organic semiconductor VCSEL (4.9 muJ/cm[superscript 2]) is achieved when excitons coherently couple, upon sub-picosecond non-resonant optical excitation. Temperature dependence of lambda/2 n device, ...
Creation of non-dispersive Bohr-like wavepackets
J. J. Mestayer; B. Wyker; F. B. Dunning; S. Yoshida; C. O. Reinhold; J. Burgdörfer
2009-01-01
The use of a periodic train of half-cycle pulses to maintain strongly-localized wavepackets in high-n (n ~ 306) atoms that travel in near-circular orbits about the nucleus is demonstrated. The wavepacket is non-dispersive for hundreds of orbits and mimics the original Bohr model of the hydrogen atom.
Quasiparticle Gaps and Exciton Coulomb Energies in Si Nanoshells
Frey, K. [University of Illinois, Chicago; Idrobo Tapia, Juan C [ORNL; Tiago, Murilo L [ORNL; Reboredo, Fernando A [ORNL; Ogut, Serdar [University of Illinois, Chicago
2009-01-01
Quasiparticle gaps and exciton Coulomb energies of H-passivated spherical Si nanoshells are computed using rst principles SCF and GW methods. We nd that the quasiparticle gap of a nanoshell depends on both its inner radius R1 (weakly) and outer radius R2 (strongly). These dependences on R1 and R2 are mostly consistent with electrostatics of a metallic shell. We also nd that the unscreened Coulomb energy ECoul in Si nanoshells has a somewhat unexpected size dependence at xed outer radius R2: ECoul decreases as the nanoshell becomes more conning, contrary to what one would expect from quantum connement eects. We show that this is a consequence of an increase in the average electron-hole distance, giving rise to reduced exciton Coulomb energies in spite of the reduction in the conning nanoshell volume.
What is complementarity?: Niels Bohr and the architecture of quantum theory
NASA Astrophysics Data System (ADS)
Plotnitsky, Arkady
2014-12-01
This article explores Bohr’s argument, advanced under the heading of ‘complementarity,’ concerning quantum phenomena and quantum mechanics, and its physical and philosophical implications. In Bohr, the term complementarity designates both a particular concept and an overall interpretation of quantum phenomena and quantum mechanics, in part grounded in this concept. While the argument of this article is primarily philosophical, it will also address, historically, the development and transformations of Bohr’s thinking, under the impact of the development of quantum theory and Bohr’s confrontation with Einstein, especially their exchange concerning the EPR experiment, proposed by Einstein, Podolsky and Rosen in 1935. Bohr’s interpretation was progressively characterized by a more radical epistemology, in its ultimate form, which was developed in the 1930s and with which I shall be especially concerned here, defined by his new concepts of phenomenon and atomicity. According to this epistemology, quantum objects are seen as indescribable and possibly even as inconceivable, and as manifesting their existence only in the effects of their interactions with measuring instruments upon those instruments, effects that define phenomena in Bohr’s sense. The absence of causality is an automatic consequence of this epistemology. I shall also consider how probability and statistics work under these epistemological conditions.
Triplet exciton dissociation in singlet exciton fission photovoltaics.
Jadhav, Priya J; Brown, Patrick R; Thompson, Nicholas; Wunsch, Benjamin; Mohanty, Aseema; Yost, Shane R; Hontz, Eric; Van Voorhis, Troy; Bawendi, Moungi G; Bulovi?, Vladimir; Baldo, Marc A
2012-12-01
Triplet exciton dissociation in singlet exciton fission devices with three classes of acceptors are characterized: fullerenes, perylene diimides, and PbS and PbSe colloidal nanocrystals. Using photocurrent spectroscopy and a magnetic field probe it is found that colloidal PbSe nanocrystals are the most promising acceptors, capable of efficient triplet exciton dissociation and long wavelength absorption. PMID:22968762
Effective Cleaning Radius Studies
Churnetski, B.V.
2001-10-15
This report discusses results of testing done in the Savannah River Laboratory half tank and full tank mockup facilities using kaolin clay slurries and the relationship between cleaning radius and pump and slurry characteristics.
Creation of nondispersive Bohr-like wave packets
J. J. Mestayer; B. Wyker; F. B. Dunning; S. Yoshida; C. O. Reinhold; J. Burgdörfer
2009-01-01
We demonstrate the use of a periodic train of half-cycle pulses to maintain strongly-localized wave packets in very-high- n (ñ300) Rydberg atoms that travel in near-circular orbits about the nucleus. This motion can be followed for hundreds of orbital periods and mimics the original Bohr model of the hydrogen atom which envisioned an electron in circular classical orbit about the
Creation of non-dispersive Bohr-like wave packets
Jeff Mestayer; B. Wyker; F. B. Dunning; C. O. Reinhold; S. Yoshida; J. Burgdörfer
2009-01-01
We demonstrate the use of a periodic train of half-cycle pulses to create strongly-localized non-dispersive wave packets in very-high-n (n ˜ 300) Rydberg atoms that travel in near-circular orbits about the nucleus. This motion can be maintained for hundreds of orbital periods mimicking the original Bohr model of the hydrogen atom which envisioned an electron in circular classical orbit about
Bohr--Sommerfeld Lagrangians of moduli spaces of Higgs bundles
Indranil Biswas; Niels Leth Gammelgaard; Marina Logares
2015-04-03
Let $X$ be a compact connected Riemann surface of genus at least two. Let $M_H(r,d)$ denote the moduli space of semistable Higgs bundles on $X$ of rank $r$ and degree $d$. We prove that the compact complex Bohr-Sommerfeld Lagrangians of $M_H(r,d)$ are precisely the irreducible components of the nilpotent cone in $M_H(r,d)$. This generalizes to Higgs $G$-bundles and also to the parabolic Higgs bundles.
Bohr--Sommerfeld Lagrangians of moduli spaces of Higgs bundles
Indranil Biswas; Niels Leth Gammelgaard; Marina Logares
2014-09-24
Let $X$ be a compact connected Riemann surface of genus at least two. Let $M_H(r,d)$ denote the moduli space of semistable Higgs bundles on $X$ of rank $r$ and degree $d$. We prove that the compact complex Bohr-Sommerfeld Lagrangians of $M_H(r,d)$ are precisely the irreducible components of the nilpotent cone in $M_H(r,d)$. This generalizes to Higgs $G$-bundles and also to the parabolic Higgs bundles.
NASA Astrophysics Data System (ADS)
Han, Zhenyu
2014-10-01
Jet radiation patterns are indispensable for the purpose of discriminating partons with different quantum numbers. However, they are also vulnerable to various contaminations from the underlying event, pileup, and radiation of adjacent jets. In order to maximize the discrimination power, it is essential to optimize the jet radius used when analyzing the radiation patterns. We introduce the concept of jet radiation radius, which quantifies how the jet radiation is distributed around the jet axis. We study the color and momentum dependence of the jet radiation radius and discuss two applications: quark-gluon discrimination and W -jet tagging. In both cases, smaller (sub)jet radii are preferred for jets with higher pT's, albeit due to different mechanisms: the running of the QCD coupling constant and the boost to a color-singlet system. A shrinking cone W -jet tagging algorithm is proposed to achieve better discrimination than previous methods.
NASA Astrophysics Data System (ADS)
Byrnes, Tim; Kim, Na Young; Yamamoto, Yoshihisa
2014-11-01
Recently a new type of system exhibiting spontaneous coherence has emerged--the exciton-polariton condensate. Exciton-polaritons (or polaritons for short) are bosonic quasiparticles that exist inside semiconductor microcavities, consisting of a superposition of an exciton and a cavity photon. Above a threshold density the polaritons macroscopically occupy the same quantum state, forming a condensate. The polaritons have a lifetime that is typically comparable to or shorter than thermalization times, giving them an inherently non-equilibrium nature. Nevertheless, they exhibit many of the features that would be expected of equilibrium Bose-Einstein condensates (BECs). The non-equilibrium nature of the system raises fundamental questions as to what it means for a system to be a BEC, and introduces new physics beyond that seen in other macroscopically coherent systems. In this review we focus on several physical phenomena exhibited by exciton-polariton condensates. In particular, we examine topics such as the difference between a polariton BEC, a polariton laser and a photon laser, as well as physical phenomena such as superfluidity, vortex formation, and Berezinskii-Kosterlitz-Thouless and Bardeen-Cooper-Schrieffer physics. We also discuss the physics and applications of engineered polariton structures.
Triplet exciton caging in two dimensions
S. Arnold; R. R. Alfano; M. Pope; W. Yu; P. Ho; R. Selsby; J. Tharrats; C. E. Swenberg
1976-01-01
It is proposed that the bimolecular process of triplet exciton fusion to form singlet excitons can be enhanced by reducing the size of the domain in which the triplet exciton pair is free to move. These small domains, or exciton cages, are much more effective when the host material is highly anisotropic, and the triplet excitons are constrained to move
Singlet exciton fission photovoltaics.
Lee, Jiye; Jadhav, Priya; Reusswig, Philip D; Yost, Shane R; Thompson, Nicholas J; Congreve, Daniel N; Hontz, Eric; Van Voorhis, Troy; Baldo, Marc A
2013-06-18
Singlet exciton fission, a process that generates two excitons from a single photon, is perhaps the most efficient of the various multiexciton-generation processes studied to date, offering the potential to increase the efficiency of solar devices. But its unique characteristic, splitting a photogenerated singlet exciton into two dark triplet states, means that the empty absorption region between the singlet and triplet excitons must be filled by adding another material that captures low-energy photons. This has required the development of specialized device architectures. In this Account, we review work to develop devices that harness the theoretical benefits of singlet exciton fission. First, we discuss singlet fission in the archetypal material, pentacene. Pentacene-based photovoltaic devices typically show high external and internal quantum efficiencies. They have enabled researchers to characterize fission, including yield and the impact of competing loss processes, within functional devices. We review in situ probes of singlet fission that modulate the photocurrent using a magnetic field. We also summarize studies of the dissociation of triplet excitons into charge at the pentacene-buckyball (C60) donor-acceptor interface. Multiple independent measurements confirm that pentacene triplet excitons can dissociate at the C60 interface despite their relatively low energy. Because triplet excitons produced by singlet fission each have no more than half the energy of the original photoexcitation, they limit the potential open circuit voltage within a solar cell. Thus, if singlet fission is to increase the overall efficiency of a solar cell and not just double the photocurrent at the cost of halving the voltage, it is necessary to also harvest photons in the absorption gap between the singlet and triplet energies of the singlet fission material. We review two device architectures that attempt this using long-wavelength materials: a three-layer structure that uses long- and short-wavelength donors and an acceptor and a simpler, two-layer combination of a singlet-fission donor and a long-wavelength acceptor. An example of the trilayer structure is singlet fission in tetracene with copper phthalocyanine inserted at the C60 interface. The bilayer approach includes pentacene photovoltaic cells with an acceptor of infrared-absorbing lead sulfide or lead selenide nanocrystals. Lead selenide nanocrystals appear to be the most promising acceptors, exhibiting efficient triplet exciton dissociation and high power conversion efficiency. Finally, we review architectures that use singlet fission materials to sensitize other absorbers, thereby effectively converting conventional donor materials to singlet fission dyes. In these devices, photoexcitation occurs in a particular molecule and then energy is transferred to a singlet fission dye where the fission occurs. For example, rubrene inserted between a donor and an acceptor decouples the ability to perform singlet fission from other major photovoltaic properties such as light absorption. PMID:23611026
An Increased Bohr Effect in Sickle Cell Anemia
Yoshihiro Ueda; Ronald L. Nagel; Robert M. Bookchin
1979-01-01
Recent findings that hemoglobin S gelation was greatly increased only between blood and sickling are pH-dependent and also pH 7.4 and 7.2 (cell pH 7.2 and 7.0. a shift influence oxygen affinity suggested that that strongly affects gelation). with Llog the red cells containing this hemoglobin p50\\/ LpH -0.92 to -0.99 (normal variant might show an abnormal Bohr 0.42 to
Second bound state of negatively charged excitons in quantum dots
NASA Astrophysics Data System (ADS)
Xie, Wenfang
2001-05-01
By using the method of few-body physics, the binding energy spectra of the second bound state of a negatively charged exciton X - in a GaAs quantum dot with a parabolic confinement have been calculated as a function of the electron-to-hole mass ratio and of the dot radius. In the case of L=1 and S12=1 our calculation predicts the existence of a second bound state of X - in QDs.
Detonation Shock Radius Experiments.
NASA Astrophysics Data System (ADS)
Lambert, David; Debes, Joshua; Stewart, Scott; Yoo, Sunhee
2007-06-01
A previous passover experiment [1] was designed to create a complex detonation transient used in validating a reduced, asymptotically derived description of detonation shock dynamics (DSD). An underlying question remained on determining the location of the initial detonation shock radius to start the DSD simulation with respect to the dynamical response of the initiation system coupling's to the main charge. This paper concentrates on determining the initial shock radius required of such DSD governed problems. `Cut-back' experiments of PBX-9501 were conducted using an initiation system that sought to optimize the transferred detonation to the desired constant radius, hemispherical shape. Streak camera techniques captured the breakout on three of the prism's surfaces for time-of-arrival data. The paper includes comparisons to simulations using constant volume explosion and high pressure hot spots. The results of the experiments and simulation efforts provide fundamental design considerations for actual explosive systems and verify necessary conditions from which the asymptotic theory of DSD may apply. [1] Lambert, D., Stewart, D. Scott and Yoo, S. and Wescott, B., ``Experimental Validation of Detonation Shock Dynamics in Condensed Explosives. J. of Fluid Mechs., Vol. 546, pp.227-253 (2006).
Elastic scattering of exciton polaritons
NASA Astrophysics Data System (ADS)
Averkiev, N. S.; Savchenko, G. M.; Seisyan, R. P.
2015-02-01
The probability of elastic scattering by impurities of exciton polaritons in thin GaAs samples has been calculated theoretically. It has been shown that thin samples of commercially pure GaAs satisfy the conditions for the existence of exciton polaritons. It has also been demonstrated that, in the case of GaAs, the inclusion of the polariton effect in the analysis leads only to a slight decrease in the scattering probability calculated for a "bare" exciton. The role of elastic scattering of exciton polaritons in the absorption of light by a semiconductor has been discussed.
Bohr - Planck quantum theory, (Tesla) magnetic monopoles and fine structure constant
Vladan Pankovic; Darko V. Kapor; Stevica Djurovic; Miodrag Krmar
2014-10-17
In this work we apply Bohr-Planck (Old quantum atomic and radiation) theory, i.e. and quasi-classical methods for analysis of the magnetic monopoles and other problems. We reproduce exactly some basic elements of the Dirac magnetic monopoles theory, especially Dirac electric/magnetic charge quantization condition. Also, we suggest a new, effective, simply called Tesla model (for analogy with positions of the solenoids by Tesla inductive motor) of the magnetic monopole instead of usual effective Dirac model (half-infinite, very tinny solenoid) of the magnetic monopole. In our, i.e. Tesla model we use three equivalent tiny solenoids connected in series with a voltage source. One end of any solenoid is placed at the circumference of a circle and solenoids are directed radial toward circle center. Length of any solenoid is a bit smaller than finite circle radius so that other end of any solenoid is very close to the circle center. Angles between neighboring solenoids equal $120^{\\circ}$. All this implies that, practically, there is no magnetic field, or, magnetic pole, e.g. $S$, in the circle center, and that whole system holds only other, $N$ magnetic pole, at the ends of the solenoids at circle circumference. Finally, we reproduce relatively satisfactory value of the fine structure constant using Planck, i.e. Bose-Einstein statistics and Wien displacement law.
Molecular Basis of the Bohr Effect in Arthropod Hemocyanin
Hirota, S.; Kawahara, T; Beltramini, M; Di Muro, P; Magliozzo, R; Peisach, J; Powers, L; Tanaka, N; Nagao, S; Bubacco, L
2008-01-01
Flash photolysis and K-edge x-ray absorption spectroscopy (XAS) were used to investigate the functional and structural effects of pH on the oxygen affinity of three homologous arthropod hemocyanins (Hcs). Flash photolysis measurements showed that the well-characterized pH dependence of oxygen affinity (Bohr effect) is attributable to changes in the oxygen binding rate constant, kon, rather than changes in koff. In parallel, coordination geometry of copper in Hc was evaluated as a function of pH by XAS. It was found that the geometry of copper in the oxygenated protein is unchanged at all pH values investigated, while significant changes were observed for the deoxygenated protein as a function of pH. The interpretation of these changes was based on previously described correlations between spectral lineshape and coordination geometry obtained for model compounds of known structure A pH-dependent change in the geometry of cuprous copper in the active site of deoxyHc, from pseudotetrahedral toward trigonal was assigned from the observed intensity dependence of the 1s ? 4pz transition in x-ray absorption near edge structure (XANES) spectra. The structural alteration correlated well with increase in oxygen affinity at alkaline pH determined in flash photolysis experiments. These results suggest that the oxygen binding rate in deoxyHc depends on the coordination geometry of Cu(I) and suggest a structural origin for the Bohr effect in arthropod Hcs.
Mathematical Analysis of a Bohr Atom Model Goong Chen1,2
Hsu, Sze-Bi
Mathematical Analysis of a Bohr Atom Model Goong Chen1,2 , Zhonghai Ding3 , Sze-Bi Hsu1,4 , Moochan Kim2 , and Jianxin Zhou1 Abstract N. Bohr proposed in 1913 a model for atoms and molecules by synthesizing Planck's quantum hypothesis with classical mechanics. When the atom number Z is small, his model
Proton Radius Puzzle 1 Muonic hydrogen and the proton radius
Pachucki, Krzysztof
Proton Radius Puzzle 1 Muonic hydrogen and the proton radius puzzle Randolf Pohl Max-681 Warsaw, Poland Key Words Laser Spectroscopy, Atomic Physics, Proton Structure, Exotic Atoms, Nuclear extraction of the proton radius by Pohl et al. from the mea- #12;Annu. Rev. Nucl. Part. Sci. 2013, Vol. 63
NSDL National Science Digital Library
2011-01-01
ChemTeacher compiles background information, videos, articles, demonstrations, worksheets and activities for high school teachers to use in their classrooms. The Atomic Radius page includes resources for teaching students about trends in atomic radius.
Bohr's correspondence principle: The cases for which it is exact
Makowski, Adam J.; Gorska, Katarzyna J. [Institute of Physics, Nicholas Copernicus University, ul.Grudziadzka 5/7, 87-100 Torun (Poland)
2002-12-01
Two-dimensional central potentials leading to the identical classical and quantum motions are derived and their properties are discussed. Some of zero-energy states in the potentials are shown to cancel the quantum correction Q=(-({Dirac_h}/2{pi}){sup 2}/2m){delta}R/R to the classical Hamilton-Jacobi equation. The Bohr's correspondence principle is thus fulfilled exactly without taking the limits of high quantum numbers, of ({Dirac_h}/2{pi}){yields}0, or of the like. In this exact limit of Q=0, classical trajectories are found and classified. Interestingly, many of them are represented by closed curves. Applications of the found potentials in many areas of physics are briefly commented.
Oral History Interviews at the Niels Bohr Library & Archives
NSDL National Science Digital Library
Housed at the American Institute of Physics, the Niels Bohr Library & Archives is a very useful place for historians of science and others with an interest in the lives and personalities of modern scientists. Here, visitors will find their rather unique collection of oral history interviews. They happen to have more than a thousand, and this site contains transcriptions of several hundred of these interviews, along with a selection of audio clips. The subjects covered in these interviews are rather diverse, and they include conversations about laser science, science education, solid state physics, and science institutions. Visitors can click on to the voice clips to hear such notable personages as Felix Bock, H.R. Crane, and Werner Heisenberg talk about their work. Moving on, visitors can click on the "Read Oral History Transcripts Online" section to browse an alphabetical list of scientists. Finally, visitors can click on the feedback form to ask questions of staff members at the Library.
Challenges to the Bohr Wave Particle Complementarity Principle
Mario Rabinowitz
2012-10-30
Contrary to the Bohr complementarity principle, in 1995 Rabinowitz proposed that by using entangled particles from the source it would be possible to determine which slit a particle goes through while still preserving the interference pattern in the Young two slit experiment. In 2000, Kim et al used spontaneous parametric down conversion to prepare entangled photons as their source, and almost achieved this. In 2012, Menzel et al. experimentally succeeded in doing this. When the source emits entangled particle pairs, the traversed slit is inferred from measurement of the entangled particle location by using triangulation. The violation of complementarity breaches the prevailing probabilistic interpretation of quantum mechanics, and benefits the Bohm pilot wave theory.
Molecular basis of the Bohr effect in arthropod hemocyanin.
Hirota, Shun; Kawahara, Takumi; Beltramini, Mariano; Di Muro, Paolo; Magliozzo, Richard S; Peisach, Jack; Powers, Linda S; Tanaka, Naoki; Nagao, Satoshi; Bubacco, Luigi
2008-11-14
Flash photolysis and K-edge x-ray absorption spectroscopy (XAS) were used to investigate the functional and structural effects of pH on the oxygen affinity of three homologous arthropod hemocyanins (Hcs). Flash photolysis measurements showed that the well-characterized pH dependence of oxygen affinity (Bohr effect) is attributable to changes in the oxygen binding rate constant, k(on), rather than changes in k(off). In parallel, coordination geometry of copper in Hc was evaluated as a function of pH by XAS. It was found that the geometry of copper in the oxygenated protein is unchanged at all pH values investigated, while significant changes were observed for the deoxygenated protein as a function of pH. The interpretation of these changes was based on previously described correlations between spectral lineshape and coordination geometry obtained for model compounds of known structure (Blackburn, N. J., Strange, R. W., Reedijk, J., Volbeda, A., Farooq, A., Karlin, K. D., and Zubieta, J. (1989) Inorg. Chem., 28, 1349-1357). A pH-dependent change in the geometry of cuprous copper in the active site of deoxyHc, from pseudotetrahedral toward trigonal was assigned from the observed intensity dependence of the 1s --> 4p(z) transition in x-ray absorption near edge structure (XANES) spectra. The structural alteration correlated well with increase in oxygen affinity at alkaline pH determined in flash photolysis experiments. These results suggest that the oxygen binding rate in deoxyHc depends on the coordination geometry of Cu(I) and suggest a structural origin for the Bohr effect in arthropod Hcs. PMID:18725416
NASA Astrophysics Data System (ADS)
Ishii, A.; Yoshida, M.; Kato, Y. K.
2015-03-01
Luminescence properties of carbon nanotubes are strongly affected by exciton diffusion, which plays an important role in various nonradiative decay processes. Here we perform photoluminescence microscopy on hundreds of individual air-suspended carbon nanotubes to elucidate the interplay between exciton diffusion, end quenching, and exciton-exciton annihilation processes. A model derived from random-walk theory as well as Monte Carlo simulations are utilized to analyze nanotube length dependence and excitation power dependence of emission intensity. We have obtained the values of exciton diffusion length and absorption cross section for different chiralities, and diameter-dependent photoluminescence quantum yields have been observed. The simulations have also revealed the nature of a one-dimensional coalescence process, and an analytical expression for the power dependence of emission intensity is given.
Quasiparticle gaps and exciton Coulomb energies in Si nanoshells: First-principles calculations
Kimberly Frey; Juan C. Idrobo; Murilo L. Tiago; Fernando A Reboredo; Serdar Ögüt
2009-01-01
Quasiparticle gaps and exciton Coulomb energies of H-passivated spherical Si nanoshells are computed using first-principles DeltaSCF method and selectively comparing to GW computations. We find that the quasiparticle gap of a nanoshell depends on both its inner radius R1 (weakly) and outer radius R2 (strongly). These dependences on R1 and R2 are mostly consistent with electrostatics of a metallic shell.
Transient excitons at metal surfaces
NASA Astrophysics Data System (ADS)
Cui, Xuefeng; Wang, Cong; Argondizzo, Adam; Garrett-Roe, Sean; Gumhalter, Branko; Petek, Hrvoje
2014-07-01
Excitons, electron-hole pairs bound by the Coulomb potential, are the fundamental quasiparticles of coherent light-matter interaction relevant for processes such as photosynthesis and optoelectronics. The existence of excitons in semiconductors is well established. For metals, however, although implied by the quantum theory of the optical response, experimental manifestations of excitons are tenuous owing to screening of the Coulomb interaction taking place on timescales of a few femtoseconds. Here we present direct evidence for the dominant transient excitonic response at a Ag(111) surface, which precedes the full onset of screening of the Coulomb interaction, in the course of a three-photon photoemission process with ~15 fs laser pulses. During this transient regime, electron-hole pair Coulomb interactions introduce coherent quasiparticle correlations beyond the single-particle description of the optics of metals that dominate the multi-photon photoemission process on the timescale of screening at a Ag(111) surface.
DNA-Controlled Excitonic Switches
2012-01-01
Fluorescence resonance energy transfer (FRET) is a promising means of enabling information processing in nanoscale devices, but dynamic control over exciton pathways is required. Here, we demonstrate the operation of two complementary switches consisting of diffusive FRET transmission lines in which exciton flow is controlled by DNA. Repeatable switching is accomplished by the removal or addition of fluorophores through toehold-mediated strand invasion. In principle, these switches can be networked to implement any Boolean function. PMID:22401838
Bohr's complementarity relation and the violation of the CP symmetry in high energy physics
Beatrix C. Hiesmayr; Marcus Huber
2007-01-01
We test Bohr's complementary relation, which captures the most\\u000acounterintuitive difference of a classical and a quantum world, for single and\\u000abipartite neutral kaons. They present a system that is naturally interfering,\\u000aoscillating and decaying. Moreover, kaons break the CP symmetry (C...charge\\u000aconjugation, P...parity). In detail we discuss the effect of the CP violation\\u000aon Bohr's relation, i.e. the effect
Memories of Crisis: Bohr, Kuhn, and the Quantum Mechanical ``Revolution''
NASA Astrophysics Data System (ADS)
Seth, Suman
2013-04-01
``The history of science, to my knowledge,'' wrote Thomas Kuhn, describing the years just prior to the development of matrix and wave mechanics, ``offers no equally clear, detailed, and cogent example of the creative functions of normal science and crisis.'' By 1924, most quantum theorists shared a sense that there was much wrong with all extant atomic models. Yet not all shared equally in the sense that the failure was either terribly surprising or particularly demoralizing. Not all agreed, that is, that a crisis for Bohr-like models was a crisis for quantum theory. This paper attempts to answer four questions: two about history, two about memory. First, which sub-groups of the quantum theoretical community saw themselves and their field in a state of crisis in the early 1920s? Second, why did they do so, and how was a sense of crisis related to their theoretical practices in physics? Third, do we regard the years before 1925 as a crisis because they were followed by the quantum mechanical revolution? And fourth, to reverse the last question, were we to call into the question the existence of a crisis (for some at least) does that make a subsequent revolution less revolutionary?
Nomenclature R mean rotor radius
Belta, Calin A.
Nomenclature R mean rotor radius A compressor duct area ; blade speed at mean radius a speed). Automatica 37 (2001) 921}931 Brief Paper Rotating stall control for axial #ow compressors Calin Belta State University, Baton Rouge, LA 70803-5901, USA Flight Dynamics Directorate, Wright Laboratory, Wright
Adiabatic preparation of a cold exciton condensate
NASA Astrophysics Data System (ADS)
Shahnazaryan, V.; Kyriienko, O.; Shelykh, I. A.
2015-02-01
We propose a scheme for the controllable preparation of a cold indirect exciton condensate using dipolaritonic setup with an optical pumping. Dipolaritons are bosonic quasiparticles which arise from the coupling between cavity photon (C), direct exciton (DX), and indirect exciton (IX) modes and appear in a double quantum well embedded in a semiconductor microcavity. Controlling the detuning between modes of the system, the limiting cases of exciton polaritons and indirect excitons can be realized. Our protocol relies on the initial preparation of an exciton polariton condensate for the far blue-detuned IX mode, with its subsequent adiabatic transformation to an indirect exciton condensate by lowering IX energy via applied electric field. The following allows for generation of a spatially localized cold exciton gas, on the contrary to currently used methods, where IX cloud appears due to diffusion of carriers from spatially separated electron- and hole-rich areas.
Excitons and charged excitons in semiconductor quantum wells
Riva, C. [Departement Natuurkunde, Universiteit Antwerpen (UIA), B-2610 Antwerpen, (Belgium)] [Departement Natuurkunde, Universiteit Antwerpen (UIA), B-2610 Antwerpen, (Belgium); Peeters, F. M. [Departement Natuurkunde, Universiteit Antwerpen (UIA), B-2610 Antwerpen, (Belgium)] [Departement Natuurkunde, Universiteit Antwerpen (UIA), B-2610 Antwerpen, (Belgium); Varga, K. [Physics Department, Argonne National Laboratories, Argonne, Illinois 60439 (United States)] [Physics Department, Argonne National Laboratories, Argonne, Illinois 60439 (United States)
2000-05-15
A variational calculation of the ground-state energy of neutral excitons and of positively and negatively charged excitons (trions) confined in a single-quantum well is presented. We study the dependence of the correlation energy and of the binding energy on the well width and on the hole mass. The conditional probability distribution for positively and negatively charged excitons is obtained, providing information on the correlation and the charge distribution in the system. A comparison is made with available experimental data on trion binding energies in GaAs-, ZnSe-, and CdTe-based quantum well structures, which indicates that trions become localized with decreasing quantum well width. (c) 2000 The American Physical Society.
Exciton dispersion in molecular solids
NASA Astrophysics Data System (ADS)
Cudazzo, Pierluigi; Sottile, Francesco; Rubio, Angel; Gatti, Matteo
2015-03-01
The investigation of the exciton dispersion (i.e. the exciton energy dependence as a function of the momentum carried by the electron–hole pair) is a powerful approach to identify the exciton character, ranging from the strongly localised Frenkel to the delocalised Wannier–Mott limiting cases. We illustrate this possibility at the example of four prototypical molecular solids (picene, pentacene, tetracene and coronene) on the basis of the parameter-free solution of the many-body Bethe–Salpeter equation. We discuss the mixing between Frenkel and charge-transfer excitons and the origin of their Davydov splitting in the framework of many-body perturbation theory and establish a link with model approaches based on molecular states. Finally, we show how the interplay between the electronic band dispersion and the exchange electron–hole interaction plays a fundamental role in setting the nature of the exciton. This analysis has a general validity holding also for other systems in which the electron wavefunctions are strongly localized, as in strongly correlated insulators.
Exciton coupling in molecular crystals
NASA Technical Reports Server (NTRS)
Ake, R. L.
1976-01-01
The implications of perfect exciton coupling and molecular vibrations were investigated, as well as the effect they have on the lifetime of singlet and triplet excitons coupled in a limiting geometry. Crystalline bibenzyl, Cl4Hl4, provided a situation in which these mechanisms involving exciton coupling can be studied in the limit of perfect coupling between units due to the crystal's geometry. This geometry leads to a coupling between the two halves of the molecule resulting in a splitting of the molecular excited states. The study reported involves an experimental spectroscopic approach and begins with the purification of the bibenzyl. The principal experimental apparatus was an emission spectrometer. A closed cycle cryogenic system was used to vary the temperature of the sample between 20 K and 300 K. The desired results are the temperature-dependent emission spectra of the bibenzyl; in addition, the lifetimes and quantum yields measured at each temperature reveal the effect of competing radiationless processes.
Localized molecular excitons in polyaniline
NASA Astrophysics Data System (ADS)
Duke, C. B.; Conwell, E. M.; Paton, A.
1986-10-01
An analysis of the UV absorption spectra of polyemeraldine and nigrosine reveals that the absorption band which appears at about 2.2 eV in both compounds is due to the creation of a localized molecular exciton with the electron on a quinoid moiety and the hole on the neighboring two benzoid moieties. The quinoid moiety is rotated approximately 90° relative to its ground-state conformation. Higher-energy absorption bands may be associated with molecular excitons localized on individual phenyl moieties.
Why has the bohr-sommerfeld model of the atom been ignoredby general chemistry textbooks?
Niaz, Mansoor; Cardellini, Liberato
2011-12-01
Bohr's model of the atom is considered to be important by general chemistry textbooks. A major shortcoming of this model was that it could not explain the spectra of atoms containing more than one electron. In order to increase the explanatory power of the model, Sommerfeld hypothesized the existence of elliptical orbits. This study has the following objectives: 1) Formulation of criteria based on a history and philosophy of science framework; and 2) Evaluation of university-level general chemistry textbooks based on the criteria, published in Italy and U.S.A. Presentation of a textbook was considered to be "satisfactory" if it included a description of the Bohr-Sommerfeld model along with diagrams of the elliptical orbits. Of the 28 textbooks published in Italy that were analyzed, only five were classified as "satisfactory". Of the 46 textbooks published in U.S.A., only three were classified as "satisfactory". This study has the following educational implications: a) Sommerfeld's innovation (auxiliary hypothesis) by introducing elliptical orbits, helped to restore the viability of Bohr's model; b) Bohr-Sommerfeld's model went no further than the alkali metals, which led scientists to look for other models; c) This clearly shows that scientific models are tentative in nature; d) Textbook authors and chemistry teachers do not consider the tentative nature of scientific knowledge to be important; e) Inclusion of the Bohr-Sommerfeld model in textbooks can help our students to understand how science progresses. PMID:24061142
Bohr's Electron was Problematic for Einstein: String Theory Solved the Problem
NASA Astrophysics Data System (ADS)
Webb, William
2013-04-01
Neils Bohr's 1913 model of the hydrogen electron was problematic for Albert Einstein. Bohr's electron rotates with positive kinetic energies +K but has addition negative potential energies - 2K. The total net energy is thus always negative with value - K. Einstein's special relativity requires energies to be positive. There's a Bohr negative energy conflict with Einstein's positive energy requirement. The two men debated the problem. Both would have preferred a different electron model having only positive energies. Bohr and Einstein couldn't find such a model. But Murray Gell-Mann did! In the 1960's, Gell-Mann introduced his loop-shaped string-like electron. Now, analysis with string theory shows that the hydrogen electron is a loop of string-like material with a length equal to the circumference of the circular orbit it occupies. It rotates like a lariat around its centered proton. This loop-shape has no negative potential energies: only positive +K relativistic kinetic energies. Waves induced on loop-shaped electrons propagate their energy at a speed matching the tangential speed of rotation. With matching wave speed and only positive kinetic energies, this loop-shaped electron model is uniquely suited to be governed by the Einstein relativistic equation for total mass-energy. Its calculated photon emissions are all in excellent agreement with experimental data and, of course, in agreement with those -K calculations by Neils Bohr 100 years ago. Problem solved!
Intramolecular radiationless transitions dominate exciton relaxation dynamics
NASA Astrophysics Data System (ADS)
Jumper, Chanelle C.; Anna, Jessica M.; Stradomska, Anna; Schins, Juleon; Myahkostupov, Mykhaylo; Prusakova, Valentina; Oblinsky, Daniel G.; Castellano, Felix N.; Knoester, Jasper; Scholes, Gregory D.
2014-04-01
Reports of long-lived exciton coherences have lead researchers to expect that model dimer systems inevitably generate exciton superposition states observable by two-dimensional electronic spectroscopy. Here we report a careful photophysical characterization of a model dimer system, a diacetylene-linked perylenediimide dimer to examine that issue. The absorption spectrum of the dimer shows molecular exciton splitting, indicating that excitation is delocalized. The assignment of exciton states was supported by other photophysical measurements as well as theoretical calculations. Ultrafast two-dimensional electronic spectroscopy was employed to identify and characterize excitonic and vibrational features, as they evolve over time. Population transfer between the two exciton states is found to happen in <50 fs, thus preventing the sustainment of exciton coherences. We show that such fast radiationless relaxation cannot be explained by coupling to a solvent spectral density and is therefore missed by standard approaches such as Redfield theory and the hierarchical equations of motion.
Subdiffusive exciton transport in quantum dot solids.
Akselrod, Gleb M; Prins, Ferry; Poulikakos, Lisa V; Lee, Elizabeth M Y; Weidman, Mark C; Mork, A Jolene; Willard, Adam P; Bulovi?, Vladimir; Tisdale, William A
2014-06-11
Colloidal quantum dots (QDs) are promising materials for use in solar cells, light-emitting diodes, lasers, and photodetectors, but the mechanism and length of exciton transport in QD materials is not well understood. We use time-resolved optical microscopy to spatially visualize exciton transport in CdSe/ZnCdS core/shell QD assemblies. We find that the exciton diffusion length, which exceeds 30 nm in some cases, can be tuned by adjusting the inorganic shell thickness and organic ligand length, offering a powerful strategy for controlling exciton movement. Moreover, we show experimentally and through kinetic Monte Carlo simulations that exciton diffusion in QD solids does not occur by a random-walk process; instead, energetic disorder within the inhomogeneously broadened ensemble causes the exciton diffusivity to decrease over time. These findings reveal new insights into exciton dynamics in disordered systems and demonstrate the flexibility of QD materials for photonic and optoelectronic applications. PMID:24807586
All conjugated copolymer excitonic multiferroics.
Lohrman, Jessica; Liu, Yueying; Duan, Shaofeng; Zhao, Xiaoyong; Wuttig, Manfred; Ren, Shenqiang
2013-02-01
A substantial magnetoelectric coupling effect of an excitonic all-conjugated block copolymer multiferroics consisting of electronically distinct polythiophene derivatives is reported. The observations open new avenues for the multifunctional all-conjugated block copolymer synthesis and electric field tunable multiferroic devices. PMID:23172730
Spatially indirect excitons in coupled quantum wells
Lai, Chih-Wei Eddy
2004-03-01
Microscopic quantum phenomena such as interference or phase coherence between different quantum states are rarely manifest in macroscopic systems due to a lack of significant correlation between different states. An exciton system is one candidate for observation of possible quantum collective effects. In the dilute limit, excitons in semiconductors behave as bosons and are expected to undergo Bose-Einstein condensation (BEC) at a temperature several orders of magnitude higher than for atomic BEC because of their light mass. Furthermore, well-developed modern semiconductor technologies offer flexible manipulations of an exciton system. Realization of BEC in solid-state systems can thus provide new opportunities for macroscopic quantum coherence research. In semiconductor coupled quantum wells (CQW) under across-well static electric field, excitons exist as separately confined electron-hole pairs. These spatially indirect excitons exhibit a radiative recombination time much longer than their thermal relaxation time a unique feature in direct band gap semiconductor based structures. Their mutual repulsive dipole interaction further stabilizes the exciton system at low temperature and screens in-plane disorder more effectively. All these features make indirect excitons in CQW a promising system to search for quantum collective effects. Properties of indirect excitons in CQW have been analyzed and investigated extensively. The experimental results based on time-integrated or time-resolved spatially-resolved photoluminescence (PL) spectroscopy and imaging are reported in two categories. (i) Generic indirect exciton systems: general properties of indirect excitons such as the dependence of exciton energy and lifetime on electric fields and densities were examined. (ii) Quasi-two-dimensional confined exciton systems: highly statistically degenerate exciton systems containing more than tens of thousands of excitons within areas as small as (10 micrometer){sup 2} were observed. The spatial and energy distributions of optically active excitons were used as thermodynamic quantities to construct a phase diagram of the exciton system, demonstrating the existence of distinct phases. Optical and electrical properties of the CQW sample were examined thoroughly to provide deeper understanding of the formation mechanisms of these cold exciton systems. These insights offer new strategies for producing cold exciton systems, which may lead to opportunities for the realization of BEC in solid-state systems.
Exciton Seebeck effect in molecular systems
NASA Astrophysics Data System (ADS)
Yan, Yun-An; Cai, Shaohong
2014-08-01
We investigate the exciton dynamics under temperature difference with the hierarchical equations of motion. Through a nonperturbative simulation of the transient absorption of a heterogeneous trimer model, we show that the temperature difference causes exciton population redistribution and affects the exciton transfer time. It is found that one can reproduce not only the exciton population redistribution but also the change of the exciton transfer time induced by the temperature difference with a proper tuning of the site energies of the aggregate. In this sense, there exists a site energy shift equivalence for any temperature difference in a broad range. This phenomenon is similar to the Seebeck effect as well as spin Seebeck effect and can be named as exciton Seebeck effect.
EPR before EPR: A 1930 Einstein-Bohr thought Experiment Revisited
ERIC Educational Resources Information Center
Nikolic, Hrvoje
2012-01-01
In 1930, Einstein argued against the consistency of the time-energy uncertainty relation by discussing a thought experiment involving a measurement of the mass of the box which emitted a photon. Bohr seemingly prevailed over Einstein by arguing that Einstein's own general theory of relativity saves the consistency of quantum mechanics. We revisit…
Why We Should Teach the Bohr Model and How to Teach it Effectively
ERIC Educational Resources Information Center
McKagan, S. B.; Perkins, K. K.; Wieman, C. E.
2008-01-01
Some education researchers have claimed that we should not teach the Bohr model of the atom because it inhibits students' ability to learn the true quantum nature of electrons in atoms. Although the evidence for this claim is weak, many have accepted it. This claim has implications for how to present atoms in classes ranging from elementary school…
What Can the Bohr-Sommerfeld Model Show Students of Chemistry in the 21st Century?
ERIC Educational Resources Information Center
Niaz, Mansoor; Cardellini, Liberato
2011-01-01
Bohr's model of the atom is considered to be important by general chemistry textbooks. A shortcoming of this model was that it could not explain the spectra of atoms containing more than one electron. To increase the explanatory power of the model, Sommerfeld hypothesized the existence of elliptical orbits. This study aims to elaborate a framework…
Bohr's complementarity relation and the violation of CP symmetry in the neutral kaon system
Beatrix C. Hiesmayr; Marcus Huber
2008-01-01
We test Bohr's complementary relation, for single and bipartite neutral kaons. They present a system that is oscillating, decaying and breaks the CP symmetry. We discuss the effect of the CP violation and moreover show that the quantity that complements the single partite information for bipartite kaons is an entanglement measure. Which is independent of CP violation, while nonlocality is
Richard Durran; Andrew Neate; Aubrey Truman
2008-01-01
We consider the Bohr correspondence limit of the Schrödinger wave function for an atomic elliptic state. We analyze this limit in the context of Nelson's stochastic mechanics, exposing an underlying deterministic dynamical system in which trajectories converge to Keplerian motion on an ellipse. This solves the long standing problem of obtaining Kepler's laws of planetary motion in a quantum mechanical
A Possible Reconciliation of the Atomic Models of Bohr and of Lewis and Langmuir
W. Hughes
1922-01-01
BROADLY speaking, the merits of Bohr's atomic model lie in its very accurate explanation of the reaction of atoms and molecules with radiation, while those of the Lewis-Langmuir model lie in its very satisfactory representation of the mechanism of chemical combination, but the merits of either model are lacking in the other. Both must therefore possess properties which are accurate
Exciton Binding Energy of Monolayer WS2
Zhu, Bairen; Chen, Xi; Cui, Xiaodong
2015-01-01
The optical properties of monolayer transition metal dichalcogenides (TMDC) feature prominent excitonic natures. Here we report an experimental approach to measuring the exciton binding energy of monolayer WS2 with linear differential transmission spectroscopy and two-photon photoluminescence excitation spectroscopy (TP-PLE). TP-PLE measurements show the exciton binding energy of 0.71 ± 0.01?eV around K valley in the Brillouin zone. PMID:25783023
Exciton Binding Energy of Monolayer WS2.
Zhu, Bairen; Chen, Xi; Cui, Xiaodong
2015-01-01
The optical properties of monolayer transition metal dichalcogenides (TMDC) feature prominent excitonic natures. Here we report an experimental approach to measuring the exciton binding energy of monolayer WS2 with linear differential transmission spectroscopy and two-photon photoluminescence excitation spectroscopy (TP-PLE). TP-PLE measurements show the exciton binding energy of 0.71 ± 0.01?eV around K valley in the Brillouin zone. PMID:25783023
Fission of singlet excitons into triplet-exciton pairs in molecular crystals
M. Chabr; D. F. Williams
1977-01-01
A kinetic model of singlet-exciton fission into pairs of triplet excitons in aromatic hydrocarbon crystals is presented. The model is based on Suna's hopping model for triplet-exciton fusion. The assumptions made in this description of fission and the relationship between fusion and fission are analyzed in detail. According to this theoretical model, the magnetic field modulation of anthracene hot singlet
Extraordinary exciton conductance induced by strong coupling
Johannes Feist; Francisco J. Garcia-Vidal
2015-01-07
We demonstrate that exciton conductance in organic materials can be enhanced by several orders of magnitude when the molecules are strongly coupled to an electromagnetic mode. Using a 1D model system, we show how the formation of a collective polaritonic mode allows excitons to bypass the disordered array of molecules and jump directly from one end of the structure to the other. This finding could have important implications in the fields of exciton transistors, heat transport, photosynthesis, and biological systems in which exciton transport plays a key role.
Cross-polarized excitons in carbon nanotubes
Kilina, Svetlana; Tretiak, Sergei; Doorn, Stephen K.; Luo, Zhengtang; Papadimitrakopoulos, Fotios; Piryatinski, Andrei; Saxena, Avadh; Bishop, Alan R.
2008-01-01
Polarization of low-lying excitonic bands in finite-size semiconducting single-walled carbon nanotubes (SWNTs) is studied by using quantum-chemical methodologies. Our calculations elucidate properties of cross-polarized excitons, which lead to the transverse optical absorption of nanotubes and presumably couple to intermediate-frequency modes recently observed in resonance Raman excitation spectroscopy. We identify up to 12 distinct excitonic transitions below the second fundamental band associated with the E22 van Hove singularity. Calculations for several chiral SWNTs distinguish the optically active “bright” excitonic band polarized parallel to the tube axis and several optically “weak” cross-polarized excitons. The rest are optically (near) forbidden “dark” transitions. An analysis of the transition density matrices related to excitonic bands provides detailed information about delocalization of excitonic wavefunction along the tube. Utilization of the natural helical coordinate system accounting for the tube chirality allows one to disentangle longitudinal and circumferential components. The distribution of the transition density matrix along a tube axis is similar for all excitons. However, four parallel-polarized excitons associated with the E11 transition are more localized along the circumference of a tube, compared with others related to the E12 and E21 cross-polarized transitions. Calculated splitting between optically active parallel- and cross-polarized transitions increases with tube diameter, which compares well with experimental spectroscopic data. PMID:18463293
Excitons in the rare gas solids
Not Available
1988-01-01
Excitons play a prominent role in the chemistry and physics of condensed matter. Excitons in the rare gas solids, the prototypical van der Waals insulators, will be the focus of the remainder of this report. The goal here is to investigate the controversies surrounding the description of excitons in insulators and, therefore the simplest class of these solids, namely the rare gas solids, is chosen as the exemplary system. Specific problems associated with molecular crystals are, therefore, avoided and only the salient features of excitons are thus considered. 47 refs., 9 figs., 4 tabs.
Electric arc radius and characteristics
Fang, T.M.
1980-09-30
The heat transfer equation of an arc discharge has been solved. The arc is assumed to be a cylinder with negligible axial variation and the dominant heat transfer process is conduction radially inside the column and radiation/convection at the outside edge. The symmetric consideration allows a simple one-dimensional formulation. By taking into account proper variation of the electrical conductivity as function of temperature, the heat balance equation has been solved analytically. The radius of the arc and its current-field characteristics have also been obtained. The conventional results that E approx. I/sup 0/ /sup 5385/ and R approx. I/sup 0/ /sup 7693/ with E being the applied field, I the current, and R the radius of the cylindrical arc, have been proved to be simply limiting cases of our more general characteristics. The results can be applied quite widely including, among others, the neutral beam injection project in nuclear fusion and MHD energy conversion.
Neutrino charge radius and substructure
A. Grau; J. A. Grifols
1986-01-01
We use the recent data on numue --> numue and numue --> numue from the CHARM Collaboration to put bounds on possible non-gauge behavior of the charged weak boson. Indeed, present data allow for a neutrino charge radius
Photoexcitation of the triplet exciton in single wall carbon nanotubes
Santos, Tiffany S.
The carbon nanotube photoexcitation spectrum is dominated by excitonic transitions, rather than interband transitions between continuum states. There are eight distinct excitonic transitions (four singlet and four triplet), ...
Exciton circular dichroism in channelrhodopsin.
Pescitelli, Gennaro; Kato, Hideaki E; Oishi, Satomi; Ito, Jumpei; Maturana, Andrés Daniel; Nureki, Osamu; Woody, Robert W
2014-10-16
Channelrhodopsins (ChRs) are of great interest currently because of their important applications in optogenetics, the photostimulation of neurons. The absorption and circular dichroism (CD) spectra of C1C2, a chimera of ChR1 and ChR2 of Chlamydomonas reinhardtii, have been studied experimentally and theoretically. The visible absorption spectrum of C1C2 shows vibronic fine structure in the 470 nm band, consistent with the relatively nonpolar binding site. The CD spectrum has a negative band at 492 nm (??(max) = -6.17 M(-1) cm(-1)) and a positive band at 434 nm (??(max) = +6.65 M(-1) cm(-1)), indicating exciton coupling within the C1C2 dimer. Time-dependent density functional theory (TDDFT) calculations are reported for three models of the C1C2 chromophore: (1) the isolated protonated retinal Schiff base (retPSB); (2) an ion pair, including the retPSB chromophore, two carboxylate side chains (Asp 292, Glu 162), modeled by acetate, and a water molecule; and (3) a hybrid quantum mechanical/molecular mechanical (QM/MM) model depicting the binding pocket, in which the QM part consists of the same ion pair as that in (2) and the MM part consists of the protein residues surrounding the ion pair within 10 Å. For each of these models, the CD of both the monomer and the dimer was calculated with TDDFT. For the dimer, DeVoe polarizability theory and exciton calculations were also performed. The exciton calculations were supplemented by calculations of the coupling of the retinal transition with aromatic and peptide group transitions. For the dimer, all three methods and three models give a long-wavelength C2-axis-polarized band, negative in CD, and a short-wavelength band polarized perpendicular to the C2 axis with positive CD, differing in wavelength by 1-5 nm. Only the retPSB model gives an exciton couplet that agrees qualitatively with experiment. The other two models give a predominantly or solely positive band. We further analyze an N-terminal truncated mutant because it was assumed that the N-terminal domain has a crucial role in the dimerization of ChRs. However, the CD spectrum of this mutant has an exciton couplet comparable to that of the wild-type, demonstrating that it is dimeric. Patch-clamp experiments suggest that the N-terminal domain is involved in protein stabilization and channel kinetics rather than dimerization or channel activity. PMID:25247388
NASA Astrophysics Data System (ADS)
Ohta, S.; Nakano, M.; Kishi, R.; Takahashi, H.; Furukawa, S.
2006-02-01
We develop the second-order Monte Carlo wavefunction (MCWF) approach to the exciton dynamics of molecular aggregate systems composed of dipole-coupled two-state monomers. The explicit form of Lindblad operator, which is indispensable for applying the MCWF approach, for population relaxation among exciton states is derived based on the quantum master equation involving weak exciton-phonon coupling. The exciton migration behaviors obtained by the MCWF approach are turned out to coincide with those by the conventional master equation approach, indicating the high potential of the MCWF approach to the dissipative exciton dynamics of extended molecular aggregates or supermolecules.
Josephson effects in condensates of excitons and exciton polaritons
Shelykh, I. A.; Solnyshkov, D. D.; Pavlovic, G.; Malpuech, G. [Physics Department, University of Iceland, Dunhaga-3, Reykjavik IS-107 (Iceland) and St. Petersburg State Polytechnical University, Polytechnicheskaya 29, St. Petersburg 195251 (Russian Federation); LASMEA, UMR CNRS-Universite Blaise Pascal 6602, 24 Avenue des Landais, Aubiere 63177 Cedex (France)
2008-07-15
We analyze theoretically the phenomena related to the Josephson effect for exciton and polariton condensates, taking into account their specific spin degrees of freedom. We distinguish between two types of Josephson effects: the extrinsic effect, related to the coherent tunneling of particles with the same spin between two spatially separated potential traps, and the intrinsic effect, related to the 'tunneling' between different spinor components of the condensate within the same trap. We show that the Josephson effect in the nonlinear regime can lead to nontrivial polarization dynamics and produce spontaneous separation of the condensates with opposite polarization in real space.
Model for an Exciton Mechanism of Superconductivity
David Allender; John Bardeen
1973-01-01
The exciton mechanism of superconductivity is discussed with respect to a particular model, a thin metal layer on a semiconductor surface. In this model, the metal electrons at the Fermi surface tunnel into the semiconductor gap where they interact with virtual excitons, producing a net attractive interaction among the electrons in direct analogy with the phonon mechanism of superconductivity. The
Diffusion of Singlet Excitons in Tetracene Crystals
G. Vaubel; H. Baessler
1970-01-01
The diffusion length ls of singlet excitons in crystalline tetracene was measured in the temperature range 293 to 160 °K utilizing the quenching effect exerted on the crystal fluorescence by exciton traps located at the crystal surface. It is found that ls = 120 ± 10 Å at room temperature. It increases exponentially with decreasing temperature until it approaches a
Multi-exciton complexes in semiconductors
Z. A. Insepov; G. E. Norman
1977-01-01
The binding energies of complexes consisting of several excitons are estimated. It is assumed that the excitons in the complexes retain their individuality and do not transform into an electron-hole system. It is shown that due to zero oscillations the free complexes are unstable (a similar result has been obtained for clusters consisting of several helium atoms). Complexes formed on
The structure and dynamics of molecular excitons.
Bardeen, Christopher J
2014-01-01
The photophysical behavior of organic semiconductors is governed by their excitonic states. In this review, I classify the three different exciton types (Frenkel singlet, Frenkel triplet, and charge transfer) typically encountered in organic semiconductors. Experimental challenges that arise in the study of solid-state organic systems are discussed. The steady-state spectroscopy of intermolecular delocalized Frenkel excitons is described, using crystalline tetracene as an example. I consider the problem of a localized exciton diffusing in a disordered matrix in detail, and experimental results on conjugated polymers and model systems suggest that energetic disorder leads to subdiffusive motion. Multiexciton processes such as singlet fission and triplet fusion are described, emphasizing the role of spin state coherence and magnetic fields in studying singlet ? triplet pair interconversion. Singlet fission provides an example of how all three types of excitons (triplet, singlet, and charge transfer) may interact to produce useful phenomena for applications such as solar energy conversion. PMID:24313684
The Structure and Dynamics of Molecular Excitons
NASA Astrophysics Data System (ADS)
Bardeen, Christopher J.
2014-04-01
The photophysical behavior of organic semiconductors is governed by their excitonic states. In this review, I classify the three different exciton types (Frenkel singlet, Frenkel triplet, and charge transfer) typically encountered in organic semiconductors. Experimental challenges that arise in the study of solid-state organic systems are discussed. The steady-state spectroscopy of intermolecular delocalized Frenkel excitons is described, using crystalline tetracene as an example. I consider the problem of a localized exciton diffusing in a disordered matrix in detail, and experimental results on conjugated polymers and model systems suggest that energetic disorder leads to subdiffusive motion. Multiexciton processes such as singlet fission and triplet fusion are described, emphasizing the role of spin state coherence and magnetic fields in studying singlet -- triplet pair interconversion. Singlet fission provides an example of how all three types of excitons (triplet, singlet, and charge transfer) may interact to produce useful phenomena for applications such as solar energy conversion.
High efficiency organic multilayer photodetectors based on singlet exciton fission
J. Lee; P. Jadhav; M. A. Baldo
2009-01-01
We employ an exciton fission process that converts one singlet exciton into two triplet excitons to increase the quantum efficiency of an organic multilayer photodetector beyond 100%. The photodetector incorporates ultrathin alternating donor-acceptor layers of pentacene and C60, respectively. By comparing the quantum efficiency after separate pentacene and C60 photoexcitation we find that singlet exciton fission in pentacene enhances the
Long-range excitons in conjugated polymers with ring torsions
Harigaya, Kikuo
Long-range excitons in conjugated polymers with ring torsions Kikuo Harigaya Physical Science) are investigated by the intermediate exciton formalism. Long-range excitons are characterized, and the long-range component of the oscillator strengths is calculated. We nd that ring torsions a ect the long-range excitons
Bandwidth of excitons in LH2 bacterial antenna chromoproteins
Kõu Timpmann; Gediminas Trinkunas; John D. Olsen; C. Neil Hunter; Arvi Freiberg
2004-01-01
The bandwidth of the exciton manifold in LH2 peripheral antenna complexes from the photosynthetic bacterium Rhodobacter sphaeroides has been determined using polarized fluorescence excitation spectroscopy and model simulations. It is shown that the fluorescence anisotropy spectra reveal a hidden structure of the exciton band that is correlated with its boundaries. The estimated exciton coupling energy and exciton bandwidth in the
Quantum Humor: The Playful Side of Physics at Bohr's Institute for Theoretical Physics
NASA Astrophysics Data System (ADS)
Halpern, Paul
2012-09-01
From the 1930s to the 1950s, a period of pivotal developments in quantum, nuclear, and particle physics, physicists at Niels Bohr's Institute for Theoretical Physics in Copenhagen took time off from their research to write humorous articles, letters, and other works. Best known is the Blegdamsvej Faust, performed in April 1932 at the close of one of the Institute's annual conferences. I also focus on the Journal of Jocular Physics, a humorous tribute to Bohr published on the occasions of his 50th, 60th, and 70th birthdays in 1935, 1945, and 1955. Contributors included Léon Rosenfeld, Victor Weisskopf, George Gamow, Oskar Klein, and Hendrik Casimir. I examine their contributions along with letters and other writings to show that they offer a window into some issues in physics at the time, such as the interpretation of complementarity and the nature of the neutrino, as well as the politics of the period.
Bohr's complementarity relation and the violation of the CP symmetry in high energy physics
Hiesmayr, Beatrix C
2007-01-01
We test Bohr's complementary relation, which captures the most counterintuitive difference of a classical and a quantum world, for single and bipartite neutral kaons. They present a system that is naturally interfering, oscillating and decaying. Moreover, kaons break the CP symmetry (C...charge conjugation, P...parity). In detail we discuss the effect of the CP violation on Bohr's relation, i.e. the effect on the "particle-like" information and the "wave-like" information. Further we show that the quantity that complements the single partite information for bipartite kaons is indeed concurrence, a measure of entanglement, strengthening our concept of entanglement. We find that the defined entanglement measure is independent of CP violation while it has been shown that nonlocality is sensitive to CP violation.
Bohr's complementarity relation and the violation of the CP symmetry in high energy physics
Beatrix C. Hiesmayr; Marcus Huber
2007-11-08
We test Bohr's complementary relation, which captures the most counterintuitive difference of a classical and a quantum world, for single and bipartite neutral kaons. They present a system that is naturally interfering, oscillating and decaying. Moreover, kaons break the CP symmetry (C...charge conjugation, P...parity). In detail we discuss the effect of the CP violation on Bohr's relation, i.e. the effect on the "particle-like" information and the "wave-like" information. Further we show that the quantity that complements the single partite information for bipartite kaons is indeed concurrence, a measure of entanglement, strengthening our concept of entanglement. We find that the defined entanglement measure is independent of CP violation while it has been shown that nonlocality is sensitive to CP violation.
On quasi-normal modes, area quantization and Bohr correspondence principle
Christian Corda
2015-03-30
In Int. Journ. Mod. Phys. D 14, 181 (2005), the author Khriplovich verbatim claims that "the correspondence principle does not dictate any relation between the asymptotics of quasinormal modes and the spectrum of quantized black holes" and that "this belief is in conflict with simple physical arguments". In this paper we analyze Khriplovich's criticisms and realize that they work only for the original proposal by Hod, while they do not work for the improvements suggested by Maggiore and recently finalized by the author and collaborators through a connection between Hawking radiation and black hole (BH) quasi-normal modes (QNMs). This is a model of quantum BH somewhat similar to the historical semi-classical model of the structure of a hydrogen atom introduced by Bohr in 1913. Thus, QNMs can be really interpreted as BH quantum levels (the "electrons" of the "Bohr-like BH model"). Our results have also important implications on the BH information puzzle.
On quasi-normal modes, area quantization and Bohr correspondence principle
Corda, Christian
2015-01-01
In Int. Journ. Mod. Phys. D 14, 181 (2005) Khriplovich verbatim claims that "the correspondence principle does not dictate any relation between the asymptotics of quasinormal modes and the spectrum of quantized black holes" and that "this belief is in conflict with simple physical arguments". In this paper we analyze Khriplovich's criticisms and realize that they work only for the original proposal by Hod, while they do not work for the improvements suggested by Maggiore and recently finalized by the author and collaborators through a connection between Hawking radiation and black hole (BH) quasi-normal modes (QNMs). This is a model of quantum BH somewhat similar to the historical semi-classical model of the structure of a hydrogen atom introduced by Bohr in 1913. Thus, QNMs can be really interpreted as BH quantum levels (the "electrons" of the "Bohr-like BH model"). Our results have also important implications on the BH information puzzle.
Electric quadrupole transitions of the Bohr Hamiltonian with the Morse potential
Inci, I. [I.N.F.N. and Dipartimento di Fisica ''Galileo Galilei,'' Universita di Padova, I-35131 Padova (Italy); Department of Physics, Akdeniz University, TR-07058 Antalya (Turkey); Bonatsos, D. [Institute of Nuclear Physics, National Centre for Scientific Research ''Demokritos,'' GR-153 10 Aghia Paraskevi, Attiki (Greece); Boztosun, I. [Department of Physics, Akdeniz University, TR-07058 Antalya (Turkey)
2011-08-15
Eigenfunctions of the collective Bohr Hamiltonian with the Morse potential have been obtained by using the asymptotic iteration method (AIM) for both {gamma}-unstable and rotational structures. B(E2) transition rates have been calculated and compared to experimental data. Overall good agreement is obtained for transitions within the ground-state band, while some interband transitions appear to be systematically underpredicted in {gamma}-unstable nuclei and overpredicted in rotational nuclei.
How Sommerfeld extended Bohr's model of the atom (1913-1916)
NASA Astrophysics Data System (ADS)
Eckert, Michael
2014-04-01
Sommerfeld's extension of Bohr's atomic model was motivated by the quest for a theory of the Zeeman and Stark effects. The crucial idea was that a spectral line is made up of coinciding frequencies which are decomposed in an applied field. In October 1914 Johannes Stark had published the results of his experimental investigation on the splitting of spectral lines in hydrogen (Balmer lines) in electric fields, which showed that the frequency of each Balmer line becomes decomposed into a multiplet of frequencies. The number of lines in such a decomposition grows with the index of the line in the Balmer series. Sommerfeld concluded from this observation that the quantization in Bohr's model had to be altered in order to allow for such decompositions. He outlined this idea in a lecture in winter 1914/15, but did not publish it. The First World War further delayed its elaboration. When Bohr published new results in autumn 1915, Sommerfeld finally developed his theory in a provisional form in two memoirs which he presented in December 1915 and January 1916 to the Bavarian Academy of Science. In July 1916 he published the refined version in the Annalen der Physik. The focus here is on the preliminary Academy memoirs whose rudimentary form is better suited for a historical approach to Sommerfeld's atomic theory than the finished Annalen-paper. This introductory essay reconstructs the historical context (mainly based on Sommerfeld's correspondence). It will become clear that the extension of Bohr's model did not emerge in a singular stroke of genius but resulted from an evolving process.
Bohr-Sommerfeld quantization condition for Dirac states derived from an Ermakov-type invariant
Thylwe, Karl-Erik [KTH-Mechanics, Royal Institute of Technology, S-10044 Stockholm (Sweden)] [KTH-Mechanics, Royal Institute of Technology, S-10044 Stockholm (Sweden); McCabe, Patrick [CCDC, 12 Union Road, CB2 1EZ Cambridge (United Kingdom)] [CCDC, 12 Union Road, CB2 1EZ Cambridge (United Kingdom)
2013-05-15
It is shown that solutions of the second-order decoupled radial Dirac equations satisfy Ermakov-type invariants. These invariants lead to amplitude-phase-type representations of the radial spinor solutions, with exact relations between their amplitudes and phases. Implications leading to a Bohr-Sommerfeld quantization condition for bound states, and a few particular atomic/ionic and nuclear/hadronic bound-state situations are discussed.
Potential surfaces and delocalization of excitons in dimers
NASA Astrophysics Data System (ADS)
Beenken, W. J. D.; Dahlbom, M.; Kjellberg, P.; Pullerits, T.
2002-09-01
In the present work we will demonstrate that the nuclear dynamics have a strong influence on the delocalization of an exciton in a dimer, even if they do not effect the excitonic interaction. It will be shown that the internal nuclear conformation of the molecules forming the dimer depends critically on the delocalization of the exciton state in the dimer and vice versa. The resulting closed loop enforces a localization of the lower excitonic state, but, contrary to the commonly accepted view, a delocalization of the upper one. Qualitatively different time-evolution of the delocalization length for the lower and upper excitonic state will be shown. Besides, it will turn out that the nuclear motions inhibit a complete delocalization of the excitonic state in any case. To accomplish nuclear and exciton dynamics, the nonadiabatic coupling between the two excitonic states will be deduced. This causes a relaxation from the upper to the lower excitonic state, which limits the maximum reachable exciton delocalization.
Radius of curvature controlled mirror
Neil, George R.; Rathke, John Wickham; Schultheiss, Thomas John; Shinn, Michelle D.; Dillon-Townes, Lawrence A.
2006-01-17
A controlled radius of curvature mirror assembly comprising: a distortable mirror having a reflective surface and a rear surface; and in descending order from the rear surface; a counter-distortion plate; a flow diverter having a flow diverter aperture at the center thereof; a flow return plate having a flow return aperture at the center thereof; a thermal isolation plate having a thermal isolation plate aperture at the center thereof and a flexible heater having a rear surface and a flexible heater aperture at the center thereof; a double walled tube defining a coolant feed chamber and a coolant return chamber; said coolant feed chamber extending to and through the flow diverter aperture and terminating at the counter-distortion plate and the coolant return chamber extending to and through the thermal isolation backplate and terminating at the flow diverter; and a coolant feed and a coolant return exit at the rear of said flexible heater.
Exciton localization and drift in tailored-potential quantum nanowires
Szeszko, J., E-mail: justyna.szeszko@epfl.ch; Rudra, A.; Kapon, E. [Laboratory of Physics of Nanostructures, École Polytechnique Fédérale de Lausanne, 1015 Lausanne (Switzerland); Belykh, V. V.; Sibeldin, N. N. [P. N. Lebedev Physical Institute, Russian Academy of Sciences, Moscow 119991 (Russian Federation)
2014-06-30
Exciton recombination dynamics in tailored-potential, site-controlled AlGaAs quantum wires (QWRs) are studied. Time-resolved photoluminescence spectra evidence exciton localization in weakly disordered “uniform” QWRs, whereas deterministic bandgap grading is shown to suppress localization and promote exciton drift along the potential gradient. Measured exciton transit times between two quantum dot probes placed at opposite ends of the potential gradient yield the effective 1D exciton mobility as >1300 cm{sup 2}/(eVs).
Thermalization of free excitons in ZnSe quantum wells
H. Kalt; M. Umlauff; J. Hoffmann; W. Langbein; J. M. Hvam; M. Scholl; J. Söllner; M. Heuken; B. Jobst; D. Hommel
1998-01-01
The thermalization dynamics of hot free excitons in ZnSe quantum wells is studied by photoluminescence excitation (PLE), steady-state (cw-PL) and time-resolved photoluminescence (TRPL). Formation of excitons by emission of LO phonons is very efficient and leads to the generation of narrow hot-exciton distributions. The LO phonon-assisted recombination of the excitons is a direct monitor of the hot-exciton dynamics. A thermalized
A variational Monte Carlo study of exciton condensation
NASA Astrophysics Data System (ADS)
Watanabe, Hiroshi; Seki, Kazuhiro; Yunoki, Seiji
2015-03-01
Exciton condensation in a two-band Hubbard model on a square lattice is studied with variational Monte Carlo method. We show that the phase transition from an excitonic insulator to a band insulator is induced by increasing the interband Coulomb interaction. To examine the character of the exciton condensation, the exciton pair amplitudes both in k-space and in real space are calculated. Using these quantities, we discuss the BCS-BEC crossover within the excitonic insulator phase.
Harvesting Excitons Through Plasmonic Strong Coupling
Gonzalez-Ballestero, Carlos; Moreno, Esteban; Garcia-Vidal, Francisco J
2015-01-01
Exciton harvesting is demonstrated in an ensemble of quantum emitters coupled to localized surface plasmons. When the interaction between emitters and the dipole mode of a metallic nanosphere reaches the strong coupling regime, the exciton conductance is greatly increased. The spatial map of the conductance matches the plasmon field intensity profile, which indicates that transport properties can be tuned by adequately tailoring the field of the plasmonic resonance. We exploit this effect in a plasmonic nanostructure composed of three metal nanospheres, showing efficient transport of excitons into a very deep sub- wavelength region.
Singlet exciton fission in solution
NASA Astrophysics Data System (ADS)
Walker, Brian J.; Musser, Andrew J.; Beljonne, David; Friend, Richard H.
2013-12-01
Singlet exciton fission, the spin-conserving process that produces two triplet excited states from one photoexcited singlet state, is a means to circumvent the Shockley-Queisser limit in single-junction solar cells. Although the process through which singlet fission occurs is not well characterized, some local order is thought to be necessary for intermolecular coupling. Here, we report a triplet yield of 200% and triplet formation rates approaching the diffusion limit in solutions of bis(triisopropylsilylethynyl (TIPS)) pentacene. We observe a transient bound excimer intermediate, formed by the collision of one photoexcited and one ground-state TIPS-pentacene molecule. The intermediate breaks up when the two triplets separate to each TIPS-pentacene molecule. This efficient system is a model for future singlet-fission materials and for disordered device components that produce cascades of excited states from sunlight.
Multiple Exciton Generation Solar Cells
Luther, J. M.; Semonin, O. E.; Beard, M. C.; Gao, J.; Nozik, A. J.
2012-01-01
Heat loss is the major factor limiting traditional single junction solar cells to a theoretical efficiency of 32%. Multiple Exciton Generation (MEG) enables efficient use of the solar spectrum yielding a theoretical power conversion efficiency of 44% in solar cells under 1-sun conditions. Quantum-confined semiconductors have demonstrated the ability to generate multiple carriers but present-day materials deliver efficiencies far below the SQ limit of 32%. Semiconductor quantum dots of PbSe and PbS provide an active testbed for developing high-efficiency, inexpensive solar cells benefitting from quantum confinement effects. Here, we will present recent work of solar cells employing MEG to yield external quantum efficiencies exceeding 100%.
Singlet exciton fission in solution.
Walker, Brian J; Musser, Andrew J; Beljonne, David; Friend, Richard H
2013-12-01
Singlet exciton fission, the spin-conserving process that produces two triplet excited states from one photoexcited singlet state, is a means to circumvent the Shockley-Queisser limit in single-junction solar cells. Although the process through which singlet fission occurs is not well characterized, some local order is thought to be necessary for intermolecular coupling. Here, we report a triplet yield of 200% and triplet formation rates approaching the diffusion limit in solutions of bis(triisopropylsilylethynyl (TIPS)) pentacene. We observe a transient bound excimer intermediate, formed by the collision of one photoexcited and one ground-state TIPS-pentacene molecule. The intermediate breaks up when the two triplets separate to each TIPS-pentacene molecule. This efficient system is a model for future singlet-fission materials and for disordered device components that produce cascades of excited states from sunlight. PMID:24256865
Can disorder enhance incoherent exciton diffusion?
Lee, Elizabeth M Y; Willard, Adam P
2015-01-01
Recent experiments aimed at probing the dynamics of excitons have revealed that semiconducting films composed of disordered molecular subunits, unlike expectations for their perfectly ordered counterparts, can exhibit a time-dependent diffusivity in which the effective early time diffusion constant is larger than that of the steady state. This observation has led to speculation about what role, if any, microscopic disorder may play in enhancing exciton transport properties. In this article, we present the results of a model study aimed at addressing this point. Specifically, we present a general model, based upon F\\"orster theory, for incoherent exciton diffusion in a material composed of independent molecular subunits with static energetic disorder. Energetic disorder leads to heterogeneity in molecule-to-molecule transition rates which we demonstrate has two important consequences related to exciton transport. First, the distribution of local site-specific diffusivity is broadened in a manner that results i...
Topologically protected excitons in porphyrin thin films
Joel Yuen-Zhou; Semion S. Saikin; Norman Y. Yao; Alán Aspuru-Guzik
2014-06-05
The control of exciton transport in organic materials is of fundamental importance for the development of efficient light-harvesting systems. This transport is easily deteriorated by traps in the disordered energy landscape. Here, we propose and analyze a system that supports topological Frenkel exciton edge states. Backscattering of these chiral Frenkel excitons is prohibited by symmetry, ensuring that the transport properties of such a system are robust against disorder. To implement our idea, we propose a two-dimensional periodic array of tilted porphyrins interacting with a homogenous magnetic field. This field serves to break time-reversal symmetry and results in lattice fluxes that that mimic the Aharonov-Bohm phase acquired by electrons. Our proposal is the first blueprint for realizing topological phases of matter in molecular aggregates and suggests a paradigm for engineering novel excitonic materials.
Correlated exciton dynamics in semiconductor nanostructures
Wen, Patrick, Ph. D. Massachusetts Institute of Technology
2013-01-01
The absorption and dissipation of energy in semiconductor nanostructures are often determined by excited electron dynamics. In semiconductors, one fundamentally important electronic state is an exciton, an excited electron ...
Triplet excitons: Bringing dark states to light
NASA Astrophysics Data System (ADS)
Bardeen, Christopher J.
2014-11-01
Semiconducting quantum dots have been used to harvest triplet excitons produced through singlet fission in organic semiconductors. These hybrid organic-inorganic materials may boost the efficiency of solar cells.
Excitonic Coherent States: Symmetries and Thermalization
NASA Astrophysics Data System (ADS)
Cirilo-Lombardo, Diego Julio
2015-04-01
In this paper we considered the theoretical treatment of a physical system of excitons and its behaviour under temperature by means of a new coherent state construction of bounded states in a quantum field theoretical context.
Spatiotemporal dynamics of quantum-well excitons
Hui Zhao; B. dal Don; S. Moehl; H. Kalt; K. Ohkawa; D. Hommel
2003-01-01
We investigate the lateral transport of excitons in ZnSe quantum wells by using time-resolved micro-photoluminescence enhanced by the introduction of a solid immersion lens. The spatial and temporal resolutions are 200 nm and 5 ps, respectively. Strong deviation from classical diffusion is observed up to 400 ps. This feature is attributed to the hot-exciton effects, consistent with previous experiments under
Exciton properties of selected aromatic hydrocarbon systems
NASA Astrophysics Data System (ADS)
Roth, Friedrich; Mahns, Benjamin; Hampel, Silke; Nohr, Markus; Berger, Helmuth; Büchner, Bernd; Knupfer, Martin
2013-02-01
We have examined the singlet excitons in two representatives of acene-type (tetracene and pentacene) and phenacene-type (chrysene and picene) molecular crystals, respectively, using electron energy-loss spectroscopy at low temperatures. We show that the excitation spectra of the two hydrocarbon families significantly differ. Moreover, close inspection of the data indicates that there is an increasing importance of charge-transfer excitons at lowest excitation energy with increasing length of the molecules.
Physical theory of excitons in conducting polymers.
Brazovskii, Serguei; Kirova, Natasha
2010-07-01
In this tutorial review, we cover the solid state physics approach to electronic and optical properties of conducting polymers. We attempt to bring together languages and advantages of the solid state theory for polymers and of the quantum chemistry for monomers. We consider polymers as generic one-dimensional semiconductors with features of strongly correlated electronic systems. Our model combines the long range electron-hole Coulomb attraction with a specific effect of strong intra-monomer electronic correlations, which results in effective intra-monomer electron-hole repulsion. Our approach allows to go beyond the single-chain picture and to compare excitons for polymers in solutions and in films. The approach helps connecting such different questions as shallow singlet and deep triplet excitons, stronger binding of interchain excitons in films, crossings of excitons' branches, 1/N energies shifts in oligomers. We describe a strong suppression of the luminescence from free charge carriers by long-range Coulomb interactions. Main attention is devoted to the most requested in applications phenyl based polymers. The specifics of the benzene ring monomer give rise to existence of three possible types of excitons: Wannier-Mott, Frenkel and intermediate ones. We discuss experimental manifestations of various excitons and of their transformations. We touch effects of the time-resolved self-trapping by libron modes leading to formation of torsion polarons. PMID:20517580
Mirror with thermally controlled radius of curvature
Neil, George R.; Shinn, Michelle D.
2010-06-22
A radius of curvature controlled mirror for controlling precisely the focal point of a laser beam or other light beam. The radius of curvature controlled mirror provides nearly spherical distortion of the mirror in response to differential expansion between the front and rear surfaces of the mirror. The radius of curvature controlled mirror compensates for changes in other optical components due to heating or other physical changes. The radius of curvature controlled mirror includes an arrangement for adjusting the temperature of the front surface and separately adjusting the temperature of the rear surface to control the radius of curvature. The temperature adjustment arrangements can include cooling channels within the mirror body or convection of a gas upon the surface of the mirror. A control system controls the differential expansion between the front and rear surfaces to achieve the desired radius of curvature.
Effect of Quantum and Dielectric Confinement on the Exciton-Exciton
Tretiak, Sergei
in the case of epitaxial quantum wells and superlattices. Heterostructuring is also becoming more common experimentally demonstrated that using ZnSe- (core)/CdSe(shell) NCs tunable between type I and type IIEffect of Quantum and Dielectric Confinement on the Exciton-Exciton Interaction Energy in Type II
Microscopic theories of excitons and their dynamics
NASA Astrophysics Data System (ADS)
Berkelbach, Timothy C.
This thesis describes the development and application of microscopically-defined theories of excitons in a wide range of semiconducting materials. In Part I, I consider the topic of singlet exciton fission, an organic photophysical process which generates two spin-triplet excitons from one photoexcited spin-singlet exciton. I construct a theoretical framework that couples a realistic treatment of the static electronic structure with finite-temperature quantum relaxation techniques. This framework is applied separately, but consistently, to the problems of singlet fission in pentacene dimers, crystalline pentacene, and crystalline hexacene. Through this program, I am able to rationalize observed behaviors and make non-trivial predictions, some of which have been confirmed by experiment. In Part II, I present theoretical developments on the properties of neutral excitons and charged excitons (trions) in atomically thin transition metal dichalcogenides. This work includes an examination of material trends in exciton binding energies via an effective mass approach. I also present an experimental and theoretical collaboration, which links the unconventional disposition of excitons in the Rydberg series to the peculiar screening properties of atomically thin materials. The light-matter coupling in these materials is examined within low-energy models and is shown to give rise to bright and dark exciton states, which can be qualitatively labeled in analogy with the hydrogen series. In Part III, I explore theories of relaxation dynamics in condensed phase environments, with a focus on methodology development. This work is aimed towards biological processes, including resonant energy transfer in chromophore complexes and electron transfer in donor-bridge-acceptor systems. Specifically, I present a collaborative development of a numerically efficient but highly accurate hybrid approach to reduced dynamics, which exploits a partitioning of environmental degrees of freedom into those that evolve "fast" and "slow," as compared to the internal system dynamics. This method is tested and applied to the spin-boson model, a two-site Frenkel exciton model, and the seven-site Fenna-Matthews-Olson complex. I conclude with a collaborative analysis of a recently developed polaron-transformed quantum master equation, which is shown to accurately interpolate between the well-known Redfield and Forster theories, even in challenging donor-bridge-acceptor arrangements.
Low-speed limit of Bohr{close_quote}s stopping-power formula
Sigmund, P. [Physics Department, Odense University, DK-5230 Odense M (Denmark)] [Physics Department, Odense University, DK-5230 Odense M (Denmark)
1996-10-01
Bohr{close_quote}s classical (nonrelativistic) model of charged-particle stopping is evaluated explicitly for arbitrary values of projectile charge and speed. This removes the logarithmic cutoff from the original expression and generates a stopping formula which can be utilized also to extend the range of practical applicability of Bloch{close_quote}s theory. As expected such a formulation appears to be a better starting point than the Born approximation for estimating stopping powers of heavy ions at velocities {ital v}{lt}{ital Z}{sub 1}{ital e}{sup 2}/{h_bar}. {copyright} {ital 1996 The American Physical Society.}
Parameter-Free Solution of the Bohr Hamiltonian for Actinides Critical in the Octupole Mode
D. Lenis; Dennis Bonatsos
2005-12-06
An analytic, parameter-free (up to overall scale factors) solution of the Bohr Hamiltonian involving axially symmetric quadrupole and octupole deformations, as well as an infinite well potential, is obtained, after separating variables in a way reminiscent of the Variable Moment of Inertia (VMI) concept. Normalized spectra and B(EL) ratios are found to agree with experimental data for 226-Ra and 226-Th, the nuclei known to lie closest to the border between octupole deformation and octupole vibrations in the light actinide region.
Exactly separable Bohr Hamiltonian with the Morse potential for triaxial nuclei
NASA Astrophysics Data System (ADS)
Inci, I.
2014-08-01
In this paper, the Morse potential is used in the ?-part of the collective Bohr Hamiltonian for triaxial nuclei. Energy eigenvalues and eigenfunctions are obtained in a closed form through exactly separating the Hamiltonian into its variables by using an appropriate form of the potential. The results are applied to generate the nuclear spectrum of 192Pt, 194Pt and 196Pt isotopes which are known to be the best candidate exhibiting triaxiality. Electric quadrupole transition ratios are calculated and then compared with the experimental data and the Z(5) model results.
Excitons in the optical properties of nanotubes
NASA Astrophysics Data System (ADS)
Spataru, Catalin
2006-03-01
We present ab initio calculation of self-energy and electron-hole interaction (excitonic) effects on the optical spectra of single-walled carbon and BN nanotubes. We employed a many-electron Green's function approach that determines both the quasiparticle and optical excitations from first principles. We found important many-electron effects that explain many of the puzzling experimental findings in the optical spectrum of these quasi-one dimensional systems, and the calculated spectra are in excellent quantitative agreement with measurements. In carbon nanotubes, excitons can bind by as much as one eV in semiconducting nanotubes^a). We discovered that bound excitons also exist in metallic carbon nanotubes with binding energy of many tens of meVs^a). Excitonic effects are shown to be even more inportant in BN nanotubes than in carbon nanotubes. Unlike the carbon nanotubes, theory predicts that excitons in some BN nanotubes are comprised of coherent superposition of transitions from several different subband pairs^b). We have also calculated the radiative lifetime of excitons in semiconducting carbon nanotubes. Assuming a thermal occupation of bright and dark exciton bands, we find an effective radiative lifetime of the order of 10 ns at room temperature, in good accord with recent experiments^c). a) C.D. Spataru, S. Ismail-Beigi, L.X. Benedict and S.G. Louie, Phys. Rev. Lett. 92, 077402 (2004). b) C.-H. Park, C.D. Spataru and S.G. Louie, to be published. c) C.D. Spataru, S. Ismail-Beigi, R.B. Capaz and S.G. Louie, in press Phys. Rev. Lett.
Exciton formation, relaxation, and decay in PCDTBT.
Banerji, Natalie; Cowan, Sarah; Leclerc, Mario; Vauthey, Eric; Heeger, Alan J
2010-12-15
The nature and time evolution of the primary excitations in the pristine conjugated polymer, PCDTBT, are investigated by femtosecond-resolved fluorescence up-conversion spectroscopy. The extensive study includes data from PCDTBT thin film and from PCDTBT in chlorobenzene solution, compares the fluorescence dynamics for several excitation and emission wavelengths, and is complemented by polarization-sensitive measurements. The results are consistent with the photogeneration of mobile electrons and holes by interband ?-?* transitions, which then self-localize within about 100 fs and evolve to a bound singlet exciton state in less than 1 ps. The excitons subsequently undergo successive migrations to lower energy localized states, which exist as a result of disorder. In parallel, there is also slow conformational relaxation of the polymer backbone. While the initial self-localization occurs faster than the time resolution of our experiment, the exciton formation, exciton migration, and conformational changes lead to a progressive relaxation of the inhomogeneously broadened emission spectrum with time constants ranging from about 500 fs to tens of picoseconds. The time scales found here for the relaxation processes in pristine PCDTBT are compared to the time scale (<0.2 ps) previously reported for photoinduced charge transfer in phase-separated PCDTBT:fullerene blends (Phys. Rev. B 2010, 81, 125210). We point out that exciton formation and migration in PCDTBT occur at times much longer than the ultrafast photoinduced electron transfer time in PCDTBT:fullerene blends. This disparity in time scales is not consistent with the commonly proposed idea that photoinduced charge separation occurs after diffusion of the polymer exciton to a fullerene interface. We therefore discuss alternative mechanisms that are consistent with ultrafast charge separation before localization of the primary excitation to form a bound exciton. PMID:21087001
F4TCNQ-Induced Exciton Quenching Studied by Using in-situ Photoluminescence Measurements
NASA Astrophysics Data System (ADS)
Zhu, Jian; Lu, Min; Wu, Bo; Hou, Xiao-Yuan
2012-09-01
The role of F4TCNQ as an exciton quenching material in thin organic light-emitting films is investigated by means of in situ photoluminescence measurements. C60 was used as another quenching material in the experiment for comparison, with Alq3 as a common organic light-emitting material. The effect of the growth sequence of the materials on quenching was also examined. It is found that the radius of Förster energy transfer between F4TCNQ and Alq3 is close to 0 nm and Dexter energy transfer dominates in the quenching process.
Solar Radius at Minimum of Cycle 23
NASA Astrophysics Data System (ADS)
Sigismondi, Costantino
2008-09-01
Observations of Baily beads in French Guyana, during 2006 September 22 annular eclipse, have been made to measure solar radius around solar minimum activity of cycle 23. The correction to standard solar radius at unit distance (1 AU) 959.63" to fit observations is ?R? = -0.01" ± 0.17". Sources of errors are outlined in view of relativistic accuracies.
Investigating exciton correlations using coherent multidimensional optical spectroscopy
Turner, Daniel Burton
2010-01-01
The optical measurements described in this thesis reveal interactions among bound electron-hole pairs known as excitons in a semiconductor nanostructure. Excitons are quasiparticles that can form when light is absorbed by ...
Hot exciton transport in ZnSe quantum wells
Zhao, Hui; Moehl, Sebastian; Wachter, Sven; Kalt, Heinz
2002-02-01
The in-plane transport of excitons in ZnSe quantum wells is investigated directly by microphotoluminescence in combination with a solid immersion lens. Due to the strong Froehlich coupling, the initial kinetic energy of the excitons is well...
Coherence Length of Excitons in a Semiconductor Quantum Well
Zhao, Hui; Moehl, Sebastian; Kalt, Heinz
2002-08-01
wells is determined to be 300–400 nm, about 25–30 times the exciton de Broglie wavelength. With increasing exciton kinetic energy, the coherence length decreases slowly. The discrepancy between the coherence lengths measured and calculated by considering...
Photoexcitation and Exciton Transport in Molecular Crystals
NASA Astrophysics Data System (ADS)
Irkhin, Pavel
This work is dedicated to the investigation of exciton transport processes in organic molecular crystals, and to the optical characterization of an important organic semiconductor, rubrene single crystal. Rubrene is a high quality molecular crystal that has been found to have exceptional functionalities in electronic applications such as field effect transistors. I present the intrinsic absorption and photoluminescence spectra of rubrene single crystals, deriving them from a series of experiments performed in different experimental geometries. I describe the absorption spectra for all three principal light polarizations in the crystal, and discuss how the strongly anisotropic absorption and emission properties affect the spectral characteristics of observed photoluminescence spectra. I identify vibronic progressions both in absorption and emission and discuss their parameters and the main vibrational modes that are responsible for them. Through careful analysis of the data, I arrive at a conclusion that absorption and emission of rubrene that is not polarized along a specific crystallographic and molecular direction (c-axis of the crystal, corresponding to the M-axis of the molecule) is not due to an electronic dipole matrix element that has components in that direction. Instead, it is caused by vibronically-induced depolarization of the electronic HOMO-LUMO transition that is described by a dipole matrix element that has components along the c-axis. Further, this work developed and demonstrated a direct imaging technique that allowed to directly observe the diffusion of excitons in rubrene single crystals. This simple and intuitive technique uses localized photoexcitation and spatially resolved detection of the excitonic luminescence to visualize the spatial distribution of excitons. The technique was then used to directly measure the diffusion length of triplet excitons in rubrene. To do this, I exploited the fact that in rubrene photoexcited singlet excitons undergo efficient fission into triplet excitons, and the latter can interact with each other to create photoluminescing singlet excitons again. I show that the exciton mobility in rubrene is strongly anisotropic, with long-range diffusion by several micrometers associated only with the direction of molecular stacking in the crystal, the same direction for which a large charge carrier mobility has been observed in experiments with field effect transistors.
Einstein–Bohr recoiling double-slit gedanken experiment performed at the molecular level
NASA Astrophysics Data System (ADS)
Liu, Xiao-Jing; Miao, Quan; Gel'Mukhanov, Faris; Patanen, Minna; Travnikova, Oksana; Nicolas, Christophe; Ågren, Hans; Ueda, Kiyoshi; Miron, Catalin
2015-02-01
Double-slit experiments illustrate the quintessential proof for wave–particle complementarity. If information is missing about which slit the particle has traversed, the particle, behaving as a wave, passes simultaneously through both slits. This wave-like behaviour and corresponding interference is absent if ‘which-slit’ information exists. The essence of Einstein–Bohr's debate about wave–particle duality was whether the momentum transfer between a particle and a recoiling slit could mark the path, thus destroying the interference. To measure the recoil of a slit, the slits should move independently. We showcase a materialization of this recoiling double-slit gedanken experiment by resonant X-ray photoemission from molecular oxygen for geometries near equilibrium (coupled slits) and in a dissociative state far away from equilibrium (decoupled slits). Interference is observed in the former case, while the electron momentum transfer quenches the interference in the latter case owing to Doppler labelling of the counter-propagating atomic slits, in full agreement with Bohr's complementarity.
The theory of the Bohr-Weisskopf effect in the hyperfine structure
F. F. Karpeshin; M. B. Trzhaskovskaya
2015-03-21
For twenty years research into the anomalies in the HF spectra was going in a wrong direction by fighting the related Bohr-Weisskopf effect. As a way out, the model-independent way is proposed of estimating the nuclear radii from the hyper-fine splitting. The way is based on analogy of HFS to internal conversion coefficients, and the Bohr-Weisskopf effect - to the anomalies in the internal conversion coefficients. This makes transparent It is shown that the parameters which can be extracted from the data are the even nuclear moments of the magnetization distribution. The radii $R_2$ and (for the first time) $R_4$ are thus obtained by analysis of the experimental HFS for the H- and Li-like ions of $^{209}$Bi. The critical prediction of the HFS for the $2p_{1/2}$ state is discussed. The moments may be determined in this way only if the higher QED effects are properly taken into account. Therefore, this set of the parameters form a basis of a strict QED test. Experimental prospects are discussed, aimed at retrieving data on the HFS values for a set of a few-electron configurations of the atom.
Theory of optical spectra of exciton condensates
Chu, H.; Chang, Y.C. [Department of Physics and Materials Research Laboratory, University of Illinois at Urbana-Champaign, 1110 West Green Street, Urbana, Illinois 61801-3080 (United States)] [Department of Physics and Materials Research Laboratory, University of Illinois at Urbana-Champaign, 1110 West Green Street, Urbana, Illinois 61801-3080 (United States)
1996-08-01
We present theoretical calculations of optical absorption-gain spectra for condensed excitons in both three dimensional (3D), 2D, and quasi-2D systems (appropriate for semiconductor quantum wells) as functions of the electron-hole pair density and temperature. The ladder diagram contribution to the vertex function (exciton effect) is included in our calculation. We found that fluctuations cannot destroy the condensation of excitons in 2D at finite temperatures as opposed to a theory of free bosons and the case of Cooper pairs in superconductivity. Such a difference is attributed to the fact that electrons and holes carry opposite charges as opposed to the same charges carried by the Cooper pairs in the case of superconductivity. Our studies show that the effects of exciton Bose condensation on the absorption-gain spectra remain present for temperatures up to 130 K for a 2D system with exciton binding energy of 30{endash}40 meV (appropriate for ZnSe systems). {copyright} {ital 1996 The American Physical Society.}
Optical signatures of a fully dark exciton condensate
NASA Astrophysics Data System (ADS)
Combescot, Monique; Combescot, Roland; Alloing, Mathieu; Dubin, François
2014-02-01
We propose optical means to reveal the presence of a dark exciton condensate that does not yield any photoluminescence at all. We show that i) the dark exciton density can be obtained from the blueshift of the excitonic absorption line induced by dark excitons; ii) the polarization of the dark condensate can be obtained from the blueshift dependence on the probe photon polarization as well as from the Faraday effect. All these effects result from carrier exchanges between dark and bright states.
Interaction of Excitonic Magnetic Polaron Pairs in Diluted Magnetic Semiconductor
NASA Astrophysics Data System (ADS)
Nakamura, Kiko
2006-05-01
Excitonic magnetic polarons in diluted magnetic semiconductor formed by the interaction between exciton carriers and magnetic ions within the range of exciton wave function are theoretically investigated. The Heitler-London model is applied to the polaron molecule. The binding energy of the exiton pair in triplet state is calculated and it is found that an attraction between triplet magnetic polaron pairs can occur.
Singlet exciton fission in pure and doped anthracene
K. von Burg; I. Zschokke-Gränacher
1979-01-01
On the basis of the kinetic model by Merrifield for triplet exciton fusion in molecular crystals, we have derived an expression which describes the reciprocal process, i.e., the anisotropy of the prompt fluorescence in the presence of singlet exciton fission in a magnetic field. For the first time a thorough comparison of Merrifield's theory with singlet exciton fission experiments is
Singlet exciton fission in pure and doped anthracene
K. von Burg
1979-01-01
On the basis of the kinetic model by Merrifield for triplet exciton fusion in molecular crystals, we have derived an expression which describes the reciprocal process, i.e., the anisotropy of the prompt fluorescence in the presence of singlet exciton fission in a magnetic field. For the first time a thorough comparison of Merrifield’s theory with singlet exciton fission experiments is
Exciton-Plasmon States in Nanoscale Materials: Breakdown of the
Marini, Andrea
Exciton-Plasmon States in Nanoscale Materials: Breakdown of the Tamm-Dancoff Approximation Myrta propagating only forward in time. However, we show that in nanoscale materials excitons and plasmons hybridize, creating exciton-plasmon states where the electron-hole pairs oscillate back and forth in time. Then
J. V. Kilmartin; L. ROSSI-BERNARDI
1969-01-01
Blocking of the alpha-amino groups by cyanate inhibits the uptake of CO2 by haemoglobin. It also inhibits the influence which changes in pCO2 at constant pH normally have on the oxygen affinity and on the Bohr effect. Blocking the alpha-amino groups of the alpha chain reduces the alkaline Bohr effect by 25 per cent.
Heat pumping with optically driven excitons
Erik M. Gauger; Joachim Wabnig
2010-06-07
We present a theoretical study showing that an optically driven excitonic two-level system in a solid state environment acts as a heat pump by means of repeated phonon emission or absorption events. We derive a master equation for the combined phonon bath and two-level system dynamics and analyze the direction and rate of energy transfer as a function of the externally accessible driving parameters. We discover that if the driving laser is detuned from the exciton transition, cooling the phonon environment becomes possible.
Device design for optimal exciton harvesting
NASA Astrophysics Data System (ADS)
Ingram, G. L.; Lu, Z. H.
2014-10-01
Organic light emitting diodes (OLEDs) show potential as the next generation solid state lighting technology. A major barrier to widespread adoption at this point is the efficiency droop that occurs for OLEDs at practical brightness (~ 5000 cd/m2) levels necessary for general lighting. We highlight recent progress in highly efficient OLEDs at high brightness, where improvements are made by managing excitons in these devices through rational device design. General design principles for monochrome OLEDs are discussed based on recent device architectures that have been successfully implemented. We expect that an improved understanding of exciton dynamics in OLEDs in combination with innovative device design will drive future development.
Triplet exciton dynamics in rubrene single crystals
NASA Astrophysics Data System (ADS)
Ryasnyanskiy, Aleksandr; Biaggio, Ivan
2011-11-01
The decay of the photoluminescence excited in rubrene single crystals by picosecond pulses is measured over 7 orders of magnitude and more than 4 time decades. We identify the typical decay dynamics due to triplet-triplet interaction. We show that singlet exciton fission and triplet fusion quantum yields in rubrene are both very large, and we directly determine a triplet exciton lifetime of 100±20 ?s, which explains the delayed buildup of a large photocurrent that has been reported earlier for low excitation densities.
Bright and dark excitons in semiconductor carbon nanotubes
Tretiak, Sergei [Los Alamos National Laboratory
2008-01-01
We report electronic structure calculations of finite-length semiconducting carbon nanotubes using the time dependent density functional theory (TD-DFT) and the time dependent Hartree Fock (TD-HF) approach coupled with semiempirical AM1 and ZINDO Hamiltonians. We specifically focus on the energy splitting, relative ordering, and localization properties of the optically active (bright) and optically forbidden (dark) states from the lowest excitonic band of the nanotubes. These excitonic states are very important in competing radiative and non-radiative processes in these systems. Our analysis of excitonic transition density matrices demonstrates that pure DFT functionals overdelocalize excitons making an electron-hole pair unbound; consequently, excitonic features are not presented in this method. In contrast, the pure HF and A111 calculations overbind excitons inaccurately predicting the lowest energy state as a bright exciton. Changing AM1 with ZINDO Hamiltonian in TD-HF calculations, predicts the bright exciton as the second state after the dark one. However, in contrast to AM1 calculations, the diameter dependence of the excitation energies obtained by ZINDO does not follow the experimental trends. Finally, the TD-DFT approach incorporating hybrid functions with a moderate portion of the long-range HF exchange, such as B3LYP, has the most generality and predictive capacity providing a sufficiently accurate description of excitonic structure in finite-size nanotubes. These methods characterize four important lower exciton bands. The lowest state is dark, the upper band is bright, and the two other dark and nearly degenerate excitons lie in-between. Although the calculated energy splittings between the lowest dark and the bright excitons are relatively large ({approx}0.1 eV), the dense excitonic manifold below the bright exciton allows for fast non-radiative relaxation leasing to the fast population of the lowest dark exciton. This rationalizes the low luminescence efficiency in nanotubes.
NASA Astrophysics Data System (ADS)
Singh, Jai; Koblov, Alexander
2013-02-01
Using the derived expression for the light yield in a scintillator, the influence of linear radiative and non-radiative (quenching) rates on the nonproportionality in light yield is studied. It is found that if the excitation created within the electron track initiated by a ?-photon incident on a scintillator remains mainly excitonic, then nonproportionality can be minimised by inventing a scintillator material with linear radiative rate >107 s-1, linear quenching rate <106 s-1 and track radius ?70 nm along with maintaining the rates of other nonlinear processes as discovered earlier. If one can increase the linear radiative rate to 109 s-1, then the nonproportionality can be eliminated at a track radius >20 nm.
High efficiency organic multilayer photodetectors based on singlet exciton fission
NASA Astrophysics Data System (ADS)
Lee, J.; Jadhav, P.; Baldo, M. A.
2009-07-01
We employ an exciton fission process that converts one singlet exciton into two triplet excitons to increase the quantum efficiency of an organic multilayer photodetector beyond 100%. The photodetector incorporates ultrathin alternating donor-acceptor layers of pentacene and C60, respectively. By comparing the quantum efficiency after separate pentacene and C60 photoexcitation we find that singlet exciton fission in pentacene enhances the quantum efficiency by (45±7)%. In quantitative agreement with this result, we also observe that the photocurrent generated from pentacene excitons is decreased by (2.7±0.2)% under an applied magnetic field of H =0.4 T, while the C60 photocurrent is relatively unchanged.
Laser differential confocal paraboloidal vertex radius measurement.
Yang, Jiamiao; Qiu, Lirong; Zhao, Weiqian; Shen, Yang; Jiang, Hongwei
2014-02-15
This Letter proposes a laser differential confocal paraboloidal vertex radius measurement (DCPRM) method for the high-accuracy measurement of the paraboloidal vertex radius of curvature. DCPRM constructs an autocollimation vertex radius measurement light path for the paraboloid by placing a reflector in the incidence light path. This technique is based on the principle that a paraboloid can aim a parallel beam at its focus without aberration and uses differential confocal positioning technology to identify the paraboloid focus and vertex accurately. Measurement of the precise distance between these two positions is achieved to determine the paraboloid vertex radius. Preliminary experimental results indicate that DCPRM has a relative expanded uncertainty of less than 0.001%. PMID:24562218
Solution to the Proton Radius Problem
D. Robson
2015-01-27
The relationship between the electric form factors for the proton in the rest frame and in the Breit momentum frame is used to provide a value for the difference in the mean squared charge radius of the proton evaluated in the two frames. Associating the muonic-hydrogen data analysis for the proton charge radius of 0.84087 fm with the rest frame and associating the electron scattering with the Breit frame yields a prediction 0f 0.87944 fm for the proton radius in the relativistic frame. The most recent value deduced via electron scattering from the proton is 0.877(6)fm so that the frame dependence used here yields a plausible solution to the proton radius puzzle.
ERIC Educational Resources Information Center
Gjedde, Albert
2010-01-01
The year 2010 is the centennial of the publication of the "Seven Little Devils" in the predecessor of "Acta Physiologica". In these seven papers, August and Marie Krogh sought to refute Christian Bohr's theory that oxygen diffusion from the lungs to the circulation is not entirely passive but rather facilitated by a specific cellular activity…
K. -H. Schmidt
2008-10-30
The problem of an apparent inconsistency between the fission rates derived on the basis of Bohr-Wheeler's transition-state method and Kramers' dynamical model of nuclear fission, first pointed out by Strutinsky in 1973, is revisited. The study is based on studying the features of individual trajectories on the fission path.
Dynamics of the excitonic coupling in organic crystals.
Aragó, Juan; Troisi, Alessandro
2015-01-16
We show that the excitonic coupling in molecular crystals undergoes a very large fluctuation at room temperature as a result of the combined thermal motions of the nuclei. This observation dramatically affects the description of exciton transport in organic crystals and any other phenomenon (like singlet fission or exciton dissociation) that originates from an exciton in a molecular crystal or thin film. This unexpected result is due to the predominance of the short-range excitonic coupling mechanisms (exchange, overlap, and charge-transfer mediated) over the Coulombic excitonic coupling for molecules in van der Waals contact. To quantify this effect we develop a procedure to evaluate accurately the short-range excitonic coupling (via a diabatization scheme) along a molecular dynamics trajectory of the representative molecular crystals of anthracene and tetracene. PMID:25635554
Ultrafast dynamics of exciton fission in polycrystalline pentacene.
Wilson, Mark W B; Rao, Akshay; Clark, Jenny; Kumar, R Sai Santosh; Brida, Daniele; Cerullo, Giulio; Friend, Richard H
2011-08-10
We use ultrafast transient absorption spectroscopy with sub-20 fs time resolution and broad spectral coverage to directly probe the process of exciton fission in polycrystalline thin films of pentacene. We observe that the overwhelming majority of initially photogenerated singlet excitons evolve into triplet excitons on an ?80 fs time scale independent of the excitation wavelength. This implies that exciton fission occurs at a rate comparable to phonon-mediated exciton localization processes and may proceed directly from the initial, delocalized, state. The singlet population is identified due to the brief presence of stimulated emission, which is emitted at wavelengths which vary with the photon energy of the excitation pulse, a violation of Kasha's Rule that confirms that the lowest-lying singlet state is extremely short-lived. This direct demonstration that triplet generation is both rapid and efficient establishes multiple exciton generation by exciton fission as an attractive route to increased efficiency in organic solar cells. PMID:21755937
Coherent coupling between exciton resonances governed by the disorder potential
NASA Astrophysics Data System (ADS)
Glinka, Yuri D.; Sun, Zheng; Erementchouk, Mikhail; Leuenberger, Michael N.; Bristow, Alan D.; Cundiff, Steven T.; Bracker, Allan S.; Li, Xiaoqin
2013-08-01
Monolayer fluctuations in the thickness of a semiconductor quantum well (QW) lead to the formation of spectrally resolved excitons located in the narrower, average, and thicker regions of the QW. Whether or not these excitons are coherently coupled via Coulomb interaction is a long-standing debate. We demonstrate that different types of disorder potential govern coherent coupling among excitons, and the coupling strength can be quantitatively measured using optical two-dimensional Fourier transform spectroscopy. Strong coherent coupling occurs between certain types of excitons but is missing between other types of excitons because the distinctive nature of excitons results in different spatial overlap. Our finding may be applicable to other disordered systems, such as photosynthesis and conjugated polymers, where exciton coupling plays a critical role in determining charge and energy transfer.
The ``graviton picture'': a Bohr model for gravitation on galactic scales?
NASA Astrophysics Data System (ADS)
Trippe, Sascha
2015-02-01
Modified Newtonian Dynamics (MOND) provides a successful description of stellar and galactic dynamics on almost all astronomical scales. A key feature of MOND is the transition function from Newtonian to modified dynamics which corresponds to the empirical mass discrepancy--acceleration (MDA) relation. However, the functional form of the MDA relation does not follow from theory in a straightforward manner; in general, empirical MDA relations are inserted ad hoc into analyses of stellar dynamics. I revisit the possibility of gravity being mediated by massive virtual particles, gravitons. Under certain reasonable assumptions, the resulting "graviton picture" implies a MDA relation that is equivalent to the - empirical - "simple mu" function of MOND which is in very good agreement with observations. I conclude that the "graviton picture" offers a simple description of gravitation on galactic scales, potentially playing a role for gravitation analogous to the role played by Bohr's model for atomic physics.
The "Graviton Picture": a Bohr Model for Gravitation on Galactic Scales?
Sascha Trippe
2014-05-06
Modified Newtonian Dynamics (MOND) provides a successful description of stellar and galactic dynamics on almost all astronomical scales. A key feature of MOND is the transition function from Newtonian to modified dynamics which corresponds to the empirical mass discrepancy--acceleration (MDA) relation. However, the functional form of the MDA relation does not follow from theory in a straightforward manner; in general, empirical MDA relations are inserted ad hoc into analyses of stellar dynamics. I revisit the possibility of gravity being mediated by massive virtual particles, gravitons. Under certain reasonable assumptions, the resulting "graviton picture" implies a MDA relation that is equivalent to the - empirical - "simple mu" function of MOND which is in very good agreement with observations. I conclude that the "graviton picture" offers a simple description of gravitation on galactic scales, potentially playing a role for gravitation analogous to the role played by Bohr's model for atomic physics.
Mass tensor in the Bohr Hamiltonian from the nondiagonal energy weighted sum rules
Jolos, R. V. [Joint Institute for Nuclear Research, RU-141980 Dubna (Russian Federation); Institut fuer Kernphysik der Universitaet zu Koeln, D-50937 Koeln (Germany); Brentano, P. von [Institut fuer Kernphysik der Universitaet zu Koeln, D-50937 Koeln (Germany)
2009-04-15
Relations are derived in the framework of the Bohr Hamiltonian that express the matrix elements of the deformation-dependent components of the mass tensor through the experimental data on the energies and the E2 transitions relating the low-lying collective states. These relations extend the previously obtained results for the intrinsic mass coefficients of the well-deformed axially symmetric nuclei on nuclei of arbitrary shape. The expression for the mass tensor is suggested, which is sufficient to satisfy the existing experimental data on the energy weighted sum rules for the E2 transitions for the low-lying collective quadrupole excitations. The mass tensor is determined for {sup 106,108}Pd, {sup 108-112}Cd, {sup 134}Ba, {sup 150}Nd, {sup 150-154}Sm, {sup 154-160}Gd, {sup 164}Dy, {sup 172}Yb, {sup 178}Hf, {sup 188-192}Os, and {sup 194-196}Pt.
Tailoring quantum dot assemblies to extend exciton coherence times and improve exciton transport
NASA Astrophysics Data System (ADS)
Seward, Kenton T.
Electron energy transfer (EET) through nanostructured assemblies plays a crucial role in a wide range of emerging technologies such as quantum dot solar cells, quantum computing, molecular electronics, excitonic transistors, and light emitting diodes. These technologies are very dependent on excitonic lifetimes which are short on the order of a nanosecond. In order to efficiently use this short time scale, EET needs to be as fast as possible. This leads to an interest in the application of coherent exciton transfer. To examine the possibility of coherent transfer, we ask a simple question: How rapidly do coherent superpositions of excitonic states dephase between quantum dots?. We assume that the major source of decoherence at room temperature is from the internal phonon modes of silicon quantum dots. The question is then addressed using a combination of ab initio calculations and a master equation formulation for the evolution of the electronic density operator for a dimer of interacting two-level systems coupled to a shared bath of harmonic oscillators. A combination of density functional theory (DFT) and frozen phonon method (FPM) analysis was used to obtain exciton-phonon coupling in various sizes of silicon quantum dots. As expected, coherent EET is faster in comparison to incoherent EET in assemblies of identical nanostructures. In cases of non-identical assemblies of nanostructures, the low energy regions act as trap states for the exciton. In this case, a combination of coherent and incoherent transport leads to the fastest transport rate.
Molecular Basis of the Bohr Effect in Arthropod Hemocyanin*S?
Hirota, Shun; Kawahara, Takumi; Beltramini, Mariano; Di Muro, Paolo; Magliozzo, Richard S.; Peisach, Jack; Powers, Linda S.; Tanaka, Naoki; Nagao, Satoshi; Bubacco, Luigi
2008-01-01
Flash photolysis and K-edge x-ray absorption spectroscopy (XAS) were used to investigate the functional and structural effects of pH on the oxygen affinity of three homologous arthropod hemocyanins (Hcs). Flash photolysis measurements showed that the well-characterized pH dependence of oxygen affinity (Bohr effect) is attributable to changes in the oxygen binding rate constant, kon, rather than changes in koff. In parallel, coordination geometry of copper in Hc was evaluated as a function of pH by XAS. It was found that the geometry of copper in the oxygenated protein is unchanged at all pH values investigated, while significant changes were observed for the deoxygenated protein as a function of pH. The interpretation of these changes was based on previously described correlations between spectral lineshape and coordination geometry obtained for model compounds of known structure (Blackburn, N. J., Strange, R. W., Reedijk, J., Volbeda, A., Farooq, A., Karlin, K. D., and Zubieta, J. (1989) Inorg. Chem.,28 ,1349 -1357). A pH-dependent change in the geometry of cuprous copper in the active site of deoxyHc, from pseudotetrahedral toward trigonal was assigned from the observed intensity dependence of the 1s ? 4pz transition in x-ray absorption near edge structure (XANES) spectra. The structural alteration correlated well with increase in oxygen affinity at alkaline pH determined in flash photolysis experiments. These results suggest that the oxygen binding rate in deoxyHc depends on the coordination geometry of Cu(I) and suggest a structural origin for the Bohr effect in arthropod Hcs. PMID:18725416
Singlet exciton fission in a hexacene derivative.
Lee, Jiye; Bruzek, Matthew J; Thompson, Nicholas J; Sfeir, Matthew Y; Anthony, John E; Baldo, Marc A
2013-03-13
Hexacene, an acene with six benzene rings, is notable for its exceptionally small triplet energy, around one third of the singlet energy. Herein, singlet fission, i.e., conversion of a singlet exciton into two triplets, is demonstrated in a thin film of hexacene derivative, employing both transient absorption spectroscopy and magnetic field effects on photocurrent. PMID:23293054
Magnetic field effects on quantum ring excitons
Jakyoung Song; Sergio E. Ulloa
2001-01-01
We study the effect of magnetic field and geometric confinement on excitons confined to a quantum ring. We use analytical matrix elements of the Coulomb interaction and diagonalize numerically the effective-mass Hamiltonian of the problem. To explore the role of different boundary conditions, we investigate the quantum ring structure with a parabolic confinement potential, which allows the wave functions to
NSDL National Science Digital Library
2010-10-01
The year 2010 is the centennial of the publication of the Â?Seven Little DevilsÂ? in the predecessor of Acta Physiologica. In these seven papers, August and Marie Krogh sought to refute Christian Bohr's theory that oxygen diffusion from the lungs to the circulation is not entirely passive but rather facilitated by a specific cellular activity substitute to secretion. The subjects of the present reevaluation of this controversy are Christian Bohr, Professor and Doctor of Medicine (1855Â?1911), nominated three times for the Nobel Prize; August Krogh, Doctor of Philosophy (1874Â?1949), Christian Bohr's assistant and later Nobel Prize laureate (1920); and Marie Krogh, nÃ©e JÃ¸rgensen, Doctor of Medicine and wife of August Krogh (1874Â?1943). The controversy concerned is the transport of oxygen from the lungs into the bloodstream: are passive transport and diffusion capacity together sufficient to secure the oxygen supply in all circumstances or is there an additional specific (Â?energy consumingÂ? or Â?activeÂ?) mechanism responsible for the transport of oxygen from the alveoli into the bloodstream? The present discussion purports to show that the contestants' views were closer than the parties themselves and posterity recognized. Posterity has judged the dispute unilaterally from the Nobel laureate's point of view, but it is evident that August Krogh's Nobel Prize was awarded for the discovery of a cellular activity (Christian Bohr's expression), represented by Krogh's discovery of capillary recruitment. Christian Bohr appears to have been correct in the narrower sense that the diffusion capacity at rest is not great enough to explain the transport during work; a special mechanism intervenes and optimizes the conditions under which diffusion acts. August Krogh, of course, was right in the wider sense that the transport mechanism itself is always entirely passive.
Excess equimolar radius of liquid drops.
Horsch, Martin; Hasse, Hans; Shchekin, Alexander K; Agarwal, Animesh; Eckelsbach, Stefan; Vrabec, Jadran; Müller, Erich A; Jackson, George
2012-03-01
The curvature dependence of the surface tension is related to the excess equimolar radius of liquid drops, i.e., the deviation of the equimolar radius from the radius defined by the macroscopic capillarity approximation. Based on the Tolman [J. Chem. Phys. 17, 333 (1949)] approach and its interpretation by Nijmeijer et al. [J. Chem. Phys. 96, 565 (1991)], the surface tension of spherical interfaces is analyzed in terms of the pressure difference due to curvature. In the present study, the excess equimolar radius, which can be obtained directly from the density profile, is used instead of the Tolman length. Liquid drops of the truncated and shifted Lennard-Jones fluid are investigated by molecular dynamics simulation in the canonical ensemble, with equimolar radii ranging from 4 to 33 times the Lennard-Jones size parameter ?. In these simulations, the magnitude of the excess equimolar radius is shown to be smaller than ?/2. This suggests that the surface tension of liquid drops at the nanometer length scale is much closer to that of the planar vapor-liquid interface than reported in studies based on the mechanical route. PMID:22587106
Holographic radius test plates for spherical surfaces with large radius of curvature.
Wang, Quandou; Griesmann, Ulf; Soons, Johannes A
2014-07-10
We describe a novel interferometric method, based on nested Fresnel zone lenses or photon sieves, for testing and measuring the radius of curvature of precision spherical surfaces that have radii in a range between several meters and a few hundred meters. We illustrate the measurement concept with radius measurements of a spherical mirror with a radius of about 10 m. The measured radius is 9877??mm±10??mm for a coverage factor k=2. Our measurements also demonstrate, for the first time to the best of our knowledge, the utility of photon sieves for precision surface metrology because they diffuse higher diffraction orders of computer generated holograms, which reduces coherent noise. PMID:25090075
Mass and radius of cosmic balloons
Wang, Y. (NASA/Fermilab Astrophysics Center, Fermi National Accelerator Laboratory, Batavia, Illinois 60510-0500 (United States))
1994-05-15
Cosmic balloons are spherical domain walls with relativistic particles trapped inside. We derive the exact mass and radius relations for a static cosmic balloon using Gauss-Codazzi equations. The cosmic balloon mass as a function of its radius, [ital M]([ital R]), is found to have a functional form similar to that of fermion soliton stars, with a fixed point at 2[ital GM]([ital R])/[ital R][congruent]0.486, which corresponds to the limit of infinite central density. We derive a simple analytical approximation for the mass density of a spherically symmetric relativistic gas star. When applied to the computation of the mass and radius of a cosmic balloon, the analytical approximation yields fairly good agreement with the exact numerical solutions.
Nuclear charge radius of $^8$He
P. Mueller; I. A. Sulai; A. C. C. Villari; J. A. Alcantara-Nunez; R. Alves-Conde; K. Bailey; G. W. F. Drake; M. Dubois; C. Eleon; G. Gaubert; R. J. Holt; R. V. F. Janssens; N. Lecesne; Z. -T. Lu; T. P. O'Connor; M. -G. Saint-Laurent; J. -C. Thomas; L. -B. Wang
2008-01-03
The root-mean-square (rms) nuclear charge radius of ^8He, the most neutron-rich of all particle-stable nuclei, has been determined for the first time to be 1.93(3) fm. In addition, the rms charge radius of ^6He was measured to be 2.068(11) fm, in excellent agreement with a previous result. The significant reduction in charge radius from ^6He to ^8He is an indication of the change in the correlations of the excess neutrons and is consistent with the ^8He neutron halo structure. The experiment was based on laser spectroscopy of individual helium atoms cooled and confined in a magneto-optical trap. Charge radii were extracted from the measured isotope shifts with the help of precision atomic theory calculations.
Exciton-exciton and exciton-phonon interactions in an interfacial GaAs quantum dot ensemble
G. Moody; M. E. Siemens; A. D. Bristow; X. Dai; D. Karaiskaj; A. S. Bracker; D. Gammon; S. T. Cundiff
2011-01-01
Using optical two-dimensional Fourier transform spectroscopy, we report temperature- and excitation-density-dependent measurements of the homogeneous linewidth of the exciton ground-state transition in a single layer of interfacial GaAs quantum dots (QDs). We show that the homogeneous linewidth increases nonlinearly with temperature from 6 to 50 K and that the thermal broadening is well described by an activation term and offset.
On the Spectral Radius of Graphs with Cut Vertices
Abraham Berman; Xiao-dong Zhang
2001-01-01
We study the spectral radius of graphs with n vertices and k cut vertices and describe the graph that has the maximal spectral radius in this class. We also discuss the limit point of the maximal spectral radius.
Optical properties of MgZnO alloys: Excitons and exciton-phonon complexes
NASA Astrophysics Data System (ADS)
Neumann, M. D.; Cobet, C.; Esser, N.; Laumer, B.; Wassner, T. A.; Eickhoff, M.; Feneberg, M.; Goldhahn, R.
2011-07-01
The characteristics of the excitonic absorption and emission around the fundamental bandgap of wurtzite MgxZn1-xO grown on c-plane sapphire substrates by plasma assisted molecular beam epitaxy with Mg contents between x = 0 and x = 0.23 are studied using spectroscopic ellipsometry and photoluminescence (PL) measurements. The ellipsometric data were analyzed using a multilayer model yielding the dielectric function (DF). The imaginary part of the DF for the alloys exhibits a pronounced feature which is attributed to exciton-phonon coupling (EPC) similar to the previously reported results for ZnO. Thus, in order to determine reliable transition energies, the spectral dependence is analyzed by a model which includes free excitonic lines, the exciton continuum, and the enhanced absorption due to EPC. A line shape analysis of the temperature-dependent PL spectra yielded in particular the emission-related free excitonic transition energies, which are compared to the results from the DF line-shape analysis. The PL linewidth is discussed within the framework of an alloy disorder model.
Ultrafast dynamics of excitons in tetracene single crystals
Birech, Zephania; Schwoerer, Heinrich, E-mail: heso@sun.ac.za [Laser Research Institute, Stellenbosch University, Stellenbosch 7600 (South Africa)] [Laser Research Institute, Stellenbosch University, Stellenbosch 7600 (South Africa); Schwoerer, Markus [Department of Physics, University of Bayreuth, Bayreuth (Germany)] [Department of Physics, University of Bayreuth, Bayreuth (Germany); Schmeiler, Teresa; Pflaum, Jens [Experimental Physics VI, University of Würzburg and Bavarian Center for Applied Energy Research, Würzburg (Germany)] [Experimental Physics VI, University of Würzburg and Bavarian Center for Applied Energy Research, Würzburg (Germany)
2014-03-21
Ultrafast exciton dynamics in free standing 200 nm thin tetracene single crystals were studied at room temperature by femtosecond transient absorption spectroscopy in the visible spectral range. The complex spectrally overlapping transient absorption traces of single crystals were systematically deconvoluted. From this, the ultrafast dynamics of the ground, excited, and transition states were identified including singlet exciton fission into two triplet excitons. Fission is generated through both, direct fission of higher singlet states S{sub n} on a sub-picosecond timescale, and thermally activated fission of the singlet exciton S{sub 1} on a 40 ps timescale. The high energy Davydov component of the S{sub 1} exciton is proposed to undergo fission on a sub-picoseconds timescale. At high density of triplet excitons their mutual annihilation (triplet-triplet annihilation) occurs on a <10 ps timescale.
Nanotransformation and current fluctuations in exciton condensate junctions.
Soller, H; Dolcini, F; Komnik, A
2012-04-13
We analyze the nonlinear transport properties of a bilayer exciton condensate that is contacted by four metallic leads by calculating the full counting statistics of electron transport for arbitrary system parameters. Despite its formal similarity to a superconductor the transport properties of the exciton condensate turn out to be completely different. We recover the generic features of exciton condensates such as counterpropagating currents driven by excitonic Andreev reflections and make predictions for nonlinear transconductance between the layers as well as for the current (cross)correlations and generalized Johnson-Nyquist relationships. Finally, we explore the possibility of connecting another mesoscopic system (in our case a quantum point contact) to the bottom layer of the exciton condensate and show how the excitonic Andreev reflections can be used for transforming voltage at the nanoscale. PMID:22587267
Fluorescence spectroscopy, exciton dynamics, and photochemistry of single allophycocyanin trimers
Ying, L.; Sie, X.S. [Pacific Northwest National Lab., Richland, WA (United States). William R. Wiley Environmental Molecular Sciences Lab.] [Pacific Northwest National Lab., Richland, WA (United States). William R. Wiley Environmental Molecular Sciences Lab.
1998-12-10
The authors report a study of the allophycocyanin trimer (APC), a light-harvesting protein complex from cyanobacteria, by room-temperature single-molecule measurements of fluorescence spectra, lifetimes, intensity trajectories, and polarization modulation. Emission spectra of individual APC trimers are found to be homogeneous on the time scale of seconds. In contrast, their emission lifetimes are found to be widely distributed because of generation of long-lived exciton traps during the course of measurements. The intensity trajectories and polarization modulation experiments indicate reversible exciton trap formation within the three quasi-independent pairs of strong interacting {alpha}84 and {beta}84 chromophores in APC, as well as photobleaching of individual chromophores. Comparison experiments under continuous-wave and pulsed excitation reveal a two-photon mechanism for generating exciton traps and/or photobleaching, which involves exciton-exciton annihilation. These single-molecule experiments provide new insights into the spectroscopy, exciton dynamics, and photochemistry of light-harvesting complexes.
Fluorescence Spectroscopy, Exciton Dynamics and Photochemistry of Single Allophycocyanin Trimers
Ying, Liming (ASSOC WESTERN UNIVERSITY) [ASSOC WESTERN UNIVERSITY; Xie, Xiaoliang (HARVARD UNIVERSITY) [HARVARD UNIVERSITY
1998-01-01
We report a study of the spectroscopy and exciton dynamics of the allophycocyanin trimer (APC), a light harvesting protein complex from cyanobacteria, by room-temperature single-molecule measurements of fluorescence spectra, lifetimes, intensity trajectories and polarization modulation. Emission spectra of individual APC trimers are found to be homogeneous on the time scale of seconds. In contrast, their emission lifetimes are found to be widely distributed, because of generation of exciton traps during the course of measurements. The intensity trajectories and polarization modulation experiments indicate reversible ixciton trap formation within the three quasi-independent pairs of strong interacting a84 and B84 chromophores in APC, as well a photobleaching of individual chromophores. Comparison experiments under continuous wave and pulsed excitation reveal a two-photon mechanism for generating exciton traps and/or photobleaching, which involves exciton-exciton annihilation. These single-molecule experiments provide new insights into exciton dynamics and photochemistry of light-harvesting complexes.
Singlet exciton fission in nanostructured organic solar cells.
Jadhav, Priya J; Mohanty, Aseema; Sussman, Jason; Lee, Jiye; Baldo, Marc A
2011-04-13
Singlet exciton fission is an efficient multiexciton generation process in organic molecules. But two concerns must be satisfied before it can be exploited in low-cost solution-processed organic solar cells. Fission must be combined with longer wavelength absorption in a structure that can potentially surpass the single junction limit, and its efficiency must be demonstrated in nanoscale domains within blended devices. Here, we report organic solar cells comprised of tetracene, copper phthalocyanine, and the buckyball C(60). Short wavelength light generates singlet excitons in tetracene. These are subsequently split into two triplet excitons and transported through the phthalocyanine. In addition, the phthalocyanine absorbs photons below the singlet exciton energy of tetracene. To test tetracene in nanostructured blends, we fabricate coevaporated bulk heterojunctions and multilayer heterojunctions of tetracene and C(60). We measure a singlet fission efficiency of (71 ± 18)%, demonstrating that exciton fission can efficiently compete with exciton dissociation on the nanoscale. PMID:21355536
Ultrafast dynamics of excitons in tetracene single crystals
NASA Astrophysics Data System (ADS)
Birech, Zephania; Schwoerer, Markus; Schmeiler, Teresa; Pflaum, Jens; Schwoerer, Heinrich
2014-03-01
Ultrafast exciton dynamics in free standing 200 nm thin tetracene single crystals were studied at room temperature by femtosecond transient absorption spectroscopy in the visible spectral range. The complex spectrally overlapping transient absorption traces of single crystals were systematically deconvoluted. From this, the ultrafast dynamics of the ground, excited, and transition states were identified including singlet exciton fission into two triplet excitons. Fission is generated through both, direct fission of higher singlet states Sn on a sub-picosecond timescale, and thermally activated fission of the singlet exciton S1 on a 40 ps timescale. The high energy Davydov component of the S1 exciton is proposed to undergo fission on a sub-picoseconds timescale. At high density of triplet excitons their mutual annihilation (triplet-triplet annihilation) occurs on a <10 ps timescale.
Ultrafast dynamics of excitons in tetracene single crystals.
Birech, Zephania; Schwoerer, Markus; Schmeiler, Teresa; Pflaum, Jens; Schwoerer, Heinrich
2014-03-21
Ultrafast exciton dynamics in free standing 200 nm thin tetracene single crystals were studied at room temperature by femtosecond transient absorption spectroscopy in the visible spectral range. The complex spectrally overlapping transient absorption traces of single crystals were systematically deconvoluted. From this, the ultrafast dynamics of the ground, excited, and transition states were identified including singlet exciton fission into two triplet excitons. Fission is generated through both, direct fission of higher singlet states S(n) on a sub-picosecond timescale, and thermally activated fission of the singlet exciton S1 on a 40 ps timescale. The high energy Davydov component of the S1 exciton is proposed to undergo fission on a sub-picoseconds timescale. At high density of triplet excitons their mutual annihilation (triplet-triplet annihilation) occurs on a <10 ps timescale. PMID:24655187
Singlet Exciton Fission in Nanostructured Organic Solar Cells
Jadhav, P. J.; Mohanty, A.; Sussman, J.; Baldo, Marc
2011-01-01
Singlet exciton fission is an efficient multiexciton generation process in organic molecules. But two concerns must be satisfied before it can be exploited in low-cost solution-processed organic solar cells. Fission must be combined with longer wavelength absorption in a structure that can potentially surpass the single junction limit, and its efficiency must be demonstrated in nanoscale domains within blended devices. Here, we report organic solar cells comprised of tetracene, copper phthalocyanine, and the buckyball C{sub 6}0. Short wavelength light generates singlet excitons in tetracene. These are subsequently split into two triplet excitons and transported through the phthalocyanine. In addition, the phthalocyanine absorbs photons below the singlet exciton energy of tetracene. To test tetracene in nanostructured blends, we fabricate coevaporated bulk heterojunctions and multilayer heterojunctions of tetracene and C{sub 60}. We measure a singlet fission efficiency of (71 ± 18)%, demonstrating that exciton fission can efficiently compete with exciton dissociation on the nanoscale.
Excitonic ring formation in ultrapure bulk GaAs
NASA Astrophysics Data System (ADS)
Bieker, S.; Henn, T.; Kiessling, T.; Ossau, W.; Molenkamp, L. W.
2014-11-01
We report on spatially resolved low-temperature photoluminescence (PL) measurements of excitons in ultrapure bulk GaAs. At moderate excitation densities we observe butterfly-shaped luminescence images in the wavelength-radial distance plane with a pronounced quench of the exciton PL intensity at the excitation center. The shapes of the PL images show a delicate dependence on excitation wavelength and pump power. We present a model that quantitatively explains the PL intensity quench by a localized overheating of the exciton ensemble due to nonresonant optical excitation. Our model allows us to extract absolute exciton temperatures and to trace the influence of excitation excess energy on the spatial dependence of the exciton energy relaxation. We observe temperature gradients in the exciton system which persist over distances ?10 ? m away from the excitation spot.
Exciton-phonon interaction and Raman intensity of carbon nanotubes
NASA Astrophysics Data System (ADS)
Saito, Riichiro; Jiang, Jie; Jorio, Ado; Sato, Kentaro; Dresselhaus, Gene; Dresselhaus, Millie
2007-03-01
Using extended tight binding framework, the exciton states and exciton-phonon interaction are calculated for understanding optical properties of single wall carbon nanotubes. Resonance Raman intensity for first and second order Raman processes are calculated as a function of (n,m) with use of exciton wavefunctions. Chirality, type and diameter dependence of Raman intensity is now fully given. In particular, the dark exciton plays an important role for second-order, intervalley, resonance Raman processes. Although the exciton-phonon interaction is not so different from the electron-phonon interaction, the optical absorption (emission) is enhanced significantly by the localized exciton wavefunctions.References: J. Jiang et al, Phys. Rev. B, in press.
Improved KAM estimates for the Siegel radius
Liverani, C.; Turchetti, G.
1986-12-01
For the Siegel center problem the authors explore the possibility of improving the KAM estimates, with a view to possible extensions to Hamiltonian systems. The use of a suitable norm and explicit perturbative computations allow estimates to within a factor 2 of the Siegel radius for the quadratic map.
Solar Radius Measurements at Mount Wilson Observatory
S. Lefebvre; L. Bertello; R. K. Ulrich; J. E. Boyden; J. P. Rozelot
2006-01-01
Possible temporal variations of the solar radius are important as an indicator of internal energy storage and as a mechanism for changes in the total solar irradiance. Variations in the total solar irradiance with an amplitude of 0.1% have been observed from space for more than two decades. Although the variability of this solar output has been definitely established, the
Photon statistics dispersion in excitonic composites
G. Ya. Slepyan; S. A. Maksimenko
2006-05-22
Linear media are predicted to exist whose relative permiability is an operator in the space of quantum states of light. Such media are characterized by a photon statistics--dependent refractive index. This indicates a new type of optical dispersion -- the photon statistics dispersion. Interaction of quantum light with such media modifies the photon number distribution and, in particular, the degree of coherence of light. An excitonic composite -- a collection of noninteracting quantum dots -- is considered as a realization of the medium with the photon statistics dispersion. Expressions are derived for generalized plane waves in an excitonic composite and input--output relations for a planar layer of the material. Transformation rules for different photon initial states are analyzed. Utilization of the photon statistics dispersion in potential quantum--optical devices is discussed.
Dependence of nonproportionality in scintillators on diffusion of excitons and charge carriers
NASA Astrophysics Data System (ADS)
Williams, R. T.; Li, Qi; Grim, Joel Q.; Ucer, K. B.
2011-09-01
The dipole-dipole and free-carrier Auger quenching processes that are generally regarded to be at the root of nonproportionality depend respectively on the 4th or 6th power of the electron track radius if modeled as cylindrical. In an initial time interval ? when nonlinear quenching and diffusion compete to reduce the density of excited states, the track radius expands as (Deff?)1/2 where Deff is the effective diffusion coefficient for the mixture of excitons and charge carriers. The range of Deff across semiconductor and scintillator radiation detectors is large, illustrated by 8 decades between mobilities of self-trapped holes in CsI:Tl and holes in high purity Ge. We present the functional form of nonlinear quenching predicted by diffusive track dilution and show that the simple model provides a surprisingly good fit of empirical nonproportionality across a wide range of semiconductor and oxide radiation detectors. We also show how diffusion drives nonlinear branching between excitons and free carriers in the track when electron and hole mobilities are unequal, and that this nonlinear branching coupled with linear trapping on defects can produce the "halide hump" seen in electron yield data for activated halide scintillators. Picosecond time-resolved spectroscopy in alkali halides, as well as quantitative comparison of recently measured 2nd order quenching rate constants K2 and results of K-dip spectroscopy, provide experimental benchmarks for consideration of carrier thermalization and the initial track or cluster radius r0 from which (nearly thermalized) diffusion is assumed to commence. The ratio of initial rate of 2nd order quenching to that of dilution by diffusion in a cylindrical track is proportional to K2/Deff and does not depend on r0 in lowest order; however, the absolute rates of both processes decrease with increasing r0.
Exciton-polariton oscillations in real space
NASA Astrophysics Data System (ADS)
Liew, T. C. H.; Rubo, Y. G.; Kavokin, A. V.
2014-12-01
We introduce and model spin-Rabi oscillations based on exciton-polaritons in semiconductor microcavities. The phase and polarization of oscillations can be controlled by resonant coherent pulses and the propagation of oscillating domains gives rise to phase-dependent interference patterns in real space. We show that interbranch polariton-polariton scattering controls the propagation of oscillating domains, which can be used to realize logic gates based on an analog variable phase.
Exciton Transport and Perfect Coulomb Drag
NASA Astrophysics Data System (ADS)
Nandi, Debaleena
2013-03-01
Exciton condensation is realized in closely-spaced bilayer quantum Hall systems at ?T=1 when the total density in the two 2D electron layers matches the Landau level degeneracy. In this state, electrons in one layer become tightly bound to holes in the other layer, forming a condensate similar to the Cooper pairs in a superconductor. Being charge neutral, these excitons ought to be free to move throughout the bulk of the quantum Hall fluid. One therefore expects that electron current driven in one layer would spontaneously generate a ``hole'' current in the other layer, even in the otherwise insulating bulk of the 2D system. We demonstrate precisely this effect, using a Corbino geometry to defeat edge state transport. Our sample contains two essentially identical two-dimensional electron systems (2DES) in GaAs quantum wells separated by a thin AlGaAs barrier. It is patterned into an annulus with arms protruding from each rim that provide contact to each 2DES separately. A current drag geometry is realized by applying a drive voltage between the outer and inner rim on one 2DES layer while the two rims on the opposite layer are connected together in a closed loop. There is no direct electrical connection between the two layers. At ?T=1 the bulk of the Corbino annulus becomes insulating owing to the quantum Hall gap and net charge transport across the bulk is suppressed. Nevertheless, we find that in the drag geometry appreciable currents do flow in each layer. These currents are almost exactly equal magnitude but, crucially, flow in opposite directions. This phenomenon reflects exciton transport within the ?T=1 condensate, rather than its quasiparticle excitations. We find that quasiparticle transport competes with exciton transport at elevated temperatures, drive levels, and layer separations. This work represents a collaboration with A.D.K. Finck, J.P. Eisenstein, L.N. Pfeiffer and K.W. West.
Dynamical line broadening in dense exciton systems
H. Haug; J. Liebler; R. Leonelli; A. Manar; J. B. Grun
1988-01-01
It is shown that a dynamical (i.e., frequency-dependent) collision broadening is necessary to explain the observed spectra of induced absorption in CuCl, which exhibit remarkable exponential absorption wings. The calculated analytical collision broadening allows us to determine the time development of the distribution of optically generated excitonic quasiparticles from transmission spectra, measured in time-resolved pump-and-probe experiments.
Enhancement of exciton binding energies in ZnO\\/ZnMgO multiquantum wells
H. D. Sun; T. Makino; Y. Segawa; M. Kawasaki; A. Ohtomo; K. Tamura; H. Koinuma
2002-01-01
The effect of confinement on the exciton binding energies has been systematically investigated for two series of ZnO\\/ZnMgO multiquantum wells with various well widths and barrier heights. The exciton binding energies were extracted from the energy difference between the stimulated emission band induced by inelastic exciton-exciton scattering and the free exciton absorption band. The binding energies of excitons are found
Spin relaxation and spin-dependent exciton interactions in ZnSe quantum wells
H. Kalt; J Hoffmann; D Tröndle; S Wachter; D Lüerßen; D Hägele; W. W Rühle; M Oestreich; I. J Blewett; I Galbraith
2000-01-01
We demonstrate the mutual influence of the dynamics in the real-space and the spin parts of the exciton wavefunction evoked by the exchange interaction. In the regime of high exciton density, the exciton–exciton interaction in ZnSe\\/ZnMgSSe quantum wells is found to be strongly affected by the exciton spin orientation and the longitudinal spin relaxation via single-fermion spin flips. The exciton's
Gordon Baym; Tomoki Ozawa
2009-02-16
We analyze Niels Bohr's proposed two-slit interference experiment with highly charged particles that argues that the consistency of elementary quantum mechanics requires that the electromagnetic field must be quantized. In the experiment a particle's path through the slits is determined by measuring the Coulomb field that it produces at large distances; under these conditions the interference pattern must be suppressed. The key is that as the particle's trajectory is bent in diffraction by the slits it must radiate and the radiation must carry away phase information. Thus the radiation field must be a quantized dynamical degree of freedom. On the other hand, if one similarly tries to determine the path of a massive particle through an inferometer by measuring the Newtonian gravitational potential the particle produces, the interference pattern would have to be finer than the Planck length and thus undiscernable. Unlike for the electromagnetic field, Bohr's argument does not imply that the gravitational field must be quantized.
Excitonic effects in oxyhalide scintillating host compounds
NASA Astrophysics Data System (ADS)
Shwetha, G.; Kanchana, V.; Valsakumar, M. C.
2014-10-01
Ab-initio calculations based on density functional theory have been performed to study the electronic, optical, mechanical, and vibrational properties of scintillator host compounds YOX (X = F, Cl, Br, and I). Semiempirical dispersion correction schemes are used to find the effect of van der Waals forces on these layered compounds and we found this effect to be negligible except for YOBr. Calculations of phonons and elastic constants showed that all the compounds studied here are both dynamically and mechanically stable. YOF and YOI are found to be indirect band gap insulators while YOCl and YOBr are direct band gap insulators. The band gap is found to decrease as we move from fluorine to iodine, while the calculated refractive index shows the opposite trend. As the band gap decreases on going down the periodic table from YOF to YOI, the luminescence increases. The excitonic binding energy calculated, within the effective mass approximation, is found to be more for YOF than the remaining compounds, suggesting that the excitonic effect to be more in YOF than the other compounds. The optical properties are calculated within the Time-Dependent Density Functional Theory (TDDFT) and compared with results obtained within the random phase approximation. The TDDFT calculations, using the newly developed bootstrap exchange-correlation kernel, showed significant excitonic effects in all the compounds studied here.
Microcavity controlled coupling of excitonic qubits
Albert, F.; Sivalertporn, K.; Kasprzak, J.; Strauß, M.; Schneider, C.; Höfling, S.; Kamp, M.; Forchel, A.; Reitzenstein, S.; Muljarov, E.A.; Langbein, W.
2013-01-01
Controlled non-local energy and coherence transfer enables light harvesting in photosynthesis and non-local logical operations in quantum computing. This process is intuitively pictured by a pair of mechanical oscillators, coupled by a spring, allowing for a reversible exchange of excitation. On a microscopic level, the most relevant mechanism of coherent coupling of distant quantum bits—like trapped ions, superconducting qubits or excitons confined in semiconductor quantum dots—is coupling via the electromagnetic field. Here we demonstrate the controlled coherent coupling of spatially separated quantum dots via the photon mode of a solid state microresonator using the strong exciton–photon coupling regime. This is enabled by two-dimensional spectroscopy of the sample’s coherent response, a sensitive probe of the coherent coupling. The results are quantitatively understood in a rigorous description of the cavity-mediated coupling of the quantum dot excitons. This mechanism can be used, for instance in photonic crystal cavity networks, to enable a long-range, non-local coherent coupling. PMID:23612288
Excitonic effects in oxyhalide scintillating host compounds
Shwetha, G.; Kanchana, V., E-mail: kanchana@iith.ac.in [Department of Physics, Indian Institute of Technology Hyderabad, Ordnance Factory Estate, Yeddumailaram 502 205, Telangana (India); Valsakumar, M. C. [School of Engineering Sciences and Technology (SEST), University of Hyderabad, Prof. C. R. Rao Road, Gachibowli, Hyderabad 500 046, Telangana (India)
2014-10-07
Ab-initio calculations based on density functional theory have been performed to study the electronic, optical, mechanical, and vibrational properties of scintillator host compounds YOX (X?=?F, Cl, Br, and I). Semiempirical dispersion correction schemes are used to find the effect of van der Waals forces on these layered compounds and we found this effect to be negligible except for YOBr. Calculations of phonons and elastic constants showed that all the compounds studied here are both dynamically and mechanically stable. YOF and YOI are found to be indirect band gap insulators while YOCl and YOBr are direct band gap insulators. The band gap is found to decrease as we move from fluorine to iodine, while the calculated refractive index shows the opposite trend. As the band gap decreases on going down the periodic table from YOF to YOI, the luminescence increases. The excitonic binding energy calculated, within the effective mass approximation, is found to be more for YOF than the remaining compounds, suggesting that the excitonic effect to be more in YOF than the other compounds. The optical properties are calculated within the Time-Dependent Density Functional Theory (TDDFT) and compared with results obtained within the random phase approximation. The TDDFT calculations, using the newly developed bootstrap exchange-correlation kernel, showed significant excitonic effects in all the compounds studied here.
Triplet diffusion in singlet exciton fission sensitized pentacene solar cells
NASA Astrophysics Data System (ADS)
Tabachnyk, Maxim; Ehrler, Bruno; Bayliss, Sam; Friend, Richard H.; Greenham, Neil C.
2013-10-01
Singlet fission sensitized photovoltaics have the potential to surpass the Shockley-Queisser limit for a single-junction structure. We investigate the dynamics of triplet excitons resulting from singlet fission in pentacene and their ionization at a C60 heterojunction. We model the generation and diffusion of excitons to predict the spectral response. We find the triplet diffusion length in polycrystalline pentacene to be 40 nm. Poly(3-hexylthiophene) between the electrode and pentacene works both to confine triplet excitons and also to transfer photogenerated singlet excitons into pentacene with 30% efficiency. The lower bound for the singlet fission quantum efficiency in pentacene is 180 ± 15%.
Multiple exciton generation and recombination in carbon nanotubes and nanocrystals.
Kanemitsu, Yoshihiko
2013-06-18
Semiconducting nanomaterials such as single-walled carbon nanotubes (SWCNTs) and nanocrystals (NCs) exhibit unique size-dependent quantum properties. They have therefore attracted considerable attention from the viewpoints of fundamental physics and functional device applications. SWCNTs and NCs also provide an excellent new stage for experimental studies of many-body effects of electrons and excitons on optical processes in nanomaterials. In this Account, we discuss multiple exciton generation and recombination in SWCNTs and NCs for next-generation photovoltaics. Strongly correlated ensembles of conduction-band electrons and valence-band holes in semiconductors are complex quantum systems that exhibit unique optical phenomena. In bulk crystals, the carrier recombination dynamics can be described by a simple model, which includes the nonradiative single-carrier trapping rate, the radiative two-carrier recombination rate, and the nonradiative three-carrier Auger recombination rate. The nonradiative Auger recombination rate determines the carrier recombination dynamics at high carrier density and depends on the spatial localization of carriers in two-dimensional quantum wells. The Auger recombination and multiple exciton generation rates can be advantageously manipulated by nanomaterials with designated energy structures. In addition, SWCNTs and NCs show quantized recombination dynamics of multiple excitons and carriers. In one-dimensional SWCNTs, excitons have large binding energies and are very stable at room temperature. The extremely rapid Auger recombination between excitons determines the photoluminescence (PL) intensity, the PL linewidth, and the PL lifetime. SWCNTs can undergo multiple exciton generation, while strong exciton-exciton interactions and complicated exciton structures affect the quantized Auger rate and the multiple exciton generation efficiency. Interestingly, in zero-dimensional NC quantum dots, quantized Auger recombination causes unique optical phenomena. The breakdown of the k-conversion rule and strong Coulomb interactions between carriers in NCs enhance Auger recombination rate and decrease the energy threshold for multiple exciton generation. We discuss this impact of the k-conservation rule on two-carrier radiative recombination and the three-carrier Auger recombination processes in indirect-gap semiconductor Si NCs. In NCs and SWCNTs, multiple exciton generation competes with Auger recombination, surface trapping of excitons, and cooling of hot electrons or excitons. In addition, we explore heterostructured NCs and impurity-doped NCs in the context of the optimization of charge carrier extraction from excitons in NCs. PMID:23421584
Coherence length of excitons in a semiconductor quantum well.
Zhao, Hui; Moehl, Sebastian; Kalt, Heinz
2002-08-26
We report on the first experimental determination of the coherence length of excitons in semiconductors using the combination of spatially resolved photoluminescence with phonon sideband spectroscopy. The coherence length of excitons in ZnSe quantum wells is determined to be 300-400 nm, about 25-30 times the exciton de Broglie wavelength. With increasing exciton kinetic energy, the coherence length decreases slowly. The discrepancy between the coherence lengths measured and calculated by considering only the acoustic-phonon scattering suggests an important influence of static disorder. PMID:12190436
Exciton management in organic photovoltaic multidonor energy cascades.
Griffith, Olga L; Forrest, Stephen R
2014-05-14
Multilayer donor regions in organic photovoltaics show improved power conversion efficiency when arranged in decreasing exciton energy order from the anode to the acceptor interface. These so-called "energy cascades" drive exciton transfer from the anode to the dissociating interface while reducing exciton quenching and allowing improved overlap with the solar spectrum. Here we investigate the relative importance of exciton transfer and blocking in a donor cascade employing diphenyltetracene (D1), rubrene (D2), and tetraphenyldibenzoperiflanthene (D3) whose optical gaps monotonically decrease from D1 to D3. In this structure, D1 blocks excitons from quenching at the anode, D2 accepts transfer of excitons from D1 and blocks excitons at the interface between D2 and D3, and D3 contributes the most to the photocurrent due to its strong absorption at visible wavelengths, while also determining the open circuit voltage. We observe singlet exciton Förster transfer from D1 to D2 to D3 consistent with cascade operation. The power conversion efficiency of the optimized cascade OPV with a C60 acceptor layer is 7.1 ± 0.4%, which is significantly higher than bilayer devices made with only the individual donors. We develop a quantitative model to identify the dominant exciton processes that govern the photocurrent generation in multilayer organic structures. PMID:24702468
Dynamics of long-living excitons in tunable potential landscapes
NASA Astrophysics Data System (ADS)
Gärtner, Andreas; Schuh, Dieter; Kotthaus, Jörg P.
2006-05-01
A novel method to experimentally study the dynamics of long-living excitons in coupled quantum well semiconductor heterostructures is presented. Lithographically defined top gate electrodes imprint in-plane artificial potential landscapes for excitons via the quantum confined Stark effect. Excitons are shuttled laterally in a time-dependent potential landscape defined by an interdigitated gate structure. Long-range drift exceeding a distance of 150 ?m at an exciton drift velocity vd?103 m/s is observed in a gradient potential formed by a resistive gate stripe.
Polarization-dependent exciton dynamics in tetracene single crystals
Zhang, Bo; Xu, Yanqing; Wang, Rui; He, Bin; Liu, Yunlong; Zhang, Shimeng; Wang, Xiaoyong; Xiao, Min
2014-01-01
We conduct polarization-dependent ultrafast spectroscopy to study the dynamics of singlet fission in tetracene single crystals. The spectrotemporal species for singlet and triplet excitons in transient absorption spectra are found to be strongly dependent on probe polarization. By carefully analyzing the polarization dependence, the signals contributed by different transitions related to singlet excitons have been disentangled, which is further applied to construct the correlation between dynamics of singlet and triplet excitons. The anisotropy of exciton dynamics provides an alternative approach to tackle the long-standing challenge in understanding the mechanism of singlet fission in organic semiconductors.
Polarization-dependent exciton dynamics in tetracene single crystals.
Zhang, Bo; Zhang, Chunfeng; Xu, Yanqing; Wang, Rui; He, Bin; Liu, Yunlong; Zhang, Shimeng; Wang, Xiaoyong; Xiao, Min
2014-12-28
We conduct polarization-dependent ultrafast spectroscopy to study the dynamics of singlet fission (SF) in tetracene single crystals. The spectrotemporal species for singlet and triplet excitons in transient absorption spectra are found to be strongly dependent on probe polarization. By carefully analyzing the polarization dependence, the signals contributed by different transitions related to singlet excitons have been disentangled, which is further applied to construct the correlation between dynamics of singlet and triplet excitons. The anisotropy of exciton dynamics provides an alternative approach to tackle the long-standing challenge in understanding the mechanism of singlet fission in organic semiconductors. PMID:25554147
Polarization-dependent exciton dynamics in tetracene single crystals
NASA Astrophysics Data System (ADS)
Zhang, Bo; Zhang, Chunfeng; Xu, Yanqing; Wang, Rui; He, Bin; Liu, Yunlong; Zhang, Shimeng; Wang, Xiaoyong; Xiao, Min
2014-12-01
We conduct polarization-dependent ultrafast spectroscopy to study the dynamics of singlet fission (SF) in tetracene single crystals. The spectrotemporal species for singlet and triplet excitons in transient absorption spectra are found to be strongly dependent on probe polarization. By carefully analyzing the polarization dependence, the signals contributed by different transitions related to singlet excitons have been disentangled, which is further applied to construct the correlation between dynamics of singlet and triplet excitons. The anisotropy of exciton dynamics provides an alternative approach to tackle the long-standing challenge in understanding the mechanism of singlet fission in organic semiconductors.
Optical lattices of InGaN quantum well excitons
NASA Astrophysics Data System (ADS)
Chaldyshev, V. V.; Bolshakov, A. S.; Zavarin, E. E.; Sakharov, A. V.; Lundin, W. V.; Tsatsulnikov, A. F.; Yagovkina, M. A.; Kim, Taek; Park, Youngsoo
2011-12-01
We demonstrate optical Bragg lattices formed by quasi-two-dimensional excitons in periodic systems of the InGaN quantum wells separated by the GaN barriers. When the Bragg resonance and exciton-polariton resonance are tuned to each-other, the medium exhibits an exciton-mediated resonantly enhanced optical Bragg reflection. The enhancement factor appeared to be larger than 2 for the system of 60 quantum wells. Owing to a high binding energy and oscillator strength of the excitons in InGaN quantum wells, the resonant enhancement was achieved at room temperature.
K-momentum dark exciton energy in carbon nanotubes
NASA Astrophysics Data System (ADS)
Torrens, O. N.; Kikkawa, J. M.; Zheng, M.
2008-03-01
Phonon sideband optical spectroscopy determines the energy of the dark K-momentum exciton for (6,5) carbon nanotubes (CNTs). One-phonon sidebands appear in absorption and emission, split by two zone-boundary (K-point) phonons. Their average energy locates the E11 K-momentum exciton 36 meV above the E11 bright level, higher than available theoretical estimates. A model for exciton-phonon coupling shows the absorbance sideband depends sensitively on the K-momentum exciton effective mass and has minimal contributions from zone-center phonons, which dominate the Raman spectra of CNTs.
Exciton-exciton scattering in vapor phase ZnO nanoparticles
NASA Astrophysics Data System (ADS)
Mohanta, Antaryami; Kung, Patrick; Thareja, Raj K.
2015-01-01
Photoluminescence (PL) properties of suspended ZnO nanoparticles formed in vapor phase due to the condensation of the Nd:YAG laser ablated ZnO plasma species are investigated by varying both ablating and excitation intensity at different time delays with respect to the ablating pulse and at different axial distances from the target surface. Emission due to inelastic exciton-exciton (X-X) scattering is observed and is found to be dependent on the size of the vapor phase ZnO nanoparticles. The PL intensity shows nonlinear behavior with increasing ablating intensity, indicating generation and participation of more excitons in X-X scattering process in lager size ZnO nanoparticles.
NASA Astrophysics Data System (ADS)
Cavigli, Lucia; Gabrieli, Riccardo; Gurioli, Massimo; Bogani, Franco; Feltin, Eric; Carlin, Jean-François; Butté, Raphaël; Grandjean, Nicolas; Vinattieri, Anna
2010-09-01
A detailed experimental investigation of the phonon-assisted emission in a high-quality c -plane GaN epilayer is presented up to 200 K. By performing photoluminescence and reflectivity measurements, we find important etaloning effects in the phonon-replica spectra, which have to be corrected before addressing the lineshape analysis. Direct experimental evidence for free exciton thermalization is found for the whole temperature range investigated. A close comparison with existing models for phonon replicas originating from a thermalized free exciton distribution shows that the simplified and commonly adopted description of the exciton-phonon interaction with a single excitonic band leads to a large discrepancy with experimental data. Only the consideration of the complex nature of the excitonic band in GaN, including A and B exciton contributions, allows accounting for the temperature dependence of the peak energy, intensity, and lineshape of the phonon replicas.
Jagdish Mehra
1987-01-01
In this paper, the main outlines of the discussions between Niels Bohr with Albert Einstein, Werner Heisenberg, and Erwin Schrödinger during 1920–1927 are treated. From the formulation of quantum mechanics in 1925–1926 and wave mechanics in 1926, there emerged Born's statistical interpretation of the wave function in summer 1926, and on the basis of the quantum mechanical transformation theory—formulated in
MASS-RADIUS RELATIONSHIPS FOR EXOPLANETS
Swift, D. C.; Eggert, J. H.; Hicks, D. G.; Hamel, S.; Caspersen, K.; Schwegler, E.; Collins, G. W. [Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, California, CA 94550 (United States); Nettelmann, N. [Institut fuer Physik, Universitaet Rostock, D-18051 Rostock (Germany); Ackland, G. J. [Centre for Science at Extreme Conditions, School of Physics, University of Edinburgh, Edinburgh EH9 3JZ (United Kingdom)
2012-01-01
For planets other than Earth, particularly exoplanets, interpretation of the composition and structure depends largely on comparing the mass and radius with the composition expected given their distance from the parent star. The composition implies a mass-radius relation which relies heavily on equations of state calculated from electronic structure theory and measured experimentally on Earth. We lay out a method for deriving and testing equations of state, and deduce mass-radius and mass-pressure relations for key, relevant materials whose equation of state (EOS) is reasonably well established, and for differentiated Fe/rock. We find that variations in the EOS, such as may arise when extrapolating from low-pressure data, can have significant effects on predicted mass-radius relations and on planetary pressure profiles. The relations are compared with the observed masses and radii of planets and exoplanets, broadly supporting recent inferences about exoplanet structures. Kepler-10b is apparently 'Earth-like', likely with a proportionately larger core than Earth's, nominally 2/3 of the mass of the planet. CoRoT-7b is consistent with a rocky mantle over an Fe-based core which is likely to be proportionately smaller than Earth's. GJ 1214b lies between the mass-radius curves for H{sub 2}O and CH{sub 4}, suggesting an 'icy' composition with a relatively large core or a relatively large proportion of H{sub 2}O. CoRoT-2b is less dense than the hydrogen relation, which could be explained by an anomalously high degree of heating or by higher than assumed atmospheric opacity. HAT-P-2b is slightly denser than the mass-radius relation for hydrogen, suggesting the presence of a significant amount of matter of higher atomic number. CoRoT-3b lies close to the hydrogen relation. The pressure at the center of Kepler-10b is 1.5{sup +1.2}{sub -1.0} TPa. The central pressure in CoRoT-7b is probably close to 0.8 TPa, though may be up to 2 TPa. These pressures are accessible by planar shock and ramp-loading experiments at large laser facilities. The center of HAT-P-2b is probably around 210 TPa, in the range of planned National Ignition Facility experiments, and that of CoRoT-3b around 1900 TPa.
Solar Radius Variations Measured in Central Eclipses
NASA Astrophysics Data System (ADS)
Sigismondi, Costantino
2008-01-01
Data on Baily beads observed in total eclipse of March 29, 2006 (Egypt) and those of annular eclipses of September 22, 2006 (French Guyana) and October 3, 2005 (Spain) are used to evaluate the variations of solar radius with respect to its standard value during a whole draconitic year. A portable observatory has to be set on the shadow limit of central eclipses, where lunar limb is grazing to the solar one and the number of beads is large. The observation of solar corona during Egyptian eclipse for several minutes during maximum eclipse on shadow's limits is studied in parallel with the eclipse observed by Clavius in 1567. From fall 2005 to fall 2006 the solar radius does not show significant changes (0.00 to -0.01 arcsecs) with respect to its standard value of 959.63 arcsec within errorbars of 0.17 arcsecs. This is its value at minimum of cycle 23 of solar activity.
Radius measurement by laser confocal technology.
Yang, Jiamiao; Qiu, Lirong; Zhao, Weiqian; Zhang, Xin; Wang, Xu
2014-05-01
A laser confocal radius measurement (LCRM) method is proposed for high-accuracy measurement of the radius of curvature (ROC). The LCRM uses the peak points of confocal response curves to identify the cat eye and confocal positions precisely. It then accurately measures the distance between these two positions to determine the ROC. The LCRM also uses conic fitting, which significantly enhances measurement accuracy by restraining the influences of environmental disturbance and system noise on the measurement results. The experimental results indicate that LCRM has a relative expanded uncertainty of less than 10 ppm for both convex and concave spheres. Thus, LCRM is a feasible method for ROC measurements with high accuracy and concise structures. PMID:24921872
The Rossby radius in the Arctic Ocean
NASA Astrophysics Data System (ADS)
Nurser, A. J. G.; Bacon, S.
2014-11-01
The first (and second) baroclinic deformation (or Rossby) radii are presented north of ~60° N, focusing on deep basins and shelf seas in the high Arctic Ocean, the Nordic seas, Baffin Bay, Hudson Bay and the Canadian Arctic Archipelago, derived from climatological ocean data. In the high Arctic Ocean, the first Rossby radius increases from ~5 km in the Nansen Basin to ~15 km in the central Canadian Basin. In the shelf seas and elsewhere, values are low (1-7 km), reflecting weak density stratification, shallow water, or both. Seasonality strongly impacts the Rossby radius only in shallow seas, where winter homogenization of the water column can reduce it to below 1 km. Greater detail is seen in the output from an ice-ocean general circulation model, of higher resolution than the climatology. To assess the impact of secular variability, 10 years (2003-2012) of hydrographic stations along 150° W in the Beaufort Gyre are also analysed. The first-mode Rossby radius increases over this period by ~20%. Finally, we review the observed scales of Arctic Ocean eddies.
Solar radius change between 1925 and 1979
NASA Technical Reports Server (NTRS)
Sofia, S.; Dunham, D. W.; Dunham, J. B.; Fiala, A. D.
1983-01-01
From an analysis of numerous reports from different locations on the duration of totality of the solar eclipses on January 24, 1925, and February 26, 1979, it is found that the solar radius at the earlier date was 0.5 arcsec (or 375 km) larger than at the later date. The correction to the standard solar radius found for each eclipse is different when different subsets of the observations are used (for example, edge of path of totality timings compared with central timings). This is seen as suggesting the existence of systematic inaccuracies in our knowledge of the lunar figure. The differences between the corrections for both eclipses, however, are very similar for all subsets considered, indicating that changes of the solar size may be reliably inferred despite the existence of the lunar figure errors so long as there is proper consideration of the distribution of the observations. These results are regarded as strong evidence in support of the occurrence of solar radius changes on shorter than evolutionary time scales.
Singlet Exciton Fission and Tri plet-Tri plet Exciton Fusion in Crystalline Tet racene
Martin Pope; Nicholas E. Geacintov; Frank Vogel
1969-01-01
The dominant radiationles decay channel in crystalline tetracene at 300 °K is a fission of an excited singlet into two triplet excitons with a rate constant ?S = 1.5 × 10 ± 5% cm -sec. The efficiency of this process at room temperature is estimated as 95%and constitutesan efficient intersystem crossing mechanism. At light intensities I ? 10 quanta-cm sec
Hot exciton transport in ZnSe quantum wells
Hui Zhao; Sebastian Moehl; Sven Wachter; Heinz Kalt
2002-01-01
The in-plane transport of excitons in ZnSe quantum wells is investigated directly by microphotoluminescence in combination with a solid immersion lens. Due to the strong Fröhlich coupling, the initial kinetic energy of the excitons is well controlled by choosing the excess energy of the excitation laser. When increasing the laser excess energy, we find a general trend of increasing transport
Hot excitons in ZnSe quantum wells
M. Umlauff; J. Hoffmann; H. Kalt; W. Langbein; J. M. Hvam; M. Scholl; J. Söllner; M. Heuken; B. Jobst; D. Hommel
1997-01-01
The optical and acoustic phonon assisted relaxation of free hot excitons in ZnSe quantum wells is reflected in photoluminescence excitation spectra. Slow thermalization of excitons on a 100 ps time scale is directly observed in time-resolved studies of luminescence in the spectral region of the phonon sideband.
Stepwise Quenching of Exciton Fluorescence in Carbon Nanotubes
Paris-Sud XI, UniversitÃ© de
1 Stepwise Quenching of Exciton Fluorescence in Carbon Nanotubes by Single Molecule Reactions with individual single-walled carbon nanotubes are observed, and luminescence quenching analysis reveals the diffusional range of excitons in semiconducting nanotubes. *To whom correspondence should be addressed
Magnetic Brightening of Dark Excitons in Carbon Nanotubes
Maruyama, Shigeo
Magnetic Brightening of Dark Excitons in Carbon Nanotubes Jonah Shaver 1 , Sasa Zaric 2 , Erik H properties of excitons in single-walled carbon nanotubes, we have studied photoluminescence from-field experiments (nanotubes at room temperature; photoluminescence peaks showed B
Mapping the exciton diffusion in semiconductor nanocrystal solids.
Kholmicheva, Natalia; Moroz, Pavel; Bastola, Ebin; Razgoniaeva, Natalia; Bocanegra, Jesus; Shaughnessy, Martin; Porach, Zack; Khon, Dmitriy; Zamkov, Mikhail
2015-03-24
Colloidal nanocrystal solids represent an emerging class of functional materials that hold strong promise for device applications. The macroscopic properties of these disordered assemblies are determined by complex trajectories of exciton diffusion processes, which are still poorly understood. Owing to the lack of theoretical insight, experimental strategies for probing the exciton dynamics in quantum dot solids are in great demand. Here, we develop an experimental technique for mapping the motion of excitons in semiconductor nanocrystal films with a subdiffraction spatial sensitivity and a picosecond temporal resolution. This was accomplished by doping PbS nanocrystal solids with metal nanoparticles that force the exciton dissociation at known distances from their birth. The optical signature of the exciton motion was then inferred from the changes in the emission lifetime, which was mapped to the location of exciton quenching sites. By correlating the metal-metal interparticle distance in the film with corresponding changes in the emission lifetime, we could obtain important transport characteristics, including the exciton diffusion length, the number of predissociation hops, the rate of interparticle energy transfer, and the exciton diffusivity. The benefits of this approach to device applications were demonstrated through the use of two representative film morphologies featuring weak and strong interparticle coupling. PMID:25682881
Fluorescence Spectroscopy, Exciton Dynamics, and Photochemistry of Single Allophycocyanin Trimers
Xie, Xiaoliang Sunney
that the photobleaching of a single B-phycoerythrin molecule occurs in a single step. With confocal microscopy, two-harvesting protein complex from cyanobacteria, by room-temperature single-molecule measurements of fluorescence/or photobleaching, which involves exciton-exciton annihilation. These single-molecule experiments provide new
NASA Astrophysics Data System (ADS)
Schröter, M.; Ivanov, S. D.; Schulze, J.; Polyutov, S. P.; Yan, Y.; Pullerits, T.; Kühn, O.
2015-03-01
The influence of exciton-vibrational coupling on the optical and transport properties of molecular aggregates is an old problem that gained renewed interest in recent years. On the experimental side, various nonlinear spectroscopic techniques gave insight into the dynamics of systems as complex as photosynthetic antennae. Striking evidence was gathered that in these protein-pigment complexes quantum coherence is operative even at room temperature conditions. Investigations were triggered to understand the role of vibrational degrees of freedom, beyond that of a heat bath characterized by thermal fluctuations. This development was paralleled by theory, where efficient methods emerged, which could provide the proper frame to perform non-Markovian and non-perturbative simulations of exciton-vibrational dynamics and spectroscopy. This review summarizes the state of affairs of the theory of exciton-vibrational interaction in molecular aggregates and photosynthetic antenna complexes. The focus is put on the discussion of basic effects of exciton-vibrational interaction from the stationary and dynamics points of view. Here, the molecular dimer plays a prominent role as it permits a systematic investigation of absorption and emission spectra by numerical diagonalization of the exciton-vibrational Hamiltonian in a truncated Hilbert space. An extension to larger aggregates, having many coupled nuclear degrees of freedom, becomes possible with the Multi-Layer Multi-Configuration Time-Dependent Hartree (ML-MCTDH) method for wave packet propagation. In fact it will be shown that this method allows one to approach the limit of almost continuous spectral densities, which is usually the realm of density matrix theory. Real system-bath situations are introduced for two models, which differ in the way strongly coupled nuclear coordinates are treated, as a part of the relevant system or the bath. A rather detailed exposition of the Hierarchy Equations Of Motion (HEOM) method will be given in terms of a stochastic decoupling ansatz. This method has become the standard in exciton-vibrational theory and illustrative examples will be presented as well as a comparison with ML-MCTDH. Applications will be shown for generic model systems as well as for small aggregates mimicking those formed by perylene bisimide dyes. Further, photosynthetic antenna complexes will be discussed, including spectral densities and the role of exciton-vibrational coupling in two-dimensional electronic spectroscopy.
NASA Astrophysics Data System (ADS)
Zhang, Li; Shi, J. J.
2013-03-01
Based on the surface optical phonon states and their electron-phonon coupling functions obtained recently, the surface polaronic exciton states in a quasi-one-dimensional (Q1D) wurtzite nanowire (NW) are investigated by using the variational method and Lee-Low-Pines transform. In order to reflect the different confined features of Q1D wurtzite NWs in axial and radial directions and anisotropy of wurtzite nitride crystals, a two-parameter variational approach is proposed and applied to describe the polaronic exciton states in the NWs. Numerical calculations are performed for GaN NWs. The quantum size effects on the binding energies of polaronic exciton and the contributions of surface phonon modes are analyzed in detail. Our results show that the binding energy of polaronic exciton and surface phonon contribution in the wurtzite GaN NWs reaches 190 and 95 meV, respectively, which are one or two orders of magnitude larger than those in cubic GaAs-based quantum wells and NWs with the same radius. This is mainly ascribed to the strong electron-phonon interaction, the large effective masses of carriers and relatively small dielectric constants in GaN material. The numerical results also show that the two-parameter variational approach is reasonable and necessary for the description of polaronic exciton states in Q1D wurtzite GaN NWs. Moreover, the behaviors of the two variational parameters for the polaronic exciton states are quite different from those for the bound polaron states in Q1D wurtzite NWs.
Exciton complexes in low dimensional transition metal dichalcogenides
Thilagam, A., E-mail: thilaphys@gmail.com [Information Technology, Engineering and Environment, University of South Australia, Adelaide 5095 (Australia)
2014-08-07
We examine the excitonic properties of layered configurations of low dimensional transition metal dichalcogenides (LTMDCs) using the fractional dimensional space approach. The binding energies of the exciton, trion, and biexciton in LTMDCs of varying layers are analyzed, and linked to the dimensionality parameter ?, which provides insight into critical electro-optical properties (relative oscillator strength, absorption spectrum, exciton-exciton interaction) of the material systems. The usefulness of ? is highlighted by its independence of the physical mechanisms underlying the confinement effects of geometrical structures. Our estimates of the binding energies of exciton complexes for the monolayer configuration of transition metal dichalcogenides suggest a non-collinear structure for the trion and a positronium-molecule-like square structure for the biexciton.
Modeling excitonic line shapes in weakly disordered semiconductor nanostructures
NASA Astrophysics Data System (ADS)
Kuznetsova, I.; G?gh, N.; Förstner, J.; Meier, T.; Cundiff, S. T.; Varga, I.; Thomas, P.
2010-02-01
Excitonic spectra of weakly disordered semiconductor heterostructures are simulated on the basis of a one-dimensional tight-binding model. The influence of the length scale of weak disorder in quantum wells on the redshift of the excitonic peak and its linewidth is studied. By calculating two-dimensional Fourier-transform spectra we are able to determine the contribution of disorder to inhomogeneous and also to homogeneous broadenings separately. This disorder-induced dephasing is related to a Fano-type coupling and leads to contributions to the homogeneous linewidth that depends on energy within the inhomogeneously broadened line. The model includes heavy- and light-hole excitons and yields smaller inhomogeneous broadening for the light-hole exciton if compared to the heavy-hole exciton, which agrees qualitatively with the experiment.
Activated singlet exciton fission in a semiconducting polymer.
Musser, Andrew J; Al-Hashimi, Mohammed; Maiuri, Margherita; Brida, Daniele; Heeney, Martin; Cerullo, Giulio; Friend, Richard H; Clark, Jenny
2013-08-28
Singlet exciton fission is a spin-allowed process to generate two triplet excitons from a single absorbed photon. This phenomenon offers great potential in organic photovoltaics, but the mechanism remains poorly understood. Most reports to date have addressed intermolecular fission within small-molecular crystals. However, through appropriate chemical design chromophores capable of intramolecular fission can also be produced. Here we directly observe sub-100 fs activated singlet fission in a semiconducting poly(thienylenevinylene). We demonstrate that fission proceeds directly from the initial 1Bu exciton, contrary to current models that involve the lower-lying 2Ag exciton. In solution, the generated triplet pairs rapidly recombine and decay through the 2Ag state. In films, exciton diffusion breaks this symmetry and we observe long-lived triplets which form charge-transfer states in photovoltaic blends. PMID:23883167
Multiphonon excitonic absorption in semiconductors and quantum wells
Bardyszewski, Witold; Prywata, Miroslaw; Yevick, David
2001-07-01
In this article, we present a straightforward procedure based on the cumulant expansion for computing the contribution of exciton{endash}phonon coupling to the exciton line-shape function and thus absorption in bulk and quantum well semiconductors. Using the functional integral technique we derive the effective equation of motion for the two-particle Green{close_quote}s function in the presence of interaction with phonons. We show that our many body formulation in the limit of linear coupling of carriers with polar longitudinal optical phonons in bulk semiconductors is equivalent to the model exciton{endash}phonon Hamiltonian approach. Our calculations for the hydrogenic model of excitons in three- and two-dimensional systems reproduce not only the correct polaron induced shift (renormalization) of the exciton energy but also the observed asymmetric spectral line shape and exponential Urbach absorption tail below the fundamental absorption edge. {copyright} 2001 American Institute of Physics.
Nonlinear photoluminescence spectroscopy of carbon nanotubes with localized exciton states.
Iwamura, Munechiyo; Akizuki, Naoto; Miyauchi, Yuhei; Mouri, Shinichiro; Shaver, Jonah; Gao, Zhenghong; Cognet, Laurent; Lounis, Brahim; Matsuda, Kazunari
2014-11-25
We report distinctive nonlinear behavior of photoluminescence (PL) intensities from localized exciton states embedded in single-walled carbon nanotubes (SWNTs) at room temperature. We found that PL from the local states exhibits strong nonlinear behavior with increasing continuous-wave excitation power density, whereas free exciton PL shows only weak sublinear behavior. The strong nonlinear behavior was observed regardless of the origin of the local states and found to be nearly independent of the local state density. These results indicate that the strong PL nonlinearity arises from a universal mechanism to SWNTs with sparse local states. The significant nonlinear PL is attributed to rapid ground-state depletion of the local states caused by an efficient accumulation of photogenerated free excitons into the sparse local states through one-dimensional diffusional migration of excitons along the nanotube axis; this mechanism is verified by Monte Carlo simulations of exciton diffusion dynamics. PMID:25331628
Spectral shape of intense exciton absorption in oligothiophene crystals
NASA Astrophysics Data System (ADS)
Kulig, Waldemar; Petelenz, Piotr
2009-03-01
The peculiar shape of c -polarized absorption spectra of crystalline oligothiophenes is attributed to the coupling between the intense upper Davydov component originating from the lowest molecular excitation and the continuum of unbound exciton-phonon states deriving from excitons off the center of the exciton Brillouin zone. This formulation is the energy-domain description of the first step of intraband exciton relaxation and, in analogy to intramolecular radiationless transitions, is couched in terms of the configuration interaction between the discrete state and the one-phonon continuum, using an exact theory originally developed by Fano. The experimental absorption shapes for individual oligothiophenes and the spectral changes along the series are very well reproduced within a unified treatment, providing a plausible explanation in terms of the interplay between the trends imposed by the changes of the exciton band width and of the effective vibronic coupling constant, with the latter parameter obtained from ab initio quantum chemistry calculations.
The Stark effect in the Bohr-Sommerfeld theory and in Schrödinger's wave mechanics
Anthony Duncan; Michel Janssen
2014-04-21
The explanation of the first-order Stark effect in hydrogen by Epstein and Schwarzschild in 1916 was seen as a great success for the old quantum theory. Yet, it also revealed some serious limitations of the theory. To recover the experimentally found line splittings, one had to make some arbitrary assumptions in addition to the basic quantum conditions to rule out certain orbits. The calculation of intensities of lines on the basis of Bohr's correspondence principle likewise required arbitrary additional assumptions. Finally, the actual orbits predicted by the old quantum theory depend on the coordinates chosen to impose the quantum conditions. Both Sommerfeld and Epstein recognized this problem but offered no solution for it. All these problems were solved in 1926 when Schr\\"odinger and Epstein explained the Stark effect on the basis of the new wave mechanics. The calculations in the two theories are similar. In particular, both the Schr\\"odinger equation in the new theory and the Hamilton-Jacobi equation in the old theory are separated in parabolic coordinates. The new quantum mechanics determines all allowed states and transitions without any additional assumptions. It also replaced the ambiguous guidelines based on the correspondence principle for calculating intensities by the straightforward prescription that intensities are given by the squares of the matrix elements of position, leading to results that agreed much better with the experimental data. Finally, the embarrassing non-uniqueness of orbits in the old quantum theory turned into the innocuous non-uniqueness of bases of eigenfunctions in wave mechanics. To this day, the Stark effect is remembered as one of the few qualified successes of the old quantum theory. We suspect that this is largely because after 1926 it became just one of the many unqualified successes of the new quantum theory.
Entangled exciton states in quantum dot molecules
NASA Astrophysics Data System (ADS)
Bayer, Manfred
2002-03-01
Currently there is strong interest in quantum information processing(See, for example, The Physics of Quantum Information, eds. D. Bouwmeester, A. Ekert and A. Zeilinger (Springer, Berlin, 2000).) in a solid state environment. Many approaches mimic atomic physics concepts in which semiconductor quantum dots are implemented as artificial atoms. An essential building block of a quantum processor is a gate which entangles the states of two quantum bits. Recently a pair of vertically aligned quantum dots has been suggested as optically driven quantum gate(P. Hawrylak, S. Fafard, and Z. R. Wasilewski, Cond. Matter News 7, 16 (1999).)(M. Bayer, P. Hawrylak, K. Hinzer, S. Fafard, M. Korkusinski, Z.R. Wasilewski, O. Stern, and A. Forchel, Science 291, 451 (2001).): The quantum bits are individual carriers either on dot zero or dot one. The different dot indices play the same role as a "spin", therefore we call them "isospin". Quantum mechanical tunneling between the dots rotates the isospin and leads to superposition of these states. The quantum gate is built when two different particles, an electron and a hole, are created optically. The two particles form entangled isospin states. Here we present spectrocsopic studies of single self-assembled InAs/GaAs quantum dot molecules that support the feasibility of this proposal. The evolution of the excitonic recombination spectrum with varying separation between the dots allows us to demonstrate coherent tunneling of carriers across the separating barrier and the formation of entangled exciton states: Due to the coupling between the dots the exciton states show a splitting that increases with decreasing barrier width. For barrier widths below 5 nm it exceeds the thermal energy at room temperature. For a given barrier width, we find only small variations of the tunneling induced splitting demonstrating a good homogeneity within a molecule ensemble. The entanglement may be controlled by application of electromagnetic field. For example, using an electric field along the molecule axis we can break the entanglement. Tunneling of carriers is prevented then and emission from intra-dot and inter-dot excitons in which electron and hole are located on the same and on opposite dots, respectively, is observed. The voltage required for the entanglement breaking increases with decreasing barrier width reflecting the increasing 'robustness' of the entanglement for narrow barriers.
Probing excitonic dark states in single-layer tungsten disulphide
NASA Astrophysics Data System (ADS)
Ye, Ziliang; Cao, Ting; O'Brien, Kevin; Zhu, Hanyu; Yin, Xiaobo; Wang, Yuan; Louie, Steven G.; Zhang, Xiang
2014-09-01
Transition metal dichalcogenide (TMDC) monolayers have recently emerged as an important class of two-dimensional semiconductors with potential for electronic and optoelectronic devices. Unlike semi-metallic graphene, layered TMDCs have a sizeable bandgap. More interestingly, when thinned down to a monolayer, TMDCs transform from indirect-bandgap to direct-bandgap semiconductors, exhibiting a number of intriguing optical phenomena such as valley-selective circular dichroism, doping-dependent charged excitons and strong photocurrent responses. However, the fundamental mechanism underlying such a strong light-matter interaction is still under intensive investigation. First-principles calculations have predicted a quasiparticle bandgap much larger than the measured optical gap, and an optical response dominated by excitonic effects. In particular, a recent study based on a GW plus Bethe-Salpeter equation (GW-BSE) approach, which employed many-body Green's-function methodology to address electron-electron and electron-hole interactions, theoretically predicted a diversity of strongly bound excitons. Here we report experimental evidence of a series of excitonic dark states in single-layer WS2 using two-photon excitation spectroscopy. In combination with GW-BSE theory, we prove that the excitons are of Wannier type, meaning that each exciton wavefunction extends over multiple unit cells, but with extraordinarily large binding energy (~0.7 electronvolts), leading to a quasiparticle bandgap of 2.7 electronvolts. These strongly bound exciton states are observed to be stable even at room temperature. We reveal an exciton series that deviates substantially from hydrogen models, with a novel energy dependence on the orbital angular momentum. These excitonic energy levels are experimentally found to be robust against environmental perturbations. The discovery of excitonic dark states and exceptionally large binding energy not only sheds light on the importance of many-electron effects in this two-dimensional gapped system, but also holds potential for the device application of TMDC monolayers and their heterostructures in computing, communication and bio-sensing.
Probing excitonic dark states in single-layer tungsten disulphide.
Ye, Ziliang; Cao, Ting; O'Brien, Kevin; Zhu, Hanyu; Yin, Xiaobo; Wang, Yuan; Louie, Steven G; Zhang, Xiang
2014-09-11
Transition metal dichalcogenide (TMDC) monolayers have recently emerged as an important class of two-dimensional semiconductors with potential for electronic and optoelectronic devices. Unlike semi-metallic graphene, layered TMDCs have a sizeable bandgap. More interestingly, when thinned down to a monolayer, TMDCs transform from indirect-bandgap to direct-bandgap semiconductors, exhibiting a number of intriguing optical phenomena such as valley-selective circular dichroism, doping-dependent charged excitons and strong photocurrent responses. However, the fundamental mechanism underlying such a strong light-matter interaction is still under intensive investigation. First-principles calculations have predicted a quasiparticle bandgap much larger than the measured optical gap, and an optical response dominated by excitonic effects. In particular, a recent study based on a GW plus Bethe-Salpeter equation (GW-BSE) approach, which employed many-body Green's-function methodology to address electron-electron and electron-hole interactions, theoretically predicted a diversity of strongly bound excitons. Here we report experimental evidence of a series of excitonic dark states in single-layer WS2 using two-photon excitation spectroscopy. In combination with GW-BSE theory, we prove that the excitons are of Wannier type, meaning that each exciton wavefunction extends over multiple unit cells, but with extraordinarily large binding energy (?0.7 electronvolts), leading to a quasiparticle bandgap of 2.7 electronvolts. These strongly bound exciton states are observed to be stable even at room temperature. We reveal an exciton series that deviates substantially from hydrogen models, with a novel energy dependence on the orbital angular momentum. These excitonic energy levels are experimentally found to be robust against environmental perturbations. The discovery of excitonic dark states and exceptionally large binding energy not only sheds light on the importance of many-electron effects in this two-dimensional gapped system, but also holds potential for the device application of TMDC monolayers and their heterostructures in computing, communication and bio-sensing. PMID:25162523
NASA Astrophysics Data System (ADS)
Szczytko, J.; Kappei, L.; Berney, J.; Morier-Genoud, F.; Portella-Oberli, M. T.; Deveaud, B.
2005-05-01
We report on the origin of the excitonic luminescence in quantum wells. This study is carried out by time-resolved photoluminescence experiments performed on a very high quality InGaAs quantum well sample in which the photoluminescence contributions at the energy of the exciton and at the band edge can be clearly separated and traced over a broad range of times and densities. This allows us to compare the two conflicting theoretical approaches to the question of the origin of the excitonic luminescence in quantum well: the model of the exciton population and the model of the Coulomb correlated plasma. We measure the exciton formation time and we show the fast exciton formation and its dependence with carrier density. We demonstrate, by comparing the temperature dependence of 1s and 2s excitonic transitions, that excitons provide the dominant contribution to the luminescence signal. Furthermore, our analysis gives evidence that the Coulomb correlated plasma contribution to the luminescence signal might be neglected for densities, temperatures, and time scales actually used in time-resolved experiments.
Malunion of distal radius fractures in children.
Jeroense, Kimberly T V; America, Tim; Witbreuk, Melinda M E H; Sluijs, Johannes A van der
2015-04-01
Background and purpose - Distal forearm fractures in children have excellent remodeling potential. The current literature states that 15° is the maximum acceptable angulation limit, though studies focusing on remodeling capacity above this value are lacking. We present data on the remodeling process in children with distal radius malunions with an angulation of ? 15°. Patients and methods - Retrospectively, we radiographically evaluated the remodeling in 33 children (aged 3-14 years) with 40 distal radius fractures healed in ? 15° angulation in the dorsovolar (DV) plane (n = 32) and/or the radioulnar (RU) plane (n = 8). Malunion angulation at the start and at last follow-up was measured on AP and lateral-view radiographs. Mean follow-up time was 9 (3-29) months. Results - All fractures showed remodeling. Mean DV malunion angulation was 23° (15-49) and mean RU malunion angulation was 21° (15-33). At follow-up, this had remodeled to mean 8° (-2 to 21) DV and 10° (3-17) RU. Mean remodeling speed (RS) was 2.5° (0.4-7.6) per month. There was a negative correlation between RS and remodeling time (RT) and a positive correlation between RS and malunion angulation. The relationship between RS and RT was exponential. RS was not found to be related to age or sex. Interpretation - Remodeling speed decreases exponentially over time. Its starting value depends on the amount of angulation of distal radius fractures. This compensates for the increased need for remodeling in severely angulated fractures. PMID:25355309
Exciton states and interband absorption of cylindrical quantum dot with Morse confining potential
NASA Astrophysics Data System (ADS)
Hayrapetyan, D. B.; Kazaryan, E. M.; Kotanjyan, T. V.; Tevosyan, H. Kh.
2015-02-01
In this paper the exciton and electron sates in cylindrical quantum dot with Morse potential made of GaAs are studied. For the regime of strong size quantization, energy spectrum with the parabolic approximation case are compared. For strong and weak size quantization regimes analytic expressions for the particle energy spectrum, absorption coefficient and dependencies of effective threshold frequencies of absorption on the geometrical parameters quantum dot are obtained. For the intermediate size quantization regime the problem solved in the framework of variation method. The selection rules corresponding to different transitions between quantum levels are found. The size dispersion distribution of growing quantum dots by the radius and height by two experimentally realizing distribution functions have been taken into account. Distribution functions of Gauss, Lifshits-Slezov have been considered.
NSDL National Science Digital Library
This is a task from the Illustrative Mathematics website that is one part of a complete illustration of the standard to which it is aligned. Each task has at least one solution and some commentary that addresses important asects of the task and its potential use. Here are the first few lines of the commentary for this task: Given the height $h$ and volume $V$ of a certain cylinder, Jill uses the formula r=\\sqrt{\\frac{V}{\\pi h}} to compute its radius to be 20 meters. If a s...
Journal of Luminescence 111 (2005) 343347 Exciton annihilation on dendrimeric trees
Mukamel, Shaul
Journal of Luminescence 111 (2005) 343Â347 Exciton annihilation on dendrimeric trees Subhadip-2025, USA Available online 1 February 2005 Abstract ExcitonÂexciton annihilation on Cayley tree likeÂdiffusion systems; Exciton annihilation; Cayley tree 1. Introduction Extended dendrimers (Fig. 1) are large nanos
Exciton binding energies in carbon nanotubes from two-photon photoluminescence
J. Maultzsch; R. Pomraenke; S. Reich; E. Chang; D. Prezzi; A. Ruini; E. Molinari; M. S. Strano; C. Thomsen; C. Lienau
2005-01-01
Excitonic effects in the linear and nonlinear optical properties of single-walled carbon nanotubes are manifested by photoluminescence excitation experiments and ab initio calculations. One- and two-photon spectra showed a series of exciton states; their energy splitting is the fingerprint of excitonic interactions in carbon nanotubes. By ab initio calculations we determine the energies, wave functions, and symmetries of the excitonic
Theory of exciton transfer and diffusion in conjugated polymers
Barford, William, E-mail: william.barford@chem.ox.ac.uk [Department of Chemistry, Physical and Theoretical Chemistry Laboratory, University of Oxford, Oxford OX1 3QZ (United Kingdom); Tozer, Oliver Robert [Department of Chemistry, Physical and Theoretical Chemistry Laboratory, University of Oxford, Oxford OX1 3QZ (United Kingdom); University College, University of Oxford, Oxford OX1 4BH (United Kingdom)
2014-10-28
We describe a theory of Förster-type exciton transfer between conjugated polymers. The theory is built on three assumptions. First, we assume that the low-lying excited states of conjugated polymers are Frenkel excitons coupled to local normal modes, and described by the Frenkel-Holstein model. Second, we assume that the relevant parameter regime is ?? < J, i.e., the adiabatic regime, and thus the Born-Oppenheimer factorization of the electronic and nuclear degrees of freedom is generally applicable. Finally, we assume that the Condon approximation is valid, i.e., the exciton-polaron wavefunction is essentially independent of the normal modes. The resulting expression for the exciton transfer rate has a familiar form, being a function of the exciton transfer integral and the effective Franck-Condon factors. The effective Franck-Condon factors are functions of the effective Huang-Rhys parameters, which are inversely proportional to the chromophore size. The Born-Oppenheimer expressions were checked against DMRG calculations, and are found to be within 10% of the exact value for a tiny fraction of the computational cost. This theory of exciton transfer is then applied to model exciton migration in conformationally disordered poly(p-phenylene vinylene). Key to this modeling is the assumption that the donor and acceptor chromophores are defined by local exciton ground states (LEGSs). Since LEGSs are readily determined by the exciton center-of-mass wavefunction, this theory provides a quantitative link between polymer conformation and exciton migration. Our Monte Carlo simulations indicate that the exciton diffusion length depends weakly on the conformation of the polymer, with the diffusion length increasing slightly as the chromophores became straighter and longer. This is largely a geometrical effect: longer and straighter chromophores extend over larger distances. The calculated diffusion lengths of ?10 nm are in good agreement with experiment. The spectral properties of the migrating excitons are also investigated. The emission intensity ratio of the 0-0 and 0-1 vibronic peaks is related to the effective Huang-Rhys parameter of the emitting state, which in turn is related to the chromophore size. The intensity ratios calculated from the effective Huang-Rhys parameters are in agreement with experimental spectra, and the time-resolved trend for the intensity ratio to decrease with time was also reproduced as the excitation migrates to shorter, lower energy chromophores as a function of time. In addition, the energy of the exciton state shows a logarithmic decrease with time, in agreement with experimental observations.
Theory of exciton transfer and diffusion in conjugated polymers.
Barford, William; Tozer, Oliver Robert
2014-10-28
We describe a theory of Förster-type exciton transfer between conjugated polymers. The theory is built on three assumptions. First, we assume that the low-lying excited states of conjugated polymers are Frenkel excitons coupled to local normal modes, and described by the Frenkel-Holstein model. Second, we assume that the relevant parameter regime is ?? < J, i.e., the adiabatic regime, and thus the Born-Oppenheimer factorization of the electronic and nuclear degrees of freedom is generally applicable. Finally, we assume that the Condon approximation is valid, i.e., the exciton-polaron wavefunction is essentially independent of the normal modes. The resulting expression for the exciton transfer rate has a familiar form, being a function of the exciton transfer integral and the effective Franck-Condon factors. The effective Franck-Condon factors are functions of the effective Huang-Rhys parameters, which are inversely proportional to the chromophore size. The Born-Oppenheimer expressions were checked against DMRG calculations, and are found to be within 10% of the exact value for a tiny fraction of the computational cost. This theory of exciton transfer is then applied to model exciton migration in conformationally disordered poly(p-phenylene vinylene). Key to this modeling is the assumption that the donor and acceptor chromophores are defined by local exciton ground states (LEGSs). Since LEGSs are readily determined by the exciton center-of-mass wavefunction, this theory provides a quantitative link between polymer conformation and exciton migration. Our Monte Carlo simulations indicate that the exciton diffusion length depends weakly on the conformation of the polymer, with the diffusion length increasing slightly as the chromophores became straighter and longer. This is largely a geometrical effect: longer and straighter chromophores extend over larger distances. The calculated diffusion lengths of ~10 nm are in good agreement with experiment. The spectral properties of the migrating excitons are also investigated. The emission intensity ratio of the 0-0 and 0-1 vibronic peaks is related to the effective Huang-Rhys parameter of the emitting state, which in turn is related to the chromophore size. The intensity ratios calculated from the effective Huang-Rhys parameters are in agreement with experimental spectra, and the time-resolved trend for the intensity ratio to decrease with time was also reproduced as the excitation migrates to shorter, lower energy chromophores as a function of time. In addition, the energy of the exciton state shows a logarithmic decrease with time, in agreement with experimental observations. PMID:25362268
Theory of exciton transfer and diffusion in conjugated polymers
NASA Astrophysics Data System (ADS)
Barford, William; Tozer, Oliver Robert
2014-10-01
We describe a theory of Förster-type exciton transfer between conjugated polymers. The theory is built on three assumptions. First, we assume that the low-lying excited states of conjugated polymers are Frenkel excitons coupled to local normal modes, and described by the Frenkel-Holstein model. Second, we assume that the relevant parameter regime is ?? < J, i.e., the adiabatic regime, and thus the Born-Oppenheimer factorization of the electronic and nuclear degrees of freedom is generally applicable. Finally, we assume that the Condon approximation is valid, i.e., the exciton-polaron wavefunction is essentially independent of the normal modes. The resulting expression for the exciton transfer rate has a familiar form, being a function of the exciton transfer integral and the effective Franck-Condon factors. The effective Franck-Condon factors are functions of the effective Huang-Rhys parameters, which are inversely proportional to the chromophore size. The Born-Oppenheimer expressions were checked against DMRG calculations, and are found to be within 10% of the exact value for a tiny fraction of the computational cost. This theory of exciton transfer is then applied to model exciton migration in conformationally disordered poly(p-phenylene vinylene). Key to this modeling is the assumption that the donor and acceptor chromophores are defined by local exciton ground states (LEGSs). Since LEGSs are readily determined by the exciton center-of-mass wavefunction, this theory provides a quantitative link between polymer conformation and exciton migration. Our Monte Carlo simulations indicate that the exciton diffusion length depends weakly on the conformation of the polymer, with the diffusion length increasing slightly as the chromophores became straighter and longer. This is largely a geometrical effect: longer and straighter chromophores extend over larger distances. The calculated diffusion lengths of ˜10 nm are in good agreement with experiment. The spectral properties of the migrating excitons are also investigated. The emission intensity ratio of the 0-0 and 0-1 vibronic peaks is related to the effective Huang-Rhys parameter of the emitting state, which in turn is related to the chromophore size. The intensity ratios calculated from the effective Huang-Rhys parameters are in agreement with experimental spectra, and the time-resolved trend for the intensity ratio to decrease with time was also reproduced as the excitation migrates to shorter, lower energy chromophores as a function of time. In addition, the energy of the exciton state shows a logarithmic decrease with time, in agreement with experimental observations.
An Exoplanet Radius and Transit Timing Survey
NASA Astrophysics Data System (ADS)
Deming, Drake; Jennings, Jonald; Sada, Pedro
2009-08-01
Many exoplanet systems contain Jupiter-mass planets on close-in orbits. Theories of planetary system formation account for these hot Jupiters as being end states of inward migration. Variants of those theories also predict terrestrial planets to be captured in mean motion resonance with the hot Jupiters. A recent explosion of discoveries by transit surveys have given us a sample of 37 hot Jupiters transiting planets brighter than V=13. A transit timing survey of these systems could detect hot Earths in resonance, via the large (~ 180 second) perturbations they induce on the giant planet transits. Moreover, the discovery photometry for these systems usually provides only relatively coarse photometric precision, but larger-aperture follow-up can determine the giant planet radius to a precision limited only by knowledge of the stellar mass, and thereby reveal the diversity of giant exoplanet structure, such as the presence of heavy element cores. The relatively large sample now available means that a radius- and transit timing-survey is well matched to classical observing and telescope scheduling. We propose continued observations to perform transit photometry using FLAMINGOS on the 2.1-meter in the J-band, where stellar limb darkening is minimal and transit photometry has excellent sensitivity to planetary radii and shifts in transit time.
An Exoplanet Radius and Transit Timing Survey
NASA Astrophysics Data System (ADS)
Deming, Drake; Jennings, Jonald; Sada, Pedro
2010-02-01
Many exoplanet systems contain Jupiter-mass planets on close-in orbits. Theories of planetary system formation account for these hot Jupiters as being end states of inward migration. Variants of those theories also predict terrestrial planets to be captured in mean motion resonance with the hot Jupiters. A continuing explosion of discoveries by transit surveys have given us a sample of 45 hot Jupiters transiting planets brighter than V=13. A transit timing survey of these systems could detect hot Earths in resonance, via the large (~ 180 second) perturbations they induce on the giant planet transits. Moreover, the discovery photometry for these systems usually provides only relatively coarse photometric precision, but larger-aperture follow-up can determine the giant planet radius to a precision limited only by knowledge of the stellar mass, and thereby reveal the diversity of giant exoplanet structure, such as the presence of heavy element cores. The relatively large sample now available means that a radius- and transit timing-survey is well matched to classical observing and telescope scheduling. We propose continued observations to perform transit photometry using FLAMINGOS on the 2.1-meter in the J-band, where stellar limb darkening is minimal and transit photometry has excellent sensitivity to planetary radii and shifts in transit time.
Robust excitons inhabit soft supramolecular nanotubes
Eisele, Dörthe M.; Arias, Dylan H.; Fu, Xiaofeng; Bloemsma, Erik A.; Steiner, Colby P.; Jensen, Russell A.; Rebentrost, Patrick; Eisele, Holger; Tokmakoff, Andrei; Lloyd, Seth; Nelson, Keith A.; Nicastro, Daniela; Knoester, Jasper; Bawendi, Moungi G.
2014-01-01
Nature's highly efficient light-harvesting antennae, such as those found in green sulfur bacteria, consist of supramolecular building blocks that self-assemble into a hierarchy of close-packed structures. In an effort to mimic the fundamental processes that govern nature’s efficient systems, it is important to elucidate the role of each level of hierarchy: from molecule, to supramolecular building block, to close-packed building blocks. Here, we study the impact of hierarchical structure. We present a model system that mirrors nature’s complexity: cylinders self-assembled from cyanine-dye molecules. Our work reveals that even though close-packing may alter the cylinders’ soft mesoscopic structure, robust delocalized excitons are retained: Internal order and strong excitation-transfer interactions—prerequisites for efficient energy transport—are both maintained. Our results suggest that the cylindrical geometry strongly favors robust excitons; it presents a rational design that is potentially key to nature’s high efficiency, allowing construction of efficient light-harvesting devices even from soft, supramolecular materials. PMID:25092336
Excitons and charges at organic semiconductor heterojunctions.
Friend, Richard H; Phillips, Matthew; Rao, Akshay; Wilson, Mark W B; Li, Zhe; McNeill, Christopher R
2012-01-01
All-organic heterojunction solar cells now provide very high quantum efficiencies for charge generation and rapidly-improving power conversion efficiencies. Charge generation and separation however, must overcome the strong Coulomb interactions between electrons and holes in these materials that is manifest also through the large exchange energies usually observed. We show for a polymer-polymer system with low charge generation efficiency that this arises through intersystem crossing from the photogenerated charge-transfer state to a lower lying triplet state, mediated by the proton hyperfine interaction, and that the activation barrier for full separation of electrons and holes is of the order of 250 meV. We observe, using transient optical spectroscopy, the processes of charge separation, recombination and sweep-out in efficient polymer-fullerene devices. We report also on the process of singlet exciton fission to form a pair of triplet excitons in pentacene that can later be dissociated against a heterojunction formed with C60. PMID:22470984
Nanoscale Charge Transport in Excitonic Solar Cells
Venkat Bommisetty, South Dakota State University
2011-06-23
Excitonic solar cells, including all-organic, hybrid organic-inorganic and dye-sensitized solar cells (DSSCs), offer strong potential for inexpensive and large-area solar energy conversion. Unlike traditional inorganic semiconductor solar cells, where all the charge generation and collection processes are well understood, these excitonic solar cells contain extremely disordered structures with complex interfaces which results in large variations in nanoscale electronic properties and has a strong influence on carrier generation, transport, dissociation and collection. Detailed understanding of these processes is important for fabrication of highly efficient solar cells. Efforts to improve efficiency are underway at a large number of research groups throughout the world focused on inorganic and organic semiconductors, photonics, photophysics, charge transport, nanoscience, ultrafast spectroscopy, photonics, semiconductor processing, device physics, device structures, interface structure etc. Rapid progress in this multidisciplinary area requires strong synergetic efforts among researchers from diverse backgrounds. Such effort can lead to novel methods for development of new materials with improved photon harvesting and interfacial treatments for improved carrier transport, process optimization to yield ordered nanoscale morphologies with well defined electronic structures.
CCMR: Exciton Blocking Layers in Organic Photovoltaics
NSDL National Science Digital Library
Becker, Katherine
2007-08-29
Organic solar cells have been attracting increased attention recently as an alternative to more costly silicon cells. Organic photovoltaics can be lightweight, processed on flexible substrates, and use less energy to produce [1]. However, significant barriers in efficiency and longevity must be overcome before such cells are commercially viable. Current organic cells can reach only up to 6% efficiency in the lab, while commercially available silicon cells average 15% efficiency [2, 3]. Cell longevity is also severely restricted by the tendency of C60 to increase in resistance when exposed to oxygen. Exciton blocking layers (EBLs) have been exploited to increase longevity [4], mechanically protect the donor and acceptor layers [5] and increase efficiency [6]. Until recently, primarily bathocuproine (BCP) has been used, with some investigation into tris-8-hydroxyquinolinato aluminum (Alq3) [5] and bathophenanthroline (BPhen) [6]. There has also been investigation into the positive effects of annealing on efficiency [7,8,9]. This research seeks to evaluate the effects of thermal annealing on 4 different exciton blocking layer types: BCP, Alq3, BPhen, and NBPhen, a close relative of BPhen.
Excitons in cavity-embedded quantum dot lattices
NASA Astrophysics Data System (ADS)
Grochol, Michal; Piermarocchi, Carlo
2009-03-01
We investigate excitons and trions in a two-dimensional quantum dot lattice embedded in a planar optical cavity. The strong exciton (trion)-photon coupling is described in terms of polariton quasiparticles. First, we focus on Bragg polariton modes obtained by tuning the exciton and the cavity modes into resonance at high symmetry points of the Brillouin zone. The effective mass of these polaritons can be extremely small, of the order of 10^8,0 (m0 is the bare electron mass) and makes them the lightest exciton-like quasiparticle in solids [1]. Second, we consider how disorder affects the properties of Bragg polariton modes. We focus on three kinds of disorder: (i) inhomogeneous exciton energy, (ii) inhomogeneous exciton-photon coupling, and (iii) deviations from an ideal lattice. It is found that in some cases weak disorder increases the light matter coupling and it leads to a larger polariton splitting [2]. Finally, each dot has one electron, and the electron spin determines the polarization of the cavity photon that couples to the dot. Such a ``spin lattice'' can be used for quantum information processing and we show that by using exciton detuning a conditional phase shift gate with high fidelity can be obtained [3]. [1] E. M. Kessler, et al., Phys. Rev. B 77, 085306 (2008). [2] M. Grochol et al., Phys. Rev. B 78, 035323 (2008). [3] M. Grochol et al., Phys. Rev. B 78, 165324 (2008).
Role of phonons in Josephson oscillations of excitonic and polaritonic condensates
Magnusson, E. B.; Flayac, H.; Malpuech, G.; Shelykh, I. A. [Science Institute, University of Iceland, Dunhagi-3, IS-107 Reykjavik (Iceland); LASMEA, Clermont Universite-Universite Blaise Pascal, BP 10448, 63000 Clermont-Ferrand (France) and LASMEA, UMR 6602, CNRS, 63177 Aubiere (France); Science Institute, University of Iceland, Dunhagi-3, IS-107 Reykjavik (Iceland) and International Institute for Physics, Av. Odilon Gomes de Lima, 1722, CEP 59078-400 Capim Macio Natal, RN (Brazil)
2010-11-15
We analyze theoretically the role of the exciton-phonon interactions in phenomena related to the Josephson effect between two spatially separated exciton and exciton-polariton condensates. We consider the role of the dephasing introduced by phonons in such phenomena as Josephson tunneling, self-trapping and spontaneous polarization separation. In the regime of cw pumping we find a remarkable bistability effect arising from exciton-exciton interactions as well as regimes of self-sustained regular and chaotic oscillations.
NASA Astrophysics Data System (ADS)
Andreev, A. V.; Kozhevnikov, A. B.; Yavelov, Boris E.
The authors describes the Soveit KGB operation of interviewing Niels Bohr by soviet scientist Yakov. P. Terletskii(1912-1993) and KGB kolonel Lev Petrovich Vasilevskii (b. 1903) on 24 september 1945-20 november 1945 concerning the American Nuclear weapons (Manhattan project)undertaken under the project of the Soviet KGB Lieder Lavrentij P. Berija and supervised by Soviet KGB generals Pavel A. Sudoplatov (b. 1907) and Nikolay S. Sazykin (1910-1985) after the detailed magnetophone interview of Professor Ya. P. Terletskij before his die in Moscow.
Excitonic AND Logic Gates on DNA Brick Nanobreadboards
2015-01-01
A promising application of DNA self-assembly is the fabrication of chromophore-based excitonic devices. DNA brick assembly is a compelling method for creating programmable nanobreadboards on which chromophores may be rapidly and easily repositioned to prototype new excitonic devices, optimize device operation, and induce reversible switching. Using DNA nanobreadboards, we have demonstrated each of these functions through the construction and operation of two different excitonic AND logic gates. The modularity and high chromophore density achievable via this brick-based approach provide a viable path toward developing information processing and storage systems.
Evidence for a surface exciton in KBr via laser desorption
Beck, Kenneth M.; Joly, Alan G.; Hess, Wayne P.
2001-03-15
We demonstrate that direct photoexcitation of the single crystal KBr surface leads to desorption of hyperthermal neutral bromine atoms. We have produced separately the hyperthermal and the near-thermal components of neutral halogen emission from an alkali halide. The source of hyperthermal bromine emission is attributed to decay of a surface exciton excited at photon energies below that of the bulk exciton. We argue that the frequently observed near-thermal component is derived from excitation within the bulk crystal. Our experimental data provide strong support to a theoretical excitonic emission model previously described in the literature.
Evidence for a Surface Exciton in KBr via Laser Desorption
Beck, Kenneth M. (BATTELLE (PACIFIC NW LAB)); Joly, Alan G. (BATTELLE (PACIFIC NW LAB)); Hess, Wayne P. (BATTELLE (PACIFIC NW LAB))
2001-01-01
We demonstrate that direct photoexcitation of the KBr surface exciton leads to desorption of hyperthermal neutral bromine atoms. We have for the first time produced separately the hyperthermal and the near-thermal components of neutral halogen emission from an alkali halide. The source of hyperthermal bromine emission is attributed to decay of a surface exciton excited at photon energies below that of the bulk exciton. We further demonstrate that the frequently observed near-thermal component is derived from excitation within the bulk crystal. Our experimental data provides strong support to a theoretical emission model previously described in the literature.
Photocurrent, photoluminescence and exciton dynamics in rubrene molecular single crystals
NASA Astrophysics Data System (ADS)
Lyu, ByungGook
This work discusses the photocurrent and photoluminescence that can be induced by short-pulse illumination in rubrene single crystals. The pulsed illumination excites a rubrene molecule from the ground state to its first optically accessible excited state, resulting in a singlet exciton state. In rubrene, a singlet exciton can transform into two triplet excitons - which together have a spin of zero - by an efficient spin-conserved fission process. On the other hand, two triplet excitons can interact to again form a singlet exciton by a fusion process. Quantitative modeling of the transformation of singlet excitons into triplet excitons and vice-versa shows that both photoconductivity dynamics and photocurrent dynamics after pulsed excitations can be understood within the same framework. The photoluminescence observed after pulsed excitation is only emitted upon radiative recombination of singlet excitons. A simple model of fission and fusion based on rate equations leads to a qualitatively different photoluminescence dynamics depending on the time scale. In particular, it predicts a fast exponential decay corresponding to the initial fission process, later a power-law (quadratic) decay corresponding to a regime when triplet-triplet interaction is dominant, and a final exponential decay with a time-constant which is half the triplet exciton lifetime. This last exponential decay corresponds to the case when only a lower density of triplet excitons is left. The same model can be used to predict the photocurrent dynamics after pulsed excitation. Experimental observations after pulsed illumination show that, for low excitation pulse energies, a large photocurrent grows exponentially with a time constant of the order of 100 microseconds. This photocurrent build-up time then becomes shorter at higher excitation energies, with the peak photocurrent also saturating. One finds that the observed photocurrent dynamics can be reproduced with the same model based on exciton fission and fusion that successfully explained photoluminescence dynamics. The only additional assumption that is required to do so is that triplet excitons be able dissociate and release free holes by direct interaction with a defect state. The 100 microsecond build-up time of the impulsively induced photocurrent then corresponds to the triplet lifetime.
Surface Effects on Wannier Excitons in Superlattices: Monte Carlo Simulation
NASA Astrophysics Data System (ADS)
Afif, K.; Benyoussef, A.; Diouri, J.
2002-08-01
Using a Monte Carlo quadrature, we calculate by a variational method the binding energy Eb of Wannier excitons in N-period m-CdMnTe/n-CdTe superlattices, including or not including the step discontinuity in the potential barrier at the interface, and with varying N as well as well width. The calculation is performed in the framework of the effective mass approximation for both heavy-hole and light-hole excitons. The degree of exciton localization dependence on these parameters is accurately illustrated.
Photoluminescence of high-density exciton-polariton condensates
Natsuko Ishida; Tim Byrnes; Tomoyuki Horikiri; Franco Nori; Yoshihisa Yamamoto
2015-01-15
We examine the photoluminescence of highly-excited exciton-polariton condensates in semiconductor microcavities. Under strong pumping, exciton-polariton condensates have been observed to undergo a lasing transition where strong coupling between the excitons and photons is lost. We discuss an alternative high-density scenario, where the strong coupling is maintained. We find that the photoluminescence smoothly transitions between the lower polariton energy to the cavity photon energy. An intuitive understanding of the change in spectral characteristics is given, as well as differences to the photoluminescence characteristics of the lasing case.
Chiral Inflation of the Pion Radius
I. A. Perevalova; M. V. Polyakov; A. N. Vall; A. A. Vladimirov
2011-05-25
We derive expression for the large b_perp asymptotic of the 3D parton distributions q(x,b_perp) in the pion. The asymptotic depends exclusively on the mass scales F_pi and m_pi. Therefore it provides us with a nice example of a strict non-perturbative result for the partonic structure of Nambu-Goldstone bosons in QCD. Analyzing the x-dependent pion transverse radius we reveal a new phenomenon of "chiral inflation"-- in the parametrically wide region of Bjorken x (m_pi^2/(4 pi F_pi)^2 inflation is at variance with the Gribov diffusion, because of long-range interaction of the Nambu-Goldstone bosons.
New charge radius relations for atomic nuclei
B. H. Sun; Y. Lu; J. P. Peng; C. Y. Liu; Y. M. Zhao
2014-11-24
We show that the charge radii of neighboring atomic nuclei, independent of atomic number and charge, follow remarkably very simple relations, despite the fact that atomic nuclei are complex finite many-body systems governed by the laws of quantum mechanics. These relations can be understood within the picture of independent-particle motion and by assuming neighboring nuclei having similar pattern in the charge density distribution. A root-mean-square (rms) deviation of 0.0078 fm is obtained between the predictions in these relations and the experimental values, i.e., a comparable precision as modern experimental techniques. Such high accuracy relations are very useful to check the consistence of nuclear charge radius surface and moreover to predict unknown nuclear charge radii, while large deviations from experimental data is seen to reveal the appearance of nuclear shape transition or coexsitence.
The Proton Radius from Bayesian Inference
Krzysztof M. Graczyk; Cezary Juszczak
2014-11-12
The methods of Bayesian statistics are used to extract the value of the proton radius from the elastic $ep$ scattering data in a model independent way. To achieve that goal a large number of parametrizations (equivalent to neural network schemes) are considered and ranked by their conditional probability $P(\\mathrm{parametrization}\\,|\\,\\mathrm{data})$ instead of using the minimal error criterion. As a result the most probable proton radii values ($r_E^p=0.899\\pm 0.003$ fm, $r_M^p=0.879\\pm 0.007$ fm) are obtained and systematic error due to freedom in the choice of parametrization is estimated. Correcting the data for the two photon exchange effect turns out to be important to obtain the agreement between the $r_E^p$ and $r_M^p$ values. The results disagree with recent muonic atom measurements.
Control of Exciton Photon Coupling in Nano-structures
NASA Astrophysics Data System (ADS)
Liu, Xiaoze
In this thesis, we study the interaction of excitons with photons and plasmons and methods to control and enhance this interaction. This study is categorized in three parts: light-matter interaction in microcavity structures, direct dipole-dipole interactions, and plasmon-exciton interaction in metal-semiconductor systems. In the microcavity structures, the light-matter interactions become significant when the excitonic energy is in resonance with microcavity photons. New hybrid quantum states named polariton states will be formed if the strong coupling regime is achieved, where the interaction rate is faster than the average decay rate of the excitons and photons. Polaritons have been investigated in zinc oxide (ZnO) nanoparticles based microcavity at room temperature and stimulated emission of the polaritons has also been observed with a low optical pump threshold. Exictons in organic semiconductors (modeled as Frenkel excitons) are tightly bound to molecular sites, and differ considerably from loosely bound hydrogen atom-like inorganic excitons (modeled as Wannier-Mott excitons). This fundamental difference results in distinct optoelectronic properties. Not only strongly coupled to Wannier-Mott excitons in ZnO, the microcavity photons have also been observed to be simultaneously coupled to Frenkel excitons in 3,4,7,8-naphthalene tetracarboxylic dianhydride (NTCDA). The photons here act like a glue combining Wannier-Mott and Frenkel excitons into new hybrid polaritons taking the best from both constituents. Two-dimensional (2D) excitons in monolayer transition metal dichalcogenides (TMDs) have emerged as a new and fascinating type of Wannier-Mott-like excitons due to direct bandgap transition, huge oscillator strength and large binding energy. Monolayer molybdenum disulfide (MoS2) has been incorporated into the microcavity structure and 2D exciton-polaritons have been observed for the first time with directional emission in the strong coupling regime. Valley polarization has also been discussed in this MoS2 microcavity for the possible applications in spin switches and logic gates. The direct dipole-dipole type excitonic interactions have also been studied in inorganic-organic nanocomposites, where ZnO nanowire is taken as the inorganic constituent and NTCDA thin films as the organic constituent. The excitonic interactions can be classified into weak coupling regime and strong coupling regime. Forster Resonant Energy Transfer (FRET), which is in the weak coupling regime, has been observed in this hybrid system. The optimized optical nonlinearity has also been determined in the hybrid system via Z-scan measurements. Exciton-plasmon polariton, another example of strongly coupled state which results from the interaction between excitons and plasmons when they are in resonance, has also been investigated in this thesis. Two rhodamine dyes spincoated on the silver thin films have separately been observed to be strongly coupled to the surface plasmon modes. With observed new polariton states, energy transfer mechanism has been discussed for nonlinear optical applications.
Mechanism for radiative recombination in ZnCdO alloys
Buyanova, I. A.; Bergman, J. P.; Pozina, G.; Chen, W. M.; Rawal, S.; Norton, D. P.; Pearton, S. J.; Osinsky, A.; Dong, J. W. [Department of Physics, Chemistry and Biology, Linkoeping University, S-581 83 Linkoeping (Sweden); Department of Materials Science and Engineering, University of Florida, Gainesville, Florida 32611 (United States); SVT Associates, Eden Prairie, Minnesota 55344 (United States)
2007-06-25
Temperature dependent cw- and time-resolved photoluminescence combined with absorption measurements are employed to evaluate the origin of radiative recombination in ZnCdO alloys grown by molecular-beam epitaxy. The near-band-edge emission is attributed to recombination of excitons localized within band tail states likely caused by nonuniformity in Cd distribution. Energy transfer between the tail states is argued to occur via tunneling of localized excitons. The transfer is shown to be facilitated by increasing Cd content due to a reduction of the exciton binding energy and, therefore, an increase of the exciton Bohr radius in the alloys with a high Cd content.
Understanding the Mass-Radius Relation for Sub-neptunes: Radius as a Proxy for Composition
NASA Astrophysics Data System (ADS)
Lopez, Eric D.; Fortney, Jonathan J.
2014-09-01
Transiting planet surveys like Kepler have provided a wealth of information on the distribution of planetary radii, particularly for the new populations of super-Earth- and sub-Neptune-sized planets. In order to aid in the physical interpretation of these radii, we compute model radii for low-mass rocky planets with hydrogen-helium envelopes. We provide model radii for planets 1-20 M ?, with envelope fractions 0.01%-20%, levels of irradiation 0.1-1000 times Earth's, and ages from 100 Myr to 10 Gyr. In addition we provide simple analytic fits that summarize how radius depends on each of these parameters. Most importantly, we show that at fixed H/He envelope fraction, radii show little dependence on mass for planets with more than ~1% of their mass in their envelope. Consequently, planetary radius is to a first order a proxy for planetary composition, i.e., H/He envelope fraction, for Neptune- and sub-Neptune-sized planets. We recast the observed mass-radius relationship as a mass-composition relationship and discuss it in light of traditional core accretion theory. We discuss the transition from rocky super-Earths to sub-Neptune planets with large volatile envelopes. We suggest ~1.75 R ? as a physically motivated dividing line between these two populations of planets. Finally, we discuss these results in light of the observed radius occurrence distribution found by Kepler.
Estimation of Vehicle Turning Radius Based on Federal Kalman Filter
Xianbin Wang; Shuming Shi
2010-01-01
This paper presents a new method for identifying turning radius using a Federal Kalman Filter and a vehicle dynamic model. A dynamic model, which includes vehicle radius, is first introduced. The Federal Kalman Filter is then implemented using the measurements of GPS and INS sensors. From the Federal Kalman Filter, the vehicle turning radius can be precisely estimated. Experimental results
N. T. Pelekanos; J. Ding; M. Hagerott; A. V. Nurmikko; H. Luo; N. Samarth; J. K. Furdyna
1992-01-01
Quasi-two-dimensional (2D) excitons in (Zn,Cd)Se\\/ZnSe quantum wells were investigated. We observed a confinement-induced enhancement of the exciton binding energy to a value greater than the longitudinal-optical (LO) -phonon energy. This unusual condition results in an effective reduction of the exciton-LO-phonon coupling. As a consequence, excitonic absorption features in this quantum-well system are well preserved to room temperature and beyond.
Exciton-phonon coupling and exciton thermalization in Mg xZn 1- xO thin films
NASA Astrophysics Data System (ADS)
Müller, Alexander; Benndorf, Gabriele; Heitsch, Susanne; Sturm, Chris; Grundmann, Marius
2008-12-01
Temperature dependent transmission measurements have been performed on Mg xZn 1- xO thin layers on sapphire with x in the range from 0 to 0.33. Multiple minima can be observed in the spectra from samples with low Mg content. These can be attributed to exciton-phonon complexes. To determine the exciton transition energy, the transmission spectra have been fitted using the model dielectric function of Adachi with Gaussian-like broadening. The exciton transition energy is compared to the photoluminescence maximum in dependence on temperature and Mg content.
Exciton dynamics in emergent Rydberg lattices
NASA Astrophysics Data System (ADS)
Bettelli, S.; Maxwell, D.; Fernholz, T.; Adams, C. S.; Lesanovsky, I.; Ates, C.
2013-10-01
The dynamics of excitons in a one-dimensional ensemble with partial spatial order are studied. During optical excitation, cold Rydberg atoms spontaneously organize into regular spatial arrangements due to their mutual interactions. This emergent lattice is used as the starting point to study resonant energy transfer triggered by driving a nS to n'P transition using a microwave field. The dynamics are probed by detecting the survival probability of atoms in the nS Rydberg state. Experimental data qualitatively agree with our theoretical predictions including the mapping onto the XXZ spin model in the strong-driving limit. Our results suggest that emergent Rydberg lattices provide an ideal platform to study coherent energy transfer in structured media without the need for externally imposed potentials.
Plasmon-exciton self-induced transparency
Zabolotskii, A. A., E-mail: zabolotskii@iae.nsk.su [Russian Academy of Sciences, Institute of Automatics and Electrometry, Siberian Branch (Russian Federation)
2011-04-15
The possibility of forming stable bound plasmon-polariton states in an extended metallic cylinder surrounded by a two-level medium has been investigated. The dynamics of plasmons is described in the hydrodynamic approximation. It has been shown that the equations of motion of charge-density bunches and the Bloch equations for the two-level medium are reduced in certain approximations to integrable equations for both transverse and longitudinal plasmons. In the former case, the initial system of equations after the application of the slow-envelope approximation is reduced to equations equivalent to the Maxwell-Bloch equations. In the latter case, the equations describe wave dynamics beyond the slow-envelope approximation. In the approximation of unidirectional wave propagation, the initial system of equations is reduced to equations related to the reduced Maxwell-Bloch equations. Soliton and breather-like solutions of the derived equations describe plasmon-exciton self-induced transparency.
Singlet exciton fission : applications to solar energy harvesting
Thompson, Nicholas John
2014-01-01
Singlet exciton fission transforms a single molecular excited state into two excited states of half the energy. When used in solar cells it can double the photocurrent from high energy photons increasing the maximum ...
Fate of the excitonic insulator in the presence of phonons
NASA Astrophysics Data System (ADS)
Zenker, B.; Fehske, H.; Beck, H.
2014-11-01
The influence of phonons on the formation of the excitonic insulator has hardly been analyzed so far. Recent experiments on Ta2NiSe5 ,1 T -TiSe2, and TmSe0.45Te0.55 , being candidates for realizing the excitonic-insulator state, suggest, however, that the underlying lattice plays a significant role. Employing the Kadanoff-Baym approach we address this issue theoretically. We show that owing to the electron-phonon coupling a static lattice distortion may arise at the excitonic instability. Most importantly such a distortion will destroy the acoustic phase mode being present if the electron-hole pairing and condensation is exclusively driven by the Coulomb interaction. The absence of off-diagonal long-range order, when lattice degrees of freedom are involved, challenges that excitons in these materials form a superfluid condensate of Bose particles or Cooper pairs composed of electrons and holes.
Cerium-, praseodymium- and terbium-trapped excitons in oxides
NASA Astrophysics Data System (ADS)
Pedrini, C.; Belsky, A.; Ivanovskikh, K. V.; Petrosyan, A. G.; Sargsyan, R. V.; Kamenskikh, I.
2011-10-01
Spectroscopic study of CaNb 2O 6 single crystals doped with Ce 3+, Pr 3+ or Tb 3+ ions shows existence of a broad excitation band attributed to a rare-earth-trapped exciton, with the hole trapped at the rare earth ion and the electron delocalized over the surrounding of the next-neighbor Nb 5+ cations. Their energies are in agreement with the Dorenbos' model determining the energy level location of trivalent lanthanides in oxides. The rare-earth-trapped excitons occurred in the Pr 3+ or Tb 3+ doped CaNb 2O 6 crystals undergo non-radiative relaxation to the 4f n states. In contrast, the exciton trapped on Ce 3+ ion was shown to exhibit luminescence from the excitonic state that is the lowest excited state of the system.
Topologically protected excitons in porphyrin thin films
NASA Astrophysics Data System (ADS)
Yuen-Zhou, Joel; Saikin, Semion K.; Yao, Norman Y.; Aspuru-Guzik, Alán
2014-11-01
The control of exciton transport in organic materials is of fundamental importance for the development of efficient light-harvesting systems. This transport is easily deteriorated by traps in the disordered energy landscape. Here, we propose and analyse a system that supports topological Frenkel exciton edge states. Backscattering of these chiral Frenkel excitons is prohibited by symmetry, ensuring that the transport properties of such a system are robust against disorder. To implement our idea, we propose a two-dimensional periodic array of tilted porphyrins interacting with a homogeneous magnetic field. This field serves to break time-reversal symmetry and results in lattice fluxes that mimic the Aharonov–Bohm phase acquired by electrons. Our proposal is the first blueprint for realizing topological phases of matter in molecular aggregates and suggests a paradigm for engineering novel excitonic materials.
Singlet exciton fission-sensitized infrared quantum dot solar cells.
Ehrler, Bruno; Wilson, Mark W B; Rao, Akshay; Friend, Richard H; Greenham, Neil C
2012-02-01
We demonstrate an organic/inorganic hybrid photovoltaic device architecture that uses singlet exciton fission to permit the collection of two electrons per absorbed high-energy photon while simultaneously harvesting low-energy photons. In this solar cell, infrared photons are absorbed using lead sulfide (PbS) nanocrystals. Visible photons are absorbed in pentacene to create singlet excitons, which undergo rapid exciton fission to produce pairs of triplets. Crucially, we identify that these triplet excitons can be ionized at an organic/inorganic heterointerface. We report internal quantum efficiencies exceeding 50% and power conversion efficiencies approaching 1%. These findings suggest an alternative route to circumvent the Shockley-Queisser limit on the power conversion efficiency of single-junction solar cells. PMID:22257168
Bose-Einstein condensation of excitons in bilayer electron systems.
Eisenstein, J P; Macdonald, A H
2004-12-01
An exciton is the particle-like entity that forms when an electron is bound to a positively charged 'hole'. An ordered electronic state in which excitons condense into a single quantum state was proposed as a theoretical possibility many years ago. We review recent studies of semiconductor bilayer systems that provide clear evidence for this phenomenon and explain why exciton condensation in the quantum Hall regime, where these experiments were performed, is as likely to occur in electron-electron bilayers as in electron-hole bilayers. In current quantum Hall excitonic condensates, disorder induces mobile vortices that flow in response to a supercurrent and limit the extremely large bilayer counterflow conductivity. PMID:15592403
Superconductivity in CuCl/Si superlattices: excitonic pairing?
NASA Astrophysics Data System (ADS)
Rhim, S. H.; Saniz, Rolando; Weinert, Michael; Freeman, A. J.
2013-03-01
Two-dimensional (2D) hetero-bonded semiconductor interfaces have been suggested as candidate geometries where excitonic superconductivity footnotetextV.L. Ginzburg, Sov. Phys. JETP 20,1549 (1965) -- and the greatly enhanced where TC compared to phonon mechanisms mediation -- can be realized. Among experimental efforts, epitaxially grown CuCl on Si (111) has reportedly exhibited excitonic superconductivity at 60˜150 K. Our first-principles calculations confirm 2D metallicity at the interfaces due to charge transfer by valence mismatch. footnotetextS.H. Rhim et al., Phys. Rev. B 76, 184505 (2007). The excitonic mechanism is investigated by calculating the kernel function, K(?), for the average of the electronic contributions to the effective interaction.footnotetextZakharov et al., J.Phys.Condes.Matter 9 8501 (1997) The attractive interaction found in the CuCl/Si superlattice indicates the feasibility of excitonic pairing for a certain frequency range.
Ultrafast exciton dynamics in colloidal aluminum phosphide nanocrystals
NASA Astrophysics Data System (ADS)
Busby, Erik; Thibert, Arthur; Fuzell, Jack; Arrington, Deisy C.; Jawaid, Ali M.; Snee, Preston T.; Larsen, Delmar S.
2013-02-01
The primary exciton dynamics of colloidal indirect bandgap AlP nanocrystals are characterized with ultrafast transient absorption spectroscopy. A 400-nm excitation results in a high yield formation of an emissive exciton with a ?1-ns lifetime and a 50-nm bandwidth red-shifted emission. Multi-wavelength target analysis is used to decompose the measured signals into sequential and parallel models, which interpret the measured data as an emissive exciton with a 1.2-ns decay time and a dark exciton which is attributed to surface trapping. Reconstructed nonlinear absorption spectra resolve a broad optical gain persisting for >1 ns. The 15% emission yield demonstrates that colloidal AlP nanocrystals are useful as a potential broadband, high efficiency optoelectronics material.
Dynamics of exciton dissociation in donor-acceptor polymer heterojunctions
NASA Astrophysics Data System (ADS)
Sun, Zhen; Stafström, Sven
2013-04-01
Exciton dissociation in a donor-accepter polymer heterojunction has been simulated using a nonadiabatic molecular dynamics approach, which allows for the coupled evolution of the nuclear degrees of freedom and the electronic degrees of freedom described by multiconfigurational electronic wavefunctions. The simulations reveal important details of the charge separation process: the exciton in the donor polymer first dissociates into a "hot" charge transfer state, which is best described as a polaron pair. The polaron pair can be separated into free polaron charge carriers if a sufficiently strong external electric field is applied. We have also studied the effects of inter-chain interaction, temperature, and the external electric field strength. Increasing inter-chain interactions makes it easier for the exciton to dissociate into a polaron pair state, but more difficult for the polaron pair to dissociate into free charge carriers. Higher temperature and higher electric field strength both favor exciton dissociation as well as the formation of free charge carriers.
The theory of simultaneous excitonic-superconductivity condensation
NASA Astrophysics Data System (ADS)
Wong, K. W.; Ching, W. Y.
1989-04-01
The theory of simultaneous excitonic-superconductivity condesation also known as the excitonic enhancement model (EEM) for high Tc superconductivity is presented in two different approaches: (a) Based on the assumption of a strong Coulomb binding between electron and hole carriers in a two-band model, we obtained the total excitation gap in terms of a Pythagorean sum of an excitonic and a BCS gap component in a mean-field approximation. The importance of near two-dimensionality in the excitonic component was emphasized. (b) The off-diagonal-long-range-ordering [ODLRO] of the superconducting quasi-particle was assumed to be a linear combination of electron and hole states due to a strong electron-hole correlation. The result gives a charge reduction in the quantized flux. An estimation to this reduction was given in the mean-field isotropic limit.
Topological Polaritons and Excitons in Garden Variety Systems
Charles-Edouard Bardyn; Torsten Karzig; Gil Refael; Timothy C. H. Liew
2014-10-06
Topological polaritons (aka topolaritons) present a new frontier for topological behavior in solid-state systems. They combine light and matter, which allows to probe and manipulate them in a variety of ways. They can also be made strongly interacting, due to their excitonic component. So far, however, their realization was deemed rather challenging. Here we present a scheme which allows to realize topolaritons in garden variety zinc-blende quantum wells. Our proposal requires a moderate magnetic field and a potential landscape which can be implemented, e.g., via surface acoustic waves or patterning. We identify indirect excitons in double quantum wells as a particularly appealing alternative for topological states in exciton-based systems. Indirect excitons are robust and long lived (with lifetimes up to milliseconds), and, therefore, provide a flexible platform for the realization, probing, and utilization of topological coupled light-matter states.
Microscopic Theory of Excitonic Signatures in Semiconductor Photoluminescence
M. Kira; F. Jahnke; S. W. Koch
1998-01-01
Luminescence in a quantum well is studied theoretically using a consistent quantum description of light interacting with Coulomb correlated carriers. The buildup of excitonic luminescence during the plasma cooling process is analyzed for an initially hot electron-hole plasma.
Exciton-dominant electroluminescence from a diode of monolayer MoS{sub 2}
Ye, Yu; Ye, Ziliang; Gharghi, Majid; Zhu, Hanyu; Wang, Yuan [NSF Nanoscale Science and Engineering Center, University of California, 3112 Etcheverry Hall, Berkeley, California 94720 (United States); Zhao, Mervin; Yin, Xiaobo; Zhang, Xiang, E-mail: xiang@berkeley.edu [NSF Nanoscale Science and Engineering Center, University of California, 3112 Etcheverry Hall, Berkeley, California 94720 (United States); Materials Sciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720 (United States)
2014-05-12
In two-dimensional monolayer MoS{sub 2}, excitons dominate the absorption and emission properties. However, the low electroluminescent efficiency and signal-to-noise ratio limit our understanding of the excitonic behavior of electroluminescence. Here, we study the microscopic origin of the electroluminescence from a diode of monolayer MoS{sub 2} fabricated on a heavily p-type doped silicon substrate. Direct and bound-exciton related recombination processes are identified from the electroluminescence. At a high electron-hole pair injection rate, Auger recombination of the exciton-exciton annihilation of the bound exciton emission is observed at room temperature. Moreover, the efficient electrical injection demonstrated here allows for the observation of a higher energy exciton peak of 2.255?eV in the monolayer MoS{sub 2} diode, attributed to the excited exciton state of a direct-exciton transition.
Studies of self-trapped exciton luminescence in KCl
A. E. Purdy; R. B. Murray; K. S. Song; A. M. Stoneham
1977-01-01
Measurements have been made of the luminescence intensity, lifetime, emission spectra, and linear polarization of self-trapped exciton luminescence in KCl (2.31 eV) at temperatures of 7oK and above. Data are interpreted in terms of an exciton energy-level structure composed of a singlet state approximately 15 meV above a triplet state. The singlet state is populated only by thermal excitation from
Substitutional Donors and Core Excitons in Many-Valley Semiconductors
NASA Astrophysics Data System (ADS)
Altarelli, M.
1981-01-01
The shallow-deep instability of substitutional donors and core excitons is discussed, with inclusion of all intervalley interactions. Shallow levels result in Si for a screened point-charge potential, because intervalley overlap and kinetic energy balance the potential-energy terms, which are severely reduced, at substitutional sites, by umklapp effects. Contrary to recent claims based on consideration of potential energy only, many-valley interactions cannot therefore be invoked to predict deep core-exciton levels in Si.
Control of exciton relaxation channels in quantum dot molecules
Kushal C. Wijesundara; Juan E. Rolon; Sergio E. Ulloa; Eric A. Stinaff; Allan Bracker; Dan Gammon
2011-01-01
We observe modulations in radiative lifetimes and intensities of the spatially indirect exciton as the InAs\\/GaAs coupled quantum dot system is tuned between molecular and atomic like states. With standard time-resolved single photon counting techniques the measured lifetimes were found to vary between 0.3 and 2.0 ns which resulted in modulations of the observed photoluminescence intensity of the indirect exciton.
Excitonic properties of MgS\\/ZnSe quantum wells
B. Urbaszek; A. Balocchi; C. Bradford; C. Morhain; C. B. O'Donnell; K. A. Prior; B. C. Cavenett
2000-01-01
Magnetic field and temperature dependent measurements are used to study the excitonic properties of high quality ZnSe quantum wells in MgS barriers grown by molecular beam epitaxy. The small inhomogeneous broadening of the samples allows the observation of higher excited exciton states. Due to the large difference in band gap between ZnSe and MgS a value of 43.9 meV was
Optical Properties of Strongly Coupled Plasmon-Exciton Hybrid Nanostructures
NASA Astrophysics Data System (ADS)
Fofang, Nche Tumasang
Strongly coupled plasmon-exciton hybrid nanostructures are fabricated and their optical properties are studied. The plasmonic and excitonic systems are gold nanoshells and J-aggregates, respectively. Gold nanoshells are tunable plasmonic core-shell nanoparticles which can sustain distinct dipole and quadrupole plasmons with resonant energies dependent on core-size/shell-thickness ratio. J-aggregates are organic semiconducting material with excitons that possess very high oscillator strength making them suitable for coherent interaction with other kinds of excitations. The J-aggregates are formed on the surface of the nanoshells when a water/ethanol (50:50) solution of the dye molecules (2,2'-dimethyl-8-phenyl-5,6,5',6'-dibenzothiacarbocyanine chloride) is added to an aqueous solution of nanoshells. These nanoshell-J-aggregate complexes exhibit coherent coupling between localized plasmons of the nanoshell and excitons of the molecular J-aggregates. Coherent coupling strengths of 120 meV and 100 meV have been measured for dipole and quadrupole plasmon interactions with excitons, respectively. Femtosecond time-resolved transmission spectroscopy studies are carried out in order to understand the possible sources of optical nonlinearities in the nanoshell-J-aggregate hybrid. Transient absorption of the interacting plasmon-exciton system is observed, in dramatic contrast to the photoinduced transmission of the pristine J-aggregate. An additional, transient Fano-shaped modulation within the Fano dip is also observable. The transient behavior of the J-aggregate-Au nanoshell complex is described by a combined one-exciton and two-exciton state model coupled to the nanoshell plasmon.
Singlet fission in pentacene through multi-exciton quantum states
Paul M. Zimmerman; Zhiyong Zhang; Charles B. Musgrave
2010-01-01
Multi-exciton generation—the creation of multiple charge carrier pairs from a single photon—has been reported for several materials and may dramatically increase solar cell efficiency. Singlet fission, its molecular analogue, may govern multi-exciton generation in a variety of materials, but a fundamental mechanism for singlet fission has yet to be described. Here, we use sophisticated ab initio calculations to show that
Singlet fission in pentacene through multi-exciton quantum states
Paul M. Zimmerman; Zhiyong Zhang; Charles B. Musgrave
2010-01-01
Multi-exciton generation-the creation of multiple charge carrier pairs from a single photon-has been reported for several materials and may dramatically increase solar cell efficiency. Singlet fission, its molecular analogue, may govern multi-exciton generation in a variety of materials, but a fundamental mechanism for singlet fission has yet to be described. Here, we use sophisticated ab initio calculations to show that
GaN exciton photovoltaic spectra at room temperature
W. Liu; M. F. Li; S. J. Chua; Y. H. Zhang; K. Uchida
1997-01-01
Clear exciton absorption peak has been observed at room temperature by photovoltaic spectra, on a GaN layer grown on (0001)-plane sapphire by metalorganic chemical vapor deposition. From the spectra, we obtained the wurtzite GaN room temperature A and B exciton transition energies, and the energy gap to be 3.401, 3.408, and 3.426 eV, respectively. We have also performed photovoltaic measurements
Excitonic emissions from hexagonal GaN epitaxial layers
S. Chichibu; T. Azuhata; T. Sota; S. Nakamura
1996-01-01
Excitonic photoluminescence (PL) peaks from hexagonal GaN epilayers were investigated making a connection with the analysis of the photoreflectance spectra. Free exciton emissions associated with transitions from the conduction (?7c) band to the A (?9v) and B (?7uv) valence bands are dominant above 100 K. Values of the full widths at half maximum of them were smaller than the thermal
NASA Astrophysics Data System (ADS)
Syrbu, N. N.; Nemerenco, L. L.; Bejan, V. N.; Tezlevan, V. E.
2007-12-01
In CuGaS 2 crystals absorption and luminescence spectra at the temperature 9 K at excitation by different wavelengths of Ar laser are investigated. A series of lines available in luminescence and absorption spectra is found. Another series of lines is found only in absorption spectra. The found series of lines of absorption and luminescence are determined by excitons bound on neutral acceptor. A model of electron transitions between the energy levels of the exciton bound on neutral acceptor is proposed.
Femtosecond THz Studies of Intra-Excitonic Transitions
Huber, Rupert; Schmid, Ben A.; Kaindl, Robert A.; Chemla, Daniel S.
2007-10-02
Few-cycle THz pulses are employed to resonantly access the internal fine structure of photogenerated excitons in semiconductors, on the femtosecond time scale. This technique allows us to gain novel insight into many-body effects of excitons and reveal key quantum optical processes. We discuss experiments that monitor the density-dependent re?normalization of the binding energy of a high-density exciton gas in GaAs/AlGaAs quantum wells close to the Mott transition. In a dilute ensemble of 3p excitons in Cu2O, stimulated THz emission from internal transitions to the energetically lower 2s state is observed at a photon energy of 6.6 meV, with a cross section of 10-14 cm2. Simultaneous interband excitation of both exciton levels drives quantum beats, which cause efficient THz emission at the difference frequency. By extending this principle to various other exciton resonances, we develop a novel way of mapping the fine structure by two-dimensional THz emission spectroscopy.
Molecular packing determines singlet exciton fission in organic semiconductors.
Kolata, Kolja; Breuer, Tobias; Witte, Gregor; Chatterjee, Sangam
2014-07-22
Carrier multiplication by singlet exciton fission enhances photovoltaic conversion efficiencies in organic solids. This decay of one singlet exciton into two triplet states allows the extraction of up to two electrons per harvested photon and, hence, promises to overcome the Shockley–Queisser limit. However, the microscopic mechanism of singlet exciton fission, especially the relation between molecular packing and electronic response, remains unclear, which therefore hampers the systematic improvement of organic photovoltaic devices. For the model system perfluoropentacene, we experimentally show that singlet exciton fission is greatly enhanced for a slip-stacked molecular arrangement by addressing different crystal axes featuring different packing schemes. This reveals that the fission process strongly depends on the intermolecular coupling: slip-stacking favors delocalization of excitations and allows for efficient exciton fission, while face-to-edge molecular orientations commonly found in the prevailing herringbone molecular stacking patterns even suppress it. Furthermore, we clarify the controversially debated role of excimer states as intermediary rather than competitive or precursory. Our detailed findings serve as a guideline for the design of next-generation molecular materials for application in future organic light-harvesting devices exploiting singlet exciton fission. PMID:24957197
Excitonic effects in the luminescence of quantum wells
NASA Astrophysics Data System (ADS)
Deveaud, B.; Kappei, L.; Berney, J.; Morier-Genoud, F.; Portella-Oberli, M. T.; Szczytko, J.; Piermarocchi, C.
2005-11-01
We report on the origin of the excitonic luminescence in quantum wells. This study is carried out by time-resolved photoluminescence experiments performed on a very high-quality InGaAs quantum well sample in which the photoluminescence contributions at the energy of the exciton and at the band edge can be clearly separated and traced over a broad range of times and densities. This allows us to compare the two conflicting theoretical approaches to the question of the origin of the excitonic luminescence in quantum wells: the model of the exciton population and the model of the Coulomb correlated plasma. We measure the exciton formation time and we show the fast exciton formation and its dependence with carrier density. We are also able to give the boundaries of the Mott transition in our system, and to show the absence of observable renormalization of the gap below the onset of this transition. We detail the characteristics of the trion formation and evidence the possible formation of both positive and negative trions in the absence of any resident free carrier populations.
PHOTOSPHERIC RADIUS EXPANSION DURING MAGNETAR BURSTS
Watts, Anna L.; Van der Klis, Michiel; Wijers, Ralph A. M. J. [Astronomical Institute 'Anton Pannekoek', University of Amsterdam, Postbus 94249, 1090 GE Amsterdam (Netherlands); Kouveliotou, Chryssa [Space Science Office, VP62, NASA Marshall Space Flight Center, Huntsville, AL 35812 (United States); Van der Horst, Alexander J. [NASA Marshall Space Flight Center, Huntsville, AL 35805 (United States); Goegues, Ersin; Kaneko, Yuki [Sabanci University, Orhanli-Tuzla, Istanbul 34956 (Turkey); Harding, Alice K. [NASA Goddard Space Flight Center, Greenbelt, MD 20771 (United States); Baring, Matthew G., E-mail: A.L.Watts@uva.n [Department of Physics and Astronomy, MS-108, Rice University, P.O. Box 1892, Houston, TX 77251-1892 (United States)
2010-08-10
On 2008 August 24 the new magnetar SGR 0501+4516 (discovered by Swift) emitted a bright burst with a pronounced double-peaked structure in hard X-rays, reminiscent of the double-peaked temporal structure seen in some bright thermonuclear bursts on accreting neutron stars. In the latter case this is due to Photospheric Radius Expansion (PRE): when the flux reaches the Eddington limit, the photosphere expands and cools so that emission becomes softer and drops temporarily out of the X-ray band, re-appearing as the photosphere settles back down. We consider the factors necessary to generate double-peaked PRE events, and show that such a mechanism could plausibly operate in magnetar bursts despite the vastly different emission process. Identification of the magnetic Eddington limit in a magnetar would constrain magnetic field and distance and could, in principle, enable a measurement of gravitational redshift. It would also locate the emitting region at the neutron star surface, constraining the burst trigger mechanism. Conclusive confirmation of PRE events will require more detailed radiative models for bursts. However, for SGR 0501+4516 the predicted critical flux (using the magnetic field strength inferred from timing and the distance suggested by its probable location in the Perseus arm of our Galaxy) is consistent with that observed in the August 24 burst.
Experimental study of finite Larmor radius effects
Struve, K.W.
1980-08-01
Linear Z-pinches in Ar, Kr, Xe, N/sub 2/, and He are experimentally studied in regimes where strong finite Larmor radius effects could provide a significant stabilizing effect. Scaling arguments show that for deuterium such a pinch has an electron line density of order 2 x 10/sup 15//cm. For higher Z plasmas a higher line density is allowed, the exact value of which depends on the average ion charge. The pinch is formed by puffing gas axially through the cathode towards the anode of an evacuated pinch chamber. When the gas reaches the anode, the pinch bank is fired. The pinch current rises in 2 to 3 ..mu..sec to a maximum of 100 to 200 kA. The pinch bank capacitance is 900 ..mu..F, and the external inductance is 100 nH. Additionally, the bank is fused to increase dI/dt. The primary diagnostics are a framing camera, a spatially resolved Mach-Zehnder interferometer, and X-ray absorption.
Midgett, Aaron G; Luther, Joseph M; Stewart, John T; Smith, Danielle K; Padilha, Lazaro A; Klimov, Victor I; Nozik, Arthur J; Beard, Matthew C
2013-07-10
Using ultrafast transient absorption and time-resolved photoluminescence spectroscopies, we studied multiple exciton generation (MEG) in quantum dots (QDs) consisting of either PbSe, PbS, or a PbSxSe1-x alloy for various QD diameters with corresponding bandgaps (Eg) ranging from 0.6 to 1 eV. For each QD sample, we determine the MEG efficiency, ?MEG, defined in terms of the electron-hole pair creation energy (?eh) such that ?MEG = Eg/?eh. In previous reports, we found that ?MEG is about two times greater in PbSe QDs compared to bulk PbSe, however, little could be said about the QD-size dependence of MEG. In this study, we find for both PbS and PbSxSe1-x alloyed QDs that ?MEG decreases lineally with increasing QD diameter within the strong confinement regime. When the QD radius is normalized by a material-dependent characteristic radius, defined as the radius at which the electron-hole Coulomb and confinement energies are equivalent, PbSe, PbS, and PbSxSe1-x exhibit similar MEG behaviors. Our results suggest that MEG increases with quantum confinement, and we discuss the interplay between a size-dependent MEG rate versus hot exciton cooling. PMID:23750998
Precessing Asteroids ftrom Radius Vector Models?
NASA Astrophysics Data System (ADS)
Drummond, Jack D.
2014-11-01
Examining a sample of asteroids (the first 99) for which radius vector models have been constructed from mostly lightcurves, located on a web site where such models are listed (http://astro.troja.mff.cuni.cz/projects/damit ; see Durech et al. (2010), DAMIT: a database of asteroid models, A&A, 513, A46), we fit their surfaces as triaxial ellipsoids and provide their three dimensions. In the process we also derive an Euler angular offset ? between each model's spin axis and its axis of maximum moment of inertia assuming a uniform distribution of mass. Most ?'s conform to a chi-squared distribution having a maximum at 3° and a mean at 5°, and with the square root of the variance being 3°. However, seven models produce ?>20°, which we interpret as indicating possibly strong precessors, tumblers, or due to incorrect models: asteroids (68), (89), (125), (162), (167), (222), and (230). Nine others produce an excess over the distribution at 12°
The Bohr Model and the Fifth Grade: A New Standards-Based Hands-On Physics Curriculum
NASA Astrophysics Data System (ADS)
Brock, Jeff; Springer, Russell; Goldberg, Bennett
2004-03-01
A semester-long, standards-based, hands-on physics curriculum appropriate for the fifth grade was developed. Previously available curricula were successful in using hands-on activities to teach basic fifth-grade physics skills and concepts, but did not attempt to foster understanding of the fundamental underlying physics. We expanded the role of inquiry-based instruction to expose students to the fundamental physics behind electricity, forces, energy, light and sound. Central to the course, the Bohr model of the atom was used as a key tool both to motivate exploration of these topics as well as to develop basic conceptual understanding of fundamental ideas in quantum and electromagnetic physics. The curriculum was designed to be compatible with both district and state-mandated standards in a high-stakes test environment. This work was supported by NSF grant DGE-0231909.
Centrifugal-Barrier Effects and Determination of the Interaction Radius
Ning Wu
2013-05-21
The interaction radius of a resonance is an important physical quantity to describe the structure of a resonance. But, for a long time, physicists do not find a reliable way to measure the magnitude of the interaction radius of a resonance. In this paper, a method is proposed to measure the interaction radius in physics analysis. It is found that the centrifugal barrier effects have great influence to physical results obtained in the PWA fit, and the interaction radius of some resonances can be well measured in the fit.
Photosynthetic light harvesting: excitons and coherence
Fassioli, Francesca; Dinshaw, Rayomond; Arpin, Paul C.; Scholes, Gregory D.
2014-01-01
Photosynthesis begins with light harvesting, where specialized pigment–protein complexes transform sunlight into electronic excitations delivered to reaction centres to initiate charge separation. There is evidence that quantum coherence between electronic excited states plays a role in energy transfer. In this review, we discuss how quantum coherence manifests in photosynthetic light harvesting and its implications. We begin by examining the concept of an exciton, an excited electronic state delocalized over several spatially separated molecules, which is the most widely available signature of quantum coherence in light harvesting. We then discuss recent results concerning the possibility that quantum coherence between electronically excited states of donors and acceptors may give rise to a quantum coherent evolution of excitations, modifying the traditional incoherent picture of energy transfer. Key to this (partially) coherent energy transfer appears to be the structure of the environment, in particular the participation of non-equilibrium vibrational modes. We discuss the open questions and controversies regarding quantum coherent energy transfer and how these can be addressed using new experimental techniques. PMID:24352671
Röder, Beate
propagation of the exciton-chain vibrational wave function. The formation of self-trapped exciton states-probe spectroscopy Ref. 7 announced the observation of a N-H stretching mode self-trapping in the poly- -benzyl
Scalable interconnections for remote indirect exciton systems based on acoustic transport
NASA Astrophysics Data System (ADS)
Lazi?, S.; Violante, A.; Cohen, K.; Hey, R.; Rapaport, R.; Santos, P. V.
2014-02-01
Excitons, quasiparticles consisting of electron-hole pairs bound by the Coulomb interaction, are a potential medium for the processing of photonic information in the solid state. Information processing via excitons requires efficient techniques for the transport and manipulation of these uncharged particles. We have carried out a detailed investigation of the transport of excitons in GaAs quantum wells by surface acoustic waves. Based on these results, we introduce here a concept for the interconnection of multiple remote exciton systems based on the long-range transport of dipolar excitons by a network of configurable interconnects driven by acoustic wave beams. By combining this network with electrostatic gates, we demonstrate an integrated exciton multiplexer capable of interconnecting, gating, and routing exciton systems separated by millimeter distances. The multiplexer provides a scalable platform for the manipulation of exciton fluids with potential applications in information processing.
Slow light enhanced singlet exciton fission solar cells with a 126% yield of electrons per photon
Thompson, Nicholas J.
Singlet exciton fission generates two triplet excitons per absorbed photon. It promises to increase the power extracted from sunlight without increasing the number of photovoltaic junctions in a solar cell. We demonstrate ...
Brightening of excitons in carbon nanotubes on dimensionality modification
NASA Astrophysics Data System (ADS)
Miyauchi, Yuhei; Iwamura, Munechiyo; Mouri, Shinichiro; Kawazoe, Tadashi; Ohtsu, Motoichi; Matsuda, Kazunari
2013-09-01
Despite the attractive one-dimensional characteristics of carbon nanotubes, their typically low luminescence quantum yield, restricted because of their one-dimensional nature, has limited the performance of nanotube-based light-emitting devices. Here, we report the striking brightening of excitons (bound electron-hole pairs) in carbon nanotubes through an artificial modification of their effective dimensionality from one dimension to zero dimensions. Exciton dynamics in carbon nanotubes with luminescent, local zero-dimension-like states generated by oxygen doping were studied as model systems. We found that the luminescence quantum yield of the excitons confined in the zero-dimension-like states can be more than at least one order larger (~18%) than that of the intrinsic one-dimensional excitons (typically ~1%), not only because of the reduced non-radiative decay pathways but also due to an enhanced radiative recombination probability beyond that of intrinsic one-dimensional excitons. Our findings are extendable to the realization of future nanoscale photonic devices including a near-infrared single-photon emitter operable at room temperature.
Efficient Exciton Harvesting through Long-Range Energy Transfer.
Wang, Yanbin; Ohkita, Hideo; Benten, Hiroaki; Ito, Shinzaburo
2015-04-27
Efficient exciton collection at charge-generation sites is one of the key requirements for the improvement in power conversion efficiency (PCE) of organic solar cells, because only excitons arriving at a donor/acceptor interface can be dissociated into free charge carriers. We evaluated the effective diffusion length in poly(3-hexylthiophene) (P3HT) by using donor/acceptor bilayers with two different exciton-quenching acceptors. One is an insoluble fullerene polymer (p-PCBVB), which is an efficient electron-accepting material with negligible absorption in the visible region. The other is a low-bandgap polymer, poly[(4,4-bis(2-ethylhexyl)-dithieno[3,2-b:2',3'-d]silole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl], (PSBTBT). This polymer has a large absorption band in the near-IR region, which overlaps well with the emission band of P3HT. The effective diffusion length of P3HT excitons is evaluated to be 15 nm for P3HT/p-PCBVB bilayers and improved to 30 nm for P3HT/PSBTBT bilayers. This improvement is ascribed to long-range energy transfer from P3HT to PSBTBT. This finding suggests that the effective diffusion length of P3HT excitons can be increased through long-range energy transfer by incorporating PSBTBT into P3HT/PCBM blends. PMID:25598451
Strongly bound excitons in gapless two-dimensional structures
NASA Astrophysics Data System (ADS)
Liang, Yufeng; Soklaski, Ryan; Huang, Shouting; Graham, Matthew W.; Havener, Robin; Park, Jiwoong; Yang, Li
2014-09-01
Common wisdom asserts that bound excitons cannot form in high-dimensional (d>1) metallic structures because of their overwhelming screening and the unavoidable resonance with nearby continuous bands. Strikingly we illustrate that this prevalent assumption is not quite true. A key ingredient has been overlooked: Destructive coherent effects are capable of thwarting the formation of resonance. As an example of this general mechanism, we focus on an experimentally relevant material and predict bound excitons in twisted bilayer graphene, which is a two-dimensional gapless structure that exhibits metallic screening. The binding energies calculated by first-principles simulations are surprisingly large. The low-energy effective model reveals that these bound states are produced by a unique destructive coherence between two alike subband resonant excitons. In particular, this coherent effect is not sensitive to the screening and dimensionality, and hence may persist as a general mechanism for creating bound excitons in various metallic structures, opening the door for excitonic applications based on metallic structures.
Exciton Effects in Optical Absorption of Boron-Nitride Nanotubes
Harigaya, Kikuo
2007-01-01
Exciton effects are studied in single-wall boron-nitride (BN) nanotubes. Linear absorption spectra are calculated with changing the chiral index of the zigzag nanotubes. We consider the extended Hubbard model with atomic energies at the boron and nitrogen sites. Exciton effects are calculated using the configuration interaction technique. The Coulomb interaction dependence of the band gap, the lowest exciton energy, and the binding energy of the exciton are discussed. The optical gap of the (5,0) nanotube is about 6 eV at the onsite interaction U=2t with the hopping integral t=1.2 eV. The binding energy of the exciton is 0.50 eV for these parameters. This energy agrees well with that of other theoretical investigations. We find that the energy gap and the binding energy are almost independent of the geometries of the nanotubes. This novel property is in contrast with that of the carbon nanotubes which show metallic and semiconducting properties depending on the chiral index.
Polaronic excitons in ZnxCd1-xSe\\/ZnSe quantum wells
Andrea de Nardis; Vittorio Pellegrini; Raffaele Colombelli; Fabio Beltram; Lia Vanzetti; Alfonso Franciosi; I. N. Krivorotov; K. K. Bajaj
2000-01-01
Magneto-optical absorption of quantum wells exhibits distinct excitonic resonances and allows the determination of exciton binding energies and diamagnetic shifts. Here, we report a magneto-absorption study on Zn0.69Cd0.31Se\\/ZnSe multiple quantum wells 4 and 5 nm thick yielding 1S exciton binding-energy values. These results are compared to calculations based on two theoretical models that include the exciton-phonon interaction potential or the
Exciton dynamics in ZnCdSe\\/ZnSe quantum-well structures
S. A. Permogorov; A. N. Reznitskii; L. N. Tenishev; A. V. Kornievskii; S. Yu. Verbin; S. V. Ivanov; S. V. Sorokin; W. von der Osten; H. Stolz; M. Jütte
1998-01-01
Exciton dynamics in ZnCdSe\\/ZnSe quantum-well structures have been studied from luminescence spectra obtained at T=2 K. The energy and phase relaxation times of localized exciton states have been determined from a study of the destruction\\u000a of exciton optical alignment by an external magnetic field and direct measurements of the polarized-radiation decay kinetics\\u000a in the picosecond range. The exciton polarization lifetimes
Mukamel, Shaul
the exciton representation Wei Min Zhang, Torsten Meier,* Vladimir Chernyak, and Shaul Mukamel Department nanostructures is calculated. The two-band model is transformed to the exciton representation, and exciton in a plane-wave representation using a field- independent basis set, compared to the Wannier-Stark repre
Light-Emitting Polymers: a First-Principles Analysis of Singlet-Exciton Harvesting in PPV
NASA Astrophysics Data System (ADS)
Caldas, Marília J.; Bussi, Giovanni; Ruini, Alice; Molinari, Elisa
2005-06-01
We study poly(para-phenylene-vinylene) PPV in a ?-stacked crystalline con£guration, through ab initio density functional techniques for the electronic structure and optical properties. We £nd that interchain interactions, while maintaining the quasi-1D characteristics of the lowest singlet and triplet excitons, introduces other bound excitons that should favor intersystem crossing and enhance the singlet exciton yield.
Spatial breathing of the exciton distribution in ZnSe quantum wells
B. Dal Don; Hui Zhao; G. Schwartz; H. Kalt
2004-01-01
We measured the transport dynamics of excitons in ZnSe quantum wells with help of a time-resolved nanophotoluminescence setup. Right after picosecond excitation, the excitons move out of the laser spot, but then they reverse the direction of propagation, and finally spread out again. We attribute this spatial breathing of the exciton population to acoustic phonon emission, which is a predominantly
Disorder Limited Exciton Transport in Colloidal Single-Wall Carbon Nanotubes
Boyer, Edmond
Disorder Limited Exciton Transport in Colloidal Single-Wall Carbon Nanotubes Jared J. Crochet@lanl.gov KEYWORDS: carbon nanotube, exciton, dephasing, transport, exchange interaction Abstract We present measurements of S1 exciton transport in (6,5) carbon nanotubes at room tem- perature in a colloidal environment
Effet d'une microonde sur la fission des excitons singulets dans le ttracne cristallin
Paris-Sud XI, Université de
of fission singlet exciton processes by high power microwaves in crystalline tetracene. The observed effects indicate transitions between pair states of triplets produced by singlet exciton fission. The study643 Effet d'une microonde sur la fission des excitons singulets dans le tétracène cristallin J
singlet- triplet exciton fission. In Pc films (as well as C60-doped films) this decay channel behavior in an organic semiconductor by effectively turning singlet- triplet exciton fission decay channelsMorphology Effectively Controls Singlet-Triplet Exciton Relaxation and Charge Transport in Organic
Long-range excitons in conjugated polymers with ring torsions: poly(para-phenylene) and polyaniline
Harigaya, Kikuo
Long-range excitons in conjugated polymers with ring torsions: poly(para-phenylene) and polyaniline (Running head: Long-range excitons in conjugated polymers) Kikuo Harigaya Physical Science Division formalism. Long-range excitons are charac- terized, and the long-range component of the oscillator strengths
Study of exciton dynamics in garnets by low temperature thermo-luminescence
Collins, Gary S.
Study of exciton dynamics in garnets by low temperature thermo- luminescence D. T. Mackay, C. R://jap.aip.org/about/rights_and_permissions #12;Study of exciton dynamics in garnets by low temperature thermo-luminescence D. T. Mackay, C. R July 2012) Shallow traps that affect exciton dynamics in undoped and Ce doped yttrium aluminum garnet
Time-domain chirally-sensitive three-pulse coherent probes of vibrational excitons in proteins
Mukamel, Shaul
; Four-wave mixing; Excitons; Nonlinear exciton equations 1. Introduction Calculating the nonlinear first developed by Spano and Mukamel [68] and applied for four-wave mixing of coupled two-level [3 of the nonlinear exciton equations (NEE), which establish a quasi particle-scattering mechanism for optical
The Origin of the Ionic-Radius Ratio Rules
ERIC Educational Resources Information Center
Jensen, William B.
2010-01-01
In response to a reader query, this article traces the origins of the ionic-radius ratio rules and their incorrect attribution to Linus Pauling in the chemical literature and to Victor Goldschmidt in the geochemical literature. In actual fact, the ionic-radius ratio rules were first proposed within the context of the coordination chemistry…
Studying the proton 'radius' puzzle with ?p elastic scattering
Gilman, R. [Rutgers, The State University of New Jersey, Piscataway, NJ 08854 (United States)
2013-11-07
The disagreement between the proton radius determined from muonic hydrogen and from electronic measurements is called the proton radius puzzle. The resolution of the puzzle remains unclear and appears to require new experimental results. An experiment to measure muon-proton elastic scattering is presented here.
Hardness of the Covering Radius Problem on Lattices Ishay Haviv
Regev, Oded
Hardness of the Covering Radius Problem on Lattices Ishay Haviv Oded Regev June 6, 2006 Abstract We provide the first hardness result for the Covering Radius Problem on lattices (CRP). Namely, we show that for any large enough p there exists a constant cp > 1 such that CRP in the p norm is 2-hard
Probing Excitonic Dark States in Single-layer Tungsten Disulfide
Ye, Ziliang; O'Brien, Kevin; Zhu, Hanyu; Yin, Xiaobo; Wang, Yuan; Louie, Steven G; Zhang, Xiang
2014-01-01
Transition metal dichalcogenide (TMDC) monolayer has recently emerged as an important two-dimensional semiconductor with promising potentials for electronic and optoelectronic devices. Unlike semi-metallic graphene, layered TMDC has a sizable band gap. More interestingly, when thinned down to a monolayer, TMDC transforms from an indirect bandgap to a direct bandgap semiconductor, exhibiting a number of intriguing optical phenomena such as valley selective circular dichroism, doping dependent charged excitons, and strong photocurrent responses. However, the fundamental mechanism underlying such a strong light-matter interaction is still under intensive investigation. The observed optical resonance was initially considered to be band-to-band transitions. In contrast, first-principle calculations predicted a much larger quasiparticle band gap size and an optical response that is dominated by excitonic effects. Here, we report experimental evidence of the exciton dominance mechanism by discovering a series of exc...
Exciton effects in boron-nitride (BN) nanotubes
NASA Astrophysics Data System (ADS)
Harigaya, Kikuo
2012-12-01
Exciton effects are studied in single-wall boron-nitride nanotubes. The Coulomb interaction dependence of the band gap, the optical gap, and the binding energy of excitons are discussed. The optical gap of the (5,0) nanotube is about 6eV at the onsite interaction U=2t with the hopping integral t=1.1eV. The binding energy of the exciton is 0.50eV for these parameters. This energy agrees well with that of other theoretical investigations. We find that the energy gap and the binding energy are almost independent of the geometries of nanotubes. This novel property is in contrast with that of the carbon nanotubes which show metallic and semiconducting properties depending on the chiralities.
Electrodynamic and excitonic intertube interactions in semiconducting carbon nanotube aggregates.
Crochet, Jared J; Sau, Jay D; Duque, Juan G; Doorn, Stephen K; Cohen, Marvin L
2011-04-26
The optical properties of selectively aggregated, nearly single chirality single-wall carbon nanotubes were investigated by both continuous-wave and time-resolved spectroscopies. With reduced sample heterogeneities, we have resolved aggregation-dependent reductions of the excitation energy of the S(1) exciton and enhanced electron-hole pair absorption. Photoluminescence spectra revealed a spectral splitting of S(1) and simultaneous reductions of the emission efficiencies and nonradiative decay rates. The observed strong deviations from isolated tube behavior are accounted for by enhanced screening of the intratube Coulomb interactions, intertube exciton tunneling, and diffusion-driven exciton quenching. We also provide evidence that density gradient ultracentrifugation can be used to structurally sort single-wall carbon nanotubes by aggregate size as evident by a monotonic dependence of the aforementioned optical properties on buoyant density. PMID:21391554
Exciton-assisted optomechanics with suspended carbon nanotubes
I. Wilson-Rae; C. Galland; W. Zwerger; A. Imamoglu
2012-11-05
We propose a framework for inducing strong optomechanical effects in a suspended carbon nanotube based on deformation potential exciton-phonon coupling. The excitons are confined using an inhomogeneous axial electric field which generates optically active quantum dots with a level spacing in the milli-electronvolt range and a characteristic size in the 10-nanometer range. A transverse field induces a tunable parametric coupling between the quantum dot and the flexural modes of the nanotube mediated by electron-phonon interactions. We derive the corresponding excitonic deformation potentials and show that this interaction enables efficient optical ground-state cooling of the fundamental mode and could allow us to realise the strong and ultra-strong coupling regimes of the Jaynes-Cummings and Rabi models.
Excitonic positronium emission from n-Si(111)
NASA Astrophysics Data System (ADS)
Cassidy, D. B.; Hisakado, T. H.; Tom, H. W. K.; Mills, A. P., Jr.
2012-10-01
Recently it was found that energetic positronium (Ps) emission from both n- and p-type Si(100) occurs primarily via an exciton-like surface state PsX [D. B. Cassidy, T. H. Hisakado, H. W. K. Tom, and A. P. Mills, Jr., Phys. Rev. B1098-012110.1103/PhysRevB.84.195312 84, 195312 (2011)]. The underlying Ps production mechanism was deduced from the observed Ps emission yield and kinetic energy, using laser and thermally generated electrons to populate the PsX state, but not all of the observed effects have been explained. In this brief report we show that excitonic Ps emission also occurs from n-Si(111), but with characteristics that more closely resemble p-Si(100) than n-Si(100). Considering both (100) and (111) crystal orientations may help in the formulation of a more complete theoretical description of the excitonic Ps production mechanism.
Exciton transport, charge extraction, and loss mechanisms in organic photovoltaics
NASA Astrophysics Data System (ADS)
Scully, Shawn Ryan
Organic photovoltaics have attracted significant interest over the last decade due to their promise as clean low-cost alternatives to large-scale electric power generation such as coal-fired power, natural gas, and nuclear power. Many believe power conversion efficiency targets of 10-15% must be reached before commercialization is possible. Consequently, understanding the loss mechanisms which currently limit efficiencies to 4-5% is crucial to identify paths to reach higher efficiencies. In this work, we investigate the dominant loss mechanisms in some of the leading organic photovoltaic architectures. In the first class of architectures, which include planar heterojunctions and bulk heterojunctions with large domains, efficiencies are primarily limited by the distance photogenerated excitations (excitons) can be transported (termed the exciton diffusion length) to a heterojunction where the excitons may dissociate. We will discuss how to properly measure the exciton diffusion length focusing on the effects of optical interference and of energy transfer when using fullerenes as quenching layers and show how this explains the variety of diffusion lengths reported for the same material. After understanding that disorder and defects limit exciton diffusion lengths, we suggest some approaches to overcome this. We then extensively investigate the use of long-range resonant energy transfer to increase exciton harvesting. Using simulations and experiments as support, we discuss how energy transfer can be engineered into architectures to increase the distance excitons can be harvested. In an experimental model system, DOW Red/PTPTB, we will show how the distance excitons are harvested can be increased by almost an order of magnitude up to 27 nm from a heterojunction and give design rules and extensions of this concept for future architectures. After understanding exciton harvesting limitations we will look at other losses that are present in planar heterojunctions. One of the primary losses that puts stringent requirements on the charge carrier mobilities in these cells is the recombination losses due to space charge build up at the heterojunction. Because electrons are confined to the acceptor and holes to the donor, net charge density always exists even when mobilities are matched, in contrast to bulk heterojunctions wherein matched mobilities lead to zero net charge. This net charge creates an electric field which opposes the built-in field and limits the current that can be carried away from this heterojunction. Using simulations we show that for relevant current densities charge carrier mobilities must be higher than 10-4 cm2/V.s to avoid significant losses due to space charge formation. In the last part of this work, we will focus on the second class of architectures in which exciton harvesting is efficient. We will present a systematic analysis of one of the leading polymer:fullerene bulk heterojunction cells to show that losses in this architecture are due to charge recombination. Using optical measurements and simulations, exciton harvesting measurements, and device characteristics we will show that the dominant loss is likely due to field-dependent geminate recombination of the electron and hole pair created immediately following exciton dissociation. No losses in this system are seen due to bimolecular recombination or space charge which provides information on charge-carrier mobility targets necessary for the future design of high efficiency organic photovoltaics.
Exciton band structure of monolayer MoS2
NASA Astrophysics Data System (ADS)
Wu, Fengcheng; Qu, Fanyao; MacDonald, A. H.
2015-02-01
We address the properties of excitons in monolayer MoS2 from a theoretical point of view, showing that low-energy excitonic states occur both at the Brillouin-zone center and at the Brillouin-zone corners, that binding energies at the Brillouin-zone center deviate strongly from the (n-1 /2 ) -2 pattern of the two-dimensional hydrogenic model, and that the valley-degenerate exciton doublet at the Brillouin-zone center splits at finite momentum into an upper mode with nonanalytic linear dispersion and a lower mode with quadratic dispersion. Although monolayer MoS2 is a direct-gap semiconductor when classified by its quasiparticle band structure, it may well be an indirect gap material when classified by its excitation spectra.
f-band condensates in exciton-polariton lattice systems
NASA Astrophysics Data System (ADS)
Kim, Na Young; Kusudo, Kenichiro; Löffler, Andreas; Höfling, Sven; Forchel, Alfred; Yamamoto, Yoshihisa
2014-02-01
We report the condensation of microcavity exciton-polaritons at ? points located on the boundary between the third and higher Brillouin zones in hexagonal lattices: triangular and honeycomb geometries. We collect experimental evidence that supports the finite momentum condensation: (1) the coherent Bragg peaks formed at nonzero ? points; (2) the nonlinear intensity increase in the exciton-polariton emission at quantum degeneracy threshold; (3) the spectral linewidth behavior: narrowing near threshold and broadening above threshold; and (4) the equivalent 4fy3-3x2y-like orbital symmetry in real space. The f-orbital state at ? points appears as a metastable momentum valley to trap exciton-polaritons, which is explained by single-particle band-structure calculations.
Tuning of exciton states in a magnetic quantum ring
NASA Astrophysics Data System (ADS)
Ghazaryan, Areg; Manaselyan, Aram; Chakraborty, Tapash
2015-02-01
The exciton states in a CdTe quantum ring subjected to an external magnetic field containing a single magnetic impurity are investigated. We have used the multiband approximation which includes the heavy hole-light hole coupling effects. The electron-hole spin interactions and the s, p-d interactions between the electron, the hole and the magnetic impurity are also included. The exciton energy levels and optical transitions are evaluated using the exact diagonalization scheme. We show that due to the spin interactions it is possible to change the bright exciton state into the dark state and vice versa with the help of a magnetic field. We propose a new route to experimentally estimate the s, p-d spin interaction constants.
Microscopic theory of singlet exciton fission. I. General formulation.
Berkelbach, Timothy C; Hybertsen, Mark S; Reichman, David R
2013-03-21
Singlet fission, a spin-allowed energy transfer process generating two triplet excitons from one singlet exciton, has the potential to dramatically increase the efficiency of organic solar cells. However, the dynamical mechanism of this phenomenon is not fully understood and a complete, microscopic theory of singlet fission is lacking. In this work, we assemble the components of a comprehensive microscopic theory of singlet fission that connects excited state quantum chemistry calculations with finite-temperature quantum relaxation theory. We elaborate on the distinction between localized diabatic and delocalized exciton bases for the interpretation of singlet fission experiments in both the time and frequency domains. We discuss various approximations to the exact density matrix dynamics and propose Redfield theory as an ideal compromise between speed and accuracy for the detailed investigation of singlet fission in dimers, clusters, and crystals. Investigations of small model systems based on parameters typical of singlet fission demonstrate the numerical accuracy and practical utility of this approach. PMID:23534622
Microscopic theory of singlet exciton fission. I. General formulation
NASA Astrophysics Data System (ADS)
Berkelbach, Timothy C.; Hybertsen, Mark S.; Reichman, David R.
2013-03-01
Singlet fission, a spin-allowed energy transfer process generating two triplet excitons from one singlet exciton, has the potential to dramatically increase the efficiency of organic solar cells. However, the dynamical mechanism of this phenomenon is not fully understood and a complete, microscopic theory of singlet fission is lacking. In this work, we assemble the components of a comprehensive microscopic theory of singlet fission that connects excited state quantum chemistry calculations with finite-temperature quantum relaxation theory. We elaborate on the distinction between localized diabatic and delocalized exciton bases for the interpretation of singlet fission experiments in both the time and frequency domains. We discuss various approximations to the exact density matrix dynamics and propose Redfield theory as an ideal compromise between speed and accuracy for the detailed investigation of singlet fission in dimers, clusters, and crystals. Investigations of small model systems based on parameters typical of singlet fission demonstrate the numerical accuracy and practical utility of this approach.
Effects of Fermion Exchange on the Polarization of Exciton Condensates
NASA Astrophysics Data System (ADS)
Combescot, Monique; Combescot, Roland; Alloing, Mathieu; Dubin, François
2015-03-01
Exchange interaction is responsible for the stability of elementary boson condensates with respect to momentum fragmentation. This remains true for composite bosons when single fermion exchanges are included but spin degrees of freedom are ignored. Here, we show that their inclusion can produce a spin fragmentation of the dark exciton condensate, i.e., an unpolarized condensate with an equal amount of spin (+2 ) and (-2 ) excitons not coupled to light. The composite boson many-body formalism allows us to predict that, for spatially indirect excitons, the condensate polarization switches from unpolarized to fully polarized when the distance between the layers confining electrons and holes increases. Importantly, the threshold distance for this switch lies in a regime fully accessible to experiments.
Exciton coupling induces vibronic hyperchromism in light-harvesting complexes
Schulze, Jan; Kühn, Oliver; Pullerits, Tõnu
2013-01-01
The recently suggested possibility that weak vibronic transitions can be excitonically enhanced in light-harvesting complexes is studied in detail. A vibronic exciton dimer model which includes ground state vibrations is investigated using multi-configuration time-dependent Hartree method with a parameter set typical to photosynthetic light-harvesting complexes. Absorption spectra are discussed in dependence on the Coulomb coupling, the detuning of site energies, and the number of vibrational mode. Calculations of the fluorescence spectra show that the spectral densities obtained from the low temperature fluorescence line narrowing measurements of light-harvesting systems need to be corrected for the exciton effects. For the J-aggregate configuration, as in most of the light-harvesting complexes, the true spectral density has larger amplitude than what is obtained from the measurement.
Excitonic giant Zeeman effect in GaN:Mn3+
NASA Astrophysics Data System (ADS)
Pacuski, W.; Ferrand, D.; Cibert, J.; Gaj, J. A.; Golnik, A.; Kossacki, P.; Marcet, S.; Sarigiannidou, E.; Mariette, H.
2007-10-01
We describe a direct observation of the excitonic giant Zeeman splitting in Ga1-xMnxN , a wide-gap III-V diluted magnetic semiconductor. Reflectivity and absorption spectra measured at low temperatures display the A and B excitons, with a shift under magnetic field due to s,p-d exchange interactions. Using an excitonic model, we determine the difference of exchange integrals between Mn3+ and free carriers in GaN, N0(?-?)=-1.2±0.2eV . Assuming a reasonable value of ? , this implies a positive sign of ? which corresponds to a rarely observed ferromagnetic interaction between the magnetic ions and the holes involved in the transition.
Long-range exciton dissociation in organic solar cells
Caruso, Domenico; Troisi, Alessandro
2012-01-01
It is normally assumed that electrons and holes in organic solar cells are generated by the dissociation of excitons at the interface between donor and acceptor materials in strongly bound hole-electron pairs. We show in this contribution that excitons can dissociate tens of angstroms away from the interface and generate partially separated electrons and holes, which can more easily overcome their coulombic attraction and form free charges. We first establish under what conditions long-range exciton dissociation is likely (using a kinetic model and a microscopic model for the calculation of the long-range electron transfer rate). Then, defining a rather general model Hamiltonian for the donor material, we show that the phenomenon is extremely common in the majority of polymer:fullerene bulk heterojunction solar cells. PMID:22869702
Biexcitonic molecules survive excitons at the Mott transition.
Shahmohammadi, Mehran; Jacopin, Gwénolé; Rossbach, Georg; Levrat, Jacques; Feltin, Eric; Carlin, Jean-François; Ganière, Jean-Daniel; Butté, Raphaël; Grandjean, Nicolas; Deveaud, Benoit
2014-01-01
When the carrier density is increased in a semiconductor, according to the predictions of Sir Nevil Mott, a transition should occur from an insulating state consisting of a gas of excitons to a conductive electron-hole plasma. This crossover, usually referred to as the Mott transition, is driven by the mutual effects of phase-space filling and Coulomb screening because of the presence of other charges nearby. It drastically affects the optical and electrical characteristics of semiconductors and may, for example, drive the transition from a polariton laser to a vertical cavity surface-emitting laser. Usually, the possible existence of excitonic molecules (or biexcitons) is neglected in the understanding of the Mott transition because the biexciton is supposed to be less robust against screening effects. Here, against common beliefs, we observe that the biexciton in a GaN quantum well is more stable towards the Mott transition than the exciton. PMID:25341721
Quantum confinement-induced tunable exciton states in graphene oxide
Lee, Dongwook; Seo, Jiwon; Zhu, Xi; Lee, Jiyoul; Shin, Hyeon-Jin; Cole, Jacqueline M.; Shin, Taeho; Lee, Jaichan; Lee, Hangil; Su, Haibin
2013-01-01
Graphene oxide has recently been considered to be a potential replacement for cadmium-based quantum dots due to its expected high fluorescence. Although previously reported, the origin of the luminescence in graphene oxide is still controversial. Here, we report the presence of core/valence excitons in graphene-based materials, a basic ingredient for optical devices, induced by quantum confinement. Electron confinement in the unreacted graphitic regions of graphene oxide was probed by high resolution X-ray absorption near edge structure spectroscopy and first-principles calculations. Using experiments and simulations, we were able to tune the core/valence exciton energy by manipulating the size of graphitic regions through the degree of oxidation. The binding energy of an exciton in highly oxidized graphene oxide is similar to that in organic electroluminescent materials. These results open the possibility of graphene oxide-based optoelectronic device technology. PMID:23872608
Decreasing the spectral radius of a graph by link removals
NASA Astrophysics Data System (ADS)
van Mieghem, Piet; Stevanovi?, Dragan; Kuipers, Fernando; Li, Cong; van de Bovenkamp, Ruud; Liu, Daijie; Wang, Huijuan
2011-07-01
The decrease of the spectral radius, an important characterizer of network dynamics, by removing links is investigated. The minimization of the spectral radius by removing m links is shown to be an NP-complete problem, which suggests considering heuristic strategies. Several greedy strategies are compared, and several bounds on the decrease of the spectral radius are derived. The strategy that removes that link l=i~j with largest product (x1)i(x1)j of the components of the eigenvector x1 belonging to the largest adjacency eigenvalue is shown to be superior to other strategies in most cases. Furthermore, a scaling law where the decrease in spectral radius is inversely proportional to the number of nodes N in the graph is deduced. Another sublinear scaling law of the decrease in spectral radius versus the number m of removed links is conjectured.
Novel Quantum Condensates in Excitonic Matter
Littlewood, P. B.; Keeling, J. M. J.; Simons, B. D. [Cavendish Laboratory, Madingley Road, Cambridge CB3 OHE (United Kingdom); Eastham, P. R. [Blackett Laboratory, Imperial College London, London SW7 2AZ (United Kingdom); Marchetti, F. M. [Departamento de Fisica Teorica de la Materia Condensada, Universidad Autonoma de Madrid, Madrid 28049 (Spain); Szymanska, M. H. [Department of Physics, University of Warwick, Coventry, CV4 7AL (United Kingdom)
2009-08-20
These lectures interleave discussion of a novel physical problem of a new kind of condensate with teaching of the fundamental theoretical tools of quantum condensed matter field theory. Polaritons and excitons are light mass composite bosons that can be made inside solids in a number of different ways. As bosonic particles, they are liable to make a phase coherent ground state - generically called a Bose-Einstein condensate (BEC) - and these lectures present some models to describe that problem, as well as general approaches to the theory. The focus is very much to explain how mean-field-like approximations that are often presented heuristically can be derived in a systematic fashion by path integral methods. Going beyond the mean field theory then produces a systematic approach to calculation of the excitation energies, and the derivation of effective low energy theories that can be generalised to more complex dynamical and spatial situations than is practicable for the full theory, as well as to study statistical properties beyond the semi-classical regime. in particular, for the polariton problem, it allows one to connect the regimes of equilibrium BEC and non-equilibrium laser. The lectures are self-sufficient, but not highly detailed. The methodological aspects are covered in standard quantum field theory texts and the presentation here is deliberately cursory: the approach will be closest to the book of Altland and Simons. Since these lectures concern a particular type of condensate, reference should also be made to texts on BEC, for example by Pitaevskii and Stringari. A recent theoretically focussed review of polariton systems covers many of the technical issues associated with the polariton problem in greater depth and provides many further references.
Exciton localization-delocalization transition in an extended dendrimer
Pouthier, Vincent, E-mail: vincent.pouthier@univ-fcomte.fr [Institut UTINAM, Université de Franche-Comté, CNRS UMR 6213, 25030 Besançon Cedex (France)] [Institut UTINAM, Université de Franche-Comté, CNRS UMR 6213, 25030 Besançon Cedex (France)
2013-12-21
Exciton-mediated quantum state transfer between the periphery and the core of an extended dendrimer is investigated numerically. By mapping the dynamics onto that of a linear chain, it is shown that a localization-delocalization transition arises for a critical value of the generation number G{sub c} ? 5. This transition originates in the quantum interferences experienced by the excitonic wave due to the multiple scatterings that arise each time the wave tunnels from one generation to another. These results suggest that only small-size dendrimers could be used for designing an efficient quantum communication protocol.
Evaluation of defects in cuprous oxide through exciton luminescence imaging
NASA Astrophysics Data System (ADS)
Frazer, Laszlo; Lenferink, Erik J.; Chang, Kelvin B.; Poeppelmeier, Kenneth R.; Stern, Nathaniel P.; Ketterson, John B.
2015-03-01
The various decay mechanisms of excitons in cuprous oxide (Cu2O) are highly sensitive to defects which can relax selection rules. Here we report cryogenic hyperspectral imaging of exciton luminescence from cuprous oxide crystals grown via the floating zone method showing the samples have few defects. Some locations, however, show strain splitting of the 1s orthoexciton triplet polariton luminescence. Strain is reduced by annealing. In addition, annealing causes annihilation of oxygen and copper vacancies, which leads to a negative correlation between luminescence of unlike vacancies.
Charged excitons and biexcitons bound to isoelectronic centers
NASA Astrophysics Data System (ADS)
Marcet, S.; Ouellet-Plamondon, C.; Éthier-Majcher, G.; Saint-Jean, P.; André, R.; Klem, J. F.; Francoeur, S.
2010-12-01
We demonstrate that the singular binding mechanism characterizing isoelectronic centers formed from two isoelectronic traps can also bind, in addition to the well-studied excitons, various number of charges. Using the emission fine structure of Te dyads in ZnSe and N dyads in GaAs, we establish that these pseudodonors and pseudoacceptors can bind positively and negatively charged excitons, respectively, and that both can bind biexcitons. This ability to bind various charge configurations, in addition to their very low inhomogeneous broadenings and perfectly defined symmetries, further establishes isoelectronic centers as an interesting alternative to epitaxial quantum dots for a number of applications.
Crossed excitons in a semiconductor nanostructure of mixed dimensionality
Owschimikow, Nina, E-mail: nina.owschimikow@physik.tu-berlin.de; Kolarczik, Mirco; Kaptan, Yücel I.; Grosse, Nicolai B.; Woggon, Ulrike [Institut für Optik und Atomare Physik, Technische Universität Berlin, Strasse des 17. Juni 135, 10623 Berlin (Germany)
2014-09-08
Semiconductor systems of reduced dimensionality, e.g., quantum dots or quantum wells, display a characteristic spectrum of confined excitons. Combining several of these systems may lead to the formation of “crossed” excitons, and thus new equilibrium states and scattering channels. We derive gain excitation spectra from two-color pump-probe experiments on an In(Ga)As based quantum dot semiconductor optical amplifier by analyzing the amplitudes of the traces. This grants access to the quantum dot response, even in the presence of strong absorption by the surroundings at the excitation energy. The gain excitation spectra yield evidence of crossed quantum dot-bulk states.
Singlet exciton fission in polycrystalline pentacene: from photophysics toward devices.
Wilson, Mark W B; Rao, Akshay; Ehrler, Bruno; Friend, Richard H
2013-06-18
Singlet exciton fission is the process in conjugated organic molecules bywhich a photogenerated singlet exciton couples to a nearby chromophore in the ground state, creating a pair of triplet excitons. Researchers first reported this phenomenon in the 1960s, an event that sparked further studies in the following decade. These investigations used fluorescence spectroscopy to establish that exciton fission occurred in single crystals of several acenes. However, research interest has been recently rekindled by the possibility that singlet fission could be used as a carrier multiplication technique to enhance the efficiency of photovoltaic cells. The most successful architecture to-date involves sensitizing a red-absorbing photoactive layer with a blue-absorbing material that undergoes fission, thereby generating additional photocurrent from higher-energy photons. The quest for improved solar cells has spurred a drive to better understand the fission process, which has received timely aid from modern techniques for time-resolved spectroscopy, quantum chemistry, and small-molecule device fabrication. However, the consensus interpretation of the initial studies using ultrafast transient absorption spectroscopy was that exciton fission was suppressed in polycrystalline thin films of pentacene, a material that would be otherwise expected to be an ideal model system, as well as a viable candidate for fission-sensitized photovoltaic devices. In this Account, we review the results of our recent transient absorption and device-based studies of polycrystalline pentacene. We address the controversy surrounding the assignment of spectroscopic features in transient absorption data, and illustrate how a consistent interpretation is possible. This work underpins our conclusion that singlet fission in pentacene is extraordinarily rapid (?80 fs) and is thus the dominant decay channel for the photoexcited singlet exciton. Further, we discuss our demonstration that triplet excitons generated via singlet fission in pentacene can be dissociated at an interface with a suitable electron acceptor, such as fullerenes and infrared-absorbing inorganic semiconducting quantum dots. We highlight our recent reports of a pentacene/PbSe hybrid solar cell with a power conversion efficiency of 4.7% and of a pentacene/PbSe/amorphous silicon photovoltaic device. Although substantive challenges remain, both to better our understanding of the mechanism of singlet exciton fission and to optimize device performance, this realization of a solar cell where photocurrent is simultaneously contributed from a blue-absorbing fission-capable material and an infrared-absorbing conventional cell is an important step towards a dual-bandgap, single-junction, fission-enhanced photovoltaic device, which could one day surpass the Shockley-Queisser limit. PMID:23656886
Plasmon and exciton superconductivity mechanisms in layered structures
NASA Technical Reports Server (NTRS)
Gabovich, A. M.; Pashitskiy, E. A.; Uvarova, S. K.
1977-01-01
Plasmon and exciton superconductivity mechanisms are discussed. Superconductivity in a three layer metal semiconductor metal and insulator semimetal insulator sandwich structure was described in terms of the temperature dependent Green function of the longitudinal (Coulomb) field. The dependences of the superconducting transition temperature on structure parameters were obtained. In a semiconducting film, as a result of interactions of degenerate free carriers with excitons, superconductivity exists only in a certain range of parameter values, and the corresponding critical temperature is much lower than in the plasmon mechanism of superconductivity.
Nanosecond excitonic spin relaxation in cubic GaN
NASA Astrophysics Data System (ADS)
Tackeuchi, Atsushi; Otake, Hirotaka; Ogawa, Yusuke; Ushiyama, Takafumi; Fujita, Taisuke; Takano, Fumiyoshi; Akinaga, Hiro
2007-04-01
The excitonic spin relaxation in cubic GaN is observed by spin-dependent pump and probe reflectance measurements with sub-picosecond time resolution. The spin relaxation times at 15 - 75 K are found to be longer than 5 ns and short spin relaxation times on the picosecond order are not present. Although these long spin relaxation times are in striking contrast to the sub-picosecond spin relaxation of A-band free excitons in hexagonal GaN, they are consistent with the dependence that spin relaxation time becomes longer for wider-band-gap zincblende semiconductors.
Nanosecond excitonic spin relaxation in cubic GaN
NASA Astrophysics Data System (ADS)
Tackeuchi, Atsushi; Otake, Hirotaka; Ogawa, Yusuke; Ushiyama, Takafumi; Fujita, Taisuke; Takano, Fumiyoshi; Akinaga, Hiro
2006-04-01
The excitonic spin relaxation process in cubic GaN is observed by spin-dependent pump and probe reflectance measurements with subpicosecond time resolution. The spin polarization presents at temperatures lower than 100K. The spin relaxation times at 15-75K are found to be longer than 5ns and short spin relaxation times on the picosecond order are not present. Although these long spin relaxation times are in striking contrast to the subpicosecond spin relaxation of A-band free excitons in hexagonal GaN, they are consistent with the dependence that spin relaxation time becomes longer for wider-band gap zinc blende semiconductors.
Excitonic collapse in semiconducting transition-metal dichalcogenides
NASA Astrophysics Data System (ADS)
Rodin, A. S.; Castro Neto, A. H.
2013-11-01
Semiconducting transition metal dichalcogenides (STMDC) are two-dimensional (2D) crystals characterized by electron-volt-size band gaps, spin-orbit coupling (SOC), and d-orbital character of its valence and conduction bands. We show that these materials carry unique exciton quasiparticles (electron-hole bound states) with energy within the gap but which can “collapse” in the strong coupling regime by merging into the band structure continuum. The exciton collapse seems to be a generic effect in these 2D crystals.
Norman F. Berk
1978-01-01
It is shown in a kinematic analysis based on Suna's theory of diffusion-mediated triplet exciton-exciton annihilation that the rate constants for triplet exciton fusion (annihilation) and singlet exciton fission have the same magnetic field dependence. The relation between Suna's theory and Noyes' stochastic theory of diffusion-mediated chemical reactions is briefly discussed.
Optical investigation of strong exciton localization in high Al composition AlxGa?-xN alloys.
Fan, Shunfei; Qin, Zhixin; He, Chenguang; Hou, Mengjun; Wang, Xinqiang; Shen, Bo; Li, Wei; Wang, Weiying; Mao, Defeng; Jin, Peng; Yan, Jianchang; Dong, Peng
2013-10-21
The exciton localization in wurtzite AlxGa?-xN alloys with x varying from 0.41 to 0.63 has been studied by deep-ultraviolet photoluminescence (PL) spectroscopy and picosecond time-resolved PL spectroscopy. Obvious S-shape temperature dependence was observed indicating that the strong exciton localization can be formed in high Al composition AlxGa?-xN alloys. It was also found that the Al composition dependence of exciton localization energy of AlGaN alloys is inconsistent with that of the excitonic linewidth. We contribute the inconsistency to the strong zero-dimensional exciton localization. PMID:24150295
Yamada, Yasuhiro; Imada, Masatoshi
2015-01-01
Energy dissipation and decoherence are at first glance harmful to acquiring long exciton lifetime desired for efficient photovoltaics. In the presence of both optically forbidden (namely, dark) and allowed (bright) excitons, however, they can be instrumental as suggested in photosynthesis. By simulating quantum dynamics of exciton relaxations, we show that the optimized decoherence that imposes a quantum-to-classical crossover with the dissipation realizes a dramatically longer lifetime. In an example of carbon nanotube, the exciton lifetime increases by nearly two orders of magnitude when the crossover triggers stable high population in the dark exciton.
Photoluminescence properties and exciton dynamics in monolayer WSe{sub 2}
Yan, Tengfei; Qiao, Xiaofen; Liu, Xiaona; Tan, Pingheng; Zhang, Xinhui, E-mail: xinhuiz@semi.ac.cn [State Key Laboratory of Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, P.O. Box 912, Beijing 100083 (China)
2014-09-08
In this work, comprehensive temperature and excitation power dependent photoluminescence and time-resolved photoluminescence studies are carried out on monolayer WSe{sub 2} to reveal its properties of exciton emissions and related excitonic dynamics. Competitions between the localized and delocalized exciton emissions, as well as the exciton and trion emissions are observed, respectively. These competitions are suggested to be responsible for the abnormal temperature and excitation intensity dependent photoluminescence properties. The radiative lifetimes of both excitons and trions exhibit linear dependence on temperature within the temperature regime below 260?K, providing further evidence for two-dimensional nature of monolayer material.
Yasuhiro Yamada; Youhei Yamaji; Masatoshi Imada
2015-02-25
Energy dissipation and decoherence are at first glance harmful to acquiring long exciton lifetime desired for efficient photovoltaics. In the presence of both optically forbidden (namely, dark) and allowed (bright) excitons, however, they can be instrumental as suggested in photosynthesis. By simulating quantum dynamics of exciton relaxations, we show that the optimized decoherence that imposes a quantum-to-classical crossover with the dissipation realizes a dramatically longer lifetime. In an example of carbon nanotube, the exciton lifetime increases by nearly two orders of magnitude when the crossover triggers stable high population in the dark exciton.
Thermoconvective vortices in a cylindrical annulus with varying inner radius.
Castaño, D; Navarro, M C; Herrero, H
2014-12-01
This paper shows the influence of the inner radius on the stability and intensity of vertical vortices, qualitatively similar to dust devils and cyclones, generated in a cylindrical annulus non-homogeneously heated from below. Little relation is found between the intensity of the vortex and the magnitude of the inner radius. Strong stable vortices can be found for both small and large values of the inner radius. The Rankine combined vortex structure, that characterizes the tangential velocity in dust devils, is clearly observed when small values of the inner radius and large values of the ratio between the horizontal and vertical temperature differences are considered. A contraction on the radius of maximum azimuthal velocity is observed when the vortex is intensified by thermal mechanisms. This radius becomes then nearly stationary when frictional force balances the radial inflow generated by the pressure drop in the center, despite the vortex keeps intensifying. These results connect with the behavior of the radius of the maximum tangential wind associated with a hurricane. PMID:25554036
Thermoconvective vortices in a cylindrical annulus with varying inner radius
NASA Astrophysics Data System (ADS)
Castaño, D.; Navarro, M. C.; Herrero, H.
2014-12-01
This paper shows the influence of the inner radius on the stability and intensity of vertical vortices, qualitatively similar to dust devils and cyclones, generated in a cylindrical annulus non-homogeneously heated from below. Little relation is found between the intensity of the vortex and the magnitude of the inner radius. Strong stable vortices can be found for both small and large values of the inner radius. The Rankine combined vortex structure, that characterizes the tangential velocity in dust devils, is clearly observed when small values of the inner radius and large values of the ratio between the horizontal and vertical temperature differences are considered. A contraction on the radius of maximum azimuthal velocity is observed when the vortex is intensified by thermal mechanisms. This radius becomes then nearly stationary when frictional force balances the radial inflow generated by the pressure drop in the center, despite the vortex keeps intensifying. These results connect with the behavior of the radius of the maximum tangential wind associated with a hurricane.
On the Variation of Solar Radius in Rotation Cycles
NASA Astrophysics Data System (ADS)
Qu, Z. N.; Kong, D. F.; Xiang, N. B.; Feng, W.
2015-01-01
The Date Compensated Discrete Fourier Transform and CLEANest algorithm are used to study the temporal variations of the solar radius observed at Rio de Janeiro Observatory from 1998 March 2 to 2009 November 6. The CLEANest spectra show several significant periodicities around 400, 312, 93.5, 86.2, 79.4, 70.9, 53.2, and 26.3 days. Then, combining the data on the daily solar radius measured at Calern Observatory and Rio de Janeiro Observatory and the corresponding daily sunspot areas, we study the short-term periodicity of the solar radius and the role of magnetic field in the variation of the solar radius. The rotation period of the daily solar radius is determined to be statistically significant. Moreover, its temporal evolution is anti-phase with that of sunspot activity, and it is found anti-phase with solar activity. Generally, the stronger solar activity is, the more obvious is the anti-phase relation of radius with solar activity. This indicates that strong magnetic fields have a greater inhibitive effect than weak magnetic fields on the variation of the radius.
Nogues, Gilles, E-mail: gilles.nogues@neel.cnrs.fr; Den Hertog, Martien [Inst. NEEL, Univ. Grenoble Alpes, F-38042 Grenoble (France); Inst. NEEL, CNRS, F-38042 Grenoble (France); Auzelle, Thomas; Gayral, Bruno; Daudin, Bruno [INAC, CEA, F-38054 Grenoble (France)
2014-03-10
We perform correlated studies of individual GaN nanowires in scanning electron microscopy combined to low temperature cathodoluminescence, microphotoluminescence, and scanning transmission electron microscopy. We show that some nanowires exhibit well localized regions emitting light at the energy of a stacking fault bound exciton (3.42?eV) and are able to observe the presence of a single stacking fault in these regions. Precise measurements of the cathodoluminescence signal in the vicinity of the stacking fault give access to the exciton diffusion length near this location.
An Asian Perspective on the Management of Distal Radius Fractures
Sebastin, Sandeep J.; Chung, Kevin C.
2012-01-01
Synopsis There is little data with regards to the epidemiology, pathology, or management of distal radius fractures from centers in Asia. Asia includes five advanced economies, namely Hong Kong SAR, Japan, Korea, Singapore, and Taiwan and a number of emerging economies prominent among which are China, India, Malaysia, Philippines, and Thailand. This article examines the available epidemiological data from Asia, and compares the management of distal radius fractures in the advanced and emerging Asian economies and how they match up to the current management in the west. It concludes by offering solutions for improving outcomes of distal radius fractures in both the advanced and emerging economies of Asia. PMID:22554658
Magnetic field tuning of exciton-polaritons in a semiconductor microcavity
NASA Astrophysics Data System (ADS)
Pietka, B.; Zygmunt, D.; Król, M.; Molas, M. R.; Nicolet, A. A. L.; Morier-Genoud, F.; Szczytko, J.; ?usakowski, J.; Zieba, P.; Tralle, I.; Stepnicki, P.; Matuszewski, M.; Potemski, M.; Deveaud, B.
2015-02-01
We detail the influence of a magnetic field on exciton-polaritons inside a semiconductor microcavity. Magnetic field can be used as a tuning parameter for exciton and photon resonances. We discuss the change of the exciton energy, the oscillator strength, and redistribution of the polariton density along the dispersion curves due to the magnetically induced detuning. We have observed that field-induced shrinkage of the exciton wave function has a direct influence not only on the exciton oscillator strength, which is observed to increase with the magnetic field, but also on the polariton linewidth. We discuss the effect of the Zeeman splitting on polaritons the magnitude of which changes with the exciton Hopfield coefficient and can be modeled by independent coupling of the two spin components of excitons with cavity photons.
Photoluminescence of “dark” excitons in CdMnTe quantum well, embedded in a microcavity
NASA Astrophysics Data System (ADS)
Brunetti, A.; Vladimirova, M.; Scalbert, D.; André, R.; Ballarini, D.; Amo, A.; Martin, M. D.; Viña, L.
2007-05-01
A diluted magnetic semiconductor (DMS) quantum well (QW) microcavity operating in the limit of the strong coupling regime is studied by magnetoptical experiments. The interest of DMS QW relies on the possibility to vary the excitonic resonance over a wide range of energies by applying an external magnetic field, typically about 30 meV for 5 T in our sample. In particular, the anticrossing between the QW exciton and the cavity mode can be tuned by the external field. We observe the anticrossing and formation of exciton polaritons in magneto-reflectivity experiments. In contrast, magneto-luminescence exhibits purely excitonic character. Under resonant excitation conditions an additional emission line is observed at the energy of the dark exciton. The creation of dark excitons is made possible due to heavy hole-light hole mixing in the QW. The emission at this energy could be due to a combined spin flip of an electron and a bright exciton recombination.
Photophysics of pentacene thin films: The role of exciton fission and heating effects
NASA Astrophysics Data System (ADS)
Rao, Akshay; Wilson, Mark W. B.; Albert-Seifried, Sebastian; di Pietro, Riccardo; Friend, Richard H.
2011-11-01
There is evidence that the photo-generated singlet exciton in polycrystalline pentacene films undergoes rapid and efficient fission to form two triplet excitons. However, the role of exciton fission in pentacene has been controversial, with previous studies putting forward alternate relaxation pathways for the singlet exciton, such as excimer or charge formation, or internal conversion to a doubly excited exciton. We report temperature- and angular-dependent ultrafast transient optical absorption measurements in a wide spectral and temporal window. Angular-dependent transient spectra identify a common origin to photo-induced absorptions at 530 and 860 nm, which we associate with triplet excitons. These constitute the dominant relaxation channel for singlet excitons. Other features, particularly near 620 nm, previously assigned to excimers or electronic charges, are shown to be caused by thermal modulation from the optical pump.
Interference of resonance luminescence of exciton polaritons in CuGaS 2 crystals
NASA Astrophysics Data System (ADS)
Syrbu, N. N.; Nemerenco, L. L.; Stamov, I. G.; Bejan, V. N.; Tezlevan, V. E.
2007-04-01
The nonmodulated and wavelength-modulated reflection spectra of CuGaS 2 crystals for the polarization EIIc of 10 K are studied. The states n = 1, 2 and 3 of the excitons ? 4 (A-excitons) and n = 1, n = 2 of B- and C-excitons are found. The nonmodulated absorption spectra for the polarization E? c at 10 K have been studied. The states n = 1, 2 and 3 of ? 5 excitons are found. The main parameters of the A (? 4, ? 5) and B, C exciton series at the energies of the longitudinal and transverse excitons ? 4 for the states n = 1 and n = 2, the effective masses of electrons (me1?) and holes (mv1?,mv2?,mv3?) are determined. The photoluminescence peaks were observed at n = 3 and n = 4 of the excitons ? 5 in the luminescence spectra excited by the line 4880 Å of Ar + laser. In the luminescence spectra the interference is found.
Direct evidence for self-trapping of excitons by indium nanowires at In/Si(111) surface
Xu, Maojie, E-mail: mjxu@sjtu.edu.cn [Institute of Applied Physics, CREST-JST, University of Tsukuba, 305-8573 Tsukuba (Japan) [Institute of Applied Physics, CREST-JST, University of Tsukuba, 305-8573 Tsukuba (Japan); Key Laboratory for Thin Film and Microfabrication of the Ministry of Education, Research Institute of Micro/Nano Science and Technology, Shanghai Jiao Tong University, Shanghai 200240 (China); Zhang, Yafei [Key Laboratory for Thin Film and Microfabrication of the Ministry of Education, Research Institute of Micro/Nano Science and Technology, Shanghai Jiao Tong University, Shanghai 200240 (China)] [Key Laboratory for Thin Film and Microfabrication of the Ministry of Education, Research Institute of Micro/Nano Science and Technology, Shanghai Jiao Tong University, Shanghai 200240 (China)
2013-11-04
We report on the real-space observation of self-trapped excitons using scanning tunneling microscope. Electrons of In nanowires transfer to the Si substrate, yielding charge-transfer excitons at In/Si interface. The strong coupling between excitons and lattice vibrations leads to the exciton localization at low carrier density and 80.0?K. Exciton condensation was observed at the proper carrier density and its microscopic origin is discussed.
Modeling ultrafast exciton deactivation in oligothiophenes via nonadiabatic dynamics.
Fazzi, Daniele; Barbatti, Mario; Thiel, Walter
2015-03-11
Ultrafast excited-state processes play a key role in organic electronics and photovoltaics, governing the way of how excitons can relax and separate. Through the use of nonadiabatic excited-state dynamics, relaxation processes were investigated at the sub-picosecond timescale in thiophene and oligothiophenes (nT, n = 2, 3, 4), prototype oligomers for efficient ?-electron conjugated polymers adopted in photovoltaics. For thiophene, TDDFT and TDA nonadiabatic excited-state dynamics revealed ultrafast nonradiative relaxation processes through ring opening and ring puckering, bringing the system to an S1/S0 conical intersection seam. The computed relaxation time is 110 fs, matching well the experimental one (?105 fs). In oligothiophenes (n = 2-4), high-energy (hot) excitations were considered. Exciton relaxation through the manifold of excited states to the lowest excited state is predicted to occur within ?150-200 fs, involving bond stretching, ring puckering, and torsional oscillations. For the longer oligomer (4T), the ultrafast relaxation process leads to exciton localization over three thiophene rings in 150 fs. These data agree with the self-localization mechanism (?100-200 fs) observed for poly(3-hexylthiophene) (P3HT) and shed light on the complex exciton relaxation dynamics occurring in ?-conjugated oligomers of potential interest for optoelectronic applications. PMID:25714233
Advanced theory of multiple exciton generation effect in quantum dots
NASA Astrophysics Data System (ADS)
Oksengendler, B. L.; Turaeva, N. N.; Rashidova, S. S.
2012-06-01
The theoretical aspects of the effect of multiple exciton generation (MEG) in quantum dots (QDs) have been analysed in this work. The statistical theory of MEG in QDs based on Fermi's approach is presented, taking into account the momentum conservation law. According to Fermi this approach should give the ultimate quantum efficiencies of multiple particle generation. The microscopic mechanism of this effect is based on the theory of electronic "shaking". According to this approach, the wave function of "shaking" electrons can be selected as Plato's functions with effective charges depending on the number of generated excitons. From the theory it is known increasing the number of excitons leads to enhancement of the Auger recombination of electrons which results in reduced quantum yields of excitons. The deviation of the averaged multiplicity of the MEG effect from the Poisson law of fluctuations has been investigated on the basis of synergetics approaches. In addition the role of interface electronic states of QDs and ligands has been considered by means of quantum mechanical approaches. The size optimisation of QDs has been performed to maximise the multiplicity of the MEG effect.
Effect of quantum confinement on exciton-phonon interactions
Zhao, Hui; Wachter, Sven; Kalt, Heinz
2002-08-01
We investigate the homogeneous linewidth of localized type-I excitons in type-II GaAs/AlAs superlattices. These localizing centers represent the intermediate case between quasi-two-dimensional (Q2D) and quasi-zero-dimensional localizations...
Screening of Excitons in Single, Suspended Carbon Nanotubes
Screening of Excitons in Single, Suspended Carbon Nanotubes Andrew G. Walsh, A. Nickolas Vamivakas spectroscopy of single carbon nanotubes suspended across trenches displays red-shifts of up to 30 me nanotubes (CNT)1-5 has been experimen- tally confirmed by recent two-photon experiments.6-8 With binding
Exciton-Plasmon Interaction Effects in Individual Carbon Nanotubes
NASA Astrophysics Data System (ADS)
Bondarev, Igor; Meliksetyan, Areg
2013-03-01
We have recently developed a theory for the electrostatically controlled coupling between excitons and low-energy inter-band plasmons in individual semiconducting carbon nanotubes [1]. Here, we report on our studies towards the applications of this effect of both applied and fundamental interest. One practical application is the electromagnetic absorption/photoluminescence control for individual nanotubes [2]. Another, fundamental one, comes from the fact that the coupling of the excitons to the same inter-band plasmon resonance results in their entanglement, a pre-requisite for strong quantum correlations/quantum phase transitions in many-particle systems [3]. Our coupled exciton-plasmon excitation is a quasi-1D Bose system and could possibly be Bose-condensed in an individual carbon nanotube under appropriately created external conditions --- despite the mathematical statements [4] of the BEC impossibility in ideal 1D and 2D quantum systems and previously reported evidence [5] for no free-exciton BEC in carbon nanotubes.[4pt] [1] I.V.Bondarev, et al, PRB80, 085407 (2009).[0pt] [2] I.V.Bondarev, PRB85, 035448 (2012).[0pt] [3] J.Anders, PRA77, 062102 (2008).[0pt] [4] R.K.Pathria, P.D.Beale, Statistical Mechanics (Elsevier, 2011).[0pt] [5] Y.Murakami, J.Kono, PRL102, 037401 (2009).
Is there an excitonic interaction or antenna system in bacteriorhodopsin?
El-Sayed, M. A.; Lin, C. T.; Mason, W. R.
1989-01-01
The presence of a biphasic circular dichroism (CD) observed in the visible absorption spectrum of retinal in bacteriorhodopsin (bR) has been believed for many years to be due to excitonic-type interaction within the trimeric structure of the retinal in the protein membrane. In the present work, we present data and discuss previous observations that strongly suggest the absence of such an excitonic interaction. The magnetic CD spectrum of the trimer is found to be similar to that of the monomer and shows no sign of absorption to the doubly degenerate state predicted to be present from the exciton theory. This, together with the previous observations on the CD spectra of the photocycle intermediates of bR as well as the linear polarization studies of the fluorescence and the daughter absorption, sheds doubt on the presence of exciton interaction and thus suggests the presence of neither an antenna system nor a viable special reaction center in bR. Possible explanation for the observed biphasic nature of the CD spectrum is given in terms of having more than one type of bR with different protein conformations around the retinals in the trimer giving each similar absorption maximum but opposite signs for its rotary dispersion power. PMID:16594053
Singlet fission in pentacene through multiple exciton quantum states
Zhiyong Zhang; Paul Zimmerman; Charles Musgrave
2010-01-01
Multi-exciton generation (MEG) has been reported for several materials and may dramatically increase solar cell efficiency. Singlet fission is the molecular analogue of MEG and has been observed in various systems, including tetracene and pentacene, however, no fundamental mechanism for singlet fission has yet been described, although it may govern MEG processes in a variety of materials. Because photoexcited states
Singlet Fission and Multi-Exciton Generation in Organic Systems
NASA Astrophysics Data System (ADS)
Musgrave, Charles
2012-02-01
Multi-exciton generation (MEG) has been observed in a variety of materials and might be exploited in solar-cells to dramatically increase efficiency. In tetracene and pentacene MEG has been attributed to singlet fission (SF), however a fundamental mechanism for SF has not been previously described. Here, we use sophisticated ab initio calculations to show that MEG in pentacene proceeds by transition of the lowest optically allowed excited state S1 to a dark state (D) of multi-exciton character, which subsequently undergoes SF to generate two triplets (2xT0). D satisfies the energy requirement for SF (ED>2ET0) and lies just below S1 in pentacene, but above S1 in tetracene, consistent with the observed thermally activated SF process in tetracene, but no thermal activation in pentacene. While S1 exhibits single exciton character, D shows multi-exciton character comprising two separated electron-hole pairs. Dimer simulations predict S1 excimer formation and that fission of D into triplets proceeds through the excimer. The predicted energetics, wavefunctions and excimer interaction support the proposed mechanism, which accounts for the observed rapid, unactivated SF in pentacene. Results for SF in polyacenes, grapheme nanoribbons, rubrene and carbon nanotubes will be presented.
Singlet fission in pentacene through multiple exciton quantum states
NASA Astrophysics Data System (ADS)
Zhang, Zhiyong; Zimmerman, Paul; Musgrave, Charles
2010-03-01
Multi-exciton generation (MEG) has been reported for several materials and may dramatically increase solar cell efficiency. Singlet fission is the molecular analogue of MEG and has been observed in various systems, including tetracene and pentacene, however, no fundamental mechanism for singlet fission has yet been described, although it may govern MEG processes in a variety of materials. Because photoexcited states have single-exciton character, singlet fission to produce a pair of triplet excitons must involve an intermediate state that: (1) exhibits multi-exciton (ME) character, (2) is accessible from S1 and satisfies the fission energy requirement, and (3) efficiently dissociates into multiple electron-hole pairs. Here, we use sophisticated ab initio calculations to show that singlet fission in pentacene proceeds through a dark state (D) of ME character that lies just below S1, satisfies the fission energy requirement (ED>2ET0), and splits into two triplets (2xT0). In tetracene, D lies just above S1, consistent with the observation that singlet fission is thermally activated in tetracene. Rational design of photovoltaic systems that exploit singlet fission will require ab initio analysis of ME states such as D.
Excitonic polariton structures in Wurtzite GaN
Kousuke Torii; Shigefusa F Chichibu; Takahiro Deguchi; Hisayuki Nakanishi; Takayuki Sota; Shuji Nakamura
2001-01-01
Reflectance, photoreflectance and photoluminescence spectra of GaN substrate prepared by lateral epitaxial overgrowth technique were measured at low temperature. All spectra were discussed based on model exciton-polariton picture, and the spectra were successfully explained by the model. Temperature dependence of transition energies was well described using a model that assume Einstein phonons.
Spatio-temporal dynamics of quantum-well excitons
Zhao, Hui; Dal Don, B.; Moehl, Sebastian; Kalt, Heinz; Ohkawa, K.; Hommel, D.
2003-01-01
We investigate the lateral transport of excitons in ZnSe quantum wells by using time-resolved micro-photoluminescence enhanced by the introduction of a solid immersion lens. The spatial and temporal resolutions are 200 nm and 5 ps, respectively...
Organic photosensitive optoelectronic device having a phenanthroline exciton blocking layer
Thompson, Mark E. (Anaheim Hills, CA); Li, Jian (Los Angeles, CA); Forrest, Stephen (Princeton, NJ); Rand, Barry (Princeton, NJ)
2011-02-22
An organic photosensitive optoelectronic device, having an anode, a cathode, and an organic blocking layer between the anode and the cathode is described, wherein the blocking layer comprises a phenanthroline derivative, and at least partially blocks at least one of excitons, electrons, and holes.
High quantum efficiency photoluminescence from localized excitons in Si, -,Ge,
High quantum efficiency photoluminescence from localized excitons in Si, -,Ge, L. C. Lenchyshyn) We report a new photoluminescence process in epitaxial Sil-,GeX layers grown on Si by rapid thermal measured an external photoluminescence quantum efficiency of 11.5 *2%. Recent progress in the epitaxial
Johann Marinsek
Energy balance in U-235 fission would be in disorder if binding energy were mass defect times c 2. In U-235 fission the alleged electronic shell structure must collapse, a self-resurrect ion of new electronic shell structures for fission products is impossible. Missing electrons in ?-decay; missing electron in plutonium shell after transmutation of uranium. The Rutherford-Bohr atomic model is based
Transient Optical Studies of Exciton Dynamics in Organic Solar Cells
NASA Astrophysics Data System (ADS)
Reynolds, Luke X.
There is increasing evidence that the initially generated excited state species in bulk heterojunction solar cell photoactive layers are critical to device performance. At present however, an understanding of the nature and dynamics of such excited states still remains limited. This thesis presents a study of the ultrafast exciton dynamics in bulk heterojunction organic and hybrid organic-inorganic solar cells. Fluorescence upconversion is used to elucidate the dynamics of such transient species allowing internal properties of the blend systems to be probed including changes in film morphology and ultrafast energy loss mechanisms. An understanding of such processes is an important step forward in the evolution of molecular semiconductor based solar cells. The first chapter focuses on the main experimental technique, fluorescence upconversion, and how this can be employed to study excited states. In particular, this section addresses one of the main unanswered questions in the field and attempts to correlate the exciton dynamics with the structure of the common photoactive polymer poly(3-hexylthiophene) (P3HT). Three structural variations of P3HT are studied and their exciton dynamics associated with differing internal processes occurring within the polymers. These include self localisation, and different types of long-range energy transfer mechanisms. The following two chapters build upon the knowledge of exciton dynamics obtained from the first chapter. First, a study is made of amorphous polymers with different acceptors, all based on phenyl-C61-butyric acid methyl ester (PCBM). The distinct interactions of the PCBM-type molecules with the polymer results in different electron transfer dynamics, from which the exciton diffusion length of the polymer in real bulk heterojunction blends is extracted using a simple model. Second, the ultrafast excited state dynamics of a crystalline polymer with the same PCBM-type acceptors is studied. Correlation of these dynamics with thermal analysis of the blend films allows the morphology of the films to be extracted and allows two different mechanisms of microstructure development to be identified. In the final chapter, the effect of acceptor aggregation on exciton dynamics and charge generation yields in hybrid organic-inorganic blend films has been studied. Such aggregation has been shown to be essential for efficient charge generation in all-organic solar cells but has often been assumed to be less important in such inorganic hybrids. More aggregated acceptor nanoparticles are shown to not only result in greater than expected exciton quenching but are also shown to result in a greater yield of long-lived charges. This study is extended to show that in-situ grown inorganic nanoparticles exhibit superior performance to standard pre-synthesised inorganics.
Squat exercise biomechanics during short-radius centrifugation
Duda, Kevin R., 1979-
2007-01-01
Artificial gravity (AG) created by short-radius centrifugation is a promising countermeasure to the physiological de-conditioning that results from long-duration spaceflight. However, as on Earth, gravity alone does not ...
Radius and Chirality Dependent Conformation of Polymer Molecule at
Wei, Chenyu
Radius and Chirality Dependent Conformation of Polymer Molecule at Nanotube Interface Chenyu Wei Manuscript Received June 8, 2006 ABSTRACT Temperature-dependent conformations of linear polymer molecules adsorbed at carbon nanotube (CNT) interfaces are investigated through molecule dynamics simulations. Model
Radius of B-8 halo from the asymptotic normalization coefficient
Carstoiu, F.; Trache, L.; Gagliardi, Carl A.; Tribble, Robert E.; Mukhamedzhanov, AM.
2001-01-01
The experimental asymptotic normalization coefficient determined from peripheral transfer reactions is used to obtain the root-mean-square radius of the wave function for the loosely bound proton in SB. It is shown that the asymptotic region...
Radius of Curvature of Off-Axis Paraboloids
NASA Technical Reports Server (NTRS)
Robinson, Brian; Reardon, Patrick; Hadaway, James; Geary, Joseph; Russell, Kevin (Technical Monitor)
2002-01-01
We present several methods for measuring the vertex radius of curvature of off-axis paraboloidal mirrors. One is based on least-squares fitting of interferometer output, one on comparison of sagittal and tangential radii of curvature, and another on measurement of displacement of the nulled test article from the ideal reference wave. Each method defines radius of curvature differently and, as a consequence, produces its own sort of errors.
Ipsilateral olecranon and distal radius fracture: A case report
Cengiz, Ömer; Polat, Gökhan; Karademir, Gökhan; Kara, Deniz; Erdil, Mehmet
2014-01-01
Introduction Concomitant ipsilateral olecranon and distal radius fracture are rare injuries. Their clinical presentation is unusual and investigation and management is poorly described. Presentation of case We present a 55-year-old woman patient who fell off sustaining a concomitant distal radius and olecranon fracture in the same extremity. On examination, there was gross swelling of the proximal and distal forearm and no neurovascular deficit. Radiographs confirmed distal radius and olecranon fracture. Patient was treated with open reduction and anatomic locking plate for olecranon and a closed reduction percuteneous K wire fixation with penning fixator for distal radius fracture. After physical therapy program, functional results were good and DASH score was 60. Discussion Several different combinations of fracture with dislocation have been described, but, to our knowledge, concurrent ipsilateral olecranon and distal radius fracture has not been reported before. In the literature review there are two similar cases in the English literature. Conclusion Ipsilateral olecranon and distal radius fracture is a very rare injury due to different trauma mechanisms. However we should keep in mind that there may be adjacent joints and structures for concomitant injuries. PMID:25544490
Fonoberov, Vladimir
Origin of ultraviolet photoluminescence in ZnO quantum dots: Confined excitons versus surface of ultraviolet photoluminescence (PL) in ZnO quantum dots with diameters from 2 to 6 nm. Two possible sources- minescence and enhance the UV emission from ZnO QDs. However, the nature of the UV photoluminescence from Zn
Cross-circularly polarized two-exciton states in one to three dimensions
NASA Astrophysics Data System (ADS)
Ajiki, Hiroshi
2015-03-01
Biexciton and two-exciton dissociated states of Frenkel-type excitons are studied theoretically using an exciton tight-binding (TB) model including a polarization degree of freedom. Because the biexciton consists of two cross-circularly polarized excitons, an on-site interaction (V) between the two excitons should be considered in addition to a nearest-neighbor two-exciton attractive interaction (?). Although there are an infinitely large number of combinations of V and ? providing the observed binding energy of a biexciton, the wave function of the biexciton and two-exciton dissociated states is nearly independent of these parameter sets. This means that all the two-exciton states are uniquely determined from the exciton TB model. There are a spatially symmetric and an antisymmetric biexciton state for a one-dimensional (1D) lattice and two symmetric and one antisymmetric biexciton states at most for two- (2D) and three-dimensional (3D) lattices. In contrast, when the polarization degree of freedom is ignored, there is one biexciton state for 1D, 2D, and 3D lattices. For this study, a rapid and memory-saving calculation method for two-exciton states is extended to include the polarization degree of freedom.
Cross-circularly polarized two-exciton states in one to three dimensions.
Ajiki, Hiroshi
2015-03-14
Biexciton and two-exciton dissociated states of Frenkel-type excitons are studied theoretically using an exciton tight-binding (TB) model including a polarization degree of freedom. Because the biexciton consists of two cross-circularly polarized excitons, an on-site interaction (V) between the two excitons should be considered in addition to a nearest-neighbor two-exciton attractive interaction (?). Although there are an infinitely large number of combinations of V and ? providing the observed binding energy of a biexciton, the wave function of the biexciton and two-exciton dissociated states is nearly independent of these parameter sets. This means that all the two-exciton states are uniquely determined from the exciton TB model. There are a spatially symmetric and an antisymmetric biexciton state for a one-dimensional (1D) lattice and two symmetric and one antisymmetric biexciton states at most for two- (2D) and three-dimensional (3D) lattices. In contrast, when the polarization degree of freedom is ignored, there is one biexciton state for 1D, 2D, and 3D lattices. For this study, a rapid and memory-saving calculation method for two-exciton states is extended to include the polarization degree of freedom. PMID:25770529
Electronic structure and absorption spectrum of biexciton obtained by using exciton basis
Shiau, Shiue-Yuan, E-mail: shiau.sean@gmail.com [Research Center for Applied Sciences, Academia Sinica, Taipei, 115, Taiwan (China) [Research Center for Applied Sciences, Academia Sinica, Taipei, 115, Taiwan (China); Department of Physics, National Cheng Kung University, Tainan, 701, Taiwan (China); Combescot, Monique [Institut des NanoSciences de Paris, Université Pierre et Marie Curie, CNRS, 4 place Jussieu, 75005 Paris (France)] [Institut des NanoSciences de Paris, Université Pierre et Marie Curie, CNRS, 4 place Jussieu, 75005 Paris (France); Chang, Yia-Chung, E-mail: yiachang@gate.sinica.edu.tw [Research Center for Applied Sciences, Academia Sinica, Taipei, 115, Taiwan (China)] [Research Center for Applied Sciences, Academia Sinica, Taipei, 115, Taiwan (China)
2013-09-15
We approach the biexciton Schrödinger equation not through the free-carrier basis as usually done, but through the free-exciton basis, exciton–exciton interactions being treated according to the recently developed composite boson many-body formalism which allows an exact handling of carrier exchange between excitons, as induced by the Pauli exclusion principle. We numerically solve the resulting biexciton Schrödinger equation with the exciton levels restricted to the ground state and we derive the biexciton ground state as well as the bound and unbound excited states as a function of hole-to-electron mass ratio. The biexciton ground-state energy we find, agrees reasonably well with variational results. Next, we use the obtained biexciton wave functions to calculate optical absorption in the presence of a dilute exciton gas in quantum well. We find an asymmetric peak with a characteristic low-energy tail, identified with the biexciton ground state, and a set of Lorentzian-like peaks associated with biexciton unbound states, i.e., exciton–exciton scattering states. Last, we propose a pump–probe experiment to probe the momentum distribution of the exciton condensate. -- Highlights: •New composite boson many-body theory is used to derive exactly the biexciton Schrödinger equation using the exciton basis. •We solved the 2D and 3D biexciton ground- and excited-state binding energies for various electron-to-hole mass ratios. •The absorption spectrum shows an asymmetric low-energy peak identified with the biexciton ground state. •High-energy Lorentzian-like peaks in the absorption spectrum are associated with the exciton–exciton scattering states. •The exciton gas momentum distribution can be determined by the absorption spectrum via the biexciton wave functions.
Ji, Haojie; Dhomkar, Siddharth; Roy, Bidisha; Kuskovsky, Igor L. [Department of Physics, Queens College of CUNY, Queens, New York 11367 (United States); The Graduate Center of CUNY, New York, New York 10016 (United States); Shuvayev, Vladimir [Department of Physics, Queens College of CUNY, Queens, New York 11367 (United States); Deligiannakis, Vasilios; Tamargo, Maria C. [The Graduate Center of CUNY, New York, New York 10016 (United States); Department of Chemistry, City College of CUNY, New York, New York 10031 (United States); Ludwig, Jonathan [National High Magnetic Field Laboratory, Tallahassee, Florida 32310 (United States); Department of Physics, Florida State University, Tallahassee, Florida 32306 (United States); Smirnov, Dmitry [National High Magnetic Field Laboratory, Tallahassee, Florida 32310 (United States); Wang, Alice [Evans Analytical Group, Sunnyvale, California 94086 (United States)
2014-10-28
For submonolayer quantum dot (QD) based photonic devices, size and density of QDs are critical parameters, the probing of which requires indirect methods. We report the determination of lateral size distribution of type-II ZnTe/ZnSe stacked submonolayer QDs, based on spectral analysis of the optical signature of Aharanov-Bohm (AB) excitons, complemented by photoluminescence studies, secondary-ion mass spectroscopy, and numerical calculations. Numerical calculations are employed to determine the AB transition magnetic field as a function of the type-II QD radius. The study of four samples grown with different tellurium fluxes shows that the lateral size of QDs increases by just 50%, even though tellurium concentration increases 25-fold. Detailed spectral analysis of the emission of the AB exciton shows that the QD radii take on only certain values due to vertical correlation and the stacked nature of the QDs.
NASA Astrophysics Data System (ADS)
Ji, Haojie; Dhomkar, Siddharth; Roy, Bidisha; Shuvayev, Vladimir; Deligiannakis, Vasilios; Tamargo, Maria C.; Ludwig, Jonathan; Smirnov, Dmitry; Wang, Alice; Kuskovsky, Igor L.
2014-10-01
For submonolayer quantum dot (QD) based photonic devices, size and density of QDs are critical parameters, the probing of which requires indirect methods. We report the determination of lateral size distribution of type-II ZnTe/ZnSe stacked submonolayer QDs, based on spectral analysis of the optical signature of Aharanov-Bohm (AB) excitons, complemented by photoluminescence studies, secondary-ion mass spectroscopy, and numerical calculations. Numerical calculations are employed to determine the AB transition magnetic field as a function of the type-II QD radius. The study of four samples grown with different tellurium fluxes shows that the lateral size of QDs increases by just 50%, even though tellurium concentration increases 25-fold. Detailed spectral analysis of the emission of the AB exciton shows that the QD radii take on only certain values due to vertical correlation and the stacked nature of the QDs.
Terahertz driven intraband dynamics of excitons in nanorods
NASA Astrophysics Data System (ADS)
Sy, Fredrik
Quantum dots and nanorods are becoming increasingly important structures due to their potential applications that range from photovoltaic devices to medicine. The majority of the research on carrier dynamics in these structures has been in the optical regime, with little work performed at Terahertz frequencies where excitonic dynamics can be more directly probed. In this work, we examine theoretically the interaction of Terahertz radiation with colloidal CdSe nanorods to determine the dynamics of excitons generated via a short optical pulse. We calculate the energies and wavefunctions for the excitons within the envelope function approximation in the low density limit where there is at most one exciton per nanorod. The linear Terahertz transmittance and absorbance is found for nanorods that are approximately 70 nm in length and 7 nm in diameter and are compared with experimental results that have shown the first observation of intra-excitonic transitions in nanorods. We find absorbance peaks at 8.5 THz and 11 THz that result from polarizations in the longitudinal (rod axis) and transverse directions respectively. Our theoretical results show that the 8.5 THz and 11 Thz peaks are due to 1s -- 2pz and 1s -- 2 px transitions respectively. The theoretical absorbance spectra is in good agreement with the experimental one and show that only the ground state is significantly populated 1 ps after optical excitation. This provides strong evidence of rapid trapping of excited holes into the ligand used to passivate the nanorods. A full set of dynamical equations were then constructed from Heisenberg's equation of motion, and were used to model the excitonic correlations as a function of time. Transmittance and absorbance were calculated for different nanorod orientations and electric field strengths in both the linear and nonlinear regime. These results were then averaged over nanorod orientation in order to more accurately reflect experimental conditions. Nonlinearity was found to become significant at peak pulse field strengths of 7 kV/cm and greater. Due to two-photon processes, we predict the 2pz -- 3dz transition that is not observed in the linear regime will be clearly seen in the nonlinear absorbance spectrum.
R. Sánchez-Cano; N. Porras-Montenegro
2010-01-01
We have study the heavy-hole exciton states in GaSb–GaInAsSb–GaSb type-I spherical Quantum Dots, using temperature-dependent static dielectric constant and electron affinity, with a finite height potential barrier, as a function of the quantum dot radius for several values of Indium concentration. Our calculations have been worked out using interpolating methods to find the temperature and Indium concentration dependence of both
Observation of non-Hermitian degeneracies in a chaotic exciton-polariton billiard
Gao, T; Bliokh, K Y; Liew, T C H; Fraser, M D; Brodbeck, S; Kamp, M; Schneider, C; Höfling, S; Yamamoto, Y; Nori, F; Kivshar, Y S; Truscott, A; Dall, R; Ostrovskaya, E A
2015-01-01
Exciton-polaritons are hybrid light-matter quasiparticles formed by strongly interacting photons and excitons (electron-hole pairs) in semiconductor microcavities. They have emerged as a robust solid-state platform for next-generation optoelectronic applications as well as fundamental studies of quantum many-body physics. Importantly, exciton-polaritons are a profoundly open (i.e., non-Hermitian) quantum system: it requires constant pumping of energy and continuously decays releasing coherent radiation. Thus, the exciton-polaritons always exist in a balanced potential landscape of gain and loss. However, the inherent non-Hermitian nature of this potential has so far been largely ignored in exciton-polariton physics. Here we demonstrate that non-Hermiticity dramatically modifies the structure of modes and spectral degeneracies in exciton-polariton systems, and, therefore, will affect their quantum transport, localisation, and dynamical properties. Using a spatially-structured optical pump, we create a chaotic ...
Dark bogolon-excitons in a linear atomic super-lattice
NASA Astrophysics Data System (ADS)
Zoubi, Hashem
2015-02-01
Dark and bright excitons are shown to appear naturally in a linear atomic super-lattice with two atoms per unit cell. In bringing the super-lattice into a strong coupling regime with a one-dimensional nanophotonic waveguide, bright excitons and photons are coherently mixed to form polaritons. Treating excitons as bosons implies a mechanism that forbids two excitations from being in the same atomic state, which is included here through a bosonization procedure with kinematic interactions. Interestingly these interactions couple dark and bright excitons, which we exploit as a new tool for exciting dark states in a controllable way. We suggest a pump-probe experiment where two polaritons scatter into two dark excitons that are found to be correlated and are represented as dark bogolon excitons. The results can be adapted for any super-lattice of active materials; for example, of organic molecules.
Bleaching in the region of exciton resonance of layered GaSe crystals
NASA Astrophysics Data System (ADS)
Kyazym-zade, A. G.; Salmanov, V. M.; Guseinov, A. G.; Salmanova, A. A.; Mamedov, R. M.; Dzhavadzade, A. A.
2014-09-01
Light absorption in the region of exciton resonance of GaSe crystal is studied experimentally at high levels of optical excitation. A picosecond YAG:Nd3+ laser emitting 30-ps light pulses and a dye laser with a pulse width of ˜3 ns tunable within the range 594-643 nm were used as light sources. It was found that, at high levels of optical excitation, the exciton absorption line of the GaSe crystal disappeared, which was attributed to increasing exciton density with arising mechanisms of their decay: exciton-exciton interactions and screening of excitons by the free charge-carrier plasma. It is shown that these mechanisms are also responsible for the arising new emission band in the long-wavelength region of the photoluminescence spectrum.
Optically Induced Rotation of an Exciton Spin in a Semiconductor Quantum Dot
NASA Astrophysics Data System (ADS)
Poem, E.; Kenneth, O.; Kodriano, Y.; Benny, Y.; Khatsevich, S.; Avron, J. E.; Gershoni, D.
2011-08-01
We demonstrate control over the spin state of a semiconductor quantum dot exciton using a polarized picosecond laser pulse slightly detuned from a biexciton resonance. The control pulse follows an earlier pulse, which generates an exciton and initializes its spin state as a coherent superposition of its two nondegenerate eigenstates. The control pulse preferentially couples one component of the exciton state to the biexciton state, thereby rotating the exciton’s spin direction. We detect the rotation by measuring the polarization of the exciton spectral line as a function of the time difference between the two pulses. We show experimentally and theoretically how the angle of rotation depends on the detuning of the second pulse from the biexciton resonance.
Observation of long-range exciton diffusion in highly ordered organic semiconductors
NASA Astrophysics Data System (ADS)
Najafov, H.; Lee, B.; Zhou, Q.; Feldman, L. C.; Podzorov, V.
2010-11-01
Excitons in polycrystalline and disordered films of organic semiconductors have been shown to diffuse over distances of 10-50nm. Here, using polarization- and wavelength-dependent photoconductivity in the highly ordered organic semiconductor rubrene, we show that the diffusion of triplet excitons in this material occurs over macroscopic distances (2-8?m), comparable to the light absorption length. Dissociation of these excitons at the surface of the crystal is found to be the main source of photoconductivity in rubrene. In addition, we observe strong photoluminescence quenching and a simultaneous enhancement of photoconductivity when the crystal surface is functionalized with exciton splitters. In combination with time-resolved measurements, these observations strongly suggest that long-lived triplet excitons are indeed generated in molecular crystals by fission of singlets, and these triplets provide a significant contribution to the surface photocurrent generated in organic materials. Our findings indicate that the exciton diffusion bottleneck is not an intrinsic limitation of organic semiconductors.
NASA Astrophysics Data System (ADS)
Reusswig, P. D.; Congreve, D. N.; Thompson, N. J.; Baldo, M. A.
2012-09-01
We demonstrate bilayer organic photovoltaic cells that incorporate a singlet exciton fission sensitizer layer to increase the external quantum efficiency (EQE). This solar cell architecture is realized by pairing the singlet exciton donor layer tris[4-(5-phenylthiophen-2-yl)phenyl]amine (TPTPA) with the singlet exciton fission layer 5,6,11,12-tetraphenylnaphthacene (rubrene). The presence of the rubrene layer at the donor-acceptor interface allows for a singlet generated in TPTPA to undergo singlet exciton fission with a corresponding doubling in the TPTPA EQE from 12.8% to 27.6%. This scheme de-couples singlet exciton fission from photon absorption, exciton diffusion, and charge transport for very high EQE organic photovoltaic cells.
Dielectric screening of excitons and trions in single-layer MoS2.
Lin, Yuxuan; Ling, Xi; Yu, Lili; Huang, Shengxi; Hsu, Allen L; Lee, Yi-Hsien; Kong, Jing; Dresselhaus, Mildred S; Palacios, Tomás
2014-10-01
Photoluminescence (PL) properties of single-layer MoS2 are indicated to have strong correlations with the surrounding dielectric environment. Blue shifts of up to 40 meV of exciton or trion PL peaks were observed as a function of the dielectric constant of the environment. These results can be explained by the dielectric screening effect of the Coulomb potential; based on this, a scaling relationship was developed with the extracted electronic band gap and exciton and trion binding energies in good agreement with theoretical estimations. It was also observed that the trion/exciton intensity ratio can be tuned by at least 1 order of magnitude with different dielectric environments. Our findings are helpful to better understand the tightly bound exciton properties in strongly quantum-confined systems and provide a simple approach to the selective and separate generation of excitons or trions with potential applications in excitonic interconnects and valleytronics. PMID:25216267
Interplay between ambient and exciton-induced degradation in organic light-emitting devices
NASA Astrophysics Data System (ADS)
Zhang, Yingjie; Wang, Qi; Aziz, Hany
2014-10-01
Organic light-emitting diode (OLED), albeit is currently used in consumer electronics, still faces challenge with its limited performance stability. The degradation mechanisms that limit the lifetime of OLEDs can generally be separated into two categories: ambient and intrinsic. Much research has been devoted to understanding thus limiting these degradation mechanisms. However, surprisingly, there has been little work on how ambient and intrinsic degradation affect each other. In this work, the interplay between the ambient and intrinsic (more specifically exciton-induced) degradation is studied by comparing the effects of four degradation schemes, namely, no ambient or exciton-induced, ambient only, exciton-induced only and ambient and exciton-induced degradation on device lifetime. The results show that there is no interplay between ambient and exciton-induced degradation. Furthermore, it is evident that no photo-oxidation is present during ambient and exciton-induced degradation.
Strong exciton-plasmon interaction in semiconductor-insulator-metal nanowires
NASA Astrophysics Data System (ADS)
Yan, Jie-Yun
2012-08-01
The interaction of one-dimensional excitons and plasmons is theoretically investigated in semiconductor-insulator-metal nanowires. With the exact potentials presented analytically, the excitonic equation of motion in electron-hole-pair representation in real space is established. The optical properties of the system are derived by numerically calculating the evolution of the excitonic wave function. Linear absorption spectra demonstrate strong exciton-plasmon coupling in the nanostructures. The redshifts of the exciton absorption are found to be a result of interaction between the self-image potential and the indirect Coulomb interaction, of which the former brings the blueshift and the latter gives the redshift. The shifts reach the scale of 10 meV, which can be easily observed in experiment. Moreover, how the exciton-plasmon interaction is controlled by the parameters of the structure is also illustrated.
Size effect on the electronic and optical band gap of CdSe QD
NASA Astrophysics Data System (ADS)
Sisodia, Namita
2014-04-01
Present paper deals with a critical and comprehensive analysis of the dependence of photo emission (PE) electronic band gap and optical absorption (OA) excitonic band gap on the size of CdSe QD, via connecting it with excitonic absorbance wavelength. Excitonic absorbance wavelength is determined through an empirical fit of established experimental evidences. Effective excitonic charge and Bohr radius is determined as a function of size. Increase in size of the CdSe QD results in greater Bohr radius and smaller effective excitonic charge. Excitonic binding energy as a degree of size of QD is also calculated which further relates with the difference in PE electronic and OA optical band gaps. It is also shown that with increase in size of CdSe QD, the excitonic binding energy decreases which consequently increases differences in two band gaps. Our results are very well comparable with the established results. Explanation for the origin of the unusual optical properties of CdSe QD has been also discussed.
Excitons bound to Te isoelectronic dyads in ZnSe
NASA Astrophysics Data System (ADS)
Marcet, S.; André, R.; Francoeur, S.
2010-12-01
We report on the excitonic photoluminescence from individual Te dyads in ZnSe. Based on the emission characteristics of these pairs of pseudodonor impurities, we identify dyads of C2v symmetry and determine their orientation with respect to the host lattice. The diamagnetic shift of the exciton indicates that the hole bound to the dyad is extremely localized. Measurements of large number of individual dyads reveal that (1) light-hole transitions located at higher energy are strongly suppressed, (2) the line shape of the observed phonon replica allows probing the phonon density of states, (3) dyads of various interatomic separations are observed, (4) the emission energy is extremely sensitive to the local environment of the dyad, and (5) dynamic variations in this local environment results in the intensity fluctuations and spectral diffusion.
Charged excitons and biexcitons bound to isoelectronic centers
NASA Astrophysics Data System (ADS)
Ethier-Majcher, Gabriel; Marcet, Stephane; Ouellet-Plamondon, Clauderic; St-Jean, Philippe; Andre, Regis; Klem, John F.; Francoeur, Sebastien
2012-02-01
We demonstrate that isoelectronic centers formed from two isoelectronic traps can bind, in addition to the well-studied excitons, various number of positive and negative charges. Two different systems are studied by microphotoluminescence: 1) tellurium dyads in ZnSe forming hole traps and 2) nitrogen dyads in GaAs forming electron traps. By analyzing their emission fine structure, polarization and diamagnetic shifts, we establish that Te and N dyads can bind, respectively, positively and negatively charged excitons. Using the power dependence of the emission intensity, we clearly demonstrate that both systems can also bind biexcitons. This ability to bind various charge configurations, in addition to their very low inhomogeneous broadenings and perfectly defined symmetries, further establishes isoelectronic centers as an interesting alternative to epitaxial quantum dots for a number of applications.
Exciton spectra in two-dimensional graphene derivatives
NASA Astrophysics Data System (ADS)
Huang, Shouting; Liang, Yufeng; Yang, Li
2013-08-01
The energy spectra and wave functions of bound excitons in important two-dimensional (2D) graphene derivatives, i.e., graphyne and graphane, are found to be strongly modified by quantum confinement, making them qualitatively different from the usual Rydberg series. However, their parity and optical selection rules are preserved. Thus a one-parameter modified hydrogenic model is applied to quantitatively explain the ab initio exciton spectra, and allows one to extrapolate the electron-hole binding energy from optical spectroscopies of 2D semiconductors without costly simulations. Meanwhile, our calculated optical absorption spectrum and enhanced spin singlet-triplet splitting project graphyne, an allotrope of graphene, as a candidate for intriguing energy and biomedical applications.
Microscopic theory of singlet exciton fission. I. General formulation
Berkelbach, Timothy C; Reichman, David R
2012-01-01
Singlet fission, a spin-allowed energy transfer process generating two triplet excitons from one singlet exciton, has the potential to dramatically increase the efficiency of organic solar cells. However, the dynamical mechanism of this phenomenon is not fully understood and a complete, microscopic theory of singlet fission is lacking. In this work, we assemble the components of a comprehensive microscopic theory of singlet fission that connects excited state quantum chemistry calculations with finite-temperature quantum relaxation theory. We elaborate on the distinction between localized diabatic and delocalized adiabatic bases for the interpretation of singlet fission experiments in both the time and frequency domains. We discuss various approximations to the exact density matrix dynamics and propose Redfield theory as an ideal compromise between speed and accuracy for the detailed investigation of singlet fission in dimers, clusters, and crystals. Investigations of small model systems based on parameters t...
Optical and excitonic properties of ZnO films
NASA Astrophysics Data System (ADS)
Mihailovic, M.; Henneghien, A.-L.; Faure, S.; Disseix, P.; Leymarie, J.; Vasson, A.; Buell, D. A.; Semond, F.; Morhain, C.; Zùñiga Pérez, J.
2009-01-01
Optical and excitonic properties of ZnO heterostructures are studied in order to observe the strong light-matter coupling in this material as it has been recently demonstrated in GaN. The optical index of ZnO is first studied as a function of wavelength through spectroscopic ellipsometric and reflectivity experiments on ZnO layers grown by molecular beam epitaxy and deposited on two different substrates: sapphire and silicon with an AlN buffer layer. The main features of the excitons: energy, oscillator strength and broadening are deduced. From the knowledge of these properties, a microcavity is then designed. The ZnO active layer is embedded between AlGaN/AlN and dielectric Bragg mirrors. The calculation of the reflectivity spectra as a function of the incident angle attests the strong coupling and a large Rabi splitting of 110 meV is expected in such a cavity.
NASA Astrophysics Data System (ADS)
Williams, R. T.; Grim, Joel Q.; Li, Qi; Ucer, K. B.; Bizarri, G. A.; Kerisit, S.; Gao, Fei; Bhattacharya, P.; Tupitsyn, E.; Rowe, E.; Buliga, V. M.; Burger, A.
2013-09-01
Models of nonproportional response in scintillators have highlighted the importance of parameters such as branching ratios, carrier thermalization times, diffusion, kinetic order of quenching, associated rate constants, and radius of the electron track. For example, the fraction ?eh of excitations that are free carriers versus excitons was shown by Payne and coworkers to have strong correlation with the shape of electron energy response curves from Compton-coincidence studies. Rate constants for nonlinear quenching are implicit in almost all models of nonproportionality, and some assumption about track radius must invariably be made if one is to relate linear energy deposition dE/dx to volume-based excitation density n (eh/cm3) in terms of which the rates are defined. Diffusion, affecting time-dependent track radius and thus density of excitations, has been implicated as an important factor in nonlinear light yield. Several groups have recently highlighted diffusion of hot electrons in addition to thermalized carriers and excitons in scintillators. However, experimental determination of many of these parameters in the insulating crystals used as scintillators has seemed difficult. Subpicosecond laser techniques including interband z scan light yield, fluence-dependent decay time, and transient optical absorption are now yielding experimental values for some of the missing rates and ratios needed for modeling scintillator response. First principles calculations and Monte Carlo simulations can fill in additional parameters still unavailable from experiment. As a result, quantitative modeling of scintillator electron energy response from independently determined material parameters is becoming possible on an increasingly firmer data base. This paper describes recent laser experiments, calculations, and numerical modeling of scintillator response.
Vibrational Exciton Splitting, Fermi Resonance, and Crystal Structure of Methyl Iodide
Raoul Kopelman
1966-01-01
The splitting of the v5 vibrational band in crystalline methyl iodide has played an important role in the interpretation of vibrational exciton splittings in molecular crystals. According to an earlier interpretation, the site-group splitting was said to be much larger than the factor-group (exciton) splitting, and the effect of isotopic substitution on the exciton splittings was said to be substantial.
Tunable exciton relaxation in vertically coupled semiconductor InAs quantum dots
Kushal C. Wijesundara; Juan E. Rolon; Sergio E. Ulloa; Allan S. Bracker; Daniel Gammon; Eric A. Stinaff
2011-01-01
Tunable exciton relaxation rates are observed in individual vertically coupled semiconductor quantum dots (CQDs). An applied electric field is used to tune the energy difference between the spatially direct (SD) and indirect (SI) excitons in InAs CQDs. The intensity and lifetime of the SI exciton is found to vary as a result of wave-function distribution, carrier tunneling, and phonon-mediated relaxation
Relaxation of hot excitons in CdZnSe\\/ZnSe quantum wells and quantum dots
R. Spiegel; G. Bacher; O. Breitwieser; A. Forchel; B. Jobst; D. Hommel; G. Landwehr
1998-01-01
The relaxation dynamics of hot excitons was studied in (Zn,Cd)Se\\/ZnSe quantum wells and quantum dots. A fast population of the radiative excitonic ground state occurs for an excitation excess energy corresponding to an integer number of optical phonon energies. This is indicated by a spectrally narrow photoluminescence peak observed immediately after the exciting laser pulse. Spatial diffusion of excitons, controlled
Coherent exciton-LO-phonon polarons in ZnSe quantum wells with strong confinement
S. Tripathy; H. P. Wagner; A. Ueta; D. Hommel
2007-01-01
We observe coherent exciton-LO-phonon polarons in a 3nm ZnSe single quantum well using four-wave mixing with 30fs light pulses. The formation of these superposition states is attributed to a strong LO-phonon coupling by the Fröhlich interaction in a system in which the exciton binding exceeds the LO-phonon energy and the 2s exciton is nearly resonant with the LO-phonon energy. The
Ultrafast Breathinglike Oscillation in the Exciton Density of ZnSe Quantum Wells
Hui Zhao; Benedicte dal Don; Gregor Schwartz; Heinz Kalt
2005-01-01
The spatial density profile of a low-density exciton ensemble in ZnSe quantum wells shows a breathinglike oscillation on a 30-ps time scale. This breathing results from the emission of the first acoustic phonon at the end of the quasiballistic transport phase of the excitons which reverses their direction of propagation. Since the scattering destroys the phase of the excitonic wave
Dephasing of Excitons in ZnSe Quantum Wells Using Ultrashort Excitation Pulses
S. Tripathy; P. Bajracharya; A. Kabir; H. P. Wagner
2007-01-01
We report on the decoherence of excitons in ZnSe single quantum wells by intensity dependent four-wave-mixing (FWM) experiments using 30 fs pulses. The FWM traces at the 11h exciton transition reveal marked quantum beats and a fast decay at pulse overlap that is attributed to a destructive interference of the polarization wavepacket and to an effective diffraction of the exciton
Magnetic field effects on triplet exciton fission and fusion in a polydiacetylene
R. H. Austin; G. L. Baker; S. Etemad; R. Thompson
1989-01-01
We have studied the origin and decay dynamics of triplet excitons in the conjugated polymer poly(4BCMU) in its sol(yellow) and gel(red) phases. Wavelength and intensity dependencies of the triplet yield show that the triplet exciton cannot be produced by excitation into the singlet exciton edge but only from higher lying states. The observed lifetime of the triplet state, coupled with
Geminate and Nongeminate Recombination of Triplet Excitons Formed by Singlet Fission
NASA Astrophysics Data System (ADS)
Bayliss, Sam L.; Chepelianskii, Alexei D.; Sepe, Alessandro; Walker, Brian J.; Ehrler, Bruno; Bruzek, Matthew J.; Anthony, John E.; Greenham, Neil C.
2014-06-01
We report the simultaneous observation of geminate and nongeminate triplet-triplet annihilation in a solution-processable small molecule TIPS-tetracene undergoing singlet exciton fission. Using optically detected magnetic resonance, we identify recombination of triplet pairs directly following singlet fission, as well as recombination of triplet excitons undergoing bimolecular triplet-triplet annihilation. We show that the two processes give rise to distinct magnetic resonance spectra, and estimate the interaction between geminate triplet excitons to be 60 neV.
Jonathan J. Burdett; Astrid M. Müller; David Gosztola; Christopher J. Bardeen
2010-01-01
The excited state dynamics in polycrystalline thin films of tetracene are studied using both picosecond fluorescence and femtosecond transient absorption. The solid-state results are compared with those obtained for monomeric tetracene in dilute solution. The room temperature solid-state fluorescence decays are consistent with earlier models that take into account exciton-exciton annihilation and exciton fission but with a reduced delayed fluorescence
S. F. Chichibu; K. Torii; T. Deguchi; T. Sota; A. Setoguchi; H. Nakanishi; T. Azuhata; S. Nakamura
2000-01-01
Photoreflectance (PR) spectra of high-purity, nearly free-standing GaN substrate were compared with emission and reflectance spectra, which were analyzed based on a model exciton-polariton picture in which A, B, and C free excitons couple simultaneously to an electromagnetic wave. The GaN substrate with reduced dislocation density was prepared by lateral epitaxial overgrowth technique and it exhibited predominant excitonic emissions with
Stability of a Wheel with Various Radius Rim
NASA Astrophysics Data System (ADS)
Kinugasa, Tetsuya; Yoshida, Koji
This paper describes the dynamics and impact model of a wheel with various radius rim. The dynamics is expressed by a rst order linear ordinary dierential equation with respect to the absolute orientation of the wheel, and an analytic solution is derived. Poincaré map is also derived analytically. Stability and basin of attraction (BoA) of the Poincaré map are discussed. Finally, the analysis is validated through some numerical simulations. As a result, the rim radius aects the stability and broadens its BoA. The analysis helps understanding of not only a geometric tracking control but also many underactuated control methods for bipeds.
The PRad experiment and the proton radius puzzle
Gasparian, Ashot H. [North Carolina Ag. and Tech. St. Univ.
2014-06-01
New results from the recent muonic hydrogen experiments seriously questioned our knowledge of the proton charge radius, r_p. The new value, with its unprecedented less than sub-percent precision, is currently up to eight standard deviation smaller than the average value from all previous experiments, triggering the well-known "proton charge radius puzzle" in nuclear and atomic physics. The PRad collaboration is currently preparing a novel, magnetic-spectrometer-free ep scattering experiment in Hall B at JLab for a new independent r_p measurement to address this growing "puzzle" in physics.
Radius constants for analytic functions with fixed second coefficient.
Nargesi, Mahnaz M; Ali, Rosihan M; Ravichandran, V
2014-01-01
Let f(z) = z + ?(n=2)(?) (a)n(z) (n) be analytic in the unit disk with the second coefficient a2 satisfying |a2| = 2b, 0 ? b ? 1. Sharp radius of Janowski starlikeness is obtained for functions f whose nth coefficient satisfies |a(n)| ? cn + d (c, d ? 0) or |a(n)| ? c/n ?(c > 0 and n ? 3). Other radius constants are also obtained for these functions, and connections with earlier results are made. PMID:25101327
Scanning exciton microscopy and single-molecule resolution and detection
NASA Astrophysics Data System (ADS)
Kopelman, Raoul; Tan, Weihong; Lewis, Aaron; Lieberman, Klony S.
1991-07-01
Subwavelength light sources have been constructed with the aid of luminescent and exciton transporting materials. These EXCITOR (exciton transmitted optical radiation) sources produce evanescent luminescence and can be used as scanning, light emitting tips of nanometer dimensions. They can also be used as scanning exciton donor tips. The theoretical resolution limit of this kind of near-field optical microscopy is on the atomic or molecular scale. The detection limit is a single molecule, but in contrast to other single molecule detection methods, this single molecule could be identified spatially as well as spectrally. Experimental examples of such an EXCITOR tip consist of gold or aluminum coated glass micropipettes with active crystal tips (anthracene, tetracene, perylene, etc.). Design considerations involve optical, excitonic, photochemical and mechanical properties of the luminescent point source. As it is scanned over a sample, it senses a variety of perturbations such as quenching or external heavy atom effects. It can also actively excite a luminescent probe. The latter process can be non-radiative (e.g., Forster) or may involve absorption and re-emission of evanescent luminescence. Spatially coupled emission and absorption processes are of both theoretical and practical interest. They open a way for reducing by many orders of magnitude the number of photons required to excite a single, isolated chromophore. Molecular exiton microscopy allows extention of near-field microscopy beyond the 50 nm limit already achieved and, thus, permits a new frontier of resolution with light based on the limits of energy transfer measurements. In essence, then, the goal of this research is a spectrally sensitive light microscope that will have the capability to zoom non-destructively and in air from the limits of resolution of lens-based confocal light microscopy (200 nm) to molecular dimensions of 1 nm.
Phases of the excitonic condensate in two-layer graphene
NASA Astrophysics Data System (ADS)
Suprunenko, Yevhen F.; Cheianov, Vadim; Fal'ko, Vladimir I.
2012-10-01
Two graphene monolayers that are oppositely charged and placed close to each other are considered. Taking into account valley and spin degeneracy of electrons, we analyze the symmetry of the excitonic insulator states in such a system and build a phase diagram that takes into account the effect of the symmetry breaking due to the external in-plane magnetic field and the carrier density imbalance between the layers.
Coexistence of Exciton Fission and Fusion in Tetracene Crystals
R. P. Groff; P. Avakian; R. E. Merrifield
1970-01-01
Triplet exciton fusion and singlet excition fission have been observed in tetracene crystals, and the magnetic field dependence and the rate constants for these processes have been measured. At 0°C the singlet fission rate constant and the triplet-triplet fusion rate constant (leading to excited singlets) are (6.3+\\/-0.7) ×108 sec-1 and 9×10-10 cm3 sec-1 (within an order of magnitude), respectively.
Quantum Dot Solar Cells with Multiple Exciton Generation
M. C. Hanna; M. C. Beard; J. C. Johnson; J. Murphy; R. J. Ellingson; A. J. Nozik
2005-01-01
We have measured the quantum yield of the multiple exciton generation (MEG) process in quantum dots (QDs) of the lead-salt semiconductor family (PbSe, PbTe, and PbS) using fs pump-probe transient absorption measurements. Very high quantum yields (up to 300%) for charge carrier generation from MEG have been measured in all of the Pb-VI QDs. We have calculated the potential maximum
Efficient multi-exciton emission from quantum dots.
Luk, Ting Shan
2010-09-01
The fundamental spontaneous emission rate an emitter can be modified by its photonic environment. By enhancing the spontaneous emission rate, there is a possibility of extracting multi-exciton energies through radiative decay. In this report, we explore using high Q and small volume cavities to enhance the spontaneous emission rate. We observed greater than 50 folds enhancement in the spontaneous emission from photonic crystal waveguide or microcavity using close-packed monolayer of PbS quantum dot emitters.
Excitons and Cavity Polaritons for Optical Lattice Ultracold Atoms
Hashem Zoubi; Helmut Ritsch
2013-01-25
Ultracold atoms uniformly filling an optical lattice can be treated like an artificial crystal. An implementation including the atomic occupation of a single excited atomic state can be represented by a two-component Bose-Hubbard model. Its phase diagram exhibits a quantum phase transition from a superfluid to a Mott insulator phase. The dynamics of electronic excitations governed by electrostatic dipole-dipole interactions in the ordered regime can be well described by wave-like collective excitations called excitons. Here we present an extensive study of such excitons for a wide range of geometries and dimensionality. Their lifetimes can vary over many orders of magnitude from metastable propagation to superradiant decay. Particularly strong effects occur in one dimensional atomic chains coupled to tapered optical fibers. For an optical lattice within a cavity the excitons are coupled to cavity photons and the resulting collective cavity QED model can be efficiently formulated in terms of polaritons. Their properties are explicitly calculated for different lattices and they constitute a non-destructive monitoring tool for important system properties. Even the formation of molecules in optical lattices manifests itself in modified polariton properties as e.g. an anisotropic optical spectrum. Partial dissipation of the exciton energy in the lattice leads to heating, which can be microscopically understood through a mechanism transferring atoms into higher Bloch bands via a resonant excitation transfer among neighboring lattice sites. The presence of lattice defects like vacancies in the Mott insulator induces a characteristic scattering of polaritons, which can be optically observed to monitor the lattice integrity. Our models can be applied to simulate and understand corresponding collective phenomena in solid crystals, where many effects are often masked by noise and disorder.
Magneto-photoluminescence of excitons in single quantum dots
J. G. Tischler; A. S. Bracker; D. Gammon
2001-01-01
We report magneto-spectroscopy of individual excitonic states localized laterally in narrow GaAs\\/AlGaAs quantum wells by interface fluctuations in the Faraday and Voigt geometries. Because of the extraordinarily narrow linewidths, splittings in the spectral lines are resolved in single dot spectra in both magnetic geometries. Measurements in the Faraday geometry probe the bright states (J=1), which depend among other factors on
Quantum Tunneling, Field Induced Injecting Contact, and Excitons
Yixin Liu
1995-01-01
This thesis consists of three parts: Quantum tunneling simulation, Schottky barrier induced injecting contact on wide band gap II-VI materials, and excitons in semiconductor heterostructures. Part I presents a new method for quantum transport calculations in semiconductor tunnel structures using multiband {bf k}cdot{bf p} theory. This method circumvents the numerical instability problems that arise in the standard transfer -matrix method.
Evidence for Conical Intersection Dynamics Mediating Ultrafast Singlet Exciton Fission
Musser, Andrew J.; Liebel, Matz; Schnedermann, Christoph; Wende, Torsten; Kehoe, Tom B.; Rao, Akshay; Kukura, Philipp
2015-01-01
-based organic photovoltaic cell. Science 340, 334–337 (2013). 5. Johnson, R. C. & Merrifield, R. E. Effects of Magnetic Fields on the Mutual Annihiliation of Triplet Excitons in Anthracene Crystals. Phys. Rev. B 1, 896–902 (1970). 6. Beljonne, D., Yamagata... -Queisser limit for single-junction solar cells. By converting high-energy photons into two low-energy excited states, singlet fission offers a means to overcome thermalisation losses. Devices based on pentacene, a fission sensitiser, have demonstrated external...
Environmental effect for exciton transition energy of single carbon nanotubes
NASA Astrophysics Data System (ADS)
Saito, Riichiro; Sato, Kentaro; Jinsung, Park; Miyauchi, Yuhei; Maruyama, Shigeo; Dresselhaus, Mildred; Dresselhaus, Gene
2008-03-01
The exiton transition energies of single wall carbon nanotubes which are observed in resonance Raman, photo-absorption, and photoluminescense spectroscopies, depend on the surrouding materials (environmental effect). The environmental effect can be explained by screeining of the excitonic states by the dielectric materials. We calculate the transition energies for many different (n,m) carbon nanotubes up to 4eV and to 3nm in diameter. The calculated results are compare with many experimental data with different conditions for samples. The energy shift for the exciton transition energies can be explained by a fitting parameter of static dielectric constants of surrouding materials. However we will show that the effective dielectric constant has a unique, type, metallicity, diameter, and energy dependence of the dielectric constants in order to reproduce the exciton energies for the wide range of diameter and excitation energies. By analyzing the data, we will give a simple formula for the dielectric constants for carbon nanotubes themselves and the surrounding materials as a function of chirality and diameter of single wall carbon nanotubes.
Direct observation of substrate induced exciton in carbon nanotube
NASA Astrophysics Data System (ADS)
Ye, Lin-Hui; Yu, Dang-Min; Wang, Sheng; Zhang, Zhiyong; Peng, Lian-Mao
2013-11-01
We have successfully measured the electroluminescence spectra of a single-walled carbon nanotube (CNT) grown to serpentine shape on quartz substrate. We observe two emission peaks: One locates at 0.85 eV and is identified as the usual E11 exciton peak, and the other locates at slightly higher energy of 0.94 eV with similar symmetrical line shape and comparable intensity. However, the extra peak is substantially wider and it broadens with increasing current at unusually faster speed. We show that the extra peak is not from interband transitions, and ascribe it to a type of exciton induced by the formation of substrate-CNT superlattice. The periodic surface potential of the substrate modulates the CNT band structure, causes degeneracy lifting and band flattering at the Brillouin zone, and generates the higher energy exciton. For confirmation, a similar device is fabricated using amorphous SiO2 substrate to avoid the formation of the superlattice. Indeed, the extra emission peak disappears.
The nature of singlet excitons in oligoacene molecular crystals
NASA Astrophysics Data System (ADS)
Yamagata, H.; Norton, J.; Hontz, E.; Olivier, Y.; Beljonne, D.; Brédas, J. L.; Silbey, R. J.; Spano, F. C.
2011-05-01
A theory for polarized absorption in crystalline oligoacenes is presented, which includes Frenkel exciton coupling, the coupling between Frenkel and charge-transfer (CT) excitons, and the coupling of all neutral and ionic excited states to the dominant ring-breathing vibrational mode. For tetracene, spectra calculated using all Frenkel couplings among the five lowest energy molecular singlet states predict a Davydov splitting (DS) of the lowest energy (0-0) vibronic band of only -32 cm-1, far smaller than the measured value of 631 cm-1 and of the wrong sign—a negative sign indicating that the polarizations of the lower and upper Davydov components are reversed from experiment. Inclusion of Frenkel-CT coupling dramatically improves the agreement with experiment, yielding a 0-0 DS of 601 cm-1 and a nearly quantitative reproduction of the relative spectral intensities of the 0-n vibronic components. Our analysis also shows that CT mixing increases with the size of the oligoacenes. We discuss the implications of these results on exciton dissociation and transport.
The nature of singlet excitons in oligoacene molecular crystals.
Yamagata, H; Norton, J; Hontz, E; Olivier, Y; Beljonne, D; Brédas, J L; Silbey, R J; Spano, F C
2011-05-28
A theory for polarized absorption in crystalline oligoacenes is presented, which includes Frenkel exciton coupling, the coupling between Frenkel and charge-transfer (CT) excitons, and the coupling of all neutral and ionic excited states to the dominant ring-breathing vibrational mode. For tetracene, spectra calculated using all Frenkel couplings among the five lowest energy molecular singlet states predict a Davydov splitting (DS) of the lowest energy (0-0) vibronic band of only -32 cm(-1), far smaller than the measured value of 631 cm(-1) and of the wrong sign-a negative sign indicating that the polarizations of the lower and upper Davydov components are reversed from experiment. Inclusion of Frenkel-CT coupling dramatically improves the agreement with experiment, yielding a 0-0 DS of 601 cm(-1) and a nearly quantitative reproduction of the relative spectral intensities of the 0-n vibronic components. Our analysis also shows that CT mixing increases with the size of the oligoacenes. We discuss the implications of these results on exciton dissociation and transport. PMID:21639463
Fluctuating exciton localization in giant ?-conjugated spoked-wheel macrocycles.
Aggarwal, A Vikas; Thiessen, Alexander; Idelson, Alissa; Kalle, Daniel; Würsch, Dominik; Stangl, Thomas; Steiner, Florian; Jester, Stefan-S; Vogelsang, Jan; Höger, Sigurd; Lupton, John M
2013-11-01
Conjugated polymers offer potential for many diverse applications, but we still lack a fundamental microscopic understanding of their electronic structure. Elementary photoexcitations (excitons) span only a few nanometres of a molecule, which itself can extend over microns, and how their behaviour is affected by molecular dimensions is not immediately obvious. For example, where is the exciton formed within a conjugated segment and is it always situated on the same repeat units? Here, we introduce structurally rigid molecular spoked wheels, 6 nm in diameter, as a model of extended ? conjugation. Single-molecule fluorescence reveals random exciton localization, which leads to temporally varying emission polarization. Initially, this random localization arises after every photon absorption event because of temperature-independent spontaneous symmetry breaking. These fast fluctuations are slowed to millisecond timescales after prolonged illumination. Intramolecular heterogeneity is revealed in cryogenic spectroscopy by jumps in transition energy, but emission polarization can also switch without a spectral jump occurring, which implies long-range homogeneity in the local dielectric environment. PMID:24153376
Electronic absorption of Frenkel excitons in topologically disordered systems
Schweizer, K.S.
1986-10-15
A self-consistent effective medium theory of the electronic absorption spectra of tightly bound dipolar excitons in simple fluids is developed within the adiabatic picture. The theoretical approach is based on the isomorphism between the path-integral formulation of quantum theory and classical statistical mechanics and is an extension of previous work (D. Chandler, K.S. Schweizer, and P.G. Wolynes, Phys. Rev. Lett. 49, 1100 (1982)). The consequences of fluid structural disorder on resonant excitation transfer and the statistical fluctuations of single molecule energy levels are simultaneously treated. Detailed numerical calculations are performed to establish the dependence of the absorption spectrum on fluid density, short range order, and the relative magnitude of the resonant transfer vs the single site disorder. The density dependence of the spectral features are found to be a sensitive function of fluid structure and the relative strength of the localizing vs the delocalizing interactions. By comparing the liquid state results with the corresponding crystalline solid behavior, the consequences of topological disorder on the exciton spectrum are identified. The relevance of the theoretical predictions to spectroscopic probes of exciton delocalization in molecular liquids and glasses is discussed.
Polarization and time-resolved photoluminescence spectroscopy of excitons in MoSe2 monolayers
NASA Astrophysics Data System (ADS)
Wang, G.; Palleau, E.; Amand, T.; Tongay, S.; Marie, X.; Urbaszek, B.
2015-03-01
We investigate valley exciton dynamics in MoSe2 monolayers in polarization- and time-resolved photoluminescence (PL) spectroscopy at 4 K. Following circularly polarized laser excitation, we record a low circular polarization degree of the PL of typically ?5%. This is about 10 times lower than the polarization induced under comparable conditions in MoS2 and WSe2 monolayers. The evolution of the exciton polarization as a function of excitation laser energy and power is monitored in PL excitation experiments. Fast PL emission times are recorded for both the neutral exciton of ?3 ps and for the charged exciton (trion) of 12 ps.
Towards Bose-Einstein condensation of semiconductor excitons: the biexciton polarization effect.
Hägele, D; Pfalz, S; Oestreich, M
2009-10-01
We theoretically predict a strong influence of stimulated exciton-exciton scattering on semiconductor luminescence. The stimulated scattering causes circularly polarized instead of unpolarized emission at the biexciton emission line in a degenerate gas of partly spin polarized excitons. The biexciton polarization effect increases with increasing exciton densities and decreasing temperatures and approaches almost unity in the ultimate case of Bose-Einstein condensation. Time- and polarization-resolved luminescence measurements evidence the biexciton polarization effect both in ZnSe and GaAs quantum wells. PMID:19905586
NASA Astrophysics Data System (ADS)
Ehrler, Bruno; Musselman, Kevin P.; Böhm, Marcus L.; Friend, Richard H.; Greenham, Neil C.
2012-10-01
Silicon dominates the solar cell market because of its abundance, mature production processes, and high efficiencies, with the best solar cells approaching the Shockley-Queisser limit. Multiple exciton photogeneration provides a route to solar cells that surpass the Shockley-Queisser limit, and we report the use of pentacene, for which photogenerated singlet excitons rapidly convert into two lower-energy spin-triplet excitons. We report solar cells that couple amorphous silicon to pentacene. We show that a thin layer of nanocrystals between silicon and pentacene allows simultaneously harnessing low-energy photons absorbed in silicon and high-energy photons absorbed in pentacene, generating two excitons via singlet fission.
Continuous-wave and ultrafast coherent reflectivity studies of excitons in bulk GaN
NASA Astrophysics Data System (ADS)
Aoudé, O.; Disseix, P.; Leymarie, J.; Vasson, A.; Leroux, M.; Aujol, E.; Beaumont, B.; Trassoudaine, A.; André, Y.
2008-01-01
We report an extensive study of the excitonic properties of freestanding and heteroepitaxial GaN samples using continuous-wave reflectivity and time-resolved autocorrelation measurements. The coherent impulse response of free excitons is recorded by using an interferometric correlation technique. Excitonic parameters of GaN are deduced from the combined analysis of both experiments. Oscillator strengths and transition energies are studied as a function of the residual biaxial strain of the sample and temperature-dependent measurements are used to determine the parameters of the exciton-phonon interaction in GaN.
Transient pump-probe measurements for polarized excitons in strained GaN epitaxial layers
NASA Astrophysics Data System (ADS)
Ishiguro, T.; Toda, Y.; Adachi, S.; Arita, M.; Arakawa, Y.
We have studied exciton spin relaxation in strained GaN epilayers on various sapphire substrates using spindependent transient reflectivity and three-pulse four-wave mixing measurements. Strikingly fast exciton spin relaxation (<1 ps) at 10 K was observed in the lowest exciton level (A-exciton) both in bulk and GaN layer on c-sapphire. In contrast, the transient signals in the GaN layer on a-sapphire show the break down of the spin polarization caused by the exchange interaction.
Effect of exciton-spin-orbit-photon interaction in the performance of organic solar cells
NASA Astrophysics Data System (ADS)
Narayan, Monishka Rita; Singh, Jai
2013-02-01
Photon absorptions leading to singlet and triplet excitonic states in organic solar cells are presented in this study. Applying Fermi's golden rule, the rates of absorption of singlet and triplet excitons are derived using singlet exciton-photon and triplet exciton-spin-orbit-photon-interaction, respectively, as perturbation operators. The rate of triplet absorption depends on the square of the atomic number and hence heavier atoms play the dominant role. Incorporation of heavy metal atoms in the donor organic material enhances the absorption rate and hence absorption, leading of higher generation of excited charge carriers. This increases the conversion efficiency of organic solar cells. The results are compared with experimental studies.
Energy of K-momentum dark excitons in carbon nanotubes by optical spectroscopy.
Torrens, O N; Zheng, M; Kikkawa, J M
2008-10-10
Phonon sideband optical spectroscopy determines the energy of the dark K-momentum exciton for (6,5) carbon nanotubes. One-phonon sidebands appear in absorption and emission, split by two zone-boundary (K-point) phonons. Their average energy locates the E11 K-momentum exciton 36 meV above the E11 bright level, higher than available theoretical estimates. A model for exciton-phonon coupling shows the absorbance sideband depends sensitively on the K-momentum exciton effective mass and has minimal contributions from zone-center phonons, which dominate the Raman spectra of carbon nanotubes. PMID:18999637
Energy of K-Momentum Dark Excitons in Carbon Nanotubes by Optical Spectroscopy
NASA Astrophysics Data System (ADS)
Torrens, O. N.; Zheng, M.; Kikkawa, J. M.
2008-10-01
Phonon sideband optical spectroscopy determines the energy of the dark K-momentum exciton for (6,5) carbon nanotubes. One-phonon sidebands appear in absorption and emission, split by two zone-boundary (K-point) phonons. Their average energy locates the E11 K-momentum exciton 36 meV above the E11 bright level, higher than available theoretical estimates. A model for exciton-phonon coupling shows the absorbance sideband depends sensitively on the K-momentum exciton effective mass and has minimal contributions from zone-center phonons, which dominate the Raman spectra of carbon nanotubes.
NASA Astrophysics Data System (ADS)
Berciaud, Stéphane; Voisin, Christophe; Yan, Hugen; Chandra, Bhupesh; Caldwell, Robert; Shan, Yuyao; Brus, Louis E.; Hone, James; Heinz, Tony F.
2010-01-01
We examine the excitonic nature of high-lying optical transitions in single-walled carbon nanotubes by means of Rayleigh scattering spectroscopy. A careful analysis of the principal transitions of individual semiconducting and metallic nanotubes reveals that in both cases the line shape is consistent with an excitonic model, but not one of free carriers. For semiconducting species, sidebands are observed at ˜200meV above the third and fourth optical transitions. These features are ascribed to exciton-phonon bound states. Such sidebands are not apparent for metallic nanotubes, as expected from the reduced strength of excitonic interactions in these systems.
Lin, Kuen-Feng; Chiang, Chien-Hung; Wu, Chun-Guey
2014-01-01
The refractive index and extinction coefficient of a triiodide perovskite absorber (TPA) were obtained by fitting the transmittance spectra of TPA/PEDOT:PSS/ITO/glass using the transfer matrix method. Cu nanoplasmonic structures were designed to enhance the exciton generation in the TPA and to simultaneously reduce the film thickness of the TPA. Excitons were effectively generated at the interface between TPA and Cu nanoparticles, as observed through the 3D finite-difference time-domain method. The exciton distribution is advantageous for the exciton dissociation and carrier transport. PMID:25295290
21 CFR 886.1430 - Ophthalmic contact lens radius measuring device.
Code of Federal Regulations, 2014 CFR
2014-04-01
...2014-04-01 false Ophthalmic contact lens radius measuring device. 886...Devices § 886.1430 Ophthalmic contact lens radius measuring device. (a) Identification. An ophthalmic contact lens radius measuring device is...
21 CFR 886.1430 - Ophthalmic contact lens radius measuring device.
Code of Federal Regulations, 2013 CFR
2013-04-01
...2013-04-01 false Ophthalmic contact lens radius measuring device. 886...Devices § 886.1430 Ophthalmic contact lens radius measuring device. (a) Identification. An ophthalmic contact lens radius measuring device is...
21 CFR 886.1430 - Ophthalmic contact lens radius measuring device.
Code of Federal Regulations, 2012 CFR
2012-04-01
...2012-04-01 false Ophthalmic contact lens radius measuring device. 886...Devices § 886.1430 Ophthalmic contact lens radius measuring device. (a) Identification. An ophthalmic contact lens radius measuring device is...
Two-body and three-body interactions in phonon-assisted exciton energy transfer between quantum dots
NASA Astrophysics Data System (ADS)
Xu, Kaijie; Piermarocchi, Carlo
2011-03-01
We theoretically study the dynamics of exciton energy transfer between semiconductor quantum dots. Phonons play a critical role in the exciton energy transfer process when the energy of the dots involved in the process is different. We find that the phonon-assisted energy transfer cannot be correctly described by two-body exciton-photon and exciton-phonon interactions if each dot is modeled as a single-level exciton system. Higher excited levels of the exciton state have to be included to properly describe the phonon-assisted process. However, excited states can be traced out by introducing a single-level exciton model with an effective three-body exciton-photon-phonon coupling term. The three-body term describes a change in the exciton-photon dipolar coupling due to phonon-induced deformations of the ground exciton wavefunction. The multi-level exciton model with two-body interactions and the single-level exciton model with three-body interaction reproduce the same exciton transfer rates to the leading order contributions of perturbation theory.
Nonlinear buckling analyses of a small-radius carbon nanotube
Liu, Ning, E-mail: liuxiao@ase.buaa.edu.cn; Li, Min; Jia, Jiao [School of Aeronautic Science and Engineering, Beihang University, Beijing 100091 (China); Wang, Yong-Gang [Department of Applied Mechanics, China Agricultural University, Beijing 100083 (China)
2014-04-21
Carbon nanotube (CNT) was first discovered by Sumio Iijima. It has aroused extensive attentions of scholars from all over the world. Over the past two decades, we have acquired a lot of methods to synthesize carbon nanotubes and learn their many incredible mechanical properties such as experimental methods, theoretical analyses, and computer simulations. However, the studies of experiments need lots of financial, material, and labor resources. The calculations will become difficult and time-consuming, and the calculations may be even beyond the realm of possibility when the scale of simulations is large, as for computer simulations. Therefore, it is necessary for us to explore a reasonable continuum model, which can be applied into nano-scale. This paper attempts to develop a mathematical model of a small-radius carbon nanotube based on continuum theory. An Isotropic circular cross-section, Timoshenko beam model is used as a simplified mechanical model for the small-radius carbon nanotube. Theoretical part is mainly based on modified couple stress theory to obtain the numerical solutions of buckling deformation. Meanwhile, the buckling behavior of the small radius carbon nanotube is simulated by Molecular Dynamics method. By comparing with the numerical results based on modified couple stress theory, the dependence of the small-radius carbon nanotube mechanical behaviors on its elasticity constants, small-size effect, geometric nonlinearity, and shear effect is further studied, and an estimation of the small-scale parameter of a CNT (5, 5) is obtained.
A Smaller Radius for the Transiting Exoplanet WASP-10b
John A. Johnson; Joshua N. Winn; Nicole E. Cabrera; Joshua A. Carter
2008-11-29
We present photometry of WASP-10 during the transit of its short-period Jovian planet. We employed the novel PSF-shaping capabilities the OPTIC camera mounted on the UH 2.2m telescope to achieve a photometric precision of 4.7e-4 per 1.3 min sample. With this new light curve, in conjunction with stellar evolutionary models, we improve on existing measurements of the planetary, stellar and orbital parameters. We find a stellar radius Rstar = 0.698 +/- 0.012 Rsun and a planetary radius Rp = 1.080 +/- 0.020 Rjup. The quoted errors do not include any possible systematic errors in the stellar evolutionary models. Our measurement improves the precision of the planet's radius by a factor of 4, and revises the previous estimate downward by 16% (2.5sigma, where sigma is the quadrature sum of the respective confidence limits). Our measured radius of WASP-10b is consistent with previously published theoretical radii for irradiated Jovian planets.
Percutaneous pinning for fractures of the distal radius
Marco ROSATI; Sara BERTAGNINI; Giuseppe DIGRANDI; Carmelo SALA
2006-01-01
The results of treatment of wrist fractures with per- cutaneous pin fixation in 106 patients are reported. Functional and radiological evaluation was made for all patients. All fractures healed ; loss of reduction occurred in only two cases. The best results were obtained in metaphyseal fractures of the radius, while good results were obtained in intra-articular fractures ; the worst
Ultrasound-Guided Reduction of Distal Radius Fractures
Brian Chinnock; Alexander Khaletskiy; Kane Kuo; Gregory W. Hendey
2011-01-01
Background: Ultrasound (US) may provide the emergency physician with the ability to do real-time assessment of fracture reduction adequacy. Objectives: To assess whether US guidance aids in determining the adequacy of distal radius fracture reduction in the emergency department (ED), and to compare the rates of successful reduction with and without US. Methods: We conducted a prospective study of patients
Bound on the neutrino charge radius from primordial nucleosynthesis
Grifols, J.A.; Masso, E.
1987-03-01
From the observational data on nucleosynthetic yields of /sup 4/He the authors derive a constraint on the neutrino charge radius. They obtain the upper limit
Nonlinear buckling analyses of a small-radius carbon nanotube
NASA Astrophysics Data System (ADS)
Liu, Ning; Wang, Yong-Gang; Li, Min; Jia, Jiao
2014-04-01
Carbon nanotube (CNT) was first discovered by Sumio Iijima. It has aroused extensive attentions of scholars from all over the world. Over the past two decades, we have acquired a lot of methods to synthesize carbon nanotubes and learn their many incredible mechanical properties such as experimental methods, theoretical analyses, and computer simulations. However, the studies of experiments need lots of financial, material, and labor resources. The calculations will become difficult and time-consuming, and the calculations may be even beyond the realm of possibility when the scale of simulations is large, as for computer simulations. Therefore, it is necessary for us to explore a reasonable continuum model, which can be applied into nano-scale. This paper attempts to develop a mathematical model of a small-radius carbon nanotube based on continuum theory. An Isotropic circular cross-section, Timoshenko beam model is used as a simplified mechanical model for the small-radius carbon nanotube. Theoretical part is mainly based on modified couple stress theory to obtain the numerical solutions of buckling deformation. Meanwhile, the buckling behavior of the small radius carbon nanotube is simulated by Molecular Dynamics method. By comparing with the numerical results based on modified couple stress theory, the dependence of the small-radius carbon nanotube mechanical behaviors on its elasticity constants, small-size effect, geometric nonlinearity, and shear effect is further studied, and an estimation of the small-scale parameter of a CNT (5, 5) is obtained.
Dark excitons due to direct Coulomb interactions in silicon quantum dots
NASA Astrophysics Data System (ADS)
Reboredo, F. A.; Franceschetti, A.; Zunger, A.
2000-05-01
Electron-hole exchange interactions can lead to spin-forbidden ``dark'' excitons in direct-gap quantum dots. Here, we explore an alternative mechanism for creating optically forbidden excitons. In a large spherical quantum dot made of a diamond-structure semiconductor, the symmetry of the valence band maximum (VBM) is t2. The symmetry of the conduction band minimum (CBM) in direct-gap material is a1, but for indirect-gap systems the symmetry could be (depending on size) a1, e, or t2. In the latter cases, the resulting manifold of excitonic states contains several symmetries derived from the symmetries of the VBM and CBM (e.g., t2×t2=A1+E+T1+T2 or t2×e=T1+T2). Only the T2 exciton is optically active or ``bright,'' while the others A1, E, and T1 are ``dark.'' The question is which is lower in energy, the dark or bright. Using pseudopotential calculations of the single-particle states of Si quantum dots and a direct evaluation of the screened electron-hole Coulomb interaction, we find that, when the CBM symmetry is t2, the direct electron-hole Coulomb interaction lowers the energy of the dark excitons relative to the bright T2 exciton. Thus, the lowest energy exciton is forbidden, even without an electron-hole exchange interaction. We find that our dark-bright excitonic splitting agrees well with experimental data of Calcott et al., Kovalev et al., and Brongersma et al. Our excitonic transition energies agree well with the recent experiment of Wolkin et al. In addition, and contradicting simplified models, we find that Coulomb correlations are more important for small dots than for intermediate sized ones. We describe the full excitonic spectrum of Si quantum dots by using a many-body expansion that includes both Coulomb and exchange electron hole terms. We present the predicted excitonic spectra.
Zhao, Hui; Moehl, Sebastian; Kalt, Heinz
2002-10-01
, originating from the LO-phonon assisted recombination of hot excitons, is measured directly. By calculating the LO-phonon assisted recombination probability, we obtain the nonthermal energy distribution of excitons and observe directly the energy relaxation...
NASA Astrophysics Data System (ADS)
Ding, Baofu; Alameh, Kamal
2014-07-01
The research field of organic spintronics has remarkably and rapidly become a promising research area for delivering a range of high-performance devices, such as magnetic-field sensors, spin valves, and magnetically modulated organic light emitting devices (OLEDs). Plenty of microscopic physical and chemical models based on exciton or charge interactions have been proposed to explain organic magneto-optoelectronic phenomena. However, the simultaneous observation of singlet- and triplet-exciton variations in an external magnetic field is still unfeasible, preventing a thorough theoretical description of the spin dynamics in organic semiconductors. Here, we show that we can simultaneously observe variations of singlet excitons and triplet excitons in an external magnetic field, by designing an OLED structure employing a singlet-exciton filtering and detection layer in conjunction with a separate triplet-exciton detection layer. This OLED structure enables the observation of a Lorentzian and a non-Lorentzian line-shape magnetoresponse for singlet excitons and triplet excitons, respectively.
Radiative recombination and ultralong exciton photoluminescence lifetime in GaN freestanding film June 2006; published online 10 July 2006 We have measured the photoluminescence PL lifetime in growth techniques generates considerable interest in the photoluminescence PL excitonic lifetime
Variable radius cartography - History and perspectives of a new discipline
NASA Astrophysics Data System (ADS)
Scalera, Giancarlo
2014-05-01
The map that Toscanelli sent to Columbus was an unconscious application of cartography at a smaller radius than the real. The first really conscious attempts to represent the geography of Earth on globes of radius less than the current one occurred after the formulation of the concept of expanding Earth through geological time. The American chemist and geologist Richard Owen (1810-1890) in his book Key to the geology of the globe (1857) described the principles of what he himself called Anatomical Geology, with the Earth growing as a biological organism. The book contained a global paleogeographic map of the Earth that would have had a radius of about 4000 kilometers. In 1928 J.A.H. Kerkhoff (under the pseudonym Aero-dilettant) published a series of paleogeographic globes on which the modern oceans disappeared. With the same artisan methods of transfer continental outlines from a sphere to a smaller one, in 1933 O.C. Hilgenberg represented three different geological epochs, and, later, for the first time mapped paleopoles with their site-pole segments of meridian. Even today the traditional method of Hilgenberg is followed by senior researchers (Klaus Vogel, 2003) and younger geologists (James Maxlow). In England Hugh Owen applied the methods of traditional cartography to the variable radius one. His Atlas of Continental Displacement was in the 70s and 80s, for this discipline, a real milestone. While in the field of constant radius paleogeography the adherents to plate tectonics created many computer codes of automatic mapping (Bullard et al., 1965; Smith & Hallam, 1970; Scotese et al., 1979; and many others), in the variable radius field few tried to reach the same task. In 1972 in United States a first very simple attempt (but was not further developed) came from a private, R.B. Perry, followed by the still not-computerized Atlas of Owen, and both them constituted inspiration for the construction of a FORTRAN variable radius mapping code at INGV, with which it is now possible to represent paleopoles, their uncertainty ellipses, and site-pole segments of meridian (Scalera, 1988, 1990). In all paleogeographic reconstructions of the different authors, variable radius cartography is used in a way more or less complex, more or less intertwined with other disciplines and databases, not as pure representation or in the spirit of the simple fits that supported plate tectonics, but as experiments of greater complexity with a value of proof in favor of the planet expansion. Today a common feeling is that is now necessary to develop an interactive and user friendly program code, which could be distributed or used in the web. The use of variable radius mapping would be a profitable tool in the field of geodesy, where a full treatment without subtle vicious loops of an expanding globe has yet to be developed.
Shin-ichiro Hataoka; Akihiro Itoh; Ichiro Tanahashi; Koichiro Tanaka
2000-01-01
We performed two-beam spectrally resolved four-wave mixing (SR-FWM) experiments around the exciton resonance in the reflection geometry in ZnSe multiple quantum wells (MQWs) to investigate the high-density excitation effects of biexcitons as well as excitons. This experiment is performed in the exciton-density region where ?(3)-process is no longer a good approximation. The exciton-density dependence of the FWM spectra reveals that
Picosecond dynamics and stimulated emissions of excitons in Zn1-xCdxSe\\/ZnSe quantum wells
Takeham Tsutsumi; Jen Y. Jen; Izuru Souma; Yasuo Oka
1994-01-01
The dynamics and stimulated emission processes of the exciton luminescence are studied in quantum wells (QWs) of the Zn1-xCdxSe\\/ZnSe system. A multiquantum well (MQW) structure shows an exciton lifetime of 150-280 ps and a stimulated emission effect due to exciton-exciton scattering as well as due to electron-hole plasma recombination. A combined-QW structure in which a single quantum well (SQW) is
Motion Perception During Variable-Radius Swing Motion in Darkness
Rader, A. A.; Oman, C. M.; Merfeld, D. M.
2009-01-01
Using a variable-radius roll swing motion paradigm, we examined the influence of interaural (y-axis) and dorsoventral (z-axis) force modulation on perceived tilt and translation by measuring perception of horizontal translation, roll tilt, and distance from center of rotation (radius) at 0.45 and 0.8 Hz using standard magnitude estimation techniques (primarily verbal reports) in darkness. Results show that motion perception was significantly influenced by both y- and z-axis forces. During constant radius trials, subjects' perceptions of tilt and translation were generally almost veridical. By selectively pairing radius (1.22 and 0.38 m) and frequency (0.45 and 0.8 Hz, respectively), the y-axis acceleration could be tailored in opposition to gravity so that the combined y-axis gravitoinertial force (GIF) variation at the subject's ears was reduced to ?0.035 m/s2 – in effect, the y-axis GIF was “nulled” below putative perceptual threshold levels. With y-axis force nulling, subjects overestimated their tilt angle and underestimated their horizontal translation and radius. For some y-axis nulling trials, a radial linear acceleration at twice the tilt frequency (0.25 m/s2 at 0.9 Hz, 0.13 m/s2 at 1.6 Hz) was simultaneously applied to reduce the z-axis force variations caused by centripetal acceleration and by changes in the z-axis component of gravity during tilt. For other trials, the phase of this radial linear acceleration was altered to double the magnitude of the z-axis force variations. z-axis force nulling further increased the perceived tilt angle and further decreased perceived horizontal translation and radius relative to the y-axis nulling trials, while z-axis force doubling had the opposite effect. Subject reports were remarkably geometrically consistent; an observer model-based analysis suggests that perception was influenced by knowledge of swing geometry. PMID:19625542
Eric Chapman Greyson
2007-01-01
This thesis investigates singlet exciton fission, a physical process that converts one singlet exciton to a pair of triplet excitons. Singlet fission was discovered nearly forty years ago, but the mechanism for this process is still not well understood. Recent work has suggested that singlet fission may be capable of enhancing the performance of dye-sensitized solar cells. This thesis proposes
Polarization states of charged excitons in coupled InAs\\/GaAs quantum dot molecules
Ramana Thota; Swati Ramanathan; Kushal Wijesundara; Eric Stinaff; Allan Bracker; Dan Gammon
2011-01-01
The polarization state of charged excitons in coupled InAs\\/GaAs dots can reveal useful information about the spin state of its charge carriers. In this study, we examine the complete polarization state through Stokes parameter measurements to relate the polarization parameters of the luminescence to the spin configurations of the various charged excitons they originate from. We demonstrate that this method
can be transformed into singlet because of the slow relaxation to the lowest triplet exciton. The net efficiency can be raised significantly within a range of optimal doping density. DOI: 10.1103/PhysRevB.68-hole pair, is composed of three factors: int ex S r . ex is the exciton formation ratio per injected pair
Rabi splitting and ac-Stark shift of a charged exciton M. Kroner,a
Ludwig-Maximilians-Universität, München
Rabi splitting and ac-Stark shift of a charged exciton M. Kroner,a C. Lux, S. Seidl, A. W online 23 January 2008 The Rabi splitting of the negatively charged exciton in a single InGaAs quantum dispersion, with a power dependent Rabi splitting on resonance, both signatures of a strongly coupled two
External quantum efficiency exceeding 100% in a singlet-exciton-fission-based solar cell
NASA Astrophysics Data System (ADS)
Baldo, Marc
2013-03-01
Singlet exciton fission can be used to split a molecular excited state in two. In solar cells, it promises to double the photocurrent from high energy photons, thereby breaking the single junction efficiency limit. We demonstrate organic solar cells that exploit singlet exciton fission in pentacene to generate more than one electron per incident photon in the visible spectrum. Using a fullerene acceptor, a poly(3-hexylthiophene) exciton confinement layer, and a conventional optical trapping scheme, the peak external quantum efficiency is (109+/-1)% at ?= 670 nm for a 15-nm-thick pentacene film. The corresponding internal quantum efficiency is (160+/-10)%. Independent confirmation of the high internal efficiency is obtained by analysis of the magnetic field effect on photocurrent, which determines that the triplet yield approaches 200% for pentacene films thicker than 5 nm. To our knowledge, this is the first solar cell to generate quantum efficiencies above 100% in the visible spectrum. Alternative multiple exciton generation approaches have been demonstrated previously in the ultraviolet, where there is relatively little sunlight. Singlet exciton fission differs from these other mechanisms because spin conservation disallows the usual dominant loss process: a thermal relaxation of the high-energy exciton into a single low-energy exciton. Consequently, pentacene is efficient in the visible spectrum at ?= 670 nm because only the collapse of the singlet exciton into two triplets is spin-allowed.
Wave-vector dependence of magnetic properties of excitons in ZnTe
NASA Astrophysics Data System (ADS)
Smith, L. C.; Davies, J. J.; Wolverson, D.; Boukari, H.; Mariette, H.; Kochereshko, V. P.; Phillips, R. T.
2011-04-01
The magnetic properties of heavy-hole excitons in wide quantum wells of ZnTe with ZnxMg1-xTe barriers have been studied with photoluminescence and reflectivity measurements. The exciton magnetic moments (as characterized by the g values) and the diamagnetic shifts of the exciton transitions are found to depend strongly on the wave-vector component Kz associated with translational motion of the exciton normal to the plane of the quantum well. The case of ZnTe differs from examples of this behavior previously reported for GaS, CdTe, and ZnSe since the ZnTe is under tensile biaxial strain, so that the heavy-hole exciton states lie higher in energy than the corresponding states of the light-hole excitons. The dependence of the magnetic properties on Kz is nevertheless still in excellent agreement with the predictions of a model proposed by Smith [Phys. Rev. BPRLTAO1098-012110.1103/PhysRevB.78.085204 78, 085204 (2008)], in which mixing of the heavy-hole 1S exciton state with light-hole nP states is found to be responsible for motion-induced changes in the internal structure of the exciton.
Low-threshold ZnSe microdisk laser based on fluorine impurity bound-exciton transitions
As, Donat Josef
-doped ZnMgSe/ZnSe quantum wells in microdisk cavities. The lasing mechanism was studied by power in resonators with quantum wells (QWs) [7] or quantum dots [8] as the active media. Bound-exciton emission in ZnSeLow-threshold ZnSe microdisk laser based on fluorine impurity bound-exciton transitions A. Pawlis a
Excitonic field screening and bleaching in InGaN/GaN multiple quantum wells
Cartwright, Alexander N.
Excitonic field screening and bleaching in InGaN/GaN multiple quantum wells Fei Chen*, W.D. Kirkey, provides evidence of both excitonic bleaching and field screening in these InGaN quantum wells (QWs from the bleaching effects. Finally, the extreme prolongation of the carrier recombination lifetime up
Engineering of optically defect free Cu2O enabling exciton luminescence
Wang, Wei Hua
and subgap density of states in p-type Cu2O thin-film transistors," Appl. Phys. Lett. 102(8), 082103 (2013Engineering of optically defect free Cu2O enabling exciton luminescence at room temperature@iphy.ac.cn Abstract: Cu2O is an interesting semiconductor with extraordinary high exciton binding energy, however
Steiner, Ullrich
December 2013; published online 20 December 2013) Room-temperature tunable excitonic photoluminescence and strong photoluminescence (PL) at room-temperature due to stable excitons formed within the low states corresponding to bromine rich sites towards the lower energy iodine sites. Apart from
NASA Astrophysics Data System (ADS)
Nieminen, J. V.; Vänskä, O.; Tittonen, I.; Koch, S. W.; Kira, M.
2015-03-01
A full quantum theory is developed for semiconductor quantum ring photoluminescence. The computations show that the orbital angular momentum (OAM) of individual excitons yields strong angle-dependent emission. The characteristic OAM signatures also survive significant amount of dephasing, suggesting efficient quantum-level coupling between quantum-ring excitons and the OAM of light.
Spectral properties and control of an exciton trapped in a multi-layered quantum dot
Mariano Garagiola; Omar Osenda
2014-12-23
The spectral properties of one exciton trapped in a self-assembled multi-layered quantum dot is obtained using a high precision variational numerical method. The exciton Hamiltonian includes the effect of the polarization charges, induced by the presence of the exciton in the quantum dot, at the material interfaces. The method allows to implement rather easily the matching conditions at the interfaces of the hetero-structure. The numerical method also provides accurate approximate eigenfunctions that enable the study of the separability of the exciton eigenfunction in electron and hole states. The separability, or the entanglement content, of the total wave function allows a better understanding of the spectral properties of the exciton and, in particular, shed some light about when the perturbation theory calculation of the spectrum is fairly correct or not. Finally, using the approximate spectrum and eigenfunctions, the controlled time evolution of the exciton wave function is analyzed when an external driving field is applied to the system. It is found that it is possible to obtain pico and sub-picoseconds controlled oscillations between two particular states of the exciton with a rather low leakage of probability to other exciton states, and with a simple pulse shape.
Roberto Gangemi; Giovanna Longhi; France Lebon; Sergio Abbate; Leo Laux
2005-01-01
Summary. A set of vibrational circular dichroism (VCD) spectra in the CH-stretching fundamental region for about twenty compounds belonging to the class of essential oils was empirically analyzed by the use of a sort of vibrational exciton mechanism, involving three centers. Through a general formula applicable to many coupled dipole oscillators, the rotational strengths of the previously identified vibrational excitons
Fission rate of singlet excitons in a tetracene crystal measured with picosecond laser pulses
R. R. Alfano; S. L. Shapiro; M. Pope
1973-01-01
The fluorescenve lifetime of a tetracene crystal at room temperature was measured directly using picosecond techniques. The value obtained was 145+\\/-50 psec. The risetime of the fluorescence was <10-11 sec. The singlet exciton fission rate at infinite temperature is -1013 sec-1 and is attributed to the hopping rate of triplet excitons in the crystal.